Influence of alkoxyethanols on the mixed micelle formation by hexadecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide surfactant mixtures

 The conductances of hexadecyltrimethylammonium bromide (HTAB) and tetradecyltrimethylammonium bromide (TTAB) mixtures over the entire mole fraction range of HTAB were measured in aqueous binary mixtures of ethylene glycol monomethyl ether, monoethyl ether, and monobutyl ether, and of diethylene glycol monomethyl ether and monoethyl ether containing 10–30 wt% additive in their respective binary mixtures at 30 °C. Each conductivity curve showed a single break over the whole mole fraction range of HTAB–TTAB mixtures. From the break in the conductivity curve, various micellar parameters were calculated and the results were discussed with respect to the alkoxyethanol's additive effect on the mixed micelle formation. The micellar parameters of HTAB, TTAB, and of their mixtures showed a strong dependence both on the amount as well as on the number of repeating units in the presence of ethylene glycol derivatives, whereas a significant dependence only on the amount of additive was observed in aqueous diethylene glycol derivatives. The results in the former case were attributed to the hydrophobic hydration of the mixed micelles by the ethylene glycol derivatives, which showed a large dependence on the increase in the alkyl chain length of the additive. The hydrophobic hydration was considerably reduced in the case of diethylene glycol derivatives owing to the presence of an extra ether oxygen. An evaluation of the nonideality in the HTAB–TTAB mixtures revealed that in spite of the strong hydrophobic hydration of the HTAB–TTAB mixtures by the alkoxyethanols, the mixed micelles remain almost free from the additive molecules. Received: 11 January 2000/Accepted: 14 April 2000