Surface-tension suppression of lamellar swelling on solid substrates

Multilayers of charged lipids immersed in distilled water swell to whatever dilution allowed them by the available volume of solvent. Multilayers of the same lipids on a solid surface will imbibe only a small amount of water from a vapor, even from a 100% relative-humidity vapor. We argue that the essential difference is in the extra work needed to create a vapor/multilayer interface. For stiff tightly packed multilayers, this interfacial energy is an additive constant of little consequence. But, in liquid water, multilayers swell to a softness where thermal excitation creates a rippled surface; the surface energy goes as the contour area not as the flat area of projected surface. The contour area grows as the multilayer swells. Vapor/liquid surface tension creates a "hard" surface that quells ripples and, usually, pulls the multilayer back to tighter packing. Tension can act to enhance direct attractive bilayer-bilayer forces such as weak van der Waals interactions to create new energy minima at very close spacings. These new energy minima might explain the limited swelling of charged lipids on substrates. What is remarkable is the long range and the strength of surface-tension perturbation on layered systems. In our statistical-thermodynamic formulation, bilayer motion is treated as the sum of undulations or waves that can pervade the entire multilayer. Disturbance of the surface can reach inward to distances comparable to the lateral extent of the multilayers. We use measured osmotic compressibility and bending rigidity to reveal the qualitative difference in affinity for water of multilayers in liquids and those on substrates exposed to vapors of the same chemical potential. (C) 1997 Elsevier Science B.V.