Micelle and Solvent Relaxation in Aqueous Sodium Dodecylsulfate Solutions

Abstract

Dielectric spectra have been measured for aqueous sodium dodecylsulfate (SDS) solutions up to 0.1 mol L-1 at 25 degrees C over the frequency range 0.005 < or = nu GHz-1 < or = 89. The spectra exhibit two relaxation processes at approximately 0.03 GHz and 0.2 GHz associated with the presence of micelles in addition to the dominant solvent relaxation process at approximately 18 GHz and a small contribution at approximately 1.8 GHz due to H2O molecules hydrating the micelles. Detailed analysis reveals that the micelles bind 20 water molecules per SDS unit, but not as strongly as trimethylalkylammonium halide surfactants do. The relaxation times and amplitudes of both micelle relaxation processes can be simultaneously analysed with the theory of Grosse, yielding the effective volume of a SDS unit in the micelle and the lateral diffusion coefficient of the bound counterions. The findings of this investigation fully corroborate recent molecular dynamics simulations on structure and dynamics of SDS micelles.

A new insight on the dynamics of sodium dodecyl sulfate aqueous micellar solutions by dielectric spectroscopy

“…In the dielectric spectroscopy literature on SDS micellar solutions at concentrations below 104 mM it is accepted that two relaxation processes arise from fluctuations of charges at the micelle-aqueous phase interface with relaxation times of 5.5 ns and 500 ps, respectively [16,17]. This interpretation agrees with Grosse's theory [9,10,13].…”

“…They are named by Grosse delta and gamma relaxations, and occur in the MHz and GHz ranges, respectively. According to [16,17] the lateral process is due to the reorientation of stable ion pairs or the hopping of counterions bound to the micellar charged surface between neighboring head groups, both for cationic and anionic SDS micellar solutions. In [16,17] a very wide frequency range (5 MHz-89 GHz) makes it possible to distinguish four relaxation processes.…”

“…According to [16,17] the lateral process is due to the reorientation of stable ion pairs or the hopping of counterions bound to the micellar charged surface between neighboring head groups, both for cationic and anionic SDS micellar solutions. In [16,17] a very wide frequency range (5 MHz-89 GHz) makes it possible to distinguish four relaxation processes. The fastest one, with time constant 8.93 ps, coincides with the hydrogen bond network relaxation of bulk water (see Section 4.1); a slower process (120 ps) is attributed to interfacial bound water.…”

“…In [16,17] SDS micellar solutions were studied at concentrations from 0.018 to 0.104 M by Broad Band Dielectric Spectroscopy (BBDS) [18]. The spectra exhibit two relaxation processes at 30 MHz and 200 MHz due to the micellar contribution, a relaxation process at 18 GHz due to the solvent and a further process at 1.8 GHz attributed to water molecules in a micelle hydration layer (interfacial or bound water).…”

Molecular dynamics simulations of helical antimicrobial peptides in SDS micelles: What do point mutations achieve?

“…It has been shown both by experiment 29 and by molecular dynamics simulations 14,[21][22][23] that the slowest relaxation times of lipids in micellar solutions are of the order of 500-2000 ps. The only exception to these are the slower relaxation times of counterions.…”