EFFECTS OF SURFACTANTS ON THE PHASE-TRANSITION OF POLY(N-ISOPROPYLACRYLAMIDE) GEL

The swelling equilibrium of N-isopropylacrylamide (NIPA) gel was studied as a function of temperature in aqueous solutions of surfactants: sodium dodecyl sulfate (SDS, anionic), dodecyltrimethylammonium chloride (DTAC, cationic), and nonaoxyethylene dodecyl ether (NODE, nonionic). In pure water the gel showed a discontinuous volume phase transition at 33.6-degrees-C with a volume change of V(swollen)/V(collapsed) = 8. With the addition of SDS, the transition temperature as well as the volume change at the transition increased. At 3.47 X 10(-2) M of SDS concentration, the transition temperature became 88-degrees-C and the volume ratio increased to V(swollen)/V(collapsed) = 32. A similar but less drastic change was observed when DTAC was used. In contrast, NODE induced little change in both the temperature and temperature phase transition for the concentration range studied (<6.94 X 10(-2) M). In order to understand these effects of ionic surfactants, the association and dissociation of SDS with the gel or homopolymer of NIPA were investigated in an aqueous system. The amount of free SDS ions was measured at different temperatures by a conductometric method. An increase in the transition temperature was brought about by SDS binding to the gel or polymer whereupon the bound SDS was completely dissociated by cooling. The association and dissociation of SDS were reversible and, respectively, occurred in the temperature ranges above and below 30-degrees-C. Thus, the ionic surfactant molecules convert the gel into an ionic gel due to binding through the hydrophobic interaction. The obtained experimental results have been qualitatively discussed in terms of the classical Flory-Huggins theory with modifications to account for the free energy of association of surfactant molecules with the gel network or polymer and surfactant self-association.