Calorimetric, viscosimetric, and light scattering studies of the aggregation of, and the solubilization of water by, Triton X-35 in n-heptane

The behavior of a nonionic surfactant TX-35 in solution in n-heptane in the presence and absence of added water has been examined using the microcalor imetric, viscosimetric, and quasielastic light scattering experimental meth ods. In this paper, we were interested in the aggregation process of the po ly(oxyethylene) glycol alkylphenyl ether in n-heptane and in the solubiliza tion of water in the reverse micelle of the surfactant (micellar solubiliza tion). The analysis of the differential molar enthalpies of dilution of TX- 35 in dried n-heptane has shown the occurrence of a gradual exothermic aggr egation process on a very wide range of concentration which takes place at particular concentration so-called "operational CMC". This operational CMC value has been confirmed by viscosities measurements. The differential mola r enthalpies of hydration of TX-35 were also measured and found to be esoth ermic, The maximal hydration ratio (w(o)) was found to be equal to 3.2 mol H2O per mole of TX-35 before the point of phase separation, The measurement s of the variation of the amount of water contained in TX-35 solutions at d ifferent concentrations in n-heptane also show the occurence of a gradual a ggregation process and confirm the value of the maximal hydration ratio alr eady determined by microcalorimetry. In the absence of added water, from th e quasielastic light scattering experiments, a mean diameter of the aggrega tes close to 45 Angstrom has been determine, while in the presence of water , a mean diameter of 61 Angstrom was detected and remained unchanged with i ncreasing the hydration ratio indicating that the size of the aggregate is more influenced by the presence of water than by the amount, In the presenc e of water, it is relevant to discuss aggregates of lamellar or filament sh ape. (C) 1999 Academic Press.