Electrophoretic properties of dodecyltrimethylammonium bromide micelles inKBr solution

Charge in ionic micelles determines the trends of their stability and their practical applications. Charge can be calculated from zeta potential (zeta ) measurements, which, in turn, can be obtained by Doppler microelectrophor esis. In this study, the electrophoretic properties of dodecyltrimethylammo nium bromide (DTAB) in KBr aqueous solution (0-6 mM) were determined by Dop pler microelectrophoresis. At very low surfactant concentrations (up to 6 m M), zeta potential was quite constant and due to the ionized monomers (DTA( +)). Above 6 mM, zeta potential increased to a maximum at surfactant concen trations still below the critical micellar concentration (CMC). This increa se could be explained by a formation of nonmicellar aggregates of DTAB. The n, above the CMC, zeta potential underwent an abrupt reduction, which was d ependent qualitatively and quantitatively on KBr concentration, and which c ould be due to an increase of the number of counterions adsorbed on the mic elle surface. Calculation of effective micellar charge from zeta potential gave the surface charge density. Comparing this value with the theoretical, obtained from geometrical considerations, a fraction of 0.29 of charged mi cellar headgroups was obtained when DTAB was in aqueous solution, which is consistent with the value obtained by conductivity measurements.