Using the ab initio total energy method based on the gradient-corrected local density approximation we have modeled the experimentally observed (332) water monolayer on the MgO (100) surface. The lateral interactions between the adsorbed water molecules, the formation of hydrogen bonds, and the resulting strong dimerization of the adsorbate promote the dissociation of two out of six water molecules in the surface unit cell. Although, on the theoretical grounds, water dissociation on a defective MgO surface has been already reported, this is the first theoretical evidence of water dissociation on the perfect MgO (100) surface. [S0031-9007(98)06827-6] PACS numbers: 68.55.Jk, 82.65.PaThe mechanism of water adsorption on oxide surfaces has been the subject of many studies owing to its importance for understanding numerous fundamental phenomena such as heterogeneous catalysis, corrosion, or passivation. Water at oxide surfaces is also a key phenomenon for modeling of earth subsurface hydrodynamics. As a model system, water on the MgO (100) surface, has been subject to intensive theoretical studies [1][2][3][4][5][6][7][8]. The general conclusion is that water adsorbs molecularly on the perfect (100) surface, dissociative adsorption being possible only on selected surface defects such as steps, kinks, or vacancies. Recent helium atom scattering (HAS) and LEED experiments on well-characterized uniform MgO single crystal surfaces have shown the existence of an ordered p(332) water monolayer structure [9,10]. Fourier transformed infrared spectroscopy (FTIR) studies performed on this structure show three sharp absorption bands at 3672, 3626, and 3513 cm 21 [11]. They were tentatively ascribed to isolated molecules adsorbed in the molecular form on terraces and dissociatively on surface steps. This, however, appears to be incoherent with the HAS and LEED results that clearly show the (332) pattern. The only existing theoretical evidence concerning the (332) phase is the semiempirical molecular dynamics simulation [10] in which the water molecules are found to physisorb flat on top of surface cation sites, the hydrogen atoms point approximately along the Mg rows forming weak hydrogen bonds. Although the experimentally determined adsorption energy is relatively high, none of the results of the semiempirical model show chemical bonding to the substrate.Here, using an ab initio approach, we show that in the (332) water monolayer deposited on the MgO (100) surface, due to interaction between the adsorbed molecules some of them dissociate. Since the dissociation is conditioned by the reconstruction of the monolayer, it was not found for adsorption of a single water molecule, nor for an unreconstructed (131) monolayer [6]. On the other hand, since the semiempirical calculations [10] cannot account for deprotonation of water molecules, no surface dissociation was found by this method. Our results show, for the first time in the case of an experimentally observed structure, the influence of the cooperative interactions between adsor...

The interaction of water with laboratory soots possessing a range of properties relevant for atmospheric studies is examined by two complementary methods: gravimetrical measurement of water uptake coupled with chemical composition and porosity analysis and HTDMA (humidified tandem differential mobility analyzer) inference of water uptake accompanied by separate TEM (transmission electron microscopy) analysis of single particles. The first method clarifies the mechanism of water uptake for bulk soot and allows the classification of soot with respect to its hygroscopicity. The second method highlights the dependence of the soot aerosol growth factor on relative humidity (RH) for quasi-monodisperse particles. Hydrophobic and hydrophilic soot are qualitatively defined by their water uptake and surface polarity: laboratory soot particles are thus classified from very hydrophobic to very hydrophilic. Thermal soot particles produced from natural gas combustion are classified as hydrophobic with a surface of low polarity since water is found to cover only half of the surface. Graphitized thermal soot particles are proposed for comparison as extremely hydrophobic and of very low surface polarity. Soot particles produced from laboratory flame of TC1 aviation kerosene are less hydrophobic, with their entire surface being available for statistical monolayer water coverage at RH approximately 10%. Porosity measurements suggest that, initially, much of this surface water resides within micropores. Consequently, the growth factor increase of these particles to 1.07 at RH > 80% is attributed to irreversible swelling that accompanies water uptake. Hysteresis of adsorption/desorption cycles strongly supports this conclusion. In contrast, aircraft engine soot, produced from burning TC1 kerosene in a gas turbine engine combustor, has an extremely hydrophilic surface of high polarity. Due to the presence of water soluble organic and inorganic material it can be covered by many water layers even below water saturation conditions. This soot demonstrates a gradual diameter growth factor (D(wet)/D(dry)) increase up to 1.22 at 93% relative humidity, most likely due to the presence of single particles with water soluble material heterogeneously distributed over their surface.

The laboratory combustion technique operating on a typical combustor of a gas turbine engine is used for soot sampling. Soot particles are derived by combustion of a hydrocarbon mixture at typical cruise C 3 H 8 Èn-C 4 H 10 conditions. Size, morphology, microstructure, surface area, porosity, and the chemical nature of the soot surface particles are studied by transmission electron microscopy (TEM), Raman and Auger electron spectroscopies (AES), volumetry and gravimetry. Structural irregularities such as micropores determine the speciÐc adsorbability of non-polar gases such as Kr, and With respect to water adsorption, CH 4 C 6 H 6. aircraft combustor soot is far from being hydrophobic. Initial water adsorption on polar heterogeneities leads to pore Ðlling at increasing pressures. The microstructure of soot particles is easily transformed under the inÑuence of adsorbates, giving rise to swelling e †ects. Due to its speciÐc physico-chemical properties aircraft combustor soot may act as contrail condensation nuclei at low sulfur content in the jet fuel.

Articles you may be interested inAdsorbed state of thiophene on Si(100)(2×1) surface studied by electron spectroscopic techniques and semiempirical methods A lowenergy electron diffraction investigation of the surface deformation induced by misfit dislocations in thin MgO films grown on Fe (001) A Heatom scattering study of the frustrated translational mode of CO chemisorbed on defects on copper surfacesThe adsorption and desorption of water on single crystal MgO(100): The role of surface defects A lowenergy electron diffraction data acquisition system for very low electron doses based upon a slow scan charge coupled device camera Rev.The interaction of water with MgO ͑100͒ single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K-230 K. At T crystal ϭ100-180 K water forms a layer with a c͑4ϫ2͒ symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at T crystal ϭ185-221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high-temperature phase has a ͑3ϫ2͒ symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Q st ϭ85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.