TURBIDITY OF OIL-IN-WATER MICROEMULSION DROPLETS STABILIZED BY NONIONIC SURFACTANTS

We have investigated oil-in-water microemulsions consisting of nanaometre-sized droplets of decane stabilised by the nonionic surfactant H-(CH2)(12)-(O-CH2CH2)(5)-OH (abbreviated to C(12)E(5)). The microemulsions are stable over a narrow temperature range, At the low temperature phase boundary, photon correlation spectroscopy and turbidity measurements indicate that the droplets are spherical and interact only weakly. With increasing temperature, the size of the particles increase owing to either a clustering or a coalescence of the microemulsion droplets. We show how simple turbidity measurements can be used to estimate the extent of this clustering/growth. The variation of turbidity with temperature is analysed in terms of a series of aggregation equilibria and the enthalpy change associated with the clustering/growth process is found to be approximately 1000 kJ per mole of droplets of radius 8.9 nm. When expressed per molecule of surfactant involved in the interaction between droplets, this enthalpy is of the order of kT.