Here, 𝜒 (+) is the second-order nonlinear susceptibility of the interface, which is given by the sum of the second-order nonlinear susceptibilities of the interfacial species (in order of abundance, these are interfacial water molecules, then the surface silanol groups, and then the anions and cations adsorbed to the protonated and deprotonated surface silanol groups, respectively, vide infra). The third-order contribution in eqn. 2 is multiplied into the total interfacial potential, F(0)tot, which includes all electrostatic contributions (Coulomb, dipole, and multipolar potentials, vide infra). The third-order term is dominated by the third-order contribution from the water molecules in the EDL, which is given by 𝜒 -.%/0