Solubilisation of hydrocarbons in surfactant monolayers

Abstract

Hydrocarbons which do not spread macroscopically on aqueous surfactant solutions can nevertheless be incorporated (solubilised) in the adsorbed surfactant films, producing mixed monolayers. The surface concentration of alkane in these systems can be obtained semi-quantitatively from the lowering of surface tension caused by the solubilisation. For surfactant solutions above the c.m.c., alkane uptake is found to decrease with increasing alkane chain length, but to increase with increasing surfactant chain length. For a given surfactant, not only is the molar uptake of alkane greater the smaller the alkane, but also the volume uptake. Solubilisation does not significantly affect the surface concentration of surfactant, but rather it increases the monolayer thickness. As close-packed monolayers are progressively diluted, the alkane solubilisation is found to rise initially but then to fall rather rapidly when the surface concentration of surfactant is reduced to ca. half that at the c.m.c. The contact angles, θ, of all the allkanes on dodecyltrimethylammonium bromide solutions close to the c.m.c. are very low. As the surfactant monolayer is diluted, θ begins to rise rapidly when the monolayer becomes less than ‘close-packed’, finally approaching the value for alkane on pure water. For the system comprising sodium diethylhexylsulphosuccinate (AOT), aqueous NaCl and cyclohexane, increase in the salt concentration up to 0.025 mol dm^–3 compresses the surfactant monolayer and as a ressult, hydrocarbon solubilisation is substantially reduced. Above this salt concentration solubilisation remains constant. Inversion of microemulsion type in this same system also occurs at 0.025 mol dm^–3 NaCl and so it appears, for this system at least, that the hydrocarbon pentration is very similar at the oil/water and the air/water interfaces.