Solute shape, effect

These provide a tie between theory and experimentally observed solute shape effects, and lead to a better understanding of SEC calibration practices. 

Effect of Solute Shape or Conformation in Size Exclusion Chromatography... 

Comparison of Aqueous and Water-Ethanol Solutions. The effect of the presence of ethanol in aqueous solutions of sodium hydroxide is usually small. This is shown by the similar shape of the dependence of J- on sodium hydroxide concentration (Figure 1) and by the small differences m J values obtained at the different constant ethanol concentrations up to 90 vol % (Table III). Even when the concentration of sodium hydroxide was kept constant (e.g., 0.1 M), the difference between J values in 90 vol % ethanol and 98 vol % ethanol was only 0.16 J- units (Figure 2). In this range of ethanol concentrations, it is necessary to consider the competitive influence of ethoxide ions, the addition of which would result in a decrease of the C6H5CO— absorbance indistinguishable from the decrease caused by hydroxide ion addition. In 90 vol % ethanol, the ratio of hydroxide and ethoxide concentrations is about 1 1, while in 98 vol % ethanol, it is possible to extrapolate (30) that about 90% of the base will be present as the ethoxide ion. 

The most evident reason is that dilute solution measurements can preferably be compared directly with the unmodified dilute solution theory as reviewed in Chapter 3. As has already been pointed out in Section 2.6.1, the form birefringence in dilute solution can effectively be suppressed by the choice of a solvent of practically the same refractive index as the polymer. In such a "matching solvent the contrast between the coil of the macromolecule and its surrounding practically disappears. This means that, at the same time, the influence of the shape (form) of the coil disappears. Also the comparison with measurements on con-... 

Shape effect of PFPE molecules or magnetic particles in suspension, including agglomeration phenomena at low concentration, interaction among these particles, and effects of floes can be examined via solution viscosity (r ) measurement. For a very dilute polymer solution [108], there is no interaction among polymer molecules, and the solution viscosity results from the contribution of the solvent plus the contribution of the individual polymer molecules. The intrinsic viscosity, therefore, is a measure of the hydrodynamic volume of a polymer molecule as well as the particle aspect ratio. Figure 1.24 shows the determination of the intrinsic viscosity for Zdol4000 in three different solvents. 

The Polarizable Continuum Model (PCM)[18] describes the solvent as a structureless continuum, characterized by its dielectric permittivity e, in which a molecular-shaped empty cavity hosts the solute fully described by its QM charge distribution. The dielectric medium polarized by the solute charge distribution acts as source of a reaction field which in turn polarizes back the solute. The effects of the mutual polarization is evaluated by solving, in a self-consistent way, an electrostatic Poisson equation, with the proper boundary conditions at the cavity surface, coupled to a QM Schrodinger equation for the solute. 

The dependence of m on (- for a localizing solvent C in mobile phases A/C or A/B/C is given by Eq. (30a). According to our theory of how these solvent-solute-localization effects arise (i.e., competition of localizing solvent and solute molecules for the same site), we expect that/( e) will increase slowly with 6c until the maximum localized-coverage is approached (6c > 0.5). The function/(flc) should then increase sharply and level out for 6c > 0.75. Figure 16 shows actual datafor/(do) in the case of alumina (Fig. 16a) and silica (Fig. 16b). The same function/( c) is shown in each plot, and it is apparent that experimental data fall close to the solid curves (best-fit values of m° for each solvent C). The shape of the solid curve is as predicted, based on the analysis of Fig. 3. 

In Figure 2 (a), we plot Cj against for the two functions of the "microscopic viscosity " The correlation curves for Eqs. (6) and (7) are roughly identical, the correlation between Cj and Cr is not so sensitive to the functional form From these correlation curves, we can estimate the significance of the effect of non-uniform solvent properties on Cr-Such effects are usually concealed by large effects of the solute shape and the surface boundary condition which is often treated as an adjustable parameter. 

Although the presentation so far has been concerned with isolated supramolecules or in homogeneous solution, hetereogeneous effects are, of course, also subject of constant theoretical developments. For instance, the requirements for the formation of a chiral template was addressed only recently [266] on the basis of DFT calculations for propylene oxide on Pd(lll) surfaces. Another example Molecular dynamics simulations on shape-persistent macrocycles revealed that... 

The molecular motions that are probed in P-phenyl ketones can be qualitatively described as "bending" motions of a rod-like solute. Since the molecular length of the solute can be easily altered by substitution in either the benzoyl or P-phenyl rings, this system offers a potentially very useful one with which to investigate the effects of liquid crystalline order on conformational motions involving a wide range of solute shape changes. 

The concentration profile in the immobile-water phase is controlled by a diffusional-transport mechanism. The transfer rate from the immobile-water phase to the flowing-water phase is the diffusive flux, which depends on the concentration gradient in the immobile-water phase at the interface. Parameters V, 6, A, and Lg in the profile side-pore model are estimated from the shape of the breakthrough curve for a nonreactive tracer. Parameters Pbf and pbs are estimated from the shape of the breakthrough curve for a reactive solute. The effective molecular diffusivity Dm is estimated from values published in the literature. 

Probably the biggest problem in analytical polarography is adsorption of species on to the surface of the electrode. This can be adsorption of the analyte, its electrolysis product, or any other species from the solution. The effects of adsorbed species can be very varied indeed. They can produce the splitting of polarographic waves, the distortion of their shapes, shifting of the half wave potentials, depression or even elimination of the wave heights, etc. The adsorbed forms may produce small waves of their own, known as pre-waves or post-waves, separate from the main diffusion controlled wave. On the other hand some adsorbed species have little or no effect. 

Ueno, K., Kim, H.-B. and Kitamura, N., Channel shape effects on the solution-flow characteristics and liquid/hquid extraction efficiency in polymer microchannel chips. Anal. Sci., 19, 391, 2003. 

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