One usually thinks of cell membrane lipids in the form of the lamellar sheets in which they are originally found. It appears from our recent work that the addition of relatively small amounts of non-polar hydrocarbons can create non-lamellar arrangements similar to those that occur during such membrane fusion processes as cellular secretion. We have determined the chemical potential of phospholipids in the inverted hexagonal structure finding that the lipid hydrocarbon chains pack to fill space with negligible strain. Measuring the work of bending lipid layers we have found that their bending elasticity modulus is essentially the same for planar structures as for layers with small spontaneous radii of curvature. In these lipid systems we find that the work of bringing bodies together is the hydration force of removing structured water solvent from their surfaces. We have been able to modify forces between DNA macromolecules by altering the entropy of water in the bathing solution. To do this we have used different anionic solutes--""""""""chaotropic"""""""" perchlorate, chloride, and order-forming sulphate--to change the entropy of the bathing medium. We have found that relatively small changes in the ionic composition of the bathing solution change the properties of water solvent enough to control molecular assembly. These experimental findings are tightly coupled to theoretical analyses to formulate and to compute interactions. We have evaluated the accuracy of the well-known Derjagin approximation for converting the interaction between curved surfaces into that between planar surfaces of the same material.
