Evidence for a second contact value formula for the electric double layer

The results of some recent Monte Carlo simulations (Bhuiyan et al. J. Electroanal. Chem., in press) for the electric double layer formed by rigid ions of equal diameter (the solute) in a continuum dielectric (the solvent), next to a uniformly charged planar hard wall are examined. A contact value formula for the sum of the density profiles of this system has long been known (Henderson et al. J. Electroanal. Chem. 102, 315 (1979)). Here we propose a simple formula for the contact value of the charge profile in the limit of low electrode charge density. The evidence obtained from the aforementioned simulations supports this formula.