Small-angle neutron scattering studies of microemulsions stabilised by aerosol-OT. Part 3.—The effect of additives on phase stability and droplet structure

Abstract

The droplet structure of water-in-oil microemulsions formed with the surfactant AOT in various hydrocarbon solvents has been studied by small-angle neutron scattering (SANS). As the temperature of the single-phase microemulsion is raised towards that at which a phase transition occurs, an increased scattering at low Q, characteristic of critical scattering, is exhibited, which is a result of the attractive interactions between the droplets. The effect on the microemulsion of small quantities of additives (toluene, octan-1-ol and benzyl alcohol) has been investigated for the n-alkane solvents heptane, decane, undecane and dodecane. Both a systematic increase or decrease in the transition temperature and a corresponding change in the magnitude of the critical scattering component of the SANS intensity profile are observed, which depend on the nature of the additive. These results are coupled to a change in the water droplet radius, consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water–surfactant interfacial region, whereas toluene remains predominantly in the bulk hydrocarbon phase and probably does not come into contact with the aqueous core of the droplet. The droplet radius and the phase stability of the microemulsion are also affected by addition of short-chain alcohols and show a systematic variation as the chain length is increased. These changes have been interpreted in terms of the partitioning of alcohols between the water core and the interface and their effect on the surfactant shell.