Microemulsions Supported by Octyl Monoglucoside and Geraniol. 2. An NMR Self-Diffusion Study of the Microstructure

The self-diffusion coefficients of all components of the quaternary microemulsion system water−cyclohexane−octyl monoglucoside (β-C8G1)−geraniol (trans-3,7-dimethyl-2,6-octadien-1-ol) have been measured along a well-defined linear path in the composition space, using the NMR pulsed-gradient spin−echo technique. The results have been used to derive information about the microstructure of the microemulsion and to calculate the composition (proportion of alcohol to surfactant molecules) of the mixed interfacial film separating the microscopic oil and water domains in the microemulsion. For the present system a phase inversion 2−3−2̄ occurs at a 1:1 oil-to-water volume ratio and constant overall mass fraction of surfactant plus alcohol (γ = 0.05) by increasing δ, the relative mass fraction of alcohol in the amphiphile mixture (surfactant plus alcohol). For δ ≤ 0.25 the relative self-diffusion coefficient D rel of water is about 2 orders of magnitude greater than D rel of oil, indicating an oil-in-water (o/w) droplet microemulsion; for δ ≥ 0.50 an opposite behavior of D rel for oil and water is found, indicating a water-in-oil (w/o) droplet microemulsion. For δ values between 0.30 and 0.45, D rel of water and oil exhibit a pronounced opposite dependence on δ, with a crossover near the middle of the three-phase body (δ ≈ 0.40), where D rel is about 0.47 for both oil and water. This behavior implies that the transition from an o/w- to a w/o-droplet microemulsion proceeds via a bicontinuous microstructure with oil and water domains separated by a mixed interfacial amphiphile film of low mean curvature. The composition of this film has been calculated as a function of δ by two mutually independent methods for the droplet regime and the bicontinuous regime of the microemulsion, respectively. With increasing δ the ratio of geraniol to β-C8G1 molecules in the mixed film increases from about 1:5 at δ = 0.15 (o/w domain) up to 1:1 at δ = 0.60 (upper limit of w/o domain), amounting to about 1:2 for the state of the balanced microemulsion. These results demonstrate in a quantitative manner how the composition of the mixed interfacial film affects the structure of the oil and water domains in a quaternary microemulsion system.