Receptacle Model of Salting-In by Tetramethylammonium Ions

Abstract

Water is a poor solvent for nonpolar solutes. Water containing ions is an even poorer solvent. According to standard terminology, the tendency of salts to precipitate oils from water is called salting-out. However, interestingly, some salt ions, such as tetramethylammonium (TMA), cause instead the salting-in of hydrophobic solutes. Even more puzzling, there is a systematic dependence on solute size. TMA causes the salting-out of small hydrophobes and the salting-in of larger nonpolar solutes. We study these effects using NPT Monte Carlo simulations of the MB + dipole model of water, which was previously shown to account for hydrophobic effects and ion solubilities in water. The present model gives a structural interpretation for the thermodynamics of salting-in. The TMA structure allows deep penetration by a first shell of waters, the dipoles of which interact electrostatically with the ion. This first water shell sets up a second water shell that is shaped to act as a receptacle that binds the nonpolar solute. In this way, a nonpolar solute can actually bind more tightly to the TMA ion than to another hydrophobe, leading to the increased solubility and salting-in. Such structuring may also explain why molecular ions do not follow the same charge density series’ as atomic ions do.

How ionic liquids can help to stabilize native proteins

“…48 Essentially the same ion series are observed in numerous other systems of largely different complexity. 35 Illustrative examples are solubilities of nonpolar gases in water, 49 surface tensions of solutions, 50 ion binding to micelles, 51 or even bacterial growth. 52 Ranking the anions according to their protein-stabilizing efficiency, a widely quoted excerpt of the Hofmeister series reads 35,36 […”

The water‐promoted Diels–Alder reaction in quaternary ammonium salts

“…More precisely, the explanation of S/I behaviour can be explained by two factors, (i) the structural change in the medium caused by indirect interactions between the hydrophobe and salt and (ii) direct interactions caused by the hydrophobic contacts between ion and hydrophobe. The S/O effect seems to be caused by enhanced cage structure of water in presence of salts, which solvate hydrocarbon chain of the salt cation inside and consequently the solubility of the hydrophobe decreases in the aqueous salt solution . The possibility of the hydrophobe to enter into the cage structure of water is reduced which has been indicated by a decrease in the solubility.…”

“…The typical S/I and S/O nature of salts depend on the ion density of water and salts in the vicinity of the hard sphere of solute . The similar trend of data is evident in our experimental observations.…”

“…The R 4 NX salts show this unusual behaviour when hydrophobic solute was added to its solution. Hydrophobic effects and ion solubility in water have been previously explained by NPT Monte Carlo simulations of the Mercedes‐Benz (MB) + dipole model of water . In this study they proposed that (CH 3 ) 4 N + cation provides enhanced access by a first shell of water where the dipole of water electrostatically interacts with (CH 3 ) 4 N + cation.…”

Synthesis and characterization of cationic colloidal unimolecular polymer (CUP) particles

“…Based on Monte Carlo simulations, Hribar-Lee et al [41] recently reported that the tetramethylammonium (TMA) cation has a minimum of two hydration shells with a further cage-like structuring of water molecules around the TMA ion [42]. García-Tarrés et al [43], using molecular simulation dynamics, reported that the hydration number, i.e., number of water molecules in the hydration shell, for the TMA ion was 23.…”