Well-defined colloidal systems

After the investigation of filtration behaviour of well defined colloidal systems and organics, colloids are now examined in solutions which are closer to surface water conditions. In a preliminary section the effect of colloid size on flux will be examined. This includes the packing of stable primary colloids. A further section examined the filtration of aggregates in the absence of organics. The aggregates are now formed under conditions closer to surface waters, at a pH 7-8 and in the presence of calcium, b inally, the colloidal systems OPS and SPO as described in Chapter 4 are considered. 

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. 

Even when well defined model systems are used, colloids are ratlier complex, when compared witli pure molecular compounds, for instance. As a result, one often has to resort to a wide range of characterization teclmiques to obtain a sufficiently comprehensive description of a sample being studied. This section lists some of tire most common teclmiques used for studying colloidal suspensions. Some of tliese teclmiques are discussed in detail elsewhere in tliis volume and will only be mentioned in passing. A few teclmiques tliat are relevant more specifically for colloids are introduced very briefly here, and a few advanced teclmiques are highlighted. 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

SO sharply defined that they are called surfaces. Well-defined surfaces occur between solids and either gases or liquids and thus are commonly found in catalytic and electrode reactions. More diffuse interfaces may occur between solids, as in microelectronic devices, and between fluids or semifluids, as in many polymeric and colloidal systems. 

In contradistinction to the spectra obtained in the mica dispersion, Pseudocyanine, when added to the colloidal silver and silver halide systems of Figures IB and 1C, yielded no H-bands but formed well defined J-bands. The latter also exhibited a weak secondary peak located near the absorption maximum of dissolved, unperturbed dye. Although the position and intensity of the J-band varied with the substrate (4, 17, 38, 61), the differential spectra obtained in these silver systems exhibited marked similarities. An increase in the concentration of the substrate produced in both cases a monotonic change in the absorbance of the M-... 

Until the last few decades colloid science stood more or less on its own as an almost entirely descriptive subject which did not appear to fit within the general framework of physics and chemistry. The use of materials of doubtful composition, which put considerable strain on the questions of reproducibility and interpretation, was partly responsible for this state of affairs. Nowadays, the tendency is to work whenever possible with well-defined systems (e.g. monodispersed dispersions, pure surface-active agents, well-defined polymeric material) which act as models, both in their own right and for real life systems under consideration. Despite the large number of variables which are often involved, research of this nature coupled with advances in the understanding of the fundamental principles of physics and chemistry has made it possible to formulate coherent, if not always comprehensive, theories relating to many of the aspects of colloidal behaviour. Since it is important that colloid science be understood at both descriptive and theoretical levels, the study of this subject can range widely from relatively simple descriptive material to extremely complex theory. 

Development of new in situ analytical methods for species determination - There is a disparity between the amount of speciation information obtained from laboratory studies on well-defined systems, and that which is applicable to the real environmental situation (Buffle, 1990). Development of in situ methods will facilitate more relevant measurements which minimise perturbation of the system. Systems particularly prone to measurement-induced artefacts include anoxic waters, the sediment-water interface and colloidal materials. Methods for accurate measurement of free metal ion concentration over a much broader concentration range than that currently available are particularly required. 

This is a method to obtain colloidal drug delivery systems from preformed, well-defined macromolecular materials with known physicochemical and biological properties. Biodegradable nanoparticles from PLA, PLG, PLGA, and poly(E-caprolactone) have been prepared by dispersing the polymers (Vauthier et al. 1991 Couvreur et al. 1995). 

Sedimentation FFF, applied in the above manner, yields highly detailed size distribution curves. It is convenient and accurate. Importantly, sedimentation FFF is a highly flexible technique. It can be adapted to nearly all particle types in virtually any suspending medium. It yields particle density as well as size and size distribution. Our recent work has shown that it can be used to probe both size and density distributions in complex colloids, defined as systems having colloidal particles of variable chemical composition. Complex colloids are important in many biological and environmental studies. 

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