Study of tetrabutylammonium bromide in aqueous solution by neutron scattering

The study of electrolyte solutions by neutron scattering is an example of the large range of possibilities of the technique. Structure and dynamics at different time and length scales, discrimination of global from local motions, separation of coherent from incoherent contributions are necessary to embrace the complexity of a subject where charge and hydrophobicity play important and competitive roles. The behaviour of aqueous solutions of tetrabutylammonium bromide is studied here by several neutron scattering techniques: Small Angle Neutron Scattering, Neutron Diffraction, Time-of-Flight and Neutron Spin Echo. We concentrate on the conformation and dynamics of the hydrophobic cations. In particular, the center-of-mass (CoM) motion of the cation at the microscopic scale is best described via the low Q coherent signal, as measured by Neutron Spin Echo. Due to a possible cage formation effect in the TBABr solution, at the scale of the distance between cations, the cationic CoM relaxation time is larger than that predicted by a simple extrapolation of results issued from the hydrodynamic regime and those obtained from the incoherent signal analysis.