In a hydrophilic octaethylene glycol dodecyl ether (C
12EO
8)-water system, a micellar cubic phase is formed below 20°C. Upon addition of decane, the maximum temperature of micellar cubic phase (I
1) is increased, and, moreover, the hexagonal liquid crystal region changes to the I
1 phase. Beyond the solubilization limit of decane in the I
1 phase, highly viscous and gel-like O/I
1-emulsions are formed in the two-phase region of I
1+O. In the case of a surfactant mixture (C
12EO
4.8) containing C
12EO
8+C
12EO
3, a single I
1 phase region is not present in a pseudo ternary phase diagram of water/C
12EO
4.8/decane system, but an I
1+O region appears in the oil-rich region. C
12EO
4.8 forms lamellar liquid crystal in the aqueous system but C
12EO
3 in the mixture is dissolved in oil and the mixing fraction of C
12EO
8 in the surfactant aggregate increases as oil is added. Finally, a two-phase region of I
1+O appears. It is considered that the O/I
1-emulsion or gel based on the cubic phase corresponds to the gel appeared in the so-called D-phase emulsification process. Upon addition of alkyl polyol such as glycerol or 1,3-butanediol, the transparency of the emulsion in the I
1+O region increases because the difference in refractive index between the cubic phase and excess oil is decreased. The maximum temperature of the I
1 phase decreases with increasing polyol content. Consequently, the addition of alky polyols is not crucial to form O/I
1 gels in an oil-rich region.
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