Handbook of Solvents - George Wypych - ChemTech - Ventech!

124 Valery Yu. Senichev, Vasiliy V. Tereshatov
35 R.A Keller, B.L. Karger and L.R. Snyder, Gas Chromtogr., Proc. Int Symp. (Eur), 8, 125(1971).
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4.2 EFFECT OF SYSTEM VARIABLES ON SOLUBILITY
Valery Yu. Senichev, Vasiliy V. Tereshatov
Institute of Technical Chemistry
Ural Branch of Russian Academy of Sciences, Perm, Russia
Solubility in solvents depends on various internal and external factors. Chemical structure,
molecular mass of solute, and crosslinking of polymer fall into the first group of factors, in
addition to temperature and pressure in the second group of factors involved.
4.2.1 GENERAL CONSIDERATIONS
The process of dissolution is determined by a combination of enthalpy and entropy factors.
The dissolution description can be based on the Flory-Huggins equation. Flory 1-3 and
Huggins 4 calculated the entropy of mixing of long-chain molecules under the assumption
that polymer segments occupy sites of a “lattice” and solvent molecules occupy single sites.
The theory implies that the entropy of mixing is combinatorial, i.e., it is stipulated by
permutations of molecules into solution in which the molecules of mixed components differ
greatly in size. The next assumption is that ΔVmix = 0 and that the enthalpy of mixing does
not influence the value of ΔSmix. The last assumptions are the same as in the Hildebrand theory
of regular solutions. 5 The expression for the Gibbs energy of mixing is
ΔG
RT
⎛ V ⎞ 2
= x + x + ⎜
x + x ⎟
1lnϕ1 2 lnϕ2
χ1ϕ1ϕ 2 1 2
⎝ V ⎟
[4.2.1]
1 ⎠
where:
x1,x2 molar fractions of solvent and polymer, respectively
χ1 Huggins interaction parameter
The first two terms result from the configurational entropy of mixing and are always
negative. For ΔG to be negative, the χ1 value must be as small as possible. The theory assumes
that the χ1 parameter does not depend on concentration without experimental confirmation.
χ1 is a dimensionless quantity characterizing the difference between the interaction
energy of solvent molecule immersed in the pure polymer compared with interaction energy
in the pure solvent. It is a semi-empirical constant. This parameter was introduced by Flory
and Huggins in the equation for solvent activity to extend their theory for athermic processes
to the non-athermic processes of mixing: