Volume penetrant effective molecular

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. 

Table V was prepared from Figure 5. The critical PEG molecular weight is that molecular weight below which some penetration can occur into the substance. For untreated wood, it is about 3000. However, the meaning of this value must be interpreted with caution. According to the theory of gel permeation chromatography, the important single solute parameter is effective molar volume rather than molecular weight (9). The molar volume of a polymer depends on polymer-polymer and polymer-solvent interactions this is illustrated schematically in Figure 9 (36). The total chain lengths are the same for all the polymer systems, yet the configurations and bulkiness vary considerably.

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. 

Hybrid MPC-MD schemes may be constructed where the mesoscopic dynamics of the bath is coupled to the molecular dynamics of solute species without introducing explicit solute-bath intermolecular forces. In such a hybrid scheme, between multiparticle collision events at times x, solute particles propagate by Newton s equations of motion in the absence of solvent forces. In order to couple solute and bath particles, the solute particles are included in the multiparticle collision step [40]. The above equations describe the dynamics provided the interaction potential is replaced by Vj(rJVs) and interactions between solute and bath particles are neglected. This type of hybrid MD-MPC dynamics also satisfies the conservation laws and preserves phase space volumes. Since bath particles can penetrate solute particles, specific structural solute-bath effects cannot be treated by this rule. However, simulations may be more efficient since the solute-solvent forces do not have to be computed. 

Decreasing the degree of crosslinking will increase the water uptake for a mass of dry gel, though compromises in the efficiency will result. The effect of crosslinks on the separation of vitamin B-12, a nonionic solute of molecular weight 1355, is shown in Fig. 4 [16]. As the crosslink density decreases, the polymer chain length between crosslinks increases, yielding a looser structure which vitamin B-12 can more easily penetrate. The behavior fits well with the prediction from Flory excluded volume theory [16] ... 

The models most frequently used to describe the concentration dependence of diffusion and permeability coefficients of gases and vapors, including hydrocarbons, are transport model of dual-mode sorption (which is usually used to describe diffusion and permeation in polymer glasses) as well as its various modifications molecular models analyzing the relation of diffusion coefficients to the movement of penetrant molecules and the effect of intermolecular forces on these processes and free volume models describing the relation of diffusion coefficients and fractional free volume of the system. Molecular models and free volume models are commonly used to describe diffusion in rubbery polymers. However, some versions of these models that fall into both classification groups have been used for both mbbery and glassy polymers. These are the models by Pace-Datyner and Duda-Vrentas [7,29,30]. 

According to the porosity data of Uchida et al. [102] the matrix of carbon grains (20-40 nm) forms an agglomerated structure with a bimodal psd. Primary pores (micropores, 5-40 nm) exist within agglomerates, between the carbon grains. Larger, secondary pores (macropores, 40-200 nm) form the pore spaces between agglomerates. The relation between the relative pore volume fractions of the two pore types depends on the contents of PFSI and PTFE. Due to their molecular size these components are not able to penetrate micropores. They affect only the macropore volume. The experimental study revealed that an increased PFSI content leads to a decrease of the macropore volume fraction. The opposite effect was found for PTFE. 

In Fig. 3 such relationships for n-octane and chloroform are presented. The relationships were determined at 40°C, namely when the monolayer can exist in a SC or LE form or be composed of SC islands surrounded by two-dimensional gas. The shape of Vs vs r relationship for n-octane may be interpreted as follows the first alcohol molecules deposited on silica gel surface block the most active centers on solid surface. This fact manifests as the retention volume decrease. However, from the surface concentration 1 nm up to 0-57 nm per n-octadecanol molecule the retention volume increases. The reason for this increase is probably the penetration of n-octane molecules into the rising structure of n-octadecanol chains. Let us assume that the n-octadecanol molecules are uniformly distributed on the silica gel surface. Therefore, at a surface concentration 0.57 nm per molecule the free spaces (0.38 nm in their cross-section) between them are formed, which are compatible with the n-octane molecule diameter. Then the maximum of n-octane retention volume is the result of a kind of molecular sieve effect. If the surface concentration of n-octadecanol increases and exceeds 0.57 nm per molecule, the... 

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