The adsorption of choline halides at a mercury— solution interface

Abstract Against the background of investigations undertaken on the importance of hydrophilic and lipophilic reactions, and more particularly in the case of quaternary ammonium cations [1–3] a study of the influence of a hydrophilic alcohol group has been carried out using aqueous choline halide solutions. Previous publications [4, 5] dealing with these solutions have shown that the solute—solvent interactions are complex. They reveal the existence of competitive effects between those due to the action of the cations on water which favours the formation of hydrogen bonds in the solvent around the ion (an effect which is modulated by the presence of the OH group in the choline) and that due to the anion which diminishes the number of these hydrogen bonds as its size increases. This competition leads to a change in the properties of the solutions when in a series one passes from the chloride to the bromide and iodide. The adsorbability of the cation as related to the properties of the OH group is indeed diminished as compared to the corresponding quaternary ammonium cation. Further one notes that the adsorption in the diffuse layer is inferior to that predicted by the Gouy-Chapman theory. The effect due to the taking into account of different planes of closest approach for the anion and the cation is not sufficient to account for this difference and one must envisage the competitive influence of the ion-solvent interactions which is all the more important that one is dealing with mixed solutions of choline and potassium chlorides. As in the case of tetrakylammonium salts one notes an effect due to the cation on the adsorption of the anion. The adsorption of the chlorides exactly neutralizes the electrode charge even for fairly high concentrations. The adsorption of the bromide and iodide vary with the concentration in a different manner for each ion. One can interpret these results taking into account both the relative hydrophobicity of the cation and the disymmetry introduced in the choline molecule as a result of the OH group, as well as, once again, the competitive effect in the influence on the solvent between the cations and anions, an effect which varies according to the nature of the anion. This study reveals in the case of organic cations the inadequacy of the usual differentiation between diffuse and compact layers which neglects the intrinsic solvent effect. It also shows, in this respect and with regards of all the results obtained, the fundamental importance of the solute—solvent interactions on the adsorbability.