Endothermic interaction

Menashi et al.153) could confirm the results of Privalov and Tiktopulo152 and inter-prete the described effects as follows In the case of native tropocollagen, the pyrrolidine residues are probably directed away from the fibrillar axis and are mostly coated by water which is structured in the immediate neighbourhood to the pyrrolidine residues. During the denaturation these pyrrolidine residues form hydrophobic bonds with each other or with other apolar residues within the same chain (endothermic interaction) while the structure of water breaks down (increase of entropy). 

The first two kinds of interactions are endothermic, requiring an input of energy to spread apart solvent molecules and to break apart crystals. Only the third interaction is exothermic, as attractive intermolecular forces develop between solvent and solute particles. The sum of the three interactions determines whether AHsoin is endothermic or exothermic. For some substances, the one exothermic interaction is sufficiently large to outweigh the two endothermic interactions, but for other substances, the opposite is true (Figure 11.4). 

Since PEC is a copolymer, description of its interaction with PS is more complex than if it were simply a homopolymer. Binary interaction models have been presented which suggest that copolymer miscibility with a homopolymer can be enhanced by endothermic interactions between the unlike repeat units of the copolymer (11-13). In its simplest form (11). the binary interaction model for the heat of mixing, AHaix, of a copolymer. A, containing repeat units 1 and 2, with a homopolymer, B, containing repeat units 3, is given by... 

Miscible blends of elastomers differ from corresponding blends of thermoplastics in two important areas. First, the need for elastic properties requires elastomers to be high molecular weight. This reduces both the kinetic rate and the thermodynamic driving force for the interdiffusion and thus formation of a miscible single phase of dissimilar elastomers. Second, elastomers are plasticized in conventional compounding with process oils. The presence of plasticizers leads to both a higher free volume for the blend components and a decrease of the endothermal interactions. 

The parallelism of heat capacity and heat of combustion with combustibility indicates that the combustibility of polymers could be estimated from calculated theoretical values [20]. The same overall result of lowering heat production can be observed where there is a possibility of endothermic interactions between decomposition products. 

From melting point depressions Harris et al. [83] estimated a value of the interaction parameter B of -10.1 J cm. These authors suggested that the interactions responsible for miscibihty involve hydrogen bonding with the carbonyl of PCL and the hydroxyl of the phenoxy, probably counteracted to some extent by endothermic interactions between other structural units. 

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

The quantity x is called the Flory-Huggins interaction parameter It is zero for athermal mixtures, positive for endothermic mixing, and negative for exothermic mixing. These differences in sign originate from Eq. (8.39) and reaction (8.A). 

Equation (8.49) accounts only for endothermic mixing. It is not too surprising that we are thus led to associate exothermic values with more specifically chemical interactions between solvent and solute as opposed to the purely physical interactions we have been describing in this approximation. 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

Is the second step of the overall reaction for R=Me (N-methylphthalimide + hydrazine —> phthalimide hydrazide + methylamine) exothermic or endothermic Will higher temperatures accelerate or inhibit the reaction Is the structure drawn above for phthalimide hydrazide its lowest-energy form or are either the imine or diimine tautomers preferred Compare energies for the hydrazide and imine and diimine tautomers. Examine the geometry of phthalimide hydrazide and any low energy tautomer, and draw the Lewis structure(s) that best describes it. Can your Lewis structures account for the energy differences Examine electrostatic potential maps for all three molecules. Which molecule(s) are stablized by favorable electrostatic interactions Which are destabilized Can this help explain the energy differences Elaborate. 

Interaction between niobium oxide and fluorides, chlorides or carbonates of alkali metals in an ammonium hydrofluoride melt, yielded monooxyfluoroniobates with different compositions, MxNbOF3+x, where they were subsequently investigated [123-127]. According to DTA patterns of the Nb205 - 6NFL HF2 - 2MF system, (Fig. 18) a rich variety of endothermic effects result from the formation of ammonium monooxyfluoroniobate, its thermal decomposition and its interaction with alkali metal fluorides. The number of effects decreases and separation of ammonium ceases at lower temperatures and when going from lithium to cesium in the sequence of alkali metal fluorides. 

The authors of [99] proposed a calorimetric method for determining the degree of the polymer-filler interaction the exothermal effect manifests itself in the high energy of the polymer-filler adhesion, the endothermal effect is indicative of a poor, if any, adhesion. The method was used to assess the strength of the PVC-Aerosil interaction with Aerosil surface subjected to different pre-treatments... 

Using calorimetry to estimate the degree of filler-polymer interaction as described in [99] the authors of [318, 319] determined that the filler reaction with PVC is exothermic, which is indicative of a stronger bond in the polymer-filler system. No thermal effect was noted for mechanical mixtures, except for a few cases where it was endothermal. 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

The configuration of the polymer molecule must depend also on its environment. In a good solvent, where the energy of interaction between a polymer element and a solvent molecule adjacent to it exceeds the mean of the energies of interaction between the polymer-polymer and solvent-solvent pairs, the molecule will tend to expand further so as to reduce the frequency of contacts between pairs of polymer elements. In a poor solvent, on the other hand, where the energy of interaction is unfavorable (endothermic), smaller configurations in which polymer-polymer contacts occur more frequently will be favored. 

Adsorption is typically exothermic (i.e., releases energy in the process of bonding), but can be endothermic, and can be classified into two groups, based on the energies involved chemical adsorption and physical adsorption. Chemical adsorption is more significant for heavy metals, either in the form of ion exchange or interactions involving metal complexes. 

Both reactions are endothermic, but the interaction of bromine atoms with HBr is much more so than the interaction with molecular hydrogen. Consequently, the former reaction will occur much less frequently than the latter. 

Mercury fulminate, readily formed by interaction of mercury(II) nitrate, nitric acid and ethanol, is endothermic (AH°f (s) +267.7 kJ/mol, 0.94 kJ/g) and was a very widely used detonator. It may be initiated when dry by flame, heat, impact, friction or intense radiation. Contact with sulfuric acid causes explosion [1], The effects of impurities on the preparation and decomposition of the salt have been described [2],... 

Interaction of the endothermic fluoride with the trioxide is very vigorous, and explosive in absence of solvent. 

Interaction of alkenes with ozonised oxygen tends to give several types of products or their polymers, some of which show more pronounced explosive tendencies than others [1]. The cyclic ge/n-diperoxides are more explosive than the true ozonides [2], It has been calculated that ozonisation of the endothermic /rara-stilbene (AH°f +135.4 kJ/mol, 0.78 kJ/g) would give, in the event of decomposition of the unstable ozonide, an exothermic release of 1.41 kJ/g which would attain an adiabatic decomposition temperature approaching 750°C with a 27-fold pressure increase in a closed vessel [3],... 

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