Dielectric Relaxation Behavior of Aqueous Dodecyldimethylamineoxide Solutions

Abstract

Dielectric relaxation behavior was examined for aqueous solutions of dodecyldimethylamineoxide (DDAO) in the range of concentration (c D) from 100 to 500 mM varying the average degree of protonation (〈α〉) by adding HBr. DDAO has an electric dipole moment due to an O←N coordination chemical bond on its headgroup, and the fraction of O←N is reduced by adding HBr, while that of HO−N+ increases. This process was detected as a change in dielectric spectra. Dielectric spectra for aqueous DDAO solutions had four types of relaxation modes. A fast relaxation time (τ1 ∼ 10-10 s) was essentially independent of c D, and the relaxation strength (Δε1) was proportional to c D at 〈α〉 = 0. Moreover, the magnitude of Δε1 decreased with 〈α〉; therefore, it was attributed to the rotational relaxation mode of O←N type headgroups in spherical micelles. A relaxation mode with c D-independent τ2 (∼10-9 s) possessed relaxation strength (Δε2) proportional to c D. Because Δε2 increased with 〈α〉 by adding HBr and the number of Br- entrapped in micelles increased with 〈α〉, the mode was assigned to the rotational relaxation mode of an ion pair formed between HO−N+ and Br- on the micellar surface. A relaxation mode with τ3 (∼10-8 s) at 〈α〉 > 0 was attributed to the rotational relaxation mode of intermolecular association particles formed between protonated and unprotonated DDAO molecules due to intermolecular forces such as hydrogen bonding. A slow mode with τ4 (∼10-7 s at 〈α〉 = 0) was assigned to the relaxation of a counterion distribution around spherical DDAO micelles. When 〈α〉 was higher than 0.5, dielectric behavior of aqueous DDAO solutions was essentially the same as that of aqueous solutions of cationic detergents. The addition of NaBr to aqueous DDAO solutions at 〈α〉 greater than 0.5 made a spherical micellar shape change into a spheroidal shape. This change in micellar shapes made dielectric relaxation spectra much broader.

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