Interaction of surfactant with polymer gel binding isotherm and swelling ratio

Abstract

Abstract Binding of a cationic surfactant, dodecylpyridinium chloride, to three kinds of anionic polymer gels (copolymers of N-isopropylacrylamide(NIPA) and sodium 2-acrylamide-2-methylpropane sulfonate (AMPS) cross-linked by 1 mol.% N,N′-methylene bis(acrylamide)) is determined in 10 mM NaCl by using a surfactant-selective electrode at 25°C, where the polymer compositions are AMPS:NIPA=100:0, 50:50, and 25:75 in mole ratio. The binding behavior is compared with three linear polymers with the same monomer composition as the polymer gels. Binding to both polymer gels and linear polymers is highly cooperative, being stronger for a polymer with a higher charged group content. Linear polymers bind the surfactants stronger than polymer gels, but the difference gets smaller as the charged group content decreases. These results imply that the extent of interaction between bound surfactant is larger than the average cross-linking distance in the 100:0 gel, but at least comparable in 25:75. The gel volume is measured in the presence of a surfactant by a microscope and found to drastically shrink when surfactant binding exceeds about 20% of binding sites (sulfonate groups). A plot of the swelling ratio vs the degree of binding gives a curve around which all the experimental points scatter showing that the swelling ratio is uniquely determined by surfactant binding, irrespective of the copolymer composition.