Cation-anion pair, interaction coefficient

Table A.l Ion interaction coefficients for cation-anion pairs derived using the extended...

Self-diffusion coefficients of distinct mobile species measured using PFG-NMR are based on spectral selectivity. In the context of IL selfdiffusion, PFG-NMR measures the time-averaged (miUisecond timescale) diffusion coefficients. Since ion-pair (cation-anion) interactions take place on a timescale faster than that, the measured diffusion coefficients are a weighted average over charged and neutral species. This is the reason for the variation between conductivity calculated from diffiasion coefficients determined by the Nemst-Einstein equation [15] (Eq. 1) and conductivity measured using impedance analysis ... 

Most water-soluble metal complexes, unlike typical organic reactants, are cations or anions and are therefore subject to Coulombic ion-ion interactions in solution. In essence, these are of two kinds the Debye-Hiickel or ionic atmosphere type, which affects the activity coefficients of the complex ion and hence the kinetics of its reactions, and ion association—usually considered simply as anion-cation pair formation.29 For cationic substrates in particular, pairing with an anionic incoming ligand may give an illusion of bimolecularity (an SN2 mechanism) when in fact the reaction may be dissociatively activated within the ion pair or encounter complex . 

Once ions are included in water, it is crucial to employ the dielectrically consistent version of the RISM theory [11, 12]. The basic equations and the algorithm for numerically solving them are described in the Appendix. In the analysis, we choose acetylglycine ethyl ester (AGE) CH3CONHCH2COOCH2CH3 because the salting-out coefficients of this peptide in various salt solutions are experimentally available [53]. Besides, such a peptide serves as one of the basic models of proteins. The conformation is fixed at the all-trans form in our calculations. The SPC/E model is employed for water. The following two sets of salt solutions are considered to examine the effects of cations and anions on the solvation properties of AGE LiCl, NaCl, and KGl (set 1) for the cation effects, and NaCl, NaBr, and Nal (set 2) for the anion effects. The temperature and the salt concentration are fixed at 298K and IM, respectively. The number density and the dielectric constant of each salt solution, which are used as part of the input data in the dielectrically consistent version of the RISM theory, are taken from the experimental data. We adopt the Coulomb plus L-J potential functions for all the interactions between water-solute and ion-solute atomic pairs. That is, the site-site pair potential Uab r) is expressed as Eq.(3.17) (for the NaCl-solution, for example, 6=H,0,Na+,Cl ). The AMBER-type potential parameters are employed for the peptide. 

The first virial coefficient /(/) is some function of the ionic strength and is not 0 as it would be for an ideal solution, but is in fact a version of the Debye-Htickel equation, which represents departure from ideality in very dilute solutions. The following term is a function of the interactions of all pairs of ions, and the third term a function of the interactions of ions taken three at a time. The second coefficient. Ay, is a function of ionic strength, but the third coefficient ju-y - is considered to be independent of ionic strength and equals zero if /, j, and k are all anions or cations. Later extensions to the model published by Pitzer and co-workers allow for an ionic strength dependence to the third coefficient. Pitzer (1987) and Harvie and Weare (1980) note that higher virial coefficients are required only for extremely concentrated solutions, so the series is stopped at the third coefficient. 

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