title: Visualizing monolayers with a water-soluble fluorophore to quantify adsorption, desorption, and the double layer.
abstract: Contrast in confocal microscopy of phase-separated monolayers at the air-water interface can be generated by the selective adsorption of water-soluble fluorescent dyes to disordered monolayer phases. Optical sectioning minimizes the fluorescence signal from the subphase, whereas convolution of the measured point spread function with a simple box model of the interface provides quantitative assessment of the excess dye concentration associated with the monolayer. Coexisting liquid-expanded, liquid-condensed, and gas phases could be visualized due to differential dye adsorption in the liquid-expanded and gas phases. Dye preferentially adsorbed to the liquid-disordered phase during immiscible liquid-liquid phase coexistence, and the contrast persisted through the critical point as shown by characteristic circle-to-stripe shape transitions. The measured dye concentration in the disordered phase depended on the phase composition and surface pressure, and the dye was expelled from the film at the end of coexistence. The excess concentration of a cationic dye within the double layer adjacent to an anionic phospholipid monolayer was quantified as a function of subphase ionic strength, and the changes in measured excess agreed with those predicted by the mean-field Gouy-Chapman equations. This provided a rapid and noninvasive optical method of measuring the fractional dissociation of lipid headgroups and the monolayer surface potential.
authors: Department of Chemical Engineering, University of California, Santa Barbara, CA 93106; and.
authors: Ian C
authors: IC
authors: Shieh
authors: Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 zasad008@umn.edu.
authors: Joseph A
authors: JA
authors: Zasadzinski
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D000327:Adsorption; D005453:Fluorescence; D005456:Fluorescent Dyes / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009994:Osmolar Concentration; D044367:Phase Transition; D010713:Phosphatidylcholines / Q000737:chemistry; D020112:Rhodamine 123 / Q000737:chemistry; D012995:Solubility; D013499:Surface Properties; D013696:Temperature; D014867:Water / Q000737:chemistry*; D014966:Xanthenes / Q000737:chemistry
chemical_list: D005456:Fluorescent Dyes; D010713:Phosphatidylcholines; D014966:Xanthenes; D014867:Water; D020112:Rhodamine 123; D015060:1,2-Dipalmitoylphosphatidylcholine; C034657:Texas red; C028694:1-palmitoyl-2-oleoylphosphatidylcholine
keywords: lipid domains; lung surfactants; phase behavior; surface potential