On Coulombic and Solvophobic Liquid‐Liquid Phase‐Separation in Electrolyte Solutions

Abstract

We have performed a systematic study of liquid‐liquid phase equilibria in aqueous and non‐aqueous solutions of tetraalkylammonium salts up to about 500 K, covering a large number of hydrogen‐bonding, dipolar aprotic and non‐polar solvents. There appear to be two different types of liquid‐liquid phase equilibria resulting from long‐range Coulombic and from short‐range specific forces, respectively. These may be denoted as Coulombic and solvophobic unmixing. Coulombic unmixing is observed in solutions of 1:1‐electrolytes in solvents of low dielectric constant, as is exemplified for solutions in hydrocarbons and long‐chain alcohols. It appears to be closely related to a general instability of ionic fluids at low reduced temperatures. Increase of the dielectric constant results in complete miscibility, as is demonstrated for salts dissolved in short‐chain alcohols and some common dipolar aprotic solvents. Solvophobic unmixing occurs if salts with large cations and anions are dissolved in water. A phenomenological analysis shows that this phase separation results from those thermodynamic pecularities which are usually attributed to the hydrophobic nature of the cations. Some cation‐anion association appears also to play a role. Similar immiscibilities have been observed with other hydrogen‐bonding solvents of high cohesive energy density, namely glycerol, ethylene glycol, 1,3‐propanediol, formamide and monoethanolamine, suggesting the existence of a more general solvophobic unmixing effect.

Supercritical Water as a Solvent

“…[99] Moreover, the compressibility of such systems is too low to generate substantial density variations. A more suitable electrolyte is NaOH, which at normal pressure melts at 594 K. NaOH is completely miscible with high-density supercritical water, thus offering a broad homogeneous region for investigations.…”

How ionic liquids can help to stabilize native proteins

“…T c largely increases with increasing length of the alkyl residues of the cations, pinpointing the decisive role of hydrophobic interactions. 75 Some anions give rise to a similarly large increase in T c as hydrophobic cations. The effects of the inorganic anions obey the Hofmeister series quoted above.…”

“…The effects of the inorganic anions obey the Hofmeister series quoted above. 75 By contrast, many low-melting protic ILs are completely miscible with water. In these cases the solvent properties can be tuned from typical electrolyte solution behaviour to molten salt behaviour.…”

Mutual Solubility of Hydrophobic Ionic Liquids and Water in Liquid-Liquid Two-phase Systems for Analytical Chemistry

“…The presence of upper and lower critical points has been confirmed for mixtures of tetraalkylammonium halides and water, [244][245][246][247] but at temperatures higher than room temperature. More recently, the phase behavior has been studied for ILs and water at lower temperatures using room-temperature ILs.…”