Surface activity and association of ABA polyoxyethylene—polyoxypropylene block copolymers in aqueous solution

The surface tension of aqueous solutions of a range of ABA polyoxyethylene-polyoxypropylene nonionic copolymers has been measured over a wide range of concentrations. The surface tension versus concentration plots show an inflection typical of that shown by conventional surfactants, but this occurs at very low concentrations (0.5–5 × 10−6 M liter−1) and it is thought to be the result of monomolecular micelle formation. At higher concentrations (0.5 × 10−4 to 1.5 × 10−2 M liter−1) polymolecular aggregation probably takes place. The extent of uptake of benzopurpurine and iodine in aqueous solutions increases at high concentrations but the inflection points do not agree with any observable in the surface tension plots. Areas per molecule calculated using the simple form of the Gibbs' equation indicate that there is considerable folding of the polymers at the air-water interface. For a given hydrophobic polyoxypropylene block, increasing the size of the hydrophilic polyoxyethylene chains causes an increase in interfacial molecular area. Increasing the length of the central hydrophobic moiety allows the molecule to fold more extensively and results in a decrease in the occupied area per molecule. Raising the temperature causes an increase in solution viscosity, especially near the lower consolute temperature (cloud point) of the system, a further indication of polymolecular association in these systems.