The transparency of oil in cubic (O/I
1) emulsions formed in water/C
12EO
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
12EO
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
12EO
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.
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