Octanol-rich and water-rich domains in dynamic equilibrium in the pre-ouzo region of ternary systems containing a hydrotrope

Ternary mixtures of medium-chain fatty alcohols, water and a hydrotrope (such as ethanol), near the immiscibility gap, make stable single phases at constant temperature. Interestingly, in this `pre-ouzo region' these single phases consist of two distinct nanoscopic pseudo-phases, one octanol-rich and one water-rich. This domain of composition, which is known to produce strong light scattering and to separate under ultracentrifugation into two phases, has been studied using contrast variation in small-angle neutron scattering (SANS) combined with small- and wide-angle X-ray scattering (SWAXS). The existence of fatty alcohol-rich domains of well defined size of the order of 2 nm radius is proven. The scattering can be approximated by an Ornstein-Zernike function, which is close to the general expression of Choi, Chen, Sottmann & Strey [Physica B, (1998), 241-243, 976-978] with vanishing quadratic Porod term. Exploitation of the relative intensities at the vanishing scattering angle in SANS demonstrates that the distribution coefficient of ethanol between the octanol-rich and the water-rich domains is close to one. WAXS of the two coexisting pseudo-phases is compared with the corresponding binary water-ethanol and octanol-ethanol samples.