Silica molecular surface

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

The first possibility is that the attractive potential associated with the solid surface leads to an increased gaseous molecular number density and molecular velocity. The resulting increase in both gas-gas and gas-wall collision frequencies increases the T1. The second possibility is that although the measurements were obtained at a temperature significantly above the critical temperature of the bulk CF4 gas, it is possible that gas molecules are adsorbed onto the surface of the silica. The surface relaxation is expected to be very slow compared with spin-rotation interactions in the gas phase. We can therefore account for the effect of adsorption by assuming that relaxation effectively stops while the gas molecules adhere to the wall, which will then act to increase the relaxation time by the fraction of molecules on the surface. Both models are in accord with a measurable increase in density above that of the bulk gas. 

Other additives such as silanes, titanates and zirconates are also used to overcome the processing characteristics of silica fillers. Silanes not only give improved processability of silica-filled compounds, but also provide improved crosslinks between the silica particle surface and the rubber molecular chains giving increased physical properties. The use of silane coupling agents at a... 

Other covalently bonded catalysts are those on inorganic supports such as silica, molecular sieves (zeolites), and alumina, the complexes being held either directly via the oxygen of surface hydroxy groups or via an intermediate functional group, again especially phosphino. 

Silica particles surface-imprinted with a TSA of a-chymotrypsin were applied for the enantio-selective hydrolyzation of amides. Surprisingly, the particles showed reverse enantio-selectivity, i. e., the sol-gel imprinted with the L-isomer of the enzyme s TSA showed a higher selectivity for the D-isomer of the substrate [125]. Also Ti02 gels have been imprinted, e.g., with 4-(4-propyloxypheny-lazo)benzoic acid. QCM coated with ultrathin films of this gel were prepared by an immersion process and showed selective binding of the template [ 126]. These examples demonstrate once more the broad applicability of the concept of molecular imprinting. 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

Further insight into molecular binding to the solid surface is possible from a determination of the surface area occupied by each molecule in a mono-layer. In a detailed study of the adsorption of a range of substances on silica gel (surface area 200 m2/g), Weis and coworkers9 found that a relatively small number of polynuclear aromatic substances occupied all available binding sites, indicating that the molecules are oriented parallel to the solid surface to... 

The contents of the current volume presents a sampling of more than 150 oral and poster papers delivered at the Symposium on Access in Nanoporous Materials II held in Banff, Alberta on May 25-28, 2000. The selected papers cover the three main themes of the symposium (i) synthesis of mesoporous silicas, framework-modified mesoporous silicas, and surface-modified mesoporous silicas, (ii) synthesis of other nanoporous and nanostructured materials, and (iii) characterization and applications of nanoporous materials. About 70% of the papers are devoted to the synthesis of siliceous mesoporous molecular sieves, their modification, characterization and applications, which represent the current research trend in nanoporous materials. The remaining contributions provide some indications on the future developments in the area of non-siliceous molecular sieves and related materials. Although the present book does not cover all topics in the area of nanoporous materials, it reflects the current trends and advances in this area, which will certainly attract the attention of materials chemists in the 21st Century. 

The CSC precursor build-up has been studied after modification of the silica gel surface from the gas phase. This gas phase modification involves the deposition of one molecular layer at the time. For thicker coatings, a cyclic procedure is needed. Liquid phase modification of the silica surface may also yield valuable ceramic precursors. The precursor molecular structure and layer thickness is controlled by other parameters compared to gas phase procedures. Parameters such as reaction solvent, silane concentrations and presence of water are of primal importance. Those have been discussed in detail in chapter 9. In this chapter, the application of silica modified with aminosilanes, will be discussed. The aminopropylsilica is used as a prototype compound for the production of ceramics by liquid phase chemical surface coating. 

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

Sahai, N. and Tossell, J.A., Molecular orbital study of apatite (Ca5(P04)30H) nucle-ation at silica bioceramic surfaces, J. Phys. Chem. B, 104, 4322, 2000. 

Many chlorine compoimds, including methji chlorosilanes, such as ClSi(CH2)2, Cl2Si(CH2)2, Cl2Si(CH2) tetrachlorosilane [10026-04-7] SiCl chlorine, CI2 and carbon tetrachloride, CCl can completely react with molecular surface hydroxji groups to form hydrochloric acid (40), which then desorbs from the gel body in a temperature range of 400—800 0, where the pores are stiU interconnected. Carbon tetrachloride can yield complete dehydration of ultrapure gel—silica optical components (3,23). 

The technical application of high-silica molecular sieves without any non-framework aluminium as protective layer restricts the water loading of the stream in sorption processes. The use of supersaturated steam for the regeneration of the adsorbent is impossible since already after a few adsorption/desorption cycles the molecular sieve is totally damaged. However, adsorbents which are modified by surface alumination are remarkably more resistant against water. 

The products made by the above synthetic processes still have large numbers of residual silanols, which lead to poor peak shapes or irreversible adsorption, because chemically bonded groups on the silica gel surface have large, bulky molecular sizes and, after the bonding, the functionalized silane cannot react with the silanols around the bonded ligands. Because such alkyl-bonded phases are used for reversed-phase separations, especially for chromatography of polar molecules, any silanol groups that remain accessible to sol-... 

From Table 2 it appears that on passing from carbon black and aerosil to carbosil the thickness of the solvation shell of benzene increases and the hydration film decreases. The studies of changes of chemical potential of water molecules at the adsorbent/bonded wa-ter/ice interface depending on water layer thickness are presented in another paper [57]. For the initial silica the surface effect is confined to the adsorbent water monolayer. Poor carbonization of aerosil surface causes the increase of water layer thickness to 40-50 molecular diameters. With the increase of carbon constituent part on the complex adsorbent surface, the thickness of interfacial water layer decreases to 15 molecular diameters. 

In Fig. 3 such relationships for n-octane and chloroform are presented. The relationships were determined at 40°C, namely when the monolayer can exist in a SC or LE form or be composed of SC islands surrounded by two-dimensional gas. The shape of Vs vs r relationship for n-octane may be interpreted as follows the first alcohol molecules deposited on silica gel surface block the most active centers on solid surface. This fact manifests as the retention volume decrease. However, from the surface concentration 1 nm up to 0-57 nm per n-octadecanol molecule the retention volume increases. The reason for this increase is probably the penetration of n-octane molecules into the rising structure of n-octadecanol chains. Let us assume that the n-octadecanol molecules are uniformly distributed on the silica gel surface. Therefore, at a surface concentration 0.57 nm per molecule the free spaces (0.38 nm in their cross-section) between them are formed, which are compatible with the n-octane molecule diameter. Then the maximum of n-octane retention volume is the result of a kind of molecular sieve effect. If the surface concentration of n-octadecanol increases and exceeds 0.57 nm per molecule, the... 

Because of its surface functionality, thermal stability, and at times porous intraparticle structure, silica gel presents an attractive surface for photophysical and photochemical study. This chapter deals with both surface functionality identification and with molecular mobility and reactivity occurring at the silica gel surface. 

The adsorption of Sn " atoms on zeolite and silica-gel surfaces has been studied [122]. The bonding appears to become stronger as the pore size of the material decreases towards molecular dimensions. The asymmetry of the tin(II) quadrupole splitting was held to indicate a Karyagin effect because of the anisotropy of surface atoms. 

The adsorptive selectivity of the gels may be influenced to a certain extent by the preparative conditions. One may, for instance, produce silica gels which adsorb a specific dye of characteristic molecular shape and charge distribution. This is done by dispersing this dye in the silicate solution and the precipitating the gel in the presence of the dye [2, 4, 9]. Similar experiments have been carried out with optically active compounds ([3], see also [1]). The silica gel surface may also be modified in a specific way by adsorbing on it appropriate substances this yields preparations with completely new adsorptive properties [14, 19]. 

The history of investigation of the molecular surface chemistry of silica can be divided into three time periods. The period between 1960 and 1970 saw the introduction of IR spectroscopy and the use of chemical reactivity to follow reactions on a silica surface at a molecular level. The period between 1970 and 1980 was less active, but the advances in computer instrumentation that occurred in that decade (in particular Fourier transform IR and NMR spectroscopy) have led to the current revitalization of the subject. The chapters that follow will question some old interpretations and will reinterpret some old ideas. 

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