SURFACE ACTION 7 i 



There are some more facts of interest, tending to show the great importance of the 

 electrical charge of the surface. Gee and Harrison (see William Harrison, 1911, p. 6 of 

 reprint) found that the maximum negative charge of cotton, wool, and silk was at a tempera- 

 ture of 40 C. Brown (1901, p. 92) had previously shown that the maximum adsorption of 

 "basic" (electro-positive) dyes by wool took place at the same temperature. 



W. Harrison (1911, p. 26) also showed that cotton treated in various ways, nitrated, 

 mercerised, and so on, had a contact potential difference against dilute sodium chloride whick 

 differed considerably in amount according to the treatment, although the charge was always 

 negative. The amount of "acid" ( = electro-negative) dye adsorbed was parallel with the 

 decrease, in the charge. 



That the deposition of an electro-positive dye on a negative surface results in a lowering of 

 the charge on this surface is shown by an experiment of Larguier des Bancels (1909). The 

 charge on wool, as measured by the number of drops of water transferred from one electrode 

 to the other, in an apparatus similar to that of Perrin (1904, p. 616), in a given time was 

 represented by 77. After staining with methylene blue, the number was reduced to 18. 



The very marked effect of electrolytes in altering the charge on surfaces 

 has been frequently referred to, as also the fact that electro-negative substances 

 are scarcely adsorbed at all by electro-negative surfaces. In order that this 

 adsorption may take place, the surface must be discharged, or the amount 

 of its charge lessened, by the action of an ion of opposite sign. That very 

 small quantities of an appropriate ion suffice is well shown by an experiment 

 of Elissafoff (1912, p. 404), whose work will be referred to in more detail in 

 the chapter on " The Colloidal State." 0-2 mgm. of thorium nitrate per litre 

 lowered the charge on the surface of a quartz capillary by 50 per cent. 



The absence of staining by " acid " dyes in the absence of electrolytes explains 

 why fresh teased nerve fibres of the frog only stain with Congo-red at their 

 cut ends, where, according to Macdonald (1905, p. 329), electrolytes are set 

 free. Emil Mayr (1906, p. 560) finds that the affinity of Nissl bodies in 

 nerve cells for " basic " dyes is reduced by previous treatment with neutral 

 salts. This fact also is in agreement with the doctrine of electrical adsorption. 

 The Nissl bodies have, in all probability, a negative charge; this charge would 

 be diminished by cations, and hence the attraction for positive substances, 

 like the " basic " dyes, would be also diminished. 



Disregard of this action of electrolytes has led to certain erroneous statements with regard 

 to dyes. It is to be remembered that commercial specimens almost invariably contain a large 

 percentage of salts, frequently as much as 20 to 30 per cent, of sodium chloride or sulphate, 

 arising from the mode of preparation. When it is said that Congo-red is a " direct " dye for 

 cotton, the statement only applies to the commercial dye, with its content of salts. When 

 adsorption, moreover, takes place under the influence of electrolytes, it is, as a rule, " faster," 

 that is, not so easily removed by the action of water, than when it takes place in their 

 absence. This applies more especially to the electro-negative dyes. 



Certain facts described as " anomalous adsorption " (Biltz, 1910) will also be found to be 

 explained by the presence of electrolytes (Bayliss, 1911, 3). 



For many purposes it is necessary to have pure dyes. The following method, due to 

 Harrison (1911, p. 17), may be recommended. It depends on the displacement of the non- 

 volatile salts, present as impurity, by a volatile one. The dye in concentrated solution is 

 precipitated by saturation with ammonium carbonate ("salted out"), redissolved in water, 

 and again salted out. After washing with a saturated solution of ammonium carbonate, the 

 precipitate is dried at 110 C., when all the ammonium carbonate is driven off. 



A curious fact was noted by Freundlich and Losev (1907, pp. 311, 312) : 

 When an " acid " dye is adsorbed, the whole of the molecule is taken up. When 

 a "basic" dye is adsorbed, the positive coloured ion only is taken up, leaving 

 the acid. Satisfactory explanation of these facts is not at present at hand (see 

 Freundlich and Neumann, 1909). There are, however, two facts to be remembered 

 in this connection. The "acid" dyes are, as a rule, sodium salts of strong 

 (sulphonic) acids and are very little, if at all, hydrolysed in solution, but 

 electrolytically dissociated to a considerable degree. The anion, containing a 

 large number of atoms, seems to behave as a colloid and has, of course, a negative 

 charge. The " basic " dyes, on the other hand, are salts of a rather feeble organic 

 colour base with a strong acid and are hydrolysed in solution. The free base is 

 insoluble, in the ordinary sense, but forms a colloidal solution, the particles having 

 a positive charge. The behaviour of the two classes may perhaps depend in some 

 way on this difference in mode of dissociation. 



