Article pubs.acs.org/JPCB
Is Solute Rotation in an Ionic Liquid Influenced by the Addition of Glucose? Rajan Maurya,† Sudhanshu Naithani,‡ Dibyendu Bandyopadhyay,§ Niharendu Choudhury,*,∥ and G. B. Dutt*,⊥ †
Atma Ram Sanatan Dharma College, Dhaula Kuan, New Delhi 110 021, India School of Physical Sciences, Doon University, Dehradun 248 001, India § Heavy Water Division, ∥Theoretical Chemistry Section, and ⊥Radiation & Photochemistry Division, Bhabha Atomic Research Centre Trombay, Mumbai 400 085, India ‡
S Supporting Information *
ABSTRACT: Fluorescence anisotropy measurements and molecular dynamics (MD) simulations have been performed to understand the specific interactions of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4c]pyrrole (DPP), with neat 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)2]) and also in the presence of glucose. It has been observed that the measured reorientation times of DMDPP in neat [BMIM][N(CN)2] follow the predictions of the Stokes−Einstein−Debye hydrodynamic theory with slip boundary condition. Addition of glucose (0.075 and 0.15 mole fraction) has no bearing on the rotational diffusion of the solute apart from the viscosity related effects. In contrast, the reorientation times of DPP in neat [BMIM][N(CN)2] obey stick boundary condition as the hydrogen bond donating solute experiences specific interactions with the dicyanamide anion. No influence of the additive can be noticed on the rotational diffusion of DPP at 0.075 mole fraction of glucose. However, at 0.15 mole fraction of glucose, the reorientation times of the solute at a given viscosity and temperature decrease by 15−40% compared to those obtained in the neat ionic liquid. MD simulations indicate that each DPP molecule hydrogen bonds with two dicyanamide anions in neat ionic liquid. The simulations also reveal that, at 0.15 mole fraction of glucose, the concentration of anions hydrogen bonded to glucose increases significantly; therefore, the percentage of solute molecules that can form hydrogen bonds with two dicyanamide anions decreases to 84, which leads to faster rotation of DPP.
1. INTRODUCTION Considerable progress has been made toward understanding the dissolution of simple and complex carbohydrates in imidazolium-based ionic liquids.1−17 These studies assume importance due to their potential implications in the conversion of biomass cellulose to fermentable sugars. The consensus that emerged from a number of investigations is that the mechanism of dissolution involves formation of hydrogen bonds between the anions of the ionic liquid and the hydroxyl groups of the carbohydrate in approximately stoichiometric ratios.10−17 Due to this reason, ionic liquids comprising anions that are strong hydrogen bond acceptors such as acetate, chloride, dicyanamide, ethylsulfate, phosphonate, and thiocyanate have been found to dissolve large quantities of carbohydrates.1−7 Although the role of anion in the dissolution of carbohydrates in ionic liquids is unambiguous, the part played by the cation has been debated. Molecular dynamics (MD) simulations suggest that weak interactions exist between the imidazolium cation and hydroxyl groups of the carbohydrate,11,12 whereas NMR studies indicate that the hydroxyl groups form hydrogen bonds with the anion as well as the cation of the ionic liquid.15 However, a recent study that © 2017 American Chemical Society
combines neutron diffraction, NMR, and molecular dynamics simulation techniques reveals that the cation of the ionic liquid plays only a minor role in the solvation of the carbohydrate and does not participate in hydrogen bonding interactions with the hydroxyl groups to any significant degree.17 It has been reported by Sheldon and co-workers2 that significant amounts of glucose, sucrose, and lactose can be dissolved in the low viscous ionic liquid, 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)2]). According to their study, 1 L of [BMIM][N(CN)2] can take up to 145 g of glucose at 298 K. We are interested in finding out how the presence of a large quantity of monosaccharide in [BMIM][N(CN)2] influences one of the elementary processes such as solute rotation. Since the dicyanamide anion forms hydrogen bonds with glucose, the mobility of the ions will be reduced, which is likely to increase the viscosity of the solution. An enhancement in solution viscosity will obviously slow down solute rotation compared to that in the neat ionic liquid. Received: October 6, 2017 Revised: November 8, 2017 Published: November 10, 2017 10965
DOI: 10.1021/acs.jpcb.7b09888 J. Phys. Chem. B 2017, 121, 10965−10973
Article
The Journal of Physical Chemistry B
time-correlated single-photon counting spectrometer that was purchased from Horiba Jobin Yvon, U.K. The instrumental details and the description concerning the measurement of anisotropy decays have been discussed in our earlier publications.27,34 The samples containing the probes DMDPP and DPP were excited with a 451 nm DeltaDiode, and the emission was monitored at 580 nm. The instrument response function was measured by collecting the scattered light from a TiO2 suspension in water, and the full-width at half-maximum was found to be 99% with
