Silica surface dissolution

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Most commonly observed pore-water concentration profiles, (a) A nonreactive substance, such as chloride (b) a chemical, such as O2, which undergoes removal in the surface sediment as a result of aerobic respiration (c) a chemical that is consumed by a reaction that occurs in a subsurface layer, such as Fe2+(aq) precipitating with S2-(aq) to form FeS2(s) (d) a chemical released in surface sediments, such as silica via dissolution of siliceous hard parts (e) a chemical released into pore waters from a subsurface layer, such as Mn +(aq) by the reduction of Mn02(s) and (f) a chemical released at one depth (reactive layer 1), such as Fe2+(aq) by reduction of FeOOFI(s), and removal at another depth (reactive layer 2), such as Fe +(aq) precipitating as FeS2(s). Source From Schulz,... 

Ionic Medium. Silica dispersions were freshly prepared for each experiment in solutions buffered with 10"3M HC03"/C02. The amount of species dissolved from the amorphous silica surface during the experiment was negligible because of the small rate of dissolution reactions. The ionic medium in which coagulation and adsorption studies were carried out was kept constant I = 1.0 to 2.0 X 10 3M. The conditions in all agglomeration and adsorption experiments were such that no Al(OH)3 precipitated within the period of observation. 

Pelmenschikov A., Leszczynski J., and Pettersson G. M. (2001) Mechanism of dissolution of neutral silica surfaces including effect of self-healing. J. Phys. Chem. A 105, 9528-9532. 

Brady, P. V. 1992. Silica surface chemistry at elevated temperatures. Geochim. Cosmochim. Acta 56 2941-46. Brady, P. V., and J. V. Walther. 1989. Controls on silicate dissolution rates in neutral and basic solutions at 25°C. Geochim. Cosrrwchim. Acta 53 2823-30. 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

It is not yet known with certainty if this occurs through surface dissolution of the silica support (eventually promoted by Ni I adsorption, cf. IV.B) followed by copredpitation of a mixed phase, or through an interface reaction between Ni complexes in the solution and the modified silica surface. The neoformed silicate layers are observed to be in narrow assodation with the silica partides, which would rather suggest an interface... 

The amount of Si ions dissolution is found to be dependent on surface modification, which was confirmed by induchvely coupled plasma-atomic emission spectrometer (ICP-AES) analysis. Table 2.2 shows the dissolution amount of Si ions with and without surface modification of fumed silica slurry. Without surface modification, the amount of Si dissoluhon was 1.370 0.002 mol/L, whereas surfaces modified with poly(vinylpyrrolidone) (PVP) polymer yielded a dissoluhon of 0.070 0.001 mol/L, almost 20 hmes less than the unmodified surface. Figure 2.6 represents the electro-kinetic behavior of silica characterized by electrosonic amplitude (ESA) with and without surface modification. When PVP polymer modified the silica surface, d5mamic mobility of silica particles showed a reduchon from -9 to -7 mobility units (10 m /Vxs). Dynamic mobility of silica particles lacking this passivation layer shows that silica suspensions exhibit negative surface potentials at pH values above 3.5, and reach a maximum potential at pH 9.0. However, beyond pH 9.0, the electrokinetic potential decreases with an increasing suspension pH. This effect is attributed to a compression of the electrical double layer due to the dissolution of Si ions, which resulted in an increase of ionic silicate species in solution and the presence of alkali ionic species. When the silica surface was modified by... 

Si-OH groups are also focal points of attack for water and other reagents in the mobile phase, causing dissolution of underlying silica with gradual deterioration in column performance and eventual reduction in column lifetime (5,6). The pH of the silica surface also varies from acidic, neutral, to basic, and is believed to influence the preparation and properties of bonded phases (7). The inclusion of traces of transition metals in silica matrix can further influence the retention of acids, bases or neutral compounds that can undergo complexation reactions (1). Thus the selectivity and retention characteristics of bonded phase columns also depend on the quality of silica used for the bonding reactions. 

Some columns require special care. Amino columns, for example, should not be exposed to solvents that contain aldehyde or keto groups. Amino columns also are only partially stable in an aqueous environment. When exposed to water, the high concentration of bond phase in the pores creates a strongly basic environment. Under these circumstances, the silica surface dissolves slowly, creating acid silanols. At some point, the acidic silanols neutralize the local basic environment, and the dissolution of the phase is slowed down or even halted. Since amino columns are often used in an aqueous environment as ion exchangers or for the analysis of carbohydrates, you should be aware of this effect. 

The surface of porous silica is covered by hydroxyl groups called surface silanols (Si-OH) [1,4]. Silanol groups are responsible for the polarity of the silica surface. They can ionize, the silanol pKa being 9.8. They are responsible for the silica dissolution in basic (pH>8) solutions. Naked silica packings are too polar to be used in RPLC. They are used in normal phase LC with apolar mobile phases. To obtain less polar packings for RPLC, it is necessary to derivatize the polar silanol groups. 

It can be noted that the brucitic layer of Ni(II) bonded to silica acts as nuclei for the growth of supported 1 1 nickel phyllosilicate or supported nickel hydroxide. The heterocondensation reaction is faster than the olation one, but it is limited by the concentration and diffusion in solution of silicic acid arising from silica dissolution, which itself depends on the silica surface area, i.e., on the extent of support-solution interface. 

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