Synthesis and properties of piperazine derivatives and their quaternary ammonium amphiphilic salts

Abstract

A series of long-chain piperazine derivatives, N-alkyl-N'-methyl piperazine and their amphiphilic salts, N-alkyl-N'-ethyl-N'-methyl piperazinium bromide, and the related N-alkyl-N,N'-dimethyl piperazinium bromide were synthesized and characterized by 1H and 13C NMR and FTIR spectroscopy. Under the experimental conditions used, the NMR data showed unequivocally that quaternarization of N-alkyl-N'-methyl piperazine by the reaction with ethyl bromide occurred exclusively at the nitrogen atom bearing the methyl group due to the steric hindrance of the long-chain alkyl group attached to the other nitrogen atom. Critical micelle concentration (cmc) and surface-active properties of the amphiphiles were investigated. Plots of surface tension, γ, against log concentration were presented. At cmc, the number of molecules adsorbed at the air-water interface was found to decrease with increasing chain length. This was attributed to the increase in electrostatic repulsion within the adsorption layer as suggested by Lunkenheimer et al. (Colloids Surf. A 101, 197 (1995)).