Capillary active

The reduction wave of peroxydisulphate at dme starts at the potential of the anodic dissolution of mercury. The current-potential curve exhibits certain anomalous characteristics under various conditions. At potentials more negative than the electrocapillary maximum, a current minimum can be observed this is due to the electrostatic repulsion of the peroxydisulphate ion by the negatively charged electrode surface. The current minimum depends on the concentration and nature of the supporting electrolyte, and can be eliminated by the adsorption of capillary active cations of the type NR4. ... 

We would anticipate from a consideration of the Gibbs equation that the addition of a solute to a solvent will cause marked changes in the composition of the surface phase if the solvent and solute possess different surface tensions. On the addition of a highly capillary active material to water the surface phase becomes rich in the solute and the surface tension of the solution will fall rapidly. 

In the case of soluble substances of low capillary activity the evaluation of = dimensional pressure must be regarded as the difference in surface tension between that of the liquid without the Gibbs film and one in which the Gibbs film is established only as a first approximation may the ideal dynamic surface tension of a solution, i.e. of a solution without a Gibbs film, be regarded as equal to the surface tension of the solvent. 

In many cases such as at water-mercury interfeices electrolytes are positively adsorbed. The application of the kinetic theory to surface films of molecules leads, as we have seen, to a ready interpretation of the lowering of the surface tension by capillary active nonelectrolytes. For electrolytes an additional fiictor has to be considered, namely the mutual interaction of the electrically charged ions adsorbed. As we shall have occasion to note the distribution of the adsorbed ions, both positive and negative, at an interface such as water-mercury is not readily determined, but it is clear from a consideration of the data of Gouy that mutual ionic electrical repulsion in the interface is an important factor. In the case of potassium iodide, for example, for very small values of F the Traube relationship... 

Langmuir has pointed out that some important conclusions may be drawn from consideration of the surface and bulk concentrations of capillary active substances. 

It will be noted that the fall in surface tension of the mercury with increase in the partial pressure of the vapour is similar to the fall observed in aqueous solutions with increase in concentration of a capillary active solute. 

The experimental apparatus consists essentially of a narrow vertical glass tube down the inner surface of which one liquid is made to flow, the other liquid emerges from a fine glass tip in the form of a narrow jet down the axis of the tube. The two solutions are connected with calomel electrodes employing potassium chloride or nitrate as junction liquids. The E.M.F. of the cell is measured by means of a sensitive quadrant electrometer. The greatest source of error in the method is the elimination of or the calculation of the exact values of the liquid-liquid junction potentials in the system. For electrolytes which are not very capillary active, the possible error may amount to as much as fifty per cent, of the observed E.M.F. 

Kruyt and van Duin (Kolloidchemie Beihefte, v. 269,1914) have examined the alteration in sensitiveness to electrolytes of a suspension of arsenious sulphide to which various non-electrolytes had been added. They found that the influence of various nonelectrolytes on the sensitiveness of the suspension ran parallel to the adsorption of the non-electrolytes from aqueous solution by powdered charcoal and that the most capillary active non-electrolyte exerted the greatest effect on the liminal concentration required for precipitation. Further it was observed that the addition of non-electrolytes lowered the liminal concentrations, i.e. increased the sensitiveness of the suspension to uni- and trivalent ions but increased the liminal concentrations, i.e. decreased the sensitiveness for divalent and tetravalent cations as will be noted from the following tables. 

For a ferric hydroxide suspension they obtained an increase in sensitivity on the addition of both isoamyl alcohol and phenol to both univalent and divalent anions, whilst Freundlich and Eona (Bioohem. Zeit lxxxi. 87, 1917) have noted similar effects on this suspension on the addition of many other capillary active nonelectrolytes such as the urethanes, camphor and thymol. 

Visser, S. (1964). Oxidation-reduction potentials and capillary activities of humic acids. 

Interaction between insoluble films and capillary active substances in solution. Penetration and displacement of insoluble... 

The process of haemolysis is probably a penetration or displacement of the films normally surrounding the red blood corpuscles by the capillary active substances which constitute the haemolytic agents. Gorter and others1 found that haemolysis was complete when just enough was present to cover an area equal in surface to the corpuscles. The correspondence between penetration in monomolecular films, and haemolysis, by various substances, has been discussed by Schulman and Rideal.2... 

The effect of the formation of the ionio micelle in the interior of the solutions of these long-chain salts, on their surface tension, is interesting. Although the ionic micelle is, itself, much more soluble and probably much less capillary active than the single ions, its presence in the interior causes the surface tension to be diminished to the minimum very much more rapidly than if no ionio micelles are present. The reason for this is not clear, but it is possible that electrostatic forces between the heavily charged micelles and the single ions tend to concentrate the latter at the surfaoe. 

Stability of isolated liquid films soap films foams. Pure liquids rarely, if ever, foam, but most solutions of capillary active substances do so to a greater or less extent, and long-chain colloidal electrolytes, particularly the soaps, have an almost miraculous power of rendering liquid films stable. Dewar1 has maintained soap films unbroken for over a year (one for three years ), with due precautions against evaporation, mechanical and thermal shocks, and absorption of carbon dioxide. 

Diagram (6) shows similar, but opposite, phenomena with the capillary active cations, tetramethyl and tetraethyl ammonium. The phenomena are reversed the rising part of the curve is not affected the falling part, and the maximum, are depressed and the maximum is shifted to more positive potentials. This is because the adsorption tends to form a double layer in the water, with the positive ions nearer the surface than the negative, and this positive adsorption potential aids in attracting electrons to the mercury surface and neutralizing it, so that the maximum is reached with the application of smaller negative potentials E. 

Measurements of the surface tensions of mixtures and solutions have been made in great numbers Musculus (1864) indicated that solutions may be classified into two groups according as the solute has little influence on the surface tension of the solvent (when Traube called it capillary inactive), or when it produces a marked lowering of surface tension (when Traube called it capillary active). A consideration of the second group is closely connected with adsorption phenomena, which are not dealt with here, the present discussion being mainly confined to solutions of the first group, especially electrolytes, and to mixtures of liquids. 

A difficulty encountered in the measurement of the surface tension of solutions is that it is often different when measured by so-called dynamic methods (vibrating jets, etc.), in which the value for a freshly-formed surface is measured rapidly, and when measured by so-called static methods (capillary rise, etc.), which determine the value for a surface which has been in existence for some time. The difference is due to the fact that the composition of the surface is different from that in the bulk of the solution, and in a fresh surface a change of concentration occurs, which, as it involyes diffusion, usually occurs slowly, so that rapid measurements give results different from those which deal with a surface which has come into equilibrium. In capillary active solutions, the surface is enriched in solute, whilst in capillary inactive it is usually richer in solvent. In the case of electrolyte solutions, the surface layer is considered to consist of a unimolecular layer of solvent molecules. The thermodynamic theory was established by Gibbs, and indicates that when the solute... 

The indicator behaves in a peculiar manner in alkaline solutions, and care must be exercised when pinachrome is used in pH measurements. The free red indicator base is very slightly soluble in water, and separates out on standing. If the indicator is shaken violently in a solution containing alkali, a red-violet foam is produced and the solution becomes almost completely decolorized. Shaking favors the decolorization process. Evidently the indicator base is strongly capillary active, and collects at the air-water interface when the solution is agitated. Consequently, when pinachrome is used in basic solutions, the tubes must be rotated carefully and the color estimated soon after addition of the indicator. 

The water used as a substrate was twice distilled and had a pH slightly below 7.0 and a specific conductivity of about 0.5 jumho. Because the pH was always below 7.0, its variation had a negligible effect on the lecithin isotherm. Thorough testing of the water surface on the film balance ensured freedom from significant capillary-active contamination. 

Substances that lower the surface tension of an aqueous solution are often designated as capillary active. 

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