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, 7 (6), 3495-3504

Computational Analysis of the Solvation of Coffee Ingredients in Aqueous Ionic Liquid Mixtures

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Computational Analysis of the Solvation of Coffee Ingredients in Aqueous Ionic Liquid Mixtures

Veronika Zeindlhofer et al. RSC Adv.

Abstract

In this paper, we investigate the solvation of coffee ingredients including caffeine, gallic acid as representative for phenolic compounds and quercetin as representative for flavonoids in aqueous mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] at various concentrations. Due to the anisotropy of the solutes we show that classical Kirkwood-Buff theory is not appropriate to study solvation effects with increasing ionic liquid content. However, excess coordination numbers as well as the mean residence time of solvent molecules at the surface of the solutes can be determined by Voronoi tessellation. Since the volume of the hydration shells is also available by this method, solvation free energies will be discussed as a function of the ionic liquid concentration to yield a physical meaningful picture of solvation for the anisotropic solutes. Hydrogen bonding capabilities of the solutes and their relevance for experimental extraction yields from spent coffee grounds are also discussed.

Figures

Fig. 1
Fig. 1. Caffeine (left), gallic acid and quercetin (right).
Fig. 2
Fig. 2. Schematic view on the solvation of an anisotropic solute and arising issues of a spherical analysis in terms of radial distribution functions g ij(r).
Fig. 3
Fig. 3. Radial distribution function g ij(r) of water around the solutes and its decomposition into first, second and third solvation shell (shaded areas). The black dashed line represents the distance of the “first minimum” of g ij(r) to determine the coordination number (black numbers at right column) via spherical integration.
Fig. 4
Fig. 4. Free solvation energy ΔA of the first solvation shell around (a) caffeine, (b) gallic acid and (c) quercetin as a function of the bulk concentration of the ionic liquid. The numbers denote CNj of the respective solvent species j in the first solvation shell.
Fig. 5
Fig. 5. Preferred positions of the solvent molecules around the solutes at c IL = 1.1 M.
Fig. 6
Fig. 6. Mean residence time and the respective normalized correlation function n j(0)·n j(t)/n j 2 of the solvent species in the first Voronoi solvation shell around the coffee solutes at c IL = 1.1 M.
Fig. 7
Fig. 7. First Voronoi shell contribution to g ij(r) of water around the solutes as a function of c IL.
Fig. 8
Fig. 8. Averaged interaction energy of a water molecule (blue), a cation (red) and an anion (green) in the first solvation shell with the coffee ingredients.
Fig. 9
Fig. 9. Comparison of hydrogen bonding of the solutes to water (blue), to C2mim (red) and to OAc (green) with respective experimental extraction yields from spent coffee. All values and their standard deviations can be found in the ESI.

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