Influence of surfactant hydrophilicity on the formation of transparent O/I(1)-type emulsions.

The transparency of oil in cubic (O/I(1)) emulsions formed in water/C(12)EO(n)/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 C(12)EO(7) system, O/I(1) emulsions prepared in the I(1)+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 I(1) 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 C(12)EO(9) system, a transparent emulsion was prepared by adding glycerol because the refractive index of the I(1) phase matches that of the oil before the phase transition. Since long EO chains are required to maintain the large curvature of the I(1) 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/I(1) emulsion decreases with the decreased viscosity of the I(1) phase obtained by adding glycerol. The low viscosity of the I(1) phase in the presence of glycerol could be related to an increase in the maximum oil solubilization into the I(1) phase.