Wurster D E, Oh E, Wang J C
University of Iowa, College of Pharmacy, Iowa City 52242, USA.
J Pharm Sci. 1995 Nov;84(11):1301-7. doi: 10.1002/jps.2600841109.
High-temperature drying is required to remove chemisorbed water from the zinc oxide surface. High-temperature drying of a very small particle size zinc oxide powder (median particle size approximately 23 nm) resulted in a substantial decrease in the surface area. The surface areas (BET analysis of 77 K nitrogen vapor adsorption data) of ZnO samples dried at 500 degrees C decreased continually as the drying time was increased. Although the surface area decrease was fastest during the first 5 h, a 64% decrease in surface area was found after 20 h. The decrease in surface area was not due to a collapse of pore structure. Comparison of nitrogen vapor adsorption and desorption isotherms as well as geometric calculations of surface area indicated that both the original and final particles were nonporous. X-ray diffractograms of the original powder and of powders dried at two temperatures were all identical. Thus, no change in crystal structure occurred as a result of drying at 500 degrees C. Atomic force microscopy provided substantial evidence that the surface area decrease was due to a shift in the particle size distribution to a larger mean size. It was verified using two different experiments that ZnO exhibited significant sublimation at 500 degrees C. It was concluded that the increase in particle size was due to a sublimation/condensation process that obeyed the Kelvin equation. The effect of ZnO particle size on the vapor pressure ratio in the Kelvin equation was modeled at 500 degrees C for several different assumed solid surface tensions. Drying conditions for ZnO were then selected which balanced maximum removal of chemisorbed water and minimum surface area decrease. Water vapor adsorption isotherms for ZnO at 25 degrees C were subsequently obtained. Differences in the isotherms resulting from the presence or absence of a chemisorption contribution could clearly be demonstrated.
需要高温干燥以去除氧化锌表面的化学吸附水。对粒径非常小的氧化锌粉末（中位粒径约为23纳米）进行高温干燥，导致表面积大幅下降。在500℃下干燥的氧化锌样品的表面积（根据77K氮气吸附数据进行BET分析）随着干燥时间的增加而持续下降。尽管在前5小时表面积下降最快，但20小时后发现表面积下降了64%。表面积的下降并非由于孔结构的坍塌。氮气吸附和解吸等温线的比较以及表面积的几何计算表明，原始颗粒和最终颗粒均无孔。原始粉末以及在两个温度下干燥的粉末的X射线衍射图完全相同。因此，在500℃下干燥不会导致晶体结构发生变化。原子力显微镜提供了大量证据，表明表面积的下降是由于粒径分布向更大的平均尺寸偏移所致。通过两个不同的实验证实，氧化锌在500℃时会发生显著升华。得出的结论是，粒径的增加是由于遵循开尔文方程的升华/冷凝过程。针对几种不同假设的固体表面张力，在500℃下对氧化锌粒径对开尔文方程中蒸气压比的影响进行了建模。然后选择了氧化锌的干燥条件，以平衡化学吸附水的最大去除量和最小表面积下降。随后获得了25℃下氧化锌的水蒸气吸附等温线。可以清楚地证明存在或不存在化学吸附贡献时等温线的差异。
