Effects of Protonation of Alkyldimethylamine Oxides on the Dissolution Temperature in Aqueous Media

Abstract

The effects of protonation (ionization) of hexadecyldimethylamine oxides on the dissolution temperature in aqueous media were investigated by differential scanning calorimetry. Only one endothermic peak was reproducibly observed at all the degrees of ionization α examined that were assigned to the transition from the solid (the gel phase) to the solution containing micelles. The dissolution temperature versus α curves showed a maximum at α=0.5, strongly suggesting the formation of a stable complex of 1-to-1 composition of the nonionic and cationic species through the proposed hydrogen bond. From the shape of the dissolution curve as well as the composition analysis of the solid phase, the solid solution was found to be formed over all α values. Effects of alkylchain length on the dissolution temperature for a homologous series of octadecyl- (C18DAO), hexadecyl- (C16DAO), and tetradecyldimethylamine oxide (C14DAO) were also examined for α=0.5 and α=1. Both the transition temperature and the associated thermodynamic quantities Δ H and Δ S increased systematically with the chain length, but for α=0.5 smaller increases in Δ H and Δ S values with the chain length were observed [Δ H/CH 2 (kJ mol −1)=7.2±0.2 and 2.2±0.5 for α=1 and α=0.5, respectively, and Δ S/CH 2 (J mol −1 K −1)=21.9±1.8 for α=1 and 4.6±1.9 for α=0.5]. By annealing procedures, the metastable nature of the gel phase was demonstrated for the C16DAO (α=1) solid.