Thermodynamics of solutions of interacting aggregates by methods similar to surface thermodynamics. Part 2.—Solutions of non-associating macromolecules

Abstract

A general formalism similar to adsorption thermodynamics but applicable to solutions of macromolecules is developed. This formalism enables measurable thermodynamic quantities to be interpreted simply in molecular terms provided only that the ratios of the activity coefficients of the various macromolecular species remain constant when the concentration of macromolecules is varied at constant T, p and chemical potentials of all other solute components. Important features of this approach are: (1) the same formalism is applicable to calorimetric density and activity data; (2) it allows the distinction between specific site binding and non-site binding to be preserved and enables amounts “bound” to be defined precisely in systems where there is no site binding of the species concerned; (3) all chemical potentials refer to electrically neutral combinations of species. It is shown how the theory can be used to define the free energy, entropy and enthalpy of binding of say divalent ions to polyions. Also an improved interpretation of polyacid titration curves is outlined which shows that the effects of simple electrolytes are analogous to the Esin–Markov effect for charged interfaces and can be interpreted without assuming either ion to be specifically bound.