Experimental probing of water and counterion concentrations inside a reversed micelle water-pool: an overlooked parameter in micellar enzymology?

Abstract

An experimental method has been developed for probing the local molar concentrations of water and bromide counterions present inside the water-pool of a cationic reversed micellar system [cetyltrimethylammonium bromide (CTAB)/isooctane/n-hexanol/water] across W₀=12-44. The present method is based on a modified phenyl cation-trapping protocol originally developed to determine the interfacial concentrations of various weak nucleophiles present in the interfacial region of aqueous cationic micelles and microemulsions. Our results show that the local molar concentrations of bromide counterions inside the water-pool of CTAB/isooctane/n-hexanol/water reversed micelles decrease from 1.91 to 0.29 M as W₀ increases from 12 to 44 with a concomitant increase in the local water-pool concentration of water from 43.8 to 55.4 M. The local molar concentration of water inside the water-pool of CTAB/isooctane/n-hexanol/water reversed micelle attains the molarity of normal water (55.5 M) at W₀ = 44. The catalytic efficiencies of trypsin compartmentalized within the water-pool of CTAB/isooctane/n-hexanol/water reversed micelle have been observed to increase with the increase in the molar concentrations of water in the water-pool and with the decrease in the bromide counterion concentrations present inside the water-pool. The implications of these results for reversed micellar enzymology are discussed. The current method should, in principle, be applicable to probe the local molar concentrations within the aqueous pool of reversed micellar systems formed by the wide array of surface-active amphiphiles including ionic, nonionic, zwitterionic, and possibly gemini surfactants.