Binding of sodium dodecylsulfate to poly( N-isopropylacrylamide) microgels at different temperatures

Abstract

The binding of sodium dodecylsulfate (SDS) to poly ( N-isopropylacrylamide), (PNIPAM) microgels at different temperatures (below and above the transition temperature from swollen to shrunken particles, T s) has been investigated by fluorescence probe methods complemented with surface tension and dynamic light scattering measurements. Pyrene and 1-anilinonaphthalene-8-sulfonate (ANS) were employed as fluorescence probes. At 21°C, the association of the surfactant with the particles occurs at a critical aggregation concentration (cac). Values obtained were (cac)=1 mM form surface tension data and (cac)≅0.75 mM from the dependence of the fine structure of pyrene fluorescence spectra with SDS addition. The binding isotherm derived from surface tension data at 21°C is highly cooperative, implying that the binding takes place with formation of surfactant aggregates. At 42°C (a temperature above T s), the (cac) shifts to 1.7 mM indicating that the interaction of SDS with shrunken PNIPAM microgels is weaker than with the swollen species. The microenvironment sensed by pyrene probe in both, the SDS aggregates formed onto PNIPAM swollen (21°C) and shrunken (42°C) species is less polar than that corresponding to SDS micelles in pure water. The T s of the particles and the effect of SDS on it can be determined from the temperature dependence of ANS fluorescence intensity or emission band position. At temperatures higher than T s, coulombic repulsion effects (probably due to the higher charge density of the collapsed particles) are evidenced by the fluorescence behaviour of both probes employed.