Densities and Viscosities of Aqueous Solutions of Tributylmethylphosphonium Dimethylphosphate

Densities and viscosities of aqueous solutions of tributylmethylphosphonium dimethylphosphate, [P₄₄₄₁][(CH₃O)₂PO₂], were measured over an entire range of compositions at atmospheric pressure at temperatures between 278.15 and 363.15 K. The isobaric thermal expansivities and partial molar volumes were derived at different compositions and temperatures. The density and expansivity showed maxima in the dilute aqueous solutions at the mole fraction of [P₄₄₄₁][(CH₃O)₂PO₂], x₂ ≈ 0.1. The experimental viscosities were well reproduced by the Vogel–Fulcher–Tammann (VFT) equation, with parameters that also indicate the minima (B and D) and maxima (ln A and T₀) in the dilute aqueous solutions at x₂ < 0.05. The volumetric and viscosity behaviors have been analyzed in terms of dilute electrolyte solution theory, suggesting that [P₄₄₄₁][(CH₃O)₂PO₂] can behave as a so-called structure-making salt. The excess molar volumes and deviation viscosities were all negative over an entire range of compositions with the minima at x₂ ≈ 0.3 and 0.6, respectively, and increased with increasing temperature. The excess molar volumes and deviation viscosities for the aqueous solutions of [P₄₄₄₁][(CH₃O)₂PO₂] were quantitatively compared to those for the aqueous solutions of ionic liquids composed of similar cations and/or anions.