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Marie-pierre Gaigeot - One of the best experts on this subject based on the ideXlab platform.
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pKa at Quartz/Electrolyte Interfaces
Journal of Physical Chemistry Letters, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre Gaigeot, Marialore SulpiziAbstract:Acidity of Silanol sites at the crystalline quartz/aqueous electrolyte (NaCl, NaI, KCl) interfaces are calculated from ab initio molecular dynamics simulations. pKa’s are found to follow a combination of the cationic and anionic Hofmeister series in the order pKa(neat solution) < pKa(NaCl) < pKa(NaI) < pKa(KCl), in agreement with experimental measurements. Rationalization of this ranking is achieved in terms of the microscopic local solvation of the protonated Silanols and their conjugated bases, the Silanolates SiO–. The change in the pKa is the result of both water destructuring by alkali halides, as well as of the specific cation/SiO– interaction, depending on the electrolyte. Molecular modeling at the atomistic level is required to achieve such comprehension, with ab initio molecular dynamics being able to model complex inhomogeneous charged interfaces and the associated interfacial chemical reactivity.
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Electrolytes at the Hydroxylated (0001) α-Quartz/Water Interface: Location and Structural Effects on Interfacial Silanols by DFT-Based MD
Journal of Physical Chemistry C, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre GaigeotAbstract:Structural properties of NaCl, KCl, and NaI electrolytes forming an electrical double layer (EDL) at the fully hydroxylated (0001) α-quartz/liquid water interface have been investigated by means of first-principles molecular dynamics simulations (FPMD). Cations are found in inner-sphere conformations, directly bonded on two in-plane Silanol groups that replace water molecules that would be present in the first solvation shell of the aqueous cations. Anions are located within the second/third water layer above the surface, fully solvated as in pure liquid water, and cation–anion adopt rather flexible solvent separated ion-pair (SSIP) geometries in the EDL. While the individual solvation shells of the aqua-ions are only slightly affected by ion-pairing at the interface, the Silanols at the quartz surface are strongly perturbed. The presence of the electrolytes in the EDL affects more deeply the Silanols’ geometrical properties than single ions do (J. Phys. Chem. C2016, 120, 4866–4880): the Silanol–Silanol intrasurface H-bonding that was observed at the neat interface is extremely weakened by the presence of the electrolytes. Further disordering of the surface Silanols is characterized by large changes in their orientation and covalent bond-lengths, regardless of their in-plane (IP) or out-of-plane (OP) orientations. Such structural changes of the surface Silanols are tentatively correlated here with an increase in the basicity of all surface sites.
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Adsorption of Singly Charged Ions at the Hydroxylated (0001) α-Quartz/Water Interface
Journal of Physical Chemistry C, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre GaigeotAbstract:Individual alkali (Na+, K+) and halide (Cl–, I–) ion effects have been characterized at the fully hydroxylated (0001) α-quartz water interface by means of ab initio molecular dynamics simulations in the framework of the electronic DFT representation (DFT-MD). We particularly focus our analyses on the ion adsorption and solvation structures (made by water and by surface Silanols), as well as on perturbations undergone by the Silanol surface sites when comparing the charged interfaces (present work) to the neat interface (our previous works, J. Chem. Theory. Comput. 2012, 8, 1037; J. Phys.: Condens. Matter 2012, 24, 124106). Both sodium and potassium cations are found adsorbed in an inner-sphere configuration, while chloride and iodide are found in between inner- and outer-sphere. Cation adsorption at the interface is found to induce more perturbation on interfacial properties than anions do. In particular, we show in details how and why the orientation of out-of-plane and in-plane surface Silanols found at the neat interface are modified by inner-sphere cations at the charged interfaces, with also consequences on the Silanol–Silanol intrasurface hydrogen bond network. All this detailed analysis provides a clear picture of a reduction of acidity of the surface Silanols at the quartz/water interface in the presence of the alkali/halide salts.
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the silica water interface how the Silanols determine the surface acidity and modulate the water properties
Journal of Chemical Theory and Computation, 2012Co-Authors: Marialore Sulpizi, Marie-pierre Gaigeot, Michiel SprikAbstract:Silica is the most abundant metal oxide and the main component of the Earth's crust. Its behavior in contact with water plays a critical role in a variety of geochemical and environmental processes. Despite its key role, the details of the aqueous silica interface at the microscopic molecular level are still elusive. Here we provide such a detailed understanding of the molecular behavior of the silica-water interface, using density functional theory based molecular dynamics (DFTMD) simulations, where a consistent treatment of the electronic structure of solvent and surface is provided. We have calculated the acidity of the Silanol groups at the interface directly from the DFTMD simulations, without any fitting of parameters to the experimental data. We find two types of Silanol groups at the surface of quartz: out-of-plane Silanols with a strong acidic character (pKa = 5.6), which consequently results in the formation of strong and short hydrogen bonds with water molecules at the interface, and in-plane Silanols with a pKa of 8.5, forming weak hydrogen bonds with the interfacial water molecules. Our estimate of the quartz point of zero charge (1.0) is found in good agreement with the experimental value of 1.9. We have also shown how the Silanols orientation and their hydrogen bond properties are responsible for an amphoteric behavior of the surface. A detailed analysis has identified two species of adsorbed water molecules at the solid-liquid interface, which using the language of vibrational spectroscopy can be identified as "liquid-like" and "ice-like" water or, in other words, water molecules forming respectively weak and strong H-bonds with the oxide surface. These two populations of water are in turn responsible for two distinct peaks in the infrared spectrum of interfacial water and thus provide a molecular explanation of the experimental sum frequency generation spectrum recorded in the literature. In the specific case of quartz, we show that the liquid-/ice-like behavior is the result of the Silanol groups ability to donate or accept hydrogen bonds with different strengths, which consequently modulates the vibrational properties of the adsorbed water layer.
Carmine Dagostino - One of the best experts on this subject based on the ideXlab platform.
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effect of al content on the strength of terminal Silanol species in zsm 5 zeolite catalysts a quantitative drifts study without the use of molar extinction coefficients
Physical Chemistry Chemical Physics, 2018Co-Authors: Pierre Brauer, Olivia Situmorang, Carmine DagostinoAbstract:The strength of terminal hydroxyl Si-OH groups (Silanols) in zeolites is important for many non-size-selective catalytic reactions occurring onto the external surface of the zeolite crystals and may often be responsible for catalyst deactivation, e.g., coke formation. A quantitative analysis of Si–OH strength and its link with the Al content, hence varying silica-to-alumina ratio (SAR = SiO2/Al2O3), has not been established yet. Various hypotheses have been proposed in the literature; nonetheless, the role of Al content in determining Silanol strength remains still unclear and the object of speculation. In this work, we have systematically investigated the effect of the Al content on the strength of terminal Silanol sites in ZSM-5 zeolite catalysts with varying SAR using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) at variable temperatures without molar extinction coefficients. Two base probe molecules with different proton affinity values, pyridine and collidine, were used. To quantify the strength of terminal Silanol sites the change of the terminal Silanol peak in the OH stretching region, together with data on elemental analysis, was used. With this experimental protocol, unlike most IR studies, the use of molar extinction coefficients, often difficult to obtain, is not needed for quantification. The results reported here show for the first time that for ZSM-5 zeolite catalysts the fraction of occupied terminal Silanol acid sites for both pyridine and collidine increases with increasing concentration of external Bronsted acid sites, hence establishing a clear link between the two types of acid sites. In summary, this work shows that the use of DRIFTS without molar extinction coefficients is able to quantitatively probe the strength of terminal Silanol acid sites and establishes a link between the external Bronsted Al content and the strength of terminal Silanol species in ZSM-5 zeolites with varying SAR at elevated temperatures.
Roman Kaliszan - One of the best experts on this subject based on the ideXlab platform.
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ionic liquids as mobile phase additives for feasible assay of naphazoline in pharmaceutical formulation by hptlc uv densitometric method
Journal of Chromatographic Science, 2013Co-Authors: Michal Piotr Marszall, Wiktor Dariusz Sroka, Dominik Mieszkowski, Marcin Koba, Aleksandra Balinowska, Roman KaliszanAbstract:A specific and reliable high-performance thin layer chromatography method with densitometry detection has been developed for the de- termination of naphazoline nitrate in nasal drops. The best separ- ation of the basic analyte, without spot tailing, was achieved by using a mobile phase composed of acetonitrile -water (60:40, v/v), adding 1.5 % (v/v) imidazolium-class ionic liquid and covering the plates with a stationary phase based on RP-18 with F254S (10 3 20 cm). The presented results confirm that imidazolium tetrafluoro- borate ionic liquids are efficient suppressors of free Silanols, which are considered to be responsible for troublesome and irreproducible chromatographic determinations of basic compounds. The devel- oped chromatographic system was found to be convenient in use and to provide a repeatable assay of naphazoline nitrate in nasal drops, which could not be obtained with the use of standard Silanol suppressing mobile phase additives such as triethylamine or dimethyloctylamine. the ILs based on the BF4 ,C l 2 and MeSO4 anions are water- stable compounds, which dissolve in typical chromatographic mobile phases. Recently, ILs have been proposed as Silanol suppressing agents (15, 16). The significant effects of imidazolium-based ILs as mobile phase modifiers in thinlayer chromatography (TLC) and high-performance liquid chromatography (HPLC) on the retention of basic compounds have been studied and described elsewhere (13). The addition of 0.5 -2.5% (v/v) of some types of ILs composed of 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-propyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl-3-methylimidazolium or 1-butyl-4-methylpyridinium cations and bromide chlorate, hexafluorophosphate, methyl sulphate, tetrafluoroborate or tosylate anions more markedly
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Ionic Liquids as Mobile Phase Additives for Feasible Assay of Naphazoline in Pharmaceutical Formulation by HPTLC–UV–Densitometric Method
2012Co-Authors: Michal Piotr Marszal=l, Wiktor Dariusz Sroka, Ra Balinowska, Dominik Mieszkowski, Marcin Koba, Roman KaliszanAbstract:A specific and reliable high-performance thin layer chromatography method with densitometry detection has been developed for the de-termination of naphazoline nitrate in nasal drops. The best separ-ation of the basic analyte, without spot tailing, was achieved by using a mobile phase composed of acetonitrile–water (60:40, v/v), adding 1.5 % (v/v) imidazolium-class ionic liquid and covering the plates with a stationary phase based on RP-18 with F254S (10 3 20 cm). The presented results confirm that imidazolium tetrafluoro-borate ionic liquids are efficient suppressors of free Silanols, which are considered to be responsible for troublesome and irreproducible chromatographic determinations of basic compounds. The devel-oped chromatographic system was found to be convenient in use and to provide a repeatable assay of naphazoline nitrate in nasal drops, which could not be obtained with the use of standard Silanol suppressing mobile phase additives such as triethylamine or dimethyloctylamine
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Evaluation of the Silanol‐suppressing potency of ionic liquids
Journal of separation science, 2006Co-Authors: Michał Piotr Marszałł, Tomasz Baczek, Roman KaliszanAbstract:Recently, increasing attention has been paid to the use of ionic liquids for high-performance liquid chromatography (HPLC) and capillary electrophoresis. In the present study, the Silanol-suppressing potency of ionic liquids was evaluated by HPLC using the two-retention site model proposed previously by Nahum and Horvath (J. Chromatogr. 1981, 203, 53-63). The binding constant, K A , in that approach has been demonstrated to reliably reflect the ability of the ionic liquids to block the Silanols of the silica support material of the stationary phase. The determinations were carried out for ionic liquids of the 1-alkyl-3-methylimidazolium group with the use of a series of basic drugs as the test analytes. Comparison of ionic liquids with standard mobile phase additives such as triethylamine showed the former to possess advantages as Silanol suppressors in HPLC. The main advantage of the method is that it provides a simple and fast determination of the Silanol complex stability, which allowed comparison of the suppressing efficiency of several ionic liquids.
Morgane Pfeiffer-laplaud - One of the best experts on this subject based on the ideXlab platform.
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pKa at Quartz/Electrolyte Interfaces
Journal of Physical Chemistry Letters, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre Gaigeot, Marialore SulpiziAbstract:Acidity of Silanol sites at the crystalline quartz/aqueous electrolyte (NaCl, NaI, KCl) interfaces are calculated from ab initio molecular dynamics simulations. pKa’s are found to follow a combination of the cationic and anionic Hofmeister series in the order pKa(neat solution) < pKa(NaCl) < pKa(NaI) < pKa(KCl), in agreement with experimental measurements. Rationalization of this ranking is achieved in terms of the microscopic local solvation of the protonated Silanols and their conjugated bases, the Silanolates SiO–. The change in the pKa is the result of both water destructuring by alkali halides, as well as of the specific cation/SiO– interaction, depending on the electrolyte. Molecular modeling at the atomistic level is required to achieve such comprehension, with ab initio molecular dynamics being able to model complex inhomogeneous charged interfaces and the associated interfacial chemical reactivity.
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Electrolytes at the Hydroxylated (0001) α-Quartz/Water Interface: Location and Structural Effects on Interfacial Silanols by DFT-Based MD
Journal of Physical Chemistry C, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre GaigeotAbstract:Structural properties of NaCl, KCl, and NaI electrolytes forming an electrical double layer (EDL) at the fully hydroxylated (0001) α-quartz/liquid water interface have been investigated by means of first-principles molecular dynamics simulations (FPMD). Cations are found in inner-sphere conformations, directly bonded on two in-plane Silanol groups that replace water molecules that would be present in the first solvation shell of the aqueous cations. Anions are located within the second/third water layer above the surface, fully solvated as in pure liquid water, and cation–anion adopt rather flexible solvent separated ion-pair (SSIP) geometries in the EDL. While the individual solvation shells of the aqua-ions are only slightly affected by ion-pairing at the interface, the Silanols at the quartz surface are strongly perturbed. The presence of the electrolytes in the EDL affects more deeply the Silanols’ geometrical properties than single ions do (J. Phys. Chem. C2016, 120, 4866–4880): the Silanol–Silanol intrasurface H-bonding that was observed at the neat interface is extremely weakened by the presence of the electrolytes. Further disordering of the surface Silanols is characterized by large changes in their orientation and covalent bond-lengths, regardless of their in-plane (IP) or out-of-plane (OP) orientations. Such structural changes of the surface Silanols are tentatively correlated here with an increase in the basicity of all surface sites.
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Adsorption of Singly Charged Ions at the Hydroxylated (0001) α-Quartz/Water Interface
Journal of Physical Chemistry C, 2016Co-Authors: Morgane Pfeiffer-laplaud, Marie-pierre GaigeotAbstract:Individual alkali (Na+, K+) and halide (Cl–, I–) ion effects have been characterized at the fully hydroxylated (0001) α-quartz water interface by means of ab initio molecular dynamics simulations in the framework of the electronic DFT representation (DFT-MD). We particularly focus our analyses on the ion adsorption and solvation structures (made by water and by surface Silanols), as well as on perturbations undergone by the Silanol surface sites when comparing the charged interfaces (present work) to the neat interface (our previous works, J. Chem. Theory. Comput. 2012, 8, 1037; J. Phys.: Condens. Matter 2012, 24, 124106). Both sodium and potassium cations are found adsorbed in an inner-sphere configuration, while chloride and iodide are found in between inner- and outer-sphere. Cation adsorption at the interface is found to induce more perturbation on interfacial properties than anions do. In particular, we show in details how and why the orientation of out-of-plane and in-plane surface Silanols found at the neat interface are modified by inner-sphere cations at the charged interfaces, with also consequences on the Silanol–Silanol intrasurface hydrogen bond network. All this detailed analysis provides a clear picture of a reduction of acidity of the surface Silanols at the quartz/water interface in the presence of the alkali/halide salts.
Gary E. Maciel - One of the best experts on this subject based on the ideXlab platform.
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Silica Gel Surface: Molecular Dynamics of Surface Silanols
The Journal of Physical Chemistry C, 2008Co-Authors: Takeshi Kobayashi, And Joseph A. Diverdi, Gary E. MacielAbstract:The molecular dynamics of the Silanols of high surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Deuterated silica gel samples were prepared by replacement of the exchangable Silanol protons upon exposure to liquid 2H2O and subsequent dehydration under vacuum at various temperatures that were selected to provide samples with varying populations of specific surface features, including (a) “isolated” Silanols, that is, those that are not hydrogen-bonded (as demonstrated by the 1H NMR chemical shift), (b) hydrogen-bonded Silanols with a wide range of hydrogen-bonding strengths (as shown by the broad 1H NMR peak), and (c) a hydrogen-bonded network of physisorbed water. On a highly dehydrated surface obtained by dehydration at 500 °C and containing only isolated Silanols, analysis of the deuterium line shapes indicates that the “isolated” Silanols exhibit a broad, inhomogeneous distribution of librational amplitudes of O−H entities about the internuclear Si−O vector, all in the ...
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PROBING HYDROGEN BONDING AND THE LOCAL ENVIRONMENT OF SilanolS ON SILICA SURFACES VIA NUCLEAR SPIN CROSS POLARIZATION DYNAMICS
Journal of the American Chemical Society, 1996Co-Authors: I-ssuer Chuang And, Gary E. MacielAbstract:By studying 1H→29Si cross-polarization dynamics of two untreated and two “dry” silica gel samples (one evacuated at 25 °C and one evacuated at 200 °C), we find that all the surface Silanols on the two untreated silicas are hydrogen bonded, either to the hydroxyl groups of adjacent Silanol(s) or to water molecule(s). About 46% and 47% of the geminal Silanols and 53% and 58% of the single Silanols that were hydrogen bonded only to water in the two untreated silicas become non-hydrogen bonded on the two “dry” silica surfaces, but the remainder of the Silanols of the untreated silicas (i.e., those hydrogen bonded to other Silanols) remain hydrogen bonded to other Silanols upon drying. The ratio of the number of hydrogen-bonding single Silanols to the number of hydrogen-bonding geminal Silanols is 17-to-1 for a Fisher silica surface evacuated at 25 °C and 16-to-1 for a Baker silica surface evacuated at 200 °C. These results can be explained in terms of a generalized silica surface model based on the β-cristoba...
