SURFACE ACTION 53 



zellen," Jena, 1911) to be more sensitive, being injured when the surface tension 

 was reduced only to 0'68 of that between water and air. These results are 

 difficult of interpretation, especially in view of the complex series of phenomena to 

 be described presently under the head of adsorption. 



ELECTRIC CHARGE 



In addition to the surface tension produced by unbalanced molecular forces, 

 there are various other ways in which the properties of substances at their 

 boundaries with other phases differ from those in the main body of the substances. 

 We have first to consider the electric charge. In any charged body, as we know 

 from Faraday's researches, the charge is accumulated at the surface. 



It is somewhat remarkable to find that the boundary surface between liquid 

 and solid, or between immiscible liquids, is nearly always the seat of electrical 

 forces. It has also been shown by Hardy and Harvey (1911, p. 220) that the 

 interface between water and air is similarly the seat of an electric charge. The 

 origin of this charge is not, in all cases, clear. Electrolytic dissociation at the 

 surface will account for the existence and the sign of the charge in perhaps the 

 majority of cases. In other cases, however, ionisation of this kind seems to be out 

 of the question. Drops of petroleum in water have a negative charge, investigated 

 by W. M'C. Lewis (1909, ii. p. 211), and those of aniline have also a negative 

 charge (Ridsdale Ellis, 1912, p. 346). If the charge in this latter case were due 

 to ionisation, it should be positive. Aniline, as a base, dissociates to a certain 

 extent into OH' ions, which pass into the water, leaving the aniline ion with a 

 positive charge. The same process must be supposed to occur at the surface of a 

 drop suspended in water : the mobile OH' ions will travel off, leaving the heavy 

 insoluble anions aggregated on the surface of the drops, which then behave as 

 huge electro-positive ions. This explanation is quite satisfactory for particles such 

 as those of aluminium hydroxide, which have a positive charge, but it does not 

 hold for aniline. W. M'C. Lewis (1910, ii. p. 64) suggests an electronic origin for 

 such cases, on the ground of the similar values (0'04 volt) found for very different 

 chemical substances, suspensions, emulsions, and filter plugs. Burton (1906) has 

 also shown that the same value is obtained for suspensions in methyl or ethyl 

 alcohol or ethyl malonate. The question cannot as yet be regarded as completely 

 solved. The work of Rudge (1914) on the electrification of dust is of interest in 

 this connection. 



The Helmholtz "double-layer" demands a word at this point, although an 

 adequate treatment is impossible. Those interested should consult the paper in his 

 "Gesammelte Abhandl.," i. p. 925; an account of the theory will be found in 

 Freundlich's chapter v.. "Die kapillarelektrischen Erscheinungen " (1909, pp. 

 184-262). It is unnecessary to remind the reader that an electric charge of 

 a particular sign cannot exist without the simultaneous presence in its proximity 

 of an equal and opposite one. The charge on the surface of a solid in a liquid, 

 therefore, implies the existence of an equal and opposite one on the liquid side 

 of the interface. This fact adds complexity to the interpretation of the 

 phenomena now under consideration, but cannot be left out of account. 



In later pages we shall see how the charge on a surface can be increased, 

 diminished, annulled or reversed in sign by the presence of ions in the liquid 

 which is in contact with it. 



The effect of an electric charge on the mechanical surface tension is to 

 reduce it. The elements of the surface, when they have charges of the same sign, 

 mutually repel one another, so that the area of the surface tends to increase, 

 in opposition to the effect of the surface tension to decrease it. The bearing 

 of this fact on the stability of emulsions will be seen in the following chapter. 



INFLUENCE ON SOLUBILITY AND ON CHEMICAL REACTION 



The solubility of certain bodies is found to be different in the surface layer 

 from what it is in the body of the liquid. For example, it was found by 

 J. J. Thomson (1888, p. 254) that potassium sulphate is 60 per cent, more soluble 



