Solvents bulk properties

Colloidal particles can be seen as large, model atoms . In what follows we assume that particles with a typical radius <3 = lOO nm are studied, about lO times as large as atoms. Usually, the solvent is considered to be a homogeneous medium, characterized by bulk properties such as the density p and dielectric constant t. A full statistical mechanical description of the system would involve all colloid and solvent degrees of freedom, which tend to be intractable. Instead, the potential of mean force, V, is used, in which the interactions between colloidal particles are averaged over... 

On one hand, there are the dielectric properties, which are especially important for polai solvents like water. Bulk properties can, on the other hand, only be modeled by using a supermolecule approach with explicitly defined solvent molecules. 

The explicit definition of water molecules seems to be the best way to represent the bulk properties of the solvent correctly. If only a thin layer of explicitly defined solvent molecules is used (due to hmited computational resources), difficulties may rise to reproduce the bulk behavior of water, especially near the border with the vacuum. Even with the definition of a full solvent environment the results depend on the model used for this purpose. In the relative simple case of TIP3P and SPC, which are widely and successfully used, the atoms of the water molecule have fixed charges and fixed relative orientation. Even without internal motions and the charge polarization ability, TIP3P reproduces the bulk properties of water quite well. For a further discussion of other available solvent models, readers are referred to Chapter VII, Section 1.3.2 of the Handbook. Unfortunately, the more sophisticated the water models are (to reproduce the physical properties and thermodynamics of this outstanding solvent correctly), the more impractical they are for being used within molecular dynamics simulations. 

Another classification of detector is the bulk-property detector, one that measures a change in some overall property of the system of mobile phase plus sample. The most commonly used bulk-property detector is the refractive-index (RI) detector. The RI detector, the closest thing to a universal detector in lc, monitors the difference between the refractive index of the effluent from the column and pure solvent. These detectors are not very good for detection of materials at low concentrations. Moreover, they are sensitive to fluctuations in temperature. 

Ultimately physical theories should be expressed in quantitative terms for testing and use, but because of the eomplexity of liquid systems this can only be accomplished by making severe approximations. For example, it is often neeessary to treat the solvent as a continuous homogeneous medium eharaeterized by bulk properties such as dielectric constant and density, whereas we know that the solvent is a molecular assemblage with short-range structure. This is the basis of the current inability of physical theories to account satisfactorily for the full scope of solvent effects on rates, although they certainly can provide valuable insights and they undoubtedly capture some of the essential features and even cause-effect relationships in solution kinetics. Section 8.3 discusses physical theories in more detail. 

Table 8-2 lists several physical properties pertinent to our concern with the effects of solvents on rates for 40 common solvents. The dielectric constant e is a measure of the ability of the solvent to separate charges it is defined as the ratio of the electric permittivity of the solvent to the permittivity of the vacuum. (Because physicists use the symbol e for permittivity, some authors use D for dielectric constant.) Evidently e is dimensionless. The dielectric constant is the property most often associated with the polarity of a solvent in Table 8-2 the solvents are listed in order of increasing dielectric constant, and it is evident that, with a few exceptions, this ranking accords fairly well with chemical intuition. The dielectric constant is a bulk property. 

The bulk properties of mixed solvents, especially of binary solvent mixtures of water and organic solvents, are often needed. Many dielectric constant measurements have been made on such binary mixtures. The surface tension of aqueous binary mixtures can be quantitatively related to composition. ... 

Bonhote and co-workers [10] reported that ILs containing triflate, perfluorocar-boxylate, and bistrifylimide anions were miscible with liquids of medium to high dielectric constant (e), including short-chain alcohols, ketones, dichloromethane, and THF, while being immiscible with low dielectric constant materials such as alkanes, dioxane, toluene, and diethyl ether. It was noted that ethyl acetate (e = 6.04) is miscible with the less-polar bistrifylimide and triflate ILs, and only partially miscible with more polar ILs containing carboxylate anions. Brennecke [15] has described miscibility measurements for a series of organic solvents with ILs with complementary results based on bulk properties. 

In the tradition of previous reviews [1-22], this section addresses various aspects of nonaqueous electrolytes, including intrinsic properties, such as local structures caused by ion-ion and ion-solvent interactions and bulk properties, such as ionic conductivity, viscosity, and electrochemical stability (voltage window), and their relationships to intrinsic properties. 

Solvents can be classified [15] according to bulk properties, such as... 

In the last two decades experimental evidence has been gathered showing that the intrinsic properties of the electrolytes determine both bulk properties of the solution and the reactivity of the solutes at the electrodes. Examples covering various aspects of this field are given in Ref. [16]. Intrinsic properties may be described with the help of local structures caused by ion-ion, ion-solvent, and solvent-solvent interactions. An efficient description of the properties of electrolyte solutions up to salt concentrations significantly larger than 1 mol kg 1 is based on the chemical model of electrolytes. 

The use of SCFs as solvents influences the reacting system because it is possible to dramatically change the density of the fluid with small perturbations of temperature and pressure and, in such a way, greatly affect the density-dependent bulk properties such as the dielectric constant, solubility and diffu-sibility of these compressible fluids. 

The mechanical properties of tyrosine-derived poly(iminocarbon-ates) were investigated using the procedures described in ASTM standard D882-83 (Table 2). Solvent-cast, thin polymer films were prepared, cut into the required shape, and tested in an Instron stress strain tester. Since the films were unoriented, noncrystalUne samples, the results are representative of the bulk properties of the polymers. In order to put these results into perspective, several commercial polymers were tested under identical conditions. In addition, some literature values were included in Table 2. 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

So far, very few attempts at improving ion conductivity have been realized via the salt approach, because the choice of anions suitable for lithium electrolyte solute is limited. Instead, solvent composition tailoring has been the main tool for manipulating electrolyte ion conductivity due to the availability of a vast number of candidate solvents. Considerable knowledge has been accumulated on the correlation between solvent properties and ion conductivity, and the most important are the two bulk properties of the solvents, dielectric constant e and viscosity rj, which determine the charge carrier number n and ion mobility (w ), respectively. 

The above cell performance order is almost the reversal of that for the low-temperature ion conductivity, which clearly shows the decoupling of interfacial properties from bulk properties. Thus, Smart et al. concluded that the nature of the SEI instead of bulk ion conductivity plays the key role when new solvents are selected. 

Spectroscopic measurements may, in certain cases, yield direct information on these interactions. On the other hand, thermodynamic values, obtained by measuring certain bulk properties of the system, require the aid of statistical mechanical methods to be related to specific interactions between the solute and the solvent. However, the thermodynamic aspects of the solute-solvent interactions reflect the preference of the solute for one solvent over another and, thereby, determine distribution of the solute in a solvent extraction system. 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Together with the relative permittivity, that is responsible for the number of charge carriers per unit volume of the solution as seen above, the solvent viscosity, r, (see Table 3.9) must also be mentioned among the bulk properties that are responsible for the differences of the conductivities of electrolyes in... 

In contrast with discrete methods, the thermal average is introduced in the continuum approach at the beginning of the procedure. Computer information on the distribution functions and related properties could be used (and in some cases are actually used), but in the standard formulation the input data only include macroscopic experimental bulk properties, supplemented by geometric molecular information. The physics of the system permits the use of this approximation. In fact the bulk properties of the solvent are slightly perturbed by the inclusion of one solute molecule. The deviations from the bulk properties (which become more important as the mole ratio increases) are small and can be considered at a further stage of the development of the model. 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

---