Enthalpic terms

In the case of no steric exclusion of the molecule from parts of the stationary phase, the retention can be described by the enthalpic term alone ... 

Limited data suggest that the cntropic term may be as important as the enthalpic term in determining polymerizability. The value of AA(, is lowered >20 J moh1 K."1 by the presence of an a-melhyl substituent (Table 4.10, compare entries for AN and MAN, S and AMS). This is likely to be a eonscqucncc of the polymers from a-methyl vinyl monomers having a more rigid, more ordered structure than those from the corresponding vinyl monomers. 

Thermodynamically, intermixing as above arises from achieving a negative free energy from the process in mm, this arises by balancing a contribution from enthalpy (heat content) with another from entropy (involving material stmctural aspects). The solubility parameter reflects the enthalpic term. 

In a real LC system, the distribution coefficient is a function of the entropic and enthalpic terms. Therefore, the retention volume is described by Equation 17.8 (Pasch and Trathnigg, 1998) ... 

The excess Gibbs energy of the regular solution, as pointed out in Chapter 3, is a purely enthalpic term ... 

A knowledge of these enthalpic terms, and therefore of the relative bond energies, would be expected to considerably clarify many of these fundamental aspects. The data in Table 4 show that, with the main exception of rhenium and osmium, the metal-metal distances in the tetranuclear clusters and in the pure metals are quite similar this relationship is generally valid for all the polynuclear carbonyls60. The metal-metal bond energies in clusters are therefore expected to be of the same order as those in the metallic state for a close-packed arrangement, these are given by the formula Z m-m = A//f M(g)/6. 

In solution, two terms need to be added to take into account the solvation effect (enthalpic term AFisoiv) and the Coulombic energy of the formed ion pair ... 

Deviation from ideality is entirely due to enthalpic terms. The entropy of mixing is that of an ideal mixture ... 

The enthalpic term A/7 is the dominant one in equation 4.37 and determines the variation of the defectual concentration with T. For instance, for a Schottky defect we can write... 

According to the experiments of Campbell and Roeder (1968), at T = 1400 °C the (Mg,Ni)2Si04 mixture is virtually ideal. More recent measurements by Seifert and O Neill (1987) at T = 1000 °C seem to confirm this hypothesis. Because the intracrystalline distribution of Mg " and on Ml and Ml sites is definitely nonrandom (NP is preferentially stabilized in Ml), the presumed ideality implies that negative enthalpic terms of mixing counterbalance the positive excess entropy arising from the configurational expression... 

However, Noolandi (1991) has noted that eqn 6.7 only contains elastic contributions to the reduction in interfacial tension, and not the major contribution from enthalpic terms which are included in the expression derived by Noolandi and Hong (eqn 6.10). Noolandi (1991) argues that the major reduction in interfacial tension arises from the energetically favoured orientation of the copolymer blocks in their compatible homopolymers. 

For isotopic blends both approaches by Bates et al. [68,69] estimate properly the T 1 dependent (enthalpic) term of the more realistic interaction parameter form ... 

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