The influence of a hydrotropic agent in the properties of aqueous solutions containing poly(ethylene oxide)–poly(propylene oxide) surfactants

Abstract

The aqueous solution behavior of diblock poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO) copolymers coupled with hydrocarbon groups was studied in the presence of the hydrotropic agent sodium p-toluenesulfonate (NaPTS). The change in phase of the aqueous systems was evaluated by building up temperature–concentration phase diagrams. The critical micelle concentrations (CMC) of the copolymers and the aggregation points of NaPTS and NaPTS/copolymer mixtures were obtained by surface tension measurements, viscometry data and dye solubilization. The copolymers and NaPTS adsorb and reduce the surface tension of the solution until the surface becomes saturated: the CMC values are related to the solubility of the copolymers. Solutions containing NaPTS/copolymer mixtures exhibit the opposite behavior: at constant copolymer concentrations and with increasing NaPTS concentration, the surface tension remains constant until the aggregates of NaPTS start to form. Above this concentration, the surface tension increases. The surface tension and the aggregation points of the NaPTS solutions are dependent on the structure of the copolymer. The influence of the length of the hydrocarbon groups and the PPO position segment in the structure of the copolymers were also studied. From viscometric data, a pronounced increase in solution viscosity was observed as aggregates began to form. The results obtained from dye solubilization are in good agreement with the surface tension and viscometric measurements.