Thermodynamic implications of confinement for a waterlike fluid

A theoretical framework is introduced for studying the thermodynamics and p hase behavior of a "waterlike" fluid film confined between hydrophobic plan e surfaces. To describe the hydrogen-bonding interactions in the fluid film , an earlier analytical theory for uniform associating fluids is generalize d. Two levels of approximation are presented. In the first, the reference f luid is assumed to be homogeneous. Here, the primary effect of the confinin g walls is to reduce the average number of favorable fluid-fluid interactio ns relative to the bulk fluid. The implications of this energetic penalty f or the phase behavior and, in particular, the low-temperature waterlike ano malies of the fluid are examined. It is shown that the reduction of favorab le fluid-fluid interactions can promote strong hydrophobic interactions bet ween the confining surfaces at nanometer length scales, induced by the evap oration of the fluid film. In the second level of approximation, the inhomo geneous nature of the reference fluid is accounted for by a density functio nal 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 c onfined fluid. (C) 2001 American Institute of Physics.