Modes of interaction of simazine with the surface of amorphous silica in water. Part II: Adsorption at temperatures higher than ambient

The conclusions of a previous study (S. Esposito et al. J. Phys. Chem. C 2013, 117, 11203-11210) concerning room temperature adsorption of simazine (Sim) on amorphous silica in water have been checked against a set of experiments in the range 40 to 60 C, where equilibrium conditions are more likely to be attained. Adsorbed amount as a function of pH has a complex behavior with temperature, confirming the presence of two types of protonated adsorbed species, respectively monomeric (SimH+) and dimeric (Sim 2H+), the latter prevailing both at high temperatures and loadings. A simple model for adsorption involving proton transfer from the solid indicates that the pH value at which the uptake is maximum (pH) is the half sum of the pKa's of both the active silanol species and the protonated entity given rise, pH* = [pKa(1) + pKa(2)]/2. From this, it results that (i) the dimer Sim2 is more basic than the monomer Sim by 2 units of pKa; (ii) adsorbed simazine is more basic then the molecule in solution also by ca. 2 units in pKa; and (iii) the pKa of the silanol species involved is probably not ca. 4 as recently proposed, but more likely ca. 7, in agreement with old classical views. From the qualitative energetic point of view, the reaction Sim(aq) + SiOH → SiO-···SimH+ is exothermic, the formation of the dimer from the monomer is endothermic (reaction SiO -···SimH+ + Sim(aq) → SiO -···Sim2H+), whereas the reaction 2 Sim(aq) + SiOH → SiO-···Sim 2H+ is slightly exothermic. At 25 C, the adsorbed monomer is irreversibly held, and the dimer only partially. The isotherm at 40 shows that adsorption of the dimer occurs almost reversibly, whereas equilibrium in the formation of the monomer is not completely reached. The isotherm at 60 C shows instead that both species are formed under near-equilibrium conditions.