Modes of Interaction of Simazine with the Surface of Amorphous Silica in Water. Part II: Adsorption at Temperatures Higher than Ambient

作者：Esposito Serena; Sannino Filomena; Armandi Marco; Bonelli Barbara; Garrone Edoardo^*

来源：Journal of Physical Chemistry C, 2013, 117(51): 27047-27051.

DOI：10.1021/jp4107487

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 degrees to 60 degrees 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(2)H(+)), the latter prevailing both at high temperatures and loadings. A simple model for 8 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 pK(a)%26apos;s of both the active silanol species and the protonated entity given rise, pH* = [pK(a)(1) + pK(a)(2)]/2. From this, it results that (i) the dimer Sim(2) is more basic than the monomer Sim by 2 units of pK(a); (ii) adsorbed sirnazine is more basic then the molecule in solution also by ca. 2 units in pK(a); and (iii) the pK(a) 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 -%26gt; SiO-center dot center dot center dot SimH(+) is exothermic, the formation of the dimer from the monomer is endothermic (reaction SiO-center dot center dot center dot SimH(+) + Sim(aq) -%26gt; SiO-center dot center dot center dot Sim(2)H(+)), whereas the reaction 2 Sim(aq) + SiOH -%26gt; SiO-center dot center dot center dot Sim(2)H(+) is slightly exothermic. At 25 degrees 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 degrees C shows instead that both species are formed under near-equilibrium conditions.

• 出版日期2013-12-26