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Comparing Surfactant Structures at "Soft" and "Hard" Hydrophobic Materials: Not All Interfaces Are Equivalent
Zengyi Wei1, Stefania Piantavigna2, Stephen A Holt2
1School of Chemical Engineering , UNSW Sydney , Sydney , NSW 2052 , Australia.
Langmuir : the ACS Journal of Surfaces and Colloids
|July 13, 2018
Summary
Amphiphilic molecules adsorb differently to soft versus hard hydrophobic surfaces. Soft surfaces allow tail penetration, unlike hard surfaces where molecules form aggregates. This impacts understanding of interfacial structures.
Area of Science:
- Surface Science
- Colloid and Interface Science
- Materials Chemistry
Background:
- Amphiphilic molecules, such as sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (C16TAB), are crucial in various applications.
- Understanding their adsorption behavior at different interfaces is key to controlling interfacial properties.
- Previous studies often focused on air/water or hard solid/water interfaces, with limited data on soft hydrophobic interfaces.
Purpose of the Study:
- To investigate the interfacial structures formed by amphiphilic molecules (SDS, C16TAB, and AM1 peptide) at both soft (poly(dimethylsiloxane)/water) and hard (octadecyltrichlorosilane/water) hydrophobic interfaces.
- To compare the adsorption behavior and structural conformations of these molecules across different substrate types.
- To determine if interfacial structures can be predicted solely by the surfactant packing parameter.
Main Methods:
- Neutron reflection (NR) was employed to probe the molecular structure and orientation at the interfaces.
- Quartz crystal microbalance with dissipation (QCM-D) measurements provided information on adsorbed mass and layer properties.
- Comparative studies were conducted using soft siloxane and hard octadecyltrichlorosilane hydrophobic surfaces.
Main Results:
- At the soft siloxane/water interface, small molecular surfactants formed loosely packed layers with significant hydrophobic tail penetration (30±8% for C16TAB, 20±5% for SDS).
- The engineered peptide surfactant AM1 showed penetration of its alpha-helix face into the siloxane layer.
- At the hard octadecyltrichlorosilane/water interface, no tail penetration was observed; C16TAB and SDS adsorbed as aggregates, not monolayers.
Conclusions:
- Interfacial structures of amphiphilic molecules are highly dependent on the nature of the hydrophobic substrate (soft vs. hard).
- The poly(dimethylsiloxane)/water interface acts as a suitable model for oil/water interfaces, allowing hydrophobic tail penetration.
- Interfacial adsorption behavior cannot be solely predicted by the surfactant packing parameter, highlighting the importance of substrate properties.