Here, 𝑦 " is the signal intensity offset, the SHG amplitude, A, corresponds to the SHG signal, Esig, f is the periodicity of our spectrometer, x is the position on the translational stage, and the SHG phase, 𝜑 &$' , is obtained from 𝜑 #$% . Both A and 𝜑 #$% are obtained by fitting eqn. 1 to the SHG interference fringes. The 𝜑 #$% obtained at 2 µM pH 5.7 is set to 60 o ± 1 o , which is the previously determined phase difference between that ionic solution condition and the one obtained for 500 mM NaCl at pH 2.5, 20 the measured point of zero charge (PZC) of silica, where the Coulombic contribution to the total interfacial potential is zero. The SHG phase, 𝜑 &$' , at 2 µM and pH 5.7 is then +60 o ± 1 o .