Preparation, Microstructure, and Thermodynamic Properties of Homogeneous and Heterogeneous Compound Monolayers of Polymerized and Monomeric Surfactants on the Air/Water Interface and on Solid Substrates

W. Frey, J. Schneider, H. Ringsdorf, E. Sackmann

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Abstract

Monolayers of amphiphilic copolymers consisting of two-chain surfactants bridged by hydrophilic chains of various lengths as well as of mixtures of these macrolipids with phospholipids are studied and discussed in terms of the de Gennes-Alexander scaling laws of adsorbed macromolecules. The thermodynamic properties are studied by film balance experiments, and the microstructure is studied by a recently developed phase contrast electron microscopy technique and by electron diffraction. It is demonstrated that heterogeneously organized monolayers of coexisting (fluid or solid) phases of monomeric and polymerized lipid may be transferred from the air/water interface onto solid substrates without structural alterations. Copolymers with long hydrophilic chains exhibit an entropy-driven transition from a two-dimensional expanded into a condensed state with chains confined to a cylinder as predicted by Alexander. The monomer adsorption energy is -0.1 kT to -O.&kT. The two-dimensional radius of the chains scales as N3/4(N - monomer number) as predicted. Monolayers of the copolymers with long hydrophilic chains swell readily by incorporation of monomeric lipid. The swelling and its saturation behavior are explained in terms of a balance between the entropy of mixing and the entropic elastic free energy associated with the expansion of the hydrophilic chain. The two-dimensional foamlike state of monolayers (gas/fluid coexistence) at low densities is explained in the scaling approach as a counteraction of (1) a hydrophobic repulsion of the hydrocarbon chains from the water surface (repulsion energies δ≈ +1 kT for CH2), (2) the chain-chain attraction arising since air is a poor solvent, and (3) the translational entropy. The interesting analogy between the swollen copolymer and the two-dimensional networks (the cytoskeleton) of spectrin which is coupled to the inner monolayer of the red cell envelope is pointed out.

Original languageEnglish
Pages (from-to)1312-1321
Number of pages10
JournalMacromolecules
Volume20
Issue number6
DOIs
StatePublished - 1 Nov 1987

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