Hydroxylated Silica Surface Research Articles

Achieving an Optimal Tg Change by Elucidating the Polymer–Nanoparticle Interface: A Molecular Dynamics Simulation Study of the Poly(vinyl alcohol)–Silica Nanocomposite System

The glass transition temperature (Tg) of a polymer-nanoparticle-composite system is strongly dependent on the local segmental dynamics of the polymers. These dynamics, in turn, are dependent on the types of interfacial interactions that exist between the nanoparticle and the polymers. In this work, we investigate the magnitude of Tg-change with respect to the polymer-nanoparticle interactions by performing molecular dynamics simulations for a full-atomistic model of PVA-Silica nanocomposite. Our segmental dynamics analysis and potential of mean force calculations indicate that the strong binding interactions between hydroxylated-silica surface and the poly(vinyl alcohol) (PVA) are able to induce an increase in the Tg of the composite system with respect to the bulk PVA polymer system. While we expect that the Tg of the system will increase with an increasing amount of hydrogen bonds arising from the increasing surface hydroxylation, the trend of increasing Tg reaches a maximum when the surface is about 75...

Adsorption characteristics of silane-functionalized perfluoropolyether on hydroxylated glassy silica surfaces: A multiscale approach

The adsorption mechanism and the structural features of perfluoropolyether (PFPE)-containing layers from silane-functionalized derivate (SPFPE) on hydroxylated glassy silica surfaces are systematically investigated by utilizing density functional theory and molecular dynamics simulations. It is found that each individual SPFPE molecule tends to be chemisorbed on the silica surface by forming a single siloxane bond regardless of the number of reactive branches in the functional end group of the SPFPE molecules. We also reveal that the formation of multiple siloxane bonds between a single SPFPE molecule and the hydroxylated silica surfaces is hindered by not only the constraint on geometrical optimization of the absorbed molecule but also the pre-existing hydrogen bonds between adjacent hydroxyl groups on the surfaces. With a structural analysis, the adsorption orientation of SPFPE molecules is predicted as parallel to the silica surface, which induces low surface coverage (~0.31 molecules/nm2) and sub-nano thickness (~8 Å) of the absorbed PFPE-containing layer. These adsorption characteristics are distinct from those of self-assembled monolayers consisting of fluorocarbon or hydrocarbon molecules. We believe that the fundamental understanding of adsorption mechanism and molecular morphology of the PFPE-containing layers demonstrated in this study can contribute to developing new advanced anti-fouling materials with improved mechanical property.

Understanding the Acidic Properties of the Amorphous Hydroxylated Silica Surface

Amorphous silica is an intrinsic challenging system to study. In the last decades, some particular chemical properties have been discovered and described, but their description and understanding at the molecular level are experimentally difficult. Therefore, theoretical quantum chemical methods and descriptors, combined with experimental input, are a very appropriate set up to tackle this topic. In this study, the acidity of silanol groups of amorphous silica in hydrated conditions is investigated. Special attention has been drawn to the chemical shift, but also Bader charges, and vibrational frequencies with their intensities. The known bimodal acidity behavior was recovered and rationalized. The findings support that the chemistry of the silica surface is not determined by a single site or a family of sites but by a broad distribution of very different geometries of silanol groups in a H-bond network that can eventually show very acidic and at the same time very basic properties.

Heterogeneous Interactions between Gas-Phase Pyruvic Acid and Hydroxylated Silica Surfaces: A Combined Experimental and Theoretical Study.

The adsorption of gas-phase pyruvic acid (CH3COCOOH) on hydroxylated silica particles has been investigated at 296 K using transmission Fourier transform infrared (FTIR) spectroscopy and theoretical simulations. Under dry conditions (<1% relative humidity, RH), both the trans-cis (Tc) and trans-trans (Tt) pyruvic acid conformers are observed on the surface as well as the (hydrogen bonded) pyruvic acid dimer. The detailed surface interactions were further understood through ab initio molecular dynamics simulations. Under higher relative humidity conditions (above 10% RH), adsorbed water competes for surface adsorption sites. Adsorbed water is also observed to change the relative populations of the different adsorbed pyruvic acid configurations. Overall, this study provides valuable insights into the interaction of pyruvic acid with hydroxylated silica surfaces on the molecular level from both experimental and theoretical analyses. Furthermore, these results highlight the importance of the environment (relative humidity and coadsorbed water) in the adsorption, partitioning, and configurations of pyruvic acid at the surface.

The interaction of proteins with silica surfaces. Part II: Free energies of capped amino acids

Binding free energies of the fifteen proteinogenic amino acids were investigated for neutral silica surfaces of varying silanol density by means of biased molecular dynamics. A new force field parameterized to reproduce the results of highly accurate ab initio calculations has been employed in atomistic simulations to provide a balanced description of the adsorbate–silica interactions. The calculated free energies increase in the order quartzOH < quartz < IPC-1P. The surface heterogeneity and the local curvature enhance the binding free energy due to the cooperative effects between the silica surface and the hydrophobic and hydrophilic parts of amino acids. The propensity towards amino acids with aromatic side chains has been observed for homogenous flat surfaces, with the effect being much stronger for the quartz without surface silanol groups than for fully hydroxylated silica surfaces.

Development of a ReaxFF Reactive Force Field for NaSiOx/Water Systems and Its Application to Sodium and Proton Self-Diffusion

We present the development of a ReaxFF reactive force field for Na/Si/O/H interactions, which enables reactive molecular dynamics simulation of the sodium silicate–water interfaces. The force field parameters were fitted against various quantum mechanical calculations, including equations of state of different NaSiOx crystalline phases, energy barriers of a sodium cation’s transport within the sodium silicate crystal structure, interactions between the hydroxylated silica surface and sodium cation–water systems, and dissociation energies of [NaOH–n(H2O)] (n = 1–6) clusters. After the optimization process, we validated the force field capability through calculating the structures of sodium silicate crystals and glasses and transport properties of sodium ions and protons within the amorphous structures. The force field was also applied to validate the dissociation behavior of sodium hydroxides within the bulk water. Our results with the developed force field are relevant to detailed chemical dissolution mec...

Molecular Dynamics Simulations of the Oil-Detachment from the Hydroxylated Silica Surface: Effects of Surfactants, Electrostatic Interactions, and Water Flows on the Water Molecular Channel Formation.

The detachment process of an oil molecular layer situated above a horizontal substrate was often described by a three-stage process. In this mechanism, the penetration and diffusion of water molecules between the oil phase and the substrate was proposed to be a crucial step to aid in removal of oil layer/drops from substrate. In this work, the detachment process of a two-dimensional alkane molecule layer from a silica surface in aqueous surfactant solutions is studied by means of molecular dynamics (MD) simulations. By tuning the polarity of model silica surfaces, as well as considering the different types of surfactant molecules and the water flow effects, more details about the formation of water molecular channel and the expansion processes are elucidated. It is found that for both ionic and nonionic type surfactant solutions, the perturbation of surfactant molecules on the two-dimensional oil molecule layer facilitates the injection and diffusion of water molecules between the oil layer and silica substrate. However, the water channel formation and expansion speed is strongly affected by the substrate polarity and properties of surfactant molecules. First, only for the silica surface with relative stronger polarity, the formation of water molecular channel is observed. Second, the expansion speed of the water molecular channel upon the ionic surfactant (dodecyl trimethylammonium bromide, DTAB and sodium dodecyl benzenesulfonate, SDBS) flooding is more rapidly than the nonionic surfactant system (octylphenol polyoxyethylene(10) ether, OP-10). Third, the water flow speed may also affect the injection and diffusion of water molecules. These simulation results indicate that the water molecular channel formation process is affected by multiple factors. The synergistic effects of perturbation of surfactant molecules and the electrostatic interactions between silica substrate and water molecules are two key factors aiding in the injection and diffusion of water molecules and helpful for the oil detachment from silica substrate.

Interaction of NaOH solutions with silica surfaces

HypothesisSodium adsorption on silica surfaces depends on the solution counter-ion. Here, we use NaOH solutions to investigate basic environments. SimulationsSodium adsorption on hydroxylated silica surfaces from NaOH solutions were investigated through molecular dynamics with a dissociative force field, allowing for the development of secondary molecular species. FindingsAcross the NaOH concentrations (0.01 M − 1.0 M), ∼50% of the Na+ ions were concentrated in the surface region, developing silica surface charges between − 0.01 C/m2 (0.01 M NaOH) and − 0.76 C/m2 (1.0 M NaOH) due to surface site deprotonation. Five inner-sphere adsorption complexes were identified, including monodentate, bidentate, and tridentate configurations and two additional structures, with Na+ ions coordinated by bridging oxygen and hydroxyl groups or water molecules. Coordination of Na+ ions by bridging oxygen atoms indicates partial or complete incorporation of Na+ ions into the silica surface. Residence time analysis identified that Na+ ions coordinated by bridging oxygen atoms stayed adsorbed onto the surface four times longer than the mono/bi/tridentate species, indicating formation of relatively stable and persistent Na+ ion adsorption structures. Such inner-sphere complexes form only at NaOH concentrations of > 0.5 M. Na+ adsorption and lifetimes have implications for the stability of silica surfaces.

Silica–silane coupling agent interphase properties using molecular dynamics simulations

In this paper, strength of the interphase between silica and glycidoxypropyltrimethoxy silane (GPS) coupling agent has been studied using molecular dynamics (MD) simulations. Silica–GPS interphase model is created by coupling the hydroxylated silica surface with monolayer-hydroxylated GPS molecules. The interphase model is subjected to mode-I (normal), mode-II (shear) and mixed-mode (normal–shear) mechanical loading to determine the interphase cohesive traction–separation (T–S) response (i.e., cohesive traction law). In MD simulations, atomic interactions are modeled with the reactive force field ReaxFF. Effects of interphase thickness and GPS bond density on the T–S response are studied. Simulation results indicate that interphase strength decreases with increase in the interphase thickness before attaining a plateau level at higher thickness. For a particular thickness, strength improves significantly with increase in the GPS bond density with the silica surface. Damage mode is adhesive at the silica interface at lower thickness and transitions to mixed mode and cohesive failure within the silane interphase at higher thickness. Mixed-mode T–S responses are bounded by the mode-I and mode-II responses. Characteristic parameters of the continuum-level potential-based cohesive zone model (PPR–CZM) are determined by fitting the MD-based mode-I and mode-II T–S responses with PPR–CZM functional. Development of the PPR–CZM parameters enables bridging length scales from the MD to the continuum scale for fracture modeling of the fiber–matrix interphase in composites subjected to mixed-mode loading. Results on mode-I and mode-II unloading are also presented.

Modeling of heterogeneous catalysts based on silica and zeolites by the hybrid quantum chemical embedded cluster method

The review is dedicated to the elaboration and application of hybrid quantum mechani-cal/molecular mechanical methods for heterogeneous catalytic systems, including single atoms and clusters of transition metals immobilized on covalent oxide supports. The following issues are considered: (1) elaboration of the hybrid covEPE method for modeling of covalent sys-tems of the zeolite and silicate types, (2) computations of the properties of atoms and small titanium, rhodium, iridium, and gold clusters localized in cavities or embedded in the zeolite framework, and (3) computations of small silver and tantalum clusters anchored at the dehydr-oxylated and hydroxylated silica surfaces. The calculations were performed by the density functional theory (DFT) with the Becke—Perdew (BP) exchange-correlation potential.

Surface Structure and Stability of Partially Hydroxylated Silica Surfaces.

Surface energies of silicates influence crack propagation during brittle fracture and decrease with surface relaxation caused by annealing and hydroxylation. Molecular-level simulations are particularly suited for the investigation of surface processes. In this work, classical MD simulations of silica surfaces are performed with two force fields (ClayFF and ReaxFF) to investigate the effect of force field reactivity on surface structure and energy as a function of surface hydroxylation. An unhydroxylated fracture surface energy of 5.1 J/m2 is calculated with the ClayFF force field, and 2.0 J/m2 is calculated for the ReaxFF force field. The ClayFF surface energies are consistent with the experimental results from double cantilever beam fracture tests (4.5 J/m2), whereas ReaxFF underestimated these surface energies. Surface relaxation via annealing and hydroxylation was performed by creating a low-energy equilibrium surface. Annealing condensed neighboring siloxane bonds increased the surface connectivity, and decreased the surface energies by 0.2 J/m2 for ClayFF and 0.8 J/m2 for ReaxFF. Posthydroxylation surface energies decreased further to 4.6 J/m2 with the ClayFF force field and to 0.2 J/m2 with the ReaxFF force field. Experimental equilibrium surface energies are ∼0.35 J/m2, consistent with the ReaxFF force field. Although neither force field was capable of replicating both the fracture and equilibrium surface energies reported from experiment, each was consistent with one of these conditions. Therefore, future computational investigations that rely on accurate surface energy values should consider the surface state of the system and select the appropriate force field.

Thermal Transport in Supported Graphene: Substrate Effects on Collective Excitations

Graphene's exceptional thermal conductivity is of great interest for heat management in advanced electronic devices, yet there are still fundamental issues regarding substrate effects. Using state-of-the-art simulations, the authors study collective phonon excitations in graphene on on partially hydroxylated silica surfaces, and show that certain vibrational modes can be influenced to increase thermal conductivity beyond that seen with an untreated substrate. This insight opens a path for systematic control of heat flow in this important system.

Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study.

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with 13C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed 13C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the 13C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of 13C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

How strong are H-bonds at the fully hydroxylated silica surfaces? Insights from the B3LYP electron density topological analysis

The calculation through the supermolecular approach of the hydrogen bond strength EHB between silanol groups at the surface of an ample class of silica-based materials is hindered by the intrinsic difficulty to define the “H-bond free” reference system. We propose, for the first time, to evaluate EHB by adopting the literature empirical correlation relating the Bader local electronic kinetic energy density Gb computed at the H⋅⋅⋅O bond critical point with EHB. Remarkably, EHB for the hydroxylated surfaces of quartz polymorphs correlates with surface formation energy, showing that the surface EHB is responsible of the surface stability. A number of correlations between hydrogen bond features are established, with that between EHB and the enhanced infrared intensity associated to surface hydrogen bond formation, obeying the literature formula semi-quantitatively. The present results are quite general and can be extended to other inorganic surfaces where hydrogen bonds between surface sites are the dominant features.

Structure and Dynamics of Water/Methanol Mixtures at Hydroxylated Silica Interfaces Relevant to Chromatography.

The spectroscopy and dynamics of water/methanol (MeOH) mixtures at hydroxylated silica surfaces is investigated from atomistic simulations. The particular focus is on how the structural dynamics of MeOH changes when comparing surface-bound and MeOH in the bulk. From analyzing the frequency frequency correlation functions it is found that the dynamics on the picosecond time scale differs by almost a factor of two. While the relaxation time is 2.0 ps for MeOH in the bulk solvent it is considerably slowed-down to 3.5 ps for surface-bound MeOH. Surface-adsorbed MeOH molecules reside there for several nanoseconds and their H-bonds are strongly oriented towards the surface-OH groups. These results are of particular relevance for chromatographic systems where the solvent may play a central role in their function. The present simulations suggest that surface-sensitive spectroscopic techniques should be useful in better characterizing such heterogeneous systems and provide detailed insight into solvent dynamics and structure relevant in chromatographic applications.

Reactive Molecular Dynamics Simulations of the Silanization of Silica Substrates by Methoxysilanes and Hydroxysilanes.

We perform reactive molecular dynamics simulations of monolayer formation by silanes on hydroxylated silica substrates. Solutions composed of alkylmethoxysilanes or alkylhydroxysilanes in hexane are placed in contact with a hydroxylated silica surface and simulated using a reactive force field (ReaxFF). In particular, we have modeled the deposition of butyl-, octyl-, and dodecyltrimethoxysilane to observe the dependence of alkylsilyl chain length on monolayer formation. We additionally modeled silanization using dodecyltrihydroxysilane, which allows for the comparison of two grafting mechanisms of alkoxysilanes: (1) direct condensation of alkoxysilane with surface-bound silanols and (2) a two-step hydrolysis-condensation mechanism. To emulate an infinite reservoir of reactive solution far away from the substrate, we have developed a method in which new precursor molecules are periodically added to a region of the simulation box located away from the surface. It is determined that the contact angle of alkyl tails bound to the surface is dependent on their grafting density. During the early stages of grafting alkoxy- and hydroxysilanes to the substrate, a preference is shown for silanes to condense with silanols further from the substrate surface and also close to neighboring surface-bound silanols. The kinetics of silica silanization by hydroxysilanes was observed to be much faster than for methoxysilanes. However, the as-deposited hydroxysilane monolayers show similar morphological characteristics to those formed by methoxysilanes.

Amorphous SiO2 surface models: energetics of the dehydroxylation process, strain, ab initio atomistic thermodynamics and IR spectroscopic signatures.

In this contribution, realistic amorphous SiO2 models of 2.1 × 2.1 nm with silanol densities ranging 1.1-7.2 OH per nm(2) are obtained by means of ab initio calculations via the dehydroxylation of a fully hydroxylated silica surface. The dehydroxyation process is considered to take place via direct condensation of adjacent silanol groups and silica migration steps. The latter reconstructions are needed in order to obtain highly dehydroxylated silica surfaces with favorable energetics and without the formation of defects. The obtained surface phase diagram of different silica models as a function of temperature and PH2O is able to correctly describe the silanol density under different conditions, and the IR spectroscopic signatures of the silanols are in qualitative agreement with the experiment. The amorphous silica models presented here have a high degree of heterogeneity as found from the big variability obtained in the energetics of the dehydroxylation steps. It was also found that the resulting average Si-O distance of the newly formed siloxane bridges serves as a descriptor of the strain introduced in the silica surface. All these factors can be crucial in order to simulate the activity of catalysts grafted onto silica with different silanol densities, especially the one containing ca. 1 OH per nm(2), which can serve as a model for the SiO2 surface pretreated under high vacuum and at 700 °C.

Density functional theory based-study of 5-fluorouracil adsorption on β-cristobalite (1 1 1) hydroxylated surface: The importance of H-bonding interactions

Silica-based mesoporous materials have been recently proposed as an efficient support for the controlled release of a popular anticancer drug, 5-fluorouracil (5-FU). Although the relevance of this topic, the atomistic details about the specific surface-drug interactions and the energy of adsorption are almost unknown. In this work, theoretical calculations using the Vienna Ab-initio Simulation Package (VASP) applying Grimme's—D2 correction were performed to elucidate the drug–silica interactions and the host properties that control 5-FU drug adsorption on β-cristobalite (111) hydroxylated surface. This study shows that hydrogen bonding, electron exchange, and dispersion forces are mainly involved to perform the 5-FU adsorption onto silica. This phenomenon, revealed by favorable energies, results in optimum four adsorption geometries that can be adopted for 5-FU on the hydroxylated silica surface. Silanols are weakening in response to the molecule approach and establish H-bonds with polar groups of 5-FU drug. The final geometry of 5-FU adopted on hydroxylated silica surface is the results of H-bonding interactions which stabilize and fix the molecule to the surface and dispersion forces which approach it toward silica (111) plane. The level of hydroxylation of the SiO2 (111) surface is reflected by the elevated number of hydrogen bonds that play a significant role in the adsorption mechanisms.

Trinuclear tantalum clusters grafted to hydroxylated silica surfaces: A density-functional embedded-cluster study

To identify the coordination modes of bare and hydrogenated trinuclear tantalum species on hydroxylated silica, we computationally examined models of Ta3Hn (n = 0, 3, 5–9) species grafted to a β-cris-tobalite surface. Ta3Hn clusters are bound to the surface by substitution of hydrogen atoms of vicinal (…O-)3SiOH and geminal (…O-)2Si(OH)2 groups via three and six, respectively, Ta–O bonds of ~193 pm on average, in both types of models. The maximum Ta–O coordination number of non-hydrogenated Ta3 species to a silica surface is seven for the second type model surface; the additional Ta–O bond is due to an oxygen atom located in a bridging position to Ta–Ta bond. In the latter case, the mean Ta–O bond distance to one of =Si(O–)2 group is increased by 15 pm. For the complexes bound via vicinal silanol groups, each additional unit of hydrogen loading on the metal elongated the average Ta–Ta distance by ~2 pm, covering a range of 258–277 pm. For the most stable hydrogenated trimers, Ta3H9, the desorption energies of hydrogen atoms are relatively high, above 70 kJ/mol. The average Ta–Ta distances increase by ~19 pm on going from the complex (=SiO–)3Ta3H9 to complex (=SiO–)3Ta3 and by ~5 pm when the hydrogen loading is increased by one unit for (=Si(O–)2)3Ta3Hn complexes, reaching the maximum value 319 pm when n = 9. The desorption energies of hydrogen atoms for the most stable tantalum trimer species grafted to the surface by geminal silanol groups, (=Si(O–)2)3Ta3H7, are rather low, less than 40 kJ/mol.

The nucleation of gas hydrates near silica surfaces

Understanding the nucleation and crystal growth of gas hydrates near mineral surfaces and in confinement are critical to the methane recovery from gas hydrate reservoirs. In this work, through molecular dynamics simulation studies, we present an exploration of the nucleation behavior of methane hydrates near model hydroxylated silica surfaces. Our simulation results indicate that the nucleation of methane hydrates can initiate from the silica surfaces despite of the structural mismatch of the two solid phases. A layer of intermediate half-cage structures was observed between the gas hydrate and silica surfaces, apparently helping to minimize the free energy penalty. These results have important implications to our understanding of the effects of solid surfaces on hydrate nucleation processes.