An NMR study of macromolecular aggregation in a model polymer-surfactant solution

Abstract

A model complex-forming nonionic polymer–anionic surfactant system in aqueous solution has been studied at different surfactant concentrations. Using pulsed-field-gradient diffusionNMR spectroscopy, we obtain the self-diffusion coefficients of poly(ethylene glycol) (PEO) and sodium dodecyl sulfate (SDS) simultaneously and as a function of SDS concentration. In addition, we obtain NMR relaxation rates and chemical shifts as a function of SDS concentration. Within the context of a simple model, our experimental results yield the onset of aggregation of SDS on PEO chains (CAC=3.5 mM), a crossover concentration (C2=60 mM) which signals a sharp change in relaxation behavior, as well as an increase in free surfactant concentration and a critical concentration (Cm=145 mM) which signals a distinct change in diffusion behavior and a crossover to a solution containing free micelles.Cm also marks the concentration above which obstruction effects are definitely important. In addition, we obtain the concentration of SDS in monomeric form and in the form of free micelles, as well as the average number of SDS molecules in a PEO-SDS aggregate(NAggr). Taken together, our results suggests continuous changes in the aggregation phenomenon over much of the concentration but with three distinct concentrations that signal changes in the nature of the aggregates.