Dependence of cellulose dissolution in quaternary ammonium acetates/DMSO on the molecular structure of the electrolyte: use of solvatochromism, micro-calorimetry, and molecular dynamics simulations

Abstract

We studied the dissolution of microcrystalline cellulose (MCC) at 60 °C in binary mixtures of dimethyl sulfoxide (DMSO) and the following quaternary ammonium acetates (QAAcOs): benzyl-trimethylammonium, NBzMe3AcO; diallyl-benzyl-methylammonium, NAl2BzMeAcO, tetra(n-butyl)ammonium (NBu4AcO), and tribenzyl-methylammonium, NBz3MeAcO (Al, Bu, Bz, Me, AcO refer to allyl, 1-butyl, benzyl, methyl and acetate group, respectively). We observed the following MCC dissolution order (given as wt%): NBu4AcO > NAl2BzMeAcO > NBz3MeAcO ≫ NBzMe3AcO. To explain this result we used the combined data of: (1) solvatochromism to calculate the following binary solvent (b-solvent) descriptors that are important for MCC dissolution: empirical polarity, Lewis acidity, Lewis basicity, dipolarity, polarizability; (2) isothermal titration micro-calorimetry to calculate the enthalpy of interaction (ΔH) of β-cyclodextrin (cellulose model) with QAAcO/DMSO-acetonitrile; (3) molecular dynamics simulations to calculate solvent-induced separation of cellulose chains; average number of hydrogen bonds between acetate ions and cellulose OH-groups, and average composition of the biopolymer solvation layer. We offer the following pieces of evidence to show the importance of hydrogen bonding for the efficiency of cellulose dissolution: the orders of b-solvent basicity and of |ΔH| are parallel to its cellulose dissolution efficiency; the biopolymer solvation layer of efficient b-solvents (e.g., NBu4AcO-DMSO) contains more ionic liquid ions and less DMSO molecules than that of NBz3MeAcO, leading to pronounced biopolymer chain separation, hence eventual dissolution. Our approach shows the power of the combined use of several techniques to rationalize the requirements for efficient cellulose solvents.