Intercepts which characterize water

Until recently, the characterization of the monolayer phases was based mainly on the shapes of the isotherms—the compressibility in each one-phase region and the packing densities of each phase, which can be taken as the zero-pressure intercepts of the steep portions of the isotherms. Since the head groups are constrained to be on the water surface and the van der Waals radii of the chains are known, these features of the isotherm can be matched to simple models for the monolayer phases. 

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