Solid-liquid interface silica

Electroosmosis or electroendosmosis is the bulk movement of the solvent (electrolyte solution) in the capillary caused by the zeta (0 potential at the wall/water interface of the capillary. Any solid-liquid interface is surrounded by solvent and solute constituents that are oriented differently compared to the bulk solution. Figure 17.2 illustrates a model of the wall-solution interface of the widely applied capillaries. Owing to the nature of the surface functional groups, in silica capillaries the silanol groups, the solid surface has an excess of negative... 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

Thermal uniformity in the cold zone was found to be from 0.01 to 0.02 °C, and that in the hot zone was found to be better than + 0.5 °C vertically and + 0.1 °C horizontally. Thermal gradients near the solid-liquid interface were achieved in excess of 30 °C cm " in the crystal region and up to 20 °C cm" in the melt. The growth of crystals was performed in a sealed transparent silica ampoule, which has two rooms for As source and GaAs polycrystalline, respectively, separated by a quartz diffusion barrier. For details of the growth process the reader is referred to Ref. 43. In this experiment the As source temperature T. was systematically reduced by 2 °C at 3 h intervals from 620 °C to 614 °C. 

TIRF at solid/liquid interfaces was introduced by Hirschfeld l49). Although this first use of TIRF was the study of bulk dissolved fluorescein in the vicinity of a fused silica-electrolyte interface, a number of advantages over the conventional transmission technique were demonstrated ... 

At either the solid/gas or solid/liquid interface the chlorosilane does not adsorb on a completely dehydrated silica and is hydrolyzed to the silanol with the surface water of a hydrated silica. 

C.P. Tripp and M.L. Hair, Direct observation of surface bonds between self-assembled monolayers of octadecyltrichlorosilane and silica surfaces a low frequency IR study at the solid/liquid interface, Langrawir, 11,1215-1219 (1995). 

In mineral-reagent systems, surface precipitation has been proposed as another mechanism for chemisorption. The solubility product for precipitation and the activities of the species at the solid-liquid interface determine the surface precipitation process. Under appropriate electrochemical conditions, the activity of certain species can be higher in the interfacial region than that in the bulk solution and such a redistribution can lead to many reactions. For example, the sharp increase in adsorption of the calcium species on silica around pH 11 has been shown to be due to surface precipitation (Somasundaran and Anan-thapadmanabhan, 1985 Xiao, 1990). Similar correlations have been obtained for cobalt-silica, alumina-dodecylsulfonate, calcite/apatite/dolomite-fatty acid, francolite-oleate and tenorite-salicylaldoxime systems. The chemical state of the surfactant in the solution can also affect adsorption (Somasundaran and Ananthapadmanabhan, 1985). 

In this section we present a brief account of the most recently published smdies and techniques concerning the solid-liquid interface involving silica and related materials. Interactions between solutions and solid surfaces play an important role in several processes such as ore flotation, colloidal stabilization, oil recovery, soil pollution and so on [105]. 

Another example of the usefulness of microcalorimetry to study the solid-liquid interface has been reported by Draoui et al. [118]. They analyzed the adsorption of Paraquat on different minerals, which are part of the soil, to study the retention process of this pesticide by soils. In the particular case of silica they concluded that the main driving force for adsorption is electrostatic. They found a linear decrease of the adsorption enthalpy in the range of -25 to -20 kJ/mol. Nevertheless, the Paraquat molecule has two charges the heat evolved during adsorption is of the same order as in the case of other single-charged molecules. This fact is explained assuming that the... 

The third paper in this subject that we were able to retrieve is due to Biswas et al. [145]. In their introduction to the paper they said that dynamic and mechanistic aspects of adsorption of surfactants at the solid-liquid interface, particularly silica surface, were rare and quoted six papers. The most recent among them was due to Tiberg [146] in 1996. Adsorption kinetics was studied by Biswas et al. [145] using classical batch experiments. They found that the adsorption follows a two-step first-order rate equation. From the calculated rate constants they obtained the activation energies and entropies concluding that both processes are entropy controlled. 

Under certain circumstances it is possible to obtain adsorption isotherms that correspond to a layer-by-layer mechanisms. This is a very well known fact in gas adsorption on homogeneous flat surfaces [151] and is known as layering. At the solid-liquid interface stepwise isotherms are also observed, nevertheless one step and sometimes two are found [114]. Sellami et al. [152] studied the adsorption of 2,5-dimethylpyridine (DMP) on silica from aqueous solutions. The experimental results [153] clearly show the stepwise character of the isotherms, which could either mean multilayer adsorption or layering. In their paper [152] they addressed the question of using the adsorption isotherm to discriminate between... 

Presented below is an example of structure development at the solid-liquid interface for the silica/dodecyltrimethylammonium bromide (C12TAB) system, at pH 4.0 and O.IM NaCl as the background electrolyte, obtained by Singh et aL (11). A combination of different techniques such as adsorption, zeta potential, contact angle and FT-IR attenuated total reflection (ATR) measurements, have been used in this study to clearly illustrate the structural transitions, and the structure of self-assembled surfactant films at different adsorbed amounts of the surfactants at the interface. 

Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH>4 monomers and NP+ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolyinerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 

The dispersed systems are mostly silicates. The author discusses interparticle interactions as a tool for evaluating the stability of dispersions. Parameters such as heat of immersion at solid-liquid interfaces and adsorption capacity are determined, and the mathematical treatment for determining the enthalpy isotherms is described. The heat of wetting in amorphous silica dispersion and on zeolites is discussed. 

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