Monitoring Micelle Breakdown by Chemical Trapping

Abstract

Chemical trapping of bromide ions by 3-methylbenzenediazonium ions, 3MBD, allowed us to investigate the effects of methanol and ethanol on the fraction of free Br- ions in water−alcohol solutions of cetyltrimethylammonium bromide, CTAB, and tetradecyltrimethylammonium bromide, TTAB, micellar systems. For the purpose, the high-performance liquid chromatography, HPLC, peak areas of 3-bromocresol, 3MBBr, obtained from reaction of 3MBD with Br- ions in ROH/H2O/CTAB or ROH/H2O/TTAB mixtures, ASURF, and those for 3MBBr obtained in ROH/H2O/NaBr mixtures, ANaBr, were determined. The ASURF/ANaBr ratio reflects the fraction of Br- ions sensed by the 3MBD probe in the water−alcohol bulk phase. When no alcohol is added, the ASURF/ANaBr values are identical to those for the degree of ionization, α, of CTAB and TTAB micelles obtained by physical methods; meanwhile, at high percentages of alcohol, ASURF/ANaBr ∼ 1, indicating that all Br- ions are essentially “free”, consistent with literature reports indicating that short-chain alcohols totally inhibit micelle formation. Hence the variation of ASURF/ANaBr with the percentage of alcohol is a measure of the fraction of Br- ions not bound to micellar or submicellar aggregates, thus allowing us to examine the micelle breakdown by employing a chemical trapping method. The data indicate that a continuous micelle breakdown takes place upon increasing the percentage of ROH. At 50% ROH, about 30% of the total Br- ions are still associated to the submicellar aggregates (i.e., they are not free ions), but at ∼80% ROH, no significant aggregation takes place.