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Bulk Water (bulk + water)
Selected AbstractsPulsed saturation of the standard two-pool model for magnetization transfer.CONCEPTS IN MAGNETIC RESONANCE, Issue 1 2004Part I: The steady state Abstract A general framework for magnetization transfer (MT) of a two-pool system with linear exchange for arbitrary saturation by periodic radio-frequency pulses was derived. It is based on a novel parameterization adapted to the time evolution of saturation recovery. The conditions in tissue permit a description in analogy to partial saturation of a homogeneous liquid. In this approximation, the direct saturation of bulk water is amplified by MT. Rapid transfer equilibrates the saturation of the pools in the early phase of free evolution. This kinetic "pre-equilibrium" relaxes slowly with a common relaxation rate. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part A 21A: 37,49, 2004 [source] Carbon Nanotubes and Nanofluidic TransportADVANCED MATERIALS, Issue 35 2009Jason Knowles Holt Abstract Recent strides have been made in both the modeling and measurement of fluid flow on the nanoscale. Carbon nanotubes, with their atomic dimensions and atomic smoothness, are ideal materials for studying such flows. This Progress Report describes recent modeling and experimental advances concerning fluid transport in carbon nanotubes. The varied flow characteristics predicted by molecular dynamics are described, as are the roles of defects and chirality on transport. Analytical models are increasingly being used to describe nanofluidic transport by relaxing many of the assumptions commonly used to describe bulk water. Recent experimental studies examine the size dependence of flow enhancements through carbon nanotubes and use varied spectroscopies to probe water structure and dynamics in these systems. Carbon nanotubes are finding increasing applications in biology, from protein filters to platforms for cell interrogation. [source] Peau sèche-rêche et "Hydratation".INTERNATIONAL JOURNAL OF COSMETIC SCIENCE, Issue 4 2004Concept de la capture de l'eau organisée comme de la glace Synopsis About sixty years ago Frank and Evans showed, by entropy measurements, that when a "non-polar molecule dissolves in water it modifies the water structure in the direction of greater ,cristallinity', the water builds a microscopic iceberg around it" Now, we propose the "concept of ice-like-water capture": a lowering of organized ice-like water promotes aggregation (loss of solubility) of the filaggrin/keratin1/keratin10 associations through their hydrophobic patches. The capture of ice-like water may be performed by the glucoceramides-rich bilayers in stratum granulosum. Probably, the same process aggregates the proteins of corneocytes envelope as well as corneodesmosomes proteins. According to the "concept of ice-like-water capture", to regulate the keratinization, it is not total water that must be added to the stratum corneum, but ice-like water that must be removed from stratum granulosum. Both petrolatum (lipophilic ingredient) and glycerol (hydrophilic ingredient) would capture the ice-like water, most probably after combination with the lipid bilayers of stratum corneum. Moisturizing cream, when organized in secondary droplets is likely to perform the same action. Measurements by near-infrared reflectance spectroscopy of the skin show that petrolatum; glycerol and/or moisturizing cream enhance the quantity of bulk water (1890,1897 nm band). As the ice-like water is the complement of bulk water, the enhanced bulk water let presume an ice-like water lessening. Some desynchronization (late or forward) of the keratinization/differentiation which confer the somatosensory problems associated with "dry and flaky skin" may be linked to an excess or lack of ice-like. For instance, the winter xerosis, very common by chilling weather, could be explained by an increase of ice-like water driven by the fall of the temperature. Résumé En s'appuyant: 1°-sur d'anciens travaux de thermodynamique montrant, d'une part que les molécules d'eau autour des zones apolaires en solution dans l'eau s'organise selon une structure d'eau-comme-de-la-glace, d'autre part qu'en l'absence de cette eau-comme-de-la-glace les molécules de protéines s'agrègent par leurs zones hydrophobes; 2°-sur des travaux récents utilisant la spectroscopie de l'infrarouge proche; technique qui permet de mesurer la quantité d'eau-en-vrac, forme que prend l'eau-comme-de-la-glace après sa fusion lors de l"établissement de liaisons/interactions hydrophobes; nous proposons le "concept de la capture de l"eau-comme-de-la-glace" selon lequel : 1° la différenciation des kératinocytes, qui se traduit à la fois par l'agrégation des trios filaggrine/K1/K10 (ainsi que notamment la formation de l'enveloppe des cornéocytes et des cornéodesmosomes) est promue par une baisse de la teneur en eau organisée dans le stratum granulosum. La captation de l'eau-comme-de-la-glace pourrait être assurée in situ par la structure lipidique riche en glucocéramides dont l'apparition dans le stratum granulosum est contemporaine du début de la baisse de la teneur en eau; 2° contrairement à la "tradition", la peau sèche-rêche n'est pas améliorée par une augmentation de l'hydratation du stratum corneum mais par la capture d'eau-comme-de-la-glace dans le stratum granulosum. 3° le glycérol, la Vaseline et les crèmes "hydratantes" peuvent concourir à cette capture d'eau-comme-de-la-glace, vraisemblablement après s"être combinés aux bicouches céramidiques du stratum corneum, et ainsi agir depuis ce stratum sur le stratum granulosum. 4°-la baisse hivernale de la température provoque une baisse de la quantité d'eau organisée et confère une aggravation de la peau sèche-rêche. 5°-une désynchronisation de la synthèse ou une modification de la structure et/ou de la composition des bicouches glucocéramidiques du stratum granulosum pourraient être à l'origine de certains types de peau sèche-rêche. [source] Cation-Enhanced Deprotonation of Water by a Strong PhotobaseISRAEL JOURNAL OF CHEMISTRY, Issue 2 2009Noga Munitz We have used picosecond fluorescence spectroscopy to study the proton-dissociation dynamics of bulk water and H2O molecules solvating Mg2+ ions in aqueous solutions. We have analyzed the photo-initiated proton-transfer reaction to a photobase 6-aminoquinoline by the Collins-Kimball approach and have modeled the ensuing bimolecular reaction dynamics by the Smoluchowski equation with radiation boundary conditions. We have found the on-contact proton transfer rate to follow the Marcus free-energy relation for proton transfer and estimate by this rate-equilibrium correlation the considerable enhancement in the acidity of the water molecules solvating the Mg2+ ion. Our findings may be used in the study of metallo-enzymes such as carbonic anhydrases (CAs), which catalyze the reversible addition reaction of OH, to CO2 by increasing the reactivity of the zinc-bound water molecules by means of stabilizing the product of water dissociation, the OH, anion. [source] Contrast analysis of the composition of ribosomes extracted with different purification proceduresJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2000Giuseppe Briganti The composition and hydration of E. coli ribosomes isolated with different purification protocols has been analysed by combining two experimental techniques: measurements of small-angle neutron scattering (SANS), for two different isotopic solvent compositions, and refractive index (RI) increments. From the contrast between the solvent and solute scattering densities and the molar polarizability, determined experimentally with SANS and RI measurements, three independent equations are obtained and three unknown quantities are determined: (i) the volume of the solute hydrated skeleton Vs, (ii) the material contained in it, namely the biological components, intrinsic (rRNA and proteins) and extrinsic, such as aminoacylsynthetase and elongation factors, (iii) the number of water molecules structurally bound to the ribosome and non-exchangeable with the solvent. From the form factor at infinite contrast, a second definition of the solute volume is obtained, , which represents the volume within the contour surface of the ribosome. This value is generally larger than Vs and can include a certain amount of water molecules, i.e. those inside the volume (,Vs). Considering the molar volume of this water to be equal to that of the bulk water, it is possible to evaluate its amount. The particle density calculated from the ribosome components in , including proteins, RNA, bound and unbound water molecules, corresponds to the buoyant density measured for E. coli 70S particles. The two ribosomal preparations display different performances in protein synthesis; hence the results indicate that the optimal condition corresponds to a wider skeleton and contour volume but containing a smaller amount of segregated water molecules. It is believed that the method provides a reliable technique to determine the composition of ribosomes under various experimental conditions. [source] Profiling bacterial survival through a water treatment process and subsequent distribution systemJOURNAL OF APPLIED MICROBIOLOGY, Issue 1 2005D. Hoefel Abstract Aims:, To profile fractions of active bacteria and of bacteria culturable with routine heterotrophic plate count (HPC) methods through a typical water treatment process and subsequent distribution system. In doing so, investigate how water treatment affects both bacterial abundance and diversity, and reveal the identities of active bacteria not detected by traditional HPC culture. Methods and Results:, Profiling active fractions was performed by flow cytometric cell sorting of either membrane-intact (BacLightTM kit) or enzymatically active (carboxyfluorescein diacetate, CFDA) bacteria, followed by eubacterial 16S rDNA-directed PCR and denaturing gradient gel electrophoresis (DGGE). Water treatment significantly reduced active bacterial numbers detected by the BacLightTM kit and CFDA assay by 2·89 and 2·81 log respectively. Bacterial diversity was also reduced from >20 DGGE bands in the active fractions of reservoir water to only two bands in the active fractions of finished water. These two bands represented Stenotrophomonas maltophila, initially culturable by HPC, and a Burkholderia -related species. Both species maintained measurable traits of physiological activity in distribution system bulk water but were undetected by HPC. Conclusions:, Flow cytometric cell sorting with PCR-DGGE, to assess water treatment efficacy, identified active bacteria from a variety of major phylogenetic groups undetected by routine HPC. Following treatment S. maltophila and a Burkholderia -related species retained activity and entered distribution undetected by HPC. Significance and Impact of the Study:, Methods used here demonstrate how water treatment operators can better monitor water treatment plant efficacy and assess distribution system instability by the detection and identification of active bacteria recalcitrant to routine HPC culture. [source] Reconstruction of atomistic details from coarse-grained structuresJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2010Andrzej J. Rzepiela Abstract We present an algorithm to reconstruct atomistic structures from their corresponding coarse-grained (CG) representations and its implementation into the freely available molecular dynamics (MD) program package GROMACS. The central part of the algorithm is a simulated annealing MD simulation in which the CG and atomistic structures are coupled via restraints. A number of examples demonstrate the application of the reconstruction procedure to obtain low-energy atomistic structural ensembles from their CG counterparts. We reconstructed individual molecules in vacuo (NCQ tripeptide, dipalmitoylphosphatidylcholine, and cholesterol), bulk water, and a WALP transmembrane peptide embedded in a solvated lipid bilayer. The first examples serve to optimize the parameters for the reconstruction procedure, whereas the latter examples illustrate the applicability to condensed-phase biomolecular systems. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Echogenic liposome compositions for increased retention of ultrasound reflectivity at physiologic temperatureJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 6 2008Kyle D. Buchanan Abstract Targetable echogenic liposomes (ELIP) for ultrasound enhancement of atheroma have recently been developed; however, their retention of echogenicity at physiological temperature is less than desirable. The purpose of this study was to improve ELIP stability and increase clinical potential. The approach utilized the original procedures but involved manipulation of the lipid composition by reducing the level of unsaturation of the phospholipids components to minimize the rate of loss of echogenicity. Echogenicity was measured using a 20 MHz intravascular ultrasound (IVUS) catheter and quantified (as mean gray scale values) using computer-assisted videodensitometry. The optimal preparation for retention of echogenicity stability at physiologic temperature was egg phosphatidylcholine/dipalmitoylphosphatidylcholine/dipalmitoylphos-phatidylethanolamine/dipalmitoylphosphatidylglycerol/cholesterol (27:42:8:8:15, molar percent). This preparation retained 51,±,3.5% of its echogenicity after 1 h at 37°C, more than 5× that retained by the previously descried preparation. In this composition nearly 2/3 of the phosphosphatidylcholine is fully saturated. Such an increase in saturation is anticipated to stiffen the lipid acyl chains. The air pockets that are responsible for reflection of ultrasound waves can be assumed to be stabilized by a lipid monolayer at the interface between the air and bulk water. The increased rigidity of that monolayer is presumed to be responsible for reducing the loss of air and extending the duration of echogenic activity. The stability of this improved formulation now appears to be more than adequate for clinical applications. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:2241,2248, 2008 [source] pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in aqueous micellar solutions: relative contributions of hydrophobic and electrostatic interactionsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2001Omar A. EI Seoud Abstract The pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate (NPDCP) in the presence of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlorides (alkyl,=,cetyl and dodecyl) and polyoxyethylene(9) nonylphenyl ether was studied spectrophotometrically. The observed rate constants, kobs, decrease in the following order: bulk water >cationic micelles >anionic micelles >non-ionic micelles. This order is different from that observed for pH-independent hydrolysis of 4-nitrophenyl chloroformate (NPCF), whose reaction is faster in cationic micelles than in bulk water. A proton NMR study on solubilization of a model ester, 4-nitrophenyl 2-chloropropionate, showed that the methylene groups in the middle of the surfactant hydrophobic chain are most affected by the solubilizate. Lower polarity and high ionic strength of interfacial water decrease the rates of hydrolysis of both NPCF and NPDCP, but the fraction of the former ester that diffuses to the interface is probably higher than that of the latter. Therefore, whereas the (negatively charged) transition state of NPCF is stabilized by cationic interfaces and destabilized by anionic interfaces, that of NPDCP is negligibly affected by ionic interfaces, which explains the observed rate retardation by all ionic micelles. Calculated activation parameters corroborate our explanation. Copyright © 2001 John Wiley & Sons, Ltd. [source] Waterlogged archaeological wood,chemical changes by conservation and degradationJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2006M. Christensen Abstract Conservation of waterlogged archaeological wooden artefacts from the Nydam Bog in the southern part of Denmark was investigated by Raman spectroscopy. Two different conservation methods were used: the cellosolve/petroleum method and the polyethylene glycol (PEG)/freeze-drying method. Conservation with cellosolve/petroleum at both room and elevated temperatures was used to preserve archaeological arrows made from pinewood and ash wood. The double bonds in lignin were affected by the warm treatment. Holocellulose in the archaeological wooden arrows had decomposed completely. A band around 180 cm,1 in the R(v)-representation of the low-wavenumber Raman spectrum was used to monitor the presence of water in PEG2000 with a structure like the one in ,free' bulk water. The lignin/PEG2000 relative band intensities were used to make a quantitative estimate of the wood/PEG2000 ratio in a PEG2000 impregnated piece of hardwood. A decreasing degradation of holocellulose was observed from the surface to the centre of an oak piece from the Kolding Cog, built around 1200 A.D. A broad background was observed in the spectra recorded close to the surface of the plank. A similar, very intense, broad background was seen in untreated collapsed ash wood from the Nydam Bog. This broad background, most probably arising from fluorescence, was too intense to allow a Raman spectrum to be seen. In these cases, ATR-FTIR spectroscopy is a valuable complementary tool in studies of waterlogged archaeological wood, confirming that holocellulose degrades more rapidly than lignin. Copyright © 2006 John Wiley & Sons, Ltd. [source] Molecular Dynamics Simulations of the Orientation and Reorientational Dynamics of Water and Polypyrrole Rings as a Function of the Oxidation State of the PolymerMACROMOLECULAR THEORY AND SIMULATIONS, Issue 1 2005Jose J. López Cascales Abstract Summary: Polypyrrole is one of the most widely-studied conducting polymers due to its steady electrochemical response and good chemical stability in different solvents, including organic and inorganic ones. In this work, we provide for the first time valuable information in atomic detail concerning the steady and dynamic properties of pyrrole rings as a function of the oxidation state of the polymer. The study was carried out by Classical molecular dynamics simulation, where the system was modelled by 256 polypyrrole chains of 10 pyrrole rings each. Water was explicitly introduced in our simulations. Besides the uncharged or reduced state, two steady oxidation states of the polymer have been simulated by introducing a net charge (+1) on 85 and 256 of the polypyrrole chains. To balance the charges emerging in these oxidised states, 85 and 256 chloride ions (Cl,1) respectively, were introduced into the system. From an analysis of the simulated trajectories, the orientation and relaxation times of water and pyrrole rings were evaluated for the different oxidation states of the polymer across the polypyrrole/water interface. The calculated densities for different oxidation states describe the swelling or shrinking process during electrochemical oxidation or reduction respectively. The rotational relaxation times calculated for the polypyrrole rings decrease with increasing oxidation of the polymer, which is in a good agreement with experimental electrochemical data. Almost no variation in pyrrole ring orientation was measured for the different oxidation states of the polymer, even compared with polypyrrole bulk. As regards the water structure in the vicinity of the polypyrrole/water interface, both the orientation and orientation relaxation time were strongly affected by the presence of charges in the polymer. Thus, the water dipole was strongly orientated in the vicinity of the water/polypyrrole interface and its orientational relaxation time increased by one order of magnitude compared with bulk water, even when only one-third of the total polymer chains were oxidised. The results attained in this work were validated with experimental results, when they were available. Polypyrrole ring orientation and water orientation at the polypyrrole/water interface. (a) 256 rPPy and (b)171 rPPy,+,85 oPPy. [source] Resistance to CO2 diffusion in cuticular membranes of amphibious plants and the implication for CO2 acquisitionPLANT CELL & ENVIRONMENT, Issue 1 2007HENNING FROST-CHRISTENSEN ABSTRACT Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 × 10,6 m s,1 corresponding to resistance values of 23 to 295 × 103 s m,1, i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged. [source] Nature of water molecules in hydrogels based on a liquid crystalline cellulose derivativePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 11-12 2003P. Wojciechowski Abstract The status and nature of water molecules in hydrogels with a liquid crystalline organization of the polymer network based on a biopolymer were investigated. Liquid crystalline (LC) hydrogels were obtained in situ by the photopolymerization of acrylic acid into the lyotropic liquid crystalline phase of (2-hydroxypropyl)cellulose in a solvent mixture of water and acrylic acid. The photopolymerization of acrylic acid in the lyotropic liquid crystalline phase at room temperature gives a hydrogel in which liquid crystalline order and water are retained. The liquid crystalline hydrogel contains water, which originates from the composition of the lyotropic liquid crystalline phase, and may also contain water after immersion in liquid water. The water molecule characteristics were examined by means of differential scanning calorimetry, dielectric relaxation spectroscopy, and differential scanning calorimetry coupled with thermo-optical measurements. The swelling data were obtained by using a weighing method. The investigations reveal the different nature of the two above-mentioned water types. For the water from the composition of the lyotropic liquid crystalline phase, the phase transitions,typical for the bulk water,were not observed, in contract to the water after swelling of the liquid crystalline hydrogel in the liquid water. The results of the measurements suggest that water, which comes from the composition of the lyotropic liquid crystalline phase, forms,together with a polymer network,a microstructure, stabilized by this water. The water, after swelling of the LC hydrogel in the liquid water, is separated in the pores of the hydrogel and exhibits the phase transitions of the bulk water. Copyright © 2003 John Wiley & Sons, Ltd. [source] Water-protein interactions in the molten-globule state of carbonic anhydrase b: An NMR spin-diffusion studyPROTEIN SCIENCE, Issue 8 2000Victor P. Kutyshenko Abstract We have used the homonuclear Overhauser effect (NOE) to characterize a model protein: carbonic anhydrase B. We have obtained NOE difference spectra for this protein, centering the on-resonance signals either at the methyl-proton or at the water-proton signals. The spin-diffusion spectra obtained as a function of protein concentration and temperature provide direct evidence of much greater protein,water interaction in the molten-globule state than in the native and denatured states. Furthermore, although the protein loses its gross tertiary structure in both the molten-globule and denatured states, it remains almost as compact in its molten-globule state as it is in the native state. The spin-diffusion spectra, obtained as a function of a variable delay time after the saturation pulse, allowed us to measure the relaxation times of several types of proton in the solution. These spectra contain enough information to distinguish between those water molecules solvating the protein and the free ones present as bulk water. [source] A Micellar Multitasking Device: Sensing pH Windows and Gauging the Lipophilicity of Drugs with Fluorescent SignalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2010Franck Denat Prof. Abstract A multitasking fluorescent device can be obtained by forming micelles of Triton,X-100, containing a lipophilic macrocyclic Cu2+ complex and the coordinating fluorophore Coumarin,343 (C343), which features a COOH moiety. At low pH the two micellised components do not interact, and the fluorescence of Courmarin,343 (C343) is intense. At intermediate pH, C343 is deprotonated and coordinates to the Cu2+ centre in its apical position, with fluorescence quenching. At higher pH the deprotonated C343 is displaced from Cu2+ by the formation of an OH, complex, and the fluorescence is revived. This allows the system to carry out its first task as it behaves as an "on,off,on" fluorescent sensor for pH windows. The "off" part of the window ranges from pH,6 to 8. In this interval, in which the carboxylate form of C343 is apically coordinated to the Cu2+ complex inside micelles, the device carries out its second task, that is, it behaves as a gauge for lipophilicity. For pHs between 6 and 8, molecules containing a COOH group are in their COO, form and distribute between bulk water and micelles proportionally to their lipophilicity. Upon entering the micelle, their COO, moiety competes for coordination with C343, displacing it from the Cu2+ centre, and this results in fluorescence revival, the intensity of which is also proportional to the lipophilicity of the examined molecule. We have chosen the physiological pH value (7.4) as the working pH, and we have examined the lipophilicity of fatty acids and of the widely used family of non-steroidal anti-inflammatory drugs (NSAIDs). The device successfully measures their lipophilicity, expressing it with an "off,on" type fluorescent signal, as demonstrated by the correlation of the fluorescence increase with the logarithmic water/octanol partition coefficient (log,P) and with the difference between the pKa observed in micelles and that measured in water for NSAIDs. [source] How Does a Membrane Protein Achieve a Vectorial Proton Transfer Via Water Molecules?CHEMPHYSCHEM, Issue 18 2008Steffen Wolf Abstract We present a detailed mechanism for the proton transfer from a protein-bound protonated water cluster to the bulk water directed by protein side chains in the membrane protein bacteriorhodopsin. We use a combined approach of time-resolved Fourier transform infrared spectroscopy, molecular dynamics simulations, and X-ray structure analysis to elucidate the functional role of a hydrogen bond between Ser193 and Glu204. These two residues seal the internal protonated water cluster from the bulk water and the protein surface. During the photocycle of bacteriorhodopsin, a transient protonation of Glu204 leads to a breaking of this hydrogen bond. This breaking opens the gate to the extracellular bulk water, leading to a subsequent proton release from the protonated water cluster. We show in detail how the protein achieves vectorial proton transfer via protonated water clusters in contrast to random proton transfer in liquid water. [source] Transport Processes at ,-Quartz,Water Interfaces: Insights from First-Principles Molecular Dynamics SimulationsCHEMPHYSCHEM, Issue 7 2008Waheed A. Adeagbo Dr. Abstract Car,Parrinello molecular dynamics (CP,MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the ,-quartz,water interface. The model system initially consists of a periodically repeated quartz slab with O-terminated and Si-terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H2O molecules into H+ and OH, is observed at the Si-terminated surface. The OH, fragments immediately bind chemically to the Si-terminated surface while Grotthus-type proton diffusion through the water film leads to protonation of the O-terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO2 present as Si(OH)4 in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)4 molecule as would have been indicated by a substantial lowering of the Si(OH)4 diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP,MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two SiO bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new SiO bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz SiO bond. The final solute species is Si(OH)4. [source] Structure and Dynamics of Water Confined in Dimethyl SulfoxideCHEMPHYSCHEM, Issue 1 2006A. Wulf Abstract We study the structure and dynamics of hydrogen-bonded complexes of H2O/D2O and dimethyl sulfoxide (DMSO) by infrared spectroscopy, NMR spectroscopy and ab initio calculations. We find that single water molecules occur in two configurations. For one half of the water monomers both OH/OD groups form strong hydrogen bonds to DMSO molecules, whereas for the other half only one of the two OH/OD groups is hydrogen-bonded to a solvent molecule. The H-bond strength between water and DMSO is in the order of that in bulk water. NMR deuteron relaxation rates and calculated deuteron quadrupole coupling constants yield rotational correlation times of water. The molecular reorientation of water monomers in DMSO is two-and-a-half times slower than in bulk water. This result can be explained by local structure behavior. [source] | ||||||||||||||||||||||||||||