Interaction of polyvinylpyrrolidone with copper(II), nickel and cadmium dodecyl sulfates: a calorimetric investigation

Abstract

The interaction of a water-soluble polymer, polyvinylpyrrolidone (PVP), with sodium, copper, nickel and cadmium dodecyl sulfates has been investigated using a calorimetric technique. It is shown that the enthalpy of dilution of PVP is exothermic at low polymer concentration for all surfactants studied and increases toward its value in pure water at higher PVP concentrations. The PVP enthalpy of dilution depends upon the surfactant concentration and is more exothermic the higher the surfactant concentration. Large quantitative differences are observed between the interaction of the polymer with the monovalent and the divalent surfactants. From the enthalpy of solution vs. PVP concentration profiles, the degree of surfactant coverage on the polymer has been evaluated. It is shown that the surfactant coverage is much larger with the monovalent (binding ratio r=0.47) than with the divalent surfactants ( r=0.17). It is indicated that at the low PVP and surfactant concentrations studied, the enthalpy changes are essentially due to the polymer conformational changes from an expanded to a coiled native state induced by the adsorbed micellar aggregates. The counterion binding of the adsorbed micellar aggregates may be the main factor which governs these polymer structural changes.