Surface Properties Of Silica Gels Research Articles

Surface properties of silica gel modified by trifunctional polyfluoroalkyl silanes according to data of adsorption–structural and gas chromatography studies

Surface properties of silica gels modified by trifunctional polyfluorosilanes n-C6F13(CH2)2SiCl3 and iso-C3F7(CH2)2SiCl3 are studied using the methods of adsorption and gas chromatography with respect to nitrogen, hydrocarbon, and water molecules. It is shown that the branched structure of the grafted group and high concentration of grafted groups (2.7 nm–2) provide better screening of both residual silanols of the support and of additional hydroxyl groups formed in the course of synthesis as a result of hydrolysis. Comparison with the properties of silica gel with grafted monofunctional silane n-C6F13(CH2)2Si(CH3)2Cl shows that, in the case of an overall decrease in the adsorption values and considerable surface lyophobization, modification by trifunctional silanes irrespective of the modifier chain length allows obtaining adsorbents containing more polar centers than those in silica gel due to the possible participation in adsorption of additional silanols and a polar fragment of the grafted chain of–CH2–CF2–. A considerable difference in the course of isotherms of nitrogen and hexane adsorption–desorption is found in the range of capillary condensation hysteresis related to liophobicity of polyfluoroalkyl layers, a significant decrease in the pore wetting and the corresponding increase in wetting angle θ by hexane. Adsorption of water on fluorinated silicas is negligible and capillary condensation is not observed due to nonwettability of the pore surface by liquid adsorbate (θ > 90°).

An improved methodology for adsorption characterization of unmodified and modified silica gels

An improvement in the adsorption characterization of the surface and structural properties of unmodified and modified mesoporous silica gels is presented. This improvement was achieved by selection of proper macroporous silica as the reference solid for adsorption characterization of porous silica gels. Experimental illustration is provided for unmodified and n-octyl-modified silica gels of different bonding density. The surface and structural properties of these silica gels were characterized by utilizing the standard adsorption data for both unmodified and octyl-modified LiChrospher Si-1000 macroporous silica gels. It was shown that the standard nitrogen adsorption data have an appreciable influence on the analysis of the pore size and surface properties of silica gels. This analysis can be improved by selecting the reference solid of the surface properties close to those of the silica gel studied.

Influence of manufacturing variables on surface properties and dynamic adsorption properties of silica gels

The surface properties of silica gel such as surface area, pore volume, and average pore diameter are affected by the manufacturing variables including pH values, gelling and dehydration temperatures. The dynamic adsorption properties of silica gel are also affected by their preparation conditions. The mass transfer zone and the amount of adsorption were discussed using the experimental data of breakthrough curve. The influence of the manufacturing variables on the trends of breakthrough curves is important in most of the adsorbent modification. It was found in this study that the manufacturing variables affected the adsorption performance significantly. Therefore, the relationship between the manufacturing variables and the corresponding adsorbent properties conducted by the dynamic adsorption apparatus was examined in this study. This discussion is rare in most of the studies of preparing silica gels, which are usually focused on the relationship between the manufacturing variables and the surface properties of silica gels only. However, it is not necessary that a silica gel with larger surface area will give a smaller mass transfer zone or mass transfer resistance. The modification of adsorbents should consider not only the improvement of their surface properties but also their adsorption performance.

The Influence of Thermal Treatment of Silica Gel on Surface–Molecule Interactions: II. The Gas Phase Titration

The effect of thermal treatment on the surface properties of silica gel was investigated by the gas phase titration method. The silica gel samples were heated at 200, 400, and 800°C. Ethyl methyl ketone was applied as a blocking reagent, andn-octane, benzene, and chloroform were used as testing probes. A new presentation of the results was proposed, which allowed their direct comparison for different sample areas. It was determined that the thermal treatment of silica resulted in different changes of testing probe retention upon adsorption of the same blocking reagent, and this was dependent on the type of surface–molecule interactions (nonspecific, electron-acceptor, and electron-donor, respectively). Certain differences in adsorption sites distribution were found. Then the changes in free enthalpy of adsorption were calculated for all testing probes. The results are discussed in terms of condensation of surface silanols and formation of siloxane bridges.

Silicic Acid Polymerization Catalyzed by Amines and Polyamines

Abstract Polymerization and subsequent gelation of silicic acid in water were catalyzed by amines and polyamines. Particularly effective were poly(allylamine), poly(lysine), and poly(arginine), which promoted spontaneous gelation. The resultant silica gel contained a sizable portion of amines, which altered the surface properties of silica gel.

Surface properties of silica gel and silica gel-supported ion-exchanged copper in transformations of various molecular probes: an infrared study

An IR study on some surface properties of silica gel and an ion-exchanged silica gel-supported Cu catalyst (Cu/S-X) using D 2, D 2O and 1,1-dimethylcyclopropane (DMCP) as molecular probes is reported. The interactions of DMCP “alone” or a mixture of DMCP and D 2 were studied over predeuterated surfaces. As far as the preparation of deuterated surfaces is concerned, it was found that the broad OH band on silica gel could be gradually transformed to a relatively sharp OD band by increasing the temperature of D 2 treatment. Using D 2O resulted in complete OHOD exchange even at room temperature. When copper was deposited on the support, the OHOD exchange with D 2 became faster and went to completion at 673 K. The shape of the OD bands resulting from this surface reaction and that of D 2O was nearly identical. Experiments carried out with DMCP or DMCP + D 2 over predeuterated catalysts provided direct information about the mode of adsorption of the cyclopropane.

Adsorption and diffusion of a Lennard-Jones vapor in microporous silica

The properties of a dilute Lennard-Jones vapor in contact with an adsorbing microporous medium are investigated using grand canonical ensemble Monte Carlo and molecular dynamics techniques. The bulk structure of the microporous system is modeled as an assembly of randomly distributed interconnected solid spheres, and vapor/surface interactions are treated in two ways: (i) using a smooth continuous interaction potential and (ii) using a molecular model for the surface structure of the solid. The microporous solid representation employed in these simulations is chosen to conform in realistic manner with the bulk and surface properties of silica gel. The results obtained from the simulations include equilibrium partition coefficients, diffusivities, and related microscopic properties. By comparing these results with available experimental data it is shown that the properties of simple nonpolar gases in microporous silica may be predicted with reasonable accuracy. This is particularly true when the molecular structure of the silica surface is taken into consideration.

Correlation of Orthopositronium Annihilation and the Surface Properties of Silica Gels as a Function of Temperature

The lifetime spectra of positrons annihilating in silica gels of various pore sizes as a function of temperature are investigated. There are two long lifetime components which are attributed to the annihilation of o-Ps under various processes. The intensities of the two long lifetime components change with temperature. These changes cannot be attributed solely to the diffusion process. Other processes, such as the formation, quenching, and thermalization of o-Ps in silica gels, must also be taken into consideration.The mean life of the longest lifetime component also changes with temperature. The pattern of this change varies with the pore size and grain size of the silica gel. It seems that positronium atoms are adsorbed on the pore surface of silica gels. A potential model including the effect of adsorption is proposed to explain this phenomenon. The change of annihilation rate of o-Ps is not found to be solely due to the vibration of surface atoms. The quenching due to the presence of the "strained" surface silica groups and its change with the temperature is offered as an explanation.

Studies on the Surface Properties of the Surface-Treated Silica Gels by the n-Pentanol through the Adsorption of Various Kinds of Vapours and the Heat of Immersion

The surface properties of silica gels treated with n-pentanol were investigated by the adsorption of argon at 77°K, water vapour at 288°K and n-heptane at 273°K and by the heat of immersion of water and n-heptane at 298°K on the samples having various numbers of surface groups. The following results were confirmed: (1) For the silica gel samples which were untreated or had a small number of surface groups, the shape of isotherms of water vapour adsorption was the type II in BET classification, whereas it was the type I for the samples with a large number of surface groups. (2) The shape of isotherms of n-heptane adsorption was the type II for both the untreated and surface-treated samples. The existence of many capillary pores less than 20Å in diameter was suggested, since the volume of pores larger than 20Å in diameter appeared to be exactly the same for both the untreated and surface-treated silica gels. (3) The surface areas obtained from the absorption of argon, water and n-heptane showed a linear dependence on the number of surface groups. From their slopes, it was concluded that each of the surface groups covered 1.5 unreacted silanol and these surface groups would presumably be distributed on the surface of silica gels uniformly. (4) The heat of immersion of water showed a linear relation with the monolayer adsorption value obtained from water vapour adsorption.

Study of Various Properties of Silica Gel by Positron Annihilation

The lifetimes of o-positronium in silica gel adsorbed with various amounts of water, n-hexane, or nitromethane were measured. The fraction of o-positronium escaping the condensed phase can be related to the specific area and the pore volume of the silica gel. The mean life of o-positronium in the pores is found to be very sensitive to the surface properties of silica gel. The formation of free radicals on the surface of silica and the reactions between these free radicals and o-positronium are suggested as the explanations.

Surface properties of silica gels. IV. Influence of acid concentration in the preparation from sodium silicate and acetic acid

AbstractSilica gels were prepared by adding sodium silicate to acetic acid, or acetic acid‐sodium acetate buffer, until the pH reached 4.64, the addition of silicate being made rapidly (over 1–2 min.) or slowly (over about 30 min.). Adsorption isotherms of nitrogen were determined on the outgassed samples at − 195.8° and used for the calculation of surface areas and pore volumes.The gels prepared either by rapid addition of silicate to acid, over the concentration range 2–30%, or from the buffer (initial pH 3.78), were all microporous and had very similar surface properties. On the other hand, the surface properties of those gels prepared by the slow addition of silicate depended on the concentration of acid used, the surface area of the sample prepared from 30% acetic acid being half that of the one from 10% acid. It is suggested that this effect is associated with the loss of some of the smaller primary particles, which occurs as the concentration of polysilicic acid is increased and the pH allowed to change slowly during the reaction.

The surface properties of silica gels. I. Importance of pH in the preparation from sodium silicate and sulphuric acid

AbstractGels were prepared by adding sodium silicate solution to sulphuric acid until a certain pH was reached, and then the products were aged, washed and dried under controlled conditions. The surface properties of the dry gels were studied by measuring adsorption isotherms of nitrogen at – 195·8° and nitrous oxide at — 78·5°. The pH reached came within the range 3·7–5·8, as this corresponded to a decrease in setting time from over an hour to a few seconds. For gels prepared over this pH range, the isotherms of the two gases also changed profoundly, both sets showing the same general pattern. In one preparation the temperature was varied, and in another the ageing period was reduced to the minimum; in neither case could any difference in the adsorption of nitrous oxide be detected. Activation, by soaking in very dilute sulphuric acid, was shown to be more important for a gel prepared at the higher‐pH end of the range.Surface areas have been calculated from both sets of isotherms by the Brunauer, Emmett and Teller (B.E.T.) method. Good agreement is obtained, in some cases, by assuming that each nitrous oxide molecule occupies an area of 20·4 Å2 in the close‐packed monolayer, and taking the usual value of 16·2 Å2 for the nitrogen molecule. The values of residual water content and B.E.T. surface area both decreased substantially over the pH range, but there appears to be no simple relationship between them.