Thermodynamic Properties of Surfactant/Polymer/Water Systems with Respect to Clustering Adsorption and Intermolecular Interaction as a Function of Temperature and Polymer Concentration

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This investigation forms part of a more extensive physicochemical study of the interaction of amphiphiles with different polymers, partially as a background for a better understanding of the function of pharmaceutical excipients. For the ethyl hydroxyethyl cellulose (EHEC)/sodium dodecyl sulfate (SDS)/water system, the dialysis equilibrium between a solution containing polymer and a solution not containing polymer has been investigated over a composition interval of the amphiphile ranging from zero up to well above cmc and for the polymer from zero up to slightly above the concentration of critical overlap. It is found that the amphiphile begins to redistribute preferentially to the polymer at a “foot point” (critical aggregation concentration) concentration of 2 mM SDS, rises to a maximum value at an SDS concentration most likely corresponding to the onset of formation of normal micelles in bulk, and then decreases. The maximum adsorption corresponds to approximately two DS- ions per structural unit of the polymer. It is proposed that the results observed could be interpreted in terms of a redistribution of surfactant to the polymer coil regions leading to locally enhanced surfactant concentration, cluster formation, and adsorption. In combination with polymer reconformation this model suggests an explanation of some specific effects observed on this system: the increase in average cluster size with increasing polymer concentration, the marked decrease in intrinsic viscosity at the onset of surfactant adsorption combined with a strong hydrodynamic interaction (high value of the Huggins' constant), and the considerable increase in macroscopic viscosity during the first phase of adsorption. The decrease in specific adsorption after the maximum probably derives from changes in DS- ion activity and saturation of the polymer, as well as the formation of normal micelles. Temperature is found to have only a small influence on the redistribution in an interval from room temperature up to the cloud point. The correction for the Donnan effect is discussed and results from conductometric measurements are used to calculate the degree of dissociation of surfactant in the micellar clusters.