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B.—CHEMISTRY. 73 
practically insoluble in another liquid B, is found to have, in very dilute 
solutions, a strong effect in lowering the tension at the interface A-B, 
the following interesting questions arise :— 
(1) What is the amount of the surface concentration or adsorption 
per sq. cm. of interface ? 
(2) Can it be calculated by means of the simplified Gibbs equation ? 
3) How does the surface adsorption vary with the concentration ? 
(4) Does the ‘ saturation’ value correspond to the formation of a 
unimolecular layer ? 
Some of these questions were experimentally investigated in my 
laboratory by W. C. McC. Lewis. For the liquid A water was chosen, 
and for B a neutral hydrocarbon oil. Working with sodium glycocholate 
as the surface-active subtance, it was found that the experimentally 
measured surface adsorption g was much greater than that calculated by 
means of the equation 
For example, a 0.2 per cent. aqueous solution at 16° C. gave a directly 
measured value of g=5x10-° grm. per sq. cm., whilst the calculated 
value was 5 x 10-* grm. per sq. cm., practically a hundred times smaller. 
A similar type of discrepancy was found in the cases of Congo Red and 
methyl orange. If we calculate from the experimentally found surface 
adsorption of sodium glycocholate the value of the surface area per mole- 
cule, we obtain about 0.9x10-"* sq. cm. A similar calculation in 
the case of Congo Red gives a correspondingly low figure. Now 
if we compare these values with those previously obtained for the air- 
liquid surface, it is clear that in the present case we are not dealing with 
simple unimolecular layers, but with adsorption layers or films many 
molecules thick. On the other hand, if we calculate from Lewis’ 
results the surface area per molecule as deduced from the surface 
tension measurements by the simplified Gibbs formula, we arrive at 
values of the order of 90 x 10-'* (sodium glycocholate) and 100 x 107-** 
(Congo Red). These are values which are consistent with the gradual 
building up of a unimolecular layer (of possibly heavily hydrated mole- 
cules or micelles). It is possible, therefore, that the Gibbs equation 
gives the surface concentration of the primary unimolecular ‘two 
dimensional ’ surface phase, and that any building up of further con- 
centrations beyond this layer does not affect the surface tension. It is 
true that in the case of substances such as sodium glycocholate, and 
especially Congo Red, in aqueous solution, there is a considerable amount 
of uncertainty as to the nature and molecular weight of these sub- 
stances as they exist, not only in the bulk of the solution, but especially 
in the surface phase. In a later investigation Lewis determined the 
‘surface adsorption of aniline at the interface mercury-aqueous alcoholic 
solution, and found in this case a very fair agreement between the 
observed and calculated results. This case is more favourable, since 
we can be in little doubt concerning the molecular weight of the solute 
units. The mean observed value for the surface adsorption was 
