Interactions between hydrophobically modified polymers and surfactants: A fluorescence study

Interactions between hydrophobically modified polymers (hm-polymers) and surfactants were investigated by steady-state fluorescence as well as rheological experiments. Hydrophobically modified hydroxyethylcellulose (hmHEC) with C₁₆ hydrophobe grafts of 0.9% mole along with tetradecyltrimethylammonium bromide and cetyltrimethylammonium bromide were studied. The presence of C₁₆HEC induced the aggregation of surfactant micelles in aqueous solution at concentrations about one-half times lower than the critical micelle concentration. As surfactant is added, the viscosity of hm-polymer solution first increases due to the bridging of hydrophobe clusters by surfactant micelles to form mixed micelles and later decreases due to the masking of hydrophobes individually by excess micelles. Fluorescence quenching experiments showed that the number of hydrophobes (N_H) in a mixed micelle decreases steadily with increasing surfactant concentration due to hydrophobe dilution by the surfactant. N_H declines from greater than two with no surfactant to around two at the concentration where the viscosity reaches a maximum and eventually falls to one or below when hydrophobes are masked. The observations were explained in terms of the effective associating junctions and are consistent with the picture of interactions between hm-polymers and surfactants for polymers with stiff backbones and no intramolecular association.