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A combined streaming-potential optical reflectometer for studying adsorption at the water/solid surface
O Theodoly1, L Cascão-Pereira, V Bergeron
1Chemical Engineering Department, University of California, Berkeley, California 94720-1462, USA.
Langmuir : the ACS Journal of Surfaces and Colloids
|October 19, 2005
Summary
A new streaming-potential optical reflectometry (SPOR) apparatus measures adsorbate molecular structure and zeta potential at solid surfaces. SPOR reveals bilayer formation and charge reversal for surfactants and polyelectrolytes on silica.
Area of Science:
- Surface Science
- Physical Chemistry
- Materials Science
Background:
- Understanding molecular adsorption at solid-liquid interfaces is crucial for various applications.
- Existing methods often lack the ability to simultaneously measure adsorption density and surface charge.
- Characterizing the structure and electrokinetic properties of adsorbates requires advanced in-situ techniques.
Purpose of the Study:
- To develop and validate a novel in-situ streaming-potential optical reflectometry (SPOR) apparatus.
- To investigate the molecular architecture and electrokinetic behavior of aqueous adsorbates on silica.
- To study the adsorption of nonionic surfactants, cationic surfactants, and cationic polyelectrolytes.
Main Methods:
- Construction and utilization of a streaming-potential optical reflectometry (SPOR) apparatus.
- Simultaneous measurement of adsorption density (gamma) and zeta potential (zeta) in a flow cell.
- Dynamic and equilibrium measurements to study sorption kinetics and reversibility.
Main Results:
- SPOR successfully probed the molecular architecture of aqueous adsorbates on a negatively charged silica surface.
- Bilayer adsorption was observed for nonionic surfactant C12E5 above its critical micelle concentration (cmc).
- Cationic surfactant CTAB and cationic polyelectrolytes (PDAEMA, MAPTAC) showed strong surface charge reversal, with thin, flat adsorbed layers for polymers.
- Adsorption of C12E5 and CTAB was reversible, while polymer adsorption was irreversible.
- Gouy-Chapman theory analysis indicated low ionization fractions for adsorbed cationic species.
Conclusions:
- The developed SPOR apparatus is a powerful new tool for in-situ characterization of adsorbate molecular structure.
- SPOR provides insights into bilayer formation, charge reversal, and ionization states of adsorbed molecules.
- The findings advance the understanding of interfacial phenomena involving surfactants and polyelectrolytes on charged surfaces.