Influence of Surfactant Hydrophilicity on the Formation of Transparent O/I1-Type Emulsions

概要

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The transparency of oil in cubic (O/I1) emulsions formed in water/C12EOn/isododecane systems (n=7, 9) with glycerol was investigated in order to understand the relation between surfactant hydrophilicity and transparency of the emulsion. In the C12EO7 system, O/I1 emulsions prepared in the I1+O region are milky when glycerol is not added. However, in the presence of glycerol, transparency increases with an increasing amount of glycerol because glycerol increases the refractive index of the I1 phase until it gradually approaches that of the oil phase. However, a phase transition to the hexagonal phase takes place before the refractive indices match; therefore, a transparent emulsion is not obtained. On the other hand, in the C12EO9 system, a transparent emulsion was prepared by adding glycerol because the refractive index of the I1 phase matches that of the oil before the phase transition. Since long EO chains are required to maintain the large curvature of the I1 phase against the addition of glycerol, a highly hydrophilic surfactant is required for the preparation of transparent emulsions. We also found that the viscosity of the O/I1 emulsion decreases with the decreased viscosity of the I1 phase obtained by adding glycerol. The low viscosity of the I1 phase in the presence of glycerol could be related to an increase in the maximum oil solubilization into the I1 phase.
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