Molecular Orbital Modeling of Water Adsorption on a Tetrasiloxane Ring
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MOLECULAR ORBITAL MODELING OF WATER ADSORPTION ON A TETRASILOXANE RING
J. K. WEST AND S. WALLACE Advanced Materials Research Center, University of Florida, One Progress Blvd.
#14,
Alachua, Fl 32615.
ABSTRACT A water molecule hydrogen bonded to a surface SiOH group produces an IR vibrational transmission peak (1/3) at 2.82 pm. Water was adsorbed into the pores of a metal alkoxide derived silica gel monolith, and the increase in the wavelength of the first vibrational overtone (2V3) of this peak was measured as a function of the adsorbed water content W (g H20/g SiO2 ). The peak shifted from 1.390 to 1.420 pm as W increased by 0.14 g/g. Intermediate Neglect of Differential Overlap (INDO) Molecular Orbital (MO) theory was used to model this process. The effect of a H20 molecule, hydrogen bonded to a hydroxylated tetrasiloxane ring, on the structure of the ring and the water molecule was investigated. The bond length of the O-H group H-bonded to the water molecule increased, as expected from the increase in wavelength of the 2V3 IR peak. INTRODUCTION The adsorption of water onto the surface of a hydroxylated a-silica surface causes changes in the vibrational modes producing the IR transmission spectrum of this surface. The production [1] of monolithic geometric samples of microporous silica gel has made the direct experimental investigation of these changes easier. A typical silica gel monolith is made by the acid catalysis of silicon tetramethoxide. After drying at 180°C, a gel typically has a surface area of =0.45 cc/g, an average pore radius of =12A and an average particle radius of =60A. This means that these fractal materials are dominated by their interfacial properties. Therefore, for a silica gel sample in the beam of an IR spectrometer, the properties of the material are dominated by the surface. An isolated surface silanol (SiOH.) group produces an IR transmission peak, vj, at 2.675 pm. v, is due to the O-H stretching vibration of isolated SiO-H1 groups (2]. A HZO molecule H-bonded to SiOH. groups produces an IR peak (V 3 ) at 2.82 pm. v 3 is due to the 0-H stretching vibration of SiOH5 groups Hbonded to H20 molecules [2]. In this investigation, the shift in the wavelength of the first vibrational overtone (2V3 - 1.39 pm at W - 0.001 g/g) of V3 was measured as a function of the HZO content W (g H2 0/g Si0 2 ). The adsorption of H20 into microporous silica gel monoliths has been shown experimentally to cause a small expansion of the gels [3-5]. It was proposed that the expansion was due to adsorption of H20 molecules onto the hydroxylated surface of the silica gel [5]. West et al. [3] investigated this hypothesis using Intermediate Neglect of Differential Overlap (INDO) Molecular Orbital (MO) theory [6,7]. They examined the adsorption of a H20 molecule on a ring of four hydroxylated silica tetrahedra, i.e. a hydroxylated tetrasiloxane ring. The H20 molecule was adsorbed via the formation of H-bonds with two hydroxyl groups [1]. The exact ring size distribution in a-silica is unknown, but consists of rings
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