The dynamical landscape in CTAB micelles

Abstract

Here we report molecular dynamics of cetyltrimethylammonium bromide (CTAB) based cationic micelles as studied using an incoherent quasielastic neutron scattering (QENS) technique. Data as obtained from two different spectrometers, IRIS at the ISIS facility, UK, and FOCUS at SINQ, PSI, Switzerland, having energy resolutions, ΔE ≈ 17.5 μeV and 55 μeV respectively, are used to investigate the existing complex dynamical features in CTAB micelles. A detailed dynamical landscape is obtained by combining the data from two different instruments providing a wide dynamical energy range. High resolution QENS data show the presence of two distinct motions; whole micellar motion or global diffusion and segmental motion of the CTAB monomers. The global diffusion associated with the whole micelle is found to be Fickian in nature. The segmental dynamics of CTAB molecules within a micelle has been described with a model considering the motion of the head group and the hydrophobic alkyl chain separately. It is found that the data are consistent with a model in which the methyl groups in the head group undergo 3-fold jump rotations and the hydrogen atoms belonging to the alkyl chain perform localized translational diffusion (LTD). LTD is the manifestation of the flexibility involved in the chain, where the hydrogen atoms move within a spherical volume, the size of which increases linearly from the top to the bottom. This linear description fits the data well. Diffusivity also follows the same architecture. In addition, a faster conformational dynamics component is found to exist ascribed to the torsional isomerisation of some of the CH₂ groups in the hydrocarbon chain. A comprehensive description of the complex dynamics existing in CTAB micelles has been provided.