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

The aqueous solution behavior of diblock poly(ethylene oxide)-poly(propylen e oxide) (PEO-PPO) copolymers coupled with hydrocarbon groups was studied i n the presence of the hydrotropic agent sodium p-toluenesulfonate (NaPTS). The change in phase of the aqueous systems was evaluated by building up tem perature-concentration phase diagrams. The critical micelle concentrations (CMC) of the copolymers and the aggregation points of NaPTS and NaPTS/copol ymer mixtures were obtained by surface tension measurements, viscometry dat a and dye solubilization. The copolymers and NaPTS adsorb and reduce the su rface tension of the solution until the surface becomes saturated: the CMC values are related to the solubility of the copolymers. Solutions containin g NaPTS/copolymer mixtures exhibit the opposite behavior: at constant copol ymer concentrations and with increasing NaPTS concentration, the surface te nsion remains constant until the aggregates of NaPTS start to form. Above t his concentration, the surface tension increases. The surface tension and t he aggregation points of the NaPTS solutions are dependent on the structure of the copolymer. The influence of the length of the hydrocarbon groups an d the PPO position segment in the structure of the copolymers were also stu died. From viscometric data, a pronounced increase in solution viscosity wa s observed as aggregates began to form. The results obtained from dye solub ilization are in good agreement with the surface tension and viscometric me asurements. (C) 1999 Elsevier Science B.V. All rights reserved.