INTERACTIONS OF IONIC SURFACTANTS WITH A NONIONIC CELLULOSE ETHER IN SOLUTION AND IN THE GEL STATE STUDIED BY PULSED-FIELD GRADIENT NMR

Surfactant and polymer NMR self-diffusion measurements were carried ou t at different temperatures on gelling and nongelling aqueous systems of ethyl(hydroxyethyl) cellulose (EHEC) in the presence of sodium dode cyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). The surfac tant self-diffusion experiments revealed strong surfactant-EHEC intera ctions. By using a two-site model it was demonstrated that the degree of surfactant binding to EHEC, at a given total surfactant concentrati on, is higher in the presence of CTAB than with SDS. In both cases, th e amount of surfactant bound to the polymer is roughly independent of temperature, even when the gel transition region is approached. The re sults for a more polar EHEC sample show that in the presence of SDS (a nongelling system in the temperature range considered) a significant increase of the binding parameter is observed at elevated temperatures , whereas no change of the parameter is found in solutions containing CTAB. in the analysis of the interaction situation, the interplay betw een surfactant-induced associations and enhanced polymer-polymer inter actions is considered. The polymer self-diffusion results suggest that the decay of the spin-echo attenuation can initially be described by a stretched exponential (''fast'' diffusion) followed by a single expo nential (''slow'' diffusion). The observations indicate that the inter actions are stronger in the EHEC/SDS system as compared with the corre sponding EHEC/CTAB system.