Binary aqueous solutions of choline salts: Determination and modelling of liquid density (298.15 or 313.15 K) and Vapour Pressure Osmometry (313.15 K)

Abstract

Salts composed of the cholinium cation (such as choline hydroxide and choline chloride) have become one of the main reactants for the synthesis of greener ionic liquids, but, for a proper description of the aqueous solutions of these strong electrolytes by thermodynamic modelling, reliable data on liquid density $\left(\rho \right)$ and vapour pressure $\left(p\right)$ are needed, which are often unavailable. So, in this work, the liquid density $\left(\rho \right)$ of binary aqueous solutions of choline bicarbonate ([Ch][Bic]), choline (2R,3R)-bitartrate ([Ch][Bit]), choline chloride ([Ch]Cl), choline dihydrogen citrate ([Ch][H₂Cit]) and choline hydroxide ([Ch]OH) were measured at 298.15 or 313.15 K and 0.1 MPa and correlated using second-degree polynomials with salt molality $\left(m\right)$, obtaining determination coefficients $\left(R^2\right)$ higher than 0.9974. Furthermore, the osmotic coefficients $\left(\varphi \right)$ of these binaries were determined using vapour pressure osmometry (VPO), at 313.15 K and 0.1 MPa, being satisfactorily described by the Extended Pitzer Model of Archer, which yielded low values of standard deviation $(3.58<{\sigma }_{\varphi }·{10}^3<12.86)$. Then, the mean molal activity coefficients $\left({\gamma }_{\pm }\right)$ and excess Gibbs free energies $\left(G^E/RT\right)$ were calculated, following the order [Ch][H₂Cit] > [Ch][Bit] > [Ch][Bic] > [Ch]OH > [Ch]Cl, which corresponds to the decreasing polarity of the choline salts and agrees with the empirical law of matching water affinities (LMWA).