58 PRINCIPLES OF GENERAL PHYSIOLOGY 



ELECTRICAL ADSORPTION 



The adsorption of electrolytes and of dyes is a more complex process than that 

 of mere reduction of surface tension, .since electrical forces come into play. 



In the experiments of Lewis, caffeine and aniline, and in those of Donnan and 

 Barker, nonylic acid and saponin, obey the Gibbs formula. These, it will be 

 noted, are all practically non-electrolytes. Lewis found, on the contrary, that 

 bile salts and dyestuffs, such as methyl orange and Congo-red, were taken up in 

 much larger amount than the Gibbs formula would indicate. These latter 

 compounds, however, are electrolytes, i.e., they are dissociated in water, with 

 the formation of electrically charged products. The non-dissociated part, more- 

 over, tends to form aggregates of a colloidal nature, which carry charges. 



We have already seen how most insoluble surfaces immersed in water have 

 a negative charge, some few a positive one. The origin of this charge does not 

 concern us here, and will be treated in future pages. The point to be noted 

 is that it gives rise to a considerable amount of free energy on the surface. If, 

 therefore, the deposition of any substance, from solution in the water, upon such 

 a surface would reduce the electrical potential there, it will, by the Gibbs principle, 

 tend to take place. Suppose that the surface is that of charcoal, which has in 

 water a negative charge, and that to the water we add substances with positive 

 charges, such as colloidal ferric hydroxide, or a salt which dissociates with produc- 

 tion of positive and negative ions. The colloid or the cation of the salt will be 

 deposited on the surface, so that its charge is neutralised. 



Perrin (1905, p. 100) was the first to suggest that electrical forces might play 

 a part in the phenomena of dyeing, and V. Henri and Larguier des Bancels (1905) 

 called in the aid of such forces to explain the fact that aniline blue, an electro 

 negative colloid, is taken up by gelatine, itself an electro-negative colloid, in very 

 small amount, because of the mutual repulsion of their charges. If, however, 

 barium nitrate, which dissociates with formation of positively charged barium 

 ions, be added, these ions discharge the dye particles (from Perrin's work, it is 

 more probable that it is the surface of the gelatine that is discharged), so that 

 there is no longer repulsion, and the gelatine becomes deeply stained. The first 

 systematic investigation of this electrical adsorption was made by myself (1906). 

 I found that the adsorption of various electrically charged bodies by electrically 

 charged surfaces depended on the sign and the amount of the respective charges. 

 An electro negative surface, say that of filter paper, will take up large quantities, 

 of an electro-positive substance, such as night-blue, but only a trace of a negative 

 dye, such as Congo-red. The amount adsorbed also depends on the amount of the 

 charge, as is indicated by its connection with the dielectric constants of the con- 

 stituents of the system ; for example, more Congo-red is taken up from dilute 

 alcohol than from water. The charge of paper is proportional to the difference 

 between its dielectric constant and that of the liquid in which it is immersed. 

 Paper itself has a dielectric constant of 2-82, water one of about 80, pure alcohol 

 one of 26 (see the article by Graetz in Winkelmann's " Physik," 2te Aufl., Bd. IV., 

 pp. 112, 144, and 137). Hence the negative charge of paper is lower in alcohol, 

 and a negative dye is more readily adsorbed. 



I found further, that when neutral salts, having no chemical action on the 

 materials concerned, such as sodium chloride in the cases of Congo-red and night- 

 blue, are added, the effect is to increase the adsorption of negative dyes and to- 

 diminish that of positive dyes. The explanation will be obvious ; the positive ion 

 (Na) of the salt diminishes the negative charge of the paper, in accordance with 

 the Gibbs principle, and consequently the adsorption of a similarly charged body 

 is facilitated while that of an oppositely charged one is retarded. The adsorption 

 of colloidal arsenious sulphide (electro-negative) was found to be affected in the 

 same way as that of Congo-red. Addition of a trace of gelatine or albumin to the 

 solution prevents the effect of electrolytes, a phenomenon whose explanation will 

 be found when we come to discuss the colloidal state. 



Similar theories with regard to dyes, but less complete, since the actions of 

 added salts and of dielectric constants were not included, were subsequently put for- 

 ward by Pelet-Jolivet (1910), Michaelis (1908), and by Gee and Harrison (1910). 



