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 out at different temperatures on gelling and nongelling aqueous systems of ethyl(hydroxyethyl) cellulose (EHEC) in the presence of sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). The surfactant self-diffusion experiments revealed strong surfactant-EHEC interactions. By using a two-site model it was demonstrated that the degree of surfactant binding to EHEC, at a given total surfactant concentration, is higher in the presence of CTAB than with SDS. In both cases, the amount of surfactant bound to the polymer is roughly independent of temperature, even when the gel transition region is approached. The results 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 between surfactant-induced associations and enhanced polymer-polymer interactions 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 exponential (''slow'' diffusion). The observations indicate that the interactions are stronger in the EHEC/SDS system as compared with the corresponding EHEC/CTAB system.