Hydrophobicity and counterion effects on the binding of ionic surfactants to uncharged polymeric hydrogels

Gel swelling experiments have been used to study the binding of ionic surfactants to a series of nonionic alkylacrylamide hydrogels of increasing hydrophobicity. The binding of hexadecyl trimethylammonium (C16TA+) to uncharged gels is sensitive to both the hydrophobicity of the gel and the counterion to the surfactant. There is a minimum hydrophobicity threshold below which binding of the surfactant does not occur, and this is influenced by the counterion to the surfactant. The surfactant concentration at the onset of binding, the critical association concentration (cac), decreases with increasing gel hydrophobicity. The maximum swelling of the gel (at intermediate network hydrophobicity) increases in the order of the Hofmeister series of anions, bromide (Br-) < chloride (Cl-) < acetate (Ac-). At higher gel hydrophobicity, differences in swelling are no longer observed on changing the counterion. A minimum hydrophobicity threshold was also found for the binding of the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl-di(ethylene oxide)-sulfate (SD-(EO)2-S). Differences in the swelling behavior with network hydrophobicity are explained in terms of the degree of saturation of the gel with surfactant at the cmc.