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

B Crawford and AE Ismail, POLYMERS, 12, 249 (2020).

DOI: 10.3390/polym12010249

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.

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