Electrostatic free enthalpy

A series of O-acetyl derivatives of pectic acid has been prepared to study the intramolecular binding of Ca + ions to pectic acids. The electrostatic free enthalpy of dissociation of these polyacids and the rate of exchange of the counter-ion from Ca + to K+ were determined by potentiometric titration measurements. The binding of Ca + ions was characterized by the production of single ion affinity coefficients determined in solutions of the calcium salts of 0-acetyl pectic acids and by the circular dichroism measurements of calcium and potassium salts of the pectate derivatives. The intramolecular binding of Ca + ions to isolated pectate macromolecules is purely of electrostatic character in contrast to the intermolecular chelate binding of Ca + ions which takes place when the pectate macromolecules aggregate. 

Michaeli and Overbeek used the Debye-Huckel approach to estimate the enthalpy of the PE-PE and PE-solvent interaction [23]. In this model, the Flory entropy is used as the entropic term of the PE chain, and the electrostatic free enthalpy is calculated from the electrostatic energy of a strong PE [23]. By thermodynamic considerations, Michaeli and Overbeek showed that for small univalent counterions no coacervation (complex-formation between PE) occurs... 

The apparent molal volume belongs, from a formal point of view, to the same group of thermodynamic properties as the enthalpy of dilution. The expression for the concentration dependence of the apparent molal volume of a polyelectrolyte can be derived from the electrostatic free enthalpy in a way similar to that shown above. In the final expression, which is analogous to Equation (24), the derivative dA/dT is replaced by d2/dP. Recently, Skerjanc [28] has studied the apparent molal volume of solutions of polystyrenesulphonates and shown that the cell model is applicable within a wide concentration range. 

The difference of the standard free enthalpies of two different solvates of a certain metal ion may be considered as due to the superimposition of electrostatic and nonelectrostatic contribution 124X... 

The electrochemical standard free enthalpy, of dissociation of the surface acid or base sites consists of the chemical standard free enthalpy, AG°, an electrostatic energy, eA, and an interaction energy, m0, for the adsorption coverage in the Frumkin adsorption model is the potential across the compact layer, 0 is the adsorption coverage, and m is the Frumkin parameter [Frumkin, 1925] ... 

Even extremophilic organisms and their proteins contain the same 20 amino acids with bonds similar to those in mesophiles. As the difference in free enthalpy between folded and unfolded states of globular proteins AG N >G is only about 45 15 kj mol-1 the sequence and structure of extremophilic proteins should differ from those of ordinary species. However, the main question, namely which properties cause the increase in denaturation temperature of thermostable proteins, is still debated (Rehaber, 1992). Theoretical and experimental analyses have shown that thermal stability is largely achieved by small but relevant changes at different locations in the structure involving electrostatic interactions and hydro-phobic effects (Karshikoff, 2001). There is no evidence for a common determinant or for just one effect causing thermostability. 

Since an ion has an electric charge, the partial molar free enthalpy gt of an ion i consists not only of the chemical potential ju, but also of the electrostatic energy zfffy of the ion where z, is the ionic valence, F is the Faraday constant, and is the electrostatic inner potential of the solution. This partial molar free enthalpy gt defines the electrochemical potential rj of an ion in an electrolyte solution as shown in Eq. 8.38 ... 

Formation of electrostatic complexes means mutual neutralisation of the macro-molecular reactant. This mutual neutralization of opposite charges and formation of the concentrated complex coacervate phase, minimizes the electrostatic free energy and reduces both the hydrophilicity and the solubility of the resultant complex. The loss of entropy on complexing may be compensated by the enthalpy contribution from interactions between macro-ions and by liberation of counter-ions and water molecules. 

Different intermolecular forces, like van der Waals forces (e.g., for acrylamide gel in acetoneAvater), hydrophobic interaction (e.g., NIPAAm in water), hydrogen bonding (ionic repulsive, e.g., IPN acryhc acid-acrylamide in water), and electrostatic interaction (ionic attractive, e.g., polyampholytes (cation-anion) in water) contribute to the various types of phase transitions in polymer gels. Their influence on to free enthalpy must be taken into account by additional terms to AG (or to X). Examples are ... 

The contributions of solvent effects on the activation free enthalpies of the reactions under study are reported in Tables 4 and 5 where also a comparison can be done among the different procedures. These effects are given by the differences between the solvent effects on the TSs and those on the reactants. The total solvent effect (TOT for reactants and TOT for the activation free enthalpy) consists of two contributions, labelled CDS(CDS for "cavitation+dispersion+structural" and ENP(ENP ) for "electronic+nuclear+polarisation" in the AMSOL procedure, CDR(CDR ) for "cavitation+dispersion+repulsion" and ELEC(ELEC for "electrostatic" in the Tomasi procedure. 

Energies in kcal mol unrelaxed HF/6-31G(d) geometries the total contribution (TOT ) to the activation free enthalpy is due to the cavitation+solute/solvent dispersion+solvent structural rearrangement term (CDS ) and to the electronic+nuclear- polarization term (ENP ) in the AMSOL model in the Tomasi model CDR is the cavitation-rdispersion+repulsion term and ELEC is the electrostatic contribution ab initio calculations are standard HF/6-31G(d). 

In all the evaluations of Table 4, the solvent effects on the activation free enthalpies are positive, increase with increasing solvent dielectric constants and tend to be larger for the endo than for the exo adducts. This behaviour, in accord with the experimental trend, is due to the electrostatic contribution the CDS and CDR contributions, in fact, are rather independent of the isomeric reaction considered and, moreover, appear to obtain comparable values in every 1,3-dipolar cycloaddition. For the Tomasi parametrisation in water, for example, the CDR" contribution for the cycloadditions of diazomethane and nitrile oxides to substituted alkenes amounts to -1.85 0.14 kcal mok This finding can be traced back to the view that the CDR term is approximately proportional to the solvent accessible surface area (the cavity area) of solutes and to the feature of TSs of having very alike structures of the new forming pentatomic ring so that the changes of the cavity areas from reactants to TSs are similar. 

On contact of different media occurs thermodynamical tug because of the inadequacy of their surface tension, which causes a rise in free enthalpy. Part of this stress is removed due to redistribution of components within media, and part as a result of mass transfer. Main forces controlling this redistribution are intermolecular van der Waals and electrostatic forces under whose effect forms the flow of components perpendicular to the interface. Such flow toward the denser medium... 

Neglecting the small difference between the Helmholtz and the Gibbs free energies for a condensed phase, one readily obtains the electrostatic (excess) enthalpy of the polyelectrolyte solution ... 

Crystallization is a phase transformation for which the free enthalpy A,rG has to be negative (Equation 2.1). The crystal will be stabilized by minimizing the enthalpy term AtrH, which is determined by the interaction of the building blocks. Interactive forces might either be van der Waals or electrostatic forces. In molecular crystals, hydrogen bonding also plays an important role. 

The thermodynamics of an outer-sphere electron-transfer reaction is represented by the Weller equation that provides the free enthalpy AG° as a function of the standard redox potentials of the donor and the acceptor and an electrostatic factor containing the sum d of the radiuses of the donor and acceptor, their charges Zd and Za, the dielectric constant e of the medium and the ionic strength factor f (often approximated to one). 

It is worth noting that, depending on the type of PE used, the electrostatic contribution for PEs are the dominating force and therefore it is possible to omit the Hory enthalpy term in this cases [23]. If the PE is very weak, the free enthalpy of dilution, AH, is only correctly obtained, when the electrostatic and Flory-Huggins polymer dilution enthalpy terms are added [49]. 

The change in free enthalpy of reactions (7.6) and (7.7) includes a purely electrostatic component corresponding to the electrical energy involved as the proton comes closer to the surface, and other chemical components AG ... 

---