Density measurements on C12Ej nonionic micellar solutions as a function ofthe head group degree of polymerization (j=5-8)

Density measurements have been performed on water solutions of nonionic sur factants oligooxyethylene glycol-monoether (C12Ej with j = 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure s urfactants in their Liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethyle ne (EO) bulk liquid phases. The deviation from ideality reduces as j increa ses and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T-cross), at which the C12Ej aq ueous solution density coincides with the solvent one up to 50 wt % has bee n found in all the E-j species. The T-cross of the different E-j results to be scaling temperatures for the excess density of these surfactants. The s phere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the Ej species. In the case of j = 6, 7, and 8 the values of t he sphere-to-rod transition temperatures are known and these differences ar e all about 20 degreesC. For C12E5 only rods were experimentally observed a nd, coherently, the previous consideration led to a sphere to-rod transitio n temperature of -7 degreesC. The difference between the critical and the c rossing temperatures is constant too, at about 15 degreesC for j < 8, where as for j = 8 it is 7 <degrees>C. In this case the densities of the separate d phases, two degrees above the critical temperature, result to be the same , within 10(-6) g/cm(3). It implies that, when slightly above the critical temperature, C12E8-water solution undergoes phase separate as it was in mic rogravitational condition. In our analysis, the C12Ej solution is considere d a three-component system: the oil core, the bulk water, and the region co ntaining the aggregate interface, a mixture of water molecules, and EO unit s. Fixing the density of the oil core and of the bulk water phase respectiv ely equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that-the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusi on follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur.