Micellization of dodecyldimethyl-N-2-phenoxyethylammonium bromide (domiphen) in aqueous solution. Comparison with other alkyl ammonium surfactants

Abstract

Dodecyldimethyl-N-2-phenoxyethylammonium bromide (domiphen) is a quaternary ammonium salt which has been used in oral hygiene long time ago. The published studies on its characterization as surfactant evidence several contradictory results. In this paper, measurements of surface tension, conductimetry, fluorescence and mainly isotermal titration calorimetry (ITC) have been performed at different temperatures. The enthalpy of demicellization varies linearly with temperature in the interval 15–45 °C, from which a value of 704 ± 39 J mol−1K−1 in water has been determined for the change of heat capacity. The obtained results are compared with published values for alkyltrimethylammonium bromides with different alkyl chain lengths (CnTAB). For instance, its critical micelle concentration, cmc, is almost ten times lower than that of C12TAB. The substitution of one methyl group by the phenoxyethyl has a strong influence on the behavior of domiphen making it equivalent to a CnTAB surfactant with n = 15–19 methylene groups, the number depending on the studied property. From ITC an average aggregation number of 45 ± 3 is obtained in good agreement with the one from fluorescence quenching of pyrene equal to 44.5–47.6. From the analysis of specific conductivity, the fraction of bound counterions to micelles was obtained, the values linearly decreasing with temperature. Below cmc domiphen behaves as a strong 1:1 electrolyte without association of monomers. Other themodynamic parameters have also been obtained. The fjord and reef hydration models are used to propose a structure for domiphen micelles.