Compensation temperatures

Fig. 4. The effect of temperature for Mng 6 Zng 3 Fe Fe on (a) initial magnetic permeabiUty, )J., measured on a polycrystalline toroid appHed as a core for a coil driven by a low (B <0.1 mT) ampHtude, low (10 kHz) frequency sinusoidal signal and (b) magnetocrystalline anisotropy constant, measured on a monocrystalline sphere showing the anisotropy/compensation temperature Tq and the Curie temperature, T. To convert joules to calories, divide by...

T omp practical compensation temperature is found ia a very narrow compositional range around... 

Change of compensation temperature with composition is Positive value of K indicates perpendicular anisotropy. 

Fig. 12. Temperature dependences of the magnetisation one curve typical for ferrimagnetic films, eg, RE-TM or garnets, the other one typical for ferromagnetic Co/Pt multilayers (39). compensation temperature = Curie temperature.

Enthalpy-entropy compensation has been investigated in reversed-phase HPLC with octylsilica stationary phase [77]. The compensation temperatures were determined for this system, and the results show that their change with the composition of the mobile phase is almost similar to that with octadecylsilica stationary phase. It can be concluded that the retention mechanisms of the separation of alkyl benzenes is the same in both systems with the mobile phase exceeding 20% water content. 

Androsch, R., Moon, I., Kreitmeier, S., and Wunderlich, B. (2000). Determination of heat capacity with a sawtooth-t rpe, power compensated temperature-modulated DSC. Thermochimica acta. 357-358,267-278. 

The partial derivatives are usually assumed to be constants that are evaluated at the steadystate operating level from the vapor-liquid equilibrium data. Thus, pressure and temperature on a tray can be measured, as shown in Fig. 8.3c, and a composition signal or pressure-compensated temperature signal generated and controlled. 

Values of AH° and AS° for complex-formation between alpha and beta cyclodextrin and a variety of guest molecules are shown in Tables II and III. If AH° is plotted against AS°, a linear relationship is observed, in which AH° and AS° are compensating (see Figs. 4 and 5). The slope of the graph is called the compensation temperature or isoequilibrium... 

Enthalpy-entropy compensation, is expected to occur only if unity of mechanism exists. From the slope of the enthalpy vs entre y plot the so-called compensation temperature, which is chm teristic for the type of transformation under investigation, can be calculated. 

Compensation temperatures have been found to be identical for the retention of a variety of eluites on three different types of reversed phase columns over a wide range of eluent composition (/ 77). The authors noted that if compensation behavior occurs in chromatography, the retention... 

Fig. 7 Dependence of activation enthalpy H, preexponential factor A and compensation temperature f, in pure Alfor 38.2° ( )and 40.5° (M)< 111 >-tilt grain boundaries.

The onset of glass formation in a polymer melt is associated with the development of orientational correlations that arise from chain stiffness. At the temperature Ta, there is a balance between the energetic cost of chain bending and the increased chain entropy, and below this temperature orientational correlations are appreciable while the melt still remains a fluid. Such a compensation temperature has been anticipated based on a field theoretic description of semiflexible polymers by Bascle et al. [120]. The temperature 7a is important for describing liquid dynamics since the orientational correlations (and dynamic fluid heterogeneities associated with these correlations) should alter the polymer dynamics for T < Ta from the behavior at higher... 

Polar interaction was recently suggested as the most important driving force for inclusion by cyclodextrins. This claim is based on the relatively high compensation temperature found in the A//-AS plotting and the linear relationships between 13C chemical shifts of C, carbon on cyclodextrin and AH of the inclusion (44). However, this is not true in most cases for several... 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

The search for stereocontrol parameters which depend on monomer constitution only has recently lead to the discovery of a compensation effect between the differences of the activation enthalpies and entropies, respectively (6). The calculated compensation temperature T0 and the compensation enthalpy can be used to evaluate the influence of the monomer constitution on the stereocontrol (6, 7, 8). The foundations and merits of this approach are discussed below. 

Within the limits of error, the slope of the function (AHf — A Hf) = /(ASf — ASf) is constant for a given monomer (Figure 3). The compensation temperature T0 is thus independent of the mode of addition. The compensation enthalpy AAHf differs however (Table W). 

Much has been said for and aginst the existence of compensation effects [for a recent literature review see Lumry and Rajender (33)]. In low molecular weight chemistry, activation enthalpies and activation entropies are calculated occasionally from rate constants at two different temperatures only. It has been claimed that many reported compensation effects are spurious because the compensation temperature equals... 

Table IV. Compensation Temperatures (T0) and Compensation Enthalpies (AAIff) of Various Monomers0...

Table V. Compensation Temperature (T0) and Compensation Enthalpy (AAJff) for the Various Monomers...

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