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

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.