Solvation process, simulation

The solvation processes for the anion systems were simulated using both simple ball models and more representational models of the solvent and solute [14,22]. 

In this work we presented the results of Molecular Dynamics simulations performed to study the solvatochromism and the dynamic stokes-shift of coumarin 153 in mixtures of solvents. We showed the ability of MD to reproduce available data of the time-dependent Stokes-shifts. Moreover, MD allowed us to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin or rotation of acetonitrile. The role of benzene in the solvation process of Cl53 seems to be more important than usually assumed. 

Such a time scale separation between system and bath may often be appropriate when dealing with intramolecular vibrational motions of molecules but is likely never appropriate for electronic transitions in solution near room temperature. In the past 10 years much effort has been devoted to dynamical aspects of the solvation process in polar liquids utilizing experiments [2-4], theory [5, 6], and computer simulations of molecular dynamics [7-10]. The... 

Recently, several authors have studied solvation dynamics of aqueous solutions using molecular dynamics (MD) computer simulations [36, 57, 58, 112], The simulations offer a detailed molecular approach to interpreting the experimental results, as they focus particularly on the microscopic, molecular aspects of the solvation process. 

Although experimental and theoretical works have been successful for studying phenomena of this type, however, for example the experimental methods cannot reveal the detailed solvation structures to describe the interaction between solvent and polymer. Theoretical methods are also either not completely atomistic or they assume a certain molecular behavior. Molecular simulation methods, on the other hand, can produce most atomistic information about the solvation process. 

In this chapter we will mostly focus on the application of molecular dynamics simulation technique to understand solvation process in polymers. The organization of this chapter is as follow. In the first few sections the thermodynamics and statistical mechanics of solvation are introduced. In this regards, Flory s theory of polymer solutions has been compared with the classical solution methods for interpretation of experimental data. Very dilute solution of gases in polymers and the methods of calculation of chemical potentials, and hence calculation of Henry s law constants and sorption isotherms of gases in polymers are discussed in Section 11.6.1. The solution of polymers in solvents, solvent effect on equilibrium and dynamics of polymer-size change in solutions, and the solvation structures are described, with the main emphasis on molecular dynamics simulation method to obtain understanding of solvation of nonpolar polymers in nonpolar solvents and that of polar polymers in polar solvents, in Section 11.6.2. Finally, the dynamics of solvation with a short review of the experimental, theoretical, and simulation methods are explained in Section 11.7. 

The solvated electron is a transient chemical species which exists in many solvents. The domain of existence of the solvated electron starts with the solvation time of the precursor and ends with the time required to complete reactions with other molecules or ions present in the medium. Due to the importance of water in physics, chemistry and biochemistry, the solvated electron in water has attracted much interest in order to determine its structure and excited states. The solvated electrons in other solvents are less quantitatively known, and much remains to be done, particularly with the theory. Likewise, although ultrafast dynamics of the excess electron in liquid water and in a few alcohols have been extensively studied over the past two decades, many questions concerning the mechanisms of localization, thermalization, and solvation of the electron still remain. Indeed, most interpretations of those dynamics correspond to phenomenological and macroscopic approaches leading to many kinetic schemes but providing little insight into microscopic and structural aspects of the electron dynamics. Such information can only be obtained by comparisons between experiments and theoretical models. For that, developments of quantum and molecular dynamics simulations are necessary to get a more detailed picture of the electron solvation process and to unravel the structure of the solvated electron in many solvents. 

The time dependent solvation funetion S(t) is a directly observed quantity as well as a convenient tool for numerical simulation studies. The corresponding linear response approximation C(t) is also easily eomputed from numerical simulations, and can also be studied using suitable theoretical models. Computer simulations are very valuable both in exploring the validity of such theoretical calculations, as well as the validity of linear response theory itself (by comparing S(t) to C(t)). Furthermore they can be used for direct visualization of the solute and solvent motions that dominate the solvation process. Many such simulations were published in the past decade, using different models for solvents such as water, alcohols and acetonitrile. Two remarkable outcomes of these studies are first, the close qualitative similarity between the time evolution of solvation in different simple solvents, and second, the marked deviation from the simple exponential relaxation predicted by the Debye relaxation model (cf Eq. [4.3.18]). At least two distinct relaxation modes are... 

Finally, it is intuitively clear that in large molecule complex solvents simple molecular rotation as seen in Figure 4.3.6 can not be the principal mode of solvation. Numerical simulations with polyether solvents show that instead, hindered intramolecular rotations that distort the molecular structure so as to bring more solvating sites into contact with the ion dominate the solvation dynamics. The bi-modal, and in fact multi-modal, character of the solvation is maintained also in such solvents, but it appears that the short time component of this solvation process is no longer inertial as in the simple small molecule solvents."... 

Numerical simulations have also been instrumental in elucidating the differences between simple and complex solvents in the way they dynamically respond to a newly created charge distribution. The importance of translational motions that change the composition or structure near the solute, the consequent early failure of linear response theory in such systems, and the possible involvement of solvent intramolecular motions in the solvation process were discovered in this way. 

---