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Ions Slow Water Dynamics at Nonionic Surfactant Interfaces.
Christopher P Baryiames1, Emily Ma2, Carlos R Baiz1
1Department of Chemistry, University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United States.
The Journal of Physical Chemistry. B
|December 16, 2020
Summary
Aqueous ions significantly slow water network dynamics at nonionic surfactant interfaces, contrary to common assumptions. This occurs due to ion-induced disorder in surfactant headgroups, altering interfacial water interactions.
Area of Science:
- Physical Chemistry
- Surface Science
- Colloid Science
Background:
- Nonionic surfactant interfaces are typically assumed to be unaffected by aqueous ions.
- However, ions can accumulate at interfaces, influencing surfactant behavior and water structure through direct contact or altered hydrogen bonding.
Purpose of the Study:
- To investigate the impact of high-concentration Na+ and Ca2+ ions on interfacial hydrogen bond dynamics.
- To understand how these ions affect the oil-water interface in the presence of sorbitan stearate reverse micelles.
Main Methods:
- Utilized two-dimensional infrared (2D IR) spectroscopy.
- Employed atomistic molecular dynamics (MD) simulations.
Main Results:
- Observed only minor changes in interfacial hydrogen bond populations upon salt addition.
- Detected a significant slowdown (nearly 300%) in interfacial water network dynamics.
- MD simulations revealed that increased disorder in surfactant headgroup orientation and packing density stabilizes water-surfactant hydrogen bonds.
Conclusions:
- Aqueous ions profoundly influence interfacial water dynamics at nonionic surfactant interfaces, despite minimal changes in hydrogen bond populations.
- Ion-induced structural changes in the surfactant layer are key to modulating interfacial water behavior.
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