Physical and rheological properties of nonionic polymers-ionic surfactants complexes

Abstract

The formation of a polymer-surfactant complex upon mixing a nonionic polymer, poly(ethylene oxide) (PEO), with a cationic surfactant, hexadecyltrimethylammoniumchloride (HTAC), was studied by observing the changes in conductivity, specific viscosity (nsp), and hydrodynamic radius (Rh). The conductivity data showed that an interaction between PEO and HTAC occurred at a temperature above 25C, as indicated by a decrease in the critical aggregation concentration in the presence of PEO relative to the critical micelle concentration of a surfactant in the absence of PEO. The binding of HTAC to PEO induced a chain expansion due to electrostatic repulsions between bound micelles. On further addition of HTAC, nsp and Rh reached a maximum at the saturation of binding, and then decreased because of the contraction of the PEO-HTAC complex due to electrostatic screening from the accumulation of free micelles and conunterions in the solution. From the static light scattering, the structures of PEO-HTAC complexes were determined by means of molecular weight measurement at different concentration ratios of HTAC to PEO. Multichain complexation was observed in aqueous solution in the formation of the complex at the maximum binding. Howver, in the presence of 0.1 M KNO3, the structure of the complex changed from multichain to unipolymer complexation. Moreover, the number of bound HTAC molecules per PEO chain increased form 0.12 mole HTAC per mole EO to 0.23 mole HTAC per mole EO in salt solution. Rhological measurements also indicated that the storage modulus G' and the loss modulus G" of the concentrated PEO-HTAC complex solution show largest values at mass concentration ratio of 1.5 which is near the maximum binding of HTAC to PEO in dilute aqueous solution. It was confirmed that the progressive increase on the PEO-HTAC aggregates induced a cross-linking between PEO and HTAC, leading to an increase in the modulus. Finally, the interaction between nonionic polymer, hydroxypropyl cellulose (HPC), and amphoteric surfactant, cocamidopropy. dimethyl glycne (CADG), was studied by means of viscosity and light scattering measurements in an isolectric point (pH = 9). The viscosity and dynamic light scattering showed that a maximum and minimum occurred [HTAC]/[PEO] ratios (cs/cp) at 0.026 and 0.43. From Zimm plot analysis, the molecular weight of complex (Mw,com)was approximately equal to the molecular weight of pure HPC at maximum point, indicating that there was no interaction between HPC and CADG at c2/cp = 0.026. The increases in Mw,com and the number of bound CADG to HPC were observed due to the binding between polymer and surfactant. At the binding condition, the minimum value was observed because of the electrostatic attractions between positive and negative charges within the polymer chain.