per unit area which can be obtained as the derivative of
with respect to the distance, where 
is the total Gibbs energy of interaction per unit area of each of the two flat and parallel plates. Source:
PAC, 1972, 31, 577
(Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry)
on page 616