Abstract
Phosphonium based phase-separable ionic liquids (PSILs) are promising green solvents for dissolution of cellulose and lignin, a necessary step for conversion of biomass to fuels and chemicals. The knowledge of interfacial behavior of ionic liquid/solvent systems is critical for designing efficient separation processes. Here, molecular dynamics simulations are carried out for aqueous interface of tetraalkylphosphonium ionic liquids with chloride and acetate as anions to investigate IL miscibility with water. The transition zone from miscible to immiscible behavior was observed for alkyl chain lengths of 6 to 8. Emulsion phase was observed for [P8888]+ ion and multiple IL/water interface was observed for [P12121212]+ phosphonium cation. IL/water interface is observed to be enriched with solvated anions with phosphorous atom oriented towards the aqueous phase. The potential of mean force calculations suggest that as the alkyl chain of the phosphonium cations increase, so does the driving force for aggregation of cations in the aqueous phase. Due to the difference in the molecular shape and size, anions are better solvated by the water molecules, and these ions diffuse into the aqueous phase. Furthermore, the analysis of selected pair interactions provides insights into the nature of intermolecular forces and the role of the alkyl side chains on the interfacial properties.
Highlights
Due to many desirable characteristics such as low vapor pressure1–5 and the ease with which new ionic liquids (ILs) can be designed, ILs provide immense opportunity to be used as solvents to dissolve cellulose and/or lignin from the biomass.6–9 As energy efficient recovery of ionic liquid post dissolution step is important for overall techno-economic aspects of the process, ILs that are amphiphilic in nature are attractive
Holding et al.9 have reported that phosphonium based ionic liquids (PBILs) with chloride and acetate as anions dissolve lignin, and microcrystalline cellulose (MCC) in the presence of dimethyl sulfoxide (DMSO) as co-solvent
Irrespective of miscible and immiscible behavior of PBILs in aqueous phase, penetration of anions in aqueous phase is observed. This can be rationalized by the fact that water is able to stabilize these ions in the aqueous phase, and while the phosphonium ions are microsolvated by water molecules minimizing the intermolecular repulsion between cations
Summary
Due to many desirable characteristics such as low vapor pressure and the ease with which new ionic liquids (ILs) can be designed, ILs provide immense opportunity to be used as solvents to dissolve cellulose and/or lignin from the biomass. As energy efficient recovery of ionic liquid post dissolution step is important for overall techno-economic aspects of the process, ILs that are amphiphilic in nature are attractive. Phase separable ionic liquids (PSILs) such as imidazolium or phosphonium based ILs can be tailored to form biphasic system with water.. Phosphonium based PSILs consist of hydrophobic moieties on phosphorus center, which govern mixing thermodynamics with water or saline solutions.9,24 Among these ILs, trioctylmethyl phosphonium acetate ([P8881][OAc]) is one of the most efficient as it can dissolve cellulose up to 19 wt% of MCC with 40 wt% DMSO. We investigate PBILs miscibility and interfacial behavior with aqueous phase, and the effect of length of the alkyl side chain on the phosphonium ion using atomistic MD simulations. Holding et al. have reported that minor change in alkyl chain length of phosphonium cation results in significant change in the miscibility of ionic liquid towards water. Structural analysis was carried out to develop molecular level understanding of local microstructure of ions in solution
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