Characterization of silicas by adsorption

Distribution of silica surface active sites and examination of their surface heterogeneity have been assessed by study of their reactivity towards alcohols, water and polyamines, and by characterization of the initial and modified samples. The results obtained provided valuable informations as for the surface structural characteristics of the solids and their dependence on mode of preparation.     

Characterization of chemically modified silica gels by DRIFT spectroscopy

Chemically modified silica gels are investigated with respect to identity, purity and degree of surface coverage using diffuse reflectance FTIR (DRIFT) Spectroscopy. We report the results for two bonded HPLC phases, pyrene butyric acid propylamido- and octadecyl-groups grafted on irregular porous silica gel. For quantitative determination calibration standards are prepared by adsorbing structurally similar ligand compounds onto the surface of the silica gel; these coated materials give linear calibration curves up to a concentration of 1 mmol/g modified silica gel.     

The CO2 adsorption performance of aminosilane-modified mesoporous silicas

Journal of Thermal Analysis and Calorimetry - Aminosilane-modified MCM-41 and SBA-15 mesoporous silicas were synthesized using sodium silicate extracted from gold mine tailings slurry in this...     

Lower closure point of adsorption hysteresis in ordered mesoporous silicas

To examine the nature of the lower closure point of adsorption hysteresis in ordered mesoporous silicas, we measured the temperature dependence of the adsorption-desorption isotherm of nitrogen for three kinds of ordered silicas with cagelike pores and three kinds of ordered silicas with cylindrical pores. The lower closure point pressure of nitrogen in the cagelike pores with sufficiently small necks, that is, the cavitation pressure of a confined liquid, did not depend appreciably on the cage size in the temperature region far away from a hysteresis critical temperature (Tch) but its cage-size dependence was noticeable in the vicinity of Tch. The lower closure point in the cylindrical pores depended on the pore size, and its thermal behavior was totally different from that in the cagelike pores. Nevertheless, the hysteresis critical points of nitrogen in the ordered mesoporous silicas, which are defined as a threshold of temperatures (Tch) and pressure above which reversible capillary condensation takes place in a given size and shape of pores, fell on a common line in a temperature-pressure diagram regardless of the pore geometries. We consider this finding as evidence that capillary evaporation in the cylindrical pores follows a cavitation process in the vicinity of Tch in the same way as that in the cagelike pores and also that the low limit of the hysteresis loop that has been long recognized since 1965 is due to the occurrence of a vapor bubble in a stretched metastable liquid confined to the pores with decreasing pressure (cavitation).     

Self-healing surface hydrophobicity by consecutive release of hydrophobic molecules from mesoporous silica

The paper reports a novel approach to achieve self-healing surface hydrophobicity. Mesoporous silica is used as the reservoir for hydrophobic molecules, i.e., octadecylamine (ODA), that can release and refresh the surface hydrophobicity consecutively. A polymdopamine layer is used to further encapsulate silica-ODA, providing a reactive layer, governing release of the underlying ODA, and improving the dispersivity of silica nanoparticles in bulk resin. The approach arrives at self-healing (super)hydrophobicity without using any fluoro-containing compounds.     

Vesicle adsorption on mesoporous silica and titania

Lipid bilayer formation via vesicle fusion on mesoporous silica and mesoporous titania was investigated using quartz crystal microbalance with dissipation monitoring (QCM-D) and fluorescent recovery after photobleaching (FRAP). Results showed that lipid bilayers were formed on mesoporous silica and that intact vesicle adsorption was obtained on mesoporous titania. From the FRAP results, it could be concluded that the lipid bilayer was fluid; however, it had a smaller diffusivity constant compared to bilayers supported on a nonporous silica.     

Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas

Grand Canonical Monte Carlo simulations are used to study the adsorption of benzene at 298 K in an atomistic cylindrical silica nanopore of a diameter 3.6 nm. The adsorption involves a transition from a partially filled pore (a two layers thick film at the pore surface) to a completely filled pore configuration. Strong layering of the benzene molecules at the pore surface is observed. It is found that the layering decays as the distance to the pore surface increases. The position of the peaks for the density of the C, H atoms and the center of mass of the molecules shows that benzene molecules prefer an orientation in which their ring is perpendicular to the pore surface. This result is corroborated by calculating orientational order parameters and examining the distribution of the distances between the H and C atoms of the benzene molecules and the H and O atoms of the silica substrate.     

Molecular simulation study on adsorption of methanol/water mixtures in mesoporous silicas modified pore surface silylation

Two types of molecular simulation techniques have been utilized to investigate adsorption of methanol/water mixtures in a mesoporous silica with a hydrophobic pore surface: the NVT-ensemble Molecular Dynamics method with the melt-quench algorithm for modeling a fully-silylated mesoporous silica and the μVT-ensemble Orientaional-Biased Monte Carlo method for calculating adsorption isotherms. Adsorption isotherms of methanol and water at 333 K are calculated for an equi-relative-pressure mixture (each component has the same relative pressure which is defined as the ratio of the partial pressure to the saturation pressure of the pure gas) together with pure gases. In the case of the pure gas, water hardly adsorb even at elevated pressures, while the adsorption isotherm for methanol shows the condensable adsorption. On the other hand, in the case of the mixture, water molecules are substantially adsorbed along with methanol molecules, showing an isotherm representing the condensation mechanism. In addition, it is found that the separation factor of methanol to water is the highest in the case of monolayer adsorption from a liquid mixture.     

Morphology control of mesoporous silicas and their template synthesis in silica gel pores

Structure-related adsorption characteristics of ordered mesoporous silicas with spherical particles were studied depending on the conditions of their synthesis, including pH of the medium, the type of the alcohol, and the concentrations of the initial components of micelle solutions. The special features of the template synthesis of mesoporous silica in large silica gel pores were studied. The synthesized silicas were characterized by low-temperature nitrogen sorption-desorption, X-ray diffraction, and scanning electron microscopy measurements.     

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 Å, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.     

Behaviour of pure water and water mixture with benzene or chloroform adsorbed onto ordered mesoporous silicas

Structural characteristics of synthesized ordered mesoporous silicas MCM-41, MCM-48 and SBA-15 were studied using XRD, nitrogen adsorption and FTIR methods. Pure water and mixtures with water/benzene and water/chloroform-d adsorbed onto silicas were studied by ¹H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial liquids. Concentrated aqueous suspensions of MCM-48 and SBA-15 were studied by thermally stimulated depolarization current (TSDC) method. Benzene and chloroform-d can displace a portion of water to broad pores from the pore walls and from narrower pores, especially in the case of a large excess of an organic solvent. This process is accompanied by diminution of both interaction energy of water with an adsorbent surface and freezing temperature depression of adsorbed water. The effect of nonpolar benzene on pore water is much stronger than that of weakly polar chloroform-d. Modifications of the Gibbs-Thomson relation to describe the freezing point depression of mixtures of immiscible liquids confined in pores allow us to determine distribution functions of sizes of structures with unfrozen pore water and benzene. Former address: Pisarzhevskii Institute of Physical Chemistry, 31 Prospect Nauki, Kiev, Ukraine     

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography

Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.     

Porous properties of silylated mesoporous silica and its hydrogen adsorption

Porous properties of mesoporous silica silylated with various organic silanes were examined and their hydrogen adsorption properties were measured at 77 K. By silylation of the mesoporous silica, the specific surface area, pore radius and pore volume steeply decreased depending on both the size of the silane and the amount actually incorporated into the mesoporous framework. The pores reduced in size depending on the amount of modifying silane and vanished completely in the samples silylated with 3-aminopropyltriethoxysilane and phenyltrichlorosilane. Hydrogen adsorption isotherms of the silylated samples were measured at 77 K. With the exception of the sample with phenyltrichlorosilane, hydrogen adsorption volumes were proportional to the surface area with a slope of around 1.1 molecules/nm². On the other hand, for the sample treated with phenyltrichlorosilane, a large hydrogen adsorption volume of around 38.1 molecules/nm² was obtained. On heating the silylated compounds at 500 °C, micropores formed and the hydrogen adsorption volume increased by 1.5 times or more due to the development of micropores.     

Nitrogen adsorption on silica surfaces of nonporous and mesoporous materials

We present an accurate comparative analysis of N 2 adsorption at 77 K on nonporous silica and the pore wall surface of MCM-41 materials. The analysis shows that in the low-pressure region of N 2 adsorption obeys a peculiar mechanism governed by short-ranged forces, which makes the surface curvature effect on the N 2 adsorption in mesopores nearly negligible. We used this observation to define more exactly compared to the BET technique the specific surface area of the reference adsorption isotherm on nonporous silica basing on XRD data and linear sections of t-plots. Calculation of the capillary evaporation and condensation pressures seems to confirm our previous finding that the capillary condensation pressure corresponds to the equilibrium transition rather than spinodal condensation at least for pore sizes less than 7 nm. It allowed us to provide more reliable pore size distribution (PSD) analysis of mesoporous silica materials. For example, the PSDs of MCM-41 samples do not show artificial peaks in the micropore range that we obtained in our earlier publications.     

Characterization of a delaminated clay and pillared clays by adsorption of probe molecules

Characterization of the textural and structural properties of a sodium form of a delaminated calcic montmorillonite, and of aluminium pillared materials prepared with and without amine pre-adsorption, was made using the adsorption of different probe molecules (nitrogen, toluene, methyl ethyl ketone and 1,1,1-trichloroethane). Due to the delaminated character of the prepared solids, the characterization by X-ray diffraction of the pillared materials was not possible. In this context, the adsorption of probe molecules revealed to be informative since, although the prepared materials were mainly mesoporous solids in consequence of their delaminated nature, when the amine pre-adsorption was used before the pillaring, microporosity was also formed.     

Atomistic model of micelle-templated mesoporous silicas: structural, morphological, and adsorption properties

The structural, morphological, and adsorption properties of MCM-41 porous silicas are investigated using a realistic numerical model obtained by means of ab initio calculations [Ugliengo, P.; et al. Adv. Mater.2008, 20, 1]. Simulated X-ray diffraction, small angle neutron scattering, and electronic microscopy for the atomistic model are in good agreement with experimental data. The morphological features are also assessed from chord length distributions and porous volume and specific geometrical surface calculations, etc. The N(2), CO(2), and H(2)O adsorption isotherms in the atomistic model of MCM-41 are also in reasonable agreement with their experimental counterpart. An important finding of the present work is that water forms a film adsorbed on specific hydrophilic regions of the surface while the rest of the surface is depleted in water molecules. This result suggests that the surface of MCM-41 materials is heterogeneous, as it is made up of both hydrophilic and hydrophobic patches. While adsorption and irreversible capillary condensation can be described using the thermodynamical approach by Derjaguin (also known as the Derjaguin-Broekhoff-De Boer model), the Freundlich equation fits nicely the data for reversible and continuous filling in small pores.     

Morphological and structural evolution of mesoporous silicas in a mild buffer solution and lysozyme adsorption

Mesoporous silicas with various morphologies and structures were synthesized with the aid of 2,2,4-trimethylpentane (TMP) in the presence of nonionic surfactant P123 [(EO)20(PO)70(EO)20] as a structure-directing agent under mild reaction conditions (HAc-NaAc buffer solution, pH 4.4). The ropelike particles formed by end-to-end interconnected nanorods were obtained at a TMP/P123 weight ratio of 0.5. It is noteworthy to mention that the mesoporous nanorods have channels running parallel to the short axis. The silica hollow spheres can be obtained at a higher TMP/P123 weight ratio because of the fusion of the primary nanorods around the interface of the O/W emulsion. Initial synthesis temperatures of 15, 25, and 40 degrees C can lead to mesoporous silicas with highly ordered 2D hexagonal mesostructure, vesicular mesostructure, and mesostructured cellular foams (MCF), respectively. The mesoporous silicas exhibit high adsorption capacity (up to 536 mg g(-1)) and very rapid (<5 min to reach equilibrium) lysozyme immobilization. More importantly, it is revealed that mesoporous silica hollow spheres with rugged surfaces can greatly accelerate the adsorption rate of the enzyme during the adsorption process.     

Peculiarities of adsorption of organic compounds and water on silicas with bonded polyfluoroalkyl groups

The adsorption isotherms of benzene revealed the low adsorption potential of polyfluoroalkyl silicas compared with both initial silica and hexadecyl silica. Hydrophobicity of all modified silicas, measured by water adsorption, was rather close at p/p(s)=1 and was significantly different in the initial p/p(s) region: polyfluoroalkyl silicas did not nearly adsorb water but alkyl silica did it gradually. Thermodynamic parameters of adsorption of organic compounds of different nature were studied at a zero surface coverage by gas chromatography (GC). The high enough contributions of specific interactions as well as polar molecular group increments to the Gibbs adsorption energy were found to be characteristic of polyfluoroalkyl silica. The selectivity of polyfluoroalkyl silicas and the commonly used fluorinecontaining liquid stationary phase for GC (OV-210) toward homologous ethers, esters, and alcohols was comparable in most cases. GC data, IR spectra, and adsorption measurements were in good agreement with each other. It was shown that residual SiOH-groups of silica do not nearly take part in the adsorption process because they are effectively screened by the attached organic groups.     

Interaction of single water molecules with silanols in mesoporous silica

Deep Inelastic Neutron Scattering measurements of water confined in mesoporous silica have been carried out. The experiment has been performed at room temperature on dry and on hydrated samples in order to investigate the interaction between the protons and the silanol groups of the confining surface. With this aim we could control the hydration of the pores in such a way as to adsorb 3.0 water molecules per nm(2), corresponding to a 1 to 1 ratio with the silanol groups of the surface. DINS measurements directly measure the mean kinetic energy and the momentum distribution of the protons. A detailed analysis of the hydrated sample has been performed in order to separate the contributions of the protons in the system, allowing us to determine the arrangement of water molecules on the silanol groups. We find that the hydrogen bond of the water proton with the oxygen of the silanol group is much stronger than the hydrogen bonds of bulk water.