PHYSICAL CHEMISTRY IN BRIEF

CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 446

Obr.13.3:Substance concentration in the vicinity of an interface.

Note:At a liquid-vapour interface, a positive value of Γi, 1 indicates that the amount-of-

substance ratio between the dissolved substanceiand the solvent in the surface layer is

higher than in the liquid phase. The dissolved substance thus concentrates on the liquid

surface. If (Γi, 1 = 0, the concentration ratio of both substances in the surface and liquid

phases is identical.

The relation between relative adsorption and surface tension is called theGibbs adsorp-
tion isotherm

Γi, 1 =−

(

∂γ

∂μi

)

T

. (13.27)

Using relations (6.71) and (6.101), we may express this equation by the relation^2

(

∂γ

∂ci

)

T

=−

RT

ci

Γi, 1



1 +ci



∂lnγ

[c]

i

∂ci





T



. (13.28)

(^2) We have to distinguish between the surface tension,γ, and the activity coefficient of componenti,γ[c]
i.