INTERACTION OF SURFACTANT WITH POLYMER GEL - BINDING ISOTHERM AND SWELLING RATIO

Binding of a cationic surfactant, dodecylpyridinium chloride, to three kinds of anionic polymer gels (copolymers of N-isopropylacrylamide(NI PA) and sodium 2-acrylamide-2-methylpropane sulfonate (AMPS) cross-lin ked by 1 mol.% N,N'-methylene bis(acrylamide)) is determined in 10 mM NaCl by using a surfactant-selective electrode at 25 degrees 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 wi th the same monomer composition as the polymer gels. Binding to both p olymer gels and linear polymers is highly cooperative, being stronger for a polymer with a higher charged group content. Linear polymers bin d the surfactants stronger than polymer gels, but the difference gets smaller as the charged group content decreases. These results imply th at the extent of interaction between bound surfactant is larger than t he average cross-linking distance in the 100:0 gel, but at least compa rable in 25:75. The gel volume is measured in the presence of a surfac tant by a microscope and found to drastically shrink when surfactant b inding exceeds about 20% of binding sites (sulfonate groups). A plot o f the swelling ratio vs the degree of binding gives a curve around whi ch all the experimental points scatter showing that the swelling ratio is uniquely determined by surfactant binding, irrespective of the cop olymer composition.