Since the electrolyte solution is ultra-dilute, 𝑐 0 = 0.1 mM, the ion size effect is unimportant near the pzc as the 𝐶 dl curves of the full model and the model without ion size effect s overlap in the region of |𝐸 SHE -𝐸 pzc,SHE 0 | < 0.2 V. As 𝜎 free grows in magnitude and the EDL becomes more crowded with counterions, the ion size effect cannot be neglected any longer. Without considering the ion size effect, the two peaks are much more elevated. The 𝐶 dl curve becomes highly asymmetric with the left peak being much higher than the right one for 𝛿𝜇 ̃= -1. A negative 𝛿𝜇 ̃ means that in the absence of electrostatic interactions, water molecules prefer the configuration of α w = 0° (the O-down or H-up configurations). Therefore, these water molecules have a higher capacity to screen negative 𝜎 free , that intends to rotate water molecules towards α w = 180°, than positive 𝜎 free . Consequently, a higher Cdl peak is observed in the negatively charged region. Interestingly, a symmetric 𝐶 dl curve is reobtained for 𝛿𝜇 ̃= -10. For the case of such a very negative 𝛿𝜇 ̃, the orientation of water molecules is dominated by the strong chemical interactions and is no longer tunable by the local electric field. Therefore, the 𝐶 dl curve resembles that of the model without water effects in Figure , except a horizontal shift. KClO4. The model results are calculated for 𝛿𝜇 ̃= -1. Consider first the case of 1 mM. Experimental data show 𝐶 dl peaks near the pzc, 0.04 V vs the saturated calomel electrode (SCE), which is +0.284 V vs the SHE. In agreement with experiments, the model also gives a 𝐶 dl peak near the 0 V referenced to 𝐸 pzc,SHE 0 . This theoretical peak is ascribed to saturation of water porlairzation as discussed previously.