Visualizing monolayers with a water-soluble fluorophore to quantify adsorption, desorption, and the double layer

Area of Science:

• Surface science
• Physical chemistry
• Biophysics

Background:

• Phase separation in monolayers at interfaces is crucial for biological and material systems.
• Confocal microscopy offers high resolution but requires contrast enhancement for interfacial studies.
• Selective dye adsorption is a potential method for visualizing these structures.

Purpose of the Study:

• To develop and validate a method for visualizing phase-separated monolayers at the air-water interface using confocal microscopy.
• To quantitatively assess dye adsorption and its relation to monolayer properties.
• To measure lipid headgroup dissociation and surface potential non-invasively.

Main Methods:

• Confocal microscopy with optical sectioning to minimize subphase signal.
• Quantitative assessment of excess dye concentration using point spread function convolution.
• Selective adsorption of water-soluble fluorescent dyes to differentiate monolayer phases.
• Varying subphase ionic strength to study electrostatic interactions.

Main Results:

• Differential dye adsorption allowed visualization of coexisting liquid-expanded, liquid-condensed, and gas phases.
• Dye adsorption patterns reflected liquid-disordered phase behavior during phase coexistence and through the critical point.
• Quantified excess cationic dye concentration correlated with Gouy-Chapman predictions, enabling measurement of lipid headgroup dissociation and surface potential.

Conclusions:

• Selective fluorescent dye adsorption provides a rapid, non-invasive optical method for characterizing phase-separated monolayers at the air-water interface.
• The technique allows for the determination of lipid headgroup fractional dissociation and monolayer surface potential.
• This approach enhances the understanding of interfacial phenomena in complex systems.