Thermodynamic equilibrium in the system macromolecular coil–binary solvent

Thermodynamic conditions of an equilibrium between a swollen coil and a solvent outside the coil, established in a dilute solution of a polymer in a binary solvent, are expressed in terms of quantities related to the unit volume. The impossibility to correlate unambiguously preferential sorption with the concentration change of overall sorption is discussed by means of an analogy of Gibbs adsorption equation. Equations expressing the preferential sorption of one of the components in the coil and the overall swelling of the coil in terms of partial derivatives of the free enthalpy of mixing with respect to composition are derived. These equations facilitate the calculation of preferential and overall sorption from theoretical or semiempirical relations expressing the composition dependence of ΔGM, the free enthalpy of mixing. The application is shown on a treatment of data obtained by the measurement of dialysis equilibrium and intrinsic viscosity in the system polymethylmethacrylate–benzene–methanol. The possibility of generalization of the equations derived to the case of osmotic and vapor‐pressure equilibrium in dilute solutions of a polymer in a binary solvent is pointed out. The relation of the equations for osmotic equilibrium to the Scatchard equations is discussed as well.