Solution molecular size

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

The larger the solute molecular size, the higher the water loss and the lower the sugar uptake under fixed process conditions. Using the right size of osmotic solute, satisfactory moisture diffusivities, with nearly zero net solute uptake, can be obtained. 

Tyrrell and Watkiss [58] showed that the numerical coefficient for 2-methylpentan-2,4-diol solvent is not 6 or 4 but nearer 1—3 depending on the solute molecular size and shape and temperature, being smaller for smaller and planar molecules. In a study of normal alkanes, Amu [59] found that this numerical factor decreased as the molecular chain length increased. Indeed, if the solute molecules are significantly different from a spherical shape, the diffusion coefficient should be regarded as (a tensor) having different values in different directions. Usage of the microfriction factor mentioned in Sect. 5.2 is an improvement, but still rather unsatisfactory (Alwatter et al. [43]). 

The correlation between aqueous solubility and molar volume discussed by McAuliffe [5] for hydrocarbons, and the importance of the cavity term in the solvatochromic approach, indicates a significant solubility dependence on the molecular size and shape of solutes. Molecular size and shape parameters frequently used in quantitative structure-water solubility relationships (QSWSRs) are molecular volume and molecular connectivity indices. Moriguchi et al. [33] evaluated the following relationship to estimate Cw of apolar compounds and a variety of derivatives with hydrophilic groups ... 

Yoshida, N.H., and M.S. Roberts. 1993. Solute molecular size and transdermal iontophoresis across excised human skin. J Control Release 25 177. 

Prerequisite for hydrodemetallisation is the diffusion of the large porphyrins into the catalyst porous texture prior to the sequential reaction mechanism. Diffusion of these large molecules can be limited by geometric exclusion and hydrodynamic drag. When the solute molecular size is significant as compared to the pore size, a restrictive factor can be introduced to account for the reduction in difftisivity. As a consequence, clarification of detailed HDM reaction kinetics may be obscured by diffusion limitations. 

New models for the prediction of molecular diffusion coefficients are described, and compared to previously established ones. These are based on solute molecular size, solvent viscosity solvent molecular size, and temperature. The data set of diffusion coefficients used was primarily the one developed by Wilke and Chang and upon which their commonly used diffusion model is based (A.I.Ch.E. Journal, 1 (1955), 264). 

The rate of solute diffusion in liquids in porous materials becomes observably lower than would be expected from Equation 3.27 when the solute molecular size becomes large with respect to the pore size. This phenomena can be important for catalytic processing of heavy liquid petroleum fractions [25]. In the case of liquid transport through finely porous materials, the effective diffusivity can be expressed as [26]... 

Site-competition delocalization manifests itself in two ways (a) an increase in the apparent solute molecular size, or value of A, in Eq. (8) and... 

Hence retention volume can be expressed in terms of two measurable quantities Fq and Ft, and Ksec which is a function of solute molecular size and of the column packing pore size. In SEC, it can only vary between zero (total exclusion) and unity (total permeation). 

The value of each contribution to the peak variance (the individual terms in equation 16) was determined for eadi protein under the base case conditions and these values are presented in Table 3. These results show that the contribution of the width of the feed band to peak variance is independent of solute molecular size, as expected. The influence of dispersion decreases with molecular size. This is because the solute residence time in the column decreases with molecular size. As expected, the contributions of external mass transfer and internal diffusion increase with molecular size because transport rates from the mobile phase to the stationary phase and within the stationary phase decrease with increasing solute size. Under the conditions of this simulation, the total variance decreases with molecular size because the dispersion term is dominant over the other terms. 

Figure 2. Dependence of diffusion coefficients on solute molecular size in p-HEMA (—) correlation of steroid diffusion in B-type domains with water-soluble solutes (---) experimental values of steroid diffusion...

On the basis of either model, ii is clear ihat dialytic transport will dectesse with increasing solute molecular size, not only beeause of the smeller solution diffusivity of a large molecule bur also because of incransmg values of q. This effect is seen in Fig. 21.1-4. where the ratio of diffusivity in the membrane (3Dm) to difliisivity in solution (3D) is plotted as a function of solute Stokes radii for several dialysis membranes. 

The short in vivo half-life of many pharmaceutically active proteins necessitates the need for multiple administrations to produce a therapeutic response, emphasizing the applicability of controlled release formulations (Lee, 1992). The key variables that affect protein transport through hydrophilic polymers depend on the delivery mechanism and device properties, in a similar manner as in the case of lower-molecular-weight drug substances. However, in the case of proteins, the role of solute molecular size is much more dramatic in hindering the diffusion and release from hydrophilic polymers (am Ende, 1993). 

In addition to the reduced cross-sectional area available for diffusion and pore space tortuosity, Grathwohl (1998) describes a factor that accounts for the constrictiveness of pores relative to solute molecular size the same idea applies to diffusing particles. The porous medium may contain small pores which are not accessible due to size exclusion... 

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