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Adsorption at liquid interfaces: the generalized Langmuir isotherm and interfacial structure
Vladislav S Markin1, Maia I Volkova-Gugeshashvili, Alexander G Volkov
1Department of Anesthesiology, UT Southwestern Medical Center, University of Texas, Dallas, Texas 75390-9068, USA.
The Journal of Physical Chemistry. B
|June 15, 2006
Summary
This study explores amphiphilic molecule adsorption at oil-water interfaces, developing a generalized isotherm. Pentafluorobenzoic acid adsorption reveals specific interfacial structures and pH-dependent interactions.
Area of Science:
- Physical Chemistry
- Surface Science
- Thermodynamics
Background:
- Classical adsorption isotherms (Frumkin, Langmuir, Henry) assume non-penetrable interfaces, limiting their applicability to systems where adsorbates replace only one solvent.
- Amphiphilic molecules at immiscible liquid interfaces, such as oil-water, can replace solvent molecules from both phases, necessitating more generalized models.
- The Markin-Volkov (MV) isotherm generalizes classical models for both permeable and non-penetrable interfaces, enabling comprehensive analysis of adsorption and interfacial structure.
Purpose of the Study:
- To investigate the adsorption thermodynamics of amphiphilic compounds at the octane-water interface.
- To measure adsorption isotherms of pentafluorobenzoic acid (PFBA) at varying pH.
- To determine interfacial structure and thermodynamic parameters governing PFBA adsorption.
Main Methods:
- Experimental measurement of adsorption isotherms using the drop-weight method.
- Application of the generalized Markin-Volkov (MV) isotherm for data analysis.
- Thermodynamic analysis of adsorption equilibrium and interfacial properties.
Main Results:
- Adsorption isotherms of pentafluorobenzoic acid (PFBA) were successfully measured at the octane-water interface across different pH values.
- Thermodynamic parameters for PFBA adsorption were determined, providing insights into the adsorption process.
- The interfacial structure was elucidated, indicating that approximately three adsorbed PFBA molecules substitute one adsorbed octane molecule, suggesting lateral orientation of octane.
- Adsorption of PFBA is associated with decreased intermolecular attraction as pH shifts towards acidic conditions.
Conclusions:
- The Markin-Volkov (MV) isotherm provides a suitable framework for analyzing adsorption at permeable interfaces, like the octane-water system.
- The study determined the thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption and characterized the octane-water interface structure.
- PFBA adsorption behavior is pH-dependent, influencing intermolecular interactions and interfacial molecular orientation.
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