Interactions between surfactants and polymers. I: HPMC

We have studied the interactions between hydroxypropylmethyl cellulose (HMPC) and the surface active substances sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (C16TAB), sodium cholate (SC0 and sodium taurocholate (STC) in solution. Cloud point (CP) measurements showed that the CP was lowered when surfactant was added at concentrations below the normal CMC, the effect being most pronounced for SDS and least for the bile salts. The effect could be enhanced with low concentrations of added salt. Experiments with ion-selective electrodes and conductdivity showed that the process was cooperative, and that small aggregates with low surface charge density were formed. The aggregates were highly hydrophobic as determined with fluorescence probe studies. The aggregation number of the SDS/HPMC aggregate was determined to be 30 with fluorescence quenching. The importance of the results from a pharmaceutical formulation point of view was emphasized.