Fig. shows the number of adsorbed ions for Na + vs Mg 2+ and Al 3+ when plotting the xaxis as ionic strength as opposed to cation concentration, signaling a detectable ion specific effect, similar to what is observed in the c values shown in Fig. . The Al 3+ surface coverage at the highest concentration used is in good agreement with results from quartz crystal microbalance measurements reported by the Kabengi group (at 1 mM Al(NO3)3 and a pH of ~4, a mass of 18 to 19 ng cm -2 was detected on silica-terminated sensors, which corresponds to 1.5 x 10 13 Al 3+ ions cm -2 if all the mass detected is due to the metal cation). The total potential, Ftot, contains the contributions from the mobile charges (the anion and cation valency, commonly modeled using Gouy-Chapman and/or Gouy-Chapman-Stern, or GCS, electrical double layer models, or EDLs) as well as from the bound charges (those on the water molecules, the silanol groups, the Si-O-Si bonds, and the ions) that are present in the SHG active region. Subtracting the GCS contribution from Ftot yields the contribution from the bound charges, according to