Dielectric constant and density of water as a function of pressure at constant temperature

In order to simulate the effects of hydrostatic pressure on protein folding/unfolding it is necessary to accurately describe the behavior of the dielectric constant and the density of the solvent (water), in the range of pressures (between 0.1 MPa and 2.0 GPa) and temperatures (below 75 degreesC) required for pressure-induced unfolding. A simple equation of the form X = X (T, P-i) + a(0) ln (a(i) + P-i)/(a(i) + Pi) [were X is the property, P-i (in MPa) is the reference pressure and ai are coefficients adjusted to fit experimental values] is proposed to describe both properties as function of pressure, at constant temperatures. The equation reproduces available data for dielectric constant and density of water to an accuracy of 0.1%. Because of its simplicity and accuracy, the proposed equation is useful for simulation studies and for any other problem where the knowledge of those properties as a function of pressure is needed.