Thermodynamic and Transport Properties of Tetrabutylphosphonium Hydroxide and Tetrabutylphosphonium Chloride-Water Mixtures via Molecular Dynamics Simulation.

Thermodynamic, structural, and transport properties of tetrabutylphosphonium hydroxide (TBPH) and tetrabutylphosphonium chloride (TBPCl)-water mixtures have been investigated using all-atom molecular dynamics simulations in response to recent experimental work showing the TBPH-water mixtures capability as a cellulose solvent. Multiple transitional states exist for the water-ionic liquid (IL) mixture between 70 and 100 mol% water, which corresponds to a significant increase in water hydrogen bonds. The key transitional region, from 85 to 92.5 mol% water, which coincides with the mixture's maximum cellulose solubility, reveals small and distinct water veins with cage structures formed by the TBP ions, while the hydroxide and chloride ions have moved away from the P atom of TBP and are strongly hydrogen bonded to the water. The maximum cellulose solubility of the TBPH-water solution at approximately 91.1 mol% water, appears correlated with the destruction of the TBP's interlocking structure in the simulations, allowing the formation of water veins and channeling structures throughout the system, as well as changing from a subdiffusive to a near-normal diffusive regime, increasing the probability of the IL's interaction with the cellulose polymer. A comparison is made between the solution properties of TBPH and TBPCl with those of alkylimidazolium-based ILs, for which water appears to act as anti-solvent rather than a co-solvent.