SURFACE ACTION 6r 



VELOCITY OF ADSORPTION 



There is every reason to suppose that, when a substance reaches the surface 

 at which it is adsorbed, the actual process of attachment itself is of very great 

 rapidity. The difference between static and dynamic surface tension, referred to 

 on p. 56 above, shows, however, that the rate of surface concentration is not 

 absolutely instantaneous. Although this is the case, it is clear that, when an 

 obvious interval of time is observed to elapse in an adsorption experiment before 

 equilibrium is attained, in many cases several hours, what is being measured is 

 the time taken for the substance to diffuse from the more distant parts of the 

 solution to the adsorbing surface. As would be expected, it is found that the 

 time taken for attainment of equilibrium is shortened by shaking. 



EFFECT OF TEMPERATURE ON ADSORPTION 



The effect of temperature on the rate of adsorption, in accordance with the 

 previous paragraph, is found to be of the same order as that which it has on 

 diffusion processes. In the case of Congo-red and filter paper, my experiments 

 (1906, p. 188) showed the coefficient to be 1'36 for 10 C. Brunner (1904, 

 p. 62) 'found that for the diffusion of benzoic acid to be 1'5. 



Although the rate of adsorption is increased by rise of temperature, in 

 agreement with the usual rule, the amount adsorbed when equilibrium is 

 reached is diminished. Heat dissociates an adsorption compound. This fact 

 is familiar in the case of charcoal, where the gas adsorbed at a low temperature 

 is given off again on heating. In the case of Congo-red, my experiments showed 

 that the amount taken up was in inverse linear proportion to the temperature 

 (1906, p. 190). When the temperature was raised to 100 C., the dye was 

 fixed in the paper and could not be removed by washing. Chemical combina- 

 tion appears to take place, and also goes on very slowly at ordinary temperatures. 

 This fact will be referred to again below. 



The decrease of adsorption by rise of temperature is, no doubt, to be 

 explained by the fact that surface energy itself is anomalous in having a negative 

 temperature coefficient. The surface tension of a particular sample of tap water 

 was found to be 73'8 dynes at 17 and 65 dynes at 60. Lactic acid in 44 per 

 cent, solution at 18 has a value of 50'5 dynes, and of 47 dynes at 67. In 

 accordance with these data, it was found that 2 grams of charcoal at adsorbed 

 51 per cent, of the lactic acid from 20 c.c. of 0'71 per cent, solution, but only 42 

 per cent, at 40. If we consider the surface tension at the interface between a 

 liquid and its vapour, we see that it must vanish at the critical temperature, since 

 the boundary surface disappears. Hence, we should expect that the surface 

 tension would decrease as the temperature rises towards this critical point. 



The physiological significance of the fact may be illustrated by the case of 

 muscular contraction, whose strength is diminished by rise of temperature. 

 Weizsilcker (1914) has shown experimentally that one of the components of the 

 process has itself a negative temperature coefficient. The conclusion rnay be 

 drawn that surface energy plays an important part in muscular contraction. 



HEAT OF ADSORPTION 



Since adsorption is decreased by rise of temperature, the van't Hoff principle 

 of mobile equilibrium implies that it takes place with evolution of heat. This 

 is easy to detect in the case of gases, as we have seen ; in that of liquids and 

 solids it is difficult to distinguish it from the heat of liquefaction or of dilution, etc. 



THE ADSORPTION FORMULA 



One of the most characteristic properties of an adsorption process is that 

 the amount taken up is not in direct linear relationship to the concentration 

 of the adsorbed substance in the solution in equilibrium with the surface. 

 Suppose a is the amount adsorbed from a certain solution, then the amount 

 adsorbed from a solution of twice the concentration will not be a x 2, but 



