Density Of Surface Hydroxyl Groups Research Articles

Effects of surface hydroxyl group density on the photocatalytic activity of Fe3+-doped TiO2

The aim of the work is to study the effects of oxygen vacancy and surface hydroxyl group density on the photocatalytic activity of Fe3+-doped TiO2, and to investigate how the titanium dioxide doped with different levels of the Fe3+ influenced their physical and chemical characterizations. The photocatalysts were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), electron spin resonance (ESR), X-ray photoelectron spectroscope (XPS) and Fourier transform infrared (FTIR) spectra. The results revealed that adsorbed hydroxyl group density significantly influenced the photocatalytic activity, and a small amount of Fe3+ can act as a photo-generated hole and a photo-generated electron trap and inhibit the electron–hole recombination. The 0.10%-Fe3+-TiO2 with the highest surface hydroxyl group density revealed the maximum rate constant of 0.716 and the optimal photocatalytic degradation of MB. As Fe3+ doping levels are larger than 0.10%, the cluster of FeTi′–V¨O–FeTi′ generated gradually. This implied that an excessive amount of Fe3+ doped into TiO2 is detrimental to the photocatalytic activity due to the formation of FeTi′–V¨O–FeTi′ clusters and enhances the recombination of photogenerated electrons and holes.

Structural and Dynamical Properties of POPC Bilayers Supported on Nanoporous Substrates

The structure, dynamics and organization of lipid bilayers depend on a variety of environmental factors. Here we examine, using molecular dynamics (MD) simulations, the specific effects of nanoporous substrates on palmitoyl-oleyl phosphatidylcholine (POPC) bilayers. We expose POPC bilayers unilaterally and separately to various model Lennard-Jones (LJ) solid substrates differing in surface chemistry. In an earlier study, we had found that substrates with surface hydroxyl densities in the range 10-20% kept POPC bilayers juxtaposed to the substrates. While the bilayers did not interact directly with these hydroxylated substrates, they were separated from substrates by thin (< 0.6 nm) layers of water. Additionally, despite such buffered interactions, these supported bilayers exhibited physical properties notably different from unsupported bilayers. Recent experiments show that zwitterionic lipid bilayers supported on partially charged, nanoporous silica wafers also do not interact with silica directly. However, in such cases the bilayer is separated from the silica substrate by a relative thicker (∼1.5 nm) layer of solvent. We report here results from our MD simulation that reconcile these two seemingly disparate observations. We find that LJ nano-substrates whose porosity and surface hydroxyl density match those of nanoporous silica wafers do not increase the substrate-bilayer distance. Instead, the introduction of partial charge on the LJ substrate increases the substrate-bilayer distance to 1.5 nm. The negative charge on the substrate attracts hydrated ions, creating an electric double layer, which is a sufficient condition to increase the substrate-bilayer distance. While the properties of the bilayer supported on a charged substrate are also different from those of unsupported bilayers, the extent of the perturbation is not as large compared to that induced by uncharged hydroxylated substrates.

UV-induced photoactive adsorption mechanism of arsenite by anatase TiO2 with high surface hydroxyl group density

Adsorption and photo-oxidation using anatase TiO2 is a promising technique for arsenite removal from aqueous solution. However, adsorption and photo-oxidation at the solid/liquid interface of TiO2 in aqueous suspensions is a complex reaction process. In this study, we use a simple and inexpensive synthesis method for this type of TiO2 and assess its arsenite adsorption and photo-oxidation performance. The prepared anatase TiO2 is rich in hydroxyl groups, with a hydroxyl concentration of approximately 12.84nm−2. Under irradiation with UV light, the adsorption capacity increases and the residual concentration of arsenic in the solution is lower than 10μgL−1. According to zeta potential, FTIR, and XPS analyses, the hydroxyl groups on the anatase TiO2 surface are responsible for arsenic adsorption. The effect of hydroxyl radicals is investigated through scavenger experiments, the results of which indicate that hydroxyl radicals play a key role in the oxidation of arsenite on anatase TiO2 under UV irradiation. The number of hydroxyl groups on the anatase TiO2 increases upon UV illumination, indicating that these are the photoactive species for arsenic adsorption.

ZnAl2O4 as a novel high-surface-area ozonation catalyst: One-step green synthesis, catalytic performance and mechanism

Abstract Due to the great importance of high-surface-area metal oxide catalysts in catalytic ozonation, green and facile synthesis of a novel robust ozonation catalyst with high surface area is greatly attractive. In this paper, using inexpensive inorganic salts as starting materials, a high-surface-area (196 m 2 /g) porous nanocrystalline ZnAl 2 O 4 was prepared by a one-step green hydrothermal method without organic template, high temperature treatment and supported substance. ZnAl 2 O 4 was developed as a novel ozonation catalyst with enhanced catalytic activity in removal of phenol with a high concentration (300 mg/L) and the removal rate of phenol reached 73.4% within 60 min. The slight decrease by 5.7% on phenol removal rate after being used 4 times of catalyst illustrated the good reusability of ZnAl 2 O 4 in catalytic ozonation. The positive linear correlation between catalytic activity and the density of surface hydroxyl groups of catalyst was established. Surface hydroxyl groups and chemisorbed water of catalyst acted as catalytic active sites for the generation of hydroxyl radicals. In the presence of catalyst, ozone transformed into high active hydroxyl radicals, leading to the enhanced removal of phenol in bulk solution. Moreover, ZnAl 2 O 4 can be applied in a wide pH range from 3.3 to 9.3 and is a promising ozonation catalyst.

Catalytic ozonation of p-chloronitrobenzene over pumice-supported zinc oxyhydroxide

The catalytic ozonation of p-chloronitrobenzene (pCNB) in an aqueous solution using pumice-supported zinc oxyhydroxide (ZMP) as the catalyst was investigated. ZMP significantly enhanced the degradation efficiency in the heterogeneous catalytic ozonation compared with ozonation alone. The decomposition rate of the aqueous ozone increased 2.84-fold in the presence of ZMP. Catalytic ozone decomposition showed that pCNB is oxidized primarily by hydroxyl radicals (•OH) in ozonation/ZMP processes. This modification increases the density of surface hydroxyl groups as well as the pH at the point of zero charge (pHPZC) of pumice, resulting in the appearance of new ZnO and Zn(OH)2 crystalline phases. An investigation of the underlying mechanism confirms that ZnOOH loading promotes •OH initiation, which enhances the degradation of pCNB.

Applications of Mesoporous Materials as Excipients for Innovative Drug Delivery and Formulation

Due to uniquely ordered nanoporous structure and high surface area as well as large pore volume, mesoporous materials have exhibited excellent performance in both controlled drug delivery with sustained release profiles and formulation of poorly aqueoussoluble drugs with enhanced bioavailability. Compared with other bulk excipients, mesoporous materials could achieve a higher loading of active ingredients and a tunable drug release profile, as the high surface density of surface hydroxyl groups offered versatility to be functionalized. With drug molecules stored in nano sized channels, the pore openings could be modified using functional polymers or nano-valves performing as stimuli-responsive release devices and the drug release could be triggered by environmental changes or other external effects. In particular, mesoporous silica nanoparticles (MSN) have attracted much attention for application in functional target drug delivery to the cancer cell. The smart nano-vehicles for drug delivery have showed obvious improvements in the therapeutic efficacy for tumor suppression as compared with conventional sustained release systems, although further progress is still needed for eventual clinical applications. Alternatively, unmodified mesoporous silica also exhibited feasible application for direct formulation of poorly water-soluble drugs to enhance dissolution rate, solubility and thus increase the bioavailability after administration. In summary, mesoporous materials offer great versatility that can be used both for on-demand oral and local drug delivery, and scientists are making great efforts to design and fabricate innovative drug delivery systems based on mesoporous drug carriers.

Mg alloy treatment for superhydrophobic anticorrosion coating formation

In this article, the authors describe an efficient process for the fabrication of a composite superhydrophobic coating on Mg-Mn-Ce alloy. The authors show that the preliminary plasma electrolytic oxidation of alloy, with controlled density of surface hydroxyl groups, allows manipulation of the adhesion, structure and water-repelling properties of superhydrophobic coatings, which is formed on top of the Mg alloy. The analysis based on both the electrochemical methods and the evolution of wettability of the investigated surface by different corrosive active media indicates the long-term durability and protective anticorrosion properties of the designed superhydrophobic coatings.

Combined in situ PM-IRRAS/QCM studies of water adsorption on plasma modified aluminum oxide/aluminum substrates

Water adsorption on plasma modified oxyhydroxide covered aluminum surfaces was analyzed by means of a set-up combining in situ photoelastic modulated infrared reflection absorption spectroscopy (PM-IRRAS) and quartz crystal microbalance (QCM) in a low-temperature plasma cell. The chemical structure of the surface before and after the plasma treatment was moreover characterized by means of X-ray photoelectron spectroscopy (XPS) analysis.The surface chemistry of oxide covered aluminum was modified by oxidative and reductive low-temperature plasma pre-treatments. The Ar-plasma treatment reduced the surface hydroxyl density and effectively removed adsorbed organic contaminations. Surface modification by means of a water plasma treatment led to an increased surface hydroxyl density as well as an increase of the thickness of the native oxide film. The adsorption of water at atmospheric pressures on plasma modified aluminum surfaces led to a superimposition of reversible water layer adsorption and a simultaneous increase of the oxyhydroxide film thickness as a result of a chemisorption process.The amount of physisorbed water increased with the surface hydroxyl density whereas the chemisorption process was most significant for the surface after Ar-plasma treatment and almost negligible for the already water plasma treated surface.

Enhanced arsenic removal from water by hierarchically porous CeO2–ZrO2 nanospheres: Role of surface- and structure-dependent properties

Arsenic contaminated natural water is commonly used as drinking water source in some districts of Asia. To meet the increasingly strict drinking water standards, exploration of efficient arsenic removal methods is highly desired. In this study, hierarchically porous CeO2–ZrO2 nanospheres were synthesized, and their suitability as arsenic sorbents was examined. The CeO2–ZrO2 hollow nanospheres showed an adsorption capacity of 27.1 and 9.2mgg−1 for As(V) and As(III), respectively, at an equilibrium arsenic concentration of 0.01mgL−1 (the standard for drinking water) under neutral conditions, indicating a high arsenic removal performance of the adsorbent at low arsenic concentrations. Such a great arsenic adsorption capacity was attributed to the high surface hydroxyl density and presence of hierarchically porous network in the hollow nanospheres. The analysis of Fourier transformed infrared spectra and X-ray photoelectron spectroscopy demonstrated that the adsorption of arsenic on the CeO2–ZrO2 nanospheres was completed through the formation of a surface complex by substituting hydroxyl with arsenic species. In addition, the CeO2–ZrO2 nanospheres were able to remove over 97% arsenic in real underground water with initial arsenic concentration of 0.376mgL−1 to meet the guideline limit of arsenic in drinking water regulated by the World Health Organization without any pre-treatment and/or pH adjustment.

Simultaneous removal of arsenate and fluoride from water by Al-Fe (hydr)oxides

Al-Fe (hydr)oxides with different Al/Fe molar ratios (4:1, 1:1, 1:4, 0:1) were prepared using a coprecipitat method and were then employed for simultaneous removal of arsenate and fluoride. The 4Al: Fe was superior to other adsorbents for removal of arsenate and fluoride in the pH range of 5.0–9.0. The adsorption capacity of the Al-Fe (hydr)oxides for arsenate and fluoride at pH 6.5±0.3 increased with increasing Al content in the adsorbents. The linear relationship between the amount of OH− released from the adsorbent and the amount of arsenate or fluoride adsorbent by 4Al: Fe indicated that the adsorption of arsenate and fluoride by Al-Fe (hydr)oxides was realized primarily through quantitative ligand exchange. Moreover, there was a very good correlation between the surface hydroxyl group densities of Al-Fe (hydr)oxides and their adsorption capacities for arsenate or fluoride. The highest adsorption capacity for arsenate and fluoride by 4Al : Fe is mainly ascribed to its highest surface hydroxyl group density besides its largest pHpzc. The dosage of adsorbent necessary to remove arsenate and fluoride to meet the drinking water standard was mainly determined by the presence of fluoride since fluoride was generally present in groundwater at much higher concentration than arsenate.

Low‐Temperature Transformation of Titania Thin Films from Amorphous Nanowires to Crystallized Nanoflowers for Heterogeneous Photocatalysis

Titania thin films with ordered nanostructures are of general interest in fields of photocatalysis, gas sensors, energy storages, energy conversions, etc. In this study, we report a low‐temperature crystallization and the simultaneously occurred morphology change of titania thin films in a dilute H2SO4 solution. Amorphous titania nanowire arrays were fabricated by a Ti–H2O2 interaction, which transformed to crystallized nanoflower arrays through a dissolution–precipitation route during the subsequent acid treatments. The nanoflowers were doped with nitrogen and also incorporated with sulfate ions. An increasing H2SO4 concentration resulted in larger nanoflowers with higher anatase content; but the crystallinity reduced. The low‐temperature‐derived nanoflower arrays possessed high density of surface hydroxyl groups and defect VO‐Ti3+ sites, which contributed to a high absorption and enhanced photodegradation efficiency of rhodamine B in water under the illumination of UV–visible light during the first several runs of photocatalytic evaluations. The beneficial surface defects diminished gradually with increasing runs; however, the average reaction rate constants of the acid treated films are still superior to that of the calcinated one, which can be attributed to several structural parameters such as the nanoflower morphology, incorporation of sulfate ions, and the coexistence of anatase and rutile.

The preparation and characterization of a three-dimensional titanium dioxide nanostructure with high surface hydroxyl group density and high performance in water treatment

A three-dimensional (3D) titanium dioxide nanostructure consisting of a nanoparticle core and needlelike surface was fabricated. The effects of Ti4+ concentration, stirring methods and stirring time for TiO2 preparation were discussed with results from transmission and scanning electron microscopy; meanwhile, the TiO2 growth mechanism was illustrated by morphology evolution and crystallization of titanium dioxide spheres. The spherical diameter could be easily controlled by altering the concentration of precursor using a mechanical stirring method. X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry and thermogravimetric analysis were used to confirm the composition of titanium dioxide spheres rich with hydroxyl groups. The product showed excellent ability for arsenic(V) and chromium(VI) removal and could be separated using sedimentation. Most of the ions could be removed in less than 1h, and TiO2 had maximum adsorption capacities of 59.7mgg−1 for arsenic(V) and 21.92mgg−1 for chromium(VI). The high performance of self-assembled 3D titanium dioxide in water treatment was due to (1) its large hydroxyl group density and high specific surface area and (2) its 3D nanostructure consisting of a nanoparticle core and needlelike surface.

Uranyl-Sorption Properties of Amorphous and Crystalline TiO2/ZrO2 Millimeter-Sized Hierarchically Porous Beads

Hierarchically porous TiO(2)/ZrO(2) millimeter-sized beads were synthesized using a sol-gel templating technique, and investigated for suitability as radionuclide sorbents using uranyl as a radionuclide-representative probe. The bead properties were varied by altering either composition (22, 36, and 82 wt % Zr in the Ti/Zr composite) or calcination temperature (500 or 700 °C). Uranyl adsorption was higher for the crystalline beads (surface area: 52-59 m(2) g(-1)) than the amorphous beads (surface area: 95-247 m(2) g(-1)), reaching a maximum of 0.170 mmol g(-1) for the 22 wt % Zr sample. This was attributed to the higher surface hydroxyl density (OH nm(-2)), presence of limited microporosity, and larger mesopores in the crystalline beads. Mass transport properties of the crystalline beads were not compromised by the large bead diameter: sorption rates comparable to those reported for powders were achieved and rates were higher than exclusively mesoporous reported systems, thereby highlighting the importance of pore hierarchy in designing materials with improved kinetics. Chemical stability of the sorbent, an important property for processes involving corrosive effluents (e.g., radioactive waste), was also assessed. Crystalline beads displayed superior resistance against matrix leaching in HNO(3). Stability varied with composition: the 22 wt % Zr sample demonstrated the highest stability.

Hydrophilization of gigaporous polystyrene microspheres with saccharide as high-speed protein chromatography base support

Gigaporous poly(styrene–divinylbenzene) (PS) microspheres were hydrophilically modified with natural saccharide to minimize their nonspecific adsorption to proteins. The microspheres were chloroacetylated through Friedel–Crafts acetylation with chloroacetyl chloride, and then coupled with diacetone-D-glucose (DAGlu) through the Williamson reaction, and the protecting groups were removed on DAGlu. Results showed that the PS microspheres were successfully coupled with DAGlu and that the gigaporous structure was well maintained. After hydrophilization (Glu-PS), nonspecific adsorption of proteins on PS microspheres was greatly reduced. The high surface density of hydroxyl groups on Glu-PS microspheres surface make it easy to derivatize the spheres by classical methods. Flow experiments showed that the Glu-PS column had low backpressure, good permeability, and mechanical stability. All results indicate that the Glu-PS microspheres have great potential applications in high-speed protein chromatography.

New Insights into the Adsorption of 3-(Trimethoxysilyl)propylmethacrylate on Hydroxylated ZnO Nanopowders

Functionalization of zinc oxide (ZnO) nano-objects by silane grafting is an attractive method to provide nanostructured materials with a variety of surface properties. Active hydroxyl groups on the oxide surface are one of the causes governing the interfacial bond strength in nanohybrid particles. Here, "as-prepared" and commercially available zinc oxide nanopowders with a wide range of surface hydroxyl density were functionalized by a well-known polymerizable silane coupling agent, i.e., 3-(trimethoxysilyl)propylmethacrylate (MPS). Fourier transform infrared (FTIR) and solid-state (13)C and (29)Si nuclear magnetic resonance (NMR) spectroscopic investigations demonstrated that the silane coupling agent was fully hydrolyzed and linked to the hydroxyl groups already present on the particle surface through covalent and hydrogen bonds. Due to a basic catalyzed condensation of MPS with water, a siloxane layer was shown to be anchored to the nanoparticles through mono- and tridentate structures. Quantitative investigations were performed by thermogravimetric (TGA) and elemental analyses. The amount of silane linked to ZnO particles was shown to be affected by the amount of isolated hydroxyl groups available to react on the particle surface. For as-prepared ZnO nanoparticles, the number of isolated and available hydroxyl groups per square nanometer was up to 3 times higher than the one found on commercially available ZnO nanoparticles, leading to higher amounts of polymerizable silane agent linked to the surface. The MPS molecules were shown to be mainly oriented perpendicular to the oxide surface for all the as-prepared ZnO nanoparticles, whereas a parallel orientation was found for the preheated commercially ZnO nanopowders. In addition, ZnO nanoparticles were shown to be hydrophobized by the MPS treatment with water contact angles higher than 60°.

Nonintercalating nanosubstrates create asymmetry between bilayer leaflets.

The physical properties of lipid bilayers can be remodeled by a variety of environmental factors. Here we investigate using molecular dynamics simulations the specific effects of nanoscopic substrates or external contact points on lipid membranes. We expose palmitoyl-oleoyl phosphatidylcholine bilayers unilaterally and separately to various model nanosized substrates differing in surface hydroxyl densities. We find that a surface hydroxyl density as low as 10% is sufficient to keep the bilayer juxtaposed to the substrate. The bilayer interacts with the substrate indirectly through multiple layers of water molecules; however, despite such buffered interaction, the bilayers exhibit certain properties different from unsupported bilayers. The substrates modify transverse lipid fluctuations, charge density profiles, and lipid diffusion rates, although differently in the two leaflets, which creates an asymmetry between bilayer leaflets. Other properties that include lipid cross-sectional areas, component volumes, and order parameters are minimally affected. The extent of asymmetry that we observe between bilayer leaflets is well beyond what has been reported for bilayers adsorbed on infinite solid supports. This is perhaps because the bilayers are much closer to our nanosized finite supports than to infinite solid supports, resulting in a stronger support-bilayer electrostatic coupling. The exposure of membranes to nanoscopic contact points, therefore, cannot be considered as a simple linear interpolation between unsupported membranes and membranes supported on infinite supports. In the biological context, this suggests that the exposure of membranes to nonintercalating proteins, such as those belonging to the cytoskeleton, should not always be considered as passive nonconsequential interactions.

Photocatalytic activity and photoelectric performance enhancement for ZnWO 4 by fluorine substitution

The formation of fluorine substitution on ZnWO 4 (ZnWO 4− x F 2 x ) can be easily attained by a two step process. It could be speculated that oxygen ion was substituted by fluorine ion in the crystal lattice of ZnWO 4 on the basis of XPS and IR results. Comparing with ZnWO 4, the photocatalytic activity of ZnWO 4− x F 2 x sample almost doubled for degradation of methylene blue under UV irradiation. The enhanced photocatalytic activity should be attributed to higher density of surface hydroxyl groups of ZnWO 4− x F 2 x which induced OH radicals to improve photocatalytic reaction. Additionally, ZnWO 4− x F 2 x possessed higher donor density and efficiency of charge separation to increase the transfer rate of charges to the photocatalyst surface and to promote photocatalytic reaction.

Energy barriers for trimethylaluminum reaction with varying surface hydroxyl density

Energy barriers for trimethylaluminum (TMA) reaction with varying surface hydroxyl (–OH) density were investigated using density functional theory. When the surface –OH density increased from 0 to 6.8/nm 2 on Si (0 0 1) surfaces, the energy barrier for TMA reaction with the surfaces decreased due to attractive interactions between TMA and –OH. When the surface –OH density further increased to 9.2/nm 2 on a-SiO 2 (0 0 1) surfaces, however, the trend was reversed. This was because the attractive interactions between TMA and –OH were decreased due to the attractive interactions of –OH's themselves via hydrogen bond at this high surface –OH density.

Interface Chemistry and Molecular Interactions of Phosphonic Acid Self-Assembled Monolayers on Oxyhydroxide-Covered Aluminum in Humid Environments

Barrier properties of self-assembled octadecylphosphonic acid (ODPA) monolayers on plasma-modified oxyhydroxide-covered aluminum surfaces were analyzed by means of in situ photoelastic modulated infrared reflection absorption spectroscopy (PM-IRRAS). The surface hydroxyl density prior to ODPA adsorption was increased by means of a low-temperature H(2)O-plasma treatment. Adsorption isotherms of H(2)O on ODPA self-assembled monolayer (SAM) modified surfaces in comparison to bare oxide covered aluminum surfaces showed that the ODPA SAM leads to a strongly reduced amount of adsorbed water based on the inability of water to form hydrogen bonds to the low-energy aliphatic surface. However, the ODPA SAM covered surfaces did not show a significant inhibition of the H(2)O/D(2)O isotope exchange reaction between the D(2)O gas phase and the hydroxyl groups of the aluminum oxyhydroxide film, as the interfacial layer between the ODPA SAM and the metal substrate, while the interfacial phosphonate group as well as the orientation of the SAM is not affected by the adsorption of water. It can be followed that the strong adhesion promoting and high corrosion resistances of organophosphonate monolayers on oxyhydroxide-covered aluminum is a result of the strong acid-base interaction of the phosphonate headgroup with the Al ions in the oxyhydroxide film, even in the presence of high interfacial water activity and the molecular interactions of the aliphatic chains. However, the barrier effect of such monolayers on the transport of water is negligible.

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.