水/硅胶吸附体系的热化学研究

我们用精密自动绝热量热计测定了几种不同吸附水含量的水/硅胶吸附体系在200～320 K温度范围内的热容. 结果表明, 当吸附水含量使表面复盖度(θ)大于1时, 在相应的C_p～T曲线上会出现吸附水的相变峰. 这说明吸附在硅胶表面上的水分子已经形成了聚集态; 而当θ<1时, 由于尚未形成聚集态水, 故没有相变过程出现, 其C_p～T曲线呈光滑状. 这些现象与H_2O/γ-Al_2O_3吸附体系是一致的. 又由于硅胶表面对水分子的吸附力较γ-Al_2O_3的要小, 故在同样的吸附量的C_p～T曲线上, 水/硅胶的峰要比H_2O/γ-Al_2O_3的尖锐, 且蜂温增高的速度要快. 这些都表明, 吸附在硅胶表面上的二维表相水会随吸附量的增加而以较快的趋势接近于体相水. 此外, 由不同含水量的C_p～T曲线外推, 求出了不含吸附水的硅胶在200～300 K范围内的热容.

THERMOCHEMICAL STUDY FOR WATER/SILICA ADSORPTION SYSTEM

By the use of a precision automatic adiabatic calorimeter we have measured the heat capacities for water/silica adsorption system with various adsorbed amoun- ts over the temperature range from 200-320 K.The results obtained show that when the amount of adsorbed water makes the surface coverage (θ) larger than 1, the peak for phase transition of adsorbed water will appear on the C_p-T curves.This means that aggregate of water molecules adsorbed on the surface of silica has for- med. On the other hand when θ<1, no phase transition is observed as the temperature of the system rise and then the C_p—T curves are smooth. These phemomena are in coincidence with molecules on silica surface is weaker than that on the γ-Al_2O_3, the peak profiles for water/silica is sharper than that for H_2O/γ-Al_2O_3 even if the two adsorption systems have the same amount of adsorbed water. The rise in the peak temperature with increase of amount of adsorbed on the surface of silica could become buk water easier as compared with the water adsorbed on the γ-Al_2O_3. Besides, the heat capacities for silica without any adsorbed water over the temperature range from 200 K to 300 K have been obtained by extrapolating the C_p—T curves with various adsorbed amounts at the same temperatures.