We speculate that the deprotonated silanol (Si-O -) and the hydrophobic siloxane (Si-O-Si) are heterogeneously distributed on the silica tetrahedral facet, forming diverse sorption sites with a range of interaction properties. Recent atomic force microscopy studies have shown that the surface of the silica tetrahedral layer has a pH-dependent charge that is negative at pH > 4. Thus, deprotonated surface functional groups are present and may contribute to the surface heterogeneity.