Adsorption at liquid interfaces: The generalized langmuir isotherm and interfacial structure

Vladislav S. Markin, Maia I. Volkova-Gugeshashvili, Alexander G. Volkov

Research output: Contribution to journalArticle

22 Citations (Scopus)

Abstract

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Langmuir, Henry) were based on the model of nonpenetrable interface, where an adsorbate can substitute only molecules of one solvent. However, at the interface between two immiscible electrolytes, like nonpolar oil-water interfaces, or liquid membrane amphiphilic molecules can substitute molecules of both solvents; therefore, classical isotherms are not applicable in these cases. The generalization of Langmuir and Frumkin isotherms for permeable and nonpermeable interfaces, known as the Markin-Volkov (MV) isotherm, gives the possibility to analyze adsorption and the interfacial structure in a general case. In the present paper, the adsorption isotherms of pentafluorobenzoic acid at the octane-water interface at various pH were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at the octane-water interface were found. From the measurements of PFBA adsorption, the structure of the octane-water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk solution. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane-water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region.

Original languageEnglish (US)
Pages (from-to)11415-11420
Number of pages6
JournalJournal of Physical Chemistry B
Volume110
Issue number23
DOIs
StatePublished - Jun 15 2006

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Isotherms
isotherms
Adsorption
Molecules
adsorption
Liquids
octanes
liquids
Acids
molecules
acids
Water
Thermodynamics
water
substitutes
thermodynamics
Liquid membranes
Gibbs free energy
Adsorbates
pentafluorobenzoic acid

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Adsorption at liquid interfaces : The generalized langmuir isotherm and interfacial structure. / Markin, Vladislav S.; Volkova-Gugeshashvili, Maia I.; Volkov, Alexander G.

In: Journal of Physical Chemistry B, Vol. 110, No. 23, 15.06.2006, p. 11415-11420.

Research output: Contribution to journalArticle

Markin, Vladislav S. ; Volkova-Gugeshashvili, Maia I. ; Volkov, Alexander G. / Adsorption at liquid interfaces : The generalized langmuir isotherm and interfacial structure. In: Journal of Physical Chemistry B. 2006 ; Vol. 110, No. 23. pp. 11415-11420.
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