ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 8, pp. 1291!1296. + Pleiades Publishing, Inc., 2006.
Original Russian Text + L.S. Litvinova, N.G. Bel’nikevich, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 8, pp. 1305!1310.
AND POLYMERIC MATERIALS
A Study of the Relationship between the Thermodynamic
Quality of Mobile Phases and Their Strength for
the Critical Conditions of Liquid Chromatography
L. S. Litvinova and N. G. Bel’nikevich
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
Received March 9, 2006
Abstract-The relationship between the thermodynamic quality of mobile phases and their strength e was
studied for the critical conditions of liquid chromatography. The chromatography of reference polystyrene
samples was carried out in the adsorption and exclusion modes, and also under critical conditions, with binary
mobile phases and a silica gel column used. The thermodynamic quality of solvent mixtures with respect to
one another was evaluated by the intrinsic viscosity [h].
Liquid chromatography under critical conditions
(LCCC) is a promising method for analysis of poly-
mers with various types of molecular polydispersity.
A common LCCC strategy is to suppress the separa-
tion selectivity for one of molecular characteristics
(usually, molecular weight), which provides separa-
tion by another parameter .
An important specific feature of LCCC is the ne-
cessity for fine adjustment of interactions within
the system. This is achieved by careful selection of
both mobile and immobile phases for the polymer
systems studied and by varying the temperature of
the experiment. Another distinctive feature of LCCC
is that mixed solvents are used as mobile phases.
As a rule, this is a mixture of adsorption-active and
adsorption-inactive solvents. Thus, ternary polymer
systems (a polymer in a binary solvent) are widely
used in LCCC.
In liquid chromatography, the distribution coef-
is related to a change in the Gibbs energy
(3DG) for polymer3solvent3adsorbent systems [1, 12]:
3DG = RT ln K
= 3DH + TDS, (1)
where 3DH is a change in the enthalpy; DS, change
in entropy upon transition of a macromolecule from
the interparticle (free) volume into the immobile
phase; T, temperature (K); and R, gas constant.
It follows from Eq. (1) that DH = TDS for K
Consequently, the change in the entropy at the critical
point, when a macromolecule passes from solution
into the adsorbent, is compensated for by a change in
the enthalpy corresponding to the interaction between
the macromolecule and active centers of the adsorbent.
It is important that the critical conditions can be
created for the same polymer3adsorbent combination
in mixtures of different solvents.
Because a macromolecule competes with solvent
molecules for the active centers of the adsorbent,
changes in the macromolecule3adsorbent interactions
are due to a change in the interaction between the mo-
bile phase and the adsorbent, provided that the system
is in equilibrium. The interaction of the mobile phase
with the adsorbent and the polarity of the former
can be evaluated using Snyder’s parameters: solvent
 and polarity index P` .
A specific feature of polymers is that the molecular
conformation (i.e., intramolecular interactions deter-
mining the coil size in solution) depends on proper-
ties of a solvent, specifically, on its thermodynamic
quality. It can be assumed that the conformational
changes in solution, which can be evaluated by
the intrinsic viscosity [h] , may be a reason for
changes in the chromatographic behavior of macro-