Water Adsorption on Pyrogenic Silica Followed by 1H MAS NMR

Leucine on Silica: A Combined Experimental and Modeling Study of a System Relevant for Origins of Life, and the Role of Water Coadsorption

Leucine on silica constitutes an interesting system from the point of view of origins of life studies since leucine coadsorbed on SiO₂ together with glutamic acid can give rise to rather long linear polypeptides upon activation. It is also an ideal system to test methods of molecular characterization of biomolecules deposited on mineral surfaces since it combines a small-scale model of peptides and proteins, which are among the most important components of biodevices, with one of the most widely used inorganic materials. We have deposited l-leucine on a high surface fumed silica in the submonolayer range and characterized it by a multipronged approach including macroscopic information (thermogravimetry, X-ray diffraction), in situ spectroscopic methods (IR, multinuclear solid-state NMR including single-pulse and CP-MAS, 2-D HETCOR), and molecular modeling by density functional theory (DFT), including calculation of NMR parameters. Specific information can be obtained on the adsorption and interaction mechanism. Leucine is rather strongly adsorbed without any covalent bonds, through the formation of a specific lattice of H-bonds that often involve coadsorbed water molecules. Its state is indeed strongly dependent on the drying procedure: insufficient drying results in liquid-like surroundings for the leucine functional groups, while vacuum drying only retains a limited number of waters (of the order of 5 per leucine molecule). The most stable models have zwitterionic leucine interacting directly with surface silanols through their ammonium group, while the carboxylate interacts through bridging waters. Experimental NMR chemical shifts are satisfactorily predicted for these models, and leucine can be viewed as a probe for specific groups of surface sites known as silanol nests.

Semiquantitative Solid-State NMR Study of the Adsorption of Soybean Oils on Silica and Its Significance for Rubber Processing

Soybean oil (SBO) is a renewable material used as an alternative to conventional petroleum-derived oils in the processing of rubber composites. Upon chemical modifications, such as epoxidation, its performance in the processing of rubber can be significantly improved, as indicated by a considerable reduction of the mixing energy. Although it has been hypothesized that hydrogen bonding between functional groups (e.g., epoxy) of SBOs and silanols present on the silica surface plays a key role, there is still a lack of direct evidence supporting this hypothesis. In this work, it is demonstrated that there is an overall correlation between the epoxy concentration of SBOs and the mixing energy, consistent with the long-held hypothesis. In particular, a correlation between the SBO-silica adsorption affinity and the degree of epoxidation is revealed by a set of surface-selective solid-state nuclear magnetic resonance (ssNMR) experiments. In addition, the surface-selective ssNMR technique demonstrated in this work could also be used to evaluate the adsorption affinity of other oils and/or additives more broadly.

Structural Changes of Hierarchically Nanoporous Organosilica/Silica Hybrid Materials by Pseudomorphic Transformation

Herein, it is reported how pseudomorphic transformation of divinylbenzene (DVB)-bridged organosilica@controlled pore glasses (CPG) offers the possibility to generate hierarchically porous organosilica/silica hybrid materials. CPG is utilized to provide granular shape/size and macroporosity and the macropores of the CPG is impregnated with organosilica phase, forming hybrid system. By subsequent pseudomorphic transformation, an ordered mesopore phase is generated while maintaining the granular shape and macroporosity of the CPG. Surface areas and mesopore sizes in the hierarchical structure are tunable by the choice of the surfactant and transformation time. Two-dimensional magic angle spinning (MAS) NMR spectroscopy demonstrated that micellar-templating affects both organosilica and silica phases and pseudomorphic transformation induces phase transition. A double-layer structure of separate organosilica and silica layers is established for the impregnated material, while a single monophase consisting of randomly distributed T and Q silicon species at the molecular level is identified for the pseudomorphic transformed materials.

Peptides from diatoms and grasses harness phosphate ion binding to silica to help regulate biomaterial structure

Many life forms generate intricate submicron biosilica structures with various important biological functions. The formation of such structures, from the silicic acid in the waters and in the soil, is thought to be regulated by unique proteins with high repeats of specific amino acids and unusual sidechain modifications. Some silicifying proteins are characterized by high prevalence of basic amino acids in their primary structures. Lysine-rich domains are found, for instance, in diatom silaffin proteins and in the sorghum grass siliplant1 protein. These domains exhibit catalytic activity in silica chain condensation, owing to molecular interactions of the lysine amine groups with the forming mineral. The use of amine chemistry by two very remote organisms has motivated us to seek other molecular biosilicification processes that may be common to the two life forms. In diatom silaffins, domains rich in phosphoserine residues are thought to assist the assembly of silaffin molecules into an organic supra-structure which serves as a template for the silica to precipitate on. This mold, held by salt bridges between serine phosphates and lysine amines, dictates the shape of the silica particles formed. Yet, silica synthesized with the dephosphorylated silaffin in phosphate buffer showed similar morphology to the one prepared with the native protein, suggesting that a defined spatial arrangement of serine phosphates is not required to generate silica with the desired shape. Concurrently, free phosphates enhanced the activity of siliplant1 in silica formation. It is therefore beneficial to characterize the involvement of these anions as co-factors in regulated silicification by functional peptides from the two proteins and to understand whether they play similar molecular role in the mechanism of mineralization. Here we analyze the molecular interactions of free phosphate ions with silica and the silaffin peptide PL12 and separately with silica and siliplant1 peptide SLP1 in the two biomimetic silica products generated by the two peptides. MAS NMR measurements show that the phosphate ions interact with the peptides and at the same time may be forming bonds with the silica mineral. This bridging capability may add another avenue by which the structure of the silica material is influenced. A model for the molecular/ionic interactions at the bio-inorganic interface is described, which may have bearings for the role of phosphorylated residues beyond the function as intermolecular cross linkers or free phosphate ions as co-factors in regulation of silicification. STATEMENT OF SIGNIFICANCE: The manuscript addresses the question how proteins in diatoms and plants regulate the biosilica materials that are produced for various purposes in organisms. It uses preparation of silica in vitro with functional peptide derivatives from a sorghum grass protein and from a diatom silaffin protein separately to show that phosphate ions are important for the control that is achieved by these proteins on the final shape of the silica material produced. It portrays via magnetic resonance spectroscopic measurements, in atomic detail, the interface between atoms in the peptide, atoms on the surface of the silica formed and the phosphate ions that form chemical bonds with atoms on the silica as part of the mechanism of action of these peptides.

Sensitivity enhancement via multiple contacts in the {1H–29Si}–1H cross polarization experiment: a case study of modified silica nanoparticle surfaces

{¹H–²⁹Si}–¹H double cross polarization inverse detection (DCPi) solid-state NMR, has recently been shown to be a powerful tool for studying molecules adsorbed on the silica surface. In this contribution, we develop an improved version (MCPi) which incorporates a block of multiple contact pulses, and quantitatively compare the sensitivities of MCPi and DCPi over a typical range of experimental parameters. The MCPi pulse sequence aims at higher sensitivity and robustness for studying samples with various relaxation characteristics. In the case of dimethyl sulfoxide (DMSO) molecules adsorbed on the silica surface, MCPi performs equally well or up to 2.5 times better than DCPi over a wide range of parameters. The applicability to and performance of MCPi on composite materials was demonstrated using a sample of polymer–silica composite, where significantly higher sensitivity could be achieved at very long total mixing times. The results also showed that both techniques are surface specific in the sense that only the groups close to the surface can be detected.

In this paper we demonstrate {¹H–²⁹Si}–¹H multiple cross polarization inverse detection (MCPi) solid state NMR as a robust technique for studying modified silica nanoparticle surfaces.

Adsorption Properties and Mechanism of Cd2+ in Water by Zr-containing Silica Residue Purification

Zirconium (Zr)-containing silica residue purification (ZSR-P) discharged from industrial production of ZrOCl₂ was used as an adsorbent, and CdCl₂ solution was used as the simulated wastewater containing cadmium ions (Cd²⁺). The properties and mechanisms of ZSR-P absorbing Cd²⁺ were studied. The results showed that ZSR-P had a good effect on the adsorption and removal of Cd²⁺ in water. The adsorption time, initial concentration of Cd²⁺, and pH of the solution had a significant effect on the adsorption behavior, whilst the pH value had the greatest effect amongst them. Under optimal conditions, the amount of Cd²⁺ adsorbed by ZSR-P was 43.1 mg/g. The isothermal adsorption conformed to the Langmuir adsorption model, and the adsorption kinetics conformed to the secondary adsorption rate model. In ZSR-P-Cd, Cd²⁺ was uniformly distributed on the surface of SiO₂ particles and in the pores formed by the accumulation of particles. Adsorption of Cd²⁺ by ZSR-P was achieved through the reaction between Si-OH on the surface of SiO₂ and Cd²⁺ hydroxyl compounds.

Distribution of water in the pores of periodic mesoporous organosilicates – a proton solid state MAS NMR study

Proposed model of water layers and pore filling in ethane substituted periodic mesoporous organosilicates (PMO_E) based on analysis of solid state magic angle spinning (MAS) proton NMR spectra.

Organoindium-modified monodisperse ellipsoid-/platelet-like periodic mesoporous silicas

Indium-modified mondisperse ellipsoid-/platelet-like large-pore periodic mesoporous silica nanoparticles (MMSNs) SBA-15 have been prepared via molecular grafting of In[N(SiMe₃)₂]₃. Surface ligand exchange led to the formation of heteroleptic In species, and the resulting surface In species were converted into crystalline In₂O₃ nanoparticles by calcination.

Solid-State NMR Characterization of Mixed Phosphonic Acid Ligand Binding and Organization on Silica Nanoparticles

As ligand functionalization of nanomaterials becomes more complex, methods to characterize the organization of multiple ligands on surfaces is required. In an effort to further the understanding of ligand-surface interactions, a combination of multinuclear ((1)H, (29)Si, (31)P) and multidimensional solid-state nuclear magnetic resonance (NMR) techniques was utilized to characterize the phosphonic acid functionalization of fumed silica nanoparticles using methylphosphonic acid (MPA) and phenylphosphonic acid (PPA). (1)H → (29)Si cross-polarization (CP)-magic angle spinning (MAS) solid-state NMR was used to selectively detect silicon atoms near hydrogen atoms (primarily surface species); these results indicate that geminal silanols are preferentially depleted during the functionalization with phosphonic acids. (1)H → (31)P CP-MAS solid-state NMR measurements on the functionalized silica nanoparticles show three distinct resonances shifted upfield (lower ppm) and broadened compared to the resonances of the crystalline ligands. Quantitative (31)P MAS solid-state NMR measurements indicate that ligands favor a monodentate binding mode. When fumed silica nanoparticles were functionalized with an equal molar ratio of MPA and PPA, the MPA bound the nanoparticle surface preferentially. Cross-peaks apparent in the 2D (1)H exchange spectroscopy (EXSY) NMR measurements of the multiligand sample at short mixing times indicate that the MPA and PPA are spatially close (≤5 Å) on the surface of the nanostructure. Furthermore, (1)H-(1)H double quantum-single quantum (DQ-SQ) back-to-back (BABA) 2D NMR spectra further confirmed that MPA and PPA are strongly dipolar coupled with observation of DQ intermolecular contacts between the ligands. DQ experimental buildup curves and simulations indicate that the average distance between MPA and PPA is no further than 4.2 ± 0.2 Å.

Photoinduced self-assembly of carboxylic acid-terminated lamellar silsesquioxane: highly functional films for attaching and patterning amino-based ligands

A facile procedure for immobilizing and photopatterning amino ligands onto a multilayer cross-linked COOH-functionalized organosilica film is described. Key features include high functionality, robustness and no restriction on the substrate.

Electric conductivity percolation in naturally dehydrating, lightly wetted, hydrophilic fumed silica powder

In studying the dehydration of surface-moistened fumed silica Aerosil powders, we found a conductivity percolation transition at low hydration levels. Both the percolation exponent and the threshold are typical for correlated site-bond transitions in complex two-dimensional (2D) systems. The exponent values, 0.94-1.10, are indicative of severe heterogeneity in the conducting medium. The surface moisture at the percolation threshold takes on a universal value of 0.65 mg([H2O])/m(2)([silica]), independent of the silica grain size, and equivalent to twice the first hydration monolayer. This level is just sufficient to sustain a quasi-2D, hydrogen-bonded water network spanning the silica surface.

Carbon dioxide sorbents with propylamine groups-silica functionalized with a fractional factorial design approach

Mesoporous silica particles (Davisil) were functionalized with aminopropyltriethoxysilane (APTES) in a fractional factorial design with 19 different synthesis and uptake experiments. The number of amino groups and the uptake of CO(2) were optimized in a 2(V)(5-1) design. Most important to functionalization was the amount of water present during synthesis, the reaction time, and pretreating the silica with a mineral acid; certain two-way interactions were shown to be statistically significant as well. Modifications performed at 110 or 80 °C showed no significant differences concerning amine content or uptake of CO(2). Properly choosing center points for the discrete variables is problematic and is somewhat related to the lack of fit with respect to CO(2) uptake; the regression was good. Solid-state (29)Si NMR showed that the APTES was mainly fully condensed. Specific surface areas did not correlate with the number of n-propylamine groups on the silica, which is indicative of differential levels of heterogeneity in the coverage of propylamines. The uptake of CO(2) and N(2) was measured from -20 to 70 °C and from 0 to 1 bar and parametrized by the Freundlich isotherm. Amine-modified silica adsorbed significant amounts of CO(2), especially at the low partial pressure, which is important for CO(2) capture from flue gas. At such pressures, samples with a high density of amine (4 amines/nm(2)) showed a much higher uptake of CO(2) than did those with densities of ∼2-3 amines/nm(2), reflecting differential tendencies to form propylammonium-propylcarbamate ion pairs; these require close proximity among amine groups to form. Water affected the uptake of carbon dioxide in different ways. Certain samples took up more moist CO(2) gas than dry CO(2), and others took up less moist CO(2) than dry CO(2), which is indicative of differential tendencies toward water adsorption. We conclude that experimental design is a time-efficient approach to the functionalization of silica with propylamine groups.

Solid-state NMR characterization of a controlled-pore glass and of the effects of electron irradiation

Controlled-pore glasses (CPGs) are silica-based materials which provide an adequate model system for a better understanding of the radiation chemistry of glasses, especially under nanoscopic confinement. This paper presents a characterization of a nanoporous CPG before and after electron irradiation using multinuclear solid-state magnetic resonance (NMR). 1H MAS NMR has been used for studying the surface proton sites and it is observed that the irradiation leads to a dehydration of the material. Accordingly, concerning the silicon sites near the surface, the observed variation of the Q4, Q3 and Q2 species from 1H-29Si CPMAS spectra shows an increase of the surface polymerization under irradiation, implying in majority a Q2 to Q3/Q4 conversion mechanism. Similarly, 1H-17 O CPMAS measurements exhibit an increase of Si-O-Si groups at the expenses of Si-OH groups. In addition, modifications of the environment of the residual boron atoms are also put in evidence from 11B MAS and MQMAS NMR These data show that MAS NMR methods provide sensitive tools for the characterization of these porous glasses and of the tiny modifications occurring under electron irradiation.

Adsorption of organic molecules on silica surface

The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review.

Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate

The production of molecular hydrogen in the radiolysis of dried or hydrated nanoporous controlled-pore glasses (CPG) has been carefully studied using 10 MeV electron irradiation at high dose rate. In all cases, the H2 yield increases when the pore size decreases. Moreover, the yields measured in dried materials are two orders of magnitude smaller than those obtained in hydrated glasses. This proves that the part of the H2 coming from the surface of the material is negligible in the hydrated case. Thus, the measured yields correspond to those of nanoconfined water. Moreover, these yields are not modified by the presence of potassium bromide, which is a hydroxyl radical scavenger. This experimental observation shows that the back reaction between H2 and HO* does not take place in such confined environments. These porous materials have been characterized before and after irradiation by means of Fourier-transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) techniques, which helps to understand the elementary processes taking place in this type of environment, especially the protective effect of water on the surface in the case of hydrated glasses.

Adsorption of ethylene glycol vapor on (alpha-AI2O3 (0001) and amorphous SiO2 surfaces: observation of molecular orientation and surface hydroxyl groups as sorption sites

Vapor adsorption is an important process influencing the migration and the fates of many organic pollutants in the environment. In this study, adsorption of ethylene glycol (EG) vapor onto single crystal alpha-Al2O3 (0001) and fused SiO2 (amorphous) surfaces was studied with sum frequency generation spectroscopy, a well-suited surface specific technique for probing interfacial phenomena atthe molecular scale. Air-aqueous EG solutions were also investigated to compare to the adsorption at the air-solid interface in the presence of water vapor. The gauche conformer of EG molecules dominates the air-aqueous EG solution interface, and EG molecules act as hydrogen acceptors at the air-liquid interface. Water and surface hydrophilic/ hydrophobic properties play important roles for the adsorption of EG onto silica and alumina surfaces. The adsorbed EG molecules interact in different ways at the two different oxide surfaces. EG molecules weakly physisorb onto the alpha-Al2O3 (0001) surface by forming relatively weak hydrogen bonds with surface water molecules. On the silica surface, the suppression of the silanol OH stretching peak indicates that EG molecules form hydrogen bonds with silanol OH groups.

Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides

Two kinds of amorphous aluminum hydroxides, a sample precipitated from admixing AlCl3 and NaOH aqueous solutions and the commercial product, were measured by 27Al and 1H solid-state NMR spectroscopy. Pentahedral and tetrahedral coordinations, as well as octahedral coordination of oxygen atoms for aluminum, are observed in 27Al magic angle spinning (MAS) spectra of both amorphous samples. In contrast, octahedral coordination is only observed in gibbsite, bayerite, and boehmite. According to 1H MAS-NMR spectra under conditions of high spinning rate (35 kHz) and high field (14.09 T), free waters and OH groups coupled with aluminum for amorphous samples are observed at approximately 5 and approximately 4.5 ppm, respectively, the latter peak being broader. This is consistent with the differential spectra between spin echo and transfer of populations in double resonance. We conclude that the subunits of AlO4, AlO5, and AlO6 in amorphous aluminum hydroxides are bound through hydrogen bonds with a wide distribution of bonding strength.

A New Straightforward Approach to Generate Si-H Groups on Silica

In this paper it is demonstrated unequivocally by solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies that, at high temperature, hydrogen reacts directly with pyrogenic silica to form stable hydride Si-H groups. Such modified silicas should exhibit specific properties far from those of silicas having only surface silanol groups. The interest for hydride-modified silicas arises from the possibility of using their Si-H groups as reducing agents or as active sites for the synthesis of new chromatographic phases. Copyright 1999 Academic Press.