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Forces between glass surfaces in mixed cationic-zwitterionic surfactant systems.
William J Lokar1, William A Ducker
1Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA. wlokar@vt.edu
Langmuir : the ACS Journal of Surfaces and Colloids
|June 23, 2005
Summary
Cationic surfactants significantly influence zwitterionic surfactant adsorption on glass, altering interfacial forces. This interaction, driven by electrostatic and hydrophobic forces, enhances zwitterionic surfactant adsorption in mixtures.
Area of Science:
- Surface science
- Colloid and interface science
- Physical chemistry
Background:
- Understanding surfactant adsorption is crucial for controlling interfacial properties.
- Cationic and zwitterionic surfactants exhibit distinct behaviors in aqueous solutions.
- Interactions between different surfactant types can lead to complex adsorption phenomena.
Purpose of the Study:
- To investigate the adsorption of cationic (dodecylpyridinium chloride, DPC) and zwitterionic (N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, DDAPS) surfactants on borosilicate glass.
- To determine how surfactant mixtures affect interfacial forces and adsorption as a function of interface separation.
- To elucidate the thermodynamic and molecular mechanisms governing these interactions.
Main Methods:
- Atomic force microscopy (AFM) to measure forces between glass surfaces.
- Application of Maxwell relations to determine surfactant adsorption from force measurements.
- Attenuated total internal reflection infrared spectroscopy (ATR-IR) for simultaneous, independent measurement of cationic and zwitterionic surfactant adsorption.
- Thermodynamic analysis to explain observed adsorption behaviors.
Main Results:
- Zwitterionic DDAPS shows minimal adsorption and force influence alone at sub-micellar concentrations.
- Cationic DPC adsorbs significantly and affects forces even at very low concentrations (cmc/300).
- In DPC presence, DDAPS adsorption becomes sensitive to interface separation, and DPC adsorption is influenced by DDAPS.
- Intermolecular coupling between electrostatic and hydrophobic interactions explains the enhanced DDAPS adsorption.
Conclusions:
- Cationic surfactants play a key role in modulating zwitterionic surfactant adsorption and interfacial forces.
- The presence of even small amounts of cationic surfactant dramatically alters the adsorption behavior of zwitterionic surfactants.
- Synergistic interactions between surfactant types are critical for understanding complex interfacial phenomena.