Quantum, classical, and multi-scale simulation of silica–water interaction: molecules, clusters, and extended systems

Over the past 6 years, we have engaged in a multi-faceted computational investigation of water–silica interactions at the fundamental physical and chemical level. This effort has necessitated development and implementation of simulation methods including high-accuracy quantum mechanical approaches, classical molecular dynamics, finite element techniques, and multi-scale modeling. We have found that water and silica can interact via either hydration or hydroxylation. Depending on physical conditions, the former process can be weak ( < 0.2 eV) or strong (near 1.0 eV). Compared to hydration, the latter process yields much larger energy gains (2–3 eV/water). Some hydroxylated silica systems can accept more water molecules and undergo further hydroxylation. We have also studied the role of external stress, effects of finite silica system size, different numbers of water molecules, and temperature dependences.     

Thermal treatment of the precipitated solid material (PSM) from the waste black liquor (WBL)

Preciptated solid materials (PSM) from waste black liquor (WBL), at pH 8·5–9·0, produced from cooking of rice straw in paper mill factories were thermally treated. The cooking process led to decrease in organic material and partial substitution in silica structure. During this cooking process, three probable stages of mass loss comprise removal of moisture, volatile release, and combustion. The chemical analysis depicts the cooking effect on leaching either the high percentage elements (Ca, Na, and K) or some metallic cations (Mn, Cd, Zn and Cu) in the parent rice straw. Silica hydrate, amorphous silica and crystalline silica were obtained at <600°C, 600–700°C and at>800°C, respectively. The infrared spectra show gradual removal of the hydrocarbon bond (C–H), molecular H₂O, and sianol group (Si-OH) with temperature. TG, DTA, XRD and SEM were used in this study.     

Low-temperature <Superscript>1</Superscript>H-NMR spectroscopic study of doxorubicin influence on the hydrated properties of nanosilica modified by DNA

The effect of the anticancer drug—doxorubicin (Dox) on hydration properties of a nanocomposite material deposited on silica and modified by small amount of DNA (0.6 wt%) was studied by means of ¹H NMR spectroscopy at low temperatures (in the range of 200–280 K). Signals of either weakly (WAW) or strongly (SAW) associated water, as well as water associated with electrondonor groups of the composite surface (ASW), were observed. The findings reveal that, depending on the temperature and the composition of the dispersion medium, fast molecular exchange takes place between different forms of interphase water. The presence of Dox (0.1–0.2 wt%) in the dispersion medium leads to change of the relative concentrations of different forms of water.     

One-pot synthesis of ordered mesoporous carbon–silica nanocomposites templated by mixed amphiphilic block copolymers

Mixed amphiphilic block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO–PPO–PEO) and polydimethylsiloxane-poly(ethylene oxide) (PDMS–PEO) have been successfully used as co-templates to prepare ordered mesoporous polymer–silica and carbon–silica nanocomposites by using phenolic resol polymer as a carbon precursor via the strategy of evaporation-induced self-assembly (EISA). The ordered mesoporous materials of 2-D hexagonal (p6m) mesostructures have been achieved, as confirmed by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and nitrogen-sorption measurements. Experiments show that using PDMS–PEO as co-template can enlarge the pore sizes and reduce the framework shrinkage of the materials without evident effect on the specific surface areas. Ordered mesoporous carbons can then be obtained with large pore sizes of 6.7 nm, pore volumes of 0.52 cm³/g, and high surface areas of 578 m²/g. The mixed micelles formed between the hydrophobic PDMS groups and the PPO chains of the F127 molecules should be responsible for the variation of the pore sizes of the resulting mesoporous materials. Through the study of characteristics of mesoporous carbon and mesoporous silica derived from mother carbon–silica nanocomposites, we think mesoporous carbon–silica nanocomposites with the silica-coating mesostructure can be formed after the pyrolysis of the PDMS–PEO diblock copolymer during surfactant removal process. Such method can be thought as the combination of surfactant removal and silica incorporation into one-step. This simple one-pot route provides a pathway for large-scale convenient synthesis of ordered mesostructured nanocomposite materials.     

Energetics of the adsorption and diffusion of hydrogen molecules in a (001) plate of nanocrystalline aluminum

The energetics of the adsorption and diffusion of hydrogen molecules in the surface of an Al plate bounded by the (001) atomic plane has been investigated by the density-functional method. The spatial configuration of hydrogen atoms and surface layers of the plate corresponding to the most stable structures was determined. It has been established that the energies of the physical and chemical adsorptions of hydrogen are equal to −0.049 eV and −0.080 eV respectively. It is shown that, as a result of the chemisorption, two stable spatial configurations with energies differing by 0.33 eV are formed. A bridge arrangement of hydrogen atoms between the apical atoms of aluminum is characterized by a minimum energy. The energy barrier of transformation of a hydrogen molecule from the physical into the chemisorbed state comprises 0.438 eV. An H₂ molecule does not dissociate completely in the process of adsorption and diffusion. On the surface and in the bulk of the metal being considered there arise diagonal-chain structures, in which the distance between the hydrogen atoms does not exceed 2.86 Å. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 1, pp. 157–164, January–February, 2008.     

Infrared spectroscopic study of water in mesoporous silica under supercritical conditions

The structure of water under high temperature–pressure conditions in mesospace was investigated by measuring the infrared spectra of water in mesoporous silica. Absorption peaks attributed to OH-stretching vibration of water in mesoporous silica were detected at lower wavenumbers as compared with bulk water, and the absorption peak positions were dependent on pore diameter. For small pore diameters (3–20 nm), absorption peak positions of water were detected at lower wavenumbers (ca. 3,300 cm⁻¹) at 400 °C, while for larger pore diameters (30–50 nm) the peaks were detected at higher wavenumbers (ca. 3,500 cm⁻¹). We attribute these features to the effects of mesoporous silica surface structure on the structural and vibrational modes of water. Furthermore, absorption peak positions changed significantly at different pore sizes (20 and 30 nm), indicating that the structure of water in small pores approaches a more ice-like structure. Based on our experimental results, the structured water layer in mesoporous silica is estimated to be at least 10 nm thick, which is thicker than that previously documented in molecular dynamic simulation studies where the thickness of structured water was found to be two or three layers from the surface.     

Water binding to biopolymers in different cereals and legumes: proton NMR relaxation, dielectric and water imbibition studies

Proton NMR relaxation time (T1), dielectric properties by means of the thermally stimulated depolarization currents (TSDC) method, and water imbibition were measured in cereal and legume grains (wheat, triticale, maize, pea, chick pea, horsebean, white lupin, lentil and beans) having different chemical composition (proteins, carbohydrates, lipids). T1 versus water content in the range 0.05–1.40 g water/g dry matter showed characteristic V-shaped curves with a sharp or a broad minimum depending on the species. Water content at T1 min was in high positive correlation with protein content of the grains (r=0.90) and in high negative correlation with soluble carbohydrates (r=−0.92), while lipids gave a very low correlation (r=0.38). The water content at T1 min (0.18–0.47 g water/g dry matter) was assigned to a primary hydration sphere around the macromolecules, since, when T1 was plotted versus per cent maximum hydration, the T1 min values for all grains fell between 25–30% of maximum hydration. The extrapolated value for zero protein content was 0.08 g water/g dry matter, which coincided with data in the literature for the water monolayer on starch. The TSDC measurements enabled us to determine the amount of tightly (irrotationally) bound water at primary hydration sites to 0.18±0.02 g water/g dry matter for beans, pea and chickpea, and, tentatively, to about 0.10 g water/g dry matter for wheat. Water imbibition data for 11 cereal and legume species gave total water hydration capacity in the range a=0.44–1.82 g water/g dry matter. This value divided by the water content of the primary hydration sphere (swelling index) was also in high positive correlation with the protein content of the grains (r=0.84). This revised version was published online in November 2006 with corrections to the Cover Date.     

The emission characteristics of low-pressure water vapor discharge UV emitters

We have studied the characteristics of UV emission sources operating on low-pressure normal (H₂O) and heavy (D₂O) water vapor excited by periodic-pulsed and glow discharges. The emission in a 300–330 nm wavelength interval has been studied in detail for water vapor pressures ranging from 50 to 2500 Pa. A comparison of the characteristics of emission from discharge plasma at low (50–150 Pa) and elevated (2.0–2.5 kPa) water vapor pressures reveals significant differences in the character of emission spectra, which can be related to the different types of emitting species (hydroxy radicals versus small clusters of such radicals and water molecules). Discharge current and emission intensity pulses in the periodic-pulsed discharge regime have been measured.     

Quantum-chemical calculation of the adhesion energy of contacting dissimilar metals in a medium

We have constructed a model of the contact interaction of dissimilar metals Al–Fe, Al–Cr, Cu–Al, and Cu–Fe in the presence of particles of a corrosive medium. We have used here the quantum-chemical method of density functionals with the exchange-correlation functional of generalized gradient approximation and LANL2DZ basic set in the cluster approximation. The adhesion energy for clusters of dissimilar metals has been calculated, and its dependence on the composition of corrosive medium has been evaluated. We have established that the adhesion energy of dissimilar metals is determined by the summary contribution of the surface energies of both contacting metals. It has a “quasichemical character,” i.e., its values are intermediate between the chemisorption energy and the energy of forces of physical nature. We have established a substantial change in the distribution of surface charges and spin electron densities of contacting metals in the course of their interaction with water molecules, chlorine ions, and glycerol molecules.     

Probing of wood–cement interactions during hydration of wood–cement composites by proton low-field NMR relaxometry

Proton NMR T ₂ relaxometry has been applied to investigate phenomena involved in wood–cement composites during hydration. The transformation of capillary pore water into hydrates and gel pore water, as well as the microstructural changes occurring in the cement matrix, was continuously monitored during the first 28 days of hydration. Water in wood and its transfer into the matrix as cement hardens were also evidenced with the method. It has been found, for example, that some of the water in the mixture is retained in wood in the form of bound or free water, depending on the initial water content. By measuring the area under the different peaks, the consumption of water during hydration can be measured and the advancement of the hydration process can be evaluated via the hydration advancement coefficient α. The cement hardening within the composite has been also studied in the presence of calcium chloride, an accelerating agent. The acceleration was clearly evidenced at the early stage of the hydration process. The influence of extractives has been evaluated by comparing the hydration behaviour of composites prepared from Eucalyptus saligna (low extractives content) and Afzelia bipendensis (high extractives content), and a new compatibility index based on NMR relaxometry measurements has been proposed.