Sol-gel colloids

Solid-liquid Wettability Sol, gel, colloidal suspension, solid emulsion, wetting, spreading, surface tension, friction, lubrication, diffusion, pervaporation, capillarity, electrochemistry, galvanic effects, corrosion, cleaning, filtration, ion electromigration, optical properties, charge transfer, nucleation and growth... 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

Monodisperse latex spheres of a controlled size can be arranged into three-dimensional arrays and are used as templates to prepare well-defined cavities and structures once the latex sphere template has been removed. These latex spheres are identical to one another in size and shape and are often prepared from colloidal clusters resulting from the aggregation of sol-gel colloids (Section 5.6). The spherical polymer clusters can be fabricated by the slow addition of an aqueous solution into a reservoir of hydrophobic silicone liquid, forming emulsion droplets. This produces a highly structured porous matrix with a well-defined structure upon polymerisation. The size of the droplets is controlled by the concentration of the aqueous latex, the speed at which the suspension is stirred and the ratio between the silicone liquid and latex. As the concentration of the latex spheres increases to its critical concentration, i.e. the concentration at which the colloidal spheres start to order themselves into a close-packed structure, the balls are filtered off and are dried, ready to be used as templates. 

Chemical Processes (Sol-Gel, Colloidal) Simple low processing temperature versatile high chemical homogeneity rigorous stoichiometry control high purity products. Weak bonding, low wear-resistance, high permeabdity md difficult control of porosity. 

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

Sol-gel is one of the most useful techniques for preparation of inorganic membranes with fine pores in the nanometer range (1-5 nm). The sol is a stable suspension of colloidal solid particles within soft uniform solution. The gel was obtained by hydrolysis with open reflux in 24 hours at 85-90 °C. The advantage of sol-gel technology is the ability to produce... 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Djabourov M., Grillon Y., Leblond J. The sol-gel Transition in gelatin viewed by Diffusing colloidal probes. Polymer Gels and Networks 3 (1995) 407-428. 

We prepared ceria on Ni substrate by sol-gel coating method. Ceria sol solution was prepared with ceria sol solution (Alfa, 20% in H2O, colloidal dispersion) mixed with ethanol (99.9%, Hayman) with weight ratio (1 2) and stirred. Ceria was deposited on Ni substrate by dip coating method. The variation number of dipping was carried out to obtain different coating ratio. The anode was completely dipped into the ceria sol solution for several seconds and dried at a temperature of 50 C for 24 hours in air atmosphere followed by calcination at 700 C for 30 minutes in 5%H2-N2 atmosphere. 

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au particles. The Mossbauer data have yielded evidence that alloying Pd with Au in bimetallic colloids leads to enhanced catalytic hydrogenation and also to improved selectivity [426]. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Figure 6 A flow diagram showing the reaction steps involved in the sol-gel preparation of Ti02 colloids...

The first belief in the possibility of enzyme stabilization on a silica matrix was stated by Dickey in 1955, but he did not give experimental evidence, only mentioning that his experiments were unsuccessful [65]. A sol-gel procedure for enzyme immobilization in silica was first developed by Johnson and Whateley in 1971 [66]. The entrapped trypsin retained about 34 % of its tryptic activity observed in solution before the encapsulation. Furthermore, the enzyme was not released from the silica matrix by washing, demonstrating the increased stability and working pH range. Unfortunately, the article did not attract attention, although their method contained all the details that may be found in the present-day common approach. This was probably due to its publication in a colloid journal that was not read by biochemists. 

Applications of sol-gel-processed interphase catalysts. Chemical Reviews, 102, 3543-3578. Pierre, A.C. (2004) The sol-gel encapsulation of enzymes. Biocatalysis and Biotransformation, 22, 145-170. Shchipunov, Yu.A. (2003) Sol-gel derived biomaterials of silica and carrageenans. Journal of Colloid and Interface Science, 268, 68-76. Shchipunov Yu.A. and Karpenko T.Yu. (2004) Hybrid polysaccharide-silica nanocomposites prepared by the sol-gel technique. Langmuir, 20, 3882-3887. 

Shchipunov, Yu.A., Kojima, A. and Imae, T. (2005) Polysaccharides as a template for silicate generated by sol-gel processes. Journal of Colloid and Interface Science, 285, 374-380. 

F. J.V.E. and Airoldi, C.A. (2006) Some features associated with organosilane groups grafted by the sol-gel process onto synthetic talc-like phyllosilicate. Journal of Colloid and Interface Science, 297, 95-103. 

A colloidal suspension of conductive vanadium pentoxide [130] can be used to perform intercalation, adsorption or encapsulation of electroactive molecules or biomolecules for electrodes or biosensor realization [131]. Encapsulation of glucose oxidase in nanocomposite films made with polyvinyl alcohol and V205 sol-gel matrix or in ferrocene intercalated V2Os sol-gel [132] were envisaged to prepare glucose biosensors. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

Sol-gel process starts with a solution or a sol that becomes a gel. The solution can be prepared from either inorganic salts or organic components which than are hydrolyzed and condensed to make a sol or a gel. One can stop at the sol stage, which refers to a dispersion of particles of colloidal dimensions in a liquid, or proceed to the gel state which refers to a three-dimensionally-linked solid network with liquid filling the pores. These pores are interconnected in the wet gel state. 

Sakka S., Kozuka H., Sol-gel preparation of coating fdms containing noble metal colloids, J. Sol-Gel Sci. Technol. 1998 13 701-705. 

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