Enthalpies of interaction between dimethyldioctadecylammonium bromide vesicles in aqueous solution and either dipicolinate or sulfate anions

Abstract

Injection of small aliquots of dipicolinate anions (sodium salt) into an aqueous solution containing dimethyl-dioctadecylammonium bromide (DOAB) vesicles is endothermic at 50 °C, becoming first more and then less endothermic. The injection process is effectively athermal for solutions containing more than equimolar amounts of DOAB and dipicolinate anions. A similar pattern is observed when small aliquots of sodium sulfate(aq) are injected into DOAB(aq). The overall patterns of enthalpy changes are attributed to the vesicle–dianion interaction which is exothermic and head-group dehydration with bromide ion displacement which is endothermic. Neverthless, a complexity emerges if the solutions include a buffer which turns out to play a less than passive role. This conclusion is supported by differential scanning microcalorimetry for DOAB(aq) in the presence and absence of HEPES buffer.