Thermodynamic implications of confinement for a waterlike fluid

A theoretical framework is introduced for studying the thermodynamics and phase behavior of a “waterlike” fluid film confined between hydrophobic plane surfaces. To describe the hydrogen-bonding interactions in the fluid film, an earlier analytical theory for uniform associating fluids is generalized. Two levels of approximation are presented. In the first, the reference fluid is assumed to be homogeneous. Here, the primary effect of the confining walls is to reduce the average number of favorable fluid–fluid interactions relative to the bulk fluid. The implications of this energetic penalty for the phase behavior and, in particular, the low-temperature waterlike anomalies of the fluid are examined. It is shown that the reduction of favorable fluid–fluid interactions can promote strong hydrophobic interactions between the confining surfaces at nanometer length scales, induced by the evaporation of the fluid film. In the second level of approximation, the inhomogeneous nature of the reference fluid is accounted for by a density functional theory. The primary effect of the density modulations is to promote or disrupt hydrogen bonding in distinct layers within the pore. Interestingly, when the reference fluid is treated as inhomogeneous, the theory predicts the possibility of a new low-temperature phase transition in the strongly confined fluid.