Surface chemistry and microemulsion formation in systems containing dialkylphthalate esters as oils

Abstract

The surface chemical behaviour of phthalate esters in oil–water mixtures, both in the presence and absence of a conventional surfactant, has been investigated. We have estimated the surface activity of phthalate esters and the extent to which this type of moderately polar oil can be coadsorbed into concentrated monolayers of conventional surfactants at the oil/water interface. In the absence of surfactant, phthalate oils are adsorbed from dilute solution in heptane at the heptane/water interface, exhibiting a surface activity typical of non-aromatic diesters and similar to that of alcohols. However, the surface chemical and microemulsion phase behaviour in mixtures of phthalate oils and aqueous NaCl solutions in the presence of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) indicates a low extent of penetration of the phthalate oils into AOT monolayers. This is interesting since the phthalates have similar surface activities (from hydrocarbon oils) to alcohols which are very effective cosurfactants capable of strong coadsorption into surfactant monolayers. The main effects of substituting phthalate oils for alkanes in mixtures of oil–AOT–aqueous NaCl are (i) to increase the critical aggregate concentration of surfactant in the oil phase, (ii) to increase the NaCl concentration required for microemulsion phase inversion and (iii) to increase the magnitude of the minimum interfacial tension obtained by varying the salt concentration. It is concluded that phthalate oils do not act as strongly adsorbed cosurfactants in AOT–water–oil systems and the extent of phthalate oil penetration of AOT monolayers is less than for alkanes.