The Excellent Water-Solubility of N-[3-(dimethyl-amino)propyl] docosanamide Chloride : The Most Eco-friendly Cationic Hair Conditioning Agent

A new hair-conditioning agent, N-[3-(dimethylamino)propyl]docosanamide (APA-22) hydrochloric acid salt (APA-22 HCl), has excellent solubility in water with a solubility limit over 10-times larger than that of other APA hydrochloric salts with shorter alkyl chain lengths. The physicochemical characteristics of APA-22 HCl were studied at 25°C by using equilibrium surface tension (γ), solubilization of an oil-soluble dye, steady-state fluorescence, and chloride ion selective electrode techniques. The APA-22 HCl salt is considered to form an aggregate at a concentration, C1, which is about twice the concentration of the solubility limit of APAs with shorter alkyl chains. At a higher concentration, C2, the aggregates of APA-22 HCl start forming another kind of aggregate, which is able to solubilize oil-soluble materials. That is, C1 is considered to represent the critical aggregation concentration (cac) and C2 a morphology transition. In the concentration region between C1 and C2 the solutions are seemingly transparent whereas at above C2 they appear bluish or translucent. Since the Krafft point of APA-22 HCl is 55°C, all the solutions, including the aggregates, are thought to exist in metastable states. Notably, however, these metastable solutions do not change significantly over a few months. The excellent solubility of APA-22 HCl in water is considered to be a result of this unique stepwise aggregation with increasing concentration.