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

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 ([Cnmim][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 (C4), heptyl (C7), and octyl (C8)), sodium dihydrogen citrate (NaH2Cit), disodium hydrogen citrate (Na2HCit), and trisodium citrate (Na3Cit) 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 (NH4)3Cit, K3Cit, Na3Cit, (NH4)2HPO4, and (NH4)3PO4 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, [C4mim][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 [C4mim][I] and apparent molar volume or isentropic compressibility of transfer of [C4mim][I] from water to aqueous solutions of the investigated salts.

特定电解质改变离子液体（ionic liquids）溶液性质的行为已有充分文献记载，尽管该效应的成因尚未得到明确界定。为厘清这一问题，本研究旨在通过考察一系列盐类对可与水混溶的离子液体1-烷基-3-甲基咪唑卤化物([Cₙmim][X])的气液平衡、液液相图、体积性质、压缩性及电导性质的影响，获取向该类离子液体水溶液中添加不同盐类所产生的盐析效应（salting-out effect）的更多佐证。本研究第一部分采用蒸气压渗透法(VPO)与改进等压法，测定了298.15 K及308.15 K下，包含1-烷基-3-甲基咪唑溴化物([Rₘim][Br]，R为丁基(C₄)、庚基(C₇)与辛基(C₈))、柠檬酸二氢钠(NaH₂Cit)、柠檬酸氢二钠(Na₂HCit)及柠檬酸三钠(Na₃Cit)的二元及三元水溶液的水活度。研究考察了温度、柠檬酸钠类阴离子电荷以及离子液体烷基链长度对所研究体系气液平衡性质的影响。所有三元体系的恒定水活度线均与基于半理想水化模型推导得到的线性等压关系——扎丹诺夫斯基-斯托克斯-罗宾逊规则（Zdanovskii−Stokes−Robinson rule）——存在显著负偏差，且三元溶液的蒸气压降幅远大于与该三元溶液具有相同质量摩尔浓度的对应二元溶液的蒸气压降幅之和。上述结果可通过溶质-水及溶质-溶质相互作用加以阐释。本研究第二部分则在不同温度下，考察了添加柠檬酸三铵((NH₄)₃Cit)、柠檬酸三钾(K₃Cit)、柠檬酸三钠(Na₃Cit)、磷酸氢二铵((NH₄)₂HPO₄)及磷酸三铵((NH₄)₃PO₄)对以模型离子液体1-丁基-3-甲基咪唑碘化物([C₄mim][I])配制的水溶液的液液相图、表观摩尔体积、等熵压缩率及电导率的影响。研究发现，与[C₄mim][I]形成两相体系的各类盐的相对浓度，与[C₄mim][I]从纯水转移至所研究盐类水溶液的表观摩尔体积或等熵压缩率之间存在相关性。