Effects of Traces of Metallic Ions on Monolayers

Abstract

IN a recent issue of NATURE, Myers and Harkins1 directed attention to the possibility of considerable errors arising in experiments on monomolecular films where small quantities of ions are introduced from the glass vessel or the metal trough. Experiments which we have carried out recently at University College, London2, show that corresponding errors may arise in the measurement of interfacial tensions as a result of the presence of minute quantities of ions in the distilled water. Thus when using ordinary laboratory distilled water, minima were obtained in the interfacial tension-concentration curves for sodium cetyl sulphate against various oils, the interfacial tension with 0.05 per cent sodium cetyl sulphate being several hundred per cent higher than the minimum which occurred at about 0.015 per cent. Similar results were obtained with sodium salts of other long-chain acids. On using very good conductivity water (specific conductivity 0.l × 106 ohm.1 cm.1 or less) these minima were no longer found. The ordinary distilled water evidently contained small quantities of a cation which strongly depresses the interfacial tension of such systems, its effect being greater in the lower concentrations of long-chain salt.