Enthalpy transition

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

When the free enthalpy of reaction AG for the transformation of the structure of a compound to any other structure is positive, then this structure is thermodynamically stable. Since AG depends on the transition enthalpy AH and the transition entropy AS, and AH and AS in turn depend on pressure and temperature, a structure can be stable only within a certain range of pressures and temperatures. By variation of the pressure and/or the temperature, AG will eventually become negative relative to some other structure and a phase transition will occur. This may be a phase transition from a solid to another solid modification, or it may be a transition to another aggregate state. 

The transition enthalpies of the s- and p-fractions obtained from the feed with a comonomer molar ratio of 85 15 were equal to 6 and 7 J/g, respectively, i.e. the values are very close. This, therefore, can be indicative of almost the same average length of oligoNVCl blocks. Moreover, as we have already stressed, the fractions also had virtually the same final comonomer composition. However, since the solution properties of these fractions are drastically different, one can draw the conclusion that this is apparently due to a specific distribution of hydrophobic and hydrophilic residues along the polymer chains. In turn, because of all the properties that are exhibited by the s-fraction, this fraction can be considered to be a protein-like copolymer [27]. 

It should be noted that the theory described above is strictly vahd only close to Tc for an ideal crystal of infinite size, with translational invariance over the whole volume. Real crystals can only approach this behaviour to a certain extent. Flere the crystal quality plays an essential role. Furthermore, the coupling of the order parameter to the macroscopic strain often leads to a positive feedback, which makes the transition discontinuous. In fact, from NMR investigations there is not a single example of a second order phase transition known where the soft mode really has reached zero frequency at Tc. The reason for this might also be technical It is extremely difficult to achieve a zero temperature gradient throughout the sample, especially close to a phase transition where the transition enthalpy requires a heat flow that can only occur when the temperature gradient is different from zero. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

Equation 6.1 provides the rate at which stable nuclei will form. It also demonstrates the clear rate dependence on the temperature, surface energy, and transition enthalpy. 

Chapter B outlines a typical statistical-mechanical formulation of polypeptide conformations in terms of these three parameters and describes its use for the evaluation of s and tr from observed helix-coil transition curves. Then the reported values of AH and a for selected polypeptide-solvent pairs are given and their implications are briefly discussed from a molecular standpoint. Here AH denotes the transition enthalpy derived from s by a thermodynamic relation. 

The free energy of hydrogen bonding, GB, is presumably dominated by its enthalpy part HB. Hence the transition enthalpy and entropy may be written [see Eq. (B-4)]... 

Table 4. Transition enthalpies and entropies of PHBA as a function of increasing molecular weight...

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

An inspection of the transition enthalpies to the plastic state reveals relatively small values, so that one may expect, as in the case of liquid crystals, that some of the enthalpy is gained continuously through increased heat capacity (see Sect. 5.1.2). 

The optical results at low salt concentration have been interpreted in terms of the presence of the two conformations given by reactions (16.24) and (16.25). The latter structure is only partially double-stranded and incorporates the hairpin loop region in which four bases are not paired. The lower transition temperature, which is concentration-dependent, with the larger melting enthalpy is ascribed to the disruption of the full duplex structure, which is a bimolecular process. The higher temperature transition (with a lower transition enthalpy) is attributed to the disruption of the hairpin structure, which is a... 

The unequivocal characterization of the mesophases is quite often very tricky and problematic. Each phase shows typical textures under a polarizing microscope a lot of them are documented in precise photos and pictures, but the formation of a texture depends strongly on the sample preparation, surface treatment, temperature control and other parameters. DSC curves and the phase-transition enthalpies yield important information, but the only unequivocal and suitable tool for the determination of the mesophases is given in the different x-ray methods, and nowadays modern resonance and atomic probe techniques attract more and more notice and acceptance9-11. A general description is given in the next section. 

Differential scanning calorimetry (DSC) Since lc s form phases in a thermodynamic sense, a transition from one phase to another is accompanied by a phase-transition enthalpy. Nevertheless, there are phase transitions of second-order character which can hardly be detected by DSC since there is no phase-transition enthalpy but just a change in heat capacity. A typical example is the transition from orthogonal phases to tilted phases. 

All phase transition enthalpies will be omitted for reasons of clarity. 

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