Equation describing the contraction (ΔVel) taking place in a dielectric medium of relative static permittivity ɛr (formerly called dielectric constant) upon introduction of an ion of charge number z and radius r: \[\Delta V_{\text{el}} = -\frac{\left ( z\,e \right )^{2}}{2\,r\,\varepsilon_{\text{r}}}\frac{\partial\left ( \text{ln}\varepsilon_{\text{r}}\right)}{\partial\,p}\] with e the elementary charge.
Note:
Inasmuch as the derivative of lnɛr with respect to pressure, ∂(lnɛr)/∂p, is not known for all media, there are approximations to evaluate this term as a function of ɛr and of the isothermal compressibility of the medium, κ T.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 327 (https://doi.org/10.1351/pac200779030293)