Langmuir 2006, 22, 7203-7207
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Thermodynamic Investigation of n-Hexane Thin Films Adsorbed on Magnesium Oxide Peter N. Yaron,† Mark T. F. Telling,‡ and J. Z. Larese*,†,§ UniVersity of Tennessee, KnoxVille, Buehler Hall, KnoxVille, Tennessee 37996-1600, Rutherford Appleton Laboratory, Chilton, Didcot, United Kingdom, and Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831 ReceiVed April 12, 2006. In Final Form: May 25, 2006 The thermodynamic properties of n-hexane adsorption on MgO(100) were determined using high-resolution volumetric adsorption isotherms in the temperature range 195-255 K. Two distinct layering transitions are observed in the isotherms. The isotherms are used to calculate the two-dimensional compressibility, the differential enthalpy and entropy, the heat of adsorption, and the isosteric heat of adsorption. Neutron Diffraction is used to identify where melting of the n-hexane monolayer takes place.
Introduction There is currently a surge of activity surrounding the synthesis and characterization of nanometer-scale materials. Metal oxides (MOs) play an important role in this emerging nanotechnology field because they are used in a wide range of applications such as in pigments for paints, supports for catalysts, base components in cosmetics and medical ointments and creams, and optoelectronics, e.g., tunable and MEMS devices, actuators, and novel sensors/detectors. This research is part of a comprehensive study of the adsorption, structural, and dynamical properties of thin films of short-chained alkanes and alkenes on magnesium oxide (MgO) surfaces. The aim of this investigation is to aid the development of accurate and realistic theoretical models of the potential energy surfaces of these small molecules with MO surfaces. MgO is an ideal representative of the MO family of solids because it has a simple crystal structure (rock salt type) and it can be produced such that the most predominant exposed face is (100).1 High-resolution volumetric isotherms were used to study the thermodynamics properties of n-hexane on MgO over a temperature range of 196-250 K. Comparison of these hexane adsorption studies with previous ones involving adsorption on graphite also leads to a better understanding of how the interplay between adsorbate-adsorbate and adsorbate-substrate interactions determine the physical and chemical properties of these adsorbed films. Numerous experimental techniques, including X-ray and neutron diffraction,2-8 heat capacity measurements,9 low-energy electron diffraction (LEED),10 and computer simulations,5,11-13 have been used to study the * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Tennessee. ‡ Rutherford Appleton Laboratory. § Oak Ridge National Laboratory. (1) Spoto, G.; et al. Prog. Surf. Sci. 2004, 76, 71. (2) Wu, S. E.; Matthies, B.; Herwig, K. W.; Dai, P.; Volkmann, U. G.; Hansen, F. Y.; Taub, H. Chem. Phys. Lett. 2001, 348, 168. (3) Arnold, T.; et al. Phys. Chem. Chem. Phys. 2002, 4, 345. (4) Boese, R.; Weiss, H.-C.; Blaser, D. Angew. Chem., Int. Ed. 1999, 38, 988. (5) Krishnan, M.; Balasubramanian, S.; Clarke, S. J. Chem. Phys. 2003, 118, 5082. (6) Wu, X. Z.; Ocko, B. M.; Deutsch, M.; Sirota, E. B.; Sinha, S. K.; Physica B (Amsterdam) 1996, 221, 261. (7) Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Sinha, S. K.; Deutsch, M. Physica A (Amsterdam) 1993, 200, 751. (8) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. ReV. Lett. 1993, 70, 958. (9) Chirnside, G. C.; Pope, C. G. J. Phys. Chem. 1964, 68, 2377. (10) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2901. (11) Hansen, F. Y.; Taub, H. NATO ASI Ser., Ser. B 1991, 267, 153.
adsorption properties of n-hexane on various solid surfaces although the investigations involving MO surfaces are more limited. Experimental Procedure The MgO powder used in this study was prepared from magnesium vapors using a method14 that facilitates the production of large batches (∼20 g) of MgO powder. The nanometer-scale cubes of MgO produced have a narrow distribution of particle size (250 ( 30 nm) as determined by transmission electron microscopy (TEM) and display uniform layering steps in a methane isotherm.15 Furthermore, they exhibit a large surface area (5-10 m2/g) and almost exclusively (100) surface exposure. Commercially available MgO powders have a variety of transition metals present at the ∼10 ppm level (e.g., nickel, iron, copper, manganese, and chromium). Our method enables us to produce extremely pure MgO with concentrations well below the 1 ppm level except for Mn, which was determined to be present in ∼4 ppm concentration as determined by electron spin resonance (ESR) spectroscopy and atomic absorption (AA) spectroscopy. The MgO powder was heat treated in vacuo (∼10-7 Torr) at 950 °C for more than 36 h before being used in an experiment. It has been shown that heat treatment at this temperature homogenizes the surface structure of the MgO and ensures that all of the exposed facets are the (100) equilibrium surface.16 Approximately 0.5 g of heat-treated MgO powder was loaded into an oxygen-free high-conductivity copper (OFHC) cell sealed with an indium gasket inside a glovebox filled with argon (
