Benzene, diffusion measurements

Table 1. Comparison of results of diffusion and self-diffusion measurements of benzene in MFI crystals under various experimental conditions.

In a separate study on a series of zirconium-based metallocenes complexed with [MeB(C6F5)3] there has been shown a good correlation between their hydrodynamic volumes estimated from diffusion measurements (using Si -tolyl)4 as internal reference in benzene) and the van der Waals volumes determined for the 1 1 ion pairs from their crystal structures [45] (Fig. 9.29), suggesting that appropriate application of the diffusion NMR methodology can be an effective means of determining the sizes of molecular complexes in solution. 

J. Hadgraft, A. J. Hyde, and R. W. Richards. Diffusion of polystyrene in polymethyl methacrylate + benzene solutions measured by photon correlation spectroscopy. Far. Trans. II, 75,1495-1505. 

N. Numasawa, K. Kuwamoto, and T. Nose. Translational diffusion of polystyrene single chains in semidilute solutions of poly methyl methacrylate/benzene as measured by quasi-elastic light scattering. Macromolecules, 19 (1986), 2593-2601. 

The non-bonded interaction energy, the van-der-Waals and electrostatic part of the interaction Hamiltonian are best determined by parametrizing a molecular liquid that contains the same chemical groups as the polymers against the experimentally measured thermodynamical and dynamical data, e.g., enthalpy of vaporization, diffusion coefficient, or viscosity. The parameters can then be transferred to polymers, as was done in our case, for instance in polystyrene (from benzene) [19] or poly (vinyl alcohol) (from ethanol) [20,21]. 

FIQ. 3 Diffusion coefficient of benzene molecules in benzene-polystyrene mixtures normalized by the diffusion coefficient of neat benzene molecular dynamics results, NMR measurements and prediction by the Mackie-Meares model [26]. 

It should be mentioned that the predicted curve at highest benzene level in Figure 13 agrees with classical kinetics (no diffusion-control). It is not clear therefore why measured data at even higher benzene concentrations do not agree with classical kinetics. There may be some subtle chemical interactions at these high solvent levels. Duerksen(lT) fomd similar effects with styrene polymerization in benzene and had to correct kp for solvent. 

The first step is so fast that it can hardly be measured experimentally, while the second step is much slower (probably as a result of the repulsion of negatively charged species, R and R2-, in the negatively charged diffuse electric layer). The reduction of an isolated benzene ring is very difficult and can occur only indirectly with solvated electrons formed by emission from the electrode into solvents such as some amines (see Section 1.2.3). This is a completely different mechanism than the usual interaction of electrons from the electrode with an electroactive substance. 

The permeability of human skin to n-hexane has been determined in vitro in flow-through diffusion cells (Loden 1986). Pieces of full-thickness human skin were exposed to 3H -hcxane in human serum, and the appearance of label in the trans compartment measured for 0.5 or 12 hours. The skin was then sectioned with a microtome into 0.25 mm slices and the quantity of label in the skin measured. The rate of resorption (uptake of substance by the receptor fluid beneath the skin [i.e., the amount that passes through the skin]) was calculated. The rate of resorption for n-hexane through human skin was calculated to be 0.83 ( g cm2/hr). The permeability of n-hexane through human skin was much lower (approximately 100-fold) than for other chemicals tested in this study. For example, rates of resorption (in g cm2/hr) were 99 for benzene and 118 for ethylene glycol. 

Fig. 2 displays a set of FTIR spectra obtained for the uptake of benzene into H-ZSM-5 at 415 K employing the experimental device and procedure as described in the Experimental Section. One recognizes the increase in absorbance of the typical benzene band at 1478 cm as a function of time (spectra 1 to 4). The maximum absorbance, A, of such bands can be used as a measure of the amount sorbed, M, at time t into the porous structure of the zeolite crystallites. Therefore, evaluation of the sequence of these spectral uptake curves can provide data which may be used in the appropriate solution (equ. 1) of Fick s second law, and this generates the desired diffusivities [22] ... 

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