Aggregation behavior of surfactants in polymer gel networks

Aggregation behavior of anionic sodium dodecyl sulfate and a nonionic surfactant, polyoxyethylene (p = 10) nonylphenyl ether, in the inside and outside of polymer hydrogels has been studied through solubilization techniques of an oil-soluble dye, measurements of surfactant concentration in the aqueous phase of the gel interior, and binding isotherms of the surfactants onto polymer chains of the gel. The surfactant aggregation is strongly affected by the presence of polymer gel networks. Interestingly, it is found that the nonionic surfactant cannot form micelles inside the gel at concentrations greater than even the critical micelle concentration in the outer bulk solution. The anionic surfactant molecules adsorb onto the polymer chains of a relatively hydrophobic N-isopropylacrylamide gel but not onto the hydrophilic acrylamide (AAm) gel. In the AAm gel, the anionic surfactant forms micelles in the aqueous solution phase inside the gel, avoiding the exclusive space of the polymer chains.