2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/101798A system of three electrolytes separated by two parallel planes is considered. Each region is described by a dielectric constant and a Coulomb fluid in the Debye-Hückel regime. In their book Dispersion Forces, Mahanty and Ninham have given the van der Waals free energy of this system. We rederive this free energy by a different method, using linear response theory and the electrostatic Maxwell stress tensor for obtaining the dispersion force.7 pages. PACS numbers updated. References updatedSoft Condensed Mattertext