Aggregates in the Absence of Organics

The effect of different aggregation regimes (rapid versus slow) was discussed in detail in section 6.5. Here, the effect of aggregation under conditions much closer to surface waters (neutral pH and background solution) will be investigated. The experiments were carried out with the smallest colloids (40 nm, 75 nm). At pH 7-8 aggregation is expected in all cases. 

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Figure 5.4 Flux ratio (flux after collection of 800 mL permeate over pure water flux) as a function of primary colloid sit e for stable colloids (pH3), aggregates in the absence of organics (pH8), stabilised colloids (OPS), and aggregates (SPO).

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. 

The experiments were carried out at pH 3 with the two smallest colloids (see Appendix 3 for preparation and characterisation). This extreme pH was required in order to obtain stable (non-aggregating) primary colloids, rather than their aggregates in the absence of organics (see Chapter 4 for characterisation of colloidal systems). Results are shown in Figure 7.42. At pH 3 the colloids have a high positive charge. This should lead to a significant repulsion between the colloids, increased backdiffusion, and therefore lower concentration polarisation of colloids (Bhattacharjee et al. (1999)). 

In MF, where sieving is believed to dominate rejection, charge effects were observed. However, it is the coUoid stability that determines rejection for colloids which were smaller than membrane pore size. Aggregates that form in the absence of organics are retained, but exhibit reduced rejection at high fluxes, pressures and after a backwash. 

Aggregation of blood platelets is the requisite first event for the maintenance of intact circulation in the face of any break in a blood vessel. It is the platelet clump that starts the long and complicated process leading to closure of the broken vessel by an organized blood clot. Though this property of platelets is vital to maintenance of the circulatory system, an excessive tendency to aggregation can also lead to problems. Thus platelet clumps formed in blood vessels in the absence of injury can lead to blockade of blood circulation and subsequent injury. Strokes and some types of myocardial infarcts have thus been associated with platelet clumps. The nonsteroid antiinflammatory... 

The history of organic radical ions is intertwined with the history of quinhy-drones , molecular aggregates between substrates that are readily oxidized and compounds that are readily reduced. In the absence of modem analytical methods, particularly magnetic resonance techniques, it was often difficult to ascertain whether one was dealing with a homogeneous radical ion salt, such as Wurster s Blue, or with a quinhydrone, such as the prototypical complex formed between benzoquinone and benzohydroquinone. Indeed, in several cases radical ions were mistaken for molecular complexes [54,55]. Furthermore, there are instances where a free radical ion and a molecular complex have a similar appearance, at least subjectively, so that it is not clear which of the two species was observed originally. 

Benzylquininium chloride has been studied as a catalyst for the asymmetric Michael reaction. Reaction of amidoma-lonate (5) and chalcone (4) with catalytic base and a variety of chiral, nonracemic ammonium salts in the absence of solvent produced (6) in yields of 41-68% and 20-68% ee (eq 2). The quinine-derived salt (1) was of intermediate effectiveness (38% ee, 47% yield) when compared to ephedrine-based catalysts. Although (1) was not specifically tested with regard to solvation effects, it is suggested that increased aggregation of reactive species under solid-liquid PTC conditions leads to enhanced organization and selec-... 

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