Characterizing the composition of an oxide surface is critical to a range of applications in separations and reaction chemistry. For silica surfaces, among the key issues is the competition for occupancy of the siloxide (SiO-) sites by H+ and other cations. We studied the competition between H+ and Na+ for surface siloxide occupancy as a function of the concentrations of each cation using the contact angle of a droplet of CCl4 on a silica surface immersed in aqueous solution as the probe of surface energy. We find the dissociation constant for SiONa to be larger than the SiOH dissociation constant, and with knowledge of the surface energy for CCl4 in contact with H2O, we are able to determine the surface energies of the SiOH-water, SiONa-water, and CCl4-SiOX interfaces.