Bromide, silica surface

Cetylpyridinium bromide, physical properties of, 4 350t Cetylpyridinium chloride, function as ingredient in cosmetics, 7 829t Cetyltrimethylammonium bromide (CTAB) surface area, of silica, 22 371, 386. 

Fused silica capillary tubes were variously coated for capillary electrophoresis. The chemical process involving a Grignard reaction is shown in equation 30. The silanol groups on the silica surface are treated with alkali, dried, convert to chlorosUanes with thionyl chloride and vinylmagnesium bromide replaces the chlorine atoms with vinyl... 

Ye et al. found a parallel between CE and CEC, in dynamically modified capillary separations [52], A column packed with bare silica was dynamically modified with cetyltrimethylaammoinium bromide (CTAB), a long-chain quaternary ammonium salt that was adsorbed onto the silica surface and produced a hydrophobic layer. The same additive, but with a completely different effect, was used Wu et al. [53]. They employed a methacrylate-based monolithic column and added in the mobile phase CTAB or sodium dodecyl sulfate (SDS) to generated EOF. Ten analytes, ranging from acidic to basic, were separated in both cases. 

For most electrophoretic separations of small ions, the smallest analysis time results when the analyte ions move in the same direction as the electroosmotic flow. Thus, for cation separations, the walls of the capillary are untreated, and the electroosmotic flow and the cation movement are toward the cathode. For the separation of anions, however, the electroosmotic flow is usually reversed by treating the walls of the capillary with an alkyl ammonium salt, such as cetyl trimethylammonium bromide. The positively charged ammonium ions become attached to the negatively charged silica surface and in turn create a negatively charged double layer of solution, which is attracted toward the anode, reversing the electroosmotic flow. 

The attachment of olefin groups through Si-C bonds can be also carried out with the aid of the reaction of the previously chlorinated silica surface with allylmagnesium bromide. For the first time the approach of this kind was employed by Neimark, Chuiko, and Slinyakova [7] ... 

Halides. Another treatment which can lower the hydroxyl population, or even eliminate it altogether, is halogenation of the silica surface.This removes hydroxyls, not by condensation as with CO and sulfur, but by replacement with halide, which prevents later attachment by Cr. The presence of halide probably also changes the electronic environment on the silica. Thus, fluoriding has long been used to increase activity but decrease RMip.i l Chloride also depresses RMIP, as well as the surface bromide and iodide of silica. However, these latter two have recently been studied, and it was possible to burn off most of the iodide or bromide with oxygen above 600 C, leaving a partially dehydroxylated surface. 

Powder DR data were collected with a Harrick DRA-2CI attachment and a HVC-DRP heatable cell, evacuable below 0.1 Pa. To collect spectra below 4000 cm-1, the silica powder was generally mixed with potassium bromide, without any pressure, so that there was no close interaction between KBr and the silica surface. 

Metal oxides. Beside mesoporous silica materials conventional metal oxides have been applied for the synthesis of solid Pd catalysts for C—C coupling reactions. Alper and coworkers [151] and Singh and coworkers [144] immobilized Pd complexes on a modified silica surface modified by covalent anchoring. The resulting solid catalyst was tested in reactions of several non- and deactivated aryl bromides (bromobenzene, p-bromotoluene, p-bromoanisole) and butyl acrylate or styrene. Only TONs around 100 were achieved after 1 day reaction. 

Early examples of the precipitation approach include the aqueous solution polymerizations reported by Chaimberg et al. [53] for the graft polymerization of polyvinylpyrrolidone onto silica. The nonporous silica particles were modified with vinyltriethoxysilane in xylene, isolated and dispersed in an aqueous solution of vinylpyrrolidone. The reaction was performed at 70°C and initiated by hydrogen peroxide, after which precipitation on the surface occurred, leading to encapsulation. Nagai et al. [54] in 1989 reported on the aqueous polymerization of the quaternary salt of dimethylaminoethyl methacrylate with lauryl bromide, a surface-active monomer, on silica gel. Although the aim was to polymerize only on the surface, separate latex particles were also formed. 

As one example, the force between a hydrophilic silica particle and an air bubble at different concentrations of dode-cyltrimethylammonium bromide (DTAB) is shown in Fig. 10. Without surfactant, the particle is repelled by the air bubble. At distances above 5 nm, the electrostatic repulsion dominates. The reason being the negative surface charges on the silica surface and at the water-air interface [187-190]. Even at close distance, a stable water remains on the particle surface and no three-phase contact is formed. Adding even small amounts of the cationic surfactant DTAB changes the interaction drastically. At concentrations between 0.1 mM and typically 5 mM DTAB (critical micellar concentration is >= 16 mM), no repulsion was observed. When the particle comes into contact with the air-water interface, it jumps into the bubble and a three-phase contact is formed. Such a behavior can be explained with the strong adsorption of long-chain alkyltrimethy-lammonium ions to silica [191]. At a concentration of 0.1 mM, DTA+ forms a monolayer on the silica surface. This... 

Experimentally the most accessible hydrophilic surface is the silica surface, although it is by no means simple. Except at very low concentrations, where there may be coulombic interactions between the charged surfactant and charged surface, adsorption of surfactants is generally expected to be dominated by the hydrophobic effect, which will cause the surfactant to adsorb in a bilayer or related structure. In principle, neutron reflection can identify a bilayer structure either from its overall thickness or from the surface coverage. In practice, for the two surfactants that have so far been most systematically studied by neutron reflection, the non-ionic surfactant Ci2Eg [17, 18] and the cationic surfactant hexadecyl trimethyl ammonium bromide. 

More recently, an interesting technique has been used by Clark and Ducker [70] to measure kinetics, in the form of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. They found that total surface exchange of a cationic ammonium bromide surfactant on a silica surface occurred in slightly less than 10 s (fig. 19.6). This technique had been used previously by Couzis and Gulari [71, 72] to look at the adsorption kinetics of anionic surfactants at the alumina-water interface with apparent timescales in the region of tens of hours. 

More recent studies of amine adsorption mechanisms at surfaces have been carried out using atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and SFVS techniques (Bakshi et al. 2004 Castro et al. 1986 C hemyshova et al. 2000 Fuerstenau and Renhe 2004 Paruchuri et al. 2004 Schrodle and Richmond 2008 Subramanian and Ducker 2000 Velegol et al. 2000). AFM studies show that spherical micelles of cetyltrimethylammonium bromide/cetyltrimethylammonium chloride (CTAB/CTAC) surfactants form at silica surfaces near the critical micelle concentration (CMC) (Bakshi et al. 2004). Compared with tertiary and quaternary amines, primary amines, such as DDA, would have a different molecular structure at the silica surface. For example, AFM studies suggest that primary amine forms a featureless bilayer at mica surfaces. 

M. Chorro, C. Chorro, O. DoUadiUe, S. Partyka, R. Zana, Adsorption mechanism of conventional and dimeric cationic surfactants on silica surface effect of the state of the surface. J. Colloid Interface Sci. 210(1), 134-143 (1999). doi 10.1006/jcis.l998.5936 R. Chaghi, L.-C. de Menorval, C. Chamay, G. Derrien, J. Zajac, Interactions of phenol with cationic micelles of hexadecyltrimethylammonium bromide studied by titration calorimetry, conductiraetry, and IH NMR in the range of low additive and surfactant concentrations. J. Colloid Interface Sci. 326(1), 227-234 (2008). doi 10.1016/j.jcis.2008.07.035... 

Table 17.2 presents results of physical-chemical testing of the silica powders. The A1 sample has the lowest Cetultrimethyammonium bromid (CTAB) and Brunaues, Emmett, and Teller (BET) surface area, higher structure (DBF) and more silanol (-OH) groups on surface per unit area. 

Figure 4.16 A model for the surface of silica gel in equilibrium with a mobile phase of (55 40 5 methanol-water-0.2 M potassium phosphate buffer (pH 7.5) with the addition ot 2.5 aM of cetyltrimethylammonium bromide. (Reproduced with permission from ref. 279. Copyright pergamon Journals Ltd).

Surface-mediated addition of HC1 or HBr can be carried out in the presence of silica or alumina.150 The hydrogen halides can be generated from thionyl chloride, oxalyl chloride, oxalyl bromide, phosphorus tribromide, or acetyl bromide. The kinetic products from HC1 and 1-phenylpropyne result from syn addition, but isomerization to the more stable Z-isomer occurs upon continued exposure to the acidic conditions. 

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