Complex formation between hydroxypropylcellulose and hexadecyltrimethylamonium bromide as studied by light scattering and viscometry

Complex formation between a semiflexible nonionic polymer, hydroxypropylcel lulose (HPC), and a cationic surfactant, hexadecyltrimethylamonium bromide (HTAB), is investigated by static and dynamic light scattering (DLS) and by viscometry. Upon addition of surfactant, at a fixed polymer concentration, the solution specific viscosity increases initially to a maximum value, an d then decreases, in parallel with the hydrodynamic radius of the polymer-s urfactant complex measured by DLS. The increase in specific viscosity and h ydrodynamic radius is interpreted to reflect chain expansion due to the ele ctrostatic interaction between bound micelles. The maximum occurs at the sa turation of micelle binding, and the subsequent decrease, as more surfactan t is added, is due to chain contraction because of electrostatic screening by free micelles and their Br- counter-ions. This behavior was observed at various polymer concentrations and we found that the maximum in the solutio n viscosity and in the hydrodynamic radius occurs always at [HTAB]/[HPC] = 0.18, from which we deduce that the average number of surfactant molecules bound to a single HPC chain is 28. The effect of electrostatic interactions between the bound micelles on the solution viscosity can be represented by the "interaction viscosity", eta(I), defined as the difference between the measured viscosity of the ternary polymer solution and that computed from the sum of solvent, polymer and surfactant. We find that the normalized int eraction viscosity eta(I)/eta(I,max), where n(I,max) is the interaction vis cosity at the maximum surfactant-polymer binding, is a common function of [ HTAB]/[HPC], independent of polymer concentration and ionic strength. The o rigin of this observation appears to lie in the fact that the fraction of c omplex formed and the relative chain expansion are functions only of the ra tio [HTAB]/[HPC]. (C) 1999 Elsevier Science Ltd. All rights reserved.