Dispersion Half time data

On one hand, TR-XAFS investigations that require the best time resolution available (such as isothermal reactions or rapid decompositions with half lives of the order of one minute) may be performed at an energy-dispersive XAFS station, with full advantage taken of the time resolution in the sub-second range. On the other hand, TR-XAFS investigations of processes with half lives of the order of several minutes may be performed in the QEXAFS mode, with advantage taken of the increased EXAFS data quality for a detailed structural analysis. 

The system then adjusts these scores to account for the way different compounds react in the different media — air, ground water, and surface water. On the basis primarily of a compound s half-life in a medium and on dispersion patterns, we assign each compound a separate inherent risk score for each medium. The scale we use is very coarse each level is 10 times greater than the previous level. The data now in the model is, therefore, insensitive to risks that are only two or thre times as great as others. We found it convenient to express the ten-fold differences on a logarithmic unit scale. 

Equation 18.12 shows that the inverse of the concentration at any time is a linear function of time, the slope of the line being determined by the coagulation constant. Experimental data from both mono-disperse and polydisperse aerosols follow this general form, at least initially. As will be discussed later, the coagulation constant may be appreciably larger than the theoretical value. If th is defined as the half-value time, i.e., the time in which the concentration decreases by a factor of 2, then... 

Figure 2 shows the spectral response functions (5,(r), Eq. 1) derived firom the spectra of Fig. 1. In order to adequately display data for these varied solvents, whose dynamics occur on very different time scales, we employ a logarithmic time axis. Such a representation is also useful because a number of solvents, especially the alcohols, show highly dispersive response functions. For example, one observes in methanol significant relaxation taking place over 3-4 decades in time. (Mdtiexponential fits to the methanol data yield roughly equal contributions from components with time constants of 0.2, 2, and 12 ps). Even in sinqrle, non-associated solvents such as acetonitrile, one seldom observes 5,(r) functions that decay exponentially in time. Most often, biexponential fits are required to describe the observed relaxation. This biexponential behavior does not reflect any clear separation between fast inertial dynamics and slower diffusive dynamics in most solvents. Rather, the observed spectral shift usually appears to sirrply be a continuous non-exponential process. That is not to say that ultrafast inertial relaxation does not occur in many solvents, just that there is no clear time scale separation observed. Of the 18 polar solvents observed to date, a number of them do show prominent fast components that are probably inertial in origin. For example, in the solvents water [16], formamide, acetoniuile, acetone, dimethylformamide, dimethylsulfoxide, and nitromethane [8], we find that more than half of the solvation response involves components with time constants of 00 fs.

The diffraction patterns of the dehydrated samples dispersed in water for 24, 48, and 72 h are shown in Figs. 8.5c—8.5e, respectively. As can be verified, the intensity of the 001 diffraction peak of the hydrated samples enhances in intensity for largest contact times. However, even after 72 h, less than half of the sample was rehydrated. So, both data sets, calorimetric and XRD, show that the dehydrated vermiculite does not behave as a hydrophilic compound, with its total surface, that is, the external and internal space of the lamella, exhibiting a very low affinity toward water molecules. This indicates that besides being unfavorable from a thermodynamic point of view (endothermic), the rehydration process, that is, adsorption of water molecules on the surfiice, is kinetically slow. 

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