Solvation, preferential

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

The nitrenium ion +NH2 has been the subject of a detailed, comprehensive calculation. Calculations on (48) with 15 different X substituents reveal a large substituent sensitivity, and also that aqueous solvation preferentially stabilizes the singlet state. This substiment sensitivity agrees with the results of a time-resolved IR study of the diphenylnitrenium ion (49), which shows that resonance contributors such as (50) and (51) are very important to the overall structure. Substituted 4-biphenyl nitrenium ions... 

A quantitative measure of interaction between polymer and monomer in solution can be expressed by a value of preferential solvation. Preferential solvation can be calculated from the measurements of refractive index increments in dialysis experiments. This experiment can be illustrated as in Figure 3.1. 

Single-ion conductors can be obtained by the intercalation of PEO on clay due to the presence of cation charge at the silicate surface. The conductivity values of electrolytes based on POEM with the addition of 2 and 5 wt% clay were found to be around 4 x 10 S/cm at 70 °CF The conductivity obtained can be anisotropic. Molecular dynamic simulation has shown that the Li" ions are solvated preferentially by the silicate oxygen atom rather than PEO. The conductivity is too low for practical applications, even with a cationic transference number equal to one. In order to increase conductivity, but with a cationic transference number different from one, lithium salts were added to PEO/clay nanocomposites. At room temperature, the nanocomposite electrolyte exhibited higher ionic conductivity than unfilled polymer due to the larger content of the PEO amorphous phase. The improvement in conductivity depends on the nature of the clay. Fan et al. have shown that 250-Li-MMT, i.e. Li-MMT heated to 250°C, was more effective in enhancing the conductivity of (PE0)i6LiC104 than Org-MMT, dodecylamine modified Li-MMT, and Li-MMT, since 250-Li-MMT forms an exfoliated structure in the PEO matrix. 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

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). 

It is also of significance that in the dilute gas phase, where the intrinsic orientating properties of pyrrole can be examined without the complication of variable phenomena such as solvation, ion-pairing and catalyst attendant on electrophilic substitution reactions in solution, preferential /3-attack on pyrrole occurs. In gas phase t-butylation, the relative order of reactivity at /3-carbon, a-carbon and nitrogen is 10.3 3.0 1.0 (81CC1177). 

Fig. 4.12. Potential energy liagrams showing effect of preferential solvation of transition state (a) and ground state (b) on the activation energy.

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

Another example of preferential solvation is provided by the inhibition by dioxane of the aiyl alkylation of phenol by r-butyl chloride. ... 

The signal from H-4 in the parent compounds underwent a greater downfield shift than that from H-2 when the solvent polarity was increased (CDCI3 MeaCO DMSO). This was ascribed to the counterbalancing effect of preferential solvation at the N-1 position. 

Solubility depends on the nature of the IL and on solvation or complex formation. Most metal ions display preferential partitioning into water in IL aqueous systems and are hence less soluble in the IL than in water. 

If S[ is a weak base (low DN) and S2 is a strong base, cations are selectively solvated by S2 conversely, if Sj is a good acceptor (high AN) it will preferentially solvate the anion. As a consequence, the Stokes radii of ions generally change with the composition of a binary solvent. 

The main problem in the study of the role of these parameters in electrolyte conductivity is their interdependence. A change in composition of a binary solvent changes viscosity, along with the permittivity, ion-ion association, and ion solvation, which may be preferential for one of the two solvents and therefore also changes the Stokes radii of the ions. 

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. 

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