Esterolytic reactivities of (dialkylamino)pyridine amphiphiles solubilized in different pseudo-three-component cationic microemulsions

Abstract

Hydrolysis reactions at pH 8.7 of p-nitrophenyl alkanoates of varying chain lengths mediated by four (dialkylamino)pyridine (DAAP)-functionalized amphiphiles were examined in three different microemulsion (ME) media. All the ME systems studied herein were stabilized by a cationic surfactant, cetyltrimethylammonium bromide (CTABr), and a cosurfactant, 1-butanol. The three MEs could also be described as (a) an oil-in-water, (b) a water-in-oil, and (c) a bicontinuous oil-water ME recipe. The DAAP surfactants employed in this study differ in their electrostatic character at the level of headgroup. In order to examine their esterolytic activities, individual DAAP surfactants were separately doped into each ME and the resulting kinetic data for esterolyses were compared with that of the nonamphilphilic (dimethylamino)pyridine. The ester-cleaving abilities of each DAAP catalyst in three different MEs were also compared. Hydrophilic substrates such as 3-acetoxy-4-nitrobenzoic acid or short chain esters like p-nitrophenyl acetate were found to be especially susceptible to cleavage compared to the alkanoate esters having long alkyl chain residues under these conditions in any of the MEs. Importantly these results are markedly different from what we found in CTABr micelles with the same DAAP catalysts, although the crucial catalytic turnover behavior of the DAAPs was maintained even in MEs.