Sur l’équilibre d’adsorption en surface des solutions aqueuses de corps aliphatiques

II. — Adsorption en phase superficielle réelle (corps non dissociés)

(Laboratoire de Biologie physico-chimique, Université des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex), France.

Résumé

On étend, à l’adsorption d’un corps tensioactif non dissocié en couche monomoléculaire réelle à la surface de ses solutions, des conceptions antérieures relatives à l'adsorption en couche idéale. L’introduction de l’activité superficielle permet d’exprimer l’équilibre d’adsorption par une relation qui conduit par différentiation à la relation de Gibbs. A titre d'exemple, on établit une courbe d’absorption calculée, à partir d’une équation d’état de la couche superficielle; on étudie sur cet exemple les particularités de la courbe.

Abstract

The present paper is related to monolayer adsorption of an undissociated tensioactive substance from its solution; the author extends his previous conceptions to real layer adsorption (thickness E of a tridimensional homogeneous surface phase thermodynamically equivalent to an ideal adsorbed layer, and immersion work Wim of an adsorbed molecule into a very dilute solution). Being a the molecular area in the adsorbed layer, p the surface pressure exerted by this layer (taken as equal to the difference between the surface tension of the solvent and the equilibrium surface tension of the solution), surface activity a_sup is defined, so that kT d ln a_sup = σ dp (a_sup tending to δ = 1/σ at large surface dilutions); then the equilibrium between a real adsorbed layer and a real solution can be expressed by a non-differential relation. By differentiating this, the classical simple form of Gibbs relation is deduced.

As an exemple, the calculated adsorption curve p (a_sol) is established for a fictive substance, from a very simple state relation of the adsorption layer and the value of the slope at the origin of the adsorption curve (the parameters are arbitrarly choosen in view of a certain similarity in surface properties between this fictive substance and dodecanoic acid), Using this exemple, particularities of the adsorption curve are studied.