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Water Sorption and Diffusivity in [C2C1im][BF4], [C4C1im][OAc], and [C4C1im][Cl]

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journal contribution
posted on 2019-01-30, 00:00 authored by M. Alejandra Rocha, Mark B. Shiflett
Measurements of in situ atmospheric water absorption and desorption in ionic liquids (ILs) (1-ethyl-3-methylimidazolium tetrafluoroborate [C2C1im]­[BF4], 1-butyl-3-methylimidazolium acetate [C4C1im]­[OAc], and 1-butyl-3-methylimidazolium chloride [C4C1im]­[Cl]) were made using a gravimetric microbalance at temperatures ranging from 283.15 to 315.15 K and relative humidity (RH) 0–70% at 101 kPa. Solubility data were well correlated using the nonrandom two-liquid (NRTL) activity model, and time dependent concentration data were used to determine the binary diffusion coefficients using one- and two-dimensional transport models of water in the IL–water systems. The solubility of water was highest in [C4C1im]­[OAc] (77.5 mol %), followed by [C4C1im]­[Cl] (68.6 mol %), and [C2C1im]­[BF4] (19.5 mol %) at equivalent conditions (303.15 K and 25.00% RH). The diffusion coefficients in order of increasing relative humidity ranged from 1.3 × 10–10 to 2.8 × 10–11 m2/s for [C2C1im]­[BF4], from 8.8 × 10–12 to 3.9 × 10–11 m2/s for [C4C1im]­[OAc], and from 4.5 × 10–12 to 2.8 × 10–11 m2/s for [C4C1im]­[Cl]. Heats of absorption were calculated and ranged from 39 to 44 kJ/mol for [C2C1im]­[BF4], from 47 to 45 kJ/mol for [C4C1im]­[OAc], and from 55 to 45 kJ/mol for [C4C1im]­[Cl] with increasing water mole fraction of 0.3–0.8. The water diffusivity increases with increasing water concentration in both [C4C1im]­[OAc] and [C4C1im]­[Cl] with respect to decreasing viscosity and heats of absorption. However, the diffusivity of water in [C2C1im]­[BF4] decreases with increasing water concentration with respect to decreasing viscosity and increasing heats of absorption. Diffusing radius calculations using the Stokes–Einstein relationship support the hypothesis that a few water molecules through hydrogen bonding form clusters with the [OAc] and [Cl] anions, but much larger water/BF4 clusters/networks are occurring in the [C2C1im]­[BF4] system which increase in size with increase in water concentration.