Globule diffusion

The bulk of the cement is extremely porous as the fractured surface of a specimen shows (Figure 6.3c). The pores are 0-5 pm in diameter and more abundant in the depth of the cement. The porosity arises from excess unbound water which separates out as globules in the cement and is trapped by the rapid setting. Subsequent diffusion of these globules leaves the cement porous. This makes the cement permeable to dyes (Wisth, 1972). 

Food products can generally be considered as a mixture of many components. For example, milk, cream and cheeses are primarily a mixture of water, fat globules and macromolecules. The concentrations of the components are important parameters in the food industry for the control of production processes, quality assurance and the development of new products. NMR has been used extensively to quantify the amount of each component, and also their states [59, 60]. For example, lipid crystallization has been studied in model systems and in actual food systems [61, 62]. Callaghan et al. [63] have shown that the fat in Cheddar cheese was diffusion-restricted and was most probably associated with small droplets. Many pioneering applications of NMR and MRI in food science and processing have been reviewed in Refs. [19, 20, 59]. 

In early 2004, Hurlimann studied several cheese samples using D-T2 correlation experiments. The D-T2 spectrum shows predominantly two signals, one with a diffusion coefficient close to that of bulk water, and the other with a D about a factor of 100 lower. The fast diffusing component is identified as water and the other as fat globules. Two components of cheese in the D-T2 map has also been observed by Callaghan and Godefroy [65]. Recently, Hurlimann et al. have performed a systematic 2D NMR study of milk, cream, cheeses and yogurts [66], Some of the preliminary results are discussed here. 

For example, in the case of PS and applying the Smoluchowski equation [333], it is possible to estimate the precipitation time, fpr, of globules of radius R and translation diffusion coefficient D in solutions of polymer concentration cp (the number of chains per unit volume) [334]. Assuming a standard diffusion-limited aggregation process, two globules merge every time they collide in the course of Brownian motion. Thus, one can write Eq. 2 ... 

In order to estimate the region of this approximation applicability, it is necessary to examine macrokinetics of a polymeranalogous reaction with explicit allowance for the diffusion of a reagent Z into a globule. In this case, the profile of its constituent monomeric units will be fuzzy rather than stepwise (see Fig. 1). This brings up two questions. The first one is how this profile depends on kinetic and diffusion parameters of a reaction system. The second question is concerned with the effect of the profile shape on the statistical characteristics of the chemical structure of the products of a polymeranalogous reaction. A rigorous theory has been developed [22,23] which enables us to answer these questions. The main concepts of this theory are outlined in the subsequent Sections. 

When considering the macrokinetics of PAR described by equations (Eq. 17), it is reasonable to focus on two limiting regimes. The first of these, the kinetically-controlled regime, takes place provided the rate of diffusion of molecules Z appreciably exceeds that of the chemical reaction. In this case, a uniform concentration Z = Ze should be established all over the globule after time interval t R2/D. Subsequently, during the interval t 1 /kZe, which is considerably larger than f[Pg.152]

Slow release rates and remarkable long shelf-life (months) were obtained compared to typical multiple emulsions stabilized by two short surfactants (SMO and polyoxyethylene (20) sorbitan monolaurate). Finally, the long lifetime of the emulsions allowed study via diffusing wave spectroscopy (DWS) of the interactions between the droplets and the globule surface [37],... 

Diffusion of cephalosporin anion (P ) from the bulk feed phase (III) to feed-membrane (III-II) interface of the emulsion globule... 

Diffusion of the solute-carrier complex, QP into the emulsion globule... 

Lactic acid Secondary amine Na2C03 and NaOH Co-transport of OH- ELM Model of mass transfer in external layer and internal diffusion in globule pH variation occmred [71]... 

ELM Model incorporating external mass transfer and diffusion in globule Selective separation of cephalexin from mixture with 7-ADCA and of CPC from deacetylCPC of broth ... 

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