The reactivity of P-nitrophenyl esters with alkylammonium surfactants in nonaqueous solvents

Abstract

The study of the behaviour of amphiphilic molecules in both aqueous and nonaqueous solvents is a rapidly expanding field. In the introduction, the aqueous micelle model and its relation to the various models proposed for aggregation of surfactants in nonaqueous solvents, and the controversy surrounding the applicability of the critical micelle concentration to these aggregates, are discussed. Previous kinetic studies involving alkylammonium carboxylate surfactants in nonaqueous solvents, and studies concerning micellar effects on the reactions of p-nitrophenyl esters are reviewed. The rate of decomposition of p-nitrophenyl acetate, PNPA, in benzene in the presence of dodecylammonium propionate, DAP, has been measured at temperatures between 15°C and 35°C. The results obtained are in reasonable agreement with those reported in similar studies by E1 Seoud et al. and Kitahara et al. The proposed rate equation consists of a term arising from micellar catalysis of the unimolecular decomposition of the ester and a term arising from a bimolecular reaction between the ester and the components of DAP. Arrhenius parameters have been estimated for the two reaction modes. In the presence of solubilized water the micellar rate constant is decreased due to hydration of the head-groups, while the bimolecular rate constant is unchanged. Hydrolysis is negligible. Changing the solvent to either toluene or cyclohexane causes an increase in both rate constants. The reactivity of longer chain esters with DAP in benzene is markedly less than that of PNPA. The rate measurements have been extended to include a series of alkylammonium propionates, CH3(CH2)n+NH3-O2CCH2CH3 (where n = 3, 5, 7, 9 or 11), and a series of dodecylammonium carboxylates, CH3(CH2)11+NH3-O2C(CH2)n,CH3 (where n' = 1, 2, 6 or 7). The micellar rate constant is affected by the alkyl chain length. The bimolecular rate constant is affected by both the chain length and the acidity of the head-groups. A series of dodecylammonium 4-substituted phenoxide salts, CH3(CH2)11+NH3-OC6H4-4X (where X = H, Cl, Br, Me or MeO), has been prepared. Absorbance measurements indicate that aggregation of these compounds occurs at concentrations greater than 10-4 mol l-1 in benzene. The decomposition of PNPA in benzene in the presence of the dodecylammonium phenoxide salts is suggested to occur by an aminolysis reaction. The observed rate constants are affected by the basicity of the phenol. In the presence of solubilized water the rate of decomposition of PNPA is increased slightly due to an increase in the effective concentration of free amine and to the occurrence of hydrolysis. An investigation of the aquation of the tris(oxalato) - chromate(III) anion in reversed micellar systems is reported in the appendix.