SURFACE ACTION 59 



That the electric charge on surfaces is in reality diminished, neutralised, or even 

 reversed by tons with charges of opposite sign, has been shown experimentally by 

 Pen-in (1904). The method used was to determine the rate at which water passes 

 through diaphragms of paper or other substance, which had been exposed to the 

 action of various electrolytes, in obedience to the attraction or repulsion of charged 

 electrodes at opposite sides of the diaphragm. If this latter, for example, is 

 negatively charged, the water in contact with it will be positively charged, and 

 therefore attracted by the negatively charged electrode (anode). 



Emil Baur (1913) describes a method of demonstrating and measuring the change of 

 potential at a lipoid-water interface when anion or cation is adsorbed thereon. A model, on 

 this principle, of the electrical organ of the fish is also described. The change of electro- 

 motive force produced in this manner is permanent and always of the sign predicted by the 

 hypothesis, so that the effect appears to be actually due to adsorption (see Chapter XXII.). 



Acids and alkalies are very active in this power of conferring electric charges 

 on surfaces, no doubt owing to the great mobility of H* and OH' ions, responsible 

 for the effect. Graphite, for example, can in this way be made positive. Lachs 

 and Michaelis have shown (1911, p. 5) that when such electro-positive graphite is 

 immersed in a solution of potassium chloride, the negative ion (Cl') is adsorbed, 

 while electro-negative graphite adsorbs, in preference, the positive ion (K*). 



It is, however, incorrect to say, as these authors do, that the Gibbs principle fails in 

 such cases. If the statement of this principle is understood to refer only to mechanical 

 surface energy, it is true that electrical energy is left out of consideration ; but this is 

 clearly not the intention of Gibbs himself, who would make it apply to all forms of 

 surface energy. In fact, it is really a deduction from the principle of Carnot and Clausius, 

 which controls all forms of energy whatever. 



From the point of view of energetics, we may formulate the main fact of 

 electrical adsorption as follows. Any process that will reduce the electrical energy 

 at a surface will tend to take place. Hence, for example, if a surface has a negative 

 charge, positively charged bodies will be concentrated upon it, so as to annul its 

 charge. These bodies may be positive ions (cations) or colloidal aggregates. It is 

 not clear, however, from this point of view alone, why the charge is, in many 

 cases, not merely reduced to zero but actually reversed in sign (Perrin, 1904, 

 p. 640). According to Harrison (1911, p. 20) the negative electrical charge on 

 " diamine-blue " is annulled by aluminium sulphate in low concentration, but, in 

 greater concentration, converted into a positive one. It is probable that, although 

 the electrical energy at the surface, in such cases of reversal of sign, is greater than 

 it is at the stage in which the original charge is abolished, other forms of surface 

 energy, such as the mechanical one due to surface tension, may be decreased. 

 The question of adsorption of ions, which decrease surface tension, has been con- 

 sidered by Freundlich (1909, p. 245), Elissafoff (1912), and Ishizaka (1913). The 

 last observer finds that, in the precipitation of aluminium hydroxide, a strongly 

 adsorbed (organic) anion, such as that of salicylic acid, is more powerful than one 

 which is weakly adsorbed, such as a univalent inorganic anion, or that of 

 sulphanilic acid. We see thus the possibility of a charge being increased, if the 

 ion conferring the charge is one that is strongly adsorbed, owing to its effect in 

 diminishing the mechanical surface energy. Similarly, Freundlich and Schucht 

 (1913, p. 646) find that, in the precipitation of a negative colloid by cations, those 

 of the heavy metals and of organic bases are more active than would be expected 

 from their valency, and that this is to be accounted for by the fact of their great 

 mechanical adsorption. 



CHEMICAL ADSORPTION 



The doctrine of energetics, as applied to chemical reactions, teaches us that 

 such reactions will be favoured at interfaces if they lower the chemical potei 

 there. The condition required for such cases is, of course, that the chemict 

 nature of the phase regarded as that one at whose surface the reaction occurs 

 such as to be capable of reaction with the substance in solution, 

 between such surface phenomena and reactions in true solution is th*t i 

 latter case the law of mass action is strictly obeyed, the total mass prese 

 equivalent to the number of molecules concerned ; whereas, in the i 



