Interaction of a Long Alkyl Chain Protic Ionic Liquid and Water

TL;DR

This study combines experimental and theoretical methods to explore how water influences the microscopic interactions and optical properties of butyl-ammonium nitrate, revealing increased local order and specific hydrogen bonding patterns.

Contribution

It provides new insights into the microscopic organization and hydrogen bonding network in BAN-water solutions through combined spectroscopic and molecular dynamics analysis.

Findings

Water causes vibrational peak shifts and shape changes in BAN spectra.

Small water amounts increase local order among ionic pairs.

Water preferentially bonds with nitrate anions over cations.

Abstract

A combined experimental/theoretical approach has been used to investigate the role of water in modifying the microscopic interactions characterizing the optical response of butyl-ammonium nitrate (BAN) water solutions. Raman spectra, dominated by the signal from the protic ionic liquid, were collected as a function of the water content, and the corresponding spatial organization of the ionic couples, as well as their local arrangement with water molecules, was studied exploiting classical molecular dynamics calculations. High quality spectroscopic data, combined with a careful analysis, revealed that water affects the vibrational spectrum BAN in solution: as the water concentration is increased, peaks assigned to stretching modes show a frequency hardening together with a shape narrowing, whereas the opposite behavior is observed for peaks assigned to bending modes. Calculation results clearly show a nanometric spatial organization of the ionic couples that is not destroyed on increasing the water content at least within an intermediate range. Our combined results show indeed that small water concentrations even increase the local order. Water molecules are located among ionic couples and are closer to the anion than the cation, as confirmed by the computation of the number of H-bonds which is greater for water-anion than for water-cation. The whole results set thus clarifies the microscopic scenario of the BAN-water interaction and underlines the main role of the extended hydrogen bond network among water molecules and nitrate anions.