Andrey A. Levchenko

Thermochemistry of microporous and mesoporous materials.

The past two decades have seen exciting advances in the discovery, improved synthesis, processing, and molecularlevel engineering of new inorganic materials having specialized electronic, ceramic, and structural applications. Many such materials share two common characteristics: they are complex in structure and composition and they must be prepared by a series of steps under carefully controlled conditions. The use of low-temperature aqueous synthesis conditions, with appropriate attention to pH, inorganic and organic structure-directing agents, and subsequent drying and calcination protocols has led to a wealth of new and often metastable crystalline polymorphs, to amorphous materials, and to fine powders with particles of nanoscale dimensions. Such materials are not constrained to be in chemical equilibrium with their surroundings and do not necessarily represent the state of lowest free energy. The abundance of possible new structures formed begins to mimic the riches of organic chemistry, where the fact that all complex organic and biological molecules are metastable under ambient conditions with respect to a mixture of carbon dioxide, water, and other simple gases is irrelevant except in a conflagration. Liberation of ceramic science from the tyranny of hightemperature equilibrium thus is leading to new materials synthesized more quickly, at lower cost, and under enVironmentally more friendly conditions. There is, of course, a price to pay. First, the synthetic procedures are more complex than traditional “mix, grind, fire, and repeat” ceramic processing. Second, and more importantly, relatiVely little is known about the long-term stability in either a thermodynamic or a kinetic sense, of the materials formed, about their degradation during use, and about materials compatibility. Two examples of such problems are the potential corrosion of materials by ambient H2O and CO2 and the collapse to inactive phases of complex zeolitic and mesoporous catalysts under operating conditions. Chemical reactions in metastable materials are governed by an intertwined combination of thermodynamic driving forces and kinetic barriers. For this rich landscape of new materials, neither the depths of the valleys nor the heights of the mountains are known; indeed, one cannot even always tell which way is energetically downhill. Families of microporous and mesoporous (synthetic and natural) framework materials promise applications for a better and greener future. Aluminosilicate zeolites are by far the most familiar members of a larger group of crystalline porous materials with pore sizes less than 2 nm. They have found many uses in industrial and domestic settings including applications in the petrochemical industry as catalysts for petroleum cracking, agriculture as soil treatments, medicine for production of medical-grade oxygen, nuclear waste disposal, water purification, and detergents. Despite tremendous efforts undertaken by many groups around the world, a complete understanding of underpinning fundamental principles that govern formation of zeolites and mesoporous materials is still missing. This presents limitations to the discovery of new frameworks with optimized properties. What, then, is the role of the thermodynamics and thermochemical measurements for these new and exciting materials? Enthalpies and free energies of formation and the energetics of metastability are useful in two major contexts. The first is thermochemical data for the calculation of phase relations, of materials compatibility, and of optimal synthesis * To whom correspondence should be addressed: anavrotsky@ucdavis.edu. † Nanomaterials in the Environment, Agriculture and Technology Organized Research Unit ‡ Current address: Setaram Inc, Newark, California 94560 Volume 109, Number 9

Interfacial effects on vitrification of confined glass-forming liquids.

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.

Inelastic Neutron Scattering Study of Confined Surface Water on Rutile Nanoparticles

The vibrational density of states (VDOS) for water confined on the surface of rutile-TiO(2) nanoparticles has been extracted from low temperature inelastic neutron scattering spectra. Two rutile-TiO(2) nanoparticle samples that differ in their respective levels of hydration, namely TiO(2) x 0.37 H(2)O (1) and TiO(2) x 0.22 H(2)O (2) have been studied. The temperature dependency of the heat capacities for the two samples has been quantified from the VDOS. The results from this study are compared with previously reported data for water confined on anatase-TiO(2) nanoparticles.

Nature of molecular rotation in supercooled glycerol under nanoconfinement.

The dynamics of glass-forming liquids under nanoconfinement is key to understanding a variety of phenomena in nature and modern technology. We report a (13)C NMR spectroscopic study that directly demonstrates that alpha-relaxation in bulk glycerol involves an isotropic rotational jump of the constituent molecules. The activation energy of this motion is approximately 78 kJ mol(-1) in the bulk, which abruptly changes to a low value of approximately 27.5 kJ mol(-1), characteristic of beta-processes, upon confinement of glycerol into approximately 2 nm pores in mesoporous silica. This observation implies that the molecular dynamics associated with structural relaxation near glass transition are inherently different in supercooled glycerol in the bulk and under extreme nanoconfinement.

Molecular dynamics in supercooled glycerol: Results from C13 NMR spectroscopy

(13)C NMR spectra of glycerol are collected over the entire temperature range of supercooling: T(g)(185 K) < or = T < or = T(m)(293 K). The temperature dependent evolution of the (13)C NMR line shape indicates dynamical averaging of the chemical shift anisotropy at the carbon sites with increasing temperature, resulting from isotropic tumbling of the constituent molecules. This isotropic reorientation dynamics involves random molecular jumps over all possible angles, and its time scale is in excellent agreement with the alpha-relaxation time scale of the supercooled liquid. The increasing activation energy of such molecular jumps with decreasing temperature and hence the fragility of supercooled glycerol are likely to be related to the corresponding temperature dependence of the average number of hydrogen bonds per molecule. The absence of any beta peak in the dielectric relaxation spectra of supercooled glycerol is possibly related to a strong coupling between intramolecular dynamics and the tumbling of the entire molecule as a whole.