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测定了25℃和35℃时活性炭自水中吸附苯甲酸、邻苯二甲酸、邻羟基苯甲酸、间羟基苯甲酸、对羟基苯甲酸、苯酚和邻苯二酚共七种芳香化合物的等温线,计算了吸附过程的ΔG°,ΔH°和ΔS°。七种芳香化合物的-ΔG°都在5.9～7.7kcal·mol~(-1)的范围内,差别不大;-ΔH°都小于-ΔG°的值;ΔS°则都是正的。这些结果表明熵变是这类体系的吸附过程的重要驱动力,而且往往是主要驱动力。在液相吸附中,溶质的吸附必伴随着溶剂的脱附,前者是熵减少的过程,后者是熵增加的过程。因为上述芳香化合物的摩尔体积约是水(溶剂)的5～7倍,也就是说,吸附1摩尔的溶质将伴随5～7摩尔的水由表面脱附。因此,由于水的脱附引起的熵增加远远超过溶质吸附所引起的熵减少。这可以解释为何这类体系的吸附过程的熵变总是相当大的正值。根据这个理论,可以设想倘若溶剂的摩尔体积与溶质的相近或比溶质的更大时,吸附过程的熵变可能出现负值,文献中的一些数据支持了这一推测。
The isotherms of seven aromatic compounds adsorbing benzoic acid, phthalic acid, o-hydroxybenzoic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, phenol and catechol from activated carbon at 25 ℃ and 35 ℃ were determined. The ΔG °, ΔH ° and ΔS ° of the adsorption process were calculated. The -ΔG ° of the seven aromatic compounds are in the range of 5.9-7.7 kcal · mol -1, with little difference; -ΔH ° is less than the value of -ΔG °; and ΔS ° is all positive. These results show that entropy change is an important driving force for the adsorption process of such systems and is often the main driving force. In the liquid phase adsorption, solute adsorption must accompany the solvent desorption, the former is the process of entropy reduction, the latter is the process of entropy increase. Since the molar volume of the above-mentioned aromatic compound is about 5 to 7 times that of water (solvent), that is, 1 mole of solute adsorbed will be desorbed from the surface with 5 to 7 moles of water. Therefore, the increase in entropy due to desorption of water far exceeds that caused by solute adsorption. This can explain why the entropy change in the adsorption process of such systems is always quite positive. According to this theory, it is conceivable that if the solvent’s molar volume is similar to or greater than the solute, the entropy change of the adsorption process may be negative, some data in the literature support this conjecture.