A neutron scattering and modelling study of aqueous solutions of tetramethylammonium and tetrapropylammonium bromide

We have investigated the properties in water of two tetraalkylammonium bromides (tetramethylammonium, TMA+, and tetrapropylammonium, TPA+), at 0.4 M, using neutron scattering coupled with empirical potential structure refinement to arrive at an atomistic description. Having both a polar and an apolar moiety, it is of interest to determine the strength of each moiety as a function of the alkyl chain length. TMA+ and TPA+, having different impact as structure directors in zeolite synthesis, were chosen for this study. Water arranges tetrahedrally around TMA+ and in an almost featureless manner around TPA+. TMA+ and TPA+ show an apolar hydration with TPA+ being slightly more apolar. TPA+ has a tendency to form small clusters of 2–4 molecules and to fold into a compact configuration. Both molecules correlate similarly with the bromide ion but do not dissociate completely at this concentration.