Qualitative Concentration Terms

Nonapparent plants were fast growing, short-lived, and occurred unpredictably in space and/or time. Thus, many of these plants would escape detection by the majority of herbivores. Because these plants allocated most of their resources to rapid growth and reproduction, they were believed to be defended by relatively low concentrations of toxic compounds that were generally effective against many herbivores but that some specialist herbivores would be able to evolve detoxification mechanisms against (and may even use the qualitative defenses as cues to locate their host). These toxic compounds such as alkaloids and terpenes were termed qualitative defenses because their potency made them effective at low concentrations qualitative defenses of the PAM are analogous to mobile defenses of the RAM. 

LeChateher s principle (Section 18.3) allows us to make qualitative predictions about the effects of changes of conditions on an equihbrium system but does not ahow quantitative calculations. However, at equihbrium at a given temperature, a certain ratio of concentration terms is very nearly constant for all solutes and gases involved in any given reaction. (Solids and pure liquids are not included in the ratio.) Because it is not exactly constant, we will use two or at most three significant digits in equihbrium constant calculahons. For the general reaction... 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

The solubility (S) of a solute is the maximum amount that dissolves in a fixed quantity of a particular solvent at a specified temperature, given that excess solute is present. Different solutes have different solubilities. For example, for sodium chloride (NaCl), S = 39.12 g/100. mL water at 100.°C, whereas for silver chloride (AgCl), S = 0.0021 g/100. mL water at 100.°C. Obviously, NaCl is much more soluble in water than AgCl is. (Solubility is also expressed in other units, as you ll see later.) Although solubility has a quantitative meaning, dilute and concentrated are qualitative terms that refer to the relative amounts of solute a dilute solution contains much less dissolved solute than a concentrated one. 

Solubility is a quantitative term, but dilute and concentrated are qualitative, referring to the relative amounts of dissolved solute the NaCl solution above is concentrated, and the AgCl solution is dilute. 

This expression can be used to describe both pore and solid diffusion so long as the driving force is expressed in terms of the appropriate concentrations. Although the driving force should be more correctly expressed in terms of chemical potentials, Eq. (16-63) provides a qualitatively and quantitatively correct representation of adsorption systems so long as the diffusivity is allowed to be a function of the adsorbate concentration. The diffusivity will be constant only for a thermodynamically ideal system, which is only an adequate approximation for a limited number of adsorption systems. 

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

For comparing the relative loss of a flavor component from a container, we have found the sniff test (6) very useful, especially when gas concentration measuring techniques were not available. Typical results of this type of test are shown in Table II. Each filled container was held in a glass jar for approximately 48 hours. The results are stated in qualitative, subjective terms such as slight, strong, or undetectable. The... 

On-line SFE coupled to GC or SFC, according to the thermal stability of the analytes, are both very competitive with classical methods of analysis in terms of sensitivity and analysis time. Since all of the extracted analytes are transferred to the GC system, much higher method sensitivities can be obtained. Several modes of operation are possible utilising on-line SFE-GC, including quantitative extraction of all analytes from a sample matrix quantitative extraction and concentration of trace analytes selective extractions at various solvating powers to obtain specific fractions and periodic sampling (multiple-step extractions) of the effluent at various pressures for qualitative characterisation of the sample matrix. 

The percutaneous absorption picture can be qualitatively clarified by considering Fig. 3, where the schematic skin cross section is placed side by side with a simple model for percutaneous absorption patterned after an electrical circuit. In the case of absorption across a membrane, the current or flux is in terms of matter or molecules rather than electrons, and the driving force is a concentration gradient (technically, a chemical potential gradient) rather than a voltage drop [38]. Each layer of a membrane acts as a diffusional resistor. The resistance of a layer is proportional to its thickness (h), inversely proportional to the diffusive mobility of a substance within it as reflected in a... 

The concentration at which a steep rise in this curve begins has been termed as the critical or threshold concentration (2,3). Figure 6 shows such typical curves for PTF and BTF in n-hexane. Despite the fact that different shear rates are involved in capillary viscometry, it can be qualitatively said that at a given concentration, PTF viscosified n-hexane better than BTF. It is clear from Figure 6 that the critical concentration for these two compounds is above 0.7%, while analogous tri-n-alkyltin fluorides showed a critical concentration of less than 0.4% (3). This may be due to the presence of bulky Me3Si-groups nearer to the Sn-F bond, which causes some steric hindrance to auto-association. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

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