Abstract:

Equilibrium adsorption of five substituted azobenzenes with pKa=1.5-5.0 from nonpolar organic solvents on 1:1 and 2:1 type layer silicates without layer charge (kaolinite, lizardite, pyrophyllite, and talc) was studied. It was shown that the shape of each adsorption isotherm belongs to L type, and, the color of the substituted azobenzene molecules adsorbed changed from basic type (yellowish) to acidic type (reddish or purplish). The higher the basicity (pKa) of the substituted azobenzene, the greater the adsorptive capacity. Moreover, the weak organic bases were more strongly adsorbed from n-hexane and carbon disulfide solutions than from benzene solution. From these facts, it was concluded that the adsorption mechanism of the weak organic bases on the minerals without layer charge is an acid-base interaction. It was also shown that the amount of acid sites of kaolinite saturated with Na, Ca, and Mg is almost identical, while that of montmorillonite shows strong dependence on exchangeable cation species. The decrease of the amount of acid sites due to the increase of RH also has quite different pattern for the minerals. These indicate that the origin of the acidity on kaolinite can not attributed to the hydrolysis of water molecules around exchangeable cations, as in the case of montmorillonite.

Key words: Clay mineral')">

Clay mineral, Organic base, Adsorption;Surface acidity, Acid sites, Non-polar organic solvent