Behavior of aqueous Tetrabutylammonium bromide - a combined approach of microscopic simulation and neutron scattering

TL;DR

This study combines neutron scattering techniques to analyze the dynamic behavior of tetrabutylammonium bromide in water, revealing different diffusion modes for ions and molecules, and clarifying discrepancies in previous measurements.

Contribution

It introduces a combined neutron scattering approach to decouple and analyze ionic and molecular dynamics in aqueous solutions, providing new insights into diffusion behaviors.

Findings

Incoherent signal analysis yields a diffusion coefficient consistent with prior methods.

Coherent signal analysis shows a slower diffusion mode, explaining previous discrepancies.

The combined approach effectively separates ionic and molecular dynamics in the system.

Abstract

Aqueous solution of tetrabutylammonium bromide is studied by quasi-elastic neutron scattering, to give information on the dynamic modes involving the ions present. Using a careful combination of two techniques, time-of-flight (TOF) and neutron spin echo (NSE), we de- couple the dynamic information in both the coherently and incoherently scattered signal from this system. We take advantage of the different intensity ratio of the two signals, as detected by each of the techniques, to achieve this decoupling. By using heavy water as the sol- vent, the tetrabutylammonium cation is the only hydrogen-containing species in the system and gives rise to a significant incoherent scattered intensity. The dynamic analysis of the incoherent signal (measured by TOF) leads to a translational diffusion coefficient of the cation as that is in good agreement with previous NMR, neutron scattering and tracer diffusion measurements. The dynamic analysis of the coherent signal observed at wave-vectors < 0.6 angstrom^(-1) (measured by NSE) leads to a significantly slower diffusive mode, by a factor of 2. This study explains that apparent difference and the reasons for that.