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Publisher Summary The chapter describes water in two different ways. The first way is to see water as a structural part of the molecular assembly itself. One part of this aqueous structure is made up of water that is energetically associated with the polar parts of the lipid molecules, as described previously and loosely called “water of hydration.” Another part is water that is required to fill space defined by the lipid assemblies. The latter is water that allows lipid bilayers to separate widely under electrostatic repulsion, or for the lipid monolayer of the reverse hexagonal phase to form large radii of curvature. Otherwise this water is unperturbed by, and some distance from, polar surfaces. The way water is used to probe the internal energies of the structures and dimensions of the assembled lipid is also discussed in the chapter. Whether or not direct hydration of polar groups may be contributing to the internal energetics of the system, by regulating water activity through osmotic stress, one can do osmotic work on the structure and determine something about its internal energetics. Two well-documented examples of using osmotic stress to measure properties other than direct hydration energy itself are the measures of electrostatic interactions between lipid bilayers and of the energy required to change the curvature of phospholipid monolayers. |
