# Prediction and elucidation of cellulose solubility in ionic liquids under high pressure using all-atom molecular dynamics simulations

## TL;DR

This study shows how high pressure increases cellulose solubility in ionic liquids by altering molecular interactions and promoting chain dispersion.

## Contribution

The paper reveals a dual mechanism of pressure-induced solubility enhancement through volumetric compression and conformational changes in cellulose.

## Key findings

- Dissolution free energy decreases with increasing pressure, indicating higher solubility.
- Pressure strengthens solute–solvent interactions and weakens cellulose–cellulose contacts.
- Conformational changes in hydroxymethyl groups promote hydrogen bonding with anions.

## Abstract

This study investigated the pressure dependence of cellulose solubility in a 60 wt% ionic-liquid mixture of 1-ethyl-3-methylimidazolium acetate and dimethyl sulfoxide ([EMIm][OAc]/DMSO) using all-atom molecular dynamics simulations conducted over a wide pressure range (P = 0.1–1000 MPa) at temperature T = 500 K. The dissolution free energy obtained via the umbrella sampling method (36 windows, 200 ns each) decreased monotonically as pressure increased, indicating that solubility enhanced at elevated pressures. The underlying molecular mechanism was elucidated by performing 100 ns NPT-MD simulations of a 36-chain cellulose system in 60 wt% [EMIm][OAc]/DMSO at each pressure. Analysis of the radial distribution functions, coordination numbers, interaction energies, chain dispersibility, hydrogen bond populations, and conformational transitions revealed a dual mechanism: (1) pressure-induced compression strengthens solute–solvent interactions and weakens cellulose–cellulose contacts, promoting chain dispersion; and (2) conformational rearrangements of the C6 hydroxyl groups disrupt intramolecular hydrogen bonds and favor new intermolecular hydrogen bonding with [OAc]− anions. This molecular-level insight demonstrated that pressure enhances cellulose solubility through both volumetric compression and active conformational mechanisms, thereby providing guidance for the design of high-pressure cellulose dissolution processes.

High pressure enhances cellulose solubility by strengthening ion–cellulose solvation through volumetric compression and by promoting new hydrogen bonding with anions via pressure-induced rotation of the hydroxymethyl groups.