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

An experimental method has been developed for probing the local molar conce ntrations of water and bromide counterions present inside the water-pool of a cationic reversed micellar system [cetyltrimethyl-ammonium bromide (CTAB )/isooctane/n-hexanol/water] across W-0 = 12-44. The present method is base d on a modified phenyl cation-trapping protocol originally developed to det ermine 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 W-0 increases from 12 to 44 with a concomit ant 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 w ater (55.5 M) at W-0 = 44. The catalytic efficiencies of trypsin compartmen talized within the water-pool of CTAB/isooctane/n-hexanol/water reversed mi celle have been observed to increase with the increase in the molar concent rations of water in the water-pool and with the decrease in the bromide cou nterion concentrations present inside the water-pool. The implications of t hese results for reversed micellar enzymology are discussed. The current me thod should, in principle, be applicable to probe the local molar concentra tions within the aqueous pool of reversed micellar systems formed by the wi de array of surface-active amphiphiles including ionic, nonionic, zwitterio nic, and possibly gemini surfactants.