Activation solvent Active surface area

We conclude that the beneficial effects of water are not necessarily limited to reactions that are characterised by a negative volume of activation. We infer that, apart from the retro Diels-Alder reaction also other reactions, in which no significant reduction or perhaps even an increase of solvent accessible surface area takes place, can be accelerated by water. A reduction of the nonpolar nature during the activation process is a prerequisite in these cases. 

The meniscus is quite small if the funnel is kept still, and partitioning is slow. Conversely, shaking the funnel generates a large number of small globules of solvent, which gready increases the active surface area of the meniscus. Therefore, we shake the funnel to increase the rate of partitioning. 

Detector Characteristics. The applied potential that produced the largest analytical signal was 0.56 V versus SCE. The decrease in analytical signal at potentials more positive than 0.56 V suggested a decrease in the active surface area of the electrode due to competitive solvent oxidation at the active sites. 

Relatively large values (in m /g) were also obtained by using the vapors of ethanol 7.3 acetic acid 18.3 and water 137 [56]. Thus, pore structures could be opened up by swelling agents but only a small portion of the pore structures opened up by water (15%) can be retained even after solvent exchange [57,58]. Drying significantly reduces the internal active surface areas... 

In all the evaluations of Table 4, the solvent effects on the activation free enthalpies are positive, increase with increasing solvent dielectric constants and tend to be larger for the endo than for the exo adducts. This behaviour, in accord with the experimental trend, is due to the electrostatic contribution the CDS and CDR contributions, in fact, are rather independent of the isomeric reaction considered and, moreover, appear to obtain comparable values in every 1,3-dipolar cycloaddition. For the Tomasi parametrisation in water, for example, the CDR" contribution for the cycloadditions of diazomethane and nitrile oxides to substituted alkenes amounts to -1.85 0.14 kcal mok This finding can be traced back to the view that the CDR term is approximately proportional to the solvent accessible surface area (the cavity area) of solutes and to the feature of TSs of having very alike structures of the new forming pentatomic ring so that the changes of the cavity areas from reactants to TSs are similar. 

It is also possible to damage a membrane by blinding its active surface area by removing the solvent present in the feed which holds a polymer or an inorganic salt in solution. 

Another structure for carbons is texture, the ways that the crystallites joined together. Texture is often characterized by the degree of orientation from random to systematic arrangement. If the crystalhte size is small enough and there is no specific orientation, the carbon appears to be amorphous. Texture control cannot change the properties of individual crystalhtes but can alter the properties of the agglomerates of these crystallites like electricity and active surface area The comparative study of mesophase-pitch-based carbon fibers with different textures showed that the radical texture is more favorable for Lb intercalation than the concentric texture, but the radical texture is more easily broken into pieces by solvent cointercalated Lb. 

The activation overpotential is the potential loss to drive the electrochemical reactions from equilibrimn state. Therefore, it is the potential loss when there is a net current production from the electrode, i.e. a net reaction rate. In PEM fuel cell, the activation overpotential at the anode is negligible compared to that of the cathode. Activation polarization depends on factors such as the properties of the electrode material, ion-ion interactions, ion-solvent interactions and characteristics of the electric double l er at the electrode-electrolyte interface. Activation polarization may be reduced by increasing operating temperature and by increasing the active surface area of the catalyst. 

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

The solvent accessible surface area (SASA) is often used as a descriptor in quantitative structure-activity relationships (Connolly 1996). For a wide variety of molecules there is an approximate linear relation between solvation free energies and SASA. However, theoretical considerations indicate that the SASA model is incapable of accurately describing non-polar solvation phenomena at length-scales comparable to the size of a water molecule. It is more useful at large length-scales when more extended hydrophobic surfaces are in contact with the solvent. 

QSAR = quantitative structure-activity relationship SASA = solvent-accessible surface area SD = standard deviation SE = standard error. 

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