Thermodynamics of ionic surfactant binding to macromolecules in solution

Abstract

The thermodynamic treatment of charged colloidal systems published recently is applied to solutions in which ionic surfactants bind to macromolecules. The aim is to develop an improved basis for the interpretation of thermodynamic information obtained from techniques such as electromotive-force measurement and dialysis equilibria. The main emphasis is on binding to neutral macromolecules or to polyions with the same sign as the surfactant ions. Expressions are derived which relate the form of binding isotherm to the distribution of bound surfactant ions among the macromolecules and which enable enthalpies of binding to be obtained from the dependence on temperature of the binding equilibria. Several interesting effects are predicted. For example, (a) the average amount bound per macromolecule may decrease with increasing surfactant concentration in the presence of free micelles, (b) the surfactant monomer concentration may exhibit two maxima separated by a minimum, (c) increasing surfactant concentration may cause the macromolecule chemical potential to exhibit two minima separated by a maximum. The theory also explains how observed binding isotherms can be expected to depend on the concentration of macromolecules. The situation in which bound surfactant aggregates coexist with free surfactant aggregates is examined in some detail. The treatment is readily extended to more complex cases such as binding to polyacids and oppositely charged macroions. It is likely to apply equally well to other types of ion binding by macromolecules.