Solvent solute collisions

Collisions in solution are diffusion-controlled and hence depend on the viscosity of the solvent. Due to Franck-Rabinowitch cage effect they occur in sets called encounters . 

Becaus-e of the similarity in the relations for osmotic pressure in dilute solutions and the equation for an ideal gas, van t Hoff proposed his bombardment theory in which osmotic pressure is considered in terms of collisions of solute molecules oil the semipeniieable membrane. This theoiy has a number of objections and has now been discarded. Other theories have also been put forward involving solvent bombardment on the semipermeable membrane, and vapor pressure effects. For example, osmotic pressure has been considered as the negative pressure which must be applied to the solvent to reduce its vapor pressure to that of the solution. It is, however, more profitable to interpret osmotic pressures using thermodynamic relations, such as the entropy of dilution,... 

The isolated binary collision (IBC) theory is one of the earliest dephasing theories developed for liquids (77). Despite criticism of its assumptions (78), it remains popular for the interpretation of experimental data, because it relates the dephasing to easily measurable solvent properties. It assumes that the solvent dynamics are essentially gas-like, consisting of uncorrelated collisions of the vibrating molecule with solvent molecules. The solvent-solute interaction is taken as purely repulsive. The dependence of... 

Normally, if you want to dissolve a compound, you first select a solvent that has a similar polarity (like dissolves like) then you try to get as much contact (large surface area) as possible between the molecules of solvent and the compound to be dissolved and heat the solvent to further increase the number of collisions. One reason liquids are preferred to gases as solvents is that a high concentration of molecules attacks the surface of the compound to be dissolved, and, therefore, more solvent-solute bonds can be formed to compete with the solute-solute bonds of the solid or liquid. The difficulty with liquid solvents is that once the compound has dissolved, it must now be separated from the solvent. If the compound would dissolve in a gas, then separating the solvent from the solute would be much easier. However, the number of molecules of gas attacking the surface of the compound is quite small compared to that of a liquid, and for that reason, few solids dissolve in gases. What must be done is to make a gas behave like a liquid. If you could compress a gas so it would approach the density of a liquid, then you should get increased solubility and be able to evaporate the gas once the compound was extracted. Under normal conditions, if you compress a gas too much, it may collapse into a liquid. 

Internal conversion is a pseudo-first-order process by which the singlet excited state Sb energy is lost by collisions with solvent molecules or else by transfer between vibrational modes. Inevitably, the rate he will increase with increasing temperature and vice versa. Quenching is a similar deactivation process, in which collision with solute molecules leads to loss in singlet... 

Solvation occurs only when the solute and solvent particles come in contact with each other. There are three common ways, shown in Figure 14.13, to increase the collisions between solute and solvent particles and thus increase the rate at which the solute dissolves agitation, increasing the surface area of the solute, and increasing the temperature of the solvent. 

Agitation Stirring or shaking—agitation of the mixture—moves dissolved solute particles away from the contact surfaces more quickly and thereby allows new collisions between solute and solvent particles to occur. Without agitation, solvated particles move away from the contact areas slowly. 

A fluorescence emission spectrum is a plot of fluorescence intensity versus wavelength (nm) or wavenumber (cm ). Emission spectra vary widely from fluor to fluor and are dependent on chemical structure and environmental conditions, e. g., pH, buffer components, solvent polarity, and dissolved oxygen. A number of processes are involved in fluorescence emission which can have an effect on the fluorescence characteristics of a fluorophore. These include collisions with quenchers, rotational and translational diffusion, and complex formation with solvent/solute. Fluorescent molecules absorb... 

The density of molecules is substantially higher in liquids than in the gas phase. However, for reactions carried out in solution under relatively dilute conditions, the concentrations of reactants are not appreciably different from in tlie gas phase. Since reactant molecules A and B must undergo collision in solution in order to react, many of the same principles developed for gas-phase reactions also apply in solution. However, the presence of solvent molecules leads to important differences between reactions in solution and in the gas phase. In solution, the rate of diffusion often limits the rate of approach of molecule A to within a sufficient distance to B for reaction to occur. Once an encounter pair AB is formed, however, the solvent may act as a cage, effectively holding them in close proximity, thereby increasing the probability of reaction. 

Heating the solvent— solvent molecules move faster and have more frequent collisions with solute at higher temperatures. 

Ues is a potential of mean force of the solvent molecules including the explicit intra- and inter-molecular interactions of the solute. Zj is a stochastic force simulating collisions of solute site i with the solvent molecules. It is assumed that Zj is uncorrelated with the positions and the velocity of the sites i. Moreover Zj has a Gaussian distribution centered on zero with variance (Zj(t) Zj t )) = 6mt ikBTBS t — t )8ij, where Tb is the temperature of the solvent, and f is a fl ic-tion parameter. In practice the components a of Zj are extracted from a Gaussian distribution with (Zj,a(t) = 2mjCj B B/Ar, where Af is the integration time step. We may solve Eq. (16)... 

Absorption bands in solution are less structured and broader than those in the gas phase. In solution, molecular rotation is hindered through collision with solvent molecules such that rotational quantisation is lost and solvent-solute interactions broaden vibrational bands further. The magnimde of the latter effect depends on the strength of the solute-solvent interaction and it is therefore most pronounced in polar solvents, so that spectra in a non-polar solvent such as hexane will generally show more distinct vibrational strucmre than those recorded in a more polar solvent such as acetone or an alcohol. While molecular absorption bands in solution may be very broad, there is almost always some electronic/vibrational structure and usually a number of electronic absorption bands can be identified. 

The physical reasons why the tracer and the binary coefficients are different can most easily by seen for the case of a dilute gas mixture of a tagged solute 1, an untagged solute 2, and a solvent 3. Diffusion in this system is described in terms of solute-solvent collisions and solute-solute collisions. Solute-solvent collisions are characterized by collision diameters ctib and 13. Solute-solute collisions are described by 0-12 and 12. With these diameters and energies, the binary diffusion coefficient can be shown from Table 7.1-1 to be a function only of solute-solvent collisions ... 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

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