Solvate system, preferential

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

Very little work has been done in this area. Even electrolyte transport has not been well characterized for multicomponent electrolyte systems. Multicomponent electrochemical transport theory [36] has not been applied to transport in lithium-ion electrolytes, even though these electrolytes consist of a blend of solvents. It is easy to imagine that ions are preferentially solvated and ion transport causes changes in solvent composition near the electrodes. Still, even the most sophisticated mathematical models [37] model transport as a binary salt. 

If the system separates, it can be extended to a model for the interface between two solutions by introducing ions. In the basic case the system contains a salt composed of cations and anions which is preferentially solvated by the solvent 5], but badly solvable in solution 2, and a salt K2A2 that is preferentially dissolved in solvent 2. This can be achieved by choosing suitable interaction parameters between the ions and the two solvents. 

Fe(5N02phen)3] + aquation in ternary water-Bu OH-polyethyleneglycol (PEG400). " Kinetic patterns for systems of these types have been the subject of theoretical analyses, as in the application of the Savage-Wood Group Additivity principle to [Fe(5N02phen)3f" " aquation in a variety of water-rich binary aqueous mixtures and in aqueous salt solutions " and in the Kirkwood-Buff treatment of preferential solvation of initial and transition states for [Fe(phen)3] + aquation in methanol-water " and for [Fe(gmi)3] " " aquation in Bu OH + water. " ... 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

Evidently, any overlooked enthalpic interaction present in the system may substantially affect the calculated results because the measured retention volumes are shifted. The already mentioned system peaks caused by preferential solvation of polymer sample (Sections 16.2.2, 16.3.2, and 16.4.3),... 

Another type of nonideal SEC behavior, which will not be covered in this chapter, is related to the use of mixed mobile phases (multiple solvents). Because solute-solvent interactions play a critical role in controlling the hydrodynamic volume of a macromolecule, the use of mixed mobile phases may lead to deviations from ideal behavior. Depending on the solubility parameter differences of the solvents and the solubility parameter of the packing, the mobile phase composition within the pores of the packing may be different from that in the interstitial volume. As a result, the hydrodynamic volume of the polymer may change when it enters the packing leading to unexpected elution results. Preferential solvation of the polymer in mixed solvent systems may also lead to deviations from ideal behavior (11). 

A method of prediction of the salt effect of vapor-liquid equilibrium relationships in the methanol-ethyl acetate-calcium chloride system at atmospheric pressure is described. From the determined solubilities it is assumed that methanol forms a preferential solvate of CaCl296CH OH. The preferential solvation number was calculated from the observed values of the salt effect in 14 systems, as a result of which the solvation number showed a linear relationship with respect to the concentration of solvent. With the use of the linear relation the salt effect can be determined from the solvation number of pure solvent and the vapor-liquid equilibrium relations obtained without adding a salt. 

Figure 3. Preferential solvation model for MeOH-EtOAc-CaCl system at 1 atm...

The salt effect in the MeOH-EtOAc-CaC system can be explained by preferential solvation. As calcium chloride dissolves readily in methanol but only sparingly in ethyl acetate, it will be sufficient to consider the interaction between methanol molecules and calcium chloride molecules only in the MeOH-EtOAc solution. Referring again to Figure 2, the free methanol molecules which are not clustered with ethyl acetate increase linearly when the liquid-phase composition of methanol is above 0.333 in mole fraction. The solubility... 

The absolute value of (l/e Ae/Axi) is the greatest in the ethyl acetate-methanol system but becomes smaller in the ethanol-water system and methanol-water system, in that order. In solvent systems, the greater the value of the right hand side of Equation 1, the greater the value of X2A20 but smaHer the value of In other words, the preferential solvation due to methanol or... 

Table IV. Salt Effect Predicted from the Preferential Solvation for Methanol—Water—CaCl2 System at 25° C...

The salt effect is attributable to the formation of preferential solvation from the standpoint of molecular structure. In other words, when calcium chloride, which dissolves readily in methanol but very little in ethyl acetate, was added to the methanol-ethyl acetate system to saturation, calcium chloride formed with methanol the preferential solvate which may be written CaCl2 6CH30H. It was also shown from the observation of solubility that the solvated methanol molecules did not participate in the vapor-liquid equilibrium. 

Normalized plots of 6/6p against x for all available data for this solvent system are shown in Figure 3. It is immediately clear from the curvature of the lines in Figure 3 that Rb+, Cs+, and F are preferentially solvated by peroxide, that Li+ is preferentially solvated by water, and hardly any preferential solvation exists for Na+ and Cl . 

Table VII collects the results for all monovalent ion systems for which spectroscopic data are available. Studies of preferential solvation are still at a stage comparable to the establishment of Raoult s and Henry s laws for binary nonelectrolyte solutions. Correlation with thermodynamic data is encouraging for isodielectric solvent systems, but further consideration of the electrostatic terms necessary in the discussion of other systems is required. It is hoped that this present work, which coordinates, correlates, and advances progress made by other workers (7, 18,19, 20, 45, 46, 61, 62, 66, 67, 68), will stimulate systematic experimental investigations of suitable systems by both spectroscopic and thermodynamic methods.

Since anions are much less solvated in dipolar aprotic solvents (23) than in water, the hydrogen ion will be more highly solvated in the mixed solvent because it is preferentially solvated by monoglyme in the monoglyme-water mixtures rather than in the pure aqueous medium. The selective solvation is an important factor in an understanding of solute-solvent interactions in mixed solvent systems. Unfortunately, the detailed compositions of the primary solvation shell and the secondary mode of solvation (ion-dipole interaction) in mixed solvents are not yet clearly understood. 

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