Solvent effect viscosity

Compared with other metal nuclei, the Li chemical shift scale is rather small and the NMR signals of > Li encompass not more than ca 6 ppm for salt solutions and about 12 ppm for organolithium compounds. This can be attributed to the relatively small paramagnetic contribution to the shielding constant of Li which leads to a near cancellation of the diamagnetic term [46]. In addition, Li shifts are sensitive to solvent effects, viscosity. 

Solvent Neat viscosity Effective viscosity in hydrocarbon solvents Effective viscosity in aqueous blends... 

Bfect of Environment olubilization and Thermal History Effect of Low Molecular Weight Monomers onformational Changes and Solvent Effect Viscosity and Conformational Effects End-Group Analysis Free Volume Copolymer Architecture pPolymer Blends Ofects of Additives, Conformational Analysis 1 Properties chanical Properties Stability of Polymers... 

Together with this solvent effect, another effect, called phase soaking, occurs in the retention gap technique if a large volume of solvent vapour has saturated the carrier gas, the properties of the stationary phase can be altered by swelling (thicker apparent film), a change in the viscosity or changed polarity. The consequence is that the column shows an increased retention power, which can be used to better retain the most volatile components. 

There have been numerous studies on the kinetics of decomposition of A IRK. AIBMe and other dialkyldiazenes.46 Solvent effects on are small by conventional standards but, nonetheless, significant. Data for AIBMe is presented in Table 3.3. The data come from a variety of sources and can be seen to increase in the series where the solvent is aliphatic < ester (including MMA) < aromatic (including styrene) < alcohol. There is a factor of two difference between kA in methanol and k< in ethyl acetate. The value of kA for AIBN is also reported to be higher in aromatic than in hydrocarbon solvents and to increase with the dielectric constant of the medium.31 79 80 Tlic kA of AIBMe and AIBN show no direct correlation with solvent viscosity (see also 3.3.1.1.3), which is consistent with the reaction being irreversible (Le. no cage return). 

The reactivity of macromonomers in copolymerizalion is strongly dependent on the particular comonomer-macromonomer pair. Solvent effects and the viscosity of the polymerization medium can also be important. Propagation may become diffusion controlled such that the propagation rate constant and reactivity ratios depend on the molecular weight of the macromonomer and the viscosity or, more accurately, the free volume of the medium. 

The validity of the above conclusions rests on the reliability of theoretical predictions on excited state barriers as low as 1-2 kcal mol . Of course, this required as accurate an experimental check as possible with reference to both the solvent viscosity effects, completely disregarded by theory, and the dielectric solvent effects. As for the photoisomerization dynamics, the needed information was derived from measurements of fluorescence lifetimes (x) and quantum yields (dielectric constant, where extensive formation of ion pairs may occur [60], the observed photophysical properties are confidently referable to the unperturbed BMPC cation. Figure 6 shows the temperature dependence of the... 

Figure 7 shows the viscosity vs. concentration curves for PTF in various organic solvents. As would be expected, in toluene, the most polar among the four solvents, the viscosity was the lowest at any given concentration. Figure 8 shows the effect of concentration on the viscosity of C02 at 25°C. The observed 50% increase in viscosity is not sufficient for enhanced oil recovery operations. 

V, is the molar volume of polymer or solvent, as appropriate, and the concentration is in mass per unit volume. It can be seen from Equation (2.42) that the interaction term changes with the square of the polymer concentration but more importantly for our discussion is the implications of the value of x- When x = 0.5 we are left with the van t Hoff expression which describes the osmotic pressure of an ideal polymer solution. A sol vent/temperature condition that yields this result is known as the 0-condition. For example, the 0-temperature for poly(styrene) in cyclohexane is 311.5 K. At this temperature, the poly(styrene) molecule is at its closest to a random coil configuration because its conformation is unperturbed by specific solvent effects. If x is greater than 0.5 we have a poor solvent for our polymer and the coil will collapse. At x values less than 0.5 we have the polymer in a good solvent and the conformation will be expanded in order to pack as many solvent molecules around each chain segment as possible. A 0-condition is often used when determining the molecular weight of a polymer by measurement of the concentration dependence of viscosity, for example, but solution polymers are invariably used in better than 0-conditions. 

In Section 3.4, structural effects were often discussed in conjunction with the nature of the solvent. As emphasized in the introduction to this book, the fluorescence emitted by most molecules is indeed extremely sensitive to their microenvironment (see Figure 1.3), which explains the extensive use of fluorescent probes. The effects of solvent polarity, viscosity and acidity deserves much attention because these effects are the basis of fluorescence probing of these microenvironmental characteristics and so, later chapters of this book are devoted to these aspects. The effects of polarity and viscosity on fluorescence characteristics in fluid media and the relevant applications are presented in Chapters 7 and 8, respectively. The effect of acidity is discussed in Sections 4.5 and 10.2. This section is thus mainly devoted to rigid matrices or very viscous media, and gases. 

In most investigations in solvents of medium or high viscosity, or in polymers above the glass transition temperature, the fluorescence quantum yields were in fact found to be a power function of the bulk viscosity, with values of the exponent x less than 1 (e.g. for p-N,N-dimethylaminobenzylidenemalononitrile, x = 0.69 in glycerol and 0.43 in dimethylphthalate). This means that the effective viscosity probed by a molecular rotor appears to be less than the bulk viscosity >/ because of free volume effects. 

These results suggest that the crystallographic determination of the structure of a productive enzyme-substrate complex is feasible for lysozyme and oligosaccharide substrates. They also provide the information of pH, temperature, and solvent effects on activity which are necessary to choose the best conditions for crystal structure work. The system of choice for human lysozyme is mixed aqueous-organic solvents at -25°C, pH 4.7. Data gathered on the dielectric constant, viscosity, and pH behavior of mixed solvents (Douzou, 1974) enable these conditions to be achieved with precision. 

The observation that branches A and B in Fig. 6.25 merge at large Q is consistent with the predictions for and T since 6ti and 18.84 deviate from 16 by less than 15% and statistical errors of the experiment and systematic uncertainties in methods to extract the cumulant exceed this difference. In [325] for both the collective concentration fluctuations and the local Zimm modes the observed rates are too slow by a factor of 2 if compared to the predictions with T (the solvent viscosity) and (the correlation length) as obtained from the SANS data. It is suggested that this discrepancy may be removed by the introduction of an effective viscosity qf that replaces the plain solvent viscosity Finally at very low Q, i.e. 1, branch C should level at the centre of mass... 

Luminescence is often much more sensitive to molecular dynamics than other optical techniques where temperature, viscosity, pH and solvent effects can have a significant influence on the emission response. Analyte degradation for light sensitive fluors and photobleaching for static measurements also influence the emission signal. Because of the wide variety of potential matrix effects, a thorough investigation should be conducted or the sample matrix well understood in terms of its potential impact on emission response. A complete discussion on the fate of the excited states and other measurement risk considerations can be found elsewhere. ... 

The buzzword polarity, derived from the dielectric approach, is certainly the most popular word concerning solvent effects. (It is the basis for the famous rule of thumb similia similibus solventur, i.e., like dissolves like in English.) We should add like helps like. Let us compare toluene and n-hexane as solvents. At temperatures when both solvents have comparable viscosity, the reaction between 1,2,4,5-tetrafluorobenzene and its anion-radical proceeds in different ways (Werst 1993), depending on the solvent. In toluene, the reaction consists of electron exchange ... 

This study addresses two questions 1) Is polymer aggregation in solutions directly related to solvent quality 2) If not, does solvent quality exert an effect on the viscosity of semidilute solutions separate from the effect of aggregation The copolymer poly(vinylbutyral) (PVB) was chosen for this investigation. PVB is known to aggregate in several solvents (IS). Light scattering and intrinsic viscosity measurements were used to assess solvent quality. Viscosities were measured at one concentration in three solvents and temperatures from 25 to 55 C. 

Eluents used in reversed-phase chromatography with bonded nonpolar stationary phases are genei ly polar solvents or mixtures) of polar solvents, such as acetonitrile, with water. The properties of numerous neat solvents of interest, their sources, and their virtues in teversed-phase chromatography have been reviewed (128). Properties of pure solvents which may be of value as eluents are summiuized in Table. VII. The most significant properties are surface tension, dielectric constant, viscosity, and eluotropic value. Horvath e/ al. 107) adapted a theory of solvent effects to consider the role of the mobile phase in determinmg the absolute retention and the selectivity found in reversed-phase chromatography. 

Note, further, that this leveling off occurs at progressively lower potentials as the concentration of electrolyte increases. Increasing both the potential and the electrolyte concentration tends to increase the field in the double layer (see Table 12.1), which in turn increases the viscosity of solvent in the double layer. As the effective viscosity of the medium increases, the surface of shear occurs progressively further from the surface. This accounts for the fact that hs falls behind J/0 as [/0 increases. These conclusions are consistent with the experimental observation that HS for Agl becomes independent of the concentration of the potentialdetermining Ag + and I ions once the concentrations of these ions are well removed from the conditions at which the particles are uncharged. 

The unperturbed chain dimensions of near-monodisperse atactic PS are evaluated from intrinsic viscosity measurements. Negative values for the temperature coefficient of chain dimensions are found. Under conditions where specific solvent effects are eliminated or minimized, measurements yield results in excellent agreement with the theoretical predictions for atactic PS. 

Suzuki,H., Kotaka,T., Inagaki,H. Shear-rate dependence of the intrinsic viscosity of flexible linear macromolecules. II. Solvent effect. J. Chem. Phys. 51,1279-1285 (1969). 

As can be seen from the numbers, the exponent a is clearly a function of barrier frequency (cob) and its value is decreasing with increase in a>b- For cob — 2 x 1013 s-1, its value almost goes to zero (a < 0.05), which clearly indicates that beyond this frequency the barrier crossing rate is entirely decoupled from solvent viscosity so that one recovers the well-known TST result that neglects the dynamic solvent effects. 

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