Formulation and stability of a soap microemulsion and the apparent pKA herein

[Display omitted] •An oil-in-water microemulsion and the apparent pKA of its soap were investigated.•High temperature favored the formation of micelles that are more hydrated.•None of the anions of sodium salts showed any specific effects.•Specific effects of the cations were observed.•Associated with ethanol, sorbitol showed a salting-in effect. The influence of composition, and added salts and polyols on the stability of an oil-in-water microemulsion formulation and on the apparent pKA (apKA) of the used oleate surfactant is investigated. High temperature favours a decrease of the apKA and leads to the formation of more hydrated micelles. The apKA decreases also when the percentage of ethanol increases. Citronellol molecules do not significantly influence the apKA at concentrations between 0% and 2% w/w. By contrast, with increasing limonene concentration, the apKA increases. It was observed that anions of sodium salts destabilize the microemulsion and high temperatures are needed to recover it. By increasing the concentration of NaCl, a slight increase of the apKA is observed, which can be associated with a non-specific, electrostatic (Debye–Hückel) effect. Cations of chloride salts have different effects depending on their ability to exchange with Na+ near the carboxylate group. Li+, Na+ and K+ have apparently a salting-out effect. Tetramethylammonium chloride and choline chloride have salting-in effects until respectively 0.6 and 0.4molkg−1. The associations of sorbitol or glycerol with ethanol lead to a salting-in effect and to a decrease of the apKA of Na-Oleate.