SURFACE ACTION 55 



that dissolved substances which lower surface tension will be concentrated in 

 this situation, on account of the fact that free energy will be lessened thereby. 



This result is of fundamental importance, and was arrived at by Willard 

 Gibbs (1906, i. p. 56) from thermodynamic considerations in 1878, and by 

 J. J. Thomson in 1888 (1888, pp. 191, 192) from the dynamical point of view. 

 It will be referred to in subsequent pages as the "Gibbs" or " Gibbs-Thomson " 

 principle. It is really a particular application of the general doctrine of 

 decrease of free energy, as shown by the headlines chosen by Gibbs himself 

 for his work on " Heterogeneous Equilibrium," viz., the formulation by Clausius 

 of the two laws of energetics, as given on p. 28 of the present volume. As 

 applied to surface energy, the Gibbs principle has a wider application than 

 may appear from the above statement of it as referring to surface tension. It 

 may be expressed thus : Any process that diminishes the free energy at an 

 interface will tend to take place, whatever be the nature of the energy con- 

 cerned, whether mechanical, electrical, chemical, or other. If the surface has 

 an electric charge, a process diminishing it will be favoured. If it possesses 

 chemical energy, a reaction reducing this energy will take place, if possible ; 

 and so on. 



Accordingly, any substance in solution in a liquid, in contact with the v / 

 surface of another phase, will be concentrated on that surface, if, by doing so, y\ 

 the free energy present there is decreased. This process is called "adsorption," 

 Its characteristic is the relation to surfaces of contact. Whatever further 

 process may follow it, chemical reaction, or diffusion into the body of the 

 other phase, the first thing to take place is the local concentration. The rate 

 at which subsequent events happen will naturally depend, by mass-action, on 

 the amount of this condensation. Given the diminution of surface energy, the 

 adsorption process is thermodynamically bound to take place, and any other 

 explanation of the phenomenon is superfluous. 



As an example, the well-known effect of charcoal in decolorising or clarifying a solution 

 may be given. If a dilute solution of a dye, such as " night-blue," be mixed with charcoal, 

 it can be almost completely decolorised. That the dye is not destroyed, or chemically 

 combined with the carbon, can easily be shown by filtering off the latter, and extracting it 

 with alcohol, which will be found to become of a deep blue colour. The process is, in fact, 

 reversible. 



In the case of the ordinary form of surface energy, Gibbs has given a 

 formula by which the amount of dissolved substance concentrated at the 

 interface can be calculated. Thus : Let F be the excess of solute in the surface 

 layer above that in the body of the solution, C the concentration of the solute, 

 R the gas constant, T the absolute temperature, and o- the surface tension at 



the interface. Then -^ represents the change of surface tension with change 

 of concentration of solute, which can be measured, and 



_r = .*i 



~ RT ' dC' 



The way in which this equation is obtained is beyond the scope of this work. 

 The appearance of R and T is due to the assumption that dilute solutions obey 

 the gas law, so that the formula cannot be of general application. 



This formula has been tested experimentally by W. C. M'C. Lewis and by Donnan and 

 Barker, and found to give values in accordance with experiment in cases where the conditions 

 arc such that no complication due to other forms of surface energy, especially electrical, 

 intervene. Lewis (1909, i. p. 486) found in the case of caffeine on the surface of petroleum, 

 and (1910, iii. p. 136) of aniline on the surface of mercury, satisfactory agreement with the 

 values calculated from the Gibbs formula. Donnan and Barker (1911, p. 573) obtained similar 

 results in the cases of nonylic (pelargonic) acid, and of saponin at the interface between water 

 and air. Nonylic acid has an extraordinarily high capacity of lowering surface tension. 



Condensation of substances at the interface between their solutions and air 

 shows itself in an interesting way in the experiments of Ramsden (1904). 

 Certain substances, of which a list will be found in the original paper, such 

 as white of egg, saponin, and quinine, are actually deposited in a solid form, 

 so that the surface film of the solution becomes rigid. 



