Alexander, Will

We will focus on one experimental study here. Monovoukas and Cast studied polystyrene particles witli a = 61 nm in potassium chloride solutions [86]. They obtained a very good agreement between tlieir observations and tire predicted Yukawa phase diagram (see figure C2.6.9). In order to make tire comparison tliey rescaled the particle charges according to Alexander et al [43] (see also [82]). At high electrolyte concentrations, tire particle interactions tend to hard-sphere behaviour (see section C2.6.4) and tire phase transition shifts to volume fractions around 0.5 [88]. 

The final chapter in this volume by Alexander Sadimenko (University of Fort Hare, South Afiica) continues a series by this author on the organometaUic chemistry of heterocycles, of which 0,S monoheterocycles and N,P,Si,B monoheterocycles were published in volumes 78 and 79, respectively. The organometaUic chemistry of pyrazole is so broad that the present overview does not include the polyfunctional, chelating frameworks containing pyrazolyl units, which are typified by the pyrazolyl borate derivatives. These will be the subject of a future chapter. 

The importance of polydispersity is an interesting clue that it may be possible to tailor the weak interactions between polymer brushes by controlled polydispersity, that is, designed mixtures of molecular weight. A mixture of two chain lengths in a flat tethered layer can be analyzed via the Alexander model since the extra chain length in the longer chains, like free chains, will not penetrate the denser, shorter brush. This is one aspect of the vertical segregation phenomenon discussed in the next section. 

Bioavailability issues have been reviewed previously (Mihelcic etal. 1993 Boesten 1993 Baveye and Bladon 1999 Ehlers and Luthy 2003). In this review, we discuss specifically the bioavailability of soil- or sediment-sorbed organic contaminants to pollutant-degrading bacteria. Direct uptake of sorbed contaminants is perhaps the most controversial and least understood process. The definition of bioavailability given by Alexander (2000) will be used in this review. The term bioaccessibility encompasses what is immediately available plus that which may become available, whereas bioavailability refers to what is available immediately. 

The above described weapons were also used by Macedonians under Alexander the Great (b356, d323 BC) during their conquest of Europe and half of Asia. No new weapons were used, but they encountered in India a new weapon which will be described below... 

This quotation from Alexander Pope s Essay on Man may well be applied to error. In a way, recognition of error in experimentation will lead to its acceptance and eventually to our almost joyful preoccupation with it as a partner in achieving our goals. To avoid misuse of error, for example, ignoring it when we wish to draw a preconceived conclusion or relying on it when we think it can be employed to avoid drawing undesirable conclusions, we first need to understand bow it behaves. 

The author wishes to thank his many collaborators and colleagues in this field, almost too numerous to mention. However, special thanks to the collaborators and colleagues at both IBM and Denmark and to Mats Persson, Geert-Jan Kroes, and Bret Jackson who have read parts of this review and offered comments. Nevertheless, the opinions presented here are the sole responsibility of the author. Discussions and arguments with many of the participants in this field, both in print and over many a bottle of wine/beer, and in many parts of the world, over the past 25 years have led to the opinions presented in this review. It has been a fun journey of discovery. Of course since this is still an active research field, some of the concepts/opinions are not yet etched in stone, but hopefully time will prove those presented here to be more fact than fiction. The author also wishes to thank the Alexander von Humboldt foundation for support of visits to Berlin where this chapter was initiated and Bonnie for her patience while this chapter was written. 

Now we consider situations in which transformation of the organic compound of interest does not cause growth of the microbial population. This may apply in many engineered laboratory and field situations (e.g., Semprini, 1997 Kim and Hao, 1999 Rittmann and McCarty, 2001). The rate of chemical removal in such cases may be controlled by the speed with which an enzyme catalyzes the chemical s structural change (e.g., steps 2, 3 and 4 in Fig. 17.1). This situation has been referred to as co-metabolism, when the relevant enzyme, intended to catalyze transformations of natural substances, also catalyzes the degradation of xenobiotic compounds due to its imperfect substrate specificity (Horvath, 1972 Alexander, 1981). Although the term, co-metabolism, may be used too broadly (Wackett, 1996), in this section we only consider instances in which enzyme-compound interactions limit the overall substrate s removal. Since enzyme-mediated kinetics were characterized long ago by Michaelis and Menten (Nelson and Cox, 2000), we will refer to such situations as Michaelis-Menten cases. 

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