Penetrant molecular volume

Evidence was presented that thallous ethylate did not penetrate or alter the crystalline parts of the fiber. Moreover, it was possible to conduct the thallation with different solvents for thallous ethylate. When this was done with normal ethers, the extent of methylation was observed to decrease as the molecular volume of the thallous ethylate solvent increased. These results suggested that accessibility is dependent upon the penetrating power of the ether solvent. Amorphous cellulose was, therefore, defined as the percentage of cellulose wetted by an ether of zero molecular volume and was estimated by determining methylation-molecular volume values for three or more straight-chain ethers, plotting the data and extrapolating to obtain methoxyl content for an ether of zero molecular volume. The amount of cellulose corre-... 

Again, a parabolic correlation between log BB and V indicated the twofold effects of molecular size on BBB penetration. Increasing molecular volume on one hand decreases the log BB value by decreasing the molecular diffusion through a lipid membrane. On the other hand, bigger molecular volume also increases lipophilicity, which, in turn, facilitates BBB penetration when <2o,n and <2h remain unchanged. 

Fu XC, Chen CX, Liang WQ, Yu QS (2001) Predicting blood-brain barrier penetration of drugs by polar molecular surface area and molecular volume. Acta Pharmacol Sin 22 663-668. 

BBB penetration can be increased by reducing the hydrophibcity of the compounds, hydrogen-bond acceptors and molecular volume are the important parameters influencing BBB permeabihty of the compounds. 

The dermal penetration coefficient Kp in this simplest case depends on both the partitioning of the chemical from its vehicle (usually water) into the stratum corneum, and its diffusion through the stratum corneum. Both of these quantities can be estimated from a chemical s properties or structure. Partitioning from water into the stratum corneum can be estimated from a chemical s octanol-water partition coefficient, Kow Diffusion through the stratum corneum is dependent on the molecular volume of the chemical, which is in turn a function of its molecular weight (MW). Perhaps the most widely used expression of the dependence of stratum corneum permeability on readily available physicochemical properties is the Potts-Guy equation ... 

The column packing in SEC comprises porous, spherical gel beads with a defined pore size distribution. Most often, these beads are made from poly(styr-ene-divinylbenzene). (For GFC, cross-linked dextran and agarose gels are often used. ) The sample is dissolved in a suitable solvent that is often used as the mobile phase as well. Separation occurs as a result of differences in accessibility of pore volume. Small molecules can freely access the whole pore volume as a result, the column retards these molecules the greatest. As molecular volume increases, less and less pore volume is accessible for molecules to sample, and elution times decrease. For all molecules with hydrodynamic volumes that are too large to penetrate into the pores of the packing, elution occurs at the (interstitial) void volume of the column. The retention volume for each solute can be described mathematically as ... 

In all cases, both permeability and diffusivity of methyl-substituted benzenes vary in an inverse manner with their molecular volumes (as calculated by dividing the molecular weight by density and Avogadro number to yield the volume per molecule). For other penetrants, namely, anisole, nitrobenzene, chlorobenzene, -dichlorobenzene and bromobenzene, the volume per molecule varies in the range 17-19. However, their diffusion trends are quite different. For instance, though nitrobenzene and chlorobenzene... 

The preceding qualitative observations about the temperature dependence of Ch and Vg — V, can be extended to a quantitative statement in cases for which the effective molecular volume of the penetrant in the sorbed state can be estimated. As a first approximation, one may assume that the effective molecular volume of a sorbed CO2 molecule is 80 A in the range of temperatures from 25 to 85 C. This molecular volume corresponds to an effective molar volume of 49 cmVmol of CO2 molecules and te similar to the partial molar volume of CO2 in various solvents, in several zeolite environments, and even as a pure subcritical liquid (see Tables 20.4-4 and 20.4-5). The implication here is not that mote than one COi molecule exists in each molecular-scale gap, but rather that the effective volume occupied by a CO2 molecule is roughly the same in the polymer sorbed state, in a saturated zeolite sorbed state, and even in a dissolved or liquidiike state since all these volume estimates tend to be similar for materials that are not too much above their critical temperatures. With the above approximation, the predictive expression given below for Cw can be compared to independently measured values for this parameter from sorption measurements ... 

Application of Eq. (20.4-12) to highly supercritical gases is somewhat ambiguous since the effective molecular volume of sorbed gases under these conditions is not estimated easily. A similar problem exists in a priori estimates of partial molar volumes of supercritical components even in low-molecular-weight iiqui. The principle on which Eq. (20.4-12) is ba remains valid, however, and while the total amount of unrelaxed volume may be available for a penetrant, the magnitude of Cj, depends strongly on how condensable the penetrant is, since this factor determines the relative efficiency with which the component can use the available volume. 

The Wilhelmy plate contact angle method might provide a sequential scanning curve or hysteresis loop that can be interpreted in terms of surface mobility, reorientation, solvent penetration, and intrinsic wettability under both water and air. Water is known to be a difficult liquid for use in contact angle studies due to its small molecular volume, resulting in penetration and local swelling of the solid surfaces [15]. 

Table 2. Various Molecular Volumes for Gaseous Penetrants...

Fig. 39. Free-volume hole distributions of various polycarbonates. Dashed lines are the molecular volumes of common penetrants. Reprinted from Ref 181. This material is used by permission of John Wiley Sons, Inc.

This law describes an ideal case of steady flow which is strongly influenced by the nature of polymer, crosslinks, plasticizers, fillers, nature of penetrant, and temperature. Figure 7.1.10 shows the effect of polymer crystallinity and temperature on diffusion coefficients of 1-butanol and 1-octanol in poly(ethylene terephthalate). Increase of temperature facilitates diffusion (see also Figure 7.1.1). There is also difference in diffusion between two penetrants studied. Butanol has higher diffusion rate than octanol, which is easy to understand since butanol has smaller molecular volume (see also Figirre 7.1.3). Increase in crystallinity causes decrease of diffusion rate of both solvents. ... 

Cross-linking polyethylene reduces chain segment motion in the disordered regions, and thus diffusion becomes more dependent upon the size and shape of the penetrant moleeules. The chain immobilization factor will have the greatest effect on penetrants that have a large molecular volume. 

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