The clay was modified by an ion exchange reaction with cetylpyridinium chloride CPC and hexadecyltrimethylammonium bromide HDTMA. The modified samples were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The basal spacing of unmodified clay determined by XRD was 12.72 Å and, after modification, increased with increasing concentration; expressed as a function of the cation exchange capacity (CEC) of the clay; to reach 21.08 and 26 Å for clays modified with CPC and HDTMA successively for an equal concentra - tion of 3CEC. FTIR studies revealed structural differences between modified and unmodified clay samples. Modified clay −1 −1 spectra showed C–N functional bands (1480 cm ) and C–H vibrations (near 2936 and 2871 cm ). The results of the SEM study reveal a difference between natural and modified clays. The purified clay has massive and curved plates. However, the modified clays show numerous small aggregate particles and plaques that become relatively flat. The arrangement of surfactants in clay is rather complicated. It depends on the nature of the surfactant molecules, the CEC of the clay and the method of preparation. According to these parameters, the inserted surfactants may be arranged in monolayer, paraffinic or admicelles structures. Keywords Tunisian clay · HDTMA · HDPyridine · Arrangements Introduction silver also as additives thickeners and in cosmetic formu- lations, as sorbents of organic compounds present (dyes, There has been growing interest in the development of inno- metals heavy) wastewater. Their main interest comes from vative adsorbent materials to solve the problem of industrial the properties conferred by the intercalation of organic cati- wastewater pollution (Nejib et al. 2014; Ltifi et al. 2017; ons in interfolliairy clay space. Substitution of surfactants Nahed and Kais 2015). Natural clays are considered among greatly increases the hydrophobicity of interfolliairy clay the available materials such as montmorillonite which space, and therefore, clay can be swollen in non-aqueous is widely used as adsorbents because of the high cation systems. The adsorption of alkylammonium molecules or exchange capacity (CEC), so that their swelling properties cations on the clay has different structures depending on are high (Ayari et al. 2005). Montmorillonite (MMT) is a 2:1 the cation exchange capacity of clay (CEC), the size of the clay mineral that has two silica–oxygen tetrahedral sheets alkyl chain or the method of preparation. These molecules with a central alumina octahedral layer (TOT layer). or cations can adopt various arrangements in the interlayer Mixed “clay-surfactant” systems are of great interest for spacing of the clay. industrial applications. Nowadays, they are used in various The hexadecyltrimethylammonium bromide HDTMA fields as thickeners in inks, in the preparation of nanoparticle intercalated in Na-montmorillonite has a paraffin-type mon - olayer arrangement parallel to the basal montmorillonite spacing. With the increase in the concentration of surfactants * Ismail Ltifi hexadecyltrimethylammonium bromide and cetylpyridinium email@example.com chloride (0–3 CEC) in montmorillonite (Tahani et al. 1999), the adsorption of the cationic surfactant has been widely Laboratory for the Application of Chemistry to Natural Resources and Substances and the Environment studied for several types of clay such as montmorillonite (LACReSNE), Faculty of Sciences Bizerte, University Carthage, Tunis, Tunisia Vol.:(0123456789) 1 3 91 Page 2 of 8 Applied Water Science (2018) 8:91 (Zhu et al. 2003), saponite (Ogawa et al. 1995), kaolinite at room temperature for 48 h. The resulting mixture was (Komoria et al. 1999) and mica (Chen et al. 1992). filtered through filter papers and washed with distilled water − − In contrast, few articles are devoted to the adsorption of until complete breakdown of Br and Cl (AgNO test). The organic ammonium cations on natural interstratified clays products are dried at 80 °C for 12 h. Finally, the adsorbents composed of smectite, kaolinite and illite. were ground in an agate mortar and stored (Ltifi et al. 2017 ). The aim of the present study is to try to elucidate the mechanism by which surfactant molecules or cations are Characterization methods intercalated in the interlayer space of these clays, thanks to an analysis based on adsorption isotherms, spectroscopy The prepared organoclays were characterized by X-ray dif- FTIR and diffraction of rays X. fraction (XRD), surface area measurement (BET), Fourier transform infrared spectroscopy (FTIR). XRD for obtaining basal spacing d(001) values was operated, and the method Experimental was described in the paper by Park et al. (2013). The miner- alogical composition was determined in the < 2 µm fraction. Materials The proportions of species in clay were estimated by the refence intensity ratio method using the High Score soft- The clay used is from the GAFSA region located south ware. The specific surface area (S ) measured by the BET of ‘TUNISIA.’ Organic surfactants used were hexadecyl- and the cation exchange capacity (CEC) is determined by the trimethylammonium bromide (HDTMA, formula weight: method of MANTIN (Mantin 1969). The zero charge point 364.45 and chemical formula: C H BrN) and cetylpyri- 19 42 (PZNPC) of the aqueous clay adsorbent was analyzed using dinium chloride (CPC, formula weight: 339.9 and chemical the solid addition method (Wibowo et al. 2007). formula: C H ClN) was obtained from Sigma-Aldrich. The 21 38 molecular structure of CPC and HDTMA is illustrated in (Fig. 1). Other chemical reagents, such as NaOH, HCl and Characterization AgNO , were utilized of analytical grade. X‑ray diffraction Preparation of organoclays X-ray diffraction (XRD) patterns were recorded using Cu The organophilic clays are prepared by the procedure which Kα radiation (n = 1.5418 Å), a Philips PANalytical X’ Pert takes place in two stages; on the one hand, a 20 g of the PRO diffractometer operating at 40 kV and 40 mA with adsorbent (Gafsa Clay) is dispersed in about 500 ml of 0.25° divergence slit. For XRD at low angle section, it was water in distilled water. On the other hand, a desired amount between 1° and 30° (2θ) at a step size of 0.0167° with vari- of surfactant (HDTMA or CPC) was stirred in 100 ml of able divergence slit and 0.125° anti-scatter slit. distilled water until it was completely dissolved and then added drop wise to the clay solutions. The amounts of each Spectroscopy IR surfactant were calculated on the basis of the CEC of the adsorbent. The reaction mixtures were mechanically stirred Infrared spectra are collected on PERKIN ELMER 66 (Fou- rier transform infrared spectrometer); OPUS software allows the band intensity to be normalized by the most intense one. −1 Spectra are collected over the spectral range 400–4000 cm . Measurement of the specific surface by the BET method and cation exchange capacity (CEC) Nitrogen adsorption measurements were performed at 77 K with an Autosorb-1 unit (Quantachrome) for the determi- nation of sample textural properties using the multipoint Brunauer–Emmett–Teller (BET) method. The samples were out gassed at 120 °C under a vacuum at 10–3 mm Hg for 3.5 h. CEC was determined by adsorption of copper ethylen- ediamine (EDA) CuCl complex (Bergaya and Vayer 1997). Fig. 1 Molecular structure of HDTMA, and CPC 2 2 1 3 Applied Water Science (2018) 8:91 Page 3 of 8 91 Morphology MEB Changes in basal spacings by organophilization The morphology of the prepared materials was evaluated One of the most important methods for studying Interlayer by transmission electron microscopy (TEM); the SEM displacement is X-ray diffraction (XRD). The decrease images were taken by a Hitachi S-3400N transmission in the angle 2θ and the widening of the peak indicate an electron microscopy. increase in interfolliairy spacing (Jahan et al. 2012). The Bragg law allows to calculate the distance of the silicate layer (nλ = 2dsinθ, d = interfolliairy distances). One of the most important properties of layered silicates is the distance Results and discussion between the clay layers (d001). This distance can be calcu- lated with data collected from X-ray diffraction. It is also Chemical composition of raw and purified clay reflected in the XRD pattern of the purified clay. When the modification of substances cannot penetrate the interlayer The results of the chemical analysis given in Table 1 show: space, the value of d001 does not change (Kozak and Domka 2004), in our study and after the ion exchange reaction, the Silica/alumina ratios equal to 3.14 characteristics of basal spacing increases from 12.72° to 26° for HDTMA and montmorillonite. Value included in the domain (2.5, 5). 21.08° for CPC indicating that both cationic surfactants have A high content of magnesium and iron. been intercalated successfully within clay (Fig. 2) (Hoidy • The clay phase consists mainly of montmorillonite with et al. 2009). a small amount of illite and kaolinite. Infrared spectroscopy (IR) The silica to alumina SiO /Al O ratio confirms mont- 2 2 3 morillonite and the essential component of GAFSA clay. Based on the results of the XRD, it was possible to confirm These results show a decrease in the silica content (SiO ) that the exchange of HDTMA and CPC polycations with the from 54.3% for the crude clay to 47.51% for the purified Na alkaline cations of the interfoliairy space is successful. −1 clay and sodium. Two absorption bands at 726–780 cm correspond to the The decrease in the CaO and MgO content is due to the mode of vibration of deformation out of the plane of the CH Ca and Mg cations exchanged by the sodium cations. In group (Vaia et al. 1994). −1 parallel during the treatment of the clay purified by sodium The group in the 950–1100 cm region corresponds to chloride (NaCl) during the washing operations, there is an the stretching vibration of the Si–O groups (Xie et al. 2001). −1 increase in the concentration of Na O. This increase is due As shown in Fig. 3, new peaks appear at 2939 cm ; −1 −1 −1 to the ion exchange between the ions of the clay and the 2842 cm for clay-CPC and at 2936 cm and 2871 cm sodium ion from the salt (NaCl). for clay-HDTMA which correspond to asymmetric vibra- tion –CH and symmetric stretching –CH . These bands are 2 2 absent in the IR spectrum of unmodified purified clay (0 CEC) which indicates the incorporation of surfactants into the organophilic clays. Since the groups are very narrow, the variation in their Table 1 Chemical composition of the catalyst intensity increases with the increase in the initial surfactant Oxide elements (%) Mass composition in % cal- content, indicating the intercalation of a larger amount of cined clay oxides surfactant in the studied clay with the increase in the initial amount of the surfactant. ArB ArP As shown in Fig. 3, when the surfactants are sandwiched SiO 54.30 47.51 −1 within the clays of the clays, a broad band at 1480 cm AI O 16.42 15.13 2 3 indicating the presence of the functional group C–N corre- Fe O 08.21 09.58 2 3 sponds to the tertiary amine as described in literature (Zhu MgO 04.75 03.47 et al. 2005). CaO 04.61 03.38 The smectite family has spectral hydration characteris- K O 01.32 00.96 tics that have been attributed to adsorption of water on the Na O 00.75 06.34 external surfaces of the clay as well as internal regions. The Total 90.38 86.37 property of these interlayer waters greatly depends on the PF 10.40 11.20 level of moisture and the intercalated cation. SiO /Al O 03.30 03.14 2 2 3 1 3 91 Page 4 of 8 Applied Water Science (2018) 8:91 Fig. 2 X-ray diffractograms of purified clay (0 CEC) and organophilic clays by HDTMA and CPC (different concentrations 1, 2 and 3 CEC) Fig. 3 FTIR spectra of purified clay (0 CEC) and organophilic clays by HDTMA and CPC (different concentrations 1, 2 and 3 CEC) The spectra show a more obvious adsorption band at changed and becomes hydrophobic and the clay becomes −1 3620 cm due to structural stretching vibrations of OH organophilic(Kung and Hayes 1993; Mandalia and Ber- groups independent of surfactant loading which is consist- gaya 2006). ent with the result indicated in the literature. −1 In the region between 3100 and 3500 cm , the spectra The study of the point of zero charge and cationic −1 show a wide band around 3400 cm corresponding to the exchange capacity symmetric and asymmetric overlapping of the vibrations (H–O–H). The PZNPC or pH zero corresponds to the pH value for When the surfactants are inserted in the smectite gal- which the net charge of the adsorbing surface is zero lery, the adsorption bands detected are mainly attributed (Wibowo et al. 2007). This parameter is very important in to the adsorbed water molecules, in particular with high the adsorption phenomena, especially when electrostatic surfactant loading. At the same time, with the intercalation forces are involved in the mechanisms. A quick and easy way of surfactants, the clay surface property is modified. As to determine the PZNPC is to place 50 ml of distilled water a result, the hydrophilic surface of the smectite has been in closed bottles and adjust the pH of each (values between 2 1 3 Applied Water Science (2018) 8:91 Page 5 of 8 91 It should be noted that for the raw sample, the CEC is 76 meq/100 g of calcined clay and that of the purified sam- ple is 91.04 meq/100 g of calcined clay. This difference is due to the presence of impurities in the untreated sample, which are removed after purification. The obtained CEC values are characteristic of smectite clay. BET analysis of clay and surfactant modified HDTMA and CPC Table 2 shows the BET surface area (m /g), the total pore Fig. 4 Point of zero charge in clays volume (cm /g) and the average pore diameter (nm) and the basal distance (d ) of the clay, clay-CPC adsorbent Table 2 The BET surface area, total pore volume and average pore and clay-HDTMA. The BET surface area decreased from 2 2 diameter for clay, HDTMA-clay and CPC-clay 54 to 1.28 m /g for CPC-modified clay and to 2.19 m /g 2 2 for HDTMA-clay modification, which can be attributed Adsorbent samples S (m /g)VP (cm /g) APD (nm) d (nm) BET 001 to blockage and pore screening of clay by surfactant alkyl Clay 54.00 0.107 4.0 1.27 chains. The average pore diameter decreases slightly from HDTMA-clay 02.19 0.011 3.3 2.60 4.0 to 3.3 nm from clay to HDTMA-clay, which is likely CPC-clay 01.28 0.007 2.7 2.10 due to the complete removal of micropores of the adsorbent structure (Gladysz-Plaska et al. 2012). S specific surface area, VP pore volume determined by BJH BET method from N desorption isotherm, APD average pore diameter determined by the curve of BJH desorption dV/dD pore volume, d Morphology and particle size by MEB Basal distance The SEM micrographs (Fig. 5) show the surface morphol- and 12) by addition of NaOH solution or HCl (0.1 M). Then ogy of montmorillonite and organophilic clay samples. It added to each flask, 50 mg of clay. The suspensions should can be seen that the original montmorillonite Arg-Na has be kept in agitation at room temperature for 24 h, and the massive and curved plates (Fig. 5a) (Lee and Kim 2002). final pH is then determined. It relates to a graph pH = f (pH ) However, clay treated with cationic surfactants (HDTMA where pH = (pH − pH ); the intersection of the curve with and HDPy) shows significant changes in morphology. f i the axis that passes through the zero gives the isoelectric Compared to the morphology of Na-montmorillonite, point (Fig. 4). there are many small aggregated particles and the plates Regarding the value of CEC of raw and purified clays, become relatively flat in Arg-HDTMA and Arg-HDPy measurements of the cation exchange capacities of the stud- organophilic clays (Fig. 5b, c). ied clay are taken by the method of MANTIN. Therefore, the present study shows that not only the basal spacing but also the morphology of the organophilic clays Fig. 5 SEM images of Arg-Na (a), Arg-HDTMA (b) and Arg-HDPy (c) 1 3 91 Page 6 of 8 Applied Water Science (2018) 8:91 strongly depends on the packing density of the surfactants amount of surfactants also plays a role on the type of in the interspace of montmorillonite. arrangement. Based on the d001 value and the dimensions of the alkylammonium, as shown in Fig. 6, the nature of the Alkyl chain arrangements alkylammonium ion arrangements in the interfolliairy space is determined. All XRD analysis values of various Calculation of the angle of inclination: organophilic clays are summarized in Table 3. According to Zhu et al. (2003), the theoretical length of We also calculated the angle of inclination (α) of the the HDTMA cation is 25.3 Å, the height of the alkyl chain alkylammonium ion between the clay sheets based on the is 4.1 Å, and the head of the nail is 5.1 Å; The HDTMA interlayer distance d001 and the length of the alkylammo- cation is flat between layers of montmorillonite. nium ion. These values were calculated assuming that the According to Gamoudi et al. (2015), the theoretical organization of ammonium ions is paraffinic. length of the cation HDPy is 23.1 Å, the height of the alkyl It is important to note that the angle of inclination (α) of chain is 2.8 Å, and the head of the nail is 4.9 Å. the alkylammonium ion between the clay layers is calculated The difference between the values of the basal space as indicated by the following equation: d001 affects the organization of the alkylammonium ions d001 − e (HDTMA and HDPy) in the interfolliairy space, and the sin = Fig. 6 Dimension of molecules (i) HDTMA+ and (ii) HDPy (Bors et al. 2001) 1 3 Applied Water Science (2018) 8:91 Page 7 of 8 91 Table 3 d00l values of CEC HDTMA HDPyridine HDTMA, HDPy and different arrangements d/nm Arrangements d/nm Arrangements 1 1.981 (α = 35.45) Paraffine-type monolayer 2.040 (α = 42.06) Paraffine-type monolayer 2 2.130 (39.79) Paraffine-type monolayer 2.103 (α = 43.63) Paraffine-type monolayer 3 2.600 (α = 55.00) Paraffine-type bilayer 2.108 (α = 44.40) Paraffine-type monolayer e thickness of the surfactant (Å); a length of the surfactant alkyl chains take the arrangement of a paraffin-type bilayer (Å) (He et al. 2006). in the studied clay, as illustrated in Fig. 7. For HDTMA: e = 5.1 Å and a = 25.3 Å. Likewise for HDPyridine and for the same concen- For HDPyridine: e = 4.9 Å and a = 23.1 Å. tration (3CEC), the alkyl chains adopt the paraffin-type The spacing of organophilic clays increases with an single-layer structure. Therefore, the type of arrangement increase in the amount of surfactant added (increase in depends on the length of the alkyl chain as well as the the concentration of the alkylammonium salts). concentration of surfactant. The alkyl chains take the paraffin-type arrangement in the interfolliairy space of the clays. The angle of incli- nation α and the spacing d increase with the increase in Conclusions the concentration of the surfactants studied, as shown in Table 3. In this study, modified organic clays were obtained with two In the case of HDTMA, for a concentration of 3CEC, different quaternary ammonium salts (HDTMA and HDPyri- the angle of inclination α is 55°. Under this condition, the dine). X-ray diffractograms show that the interlayer distance (d001) of the organophilic clay increases after treatment Fig. 7 Representation of modi- fied smectite arrangements by HDTMA (a) and HDPy (b). The lateral distances between the alkylammonium units are inversely related to the charge density of the layer 1 3 91 Page 8 of 8 Applied Water Science (2018) 8:91 Kung K, Hayes KF (1993) Fourier transform infrared spectroscopic with the two quaternary ammonium salts (HDTMA and study of the adsorption of cetyltrimethylammonium bromide and HDPyridine), confirming the intercalation of the ammo- Cetylpyridinium chloride on silica. Langmuir 9:263–267 nium cations within clay. The greatest interlayer distance is Lee SY, Kim SJ (2002) Expansion characteristics of organoclay as a observed in clays treated with HDTMA salt. Infrared spec- precursor to nanocomposites. 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Published: May 30, 2018
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