Solubilisation of water in alkanes using nonionic surfactants

Abstract

AbstractNonionic surfactants are frequently used as emulsifiers in nonpolar oil + water systems and as solubilisation agents for oil in water, or vice versa. In the latter application the amount of, say, water that can be solubilised in nonpolar oil (to give microemulsion droplets) depends on: (a) the capacity of the micelles to incorporate water; and (b) the fraction of surfactant originally present as micelles. This paper is concerned with the single‐phase water‐in‐oil (W/O) microemulsion regions enclosed by the haze and solubilisation boundaries at the oil‐rich end of Shinoda‐type phase diagrams. The systems studied contain the nonionic surfactant C12H25(OCH2CH2)5OH (C12E5), normal alkane (heptane, decane or tetradecane) and water. Critical microemulsion concentrations (cμc) and droplet compositions for w/o microemulsions formed from C12E5 in alkane have been determined at phase boundaries over a range of temperatures. The results show how the maximum extent of water solubilisation is determined jointly by the cμc and the maximum droplet size for a given temperature. It appears that for larger (microemulsion) droplets, the cμc is determined by temperature rather than by droplet size. However, along part of the haze curves, aggregates form with only small amounts of water (less than four molecules per ethyleneoxy group on the surfactant head groups). For a given temperature, in the small range where either micelles or microemulsion droplets can exist, reverse hydrated micelles have much higher critical micelle concentrations (cmc) than the cμc of the larger microemulsion droplets.