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As a basis for the study of the adsorption of a surface-active solute to a fresh liquid surface, it is necessary to establish with accuracy the changes in the physical properties of the surface which occur immediately after its formation, and throughout the adsorption process. Two important properties, which undergo considerable change as adsorption proceeds, are the surface tension and the surface potential. We have recently observed that the surface potential of aqueous solutions of many long-chain compounds, at the instant when adsorption is complete, differs widely from the ultimate equilibrium value. The changes in potential following establishment of concentration equilibrium between surface and bulk solution are comparable in magnitude with those occurring during adsorption, and erroneous deductions may result from the assumption that dynamic surface tensions and potentials change in a similar manner with age of surface.

THE fact that liquids other than water may serve as media for inorganic reactions is widely recognized; reactions in the well-known liquid ammonia and liquid sulphur dioxide systems are explained by postulating the ionization: although neither liquid has an appreciable electrical conductivity. Frankland and Farmer1in 1901 first considered the possibility that liquid dinitrogen tetroxide might serve as an ionizing medium, but were unable to find any inorganic salt which was soluble in the liquid. In view of the high solubility of many organic compounds, they concluded that the liquid was of a non-ionizing character and resembled in behaviour such organic solvents as benzene.

WHEN a single solute is adsorbed from one phase to a water – oil interface, the interfacial tension usually falls in a regular manner with increasing adsorbate concentration, to a limiting value corresponding to a close-packed monomolecular surface layer. In the very few cases1where exceptions have been found to this rule, the adsorbates have been soaps or similar hydrolysable molecules, and inter-attraction between the long-chain molecules and their products by hydrolysis has been given as explanation of the sharp breaks which occur in the interfacial tension – adsorbate concentration curves.

IN recent years various authors1have applied static methods to the study of the slow changes which occur in the surface tension of aqueous solutions of some long-chain compounds; and in general the possibility that the accumulation of solute on the surface by diffusion from bulk solution might be responsible for the drift in tension is dismissed on the grounds that the periods of time involved may be as much as 109times greater than those anticipated on classical diffusion theory. My recent quantitative studies of the adsorption process2using vibrating jets of aqueous alcohol solutions have confirmed that diffusion to the surface is complete within 10−4to 10−1sec. over the available range of chain-length and concentration, and similar results have been obtained with aqueous solutions of short-chain carboxylic acids. It seems clear that changes in tension over periods of hours, or even days, which occur in solutions of appreciable concentration (and which are usually smaller than the initial fall from the pure water value which occurs during development of the surface layer) are the result of rearrangements in the equilibrium adsorbed layer rather than of diffusion1.

SALTS having the general formula M[UO2.(NO3)3] (where M is NH4, K, Rb, Cs) have been referred to in recent literature1. The tetramethylammonium salt has been prepared by reaction of the appropriate nitrates in pure nitric acid medium2, and the identity of a trinitratouranyl ion has been established in solutions of uranyl and tetra-n-butylammonium nitrates in acetone or methyl isobutyl ketone3.

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THE surface tension of an aqueous solution has long been recognized as one of the more important physical properties controlling its wetting and detergent powers, though early attempts to correlate these powers with equilibrium surface tension met o with limited success. It is becoming generally appreciated that factors other than solute concentration influence surface tension, and that the condition of the surface must also be taken into account. Thus, the surface age involved in many detergent processes does not exceed a few seconds, and much of this time may be required for the establishment of diffusion equilibrium. For dilute solutions of long-chain compounds the operative dynamic tension may vary over a range of 40 dynes/cm. and acknowledgment of the time required for surface equilibrium is clearly essential. A further factor, the change in equilibrium surface tension which occurs during change in area of a soluble film, has not previously been considered, but may prove to have considerable significance.

IT is well known that when two long-chain compounds, stereoisomeric with one another around a double bond, are adsorbed ata surface, they infiuence the tension there to extents which differaccording the the effective chain length. Recent experiments by me have shown that although the thermal agitation of short-chain adsorbate molecules at a water-air interface prevents any appreciable fall in the surface tension of the solvent, orientation of many small molecules does occur at a water - oil interface. In many cases this is sufficiently marked to reveal interesting differences in interfacial activity between short-chain isomeric compounds, of classes not usually regarded as surface active. Further, these differences can be reproduced with an accuracy which enables such measurements to be used as a means of distinguishing between the isomers.