Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Langmuir. 2012 Mar 20;28(11):5070-8. doi: 10.1021/la204517v. Epub 2012 Mar 8.
The densities of pore-confined fluids were measured for the first time by means of vibrating tube densimetry (VTD). A custom-built high-pressure, high-temperature vibrating tube densimeter was used to measure the densities of propane at subcritical and supercritical temperatures (between 35 and 97 °C) and carbon dioxide at supercritical temperatures (between 32 and 50 °C) saturating hydrophobic silica aerogel (0.2 g/cm(3), 90% porosity) synthesized inside Hastelloy U-tubes. Additionally, supercritical isotherms of excess adsorption for CO(2) and the same porous solid were measured gravimetrically using a precise magnetically coupled microbalance. Pore fluid densities and total adsorption isotherms increased monotonically with increasing density of the bulk fluid, in contrast to excess adsorption isotherms, which reached a maximum and then decreased toward zero or negative values above the critical density of the bulk fluid. The isotherms of confined fluid density and excess adsorption obtained by VTD contain additional information. For instance, the maxima of excess adsorption occur below the critical density of the bulk fluid at the beginning of the plateau region in the total adsorption, marking the end of the transition of pore fluid to a denser, liquidlike pore phase. Compression of the confined fluid significantly beyond the density of the bulk fluid at the same temperature was observed even at subcritical temperatures. The effect of pore confinement on the liquid-vapor critical temperature of propane was less than ~1.7 K. The results for propane and carbon dioxide showed similarity in the sense of the principle of corresponding states. Good quantitative agreement was obtained between excess adsorption isotherms determined from VTD total adsorption results and those measured gravimetrically at the same temperature, confirming the validity of the vibrating tube measurements. Thus, it is demonstrated that vibrating tube densimetry is a novel experimental approach capable of providing directly the average density of pore-confined fluids, and hence complementary to the conventional gravimetric or volumetric/piezometric adsorption techniques, which yield the excess adsorption (the Gibbsian surface excess).
首次采用振动管密度计（VTD）测量了孔隙限制流体的密度。使用定制的高压、高温振动管密度计测量了在亚临界和超临界温度（35 至 97°C 之间）下丙烷和在超临界温度（32 至 50°C 之间）下二氧化碳在疏水二氧化硅气凝胶（0.2 g/cm(3)，孔隙率 90%）中的饱和密度，该气凝胶在哈氏合金 U 型管内合成。此外，使用精密磁耦合微天平通过重量法测量了 CO(2)的超临界过剩吸附等温线和相同多孔固体。与过剩吸附等温线相反，孔隙流体密度和总吸附等温线随本体流体密度的增加单调增加，后者在达到最大值后，在本体流体的临界密度之上，朝着零或负值减小。通过 VTD 获得的受限流体密度和过剩吸附等温线包含了更多的信息。例如，在总吸附的平台区域开始时，过剩吸附的最大值出现在本体流体的临界密度之下，标志着孔隙流体向更密集的液态相的转变结束。即使在亚临界温度下，也观察到受限流体在相同温度下被压缩到超过本体流体密度的程度。孔隙限制对丙烷的液-气相临界温度的影响小于约 1.7 K。丙烷和二氧化碳的结果在对应状态原理的意义上表现出相似性。从 VTD 总吸附结果确定的过剩吸附等温线与在相同温度下通过重量法测量的等温线之间获得了良好的定量一致性，证实了振动管测量的有效性。因此，证明了振动管密度计是一种能够直接提供孔隙限制流体的平均密度的新型实验方法，与传统的重量法或体积/压力吸附技术互补，后者提供过剩吸附（吉布斯表面过剩）。
