In surface pressure vs. headgroup area isotherms are typically measured. How is it possible that in the condensed phase, the average surfactant area doesn't change but the surface pressure increases? Does the increased surface pressure mean more surfactant populate the interface, the headgroups are rearranging, or something else?
Konnor K Jones
Indeed, the area of the hydrophilic group does not change. It has a certain structure. When surfactant molecules are arranged over a large area, they lie almost horizontally at the interface. Such a monomolecular layer has the properties of a gas. Then, with an increase in surface pressure, the surfactant molecules approach and the film becomes liquid, and finally, when the area per molecule is equal to the area of the hydrophilic group, the monolayer will have the properties of a crystalline body with close packing of molecules.
Thank you Yuri Mirgorod for the explanation--that makes sense. Can you also explain why the surface pressure can increase even when the area per molecule is equal to the area of the hydrophilic group? An example is the solid state (s) that is depicted in the figures at https://www.biolinscientific.com/blog/what-is-surface-pressure-area-isotherm
Konnor K Jones
Transition S>L2 transition from solid to liquid state. The area of the hydrophilic group is obtained from the area per molecule when the film is in the solid state. In the liquid state, there is a translational movement that slightly increases the area of the hydrophilic group.
Focusing on the solid state only (the region after the S>L2 transition and before the monolayer collapses/the plateau), why does the surface pressure increase even though the area per molecule does not change? I do not understand how the surface pressure (free energy of the interface) can change if the molecules do not pack more tightly.
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