Microemulsions Supported by Octyl Monoglucoside and Geraniol. 1. The Role of the Alcohol in the Interfacial Layer

The formation of balanced microemulsions in the quaternary system water + cyclohexane + octyl monoglucoside (C8G1) + geraniol (trans-3,7-dimethyl-2,6-octadien-1-ol) is reported. The contour of the three-phase body at a 1:1 oil-to-water volume ratio has been mapped at 25 °C in terms of the variables γ (the sum of the mass fractions of surfactant and alcohol in the quaternary system) and δ (the mass fraction of alcohol in the surfactant plus alcohol mixture). Increasing δ at constant γ causes a phase inversion from an oil-in-water droplet microemulsion in contact with excess oil (2) to a water-in-oil microemulsion in contact with excess water ( ) via a middle-phase microemulsion in contact with excess oil and water (3). On decreasing γ the three-phase region becomes distorted toward larger δ. This distortion is attributed to the different solubilities of C8G1 and geraniol in the oil phase. By taking into account these different solubilities, the composition of the interfacial film in the middle of the three-phase body can be calculated. For the balanced interfacial layer (corresponding to a spontaneous curvature of H = 0) we find that the mole ratio of geraniol in the interfacial film is ca. 0.28, corresponding to a ratio of geraniol to C8G1 molecules of about 2:5. We compare this result with related findings for the quaternary microemulsion water + n-octane + C8E5 + n-octanol and discuss the difference of these two systems in terms of the size and the shape of the surfactant and the alcohol molecules.