Studies in electrocapillarity

Abstract

PART I. The Electrocapillary Curves of Organic Acids and their Salts

SUMMARY: #1. The electrocapillary curves of a number of organic acids and their salts have been determined in M/2 sodium sulphate. The maximum adsorption of salts of weak acids occurs at potential differences about 0.1 volt more positive than those of the acids themselves, and strong acids give maxima in the same region. The displacement of the potential of maximum adsorption of organic anions towards positive potentials, is thus much less than the corresponding displacement towards negative potentials observed with organic cations.

#2. A study of mixtures of benzoic acid and sodium benzoate shows that this behaviour cannot be due to hydrolysis of the salts. It is suggested that the conditions under which adsorption occurs must differ on the positive and negative branches of the electrocapillary curve.

#3. The electrocapillary viscosity exhibited by solutions of the acids, appears to be due to the formation Of alkali in the solution, owing to discharge of hydrogen ions at the mercury surface.

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PART II. Selective Adsorption in Solutions containing Two Active Substances,

SUMMARY: #1, The electrocapillary curves of a number of solutions containing two activo substances have been determined.

#2. It is shown that in mixtures of substances of the same type (ions or non_ionised molecules), if one compound causes a greater surface tension lowering than the other over the whole range of potential differences, it alone is adsorbed.

#3. When the components cause nearly the same surface tension lowerings, both substances are adsorbed from the mixture.

#4. When the adsorption curves cut each other, several cases have been found at which one component is exclusively adsorbed at potential differences below the point of intersection and the other at p.ds. above the point of intersection.

#5. Except in the case of a solution containing active positive and negative ions, the surface tension lowerings produced by the mixtures are less than the sum of those produced by the components separately.

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PART III. Adsorption Effects in Mixtures of Two Active Components. The effect of varying the concentration of the Components.

SUMMARY: #1. The electrocapillary curves of a number of aqueous solutions containing mixtures at various concentrations of two surface active substances have been determined. The electrocapillary curves of the separate components, at the same concentrations as in the mixtures, have also been determined.

#2. Mixtures of a wide range of concentrations of sodium cinnamate and a constant amount of sodium o_toluate show that the effect of the latter component can be measured within a narrow range of concentrations of the former. In a mixture containing M/30 sodium cinnamate the effect on the surface tension lowering of the addition of M /20 sodium o_toluate is inappreciable, but with M/60 sodium cinnamate the effect becomes appreciable.

#3. The adsorption curves of mixtures of Phenol_ Caffein solutions indicate that as the Phenol concentration is reduced, the tendency for in creased selective adsorption of the Caffein occurs at an ever widening range of applied potential differences. The same tendency is observed for Phenol as the concentration of caffein is reduced.

#4, With a mixture of salicin and saccharose a new type of selective adsorption is obtained; the effect of saccharose on the salicin adsorption curve being only appreciable at both extreme ends,

#5. Equations developed from adsorption theory apply reasonably well to the observations made in the sodium cinnamate - sodium o_toluate mixtures, but indicate a greater effect for the surface tension lowering produced by the toluate than is actually observed,

#6. The equations applied to cases of other mixtures indicate that practically complete selective adsorption of the more surface active component is to be expected. The surface tension lowering produced by the second component in these cases is not measurable, and this is adequately confirmed by the calculations,