Solubilisation of stearic acid by the organic base choline hydroxide

In a previous study we have shown that the substitution of alkali ions in common fatty acid soaps by choline as a counterion of biological origin increases the solubility of the respective soaps without lowering the biocompatibility. Nevertheless, while choline dodecanoate (ChC12), myristate (ChC14), and palmitate (ChC16) have Krafft points below room temperature or even under 0 °C, choline stearate (ChC18) was not soluble below 40 °C. In the present contribution we show that an excess of choline hydroxide is able to solubilise choline stearate at temperatures as low as 14 °C. Furthermore, we compare our results to those obtained for the sodium and potassium salts of fatty acids, with molar ratios of base to acid higher than 1:1. In order to elucidate the solubilisation process regarding the different ion binding to the carboxylic headgroup, we further investigated the effect of different added chloride salts on the solubility of choline stearate. Our findings indicate that the cation affinity to the carboxylate headgroup follows the trend Na+ > K+ ≫ Ch+. The results are discussed in terms of hydrolysis of the fatty acids in combination with Collins’ concept of “matching water affinities”. As a feasible application of choline base, we present the saponification and simultaneous solubilisation of butter as an example of a hardly soluble triglyceride.