Phase behavior and size variation of AOT-based W/O microemulsions by substituting H⁺ for Na⁺ as the counterion

The purpose of this study is to investigate how the difference in screening of electrostatic repulsions between polar headgroups by counterions affects the phase behavior and size of AOT-based W/O microemulsions. Na-AOT and H-AOT were used as the surfactants. The counterion binding constants determined by the relationship between the CMC and the counterion concentration indicate that the screening effect of H⁺ is weaker than that of Na⁺. A mixture of water/Na-AOT and/or H-AOT/isooctane was emulsified by changing the mixing ratio of H-AOT, X_H-AOT = 0-1, keeping the water content (w₀ = [ water ] / [ surfactant ]) = 10 and the total AOT^- concentration = 0.1 M. FT-IR and DLS measurements were carried out for the upper content of the emulsified mixture to investigate phase behavior and size variation. It was found that the phase behavior can be divided into three regions: (1) the W/O microemulsion region at 0 ≤ X_H-AOT ≤ 0.52, (2) the phase transition region at 0.52 < X_H-AOT < 0.7, and (3) the phase separation region at 0.7 ≤ X_H-AOT ≤ 1. In region (1), the size of the W/O microemulsions increases from 8 to 110 nm, although the amounts of water and surfactant in the W/O microemulsions are constant. In region (2), the amounts of water and surfactant decrease to form the lower phase, whereas the upper phase still contains the W/O microemulsions and the size decreases from 110 to 15 nm with increasing X_H-AOT. In region (3), the mixture separates into two phases. The possible causes of the phase behavior and the size variation were discussed by focusing on the FT-IR spectra of symmetric sulfonate stretching vibration band due to the polar headgroup of AOT^-.