Molecular and macroscopic aspects of cavity formation in the hydrophobic effect

Abstract

The formation of a cavity in water is a conceptual preliminary to the dissolution of a nonpolar solute. The process is of biophysical importance because of its assumed primitive relation to hydrophobic factors affecting biological structure and function. In this paper, cavity formation by the isochoric deformation of a fluid specimen is investigated. Using both molecular and macroscopic descriptions of this process, the volume occupied by the fluid is held constant while the spherical specimen is reversibly deformed in differential steps until it surrounds a spherical cavity. The work of cavity formation results from the integration of energy changes generated directly from the forced, differential deformations of the equilibrium fluid structure; the heat of cavity formation is the integrated result of energy changes that accompany the nonforced, differential adjustments in the distribution of matter which modulate the average fluid structure during the process. This reversible isochoric deformation procedure for introducing the nonpolar system into the polar fluid is compared with the more familiar method that uses a reversible coupling parameter. Simple calculations based on the isochoric deformation method provide estimates of the free energy, internal energy, heat and entropy of cavity formation that agree with values from the scaled particle method.