Amphi Joliot, ESPCI, 10 rue Vauquelin, 75005, Paris
Symmetric tetraalkylammonium (TAA) cations are model systems to study the behaviour of hydrophobic ions. In this work, concentrated aqueous solutions of TAA bromides are investigated to obtain information on microscopic structure and dynamics of both the ions and solvent, by a combination of Neutron Scattering and Molecular Dynamics (MD) simulations. It is shown that TAA cations do not aggregate in aqueous solution even at high concentrations, they are penetrable for both the Br anions and solvent water molecules. The average water orientation is tangential around the cation surface, which contrasts with the simple alkali cations, such as Na+. Using quasi-elastic neutron scattering (Neutron Spin Echo and Time of Flight techniques) and with the aid of MD simulations, the dynamics in the coherent and incoherent neutron scattering signal is decoupled. The former is identified with the center-of-mass (CoM) motion of a single TAA cation, while the latter, based on the signal of individual H atoms of the TAA cation, is a complex combination of the CoM motion and H movements internal to the cation. MD helps to identify the timescale of the global cation rotation. The slowing down of water dynamics in these solutions relative to bulk water is also made evident, though the effect is lower than might be expected.
Key Words: tetraalkylammonium bromides, neutron scattering, Neutron Spin Echo, Time of Flight, microscopic simulation, coherent and incoherent signal, dynamics, translation, water