Assessment of the Thermodynamic Properties of DL-p-Mentha-1,8-diene, 4-Isopropyl-1-Methylcyclohexene (DL-limonene) by Inverse Gas Chromatography (IGC)

Assessment of the Thermodynamic Properties of DL-p-Mentha-1,8-diene,... Abstract Limonene is a colorless liquid hydrocarbon and had been investigated as a plasticizer for many plastics. Prediction of solubility between different materials is an advantage in many ways, one of the most convenient ways to know the compatibility of materials is to determine the degree of solubility of them in each other. The concept of “solubility parameter” can help practitioners in this way. In this study, inverse gas chromatography (IGC) method at infinite dilution was used for determination of the thermodynamic properties of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). The interaction between DL-limonene and 13 solvents were examined in the temperature range of 63–123°C through the assessment of the thermodynamic sorption parameters, the parameters of mixing at infinite dilution, the weight fraction activity coefficient and the Flory–Huggins interaction parameters. Additionally, the solubility parameter for DL-limonene and the temperature dependence of these parameters was investigated as well. Results show that there is a temperature dependence in solubility parameter, which increases by decreasing temperature. However, there were no specific dependence between interaction parameters and temperature, but chemical structure appeared to have a significant effect on them as well as on the type and strength of intermolecular interactions between DL-limonene and investigated solvents. The solubility parameter δ2 of DL-limonene determined to be 19.20 (J/cm3)0.5 at 25°C. Introduction Limonene is a colorless liquid hydrocarbon and it is classified as a cyclic terpene. Its name comes from lemon, as lemon peel, as well as other citrus fruits that hold substantial amounts of this compound, which contributes to their odor. It is a chiral molecule and one enantiomer is produced by biological sources (1). As the main odor constituent of citrus, limonene is common in cosmetic products, also in food manufacturing and some medicines. Because its ability to dissolve oils, it is added to cleaning products. It is used as a paint stripper and is also useful as a pleasant-smelling alternative to turpentine (2). Limonene is also gradually used as a solvent for filament-fused 3D printing. Printers can print the plastic of choice for the model, but erect supports and binders from high impact polystyrene, a polystyrene plastic that is easily soluble in limonene (3). As it is combustible, limonene has also been considered as a biofuel (4). Prediction of solubility between different materials is an advantage in many ways, taking the example of polymer blends, polymer compounds, adhesives or paints. It also helps engineers in order to choose the right materials for compounding of polymer mixtures. For compounding, one needs to know the compatibility between different compounding ingredients, for example, polymer–plasticizer, polymer–polymer, reinforcing agent–plasticizer and so on to select the correct materials to compound them (5). Recently, studies have been done on the use of limonene as a new monomer to obtain polyterpenes (6). Limonene had also been investigated as a plasticizer for many plastics such as polyethylene (7), polystyrene (8) and poly (lactic acid) in the food packaging industry (9). One of the most convenient ways to know the compatibility of materials is to determine the degree of solubility of them in each other. The concept of “solubility parameter” can help practitioners in this way. Inverse gas chromatography (IGC) has been shown to be a reliable and very useful technique for the measurement of thermodynamic parameters in a range of non-volatile materials, over a wide range of conditions (10). In gas-liquid IGC, the non-volatile binary solution component is distributed on a chromatographic packing material which is held under specified conditions in the chromatographic column. By injecting a small quantity of solvents (probes) into the column, the thermodynamic parameters of stationary phase can be calculated from the specific retention volume of probes as they flow through the chromatographic column. The IGC method has been applied for the assessment of the surface and thermodynamic characteristics of different materials such as polymers (11, 12), ionic liquids (13), carbon nanotubes (14), liquid crystalline systems (15), natural oils (16) and pharmaceuticals (17). Despite the fact that many authors used IGC, studies concerning the thermodynamic properties of limonene have not yet been reported. Here, we applied the IGC technique to determine the thermodynamic properties and solubility parameter of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Materials and methods Materials DL-limonene (DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene) sample from Merck (USA), with boiling point of 178°C (1013 hPa), density of 0.84 g/cm3 (20°C) and melting point of −89°C, is used as received (pure). Its chemical structure is shown in Figure 1. Chromosorb P AW-DMCS (60–80 mesh, Merck) was used as solid support. The solvents that are listed below were used as probes for IGC measurements. They were selected with regard to their ability to interact with three different types of interaction forces, that is dispersive, polar and hydrogen bonding. All probes (Aldrich or Merck) were highly pure grade (i.e., 99%). The probes used were n-alkanes (n-hexane, n-heptane, n-octane and n-nonane), alcohols (chloroform, methanol, ethanol and 1-butanol), polar solvents (acetone and ethyl acetate) and non-polar solvents (diethyl ether, cyclohexane and toluene). Chromatographic injections were made using syringes obtained from the Hamilton Company (Reno, NV, USA). Figure 1. View largeDownload slide Chemical structure of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Figure 1. View largeDownload slide Chemical structure of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Column preparation and IGC setup The IGC measurements were performed on a commercial Shimadzu GC-14A gas chromatograph equipped with a flame ionization detector (FID) and two manometers to determine column’s inlet and outlet pressures. Dried nitrogen was used as carrier gas. According to our previous work, flow rate set to be 30 mL/min. The injector and detector temperatures were kept at 200°C during the experiment. Diethyl ether were used as non-interacting markers in order to determine the void volume of the column. To achieve infinite dilution, 0.05 μL of each probe was injected with a 10 μL Hamilton syringes. The column temperatures were 63–123 varied in 20°C steps. The boiling point of limonene is 178°C and the highest operating temperature used in this study was 123°C that is low enough for prevent any loss of limonene due to volatilization. The weight of the column before and after the IGC procedure showed no difference. Each probe injection was repeated three times and average retention time, tR was used for calculation. The column used in this study was prepared using a stainless still column (SS 316 ASTM A-269) with 3.66 mm inner diameter, 4.60 mm outer diameter and having an approximate length of 2 m. Column packing was done by mixing a selected weight percentage of DL-limonene dissolved in hexane (10% weight) with the chromosorb followed by solvent removal using a rotary evaporator for 6 h. The packed column then preconditioned (highest temperature and nitrogen flow rate) overnight in order to remove any residual solvent left in the packing material. The coated mass was 0.68 g with 10.2% weight loading of DL-limonene. SEM measurements Scanning electron microscope (SEM) measurements were done on an AIS2100 device from Seron technologies Co., Ltd. The SEM micrographs were used to determine the stationary phase morphology. Moreover, these micrographs can show the accuracy in the preparation of stationary phase. Theory/calculations Inverse gas chromatography theory Hildebrand introduced the concept of the cohesive energy density, CED and solubility in a series of articles starting in 1916. The solubility parameter, δ, introduced later in 1949 related to CED (18).   δ=(ΔEvapV)12 (1)where ΔEvap is enthalpy of vaporization. CED represents the required energy to detach the liquid molecules into the ideal gas state. Smidsrob and Guillet developed the IGC technology in 1969 (19). This technique was found to be a convenient method to obtain thermodynamic quantities and investigate the physicochemical matter properties. The technique involves filling a column with a stationary phase of the solid material as the object of investigation, while probes of known physicochemical properties are injected. By determining the retention times of the probes, the specific retention volume, Vg0, of the probes which stands for the elution behavior of probes may be obtained according to the following equation (20):   Vg0=273.15mTaFP0−PWP0(tr−t0)32(Pi/P0)2−1(Pi/P0)3−1 (2)where F is the flow rate of the carrier gas measured at room temperature; m is the mass of stationary phase; Ta is the flow meter temperature; Pw is the saturated vapor pressure of water at ambient temperature; tr is the retention times of the probes; t0 is the retention time of the non-interacting probe and Pi and P0 are inlet and outlet pressure of the column, respectively. The molar heat of sorption, ΔH1s and the molar free energy of sorption, ΔG1s of the probe absorbed by the solid phase are calculated by the following equations (9):   ΔH1s=−R∂lnVg0∂(1T) (3)  ΔG1s=−RTln(M1Vg0273⋅15R) (4)where T is the column temperature, M1 the molecular weight of the probe and R the gas constant. The calculation of the entropy of sorption, ΔS1s of the probes is possible through combination of Equations (3) and (4) (21):   ΔG1s=ΔH1s−TΔS1s (5) Many thermodynamic properties can be determined from the specific retention volume, Vg0, such as the weight fraction activity coefficient, Ω1∞, the molar heat of mixing at infinite dilution, ΔH1∞ and the corresponding molar free energy of mixing, ΔG1∞ of each probe (22):   Ω1∞=273.15RVg0P10M1exp(−P10(B11−V1)RT) (6)  ΔH1∞=−R∂lnΩ1∞∂(1T) (7)  ΔG1∞=RTlnΩ1∞ (8)where P10 is the vapor pressure of the probe at temperature T, B11 is the second virial coefficient and V1 is the probe’s molar volume. Values of the weight fraction activity coefficient, Ω1∞, reflect compatibility between solvents of the intended material, thus values between 5 and 10 shows moderate compatibility, whereas values smaller than 5 shows good solvency and values >10 are characteristic for poor solvency (23). Experimental values of the heats of vaporization, ΔHv of the probes can be obtained from the heats of mixing and the heats of sorption with the following relationship:   ΔHv=ΔH1∞−ΔH1s (9) Flory–Huggins interaction parameter, χ12∞, is a reflection of how strong the interaction between the solid phase and the probe is (23, 24). It can be calculated through the following equation (25):   χ12∞=ln(273.15RV2/P10Vg0V1)−1−P10(B11−V1)RT (10)where V1 is molar volume of probes; V2 is specific volume of the investigated material; R is the gas constant; T is the column temperature; B11 and P10 are the second virial coefficient and the saturated vapor pressure at the column temperature which can be calculated by the following formulas, respectively:   B11Vc=0.430−0.886(Tc/T)−0.694(Tc/T)2−0.0375(n−1)(Tc/T)4.5 (11)  P10=A−B(T+C) (12)where Vc is the critical molar volume; Tc is the critical temperature of probe molecules and n is the hypothetical number of carbon atoms for the given probes molecules that yields P10 equivalent to that of a corresponding n-alkane probes. A, B and C are constants for well-known Antoine equation. In order to compute the second virial coefficient, the McGlasham and Potter Equation (26) where used [equation (5)]. The parameter n was estimated through the procedure of Guggenheim and Wormald (27) as follows:   A=T(Pc/P10)Tc−T (13)where A is vapor pressure parameter, and Pc is the critical pressure of the probe. Having calculated A values for probes and n-alkanes, the matched n-alkane carbon number is used to determine n for the probes. If the equivalent value could not be found, then the closest value to the solute’s A value was used. Probe’s vapor pressure was calculated by Equation (12), and their Antoine constants and molar volume values were taken from the standard handbooks (28). The specific volume of DL-limonene was determined through standard methods ASTM D-1193 and ASTM D-1217. For materials that their molar volumes are not accurately known and also have no appreciable vapor pressure, the definition in Equation (1) cannot be used to estimate their solubility parameter. Instead, the experimental values of χ from the IGC method can be used as follows:   (δ12RT−χV1)=(2δ2RT)δ1−(δ22RT+χSV1) (14)where the dimensionless χS is an entropy term used to overcome the fundamental problem with the solubility parameter model (i.e., this model only estimates positives value for χ). By using a series of probes with different solubility parameters, and plotting the right-hand side of above equation against δ1, the solubility parameter of the investigated material, δ2, can be simply calculated from the slope or the intercept. Generally, a linear regression method is used to determine δ2 and 𝜠(η=χSV1) (29, 30). Solubility parameter values, δ1, for probes used in this investigation were collected from Hansen and Beerbower handbook (31) for ambient temperature. Then, according to the correlation by Jayasri and Jaseen (32) the solubility values at temperature T2 was calculated as follows:   δ1,T2=δ1,T((1−T2)/(1−T1))0.34 (15)where T1 = Tref/Tc and T2 = Texp/Tc. Results The specific retention volume, Vg0, is the basic parameter in IGC measurements as it is essential in order to determine physicochemical or thermodynamic properties of materials by this method. To obtain this data, proper selection of the probes has a significant importance. Probes should be chosen so that all the three types of interactions (Hydrogen bonding, dispersive and polar) be considered in the study. Specific retention volume of probes on DL-limonene dependency on temperature is shown in Figure 2. Figure 2. View largeDownload slide Specific retention volume against 1/T. Figure 2. View largeDownload slide Specific retention volume against 1/T. The molar heats of sorption, ΔH1s were calculated from the slopes of lines shown in Figure 2. The values of ΔH1s are listed in Table I. The molar free energy of sorption, ΔG1s was calculated according to Equation (4). The entropy of sorption can be easily computed via Equation (5) and the values provided in Tables I and II. Table I. The Molar Heat of Sorption, ΔH1s, the Partial Molar Heat of Mixing, ΔH1∞, of Various Probes on Dl-Limonene and the Heats of Vaporization, ΔHv, at 63–123°C Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Table I. The Molar Heat of Sorption, ΔH1s, the Partial Molar Heat of Mixing, ΔH1∞, of Various Probes on Dl-Limonene and the Heats of Vaporization, ΔHv, at 63–123°C Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Table II. The Molar Energy of Sorption, ΔH1s (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Table II. The Molar Energy of Sorption, ΔH1s (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  The calculated interaction parameters (weight activity coefficient and Flory–Huggins parameter) between probes and DL-limonene according to equations (6) and (10), are listed in Tables III and IV. Table III. Weight Fraction Activity Coefficient, Ω1∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Table III. Weight Fraction Activity Coefficient, Ω1∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Table IV. Flory–Huggins Interaction Parameter, χ12∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Table IV. Flory–Huggins Interaction Parameter, χ12∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  The molar heats of mixing at infinite dilution of the probes, ΔH1∞ values were calculated from the slope of lines obtained from plotting lnΩ1∞ versus 1/T [Equation (7)] and they are listed in Table I. The values of molar free energy of mixing, ΔG1∞ are calculated according to Equation (8) and are presented in Table V. The values of the heats of vaporization, ΔHv are also computed from Equation (9) and presented in Table I. Table V. The Molar Free Energy of Mixing, ΔH1∞ (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Table V. The Molar Free Energy of Mixing, ΔH1∞ (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  The solubility parameter of DL-limonene is measured from the slope of the straight line obtained from plotting the left-hand side of Equation (14) versus δ1 as illustrated in Figure 3 (at 63°C). The regression coefficient corresponding to lines used to calculate δ2 are also listed in Table VI. As solubility parameter in literatures is usually reported in ambient temperature, the DL-limonene solubility parameter in 25°C obtained from extrapolation of experimental equation of Figure 4, is 19.20 (J/cm3)0.5. Figure 5 shows calculated values of solubility parameter for DL-limonene. Figure 3. View largeDownload slide Solubility parameter of the probes against (δ1)2/RT−χ/V1. Figure 3. View largeDownload slide Solubility parameter of the probes against (δ1)2/RT−χ/V1. Table VI. Solubility Parameters, δ2 of DL-Limonene Calculated from Equation (14) at 63, 83, 103 and 123°C T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  View Large Table VI. Solubility Parameters, δ2 of DL-Limonene Calculated from Equation (14) at 63, 83, 103 and 123°C T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  View Large Figure 4. View largeDownload slide Solubility parameter of DL-limonene against temperature. Figure 4. View largeDownload slide Solubility parameter of DL-limonene against temperature. Figure 5. View largeDownload slide Calculated values of solubility parameter for DL-limonene. Figure 5. View largeDownload slide Calculated values of solubility parameter for DL-limonene. Discussion Scanning electron microscope The SEM micrographs shows the effectiveness of DL-limonene loading on stationary phase. As it is clear in Figure 6 that the DL-limonene has efficiently covered the voids of the inert chromosorb. The change in chromosorb particles morphology from elliptical to spherical form is the result of DL-limonene coverage on chromosorb particles. Since the chromosorb P is not polymeric and cannot swell with an organic compound and also there is no sign of breakage in studied samples during SEM, so the probable reasonable cause for this change of morphology (after addition of DL-limonene), may be the DL-limonene’s surface tension around the particle surface. In the 1-μm micrograph, full coverage of stationary phase is clearly shown. In conclusion, SEM micrographs shows that there is no significant source of error associated to stationary phase preparation. Figure 6. View largeDownload slide SEM micrographs of chromosorb P/AW before and after DL-limonene loading at 10 μm and 100 μm show the coverage of support voids by DL-limonene. Figure 6. View largeDownload slide SEM micrographs of chromosorb P/AW before and after DL-limonene loading at 10 μm and 100 μm show the coverage of support voids by DL-limonene. IGC The IGC measurements of DL-limonene performed at 63, 83, 103 and 123°C and flow rate of 30 mL/min. The selection of specific temperature range was done due to the thermal characterization of DL-limonene (33). The lowest working temperature chosen as to be well higher than melting point and the highest one to be below its starting the decomposition temperature. Specific retention volume According to Figure 2, it is obvious that by increasing temperature, the specific retention volume reduces. Apart from their temperature dependence, the values of Vg0 are also affected by the chemical structure of the probes. In the case of the tested n-alkanes, alcohols and ketone, the Vg0 values increase by increasing the number of alkyl groups. When comparing the specific retention volume of different probes with similar boiling point such as n-hexane, ethanol and ethyl acetate, the lowest value is observed for n-hexane. This behavior is due to the fact that polar and hydrogen bonding solvents like alcohols and ketones have an additional interaction with respect to hydrocarbons (34, 35). It will be further discussed in the following sections. Thermodynamic sorption parameters In general, the relationship between lnVg0 and (1/T) for the probes was linear. This linearity shows the establishment of equilibrium between the probes and the solid phase. The molar enthalpies of sorption of all probes are exothermic (negative). As listed in Table I, in the case of n-alkanes, ΔH1s becomes more exothermic by increasing the number of alkyl (CH2) groups. The molar heats of sorption of probes are directly affected by their chemical structure. It can be realized while comparing probes with similar boiling points but different functional groups. Ethyl acetate and ethanol present more exothermic values than n-hexane. n-Alkanes can interact with DL-limonene only through dispersive forces, whereas alcohols and polar solvents have additional interactions with DL-limonene which are combination of dispersive and hydrogen bonding and polar forces, respectively. All the probes investigated here, showed an endothermic (positive) values of ΔG1s. Probe–limonene interaction parameter As mentioned earlier, probes with similar boiling point values have different interaction parameters such as ethanol, ethyl acetate and hexane. The weight coefficient activity parameter for these probes shows this fact that ethyl acetate has a better compatibility with DL-limonene rather than n-hexane and ethanol which are classified as poor solvent. Values of Flory–Huggins interaction parameter are in agreement with this result. In the case of probes from the same group (alcohol, n-alkane), by increasing the CH2 groups the compatibility with DL-limonene increases that is also clear in the values of interaction parameters. Most of the probes tested here, showed a poor compatibility with DL-limonene except for n-nonane and 1-butanol which exhibit better interaction with DL-limonene, as reflected in Ω1∞ and χ12∞ parameter values. The aforementioned results can be further reorganized when considering the chemical structures of the probes and DL-limonene as shown in Figure 1. With regard to the asymmetric double bond in DL-limonene, it is a semi-polar molecule. On the other hand, the effect of oxygen atom on electron cloud of alcohol molecules makes it semi-polar. So, it is rational to see a stronger interaction between polar solvents and DL-limonene. There was no clear trend for the temperature dependence of interaction parameters for the material tested, which was seen in previous studies as well (36). Thermodynamic parameters of mixing The values of enthalpies and free energies of mixing at infinite dilution of probes are positive and show unfavorable miscibility between DL-limonene and investigated solvents. These results are in agreement with the results obtained from the interaction parameters Ω1∞ and χ12∞. Solubility parameter The deviation seen in Figure 3 for some probes may be due to the higher inaccuracy along with δ1s as the case of polar, hydrogen-bonding probes. The values in Table III were used to calculate DL-limonene solubility parameter, δ2. It is concluded that the solubility parameter decreases by increasing temperature in almost a linear trend, in the range of investigated temperatures as shown in Figure 4. The deviation in values obtained by IGC and other common methods for solubility parameter at ambient temperature are corresponding to the shortcomings of these methods that they are usually useful for simple systems where there are no specific interactions such as hydrogen bonding and polar interactions. Also, the inherent limitations of these methods are mostly associated with molecular mechanical force field utilized. Furthermore, the sample density may be a source of discrepancy in solubility parameter measurements. Conclusion IGC at infinite dilution was applied to estimate the thermodynamic properties of DL-limonene by using thirteen different probes. SEM micrographs indicated accurate preparation of solid phase. The computed values of interaction parameters between probe and DL-limonene indicated immiscibility between DL-limonene with the majority of the investigated probes. In particular, polar solvents and n-alkanes present moderate compatibility with DL-limonene, especially at high temperatures, while hydrogen bonding solvents such as alcohols are proved non-solvent for DL-limonene. According to values of Ω1∞ and χ12∞, increasing the numbers of CH2 groups prompted an improved compatibility with DL-limonene among n-alkanes and alcohols. n-Nonane and n-octane appear to be best solvents for DL-limonene among investigated solvents, whereas toluene and 1-butanol are moderate solvents of it. Values of molar heats of sorption and mixing of probes as well as the molar free energies of sorption and mixing of probes values were in agreement with results obtained from Ω1∞ and χ12∞ calculations. 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Assessment of the Thermodynamic Properties of DL-p-Mentha-1,8-diene, 4-Isopropyl-1-Methylcyclohexene (DL-limonene) by Inverse Gas Chromatography (IGC)

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Abstract

Abstract Limonene is a colorless liquid hydrocarbon and had been investigated as a plasticizer for many plastics. Prediction of solubility between different materials is an advantage in many ways, one of the most convenient ways to know the compatibility of materials is to determine the degree of solubility of them in each other. The concept of “solubility parameter” can help practitioners in this way. In this study, inverse gas chromatography (IGC) method at infinite dilution was used for determination of the thermodynamic properties of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). The interaction between DL-limonene and 13 solvents were examined in the temperature range of 63–123°C through the assessment of the thermodynamic sorption parameters, the parameters of mixing at infinite dilution, the weight fraction activity coefficient and the Flory–Huggins interaction parameters. Additionally, the solubility parameter for DL-limonene and the temperature dependence of these parameters was investigated as well. Results show that there is a temperature dependence in solubility parameter, which increases by decreasing temperature. However, there were no specific dependence between interaction parameters and temperature, but chemical structure appeared to have a significant effect on them as well as on the type and strength of intermolecular interactions between DL-limonene and investigated solvents. The solubility parameter δ2 of DL-limonene determined to be 19.20 (J/cm3)0.5 at 25°C. Introduction Limonene is a colorless liquid hydrocarbon and it is classified as a cyclic terpene. Its name comes from lemon, as lemon peel, as well as other citrus fruits that hold substantial amounts of this compound, which contributes to their odor. It is a chiral molecule and one enantiomer is produced by biological sources (1). As the main odor constituent of citrus, limonene is common in cosmetic products, also in food manufacturing and some medicines. Because its ability to dissolve oils, it is added to cleaning products. It is used as a paint stripper and is also useful as a pleasant-smelling alternative to turpentine (2). Limonene is also gradually used as a solvent for filament-fused 3D printing. Printers can print the plastic of choice for the model, but erect supports and binders from high impact polystyrene, a polystyrene plastic that is easily soluble in limonene (3). As it is combustible, limonene has also been considered as a biofuel (4). Prediction of solubility between different materials is an advantage in many ways, taking the example of polymer blends, polymer compounds, adhesives or paints. It also helps engineers in order to choose the right materials for compounding of polymer mixtures. For compounding, one needs to know the compatibility between different compounding ingredients, for example, polymer–plasticizer, polymer–polymer, reinforcing agent–plasticizer and so on to select the correct materials to compound them (5). Recently, studies have been done on the use of limonene as a new monomer to obtain polyterpenes (6). Limonene had also been investigated as a plasticizer for many plastics such as polyethylene (7), polystyrene (8) and poly (lactic acid) in the food packaging industry (9). One of the most convenient ways to know the compatibility of materials is to determine the degree of solubility of them in each other. The concept of “solubility parameter” can help practitioners in this way. Inverse gas chromatography (IGC) has been shown to be a reliable and very useful technique for the measurement of thermodynamic parameters in a range of non-volatile materials, over a wide range of conditions (10). In gas-liquid IGC, the non-volatile binary solution component is distributed on a chromatographic packing material which is held under specified conditions in the chromatographic column. By injecting a small quantity of solvents (probes) into the column, the thermodynamic parameters of stationary phase can be calculated from the specific retention volume of probes as they flow through the chromatographic column. The IGC method has been applied for the assessment of the surface and thermodynamic characteristics of different materials such as polymers (11, 12), ionic liquids (13), carbon nanotubes (14), liquid crystalline systems (15), natural oils (16) and pharmaceuticals (17). Despite the fact that many authors used IGC, studies concerning the thermodynamic properties of limonene have not yet been reported. Here, we applied the IGC technique to determine the thermodynamic properties and solubility parameter of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Materials and methods Materials DL-limonene (DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene) sample from Merck (USA), with boiling point of 178°C (1013 hPa), density of 0.84 g/cm3 (20°C) and melting point of −89°C, is used as received (pure). Its chemical structure is shown in Figure 1. Chromosorb P AW-DMCS (60–80 mesh, Merck) was used as solid support. The solvents that are listed below were used as probes for IGC measurements. They were selected with regard to their ability to interact with three different types of interaction forces, that is dispersive, polar and hydrogen bonding. All probes (Aldrich or Merck) were highly pure grade (i.e., 99%). The probes used were n-alkanes (n-hexane, n-heptane, n-octane and n-nonane), alcohols (chloroform, methanol, ethanol and 1-butanol), polar solvents (acetone and ethyl acetate) and non-polar solvents (diethyl ether, cyclohexane and toluene). Chromatographic injections were made using syringes obtained from the Hamilton Company (Reno, NV, USA). Figure 1. View largeDownload slide Chemical structure of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Figure 1. View largeDownload slide Chemical structure of DL-p-mentha-1,8-diene, 4-Isopropyl-1-methylcyclohexene (DL-limonene). Column preparation and IGC setup The IGC measurements were performed on a commercial Shimadzu GC-14A gas chromatograph equipped with a flame ionization detector (FID) and two manometers to determine column’s inlet and outlet pressures. Dried nitrogen was used as carrier gas. According to our previous work, flow rate set to be 30 mL/min. The injector and detector temperatures were kept at 200°C during the experiment. Diethyl ether were used as non-interacting markers in order to determine the void volume of the column. To achieve infinite dilution, 0.05 μL of each probe was injected with a 10 μL Hamilton syringes. The column temperatures were 63–123 varied in 20°C steps. The boiling point of limonene is 178°C and the highest operating temperature used in this study was 123°C that is low enough for prevent any loss of limonene due to volatilization. The weight of the column before and after the IGC procedure showed no difference. Each probe injection was repeated three times and average retention time, tR was used for calculation. The column used in this study was prepared using a stainless still column (SS 316 ASTM A-269) with 3.66 mm inner diameter, 4.60 mm outer diameter and having an approximate length of 2 m. Column packing was done by mixing a selected weight percentage of DL-limonene dissolved in hexane (10% weight) with the chromosorb followed by solvent removal using a rotary evaporator for 6 h. The packed column then preconditioned (highest temperature and nitrogen flow rate) overnight in order to remove any residual solvent left in the packing material. The coated mass was 0.68 g with 10.2% weight loading of DL-limonene. SEM measurements Scanning electron microscope (SEM) measurements were done on an AIS2100 device from Seron technologies Co., Ltd. The SEM micrographs were used to determine the stationary phase morphology. Moreover, these micrographs can show the accuracy in the preparation of stationary phase. Theory/calculations Inverse gas chromatography theory Hildebrand introduced the concept of the cohesive energy density, CED and solubility in a series of articles starting in 1916. The solubility parameter, δ, introduced later in 1949 related to CED (18).   δ=(ΔEvapV)12 (1)where ΔEvap is enthalpy of vaporization. CED represents the required energy to detach the liquid molecules into the ideal gas state. Smidsrob and Guillet developed the IGC technology in 1969 (19). This technique was found to be a convenient method to obtain thermodynamic quantities and investigate the physicochemical matter properties. The technique involves filling a column with a stationary phase of the solid material as the object of investigation, while probes of known physicochemical properties are injected. By determining the retention times of the probes, the specific retention volume, Vg0, of the probes which stands for the elution behavior of probes may be obtained according to the following equation (20):   Vg0=273.15mTaFP0−PWP0(tr−t0)32(Pi/P0)2−1(Pi/P0)3−1 (2)where F is the flow rate of the carrier gas measured at room temperature; m is the mass of stationary phase; Ta is the flow meter temperature; Pw is the saturated vapor pressure of water at ambient temperature; tr is the retention times of the probes; t0 is the retention time of the non-interacting probe and Pi and P0 are inlet and outlet pressure of the column, respectively. The molar heat of sorption, ΔH1s and the molar free energy of sorption, ΔG1s of the probe absorbed by the solid phase are calculated by the following equations (9):   ΔH1s=−R∂lnVg0∂(1T) (3)  ΔG1s=−RTln(M1Vg0273⋅15R) (4)where T is the column temperature, M1 the molecular weight of the probe and R the gas constant. The calculation of the entropy of sorption, ΔS1s of the probes is possible through combination of Equations (3) and (4) (21):   ΔG1s=ΔH1s−TΔS1s (5) Many thermodynamic properties can be determined from the specific retention volume, Vg0, such as the weight fraction activity coefficient, Ω1∞, the molar heat of mixing at infinite dilution, ΔH1∞ and the corresponding molar free energy of mixing, ΔG1∞ of each probe (22):   Ω1∞=273.15RVg0P10M1exp(−P10(B11−V1)RT) (6)  ΔH1∞=−R∂lnΩ1∞∂(1T) (7)  ΔG1∞=RTlnΩ1∞ (8)where P10 is the vapor pressure of the probe at temperature T, B11 is the second virial coefficient and V1 is the probe’s molar volume. Values of the weight fraction activity coefficient, Ω1∞, reflect compatibility between solvents of the intended material, thus values between 5 and 10 shows moderate compatibility, whereas values smaller than 5 shows good solvency and values >10 are characteristic for poor solvency (23). Experimental values of the heats of vaporization, ΔHv of the probes can be obtained from the heats of mixing and the heats of sorption with the following relationship:   ΔHv=ΔH1∞−ΔH1s (9) Flory–Huggins interaction parameter, χ12∞, is a reflection of how strong the interaction between the solid phase and the probe is (23, 24). It can be calculated through the following equation (25):   χ12∞=ln(273.15RV2/P10Vg0V1)−1−P10(B11−V1)RT (10)where V1 is molar volume of probes; V2 is specific volume of the investigated material; R is the gas constant; T is the column temperature; B11 and P10 are the second virial coefficient and the saturated vapor pressure at the column temperature which can be calculated by the following formulas, respectively:   B11Vc=0.430−0.886(Tc/T)−0.694(Tc/T)2−0.0375(n−1)(Tc/T)4.5 (11)  P10=A−B(T+C) (12)where Vc is the critical molar volume; Tc is the critical temperature of probe molecules and n is the hypothetical number of carbon atoms for the given probes molecules that yields P10 equivalent to that of a corresponding n-alkane probes. A, B and C are constants for well-known Antoine equation. In order to compute the second virial coefficient, the McGlasham and Potter Equation (26) where used [equation (5)]. The parameter n was estimated through the procedure of Guggenheim and Wormald (27) as follows:   A=T(Pc/P10)Tc−T (13)where A is vapor pressure parameter, and Pc is the critical pressure of the probe. Having calculated A values for probes and n-alkanes, the matched n-alkane carbon number is used to determine n for the probes. If the equivalent value could not be found, then the closest value to the solute’s A value was used. Probe’s vapor pressure was calculated by Equation (12), and their Antoine constants and molar volume values were taken from the standard handbooks (28). The specific volume of DL-limonene was determined through standard methods ASTM D-1193 and ASTM D-1217. For materials that their molar volumes are not accurately known and also have no appreciable vapor pressure, the definition in Equation (1) cannot be used to estimate their solubility parameter. Instead, the experimental values of χ from the IGC method can be used as follows:   (δ12RT−χV1)=(2δ2RT)δ1−(δ22RT+χSV1) (14)where the dimensionless χS is an entropy term used to overcome the fundamental problem with the solubility parameter model (i.e., this model only estimates positives value for χ). By using a series of probes with different solubility parameters, and plotting the right-hand side of above equation against δ1, the solubility parameter of the investigated material, δ2, can be simply calculated from the slope or the intercept. Generally, a linear regression method is used to determine δ2 and 𝜠(η=χSV1) (29, 30). Solubility parameter values, δ1, for probes used in this investigation were collected from Hansen and Beerbower handbook (31) for ambient temperature. Then, according to the correlation by Jayasri and Jaseen (32) the solubility values at temperature T2 was calculated as follows:   δ1,T2=δ1,T((1−T2)/(1−T1))0.34 (15)where T1 = Tref/Tc and T2 = Texp/Tc. Results The specific retention volume, Vg0, is the basic parameter in IGC measurements as it is essential in order to determine physicochemical or thermodynamic properties of materials by this method. To obtain this data, proper selection of the probes has a significant importance. Probes should be chosen so that all the three types of interactions (Hydrogen bonding, dispersive and polar) be considered in the study. Specific retention volume of probes on DL-limonene dependency on temperature is shown in Figure 2. Figure 2. View largeDownload slide Specific retention volume against 1/T. Figure 2. View largeDownload slide Specific retention volume against 1/T. The molar heats of sorption, ΔH1s were calculated from the slopes of lines shown in Figure 2. The values of ΔH1s are listed in Table I. The molar free energy of sorption, ΔG1s was calculated according to Equation (4). The entropy of sorption can be easily computed via Equation (5) and the values provided in Tables I and II. Table I. The Molar Heat of Sorption, ΔH1s, the Partial Molar Heat of Mixing, ΔH1∞, of Various Probes on Dl-Limonene and the Heats of Vaporization, ΔHv, at 63–123°C Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Table I. The Molar Heat of Sorption, ΔH1s, the Partial Molar Heat of Mixing, ΔH1∞, of Various Probes on Dl-Limonene and the Heats of Vaporization, ΔHv, at 63–123°C Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Probe  ΔH1s (KJ/mol)  ΔH1∞ (KJ/mol)  ΔHv (KJ/mol)  Hexane  −2.07  0.41  2.48  Heptane  −2.43  0.56  2.99  Octane  −2.49  0.37  2.86  Nonane  −2.53  0.15  2.69  Toluene  2.61  0.64  3.25  Cyclohexane  −1.51  0.20  1.71  Chloroform  −2.31  0.70  3.00  Acetone  −1.11  0.52  1.63  Ethyl acetate  −2.51  0.72  3.24  Methanol  −1.95  0.06  2.01  Ethanol  −2.79  0.58  3.37  1-Butanol  −3.36  0.69  4.06  Table II. The Molar Energy of Sorption, ΔH1s (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Table II. The Molar Energy of Sorption, ΔH1s (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  Probe  63°C  83°C  103°C  123°C  Hexane  28.15  29.96  36.30  38.83  Heptane  21.26  24.68  28.78  32.69  Octane  17.38  20.79  24.82  28.22  Nonane  14.38  17.65  21.26  24.97  Toluene  18.93  22.50  26.87  30.36  Cyclohexane  26.77  28.66  32.42  36.46  Chloroform  26.25  27.82  34.44  37.64  Acetone  27.87  30.44  33.22  36.47  Ethyl acetate  22.88  26.35  30.86  34.78  Methanol  27.62  30.58  35.14  38.46  Ethanol  24.08  29.02  33.31  37.02  1-Butanol  17.12  21.05  27.00  30.23  The calculated interaction parameters (weight activity coefficient and Flory–Huggins parameter) between probes and DL-limonene according to equations (6) and (10), are listed in Tables III and IV. Table III. Weight Fraction Activity Coefficient, Ω1∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Table III. Weight Fraction Activity Coefficient, Ω1∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Probe  63°C  83°C  103°C  123°C  Hexane  38.71  22.53  59.76  46.19  Heptane  8.69  9.09  11.98  14.78  Octane  5.66  5.82  7.42  7.78  Nonane  4.99  4.74  5.16  5.87  Toluene  5.64  6.30  9.10  9.80  Cyclohexane  34.38  20.42  23.87  30.16  Chloroform  15.32  8.57  25.92  25.08  Acetone  23.44  17.75  14.94  14.92  Methanol  27.74  20.78  27.58  24.52  Ethanol  13.94  20.19  23.33  23.00  1-Butanol  6.66  6.64  13.05  10.93  Table IV. Flory–Huggins Interaction Parameter, χ12∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Table IV. Flory–Huggins Interaction Parameter, χ12∞, of Various Probes at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  Probe  63°C  83°C  103°C  123°C  Hexane  2.38  1.83  2.78  2.51  Heptane  0.93  0.97  1.23  1.42  Octane  0.53  0.56  0.79  0.82  Nonane  0.43  0.38  0.45  0.58  Toluene  0.74  0.85  1.21  1.28  Cyclohexane  2.43  1.92  2.05  2.27  Chloroform  2.27  1.69  2.78  2.73  Acetone  2.06  1.77  1.58  1.55  Ethyl acetate  1.10  1.15  1.53  1.71  Methanol  2.24  1.94  2.21  2.07  Ethanol  1.54  1.90  2.03  2.00  1-Butanol  0.84  0.84  1.51  1.32  The molar heats of mixing at infinite dilution of the probes, ΔH1∞ values were calculated from the slope of lines obtained from plotting lnΩ1∞ versus 1/T [Equation (7)] and they are listed in Table I. The values of molar free energy of mixing, ΔG1∞ are calculated according to Equation (8) and are presented in Table V. The values of the heats of vaporization, ΔHv are also computed from Equation (9) and presented in Table I. Table V. The Molar Free Energy of Mixing, ΔH1∞ (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Table V. The Molar Free Energy of Mixing, ΔH1∞ (KJ/mol) of Various Probes on DL-Limonene at 63, 83, 103 and 123°C Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  Probe  63°C  83°C  103°C  123°C  Hexane  10.22  9.22  12.79  12.62  Heptane  6.04  6.54  7.77  8.87  Octane  4.84  5.21  6.27  6.76  Nonane  4.49  4.61  5.13  5.83  Toluene  4.83  5.44  6.90  7.52  Cyclohexane  9.89  8.93  9.92  11.22  Chloroform  7.62  6.36  10.18  10.61  Acetone  8.81  8.52  8.46  8.90  Ethyl acetate  5.75  6.27  7.89  8.97  Methanol  9.29  8.98  10.37  10.54  Ethanol  7.36  8.90  9.85  10.33  1-Butanol  5.30  5.60  8.03  7.87  The solubility parameter of DL-limonene is measured from the slope of the straight line obtained from plotting the left-hand side of Equation (14) versus δ1 as illustrated in Figure 3 (at 63°C). The regression coefficient corresponding to lines used to calculate δ2 are also listed in Table VI. As solubility parameter in literatures is usually reported in ambient temperature, the DL-limonene solubility parameter in 25°C obtained from extrapolation of experimental equation of Figure 4, is 19.20 (J/cm3)0.5. Figure 5 shows calculated values of solubility parameter for DL-limonene. Figure 3. View largeDownload slide Solubility parameter of the probes against (δ1)2/RT−χ/V1. Figure 3. View largeDownload slide Solubility parameter of the probes against (δ1)2/RT−χ/V1. Table VI. Solubility Parameters, δ2 of DL-Limonene Calculated from Equation (14) at 63, 83, 103 and 123°C T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  View Large Table VI. Solubility Parameters, δ2 of DL-Limonene Calculated from Equation (14) at 63, 83, 103 and 123°C T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  T (°C)  Slope  δ2 (J/cm3)0.5  R2  63  0.0123  17.19∓0.24  0.9967  83  0.0109  16.14∓0.31  0.9986  103  0.0094  15.32∓0.23  0.9996  123  0.0085  14.00∓0.27  0.9957  View Large Figure 4. View largeDownload slide Solubility parameter of DL-limonene against temperature. Figure 4. View largeDownload slide Solubility parameter of DL-limonene against temperature. Figure 5. View largeDownload slide Calculated values of solubility parameter for DL-limonene. Figure 5. View largeDownload slide Calculated values of solubility parameter for DL-limonene. Discussion Scanning electron microscope The SEM micrographs shows the effectiveness of DL-limonene loading on stationary phase. As it is clear in Figure 6 that the DL-limonene has efficiently covered the voids of the inert chromosorb. The change in chromosorb particles morphology from elliptical to spherical form is the result of DL-limonene coverage on chromosorb particles. Since the chromosorb P is not polymeric and cannot swell with an organic compound and also there is no sign of breakage in studied samples during SEM, so the probable reasonable cause for this change of morphology (after addition of DL-limonene), may be the DL-limonene’s surface tension around the particle surface. In the 1-μm micrograph, full coverage of stationary phase is clearly shown. In conclusion, SEM micrographs shows that there is no significant source of error associated to stationary phase preparation. Figure 6. View largeDownload slide SEM micrographs of chromosorb P/AW before and after DL-limonene loading at 10 μm and 100 μm show the coverage of support voids by DL-limonene. Figure 6. View largeDownload slide SEM micrographs of chromosorb P/AW before and after DL-limonene loading at 10 μm and 100 μm show the coverage of support voids by DL-limonene. IGC The IGC measurements of DL-limonene performed at 63, 83, 103 and 123°C and flow rate of 30 mL/min. The selection of specific temperature range was done due to the thermal characterization of DL-limonene (33). The lowest working temperature chosen as to be well higher than melting point and the highest one to be below its starting the decomposition temperature. Specific retention volume According to Figure 2, it is obvious that by increasing temperature, the specific retention volume reduces. Apart from their temperature dependence, the values of Vg0 are also affected by the chemical structure of the probes. In the case of the tested n-alkanes, alcohols and ketone, the Vg0 values increase by increasing the number of alkyl groups. When comparing the specific retention volume of different probes with similar boiling point such as n-hexane, ethanol and ethyl acetate, the lowest value is observed for n-hexane. This behavior is due to the fact that polar and hydrogen bonding solvents like alcohols and ketones have an additional interaction with respect to hydrocarbons (34, 35). It will be further discussed in the following sections. Thermodynamic sorption parameters In general, the relationship between lnVg0 and (1/T) for the probes was linear. This linearity shows the establishment of equilibrium between the probes and the solid phase. The molar enthalpies of sorption of all probes are exothermic (negative). As listed in Table I, in the case of n-alkanes, ΔH1s becomes more exothermic by increasing the number of alkyl (CH2) groups. The molar heats of sorption of probes are directly affected by their chemical structure. It can be realized while comparing probes with similar boiling points but different functional groups. Ethyl acetate and ethanol present more exothermic values than n-hexane. n-Alkanes can interact with DL-limonene only through dispersive forces, whereas alcohols and polar solvents have additional interactions with DL-limonene which are combination of dispersive and hydrogen bonding and polar forces, respectively. All the probes investigated here, showed an endothermic (positive) values of ΔG1s. Probe–limonene interaction parameter As mentioned earlier, probes with similar boiling point values have different interaction parameters such as ethanol, ethyl acetate and hexane. The weight coefficient activity parameter for these probes shows this fact that ethyl acetate has a better compatibility with DL-limonene rather than n-hexane and ethanol which are classified as poor solvent. Values of Flory–Huggins interaction parameter are in agreement with this result. In the case of probes from the same group (alcohol, n-alkane), by increasing the CH2 groups the compatibility with DL-limonene increases that is also clear in the values of interaction parameters. Most of the probes tested here, showed a poor compatibility with DL-limonene except for n-nonane and 1-butanol which exhibit better interaction with DL-limonene, as reflected in Ω1∞ and χ12∞ parameter values. The aforementioned results can be further reorganized when considering the chemical structures of the probes and DL-limonene as shown in Figure 1. With regard to the asymmetric double bond in DL-limonene, it is a semi-polar molecule. On the other hand, the effect of oxygen atom on electron cloud of alcohol molecules makes it semi-polar. So, it is rational to see a stronger interaction between polar solvents and DL-limonene. There was no clear trend for the temperature dependence of interaction parameters for the material tested, which was seen in previous studies as well (36). Thermodynamic parameters of mixing The values of enthalpies and free energies of mixing at infinite dilution of probes are positive and show unfavorable miscibility between DL-limonene and investigated solvents. These results are in agreement with the results obtained from the interaction parameters Ω1∞ and χ12∞. Solubility parameter The deviation seen in Figure 3 for some probes may be due to the higher inaccuracy along with δ1s as the case of polar, hydrogen-bonding probes. The values in Table III were used to calculate DL-limonene solubility parameter, δ2. It is concluded that the solubility parameter decreases by increasing temperature in almost a linear trend, in the range of investigated temperatures as shown in Figure 4. The deviation in values obtained by IGC and other common methods for solubility parameter at ambient temperature are corresponding to the shortcomings of these methods that they are usually useful for simple systems where there are no specific interactions such as hydrogen bonding and polar interactions. Also, the inherent limitations of these methods are mostly associated with molecular mechanical force field utilized. Furthermore, the sample density may be a source of discrepancy in solubility parameter measurements. Conclusion IGC at infinite dilution was applied to estimate the thermodynamic properties of DL-limonene by using thirteen different probes. SEM micrographs indicated accurate preparation of solid phase. The computed values of interaction parameters between probe and DL-limonene indicated immiscibility between DL-limonene with the majority of the investigated probes. In particular, polar solvents and n-alkanes present moderate compatibility with DL-limonene, especially at high temperatures, while hydrogen bonding solvents such as alcohols are proved non-solvent for DL-limonene. According to values of Ω1∞ and χ12∞, increasing the numbers of CH2 groups prompted an improved compatibility with DL-limonene among n-alkanes and alcohols. n-Nonane and n-octane appear to be best solvents for DL-limonene among investigated solvents, whereas toluene and 1-butanol are moderate solvents of it. Values of molar heats of sorption and mixing of probes as well as the molar free energies of sorption and mixing of probes values were in agreement with results obtained from Ω1∞ and χ12∞ calculations. 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Published: May 10, 2018

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