Figure 4 The osmotic stress method (18). DNA liquid crystals are equilibrated against solutions of a neutral polymer (e.g., PEG or PVP, depicted as disordered coils). These solutions are of known osmotic pressure, pH, temperature and ionic composition (54). Equilibration of DNA under the osmotic stress of external polymer solution is effectively the same as exerting mechanical pressure on the DNA subphase with a piston that passes water and small solutes but not DNA. After equilibration under this known stress, DNA separation is measured either by X-ray scattering, if the DNA subphase is sufficiently ordered, or by densito-metry (55). DNA density and osmotic stress thus determined immediately provide an equation of state (osmotic pressure as a function of the density of the DNA subphase) to be codified in analytical form over an entire phase diagram. See the color insert for a color version of this figure.

determines the absolute magnitude of the hydration repulsion, which can be in the hundreds of atmospheres.

As already noted in this simple theoretical approach, the hydration decay length depends only on the bulk properties of the solvent, and not on the properties of the surface. To generalize this simplification, Kornyshev and Leikin (21) formulated a variant of the hydration force theory to also take into account explicitly the nature of surface ordering. They derive a modified hydration decay length that clearly shows how the surface order couples with the bare hydration decay length. Without going too deeply into this theory, we note that if the interacting surfaces have 2-dimensional ordering patterns characterized by a wave vector Q = 2/X, where X is the characteristic scale of the spatial variations of these patterns, then the effective hydration force decay length would be XKL = y/2 lH (1 + 42(XH/X)2) "1/2. Inserting numbers for luE 8.0

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