Toward an Understanding of the Salting-Out Effects in Aqueous Ionic Liquid Solutions: Vapor−Liquid Equilibria, Liquid−Liquid Equilibria, Volumetric, Compressibility, and Conductivity Behavior

Abstract

The action of particular electrolytes in altering the solution properties of ionic liquids is well documented, although the origin of this effect is not clearly defined. In order to clarify this point, the aim of this work is to obtain further evidence about the salting-out effect produced by the addition of different salts to aqueous solutions of water miscible ionic liquids by evaluating the effect of a large series of salts on the vapor-liquid equilibria, liquid-liquid phase diagram, volumetric, compressibility, and conductometric properties of ionic liquids 1-alkyl-3-methylimidazolium halide ([C(n)mim][X]). In the first part of this work, the experimental measurements of water activity at 298.15 and 308.15 K for aqueous binary and ternary solutions containing 1-alkyl-3-methylimidazolium bromide ([Rmim][Br], R = butyl (C(4)), heptyl (C(7)), and octyl (C(8))), sodium dihydrogen citrate (NaH(2)Cit), disodium hydrogen citrate (Na(2)HCit), and trisodium citrate (Na(3)Cit) are taken using both vapor pressure osmometry (VPO) and improved isopiestic methods. The effect of temperature, charge on the anion of sodium citrate salts, and alkyl chain length of ionic liquids on the vapor-liquid equilibria properties of the investigated systems are studied. The constant water activity lines of all the ternary systems show large negative deviation from the linear isopiestic relation (Zdanovskii-Stokes-Robinson rule) derived using the semi-ideal hydration model, and the vapor pressure depression for a ternary solution is much larger than the sum of those for the corresponding binary solutions with the same molality of the ternary solution. The results have been interpreted in terms of the solute-water and solute-solute interactions. In the second part of this work, the effects of the addition of (NH(4))(3)Cit, K(3)Cit, Na(3)Cit, (NH(4))(2)HPO(4), and (NH(4))(3)PO(4) on the liquid-liquid phase diagram, apparent molar volume, isentropic compressibility, and conductivity of aqueous solutions containing the model ionic liquid 1-butyl-3-methylimidazolium iodide, [C(4)mim][I], are investigated at different temperatures. It was found that there is a relation between the relative concentration of various salts to form two-phase systems with [C(4)mim][I] and apparent molar volume or isentropic compressibility of transfer of [C(4)mim][I] from water to aqueous solutions of the investigated salts.

Volumetric and compressibility studies and phase equilibria of aqueous biphasic systems of alcohols using phase diagram

“…Whereas after phase separation conductance dropped due to ion-pair formation as well as decreasing hydrogen bonding ability of water. The degree of dissociation of salt in each solvent system can be considered as efficient criteria to represent the dissociation of salt [37]. The degree of dissociation of Na 2 S 2 O 3 is in solvent, the order of water + 2-methyl-2-propanol (0.7766) > water + 1-propanol (0.66710) > water + 2-propanol (0.5993) > water + ethanol (0.5579).…”

Volumetric studies to examine the interactions of imidazolium based ionic liquids with water by means of density and speed of sound measurements

“…In some cases, small differences may originate from the method of determination of V 0 U and K 0 SU values. For example Sadeghi et al [19] treated the data of V U versus m and also K SU versus m according to equations generally used for non-electrolytes for determining V 0 U and K 0 SU values, instead of treating them according to Redlich Meyer type equation (equation (2)) or equations as in equation (11) (table 3). This trend was tried to be correlated with the effective molar volumes of anions of these ionic liquids, since they all have a common cation.…”

Aggregational Attitude of Hexadecyltrimethylammonium Bromide in Aqueous Solutions of Sodium Salt at 298.15k

“…This also led to easier formation of micelles (lowering of the CMC value). Anions with higher valency hydrated more water molecule and therefore they are more effective than anions with lower valency to promote the surfactant to go to micellar phase (pseudo phase) similar to the one observed in the case of aqueous two-phase system formulation [32,33]. …”