SURFACE PHASE-TRANSITIONS AND HYDRATION FORCES

The interplay between surface phase transitions and hydration forces (considered to be important in multilamellar lipid systems, DNA assemblies, and proteins) is studied using a mean-field Landau theory with two coupled order parameters. The latter represent water ordering at two surfaces separated by a thin water film. A complete analytical solution gives a phase diagram in the temperature/surface-separation plane, phase trajectories, and force-distance curves. The phase diagram shows nontrivial effects on surface phase transitions from hydration interaction between the surfaces. For example, these are second- and first-order transitions at finite separation, which are absent at isolated surfaces (infinite separation). The force-distance diagram displays correspondingly drastic effects of the phase transitions on hydration forces. This is a spike in the force at the second-order transition and a jump crossover from repulsion to attraction at the first-order transition.