Dielectric behavior of aqueous micellar solutions of betaine-type surfactants

Dielectric behavior was examined for aqueous solutions of the betaine-type surfactants dodecyldimethylcarbobetaine (C12DCB), tetradecyldimethylcarbobetaine (C14DCB), cetyldimethylcarbobetaine (C16DCB), and oleyldimethylcarbobetaine (OleyDCB) as a function of frequency from 1.00 x 10(6) to 2.00 x 10(10) Hz (6.28 x 10(6) to 1.26 x 10(11) rad s(-1)) with changing surfactant concentration (c(D)). Rotational relaxation times (tau) of the zwitterionic headgroups of the surfactants in aqueous solutions of C12DCB and C14DCB, which form spherical micelles, are determined to be 0.26 and 0.30 ns, respectively. Values of tau for aqueous solutions of C16DCB and OleyDCB, which form threadlike micelles, are identical at 0.44 ns. The tau values of all micellar solutions are constant irrespective of c(D). The increase in T with increasing alkyl chain length is assigned to an increase of molecular density at the micellar surface. The magnitude of the relaxation strength for the surfactant solutions increases in proportion to c(D) and is not so different from that of an aqueous solution of glycine betaine (GB), which has the same chemical structure as betaine-type surfactants with zwitterionic headgroups but never forms micelles. This finding suggests that the zwitterionic headgroup rotating on the micellar surface possesses a dipole moment with a magnitude essentially the same as that of GB in aqueous solutions.