Hole-mediated stability and permeability of bilayers

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Abstract
Membrane, foam and emulsion bilayers are considered from a unified point of view as two-dimensional phases and their stability with respect to rupture and permeability with regard to molecular species is analysed with the help of the fluctuation theory of nucleation of holes in them. Molecular models of bilayers are described and both the thermodynamics and the kinetics of the ‘condensation’ of the two-dimensional ‘gas’ of vacancies in a bilayer are outlined. It is shown how the bilayer mean lifetime τ, the probability W of observing a foam bilayer in a thicker foam film and the bilayer diffusion permeability coefficient P d depend on the concentration C of monomer amphiphiles in the surrounding solution. The effect of various factors (particularly α-particle irradiation and an externally applied electric field) on τ and P d is also discussed. Experimental techniques for investigating foam films are described and experimental data on τ(C), W(C) and Pd(C) are presented for bilayers of lipids and synthetic surfactants. An agreement between theory and experiment is established and some parameters of the process of hole nucleation are evaluated. Finally, technological and medical applications are briefly discussed, in particular the use of the W(C) dependence of lipid foam bilayers in the development of new diagnostic methods for assessing human foetal lung maturity.

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