Phase Transitions in Pores: Experimental and Simulation Studies of Melting and Freezing
详细信息 查看全文
- 作者：Malgorzata Sliwinska-Bartkowiak ; Jolanta Gras ; Roman Sikorski ; Ravi Radhakrishnan ; Lev Gelb ; and Keith E. Gubbins
- 刊名：Langmuir
- 出版年：1999
- 出版时间：August 31, 1999
- 年：1999
- 卷：15
- 期：18
- 页码：6060 - 6069
- 全文大小：198K
- 年卷期：v.15,no.18(August 31, 1999)
- ISSN：1520-5827
文摘
We report both experimental measurements and molecular simulations of the melting and freezingbehavior of simple fluids in porous media. The experimental studies are for carbon tetrachloride andnitrobenzene in controlled pore glass (CPG) and Vycor. Differential scanning calorimetry (DSC) was usedto determine the melting point in the porous materials for each of the glass samples. In the case ofnitrobenzene (which has a nonzero dipole moment), dielectric spectroscopy was also used to determinemelting points. Measurements by the two methods were in excellent agreement. The melting point wasfound to be depressed relative to the bulk value for both fluids. With the exception of smallest pores, themelting point depression was proportional to the reciprocal of the pore diameter, in agreement with theGibbs-Thomson equation. Structural information about the different confined phases was obtained bymeasuring the dielectric relaxation times using dielectric spectroscopy. Monte Carlo simulations wereused to determine the shift in the melting point, Tm, for a simple fluid in pores having both repulsive andstrongly attractive walls. The strength of attraction to the wall was shown to have a large effect on theshift in Tm, with Tm being reduced for weakly attracting walls. For strongly attracting walls, such asgraphitic carbon, the melting point increases for slit-shaped pores. For such materials, the adsorbed contactlayer is shown to melt at a higher temperature than the inner adsorbed layers. A method for calculatingthe free energies of solids in pores is presented, and it is shown that the solid-liquid transition is firstorder in these systems.