Solute-Solvent System

Prediction of pKaS of Titratable Residues in Proteins Using a Poisson-Boltzmann Model of the Solute-Solvent System... 

The effects of a solvent on growth rates have been attributed to two sets of factors (28) one has to do with the effects of solvent on mass transfer of the solute through adjustments in viscosity, density, and diffusivity the second is concerned with the stmcture of the interface between crystal and solvent. The analysis (28) concludes that a solute-solvent system that has a high solubiUty is likely to produce a rough interface and, concomitandy, large crystal growth rates. 

Solute/Solvent Systems The gamma/phi approach to X T.E calculations presumes knowledge of the vapor pressure of each species at the temperature of interest. For certain binary systems species I, designated the solute, is either unstable at the system temperature or is supercritical (T > L). Its vapor pressure cannot be measured, and its fugacity as a pure liquid at the system temperature/i cannot be calculated by Eq. (4-281). 

Whatever method is used to provide an adequate supply of diffusing metal, the diffusion mechanism remains identical for any given solute/solvent system, and it is appropriate to discuss the diffusion aspect in the first place. 

In the previous chapter we considered a rather simple solvent model, treating each solvent molecule as a Langevin-type dipole. Although this model represents the key solvent effects, it is important to examine more realistic models that include explicitly all the solvent atoms. In principle, we should adopt a model where both the solvent and the solute atoms are treated quantum mechanically. Such a model, however, is entirely impractical for studying large molecules in solution. Furthermore, we are interested here in the effect of the solvent on the solute potential surface and not in quantum mechanical effects of the pure solvent. Fortunately, the contributions to the Born-Oppenheimer potential surface that describe the solvent-solvent and solute-solvent interactions can be approximated by some type of analytical potential functions (rather than by the actual solution of the Schrodinger equation for the entire solute-solvent system). For example, the simplest way to describe the potential surface of a collection of water molecules is to represent it as a sum of two-body interactions (the interac-... 

Exercise 3.6. Write a program that evaluates ex, e2, and Hl2 for the SN2 reaction ClCl-Cl- - Cl-C Cl-, neglecting the hydrogens on the carbon so that the 6 term in eq. (3.26) is not needed. Also, neglect the Uind term. Next, surround this solute system by 20 dipoles and simulate the resulting solute + solvent system with the potential U = s 1 (examine the distances between the three atoms during the simulation). 

Vistoli, G., Pedretti, A., Villa, L, Testa, B. The solute-solvent system solvent constraints on the conformational dynamics of acetylcholine, f Am. Chem. Soc. 2002, 124, 7472-7480. 

As well known, the electronic spectral bands show shifts as a whole in solvents of different nature. This phenomenon called solvatochromism is directly connected with the intermolecular interactions in the solute-solvent system. 

The spectroscopic properties of the solute-solvent system accounting fluctuations of solution structures can be analyzed using a simple model that includes a fluorescent solute and its immediate surrounding contributing to full potential of... 

Electronic transitions in a solute take place very fast, i.e., almost immediately in comparison with the movement of the molecules as a whole and vibrations of atoms in organic molecules. Hence, absorption and fluorescence can be denoted in Fig. 5 by vertical arrows, in accordance with Franck-Condon principle. Both these processes are separated by relaxations, which are intermolecular rearrangements of the solute-solvent system after the excitation. 

The solute-solvent system, from the physical point of view, is nothing but a system that can be decomposed in a determined collection of electrons and nuclei. In the many-body representation, in principle, solving the global time-dependent Schrodinger equation with appropriate boundary conditions would yield a complete description for all measurable properties [47], This equation requires a definition of the total Hamiltonian in coordinate representation H(r,X), where r is the position vector operator for all electrons in the sample, and X is the position vector operator of the nuclei. In molecular quantum mechanics, as it is used in this section, H(r,X) is the Coulomb Hamiltonian[46]. The global wave function A(r,X,t) is obtained as a solution of the equation ... 

The treatment of the solute-solvent system with the classical Generalized Langevin equation formalism [127], with especial attention to the present problem, has been examined by us [6] a wealth of information can be found in references [128-131],... 

The solute-solvent system is coupled via solvent operators (b+bf)k so that the equation of motion for the solvent operator is to be solved first. Using the commutation relations one gets for the linear term components the equation ... 

Quantitative models of solute-solvent systems are often divided into two broad classes, depending upon whether the solvent is treated as being composed of discrete molecules or as a continuum. Molecular dynamics and Monte Carlo simulations are examples of the former 8"11 the interaction of a solute molecule with each of hundreds or sometimes even thousands of solvent molecules is explicitly taken into account, over a lengthy series of steps. This clearly puts a considerable demand upon computer resources. The different continuum models,11"16 which have evolved from the work of Bom,17 Bell,18 Kirkwood,19 and Onsager20 in the pre-computer era, view the solvent as a continuous, polarizable isotropic medium in which the solute molecule is contained within a cavity. The division into discrete and continuum models is of course not a rigorous one there are many variants that combine elements of both. For example, the solute molecule might be surrounded by a first solvation shell with the constituents of which it interacts explicitly, while beyond this is the continuum solvent.16... 

The free energy of solvation is a key property of solute-solvent systems, since it quantifies the tendency of the solute to enter into solution. We have addressed this for aqueous solvation via the GIPF approach on several occasions.46 48 Our best correlation is Eq. (10), obtained using density functional B3P86/6-31+G Vs(r) 46... 

The inter/intramolecular potentials that have been described may be viewed as classical in nature. An alternative is a hybrid quantum-mechanical/classical approach, in which the solute molecule is treated quantum-mechanically, but interactions involving the solvent are handled classically. Such methods are often labeled QM/MM, the MM reflecting the fact that classical force fields are utilized in molecular mechanics. An effective Hamiltonian Hefl is written for the entire solute/solvent system ... 

Once the energy of the solute/solvent system has been determined, using some variant of a classical or a QM/MM force field, it can be used to find the free energy of solvation, which is the property of real interest. This is given, at constant temperature and pressure or at constant temperature and volume, respectively, by Eqs. (20) and (21) ... 

When a solute particle is introduced into a liquid, it interacts with the solvent particles in its environment. The totality of these interactions is called the solvation of the solute in the particular solvent. When the solvent happens to be water, the term used is hydration. The solvation process has certain consequences pertaining to the energy, the volume, the fluidity, the electrical conductivity, and the spectroscopic properties of the solute-solvent system. The apparent molar properties of the solute ascribe to the solute itself the entire change in the properties of the system that occur when 1 mol of solute is added to an infinite amount of solution of specified composition. The solvent is treated in the calculation of the apparent molar quantities of the solute as if it had the properties of the pure solvent, present at its nominal amount in the solution. The magnirndes of quantities, such as the apparent molar volume or heat content, do convey some information on the system. However, it must be realized that both the solute and the solvent are affected by the solvation process, and more useful information is gained when the changes occurring in both are taken into account. 

F. Lewiner, J.P. Klein, F. Puel and G. Fevotte, On-hne ATR-FTIR measurement of supersaturation during solution crystallization processes. Calibrations and applications on three solute/solvent systems, Chem. Eng. Sci., 56, 2069-2084 (2001). 

There is an additional layer of complexity that increases the coupling further. As described above, the universe is external to the solute/solvent system. Perhaps the universe represents some collective normal mode(s) of the solute/solvent system, whose response is separable (or approximately so) from the local solvation of the solvent to the solute. In particular, this occurs when the solute/solvent system contains a large number of solute particles whose properties change as a function of their individual dynamics. In the limit of high enough solute concentrations, the collective (macroscopic) change of these solutes thus leads to a change in the solvation for each of them individually. 

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