Investigation of the poly(N-isopropylacrylamide) sodium dodecyl sulfate complexation with viscosity, dialysis, and time-resolved fluorescence-quenching measurements

Interaction of poly(N-isopropylacrylamide) (PNIPAM) with sodium dodecyl sulfate (SDS) has been studied by viscometry, time-resolved fluorescence-quenching (TRFQ), and dialysis measurements. The reduced viscosity of PNIPAM in SDS aqueous solutions presents an abrupt increase in the low SDS concentration region, lower than the surfactant critical micelle concentration, whereas TRFQ measurements show the formation of an increasing number of polymer-surfactant aggregates in the same SDS concentration region. The aggregation number of these aggregates, determined by combining TRFQ with dialysis data, remains constant and equal to 7-8 surfactant molecules per polymer-bound surfactant aggregate. Moreover, the values of the inverse of the intramicellar quenching rate constant, expressing the effective microviscosity of the aggregates, indicate that these microdomains, formed in the low SDS concentration region, are very rigid. After saturation of the PNIPAM chain with these aggregates, a further quantity of surfactant is adsorbed on the polymer, and this is reflected in an increase of their aggregation number. However, the aggregation number of these polymer-surfactant aggregates remains much lower than that of free surfactant micelles.