Specific interactions model

The main advantage of the specific interactions model lies in the simplicity of its calculations. Also, considering dissolved salts (in their neutral salt stoichiometry—e.g., NaCl) as components, activities and total activity coefficients are experimentally observable magnitudes. 

Whitfield M. (1975a). An improved specific interaction model for sea water at 25°C and 1 atmosphere total pressure. Mar. Chem., 3 197-213. 

Molecular Materials with Specific Interactions Modeling and Design... 

Runde et al. (2002a) compiled an internally consistent database to calculate solubility and speciation of plutonium in more complex low-ionic-strength waters. A specific interaction model (Grenthe et al., 1992) was used for ionic strength corrections. The reader is referred to that work for... 

Our discussion here is based on Bjemim s ion-association model. An alternative treatment of short-range interaction, the specific interaction model, will be discussed in Appendix 6.2 to this chapter. 

Next we turn to the evaluation of these expressions within specific interaction models. 

Baita, L. and Bradley, D. J., 1985, Extension of the specific interaction model to include gas solubfiities in high temperature brines. Geochim. et Cosmochim. Acta, 49 195-203. Bassett, R. L. and Melchior, D. C., 1990, Chemical modehng of aqueous systems an overview, In D. C. Melchior and R.L. Bassett, eds.. Chemical Modeling of Aqueous... 

Millero, F.J., "The Use of the Specific Interaction Model to Estimate the Partial Molal Volumes of Electrolytes in Sea Water", Geoch. Cosmo. Acta, 41, 215 (1977)... 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

It is only the contribution of AH to AG that we are discussing here, but we see the effect of this contribution-in the systems for which the approximation is valid-is that a solvent becomes less suitable to dissolve a polymer the greater the difference is between their 6 values. At best, when 61 = 62, the solvent effect is neutral. Cases for which a favorable specific interaction between solvent and polymer actually promotes solution are characterized by negative values of AH and are therefore beyond the capabilities of this model. 

Genetics and model building provide a framework for the interpretation of structural data, but they cannot reliably be used to predict the details of the stereochemistry of specific interactions within or between molecules. 

To conclude this section let us note that already, with this very simple model, we find a variety of behaviors. There is a clear effect of the asymmetry of the ions. We have obtained a simple description of the role of the major constituents of the phenomena—coulombic interaction, ideal entropy, and specific interaction. In the Lie group invariant (78) Coulombic attraction leads to the term -cr /2. Ideal entropy yields a contribution proportional to the kinetic pressure 2 g +g ) and the specific part yields a contribution which retains the bilinear form a g +a g g + a g. At high charge densities the asymptotic behavior is determined by the opposition of the coulombic and specific non-coulombic contributions. At low charge densities the entropic contribution is important and, in the case of a totally symmetric electrolyte, the effect of the specific non-coulombic interaction is cancelled so that the behavior of the system is determined by coulombic and entropic contributions. 

Calculate activation energies for the preferred addition mode of each reagent. (Data for borane, 9-BBN and cis-4-methylpent-2-ene are available.) Which reaction will be faster Is the faster reaction more or less regioselective than the slower reaction Compare the structures of the two transition states and identify specific interactions that can account for differences in regioselectivity and reactivity. Use space-filling models. 

Display space-filling models of endo adduct and exo adduct. Which appears to be the less crowded Identify specific interactions which disfavor the higher-energy adduct. Next, compare energies of the two adducts. Which is the more stable Were the reaction under thermodynamic control, which would be the major product and what would be the ratio of major to minor products Use equation (1). 

Nucleic acids are anionic under the neutral conditions. Thus, the syntheses of model compounds of the opposite charge are interesting for the discussion of electrostatic contributions in specific interactions of nucleic acids. We have tried to synthesize cationic models by the Menschutkin reaction of poly-4-vinylpyridine with 9-(2-chlo-roethyl)adenine, l-(2-chloroethyl)thymine, and 7-(2-chloroethyl)theophylline15,16 The obtained polymers are poly l-[2-(adenin-9-yl)ethyl]-4-pyridinioethylene chloride 7(APVP), poly l-[2-(thymin-l-yl)ethyl]-4-pyridinioethylene chloride 8 (TPVP), and poly l-[2-(theophyllin-7-yl)ethyl]-4-pyridiniothylene chloride 9 (THPVP), respectively. 

The simplest discrete approach is the solvaton method 65) which calculates above all the electrostatic interaction between the molecule and the solvent. The solvent is represented by a Active molecule built up from so-called solvatones. The most sophisticated discrete model is the supermolecule approach 661 in which the solvent molecules are included in the quantum chemical calculation as individual molecules. Here, information about the structure of the solvent cage and about the specific interactions between solvent and solute can be obtained. But this approach is connected with a great effort, because a lot of optimizations of geometry with ab initio calculations should be completed 67). A very simple supermolecule (CH3+ + 2 solvent molecules) was calculated with a semiempirical method in Ref.15). 

In Eq. (6) Ecav represents the energy necessary to create a cavity in the solvent continuum. Eel and Eydw depict the electrostatic and van-der-Waals interactions between solute and the solvent after the solute is brought into the cavity, respectively. The van-der-Waals interactions divide themselves into dispersion and repulsion interactions (Ed sp, Erep). Specific interactions between solute and solvent such as H-bridges and association can only be considered by additional assumptions because the solvent is characterized as a structureless and polarizable medium by macroscopic constants such as dielectric constant, surface tension and volume extension coefficient. The use of macroscopic physical constants in microscopic processes in progress is an approximation. Additional approximations are inherent to the continuum models since the choice of shape and size of the cavity is arbitrary. Entropic effects are considered neither in the continuum models nor in the supermolecule approximation. Despite these numerous approximations, continuum models were developed which produce suitabel estimations of solvation energies and effects (see Refs. 10-30 in 68)). 

Of special meaning for ionic reactions like cationic polymerization is the consideration of the interaction between reactants and solvent. This was attained by use of the extended solvent continuum model introduced by Huron and Claverie 69,70). Specific interactions between molecule and solvent cannot be taken into account by this model. For the above reason, the solvent is not considered to be an interacting partner, rather as a factor influencing the reacting species (see part 2.3.4). 

As a result of their size and of specific interactions, hydrophilic macromolecules or solid nanoparticles cause strong changes in micellar size and dynamics, and their structural and dynamic properties are strongly affected. In these cases, the distribution among reversed micelles can be only described by ad hoc models [13,123]. 

With different pectins, one found that the activity coefficient of calcium has a value half that of magnesium this is interpreted as the basis of a dimer formation in presence of calcium. The specific interaction of calcium was described as the egg-box model first proposed for polyguluronate in which oxygen atoms coordinated to calcium [46]. Recently, the comparative behaviour of Mg and Ca with homogalacturonan was reexamined [47]. 

A CRO may also allow for the in-house introduction of specialized lipophilic scales by transferring routine measurements. While the octanol-water scale is widely applied, it may be advantageous to utilize alternative scales for specific QSAR models. Solvent systems such as alkane or chloroform and biomimetic stationary phases on HPLC columns have both been advocated. Seydel [65] recently reviewed the suitabihty of various systems to describe partitioning into membranes. Through several examples, he concludes that drug-membrane interaction as it relates to transport, distribution and efficacy cannot be well characterized by partition coefficients in bulk solvents alone, including octanol. However, octanol-water partition coefficients will persist in valuable databases and decades of QSAR studies. 

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