From regular solutions to microemulsions

We address the structure of homogeneous mixtures of equal volumes of water and oil rendered miscible by the addition of amphiphilic molecules. Specifically, we study the pathway from regular ternary solutions to structured microemulsions with increasing amphiphilic chain length, <i>i.e.</i>, increasing <i>i</i> and <i>j</i> for alkyl polyglycol ether surfactants (C_<i>i</i>E_<i>j</i>) from C₃E₀ (<i>i.e.</i>, propanol) to C₅E₂, using contrast variation small angle neutron scattering (SANS). The bulk-contrast SANS pattern of the symmetric system water-cyclohexane-(2-propanol/1-propanol) with nearly horizontal tie-lines matches Ornstein-Zernike behavior with characteristic <i>q</i>^-2 decay, implying critical composition fluctuations occurring predominantly in the water-oil ratio. Weak scattering signals were detected in film-contrast SANS possibly due to the existence of the nearby tricritical point (TCP). After passing the TCP, the increasing amphiphilic strength leads to the gradual build-up of well-defined amphiphilic interfaces stabilizing water- and oil-rich domains of colloidal dimensions, as evidenced by the well-known <i>q</i>^-4 behavior observed in bulk-contrast SANS pattern at large <i>q</i>. The amphiphilicity factor <i>f</i>_a ranging from +1.15 to -0.60 reveals the crossing of the disorder line, the Lifshitz line, and the wetting to non-wetting transition. Precursors of structuring in those mixtures with compositions, where the lamellar phase appears for even longer-chain amphiphiles, are also borne out by the experiments.