Measurement of Forces between Supported Cationic Bilayers by Colloid Probe Atomic Force Microscopy: Electrolyte...

Area of Science:

• Surface science
• Colloid science
• Physical chemistry

Background:

• Cationic surfactant bilayers are crucial in various applications.
• Understanding their interactions is key to controlling material properties.
• The influence of electrolytes on these interactions requires detailed investigation.

Purpose of the Study:

• To measure interactions between supported cationic surfactant bilayers.
• To investigate the effect of different halide salts (NaCl, NaBr, NaI, CaCl2) on these interactions.
• To compare experimental results with theoretical models like DLVO theory.

Main Methods:

• Fabrication of Di(alkylisopropylester)dimethylammonium methylsulfate (DIPEDMAMS) bilayers on muscovite mica via vesicle fusion.
• Measurement of bilayer interactions using colloidal probe atomic force spectroscopy.
• Systematic variation of electrolyte concentrations (NaCl, NaBr, NaI, CaCl2).

Main Results:

• Bilayer interactions were repulsive across all tested NaCl concentrations.
• Debye length and surface potential decreased with increasing salt concentration.
• Electrical double layer (EDL) theory accurately described the interactions, with no detected van der Waals forces but a significant hydration repulsion at short separations.
• CaCl2 showed effects similar to NaCl.
• NaBr and NaI were more effective than chloride salts in reducing surface potential, with efficacy increasing with ionic radius.

Conclusions:

• Supported cationic surfactant bilayers display repulsive interactions governed by EDL forces.
• Electrolyte type and concentration modulate bilayer interactions, with halide efficacy correlating with ionic radius.
• Hydration forces play a significant role at close proximity, complementing EDL contributions.