Ionic solvation continuum solvent models

In contrast, there will be many cases where continuum solvent models are less useful. These include situations where one of the goals of the simulation is to obtain a detailed picture of solvent structure, or where there is evidence that a particular structural feature of the solvent is playing a key role (for example, a specific water-macromolecule hydrogen bond). In these situations, however, explicit representation of some water combined with implicit solvation may suffice. Another example is when molecular dynamics simulations are used to study kinetic, or time-dependent phenomena. The absence of the frictional effects of solvent will lead to overestimation of rates. In addition, more subtle time-dependent effects arising from the solvent will be missing from continuum models. Continuum solvent models are in effect frilly adiabatic, in the sense that for any instantaneous macromolecular conformation, the solvent is taken to be completely relaxed. For electrostatic effects, this implies instantaneous dielectric and ionic double layer relaxation rates, and for the hydrophobic effect, instantaneous structural rearrangement. An exception would be dielectric models that involve a frequency-dependent dielectric. Nevertheless, continuum solvent models should be used with caution in studying the time dependence of macromolecular processes. 

In dilute electrolyte solutions ion-ion interaction as function of electrolyte concentration is fully explained by the Debye-Hiickel-Onsager theory and its further development. The contribution of ion solvation is noticed, if, for instance, the mobilities at infinite dilution of an ion in different solvent media or as function of ionic radii as considered. Up till now the calculation of that dependence has been only rather approximateAn improvement is quite probable, though, theoretically very involved if the solvent is not regarded as a continuum, but the number and arrangement of solvent molecules in the primary solvation shell of an ion is taken into consideration. Also the lifetime of molecules in the solvation shell must be known. Beyond this region a continuum model of ion-solvent interaction may be sufficient to account for the contributions of solvent molecules in the second or third sphere. 

In the early 50 s, an ion pair model was introduced by Winstein to rationalize the mechanism and stereochemistry of solvolysis of sulfonates72). This research of carbocationic intermediates and the role of ion solvation equilibrium in reaction mechanisms represents a landmark in the study of charged species. These thermodynamically different ionic species were coined as free ions, contact ion-pairs (c.i.p.), and solvent-separated ion pairs (s.s.i.p.). The ion pair situation can be described as an equilibrium between thermodynamically distinct contact (c.i.p.) and solvent-separated ion pairs (s.s.i.p.) 2-l3 16 The situation should be represented by a continuum of ion-solvation equilibria states in which the two extreme states are the c.i.p. and the s.s.i.p. 2 76) (Eq. 12)... 

In the gas phase the neutral form is energetically favored, while in condensed phases, such as solutions in polar solvents and crystals, the ionic form is more stable. Glycine and alanine are zwitterionic in water solutions and their description is a rather challenging task for continuum solvation models. In the following we summarize the results of a study we have published on Theochem on structures and energies of tautomers of glycine and alanine in the framework of the lEF version of the Polarizable Continuum Model. 

Solvation behavior can be effectively predicted using electronic structure methods coupled with solvation methods, for example, the combination of continuum solvation methods such as COSMO with DFT as implemented in DMoF of Accelrys Materials Studio. An attractive alternative is statistical-mechanical 3D-RISM-KH molecular theory of solvation that predicts, from the first principles, the solvation structure and thermodynamics of solvated macromolecules with full molecular detail at the level of molecular simulation. In particular, this is illustrated here on the adsorption of bitumen fragments on zeolite nanoparticles. Furthermore, we have shown that the self-consistent field combinations of the KS-DFT and the OFE method with 3D-RISM-KH can predict electronic and solvation structure, and properties of various macromolecules in solution in a wide range of solvent composition and thermodynamic conditions. This includes the electronic structure, geometry optimization, reaction modeling with transition states, spectroscopic properties, adsorption strength and arrangement, supramolecular self-assembly,"and other effects for macromolecular systems in pure solvents, solvent mixtures, electrolyte solutions, " ionic liquids, and simple and complex solvents confined in nanoporous materials. Currently, the self-consistent field KS-DFT/3D-RISM-KH multiscale method is available only in the ADF software. 

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