Negative temperature sensitivity

Negatively, temperature-sensitive hydrogels possess a LCST and contract upon heating above the LCST. This type of swelling behavior is known as the inverse (or negative) temperature dependence. Inverse temperature-dependent hydrogels are comprised of polymer chains which either possess moderately hydrophobic groups or a mixture of hydrophilic... 

Calcium sulfate, the substance used to absorb water in desiccators, provides an example of this temperature sensitivity. Anhydrous calcium sulfate absorbs water vapor from the atmosphere to give the hydrated salt. The reaction has a negative AS° because water molecules become more constrained when gaseous water molecules move into the solid state. The reaction also has a negative AH ° because of the electrical forces of attraction... 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

Adverse effects of fenvalerate on survival of terrestrial arthropods were observed at 0.002 to 0.015 pg whole-body topical application, O.llkg/ha aerial application, 5.4 mg/kg in the soil, 50 mg/kg in the diet, and 1.4 g/ant mound (Table 20.4). Synthetic pyrethroids are more effective in biological systems at low temperatures. The relative sensitivity of insects when compared with mammals is attributed in part to this negative temperature coefficient. Thus, warm-blooded animals are less affected than insects and other poikilotherms (Klaassen etal. 1986). Fenvalerate, for example, showed a negative correlation between temperature and toxicity to crickets (Acheta pennsylvanicus), being up to 1.9 times more toxic at 15°C than at 32°C (Harris etal. 1981). A similar case is made for honey bees (Apis mellifera) (Mayer et al. 1987) and for many species of aquatic invertebrates and fish (Mayer 1987). 

Pyrethroids have low oral toxicity to mammals, and in general their insect (topical) to mammal (oral) toxicity ratio is much higher than that of the other major classes of insecticides [25]. As the reason, at least the following mechanisms are conceivable (1) negative temperature dependence - differences in body temperature between insects and mammals makes the insect nerves much more sensitive, (2) metabolic rate - insects metabolize the insecticide more slowly than mammals, and the metabolizing enzyme systems are different, and (3) differences in body size - insects will have less chance to metabolize the insecticides before reaching the target site [26]. 

Generally they have a negative temperature coefficient which may be as large as several percent per °C. They have large temperature sensitivity and may detect very small temperature changes. 

It should be noted that the enantiomeric excess in the product was determined in most cases by measuring the specific rotation of the neat liquid11, although GC methods are now available13. It is known that this rotation value is temperature sensitive ( + 0,18/deg) so that the temperature of measurement must be measured carefully and specified a value for enantiomer-ically pure 3-phenyl-l-butene of [a] 2 +5.92 (neat) is recommended (the / -enantiomer has a negative rotation). 

Thermistors, or thermally sensitive resistors," are semiconductors which have high negative temperature coefficients of resistance. There is no simple re-... 

From the previous discussion about the temperature sensitivity of reaction rate as a function of activation energy, we can understand why the chemical equilibrium constant of an exothermic reversible reaction decreases with increasing temperature. An exothermic reaction has a negative heat of reaction, since the activation energy of the reverse reaction exceeds that of the forward reaction. As temperature increases, the reverse reaction increases relatively more rapidly than the forward reaction, which means that at chemical equilibrium we have relatively more reactants than products and a lower equilibrium constant. 

There are numerous uses for resistors with high values of the temperature coefficient of resistance (TCR) and they may be negative (NTC) or positive (PTC). An obvious application is in temperature indicators that use negligible power to monitor resistance changes. Compensation for the variation of the properties of other components with temperature may sometimes be possible in this case the applied power may be appreciable and the resulting effect on the temperature-sensitive resistor (TSR) must be taken into account. 

INTEX DISPERSANT 33 will coat insoluble particles imparting a negative charge to assure dispersion stability. It is not temperature sensitive and will work at the boil or under pressure in jets or packages. 

---