Sols, discussion

Reerink H and Overbeek J Th G 1954 The rate of coagulation as a measure of the stability of silver iodide sols Discuss. Faraday Soc. 18 74-84... 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

Membranes produced following a template approach are reported by Raman and Brinker [42,51] using precursor sols discussed in the preceding section and produced by copolymerisation of methyltriethoxysilane (MTES) and TEOS using a two-step acid catalysed process. 

The attractive van der Waals energy of interaction (Va) for spheres in the 10- to l(X3-nm size range for silica sols discussed here varies as the inverse of the separation distance, and at any separation Va is directly proportional to particle size. The Hamaker constant (A), which controls the magnitude of the variation of van der Waals attraction with particle radius (a) and separation (Hq) between surfaces, is for silica-water-silica not a large number. Further, the known hydration-polysilicic acid formation at silica-water interfaces will further reduce the overall Hamaker constant in the silica sol-water-silica sol system. 

The sols discussed in this article are based on GPTMSGBPA. One mole of GPTMS was prehydrolysed with 1.5 mol water (0.01 M hydrochloric acid) and stirred for at least 2 h... 

In practice, e.g., in nature or in fonnulated products, colloidal suspensions (also denoted sols or dispersions) tend to be complex systems, consisting of many components that are often not very well defined, in tenns of particle size for instance. Much progress has been made in the understanding of colloidal suspensions by studying well defined model systems, which allow for a quantitative modelling of their behaviour. Such systems will be discussed here. 

The definition above is a particularly restrictive description of a nanocrystal, and necessarily limits die focus of diis brief review to studies of nanocrystals which are of relevance to chemical physics. Many nanoparticles, particularly oxides, prepared dirough die sol-gel niediod are not included in diis discussion as dieir internal stmcture is amorjihous and hydrated. Neverdieless, diey are important nanoniaterials several textbooks deal widi dieir syndiesis and properties [4, 5]. The material science community has also contributed to die general area of nanocrystals however, for most of dieir applications it is not necessary to prepare fully isolated nanocrystals widi well defined surface chemistry. A good discussion of die goals and progress can be found in references [6, 7, 8 and 9]. Finally, diere is a rich history in gas-phase chemical physics of die study of clusters and size-dependent evaluations of dieir behaviour. This topic is not addressed here, but covered instead in chapter C1.1, Clusters and nanoscale stmctures, in diis same volume. 

The need to identify replacements for chromate-based technologies has also resulted in the development of alternatives to resin-based corrosion-inhibiting primers. These are primarily inorganic barrier coating techniques, such as sol-gel processes, which may or may not include corrosion inhibitors. These are discussed in a later section. 

Sol-gel primers use inorganic or metal-organic precursors (generally aluminum, silicon or titanium alkoxides) whose chemistry is closely related to the silane coupling agents discussed previously. These precursors are dissolved in alcohol, then hydrolyzed by the addition of water ... 

In Section 4.3 several recently developed and experimental surface preparation methods for steel such as conversion coatings, plasma spray and sol-gel that attempt to simultaneously improve durability and bond strength over grit blasting will be discussed. 

Sol-gel. A limited evaluation of sol-gel coatings on stainless steel has been reported with the goal of reducing hazardous wastes and other environmentally undesirable materials [22]. In this study, the GTMS/TPOZ sol-gel discussed... 

Barium Oxide (Barium Monoxide, Baryta, Barium Protoxide, Calcined Baryta). BaO, mw 153.36 colorl or white to yelsh-white deliq powd mp 1923°, bp 2000°, d 5.72g/cc. Sol in dil acids, ethanol, methanol and w (decomps with a large amt of heat being evolved). Prepn is by thermal decompn of BaCOj under high vacuum. It is a product of the burning of the many barium compds present in pyrots and, as discussed above, it is used as an anti-barrel wear additive in proplnt compns... 

The pre-gel model calculates the weight average molecular weight of the reaction mixture, while the post-gel model calculates the weight of the sol fraction and the effective crosslink density. A simple computer program using the derived expressions has been written in BASIC and runs on IBM-PC compatible computers. The importance of secondary reactions on cure in typical coatings is discussed. 

This paper concerns the main properties of water soluble pectins in sol and gel states. First of all, the methods of purification and characterization are discussed. The method of steric exclusion chromatography equipped with different detectors is demonstrated as the most useful to determine the macromolecular characteristics of these polymers the role of aggregation is pointed out. 

Then, the ionic selectivity is discussed and related to the mechanism of crosslinking with divalent counterions. The sol-gel transition is then examined for LM and HM pectins and the mechanisms described in these two cases. The physical properties of the gels are related to the microstructure of the polymers and few data are examined. 

It is important to note, however, that beyond a = 1/2 by no means all of the material will be combined into infinite molecules. For example, in spite of the favorable probability of branching, a chain selected at random may be terminated at both ends by unreacted functional groups. Or it may possess a branch at only one end, and both of the succeeding two chains may lead to unreacted dead ends. These and other finite species will coexist with infinite networks as long as l/2[Pg.353]

Monitoring the pH value during the preparation of gold sol, which leads to the below reported results, it has been observed that pH moves from ca. 3.2, before NaBH4 addition, to ca. 6.9, after NaBH4 addition. In this section a discussion of the influence of the initial pH value on the properties of the colloidal dispersion stabilized by a large amount (PVA/Au = 0.67) or a low amount (PVA/ Au = 0.05) of stabilizer is presented. Proper amounts of HCl or NaOH were used to produce the reported pH values. 

The formation of polymeric silica or a gel from this process is clearly complicated (9) and will not be discussed further in this article for reviews of the sol-gel process, see Ref. 10. The important point for this general discussion is that silanol species play a vital role in the formation of numerous industrially important materials. 

The sol-gel process to prepare SIO2 glass fibers and T102 films has been reviewed. It has been known that the hydrolysis conditions such as molar ratio of water to alkoxide and reaction temperature are critical to the desired forms of the gel product (fiber, film or bulk). Some properties of the resultant products have been examined. Especially, Ti02 films have been attempted to use as a photoanode for decomposition of water, and their photoelectrochemical properties are described in comparison with the results previously obtained for single crystal and polycrystalline TiC>2, and are discussed in terms of the microstructure of the film. 

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