Up-hill diffusion

Let us now consider a system composed of a polymer and a solvent. For compositions in between the inflection points, solvent molecules will diffuse into the solvent-rich phase, and the polymer molecules diffuse in the polymer-rich phase. Thus diffusion occurs against a concentration gradient. Therefore, this type of phase separation is known as up-hill diffusion. The up-hill diffusion leads to a spontaneous decomposition and it is therefore also named spinodal decomposition. The formation of two phases via spinodal decomposition occurs immediately upon reaching the spinodal decomposition region and does not require any activation energy. 

The effective diffusion coefficient becomes negative for some range of composition when x is sufficiently large. The separation of phases requires an up-hill diffusion (characterized by negative Deff) in which the material moves against concentration gradients. The kinetics of the separation and the morphology of separated phases depend on the details of such diffusion. 

In order now to apply the preceding general considerations to a concrete but still relatively simple case, we shall discuss the Fe-Si-C system. We find here a situation in which a component can diffuse locally against its concentration gradient. This is known as up-hill diffusion Austenitic Fe-Si-C consists of a face-centered-cubic iron lattice with carbon on the interstitial sites. The silicon atoms are substituted on iron sites, and so the mobility of the silicon atoms is orders of magnitude smaller than that of the carbon atoms [22]. In Fig. 7-3 are shown the results of an experiment in which two iron cylinders with about the same carbon contents but with very different silicon contents were welded flush against one another and held for 13 days at 1050 °C. The experimental arrangement as well as the carbon concentration (iVc), the carbon activity (flc)> nd the silicon concentration (iVsi) the end of the experiment are shown. 

Separation of electrolytes based on the difference in chemical potential across the ion exchange membrane can be classified into (i) diffusion dialysis, (ii) Donnan dialysis, (iii) neutralization dialysis and (iv) up-hill transport. The natural flux of electrolytes through the membrane is low compared with that in the presence of an electrochemical potential. 

By reacting dicarboxylic acids with 5 % excess of diols, Carothers and Arvin obtained a range of polyesters with molecular weights up to about 4000 [19]. One of the collaborators in this work was J. W. Hill, who constructed a molecular still attached to a mercury diffusion pump that was capable of reducing the pressure in the reaction vessel to 10 5 mm of mercury [20], He made a polyester by reacting... 

Use of modified gold electrodes is not the only approach to achieve cytochrome c electrochemistry. Indeed, a number of studies have been reported on a variety of electrode surfaces. In 1977, Yeh and Kuwana illustrated (23) well-behaved voltammetric response of cytochrome c at a tin-doped indium oxide electrode the electrode reaction was found to be diffusion-controlled up to a scan rate of 500 mV sec Metal oxide electrodes were further studied (24, 25) independently in Hawkridge and Hill s groups. The electrochemical response of cytochrome c at tin-doped indium oxide and fluoride-doped tin oxide was very sensitive to the pretreatment procedures of the electrode surface. At thin-film ruthenium dioxide electrodes, variation of the faradaic current with pH correlating with the acid-base protonation of the electrode surface was observed. 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

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