Effects of thermal treatment on the surface properties of a wide pore silica gel

A mesoporous silica gel Davidson 59 was thermally treated in vacuo, in the temperature range 20–1000°C. Effects of thermal treatment on the water contents, nitrogen surface areas, pore structure and heats of immersion in water were investigated and discussed. The temperatures selected were 20, 110, 200, 290, 380, 480, 510 and 1000°C. These temperatures were found to cover all the various textural changes resulting from the heat effect.It could be shown that the heats of immersion in water depend primarily on the water content of the sample and are proportional, at least qualitatively to the number of hydroxyl groups on the surface and their availability for interaction with liquid water. The interesting result obtained is that a second factors is involved, namely the pore structure of the adsorbent. A qualitative parallelism exists between the normalized heat of immersion per unit area, and the average pore radius. Apparently the packing of water molecules in narrow pores leads to a decrease in the heat of immersion due to repulsion between the permanent dipoles of the molecules. In narrower pores, the heat of immersion in water is smaller than in wide pores.     

Validity of the Kelvin equation in estimation of small pore size by nitrogen adsorption

 We have investigated a practical lower limit of a pore-size estimation by the nitrogen desorption isotherms at 77 K using the Kelvin equation. Changes in pore size of porous silica glasses before and after the monolayer preadsorption of n-propylalcohol were estimated by measuring the nitrogen adsorption and desorption isotherms. These changes should correspond to the thickness of monolayer of adsorbed n-propylalcohol. The thickness of monolayers obtained for the samples whose pore sizes are below ca. 2 nm were underestimated, when the Kelvin equation was applied to the nitrogen desorption isotherms using the values of surface tension and molar volume of bulk liquid nitrogen at 77 K. Below ca. 2 nm pore radius a careful application of the Kelvin equation is required to estimate a pore size. These results suggest that a change in the physical properties of liquid nitrogen in such a small pore occurs. It is supposed that the interaction between the solid surface and adsorbate molecules causes the changes in the surface tension and density of liquid nitrogen in such a narrow pore. Received: 21 March 1997 Accepted: 18 July 1997     

Preparation and properties of polyvinyl acetal sponge modified by chitosan

The polyvinyl acetal sponge modified by chitosan was prepared by adding chitosan/polyvinyl alcohol (PVA) solution during the acetalation reaction of PVA and formaldehyde. The effect of vesicant and chitosan to the pore morphology, water absorption ratio, water absorption rate, expansion time and mechanical properties were studied. The polyvinyl acetal sponge modified by chitosan was used as a hemostatic packing material for the injured rabbit nasal tissue. The hemostatic effect and the healing effect of the modified sponge on the nasal mucosa after nasal surgery were studied. The results indicated that the polyvinyl acetal sponge modified by chitosan has an interconnected pore structure and the wall between large pores also has small pores. The chitosan adhered on the inner surface of the pores. The increased content of vesicant led to an increase in pore diameter, in the water absorption ratio and in expansion time. However, there was only a small change in the water absorption rate and a decrease in tensile strength and compression strength were noted. With an increase in chitosan content, the pore diameter and interconnection of the sponge was reduced. Water absorption ratio, expansion time and water absorption rate decreased, but tensile strength and compression strength improved. Observation of the rabbit nasal tissue after surgical operation suggested that polyvinyl acetal sponge modified by chitosan has an anti-inflammatory, hemostatic and anti-adherent characteristic and could promote the healing and functional recovery of rabbit nasal mucosa. __________ Translated from Journal of Jinan University (Natural Science & Medicine Edition), 2007, 28(3): 283–287 [译自: 暨南大学学报(自然科学与医学版)]     

国产硅藻土结构的研究

系统地研究了硅藻土结构, 发现除浙江白土表面具有双生的硅羟基外, 其它只有连生的与孤立的硅羟基; 经IR谱证明, 其内部尚有Si-O与Al-O振动, 酸洗或焙烧都能使硅藻体的孔径、孔径分布和结构发生改变; 经焙烧后, 结构均发生改变, 其自由水消失, 束缚水逐渐减少; 焙烧达1150℃时, 原组成无定形二氧化硅则晶化为α-方石英, 除吉林土具有一级孔洞, 云南土具有三级孔洞外, 本文中其它硅藻土均有二级孔洞。     

The kinetics of silylation of hydroxylated silica II CPG-10 porous glasses

 The kinetics of chemisorption of trimethylchlorosilane, trimethylbromosilane, and triethylchlorosilane on samples of porous silica, CPG-10 porous glasses, of varying pore diameter were investigated and found to follow a first-order kinetics law in which the reaction is retarded by one of its products. At constant temperature, the rate was found to increase with increasing diameter of the pores in the range used. Increasing the molecular cross-sectional area of the adsorptive, the silane molecule, in the range 0.35–0.465 nm² decreased the rate of reaction. Received: 3 November 1999/In revised form: 21 January 1999/Accepted: 27 January 1999     

Density minimum of confined water at low temperatures: a combined study by small-angle scattering of X-rays and neutrons

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.     

On interfacial tension calculation from the test-area methodology in the grand canonical ensemble

We propose the extension of the test-area methodology, originally proposed to evaluate the surface tension of planar fluid-fluid interfaces along a computer simulation in the canonical ensemble, to deal with the solid-fluid interfacial tension of systems adsorbed on slitlike pores using the grand canonical ensemble. In order to check the adequacy of the proposed extension, we apply the method for determining the density profiles and interfacial tension of spherical molecules adsorbed in slitlike pore with different pore sizes and solid-fluid dispersive energy parameters along the same simulation. We also calculate the solid-fluid interfacial tension using the original test-area method in the canonical ensemble. Agreement between the results obtained from both methods indicate that both methods are fully equivalent. The advantage of the new methodology is that allows to calculate simultaneously the density profiles and the amount of molecules adsorbed onto a slitlike pore, as well as the solid-fluid interfacial tension. This ensures that the chemical potential at which all properties are evaluated during the simulation is exactly the same since simulations can be performed in the grand canonical ensemble, mimicking the conditions at which the adsorption experiments are most usually carried out in the laboratory.     

Principles of the theory of adsorption of large molecules in pores with nonuniform walls

Principles of the theory of adsorption of large molecules blocking more than one adsorption center on a surface in slit-like pores are proposed. The theory takes into account lateral adsorbate—adsorbate interactions and nonuniformity of the pore walls. The equations of adsorption isotherms are derived using the cluster approach. The lateral interactions are taken into account in the quasi-chemical approximation, preserving effects of direct correlations, and in the mean field approximation without effects of correlations. The following problems are discussed: 1) distinguishing of partial contributions of nonuniform adsorption centers on the pore walls; 2) exact solution for dimer adsorption in a two-layer pore with uniform walls; 3) basic types of adsorption isotherms, for which the differences are due to various orientations of the adsorbate in micropores with uniform walls; 4) estimates of the pressure responsible for volume filing of micropores; and 5) the effect of nonuniformity of the pore walls on the pressure values. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1467–1478, August, 1999.     

Surface Chemistry and Structure of Silicon Oxycarbide Gels and Glasses

In this study, the surface chemistry and structure of methyl-substituted silica gels and porous oxycarbide glasses were investigated. FTIR was used to measure the relative concentration of Si−CH₃ and Si−OH as a function of the degree of methyl-substitution and the pyrolysis temperature. The gels and glasses were further heated, dehydrated or hydrated, in situ, within the FTIR spectrometer. In the temperature range of 800–850°C, high surface area oxycarbide glasses were created with no detectable surface hydroxyl groups. Oxycarbide glasses synthesized in argon at 700°C displayed a weak band for surface hydroxyl groups and reversible physisorption of water, while those synthesized at 850/900°C showed a complete absence of surface hydroxyl groups and the formation of vicinal silanols upon chemisorption of water. Isolated silanols were observed upon heat treatment in vacuum. Formation of aromatic carbon species was found to correlate with the decomposition of the methyl groups. The oxycarbide surface is quite stable to densification (presumably due to elemental carbon on the pore surfaces). In the absence of oxygen, porous silicon oxycarbide glass powders maintain surface areas >200 m²/g at 1200°C. However, oxidizing species in the atmosphere deplete the aromatic carbon species, and the glasses lose surface area.     

Dodecyltrimethylammonium bromide–disodium dodecanephosphonate mixed micelles

 The aqueous catanionic system dodecyltrimethylammonium bromide (DTAB)–disodiumdodecanephosphonate (DSDP) was studied by potentiometry, conductivity, surface tension, spectrometry and dye solubilization. No precipitation of neutral salts was found in the entire range of compositions studied. Up to four transitions were detected. The first transition, at about 0.001 mol dm⁻³, was probably related to a state change in the adsorption monolayer at the air/water interface. The second, at about 0.0065 mol dm⁻³, was probably related to the formation of ion pairs. The third transition was the critical micelle concentration which was analyzed with the pseudophase separation model and regular solution theory. The interaction between DTAB and DSDP molecules in micelles was weaker than in other cationic–anionic surfactant mixed micelles. Large, probably rodlike, micelles formed at the fourth transition at higher surfactant concentration. No vesicles or lamellar liquid crystals were detected. The adsorbed monolayer at the air/water interface was also studied by means of regular solution theory. It was much richer in DTAB than the micelles and the intermicellar solution. The interaction between DTAB and DSDP molecules at the air/water interface was very low. The results were explained on the basis of steric factors. Received: 6 January 1999 Accepted in revised form: 13 April 1999     

Herstellung oberflächenmodifizierter Adsorbentien

Zusammenfassung Die Reaktion von Phenyltrichlorsilan mit Silicagel wurde an drei Pr?paraten (eng-, mittel- und grobporig) mit bekannter Hydroxylgruppenkonzentration im Temperaturbereich von 20–350 °C untersucht. Die Analyse der modifizierten Produkte ergab bei jeder Reaktionstemperatur eine Phenyl-monochlorsilyl-Gruppierung als neue Oberfl?chenschicht. Daraus folgt, da? eine Phenyltrichlorsilanmolekel selektiv mit zwei oberfl?chenst?ndigen Hydroxylgruppen reagiert. Dieses Ergebnis wurde best?tigt durch Bestimmungen der Hydroxylgruppenkonzentration der Silicagele vor und nach der Reaktion. Es konnten maximal 75% der vorhandenen Hydroxylgruppen umgesetzt werden. Dies entspricht einer Belegung von 3,6μmol Phenylchlorsilylgruppen pro m². Nach der Hydrolyse des modifizierten Gels fiel ein weitgehend hydrophobes Produkt an. Bei dem mittelund grobporigen Pr?parat wurde die spezifische Oberfl?che und die mittlere Porenweite durch die Umsetzung nicht ver?ndert. Bei dem engporigen Pr?parat führte die Umsetzung mit Phenyltrichlorsilan zu einem nichtpor?sen Produkt. Die phenylierten Silicagele k?nnen als Gerüstsubstanzen in der Gas-Fest-Chromatographie und in der Gelpermeationschromatographie eingesetzt werden. Sie dienen weiterhin als Ausgangsprodukte zur Darstellung synthetischer Kationenaustauscher.

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Summary The surface reaction of phenyltrichlorsilane with three different species of silica gel of definite pore structure and known concentration of hydroxyl groups on the surface was studied in the temperature range of 20–350 °C. On the basis of the analytical results of the modified silica gel species it is suggested that phenyltrichlorosilane reacts selectively with two hydroxyl groups forming a phenyl-monochloro-silyl surface layer at all reaction temperatures under investigation. This result was confirmed by the determination of the concentration of the hydroxyl groups on the surface before and after the reaction. The maximum yield of the surface coverage attained under the best reaction conditions was 3,6 μmole phenyl-monochloro-silyl groups m⁻² indicating that 75% of the total amount of the hydroxyl groups had reacted. Hydrolysis of the modified silica gel yielded a white hydrophobic adsorbens. For the wide-pored silical gel and for the species with pores in the transitional range (20<d<400 ?) the value of the specific surface and the mean pore diameter remained unchanged by the modification. The silica gel with narrow pores gave a non porous adsorbent by the modification. The phenylated silica gels were tested as packing materials for GSC- and GPC columns. Furthermore, they were employed for the preparation of synthetic cationexchangers.

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Wir danken Herrn Prof.H. W. Kohlschütter für die F?rderung dieser Arbeit sowie für wertvolle Diskussionen.     

Small solid particles and liquid lenses at fluid/fluid interfaces

The behaviour of small solid particles and liquid droplets at fluid interfaces is of wide interest, in part because of the roles they play in the stability of foams and emulsions. Here we focus on solid particles at liquid interfaces, both singly and in highly structured monolayers. We briefly mention small oil lenses on water in connection with the determination of line tension, τ. Particles are surface-active in the sense that they often adhere quite strongly to liquid surfaces, although of course they are not usually amphiphilic. The three-phase contact line around a particle at an interface is associated with an excess free energy resulting in a tendency of the line to contract (positive τ, which is a 1D analogue of surface tension) or to expand (negative τ). Positive line tension acts so as to push the contact angle of a particle with the fluid interface further away from 90°, i.e. to force the particle towards the more “wetting” of the two bulk phases. It also leads to activation barriers to entry and departure of particles from an interface. The behaviour of particle monolayers at octane/water interfaces is also discussed . It is found that, for monodisperse spherical polystyrene particles containing ionisable sulphate groups at the surface, highly ordered monolayers are formed. This appears to result from very long range electrostatic repulsion mediated through the oil phase. Surface pressure–surface area isotherms are discussed for particle monolayers and it is shown, using light microscopy, that at monolayer “collapse” particles are not expelled from the monolayers but rather the monolayer folds, remaining intact. This has an important bearing on methods, involving the use of the Langmuir trough, for the experimental determination of contact angles and line tensions in particulate systems. Received: 18 July 1999/Accepted: 30 August 1999     

Complexation in the heptanoic acid–heptylamine system

The complexation between heptylamine and heptanoic acid has been elucidated at 298.15 K using spectroscopic methods and also by measuring macroscopic quantities such as viscosity, conductivity and surface tension. Fourier transform IR and ¹³C NMR measurements point towards the existence of a compound consisting of one amine molecule and one acid molecule in an equimolecular mixing ratio. These suggestions are supported by viscosity and surface tension measurements. This compound is further able to interact with excess acid, but similar behaviour is not observed with excess amine. The equimolecular compound behaves like a catanionic surfactant; this is seen in the phase diagram for the heptylamine–heptanoic acid–water system at 298.15 K, where the dominating phase is the lamellar liquid-crystalline phase. This phase is in equilibrium with almost pure water. At low water content a solution phase extending from the binary heptylamine–heptanoic acid axis and covering all mixing ratios between the amine and the acid is also present. Received: 15 March 1999/Accepted in revised form: 5 July 1999     

Molecular dynamics simulations on the adsorption and surface phenomena of simple fluid in porous media

Equilibrium molecular dynamics simulations have been used to investigate the fluid adsorption phenomena and calculate the surface tension in porous media at different temperatures, densities and pore widths. The facts that most of the fluid particles are adsorbed adherent to the pore walls and the surface forms near the walls have been found, and that the surface is not stable which means there exists an oscillation phenomenon in pores have been also found. The surface tension in pores is much bigger than that in macrovolume systems like normal liquid–liquid and liquid–vapor interfaces, and it will increase with the increase of density and pore width, but will decrease with the increase of temperature.     

Wetting and non-wetting fluids in surface-functionalised activated carbons

Pore filling by non-polar and polar molecules is investigated by small-angle X-ray scattering in activated carbons that had been surface-functionalised to different degrees by oxidation. A pseudo-binary model for the scattering response is used to trace the filling characteristics of water and n-hexane in the pore structure. While the pores are uniformly filled by n-hexane, pore filling by water is only partial. In the latter case, a significant contribution from liquid–vapour interfaces appears and the system becomes fully ternary. This feature is direct evidence of the development of water droplets, which form even in the most oxidised carbon. The appreciable differences between the carbons illustrate the influence on the small-angle X-ray scattering response of surface chemistry and of the polarity of the adsorbed molecules.     

Influence of Hydronium Ions in Zeolites on Sorption

In the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm³. In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level.     

The influence of protecting the hydroxyl group of β-oxy-α-diazo carbonyl compounds in the competition between Wolff rearrangement and [1,2]-hydrogen shift. Density functional theory study and topological analysis of the charge density

The influence of protecting the hydroxyl group of a β-oxy-α-diazo carbonyl compound on the competition between the Wolff rearrangement (WR) and the [1,2]-hydrogen shift (HS) was investigated theoretically. Stationary points on the potential-energy surface were located with the B3LYP density functional and the 6-31G** basis set. For the basic system geometry optimisations at B3LYP/6-311+G** were performed to validate the reliability of the B3LYP/6-31G** calculations. Single-point energy calculations were carried out at the B3LYP/6-311+G** level on the B3LYP/6-31G**-optimised geometries. Further insight into the processes was achieved with the aid of the theory of “atoms in molecules” of Bader. The calculated energy barriers qualitatively predicted the yields of HS and WR obtained experimentally. In order to rationalise the calculated energy barriers, it was necessary to take into account not only the electronegativity of the protective groups but also the alignment of the migrating groups with the depletion sites at the carbene centre. Further, when the hydroxyl group was not protected the existence of an intramolecular hydrogen bond played an important role in both HS and WR. Received: 30 December 1998 / Accepted: 7 May 1999 / Published online: 4 October 1999     

IR investigation of hydrogen bonds in weakly hydrated films of poly(vinyl alcohol)

A system of hydrogen bonds in weakly hydrated PVA films containing up to ≤8.5 wt % water is investigated via IR spectroscopy. It is shown that water molecules bind to only part of the hydroxyl groups of the polymer that are available for hydration and form the first hydrating layer. In a completely dehydrated film, practically every hydroxyl group of PVA forms hydrogen bonds with two other hydroxyl groups and serves as both a proton donor and a proton acceptor. In the hydrated film, one to three water molecules directly bind with one hydroxyl group of PVA.     

The effects of electropolishing on the nanochannel ordering of the porous anodic alumina prepared in oxalic acid

A series of porous anodic alumina has been prepared by anodizing aluminum surface in 0.3 M oxalic acid at different voltages. Prior to anodizing, the surface was pretreated in two different electropolishing electrolytes. One was Brytal solution (15% Na₂CO₃ and 5% Na₃PO₄) at 80 °C in which the electropolishing was performed at 2 V. This resulted in about 100–150 nm apart random features of 4–5 nm height. The other was the commonly employed perchloric acid–alcohol solution (1:4 ratio by volume), in which the electropolishing was performed at 20 V. The resulting surface comprised nanostripes of 1–2 nm amplitude with a wavelength of about 50 nm. The former pretreatment proved better for self-ordering of the pores at the anodizing voltage of 50–60 V, while the latter pretreatment was found better at the anodizing voltage of 40 V. The improved pore ordering at a given voltage was attributed to the higher pore density as associated with greater repulsive interactions among the pores.     

Pore nucleation in mechanically stretched bilayer membranes

We report a computer-simulation study of the free-energy barrier for the nucleation of pores in the bilayer membrane under constant stretching lateral pressure. We find that incipient pores are hydrophobic but as the lateral size of the pore nucleus becomes comparable with the molecular length, the pore becomes hydrophilic. In agreement with previous investigations, we find that the dynamical process of growth and closure of hydrophilic pores is controlled by the competition between the surface tension of the membrane and the line tension associated with the rim of the pore. We estimate the line tension of a hydrophilic pore from the shape of the computed free-energy barriers. The line tension thus computed is in a good agreement with available experimental data. We also estimate the line tension of hydrophobic pores at both macroscopic and microscopic levels. The comparison of line tensions at these two different levels indicates that the "microscopic" line tension should be carefully distinguished from the "macroscopic" effective line tension used in the theoretical analysis of pore nucleation. The overall shape of the free-energy barrier for pore nucleation shows no indication for the existence of a metastable intermediate during pore nucleation.