SURFACE ACID-BASE PROPERTIES OF KAOLINITE
采用双位模式 (即假定高岭石表面存在AlOH和SiOH基团 )拟合高岭石表面的酸碱滴定数据并描述表面上发生的质子化反应 ,Al位和Si位的表观酸度常数拟合值分别为pKa1,Al=1 .78、pKa2 ,Al=8.47和pKa2 ,Si=5 .1 2 ,它们的酸性比对应的 (氢 )氧化物表面位的更强。高岭石的总表面位密度远大于氧化铝和二氧化硅 ,其原因很可能是溶液中的质子或羟离子能够渗入高岭石的层间 ,与层间的羟基发生反应。此外 ,Al位密度也比Si位大近一个数量级 ,这种与理论化学式偏离的现象可受多种因素的影响。高岭石表面总体在pH低于 4.0时带正电荷 ,在pH高于 4.0时带负电荷。正电荷仅由AlOH基团通过质子化作用形成AlOH2 + 表面化合态来提供 ,而负电荷则由AlOH和SiOH基团的去质子化作用产生 ,分别形成AlO-和SiO-表面化合态。A two site model, i.e., assuming the presence of AlOH and SiOH groups on the kaolinite surface, was used to simulate the acid base titration data and to describe the protonation/deprotonation of the surface groups. The fitted apparent acidity constants for Al sites and Si sites were p K a1, Al =1.78, p K a2, Al =8.47 and p K a2, Si =5.12, respectively. Comparison with the p K values for aluminum (hydro) oxides and silica revealed that both Al sites and Si sites at the kaolinite surface exhibited a more acidic character than those on the corresponding (hydro)oxides. The total surface site density of kaolinite was much greater than that of alumina and silica. The possible explanation was that protons or hydroxyl ions from the solution could penetrate between the layers of kaolinite and react with hydroxyls in the sheets. In addition, the density of Al sites was about one order of magnitude greater than that of Si sites. This deviation from bulk stoichiometry would be controlled by various factors. The overall surface of kaolinite was positively and negatively charged respectively below and above pH 4.0. The positive charge was only due to the formation of AlOH 2 + surface complexes by protonation of AlOH. On the other hand, the negative charge was contributed by deprotonation of AlOH and SiOH groups, resulting in the formation of AlO - and SiO respectively.