Electrostatic model for ion

Tremaine, P.R. and Goldman, S., "Calculations of Gibbs Free Energies of Aqueous Electrolytes to 350°C from an Electrostatic Model for Ion Hydration", Jr. Phys. Chem., in press. Helgeson, H.C., "Evaluation of Irreversible Reactions in Geochemical Processes Involving Minerals and Aqueous Solutions - I Thermodynamic Relations", Geochimica et Cosmochimica Acta, (1968), 3, 853. 

Reversed-phase ion-pair chromatography is primarily used for the separation of mixtures of ionic and ionizable compounds. In this chromatographic mode, a pairing ion is added to the mobile phase in order to modulate the retention of the ionic solutes. The pairing ion is an organic ion such as alkylsulfonate, alkylsulfate, alkylamine, tetraalkylammonium ion, etc. Here, only a very brief description of the main ideas behind the electrostatic model for ion-pair chromatography is presented. For a complete discussion, the reader is referred to Ref. [7,8] and the references therein. 

Tipping E., 1994, WHAM - a chemical equilibrium model and computer code for waters, sediments and soils incorporating a discrete site/electrostatic model of ion-binding by humic substances. Computers and Geosciences 20,973-1023. 

No explanation is provided by the electrostatic model for the different behavior of ions of equal size and equal charge the enthalpy of hydration is larger for Hg2 + than for Sr2 + and the hydrated Sr2+ is an extremely weak acid whereas the hydrated Hg2+ is a much stronger acid. 

Reversed-phase chromatography is often used to separate both neutral and ionic organic compounds. In this section, some important aspects for the understanding of the behavior of ionic compounds in reversed-phase chromatography are discussed. The important concepts introduced here are the electrical double layer and the electrostatic surface potential. It will be shown that they are essential for the understanding of the elution profile of ionic compounds. These concepts are further explored in the next section where theoretical models for ion-pair chromatography are discussed. 

There can be resonance between covalent and ionic states. In the molecule H H a complete shift of the electron pair to the left would have the effect to make the left-hand H atom a negative ion, leaving the right-hand one as a positive ion. Next to the state H H there will be two others, H H+ and H+H , which closely resemble the electrostatic model for the H2 molecule. Since the three states are resonating, the states H H+ and H+H will make a contribution to the bonding energy, too in this case, however, their contribution will be relatively small, because the energy of the covalent state H H certainly is much lower than that of the ionic states. H H+ and H+H-. 

The electrostatic attraction step can be examined using the Solution and Electrostatic (SE) model for ion adsorption.25 The model uses a thermodynamic approach based on crystal chemistry and interfacial solvation. The SE model was developed originally for oxide surface-small ion interactions,26 30 and cannot pretend to... 

A critical review of the history of the ion-pair concept indicates that considerations other than electrostatic were scarcely provided by model makers. Clearly, for chromatography, solvophobic interactions usually neglected by ion-pair model makers are aucial. Chromatographers involved in ion-pair strategies realized that the theoretical description of the ion-pair was critical at the time of the ion-pair chromatography introduction in the late 1970s and subsequently, the electrostatic model of ion association dominated scientific debate. 

J. Stahlberg, Electrostatic retention model for ion-exchange chromatography, Anal. Chem. 66 (1994), 440-449. 

Also the choice of the electrostatic model for the interpretation of primary surface charging plays a key role in the modeling of specific adsorption. It is generally believed that the specific adsorption occurs at the distance from the surface shorter than the closest approach of the ions of inert electrolyte. In this respect only the electric potential in the inner part of the interfacial region is used in the modeling of specific adsorption. The surface potential can be estimated from Nernst equation, but this approach was seldom used In studies of specific adsorption. Diffuse layer model offers one well defined electrostatic position for specific adsorption, namely the surface potential calculated in this model can be used as the potential experienced by specifically adsorbed ions. The Stern model and TLM offer two different electrostatic positions each, namely, the specific adsorption of ions can be assumed to occur at the surface or in the -plane. 

Saito, T. et al.. Electrostatic interaction models for ion binding to humic substances, Colloids Surf. A, 265, 104, 2005. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

Fig. 7. Electrostatic model for the interpretation of alkali metal ion — solvent dipole interactions b distances Z1-Z4 and Zi-Z a distance Z1-Z2...

The lattice energy can be estimated by assuming an electrostatic model for the solid state ionic lattice the ions are considered to be point charges. Later in this chapter, we consider to what extent this approximation is true. 

Figure 2 A schematic representation of the electrostatic potential as a function of the distance from the surface according to the Stern-Gouy-Chapman model. (Reprinted with permission from Stahiberg J (1999) Retention models for ions in chromatography (review). Journal of Chromatography A 855 3 Elsevier.)...

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