SURFACE-TENSION SUPPRESSION OF LAMELLAR SWELLING ON SOLID SUBSTRATES

Multilayers of charged lipids immersed in distilled water swell to wha tever dilution allowed them by the available volume of solvent. Multil ayers of the same lipids on a solid surface will imbibe only a small a mount 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 multila yers, this interfacial energy is an additive constant of little conseq uence. But, in liquid water, multilayers swell to a softness where the rmal excitation creates a rippled surface; the surface energy goes as the contour area not as the flat area of projected surface. The contou r area grows as the multilayer swells. Vapor/liquid surface tension cr eates a ''hard'' surface that quells ripples and, usually, pulls the m ultilayer back to tighter packing. Tension can act to enhance direct a ttractive bilayer-bilayer forces such as weak van der Waals interactio ns to create new energy minima at very close spacings. These new energ y minima might explain the limited swelling of charged lipids on subst rates. What is remarkable is the long range and the strength of surfac e-tension perturbation on layered systems. In our statistical-thermody namic formulation, bilayer motion is treated as the sum of undulations or waves that can pervade the entire multilayer. Disturbance of the s urface can reach inward to distances comparable to the lateral extent of the multilayers. We use measured osmotic compressibility and bendin g 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.