Formation of Cubic-Phase Microemulsions with Anionic and Cationic Surfactants at Equal Amounts of Oil and Water

Abstract

The formation and microstructure of cubic phases were investigated in anionic and cationic surfactant-containing systems at 25°C. In the system sodium dodecyl sulfate(SDS)–dodecyltrimethylammonium bromide(DTAB)–water, mixing of two surfactants shows the phase transition hexagonal phase (H1)→surfactant precipitate, accompanied by an obvious decrease in the cross-sectional area per surfactant in the rod micelles of the hexagonal liquid crystal. In the mixed systems brine(A)–dodecane(B)–SDS(C)–DTAB(D)–hexanol(E), the isotropic discontinuous cubic phase is formed from the H1 phase at a low cationic surfactant weight fraction, Y=D/(C+D), and from the lamellar phase at high Y upon dilution with equal amounts of oil and brine, respectively. The minimum surfactant concentration to form the cubic phase decreases with increases both in cationic surfactant weight fraction Y from 0 to 0.30 and in hexanol weight fraction, W1=E/(C+D+E), accordingly. The maximum solubilization for oil of the cubic phase reaches 43 wt% at 14 wt% of mixed surfactants and alcohol.