Internal Dynamics in SDS Micelles: Neutron Scattering Study

Abstract

The molecular dynamics of sodium dodecyl sulfate (SDS) micelle has been investigated using high-resolution incoherent quasielastic neutron scattering technique. Data analysis clearly shows presence of two distinct motions: whole micellar motion or global diffusion and faster internal motion of the SDS monomer. The global diffusion associated with the whole micelle is found to be Fickian in nature, and the corresponding diffusion coefficients are found to be consistent with those obtained from dynamic light scattering measurements. The internal motion is described with a model consistent with the structure of the micelle and which accounts for the flexibility of the chains. The SDS monomer consists of a head group, which lies on the surface of the globular micelle, and a tail that hangs from the head toward the center of the globule. Considering various factors like conformational changes of the SDS chains, bending, stretching of the chemical bonds, etc., the dynamics of the SDS molecules is successfully described by a model in which the hydrogen atoms undergo localized translational motion confined within spherical volumes. This volume increases linearly along the SDS chain such that the hydrogen atoms closer to the head group move within smaller spheres with lower diffusion constant than the hydrogen atoms away from the head group. This model is found to be consistent with the data over the whole temperature and concentration range. Diffusivity and the volume of the spheres are also found to increase with temperature. The effect of lowering the SDS concentration is found to be similar to that of increasing the temperature.

Molecular Dynamics Simulations of Cetyltrimethylammonium Bromide (CTAB) Micelles and their Interactions with a Gold Surface in Aqueous Solution

“…6 Variations of these factors yield aggregates with different morphologies such as spherical or ellipsoidal micelles, cylindrical or thread-like micelles, disk-like micelle, membranes and vesicles. 7 These self-assembled structures have been characterized by a number of techniques, such as dynamic light scattering (DLS), 8 nuclear magnetic resonance (NMR), 9,10 fluorescence spectroscopy, 11 quasielastic neutron scattering (QENS), 12,13 small-angle X-ray scattering (SAXS), 14,15 and small-angle neutron scattering (SANS). 16,17 Computational simulations have also been employed to explore the structures and dynamical behaviors of micelles for different surfactants.…”

Dynamics of Molecular Species in Confined Geometry

“…Hence, it is of great interest to obtain a detailed description of the monomer dynamics within the micelles. Monomer in micelles exhibit complex dynamical behaviour of different motions which include segmental motions, torsional motions, bond isomerizations, librations and lateral diffusion on a curved surface, covering a wide time scale from nanoseconds to picoseconds [10][11][12][13][14][15][16][17][18][19][20]. The dynamics of ganglioside micellar solutions have been described in terms of a simple model of confined diffusion within a sphere [15].…”

“…They undergo self-association under specific conditions to form aggregates such as micelles, vesicles, bilayers, wormlike micelles, etc. Dynamics of various ionic micelles such as anionic sodium dodecyl sulfate (SDS) micelles, cationic alkyl trymethylammonium bromide (C n TAB) micelles have been studied by us using QENS technique [18][19][20]. Here first the dynamics of anionic SDS micelles will be discussed.…”

“…Scattering function for micelles is obtained by subtracting D 2 O contribution [18][19][20]. Micelles in solution can undergo a variety of motions including global motion of the micelles and internal motions within the micelle corresponding to the dynamics of the individual monomers [18][19][20].…”

“…Micelles in solution can undergo a variety of motions including global motion of the micelles and internal motions within the micelle corresponding to the dynamics of the individual monomers [18][19][20]. To interpret the data the different dynamical processes are assumed to be independent of each other, such that the scattering law S micelles (Q,ω) can be expressed as [18] …”