Melting temperatures deduced from molar volumes: a consequence of the combination of enthalpy/entropy compensation with linear cohesive free-energy densities

Abstract

Enthalpy/entropy compensation is a general issue of intermolecular binding processes when the interaction between the partners can be roughly modelled with a single harmonic potential. Whereas linear H/S correlations are wished for by experimentalists, and often graphically justified, no inflexible law of thermodynamics supports the latter statement. On the contrary, the non-directional Ford's approach suggests logarithmic H/S relationships, which can be linearized only over narrow enthalpy/entropy ranges. Predictions covering larger domains require mathematical mapping obeying specific boundary conditions which are not compatible with linear plots. The analysis of solvent-free melting processes operating in six different classes of organic and inorganic materials shows that reciprocal Hill plots are acceptable functions for correlating melting enthalpies and entropies. The combination of H/S compensation with the observed linear dependence of the cohesive free energy densities with respect to the melting temperature eventually provides an unprecedented interdependence between melting temperatures and molar volumes. This procedure is exploited for the prediction of melting temperatures in substituted cyanobiphenyls

Chemical Programming of the Domain of Existence of Liquid Crystals

“… Δ S tr versus Δ H tr plots for a) melting and b) isotropization of n ‐LC 0′ , 0 ( n symbol) and 12‐LC i ′ , j (monomethylated compounds and dimethylated compounds with i ′, j symbols), showing local linear H/S compensation. The dashed lines represent reciprocal Hill fits, which obey thermodynamic boundary conditions when Δ H tr →0 …”

“…Consequently, the square of the Hildebrand solubility parameter, δ 2 , pertinent to the vaporization of liquids, has been replaced by the concept of cohesive free energy density, CFED=−Δ / in liquid crystals, in which Δ =Δ − Δ measures the intermolecular cohesion free energy in the solid or in the liquid‐crystalline phase estimated at a fixed reference temperature, T ref (Δ H tr and Δ S tr are the enthalpy and entropy changes of the incriminated phase transition, respectively) . Applied to the melting of solid linear alkanes, alkanoic acids, organosilanes, transition metals, or metal oxides, plots of CFED with respect to the melting temperatures systematically display linear correlations, as further illustrated in Figure for methyl‐substituted cyanobiphenyls 12‐OCB i ′, j , which simply melt to give isotropic liquids, despite their amphiphilic character . To extend the use of the CFED concept into the field of thermotropic liquid crystals, which display successive melting and isotropization transitions, it is necessary to increase the polarity difference (Δ δ = δ tail − δ head ) between the antagonist segments originally included in the substituted cyanobiphenyls 12‐OCB i ′, j .…”

“…[17] Applied to the melting of solid linear alkanes, alkanoic acids, organosilanes, transition metals,o rm etal oxides, plots of CFED with respectt ot he meltingt emperatures systematically display linear correlations,a sf urther illustrated in Figure 1f or methyl-substituted cyanobiphenyls 12-OCB i',j ,w hich simply melt to give isotropic liquids,d espite their amphiphilic character. [18] To extend the use of the CFED concept into the field of thermotropic liquid crystals, which display successive melting and isotropization transitions, it is necessary to increase the polarity difference( Dd = d tail Àd head )b etween the antagonist segments originally included in thes ubstituted cyanobiphenyls 12-OCB i',j .T he introduction of an additional para-substituted benzoate group gives the n-LC i',j family ( Figure 2), for which an 8-29 %i ncrease in Dd can be computed (Appendix 1i nt he Supporting Information). We report herein on the syntheses and structures of the mesogenice xtended cyanobiphenyls n-LC i',j ,together with their thermal behaviorand liquid-crystalline properties, which are analyzedw ithin the frame of the CFED concept.…”

Enthalpy–entropy compensation for isosteric state adsorption at near ambient temperatures

“…In many thermodynamic analyses of chemical reactions/ processes, it has been experimentally demonstrated that there are linear dependencies for the standard thermodynamic function, named the EEC compensation effect (enthalpy-entropy compensation) [ (Sharp 2001;Marco and Linert 2002;Ryde 2014;Olsson et al 2008;Dutronc et al 2014;Klebe 2015) KCE (Norwisz and Musielak 2007;Lvov 2007)]. According to Starikov, the relevant entry is expressed as follows (Starikov 2014): where the slope coefficient is constant and is named the isokinetic temperature, and the intercept ∆H iso which is part of enthalpy, is replaceable in isoentropic conditions (∆S = 0).…”

Thermal dissociation in terms of the second law of chemical thermodynamics

“…Equation (30) in the EEC model corresponds to the concepts of de Marco and Linert [20], as interpreted by Starikov [24], as well as the concept of an iso-equilibrium relationship [25, p. 781]. Meanwhile, according to the work in this field [19][20][21][22], the dependency of Eq. (30) might also include an individual chemical compound.…”