The dielectric function for water and its application to van der Waals forces

The dielectric response, epsilon>(*) over bar * (i xi), for water (which is required in Lifshitz theory to calculate the van der Waals interactions in aqueous systems) is commonly constructed, in the absence of complete spectral data, by fitting a damped-harmonic-oscillator model to absorption data. Two sets of parameters for the model have been developed corresponding to different constraints: Parsegian and Weiss (J. Colloid Interface Sci., 1981, 81, 285) and Roth and Lenhoff (J. Colloid Interface Sci., 1996, 179, 637). These different representations of the dielectric response lead to significant differences in the van der Waals force calculated from Lifshitz theory. In this work, more recent and complete spectral data for water were compiled from the literature and direct integration of the Kramers-Kronig relations was used to construct a new epsilon>(*) over bar * (i xi) for water at 298 degreesK. This approach also allows a number of different types of spectral measurements (such as infrared spectroscopy, microwave resonance techniques, and x-ray inelastic scattering) in the compilation of absorption data over a large frequency range (on the order of 8 to 10 decades in frequency). A Kramers-Kronig integration was employed to construct the real and imaginary parts of epsilon>(*) over bar *(omega), epsilon'(omega), and epsilon double prime>(*) over bar *(omega) for water from the different spectral measurements before calculation of epsilon>(*) over bar * (i xi) from its integral definition. The resulting new epsilon>(*) over bar * (i xi) is intermediate between the Parsegian-Weiss and Roth-Lenhoff representations of epsilon>(*) over bar * (i xi), does not use a model, and treats the conversion of absorption data as rigorously as possible. We believe the epsilon>(*) over bar * (i xi) from the present work is the most reliable construction for use in van der Waals force calculations using Lifshitz theory. The extension of the epsilon>(*) over bar * (i xi) construction to other temperatures is also discussed. (C) 2000 Academic Press.