# Determination of standard molar volume of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide on titanium dioxide surface

## TL;DR

This study measures how the volume of a specific ionic liquid changes when it interacts with titanium dioxide, revealing significant reductions compared to its bulk form.

## Contribution

The first experimental determination of standard molar volume of an ionic liquid on a solid surface using a pycnometer and colligative law.

## Key findings

- The molar volume of [Hmim][NTf2] on P25 surface decreases by 20.8%–23.7% compared to the bulk phase.
- Reduction in molar volume decreases with increasing temperature from 293.15 to 323.15 K.
- ePC-SAFT-DFT model predictions for interfacial ionic liquid density align well with experimental results.

## Abstract

The fluids near the solid substrate display different properties compared to the bulk fluids owing to the asymmetric interaction between the fluid and substrate; however, to the best of our knowledge, no work has been conducted to determine the interfacial properties of fluids experimentally. In this work, we combined a pycnometer with experimental measurements and data processing to determine the standard thermodynamic properties of interfacial fluids for the first time. In the study, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][NTf2]) and titanium dioxide (P25) were chosen as the probes to prove the concept. It was found that, with the combination of the Gay-Lussac pycnometer and the colligative law, together with selecting a suitable solvent, it is possible and reliable to determine the standard molar volume of the immobilized [Hmim][NTf2]. Compared to the bulk phase, the molar volumes of [Hmim][NTf2] on the P25 surface reduce by 20.8%–23.7% at temperatures from 293.15 to 323.15 K, and the reduction degrees decrease with increasing temperatures. The newly determined standard thermodynamic data was used to obtain the model parameters of hybrid electrolyte perturbed-chain statistical associating fluid theory density functional theory (ePC-SAFT-DFT), and further predictions of the density of interfacial ionic liquids with different film thicknesses were proved to be reliable in comparison with the experiment results.