Water Structure and Dynamics at a Silica Surface: Pake Doublets in 1H NMR Spectra (original) (raw)
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Impact of Hydrophilic Surfaces on Interfacial Water Dynamics Probed with NMR Spectroscopy
The Journal of Physical Chemistry Letters, 2011
In suspensions of Nafion beads and of cationic gel beads, NMR spectroscopy showed two water–proton resonances, one representing intimate water layers next to the polymer surface, the other corresponding to water lying beyond. Both resonances show notably shorter spin–lattice relaxation times (T1) and smaller self-diffusion coefficients (D) indicating slower dynamics than bulk water. These findings confirm the existence of highly restricted water layers adsorbed onto hydrophilic surfaces and dynamically stable water beyond the first hydration layers. Thus, aqueous regions on the order of micrometers are dynamically different from bulk water.
Water Adsorption on Pyrogenic Silica Followed by1H MAS NMR
Journal of Colloid and Interface Science, 1997
physisorbed water), and their accessibility to water (internal On the surface of two commercial pyrogenic silicas (Degussa or external silanols). Study of the state of physisorbed water and Cabot), five resonances were identified on the basis of the itself led to the distinction between structured monolayers, chemical shift, homonuclear coupling ( T 2 ), and spin-lattice relaxclusters, and weakly bound liquid-like water molecules (14). ation behavior (T 1 ). In accordance with previous studies we ob-Nevertheless, several difficulties remain when studying served three different types of silanol groups: (i) weakly coupled silica surfaces. First, the surface is not structured. Therefore, (long T 2 ), water inaccessible, isolated ''internal'' silanols at 1.8 although models based on small crystalline domains have ppm; (ii) weakly coupled, external ''free'' silanols revealed upon been proposed (7, 10, 15), surface irregularity leads to a dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between dispersion of the physicochemical properties of the hydroxyl 3 and 7 ppm. The resonance of water, whose position between 2.6 sites. Second, thermogravimetric dosing of structural and and 4.6 ppm depended on water content, corresponded to two adsorbed water is difficult as (de)hydration overlaps with unresolved species of slightly different T 1 . By equating this reso-(de)hydroxylation. Third, amorphous silica is a metastable nance to the weighted average of two distinct populations of water, phase, and its surface is heavily reconstructed during the we were able to distinguish the first layer of strongly hydrogen conditioning of the samples. bound water at 2.7 ppm from liquid-like water at 5 ppm. The first Among the different techniques mobilized to scrutinize layer is complete for water relative humidity as low as 3.6% and protons on the surface of silica in particular, and oxides in corresponds to a surface coverage of 4.75 H 2 O/nm 2 . If we assumed general, 1 H NMR is certainly among the most promising a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. because of its great sensitivity and dependency on hydrogen This ratio was the same for the two silicas regardless of surface bonds. Despite severe line broadening due to a combination area. ᭧ 1997 Academic Press of strong homodipolar coupling and chemical shift disper-Key Words: 1 H MAS-NMR; silica.
Chemistry (Weinheim an der Bergstrasse, Germany), 2004
The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), po...
Dynamics and vibrational spectroscopy of water at hydroxylated silica surfaces
Faraday Discussions, 2013
In the present study, the structural and dynamical properties of water at hydroxylated silica surfaces are investigated with classical molecular dynamics simulations. Depending on the nature of the interface, water molecules are observed to have well defined ordering and slower dynamics compared to bulk water. These properties include the orientation of water near the surface, reorientational relaxation times, translational diffusion coefficients, planar density distribution and vibrational spectroscopic features. The dynamical and structural features are affected up to z6-10Å away from the surface, depending on the properties considered (lateral diffusion coefficient, charge density profile, rotational orientational time). Water molecules at the silica surface show a marked decrease in the diffusion coefficient and an increase in rotational correlation times. The presence of the polar-OH group on the hydroxylated silica surface provides adsorption sites for water with preferred orientations. In addition to the known broadening of the water spectrum in the bonded OH-stretch region for first-layer water molecules, the present simulations find a characteristic band at 1150 cm À1 , which is assigned to the HO(water)-H(SiOH) bending vibration. Because this signal only occurs for water molecules in the first layer, it should be experimentally accessible through surface sensitive techniques such as vibrational sum frequency generation spectroscopy (VSFG).
Proton NMR study on the structure of water in the Stern layer of negatively charged micelles
The Journal of Physical Chemistry, 1987
The proton chemical shift of water was studied as a function of the concentration of three negatively charged surfactants (sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and sodium perfluorooctanoate) and the deuterium content of the solvent. The effect of the simple presence of ionic head groups of the surfactants (Le., in the absence of the micellar interface) on the structure of water was determined by studying the behavior of the model short-chain compounds: sodium butyl sulfate, sodium p-toluenesulfonate, and sodium perfluorobutyrate. Graphs of the chemical shifts vs. the mole fractions of the solubilizates were linear in all cases and the slopes were used to calculate the so-called "fractionation factor, cp" (cp = 1 for the bulk solvent) of the water of hydration. Little structure perturbation was produced by the solubilization of the short-chain compounds, the sulfate and the sulfonate anions being slight structure breakers (cp = 0.97 f 0.04) whereas the perfluorobutyrate was found to marginally enhance the structuring (cp = 1.02 f 0.03). The water in the Stern layer of the micelles was found, however, to be more organized than bulk water (cp = 1.07 f 0.04), in agreement with other measurements of NMR chemical shifts, relaxation times, and self-diffusion coefficients. The utility as well as some possible limitations of this new approach is discussed.
Journal of the American Chemical Society, 1996
Two silicas obtained by precipitation have been compared using two 1 H NMR techniques: broad-line at 4 K with simulation of the spectra and MAS at room temperature. Previous 29 Si high resolution NMR results have been used to interpret the spectra. Geminal silanols, single silanols, water molecules relatively strongly hydrogenbonded to silanols, and water not interacting with OH groups have been characterized and quantified. Differences are found between the two precipitated silicas. Silanols, 70% of which can hydrogen-bond a water molecule, occupy all the surface of one of them. The other sample surface is not so crowded with silanols, and only 37% of them hydrogen-bond water molecules. Both geminal and single silanols hydrogen-bond water. Physisorbed water molecules not interacting with silanols can be equilibrated on both sample surfaces. Room temperature dehydration of the samples in vacuum is studied.
The Journal of Physical Chemistry C, 2018
The behavior of mixtures of octanol-isomers with water with different molar ratio confined inside the mesoporous silica SBA-15 was investigated by a combination of solid-state NMR spectroscopy and molecular dynamics (MD) simulations. Two dimensional 1 H-29 Si FSLG-HETCOR NMR spectra revealed the orientation of the 1octanol relative to the pore walls. These arrangements are in good agreement with the preferred structures found by MD. In addition, MD simulations also shed light on molecular orientations and interactions in the pore center region, which are not resolvable by solid-state NMR. 2 H-solid-state measurements All 2 H-solid-state spectra were measured with a laboratory-built 2 H-solid-state NMR spectrometer operating at 7 T Since the spectrometer was previously described, 65 here
Ordering effect of protein surfaces on water dynamics: NMR relaxation study
Biophysical Chemistry, 2019
Proteins in solution affect the structural and dynamic properties of the bulk water at the proteinwater interface, resulting in a contribution to the order of the hydration water. Theoretical and experimental NMR relaxation methods were developed to study the dynamic properties of water molecules in the protein hydration shell. Water non-selective and selective relaxation rates, were shown to be sensitive to contributions from ordered solvent molecules at protein surface. The average rotational correlation time of water molecules in the protein hydration shell was determined for three protein systems of different size: ribonuclease A, human serum albumin and fibrinogen. The knowledge of these properties is an important step towards the determination of the size of the water ordering contributions originate in proteins systems.
Zeitschrift für Physikalische Chemie, 2018
A series of novel functionalized mesoporous silica-based materials with well-defined pore diameters, surface functionalization and surface morphology is synthesized by co-condensation or grafting techniques and characterized by solid-state NMR spectroscopy, DNP enhanced solid state-NMR and thermodynamic techniques. These materials are employed as host-systems for small-guest molecules like water, small alcohols, carbonic acids, small aromatic molecules, binary mixtures and others. The phase-behavior of these confined guests is studied by combinations of one dimensional solid-state NMR techniques (1H MAS, 2H-line shape analysis, 13C CPMAS) and two-dimensional correlation experiments like 1H-29Si- solid-state HETCOR.