Concentration, threshold

At low concentrations surfactant molecules adsorbed at the surface are in equilibrium with other molecules in solution. Above a threshold concentration, called the critical micelle concentration (cmc, for short), another equilibrium must be considered. This additional equilibrium is that between individual molecules in solution and clusters of emulsifier molecules known as micelles. 

Calcium and Vascular Smooth Muscle Contraction. Calcium acts on a number of sites associated with the control of the cytoplasmic calcium concentration. Vascular smooth muscle contraction can be initiated by the opening of the slow calcium channel aUowing influx of extraceUular calcium through the sarcolemmal membrane into the cytoplasmic compartment. The iatraceUnlar calcium concentration increases to 1 x 10 Af, a threshold concentration necessary to initiate contraction. 

VOCs are released during chemical cleaning of bonding surfaces. The extract system is designed on the basis of the steady-state concentration determined for maximum source strength and considering the mechanical extract ventilation only and no air-exchange with the assembly hail. Ehis concentration must be kept below the threshold concentration (TVL) which is set to 300 mg/kg in this example. 

Experimental dependences of conductivity cr of the CPCM on conducting filler concentration have, as a rule, the form predicted by the percolation theory (Fig. 2, [24]). With small values of C, a of the composite is close to the conductivity of a pure polymer. In the threshold concentration region when a macroscopic conducting chain appears for the first time, the conductivity of a composite material (CM) drastically rises (resistivity Qv drops sharply) and then slowly increases practically according to the linear law due to an increase in the number of conducting chains. 

The composites with the conducting fibers may also be considered as the structurized systems in their way. The fiber with diameter d and length 1 may be imagined as a chain of contacting spheres with diameter d and chain length 1. Thus, comparing the composites with dispersed and fiber fillers, we may say that N = 1/d particles of the dispersed filler are as if combined in a chain. From this qualitative analysis it follows that the lower the percolation threshold for the fiber composites the larger must be the value of 1/d. This conclusion is confirmed both by the calculations for model systems [27] and by the experimental data [8, 15]. So, for 1/d 103 the value of the threshold concentration can be reduced to between 0.1 and 0.3 per cent of the volume. 

In pressing, the threshold concentration of the filler amounts to about 0.5% of volume. The resulting distribution of the filler corresponds, apparently, to the model of mixing of spherical particles of the polymer (with radius Rp) and filler (with radius Rm) for Rp > Rm as the size of carbon black particles is usually about 1000 A [19]. During this mixing, the filler, because of electrostatical interaction, is distributed mainly on the surface of polymer particles which facilitates the forming of conducting chains and entails low values of the percolation threshold. 

For the second method the threshold concentration of the filler in a composite material amounts to about 5 volume %, i.e. below the percolation threshold for statistical mixtures. It is bound up with the fact that carbon black particles are capable (in terms of energy) of being used to form conducting chain structures, because of the availability of functional groups on their surfaces. This relatively sparing method of composite material manufacture like film moulding by solvent evaporation facilitates the forming of chain structures. 

As already noted, the main merit of fibers used as a filler for conducting composite materials is that only low threshold concentrations are necessary to reach the desired level of composite conductivity. However, introduction of fiber fillers into a polymer with the help of ordinary plastic materials processing equipment presents certain difficulties which are bound up mainly with significant shearing deformations entailing fiber destruction and, thereby, a decrease of parameter 1/d which determines the value of the percolation threshold. 

If the Hill coefficient is less than one (H < 1), the maximum effect will never be obtained with increasing concentrations (E < max). If the Hill coefficient is more than ten (H > 10), an on-off phenomenon can be described at CE50. The effect can be assumed to be marginal and less than 5% of max for the threshold concentration (CE05) but almost 95% of max for the ceiling concentration (CE95). 

Pharmacokinetics. Figure 2 Sigmoid Emax model of pharmacodynamics with Hill coefficient (H), concentration producing half-maximum effect (CE50), threshold concentration (CE05), and ceiling concentration (CE95). 

Data for threshold concentrations of anionic surfactants, or highest concentrations that are tolerated without any irritation (A) and lowest concentrations that still cause mucus membrane irritation (B), have been collected by Kastner... 

FIGURE 5.6 Dieldrin intoxication in hnmans and its relationship to blood levels. The hatched area represents the sublethal effects seen at 15-30% of lethal threshold concentration in blood (after Jager 1970). 

The importance of lipophilicity to bitterness has been well established, both directly and indirectly. The importance of partitioning effects in bitterness perception has been stressed by Rubin and coworkers, and Gardner demonstrated that the threshold concentration of bitter amino acids and peptides correlates very well with molecular connectivity (which is generally regarded as a steric parameter, but is correlated with the octanol-water partition coefficient ). Studies on the surface pressure in monolayers of lipids from bovine, circumvallate papillae also indicated that there is a very good correlation between the concentration of a bitter compound that is necessary in order to give an increase in the surface pressure with the taste threshold in humans. These results and the observations of others suggested that the ability of bitter compounds to penetrate cell membranes is an important factor in bitterness perception. 

The effect of substrate concentration and the important issue of the existence or otherwise of threshold concentrations below which degradation is not effectively accomplished. 

There is substantial evidence from the aquatic environment for the existence of threshold concentrations below which rates of degradation of xenobiotics are slow or even negligible (Alexander 1985). A number of hypotheses have been put forward including the critical concentrations required for... 

Standards for individual substances are usually set to reflect ceiling values, a threshold concentration value never to be exceeded even momentarily, and also time-weighted average values,... 

The limit of tolerable error is generally smallest in a minor component assay. It will have been determined that a particular minor component must be at or below a threshold concentration for the product to be usable. Therefore, the decision to accept or reject an entire production batch may depend on the analytical result. Typical batches may contain the contaminant at a concentration very similar to the specification limit. In a minor component assay, the major component may be overloaded and out of the proper range of detection of the assay. Even so, the minor component may be at such low levels that assay noise interferes. 

Kimura, Y. Hosoda, Y. Shima, M. Adachi, S. Matsuno, R., Physicochemical properties of fatty acids for assessing the threshold concentration to enhance the absorption of a hydrophilic substance, Biosci. Biotechnol. Biochem. 62, 443 447 (1998). 

Gerike, P. and Gode, P, The biodegradability and inhibitory threshold concentration of some disinfectants, Chemosphere, 21, 799-812, 1990. 

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. 

People with Neurologic Dysfunction or Kidney Disease. This population is unusually susceptible to lead exposure. The neurologic and renal systems are the primary target organs of lead intoxication, which may become overburdened at much lower threshold concentrations to elicit manifestations of lead intoxication (Benetou-Marantidou et al. 1988 Chisolm 1962, 1968 Lilis et al. 1968 Pollock and Ibels 1986). 

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