Phase transitions calorimetric studies

The Effect of Polymer Heterogeneity on the Enthalpy. The kinetics of the isotropic-smectic phase transition were studied for two of the polymers HPX-C9 and HPX-C11, and in Figure 8 a summary of the calorimetric data for the former is presented. The behaviour of the HPX-C11 polymer was similar. Two processes are in fact revealed by these data (a) at high temperatures (7 471.9 K) the two processes have approximately the same induction time and rate and are therefore not resolved, (b) at intermediate temperatures (468.9heat evolved in the slower process is 30-50% of that evolved in the more rapid process. There is a tendency for an increase in Ah0 for the slow process with increasing temperature for HPX-C9, Ah0 was equal to 13 kJ/kg at 468.9 K and 20kJ/kg at 470.9 K. The same trend was observed for HPX-C11. (c) at low temperatures (TS467.9 K), the slower process was much retarded and not observed within the experimental window. 

In calorimetric studies of KOH doped ice V, endothermic peaks observed by Handa et al. on annealing had been attributed previously to an order-disorder phase transition. Subsequent studies of undoped or KOH doped ice V could not confirm a phase transition. Thus, the previous evidence for partial hydrogen ordering in undoped ice V, or in KOH doped ice V, must have been obtained within the ice V... 

Low melting metals (Sn and also Bi, In, Pb, and Cd) are extensively used as solvents in calorimetric studies of metallic phases [35]. Transition metals do not, however, dissolve readily in tin [43] and other solvents such as Cu and A1 have been used. An experimental probe for high-temperature solution calorimetry is shown in Figure 10.8. 

Privalov and Mrevlishvili14,188,189) made calorimetric studies on collagen. Up to 0.3 g water/g collagen is strongly bonded, this water shows no crystallisation until at liquid He T. No phase transition in collagen containing 2-3 mole water/100 g... 

Displaying a grasp of chemistry remarkable even among chemical engineers, the authors ascribe the hazardous side reaction consequent upon mono-nitration of toluene in mixed acid, to a decomposition of nitric acid (science has hitherto regarded nitric acid as thermodynamically more stable than conceivable decomposition products). This is favoured by poor mixing in what they describe as a three phase mixture (m/xo-nitrotoluenes being apparently immiscible with toluene). What the calorimetric study described seems to have observed is the transition from nitration to oxidation of the substrate. 

Seddon, J.M., Cevc, G., and Marsh, D. (1983) Calorimetric studies of the gel-fluid transition (Lj) —> La) and lamellar-inverted hexagonal (La — H ) phase transition in dialkyl- and diacyl-phosphatidylethanolamines. Biochemistry 22 1280-1289. 

H. Kawasaki, S. Sasaki, and H. Maeda, Effects of the gel size on the volume phase transition of poly(N-isopropylacrylamide) gels A calorimetric study, Langmuir 14(4), 773-776 (1998). 

Although studies of the thermotropic phase behavior of singlecomponent multilamellar phospholipid vesicles are necessary and valuable, these systems are not realistic models for biological membranes that normally contain at least several different types of phospholipids and a variety of fatty acyl chains. As a first step toward understanding the interactions of both the polar and apolar portions of different lipids in mixtures, DSC studies of various binary and ternary phospholipid systems have been carried out. Phase diagrams can be constructed by specifying the onset and completion temperatures for the phase transition of a series of mixtures and by an inspection of the shapes of the calorimetric traces. A comparison of the observed transition curves with the theoretical curves supports... 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

Seddon JM, Cevc G, Marsh D. Calorimetric studies of the gel-fluid (L-Beta-L-Alpha) and lamellar-inverted hexagonal (L-Alpha-HB) phase-transitions in dialkyl and diacylphosphatidylethanolamines. Biochemistry 1983 22 1280-1289. 

Polymorphism. The structural stabihty of margarine is influenced by the properties of the crystal lattice and by the actual amount of solid fat present. Many organic compounds or mixtures such as fats can sohdify in more than one crystalline pattern. The primary crystal forms of triglycerides are designated a, p, and p, which correspond to three principal cross-sectional arrangements of the fatty acid chains (71). These may be differentiated by characteristic x-ray diffraction patterns (72) and heats of transition observed in calorimetric studies (73). Phase behavior of... 

Tkeda, T., Horiuchi, S., Karanjit, D. B., Kurihara, S., and Tazuke, S. Photochcmically induced isothermal phase transition in polymer liquid crystals with mesogenic phenyl K nzoate side chains. 1. Calorimetric studies and order parameters. Macromolecules 23. 16 (1990). 

Calorimetric studies of surface and nanoparticle energetics fall into several classes enthalpies of wetting and hydration/dehydration, heat capacity measurements, thermal analysis of coarsening and phase transition, and enthalpy differences by solution calorimetry. These methods measure different quantities, suffer from different potential difficulties, and are generally regarded as complementary. 

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