Transport of Ions Across Adsorbed Monolayers at the Amalgam | Water and Calomel | Water Interfaces

Abstract

In order to understand the physical processes of ion transport across thin membranes we have studied the transport of ions (K+, Na+, Cs+, Cl−) across monolayers of n-alkyl quaternary ammonium salts, with alkyl chain lengths from 10–18 carbon atoms, adsorbed at the amalgam/water interface. The amalgams were formed from potassium, sodium and cesium chlorides in 0.1 N solution at 25°C. Galvanostatic measurements were carried out using a stationary mercury drop electrode, a platinum counter electrode and a silver-silver chloride reference electrode. Quaternary ammonium compounds with alkyl chain lengths of 10, 14 and 16 carbon atoms did not develop measurable potential differences across their monolayers for both cathodic and anodic currents. Trimethyl octadecyl ammonium chloride adsorbed at the amalgam/water interface developed a potential difference of about 300 mV for cathodic currents, although no specificity was observed among the various cations. An anodic oxidation current across a monolayer of trimethyl octadecyl ammonium chloride developed a potential difference of about 250 mV for transport of Na+ and K+, and 97 mV for the transport of Cs+. The transport of Na+ and K+, but not Cs+, across adsorbed monolayers of trimethyl octadecyl ammonium chloride, developed sustained oscillations in the potential difference across the monolayer, between 0 and 240 mV, and at a frequency varying between 1 and 4 Hz during the anodic oxidation of the amalgams. With the same technique we studied the ionic transport of Cl− across adsorbed monolayers of trimethyl octadecyl ammonium chloride and trimethyl oleyl ammonium chloride adsorbed at the calomel/water interface. The trimethyl octadecyl ammonium chloride developed a higher potential difference than the unsaturated compound. The potential differences developed across the various adsorbed monolayers for the transport of cations and anions reflect differences in their ionic permeabilities.