The permeation of benzene and acetone vapors through sulfur‐cured natural rubber was studied by the time‐lag method. The experimental results were analyzed by a method suggested by Meares. The zero concentration diffusion coefficient D0 was obtained by the early‐time method. The Frisch time‐lag equation was utilized to estimate both the solubility coefficient s and the additional parameter b required to define the concentration dependence of the diffusion coefficient: D(c) = D0 exp {bc}. This form of concentration dependence was manifested by the corresponding permeability coefficient values. At low entering penetrant pressure, where the transport coefficients are constant, indirect evidence was obtained that D0 is the mechanistically correct diffusion coefficient. The solubility coefficient values calculated for benzene vapor in natural rubber are in reasonable agreement with published equilibrium sorption data for a similar rubber compound. At higher entering penetrant pressures, average diffusion coefficients obtained at steady state tended to be larger than the corresponding average diffusion coefficients derived from the time lags. This same effect has been detected by other experimental approaches. Permeation experiments designed for this rapid method of analysis appear capable of yielding information consistent with that obtained by more time‐consuming traditional methods.
below that of many of the liquids of interest. It was this circumstance which necessitated the alteration of the range of the refractometer described in the preceding paragraph. Even with this range extension it was necessary to use formamide as a secondary reference for measuring the refractive index of carbon tetrachloride, dichloracetic acid and hydrazine.Bayen's dispersion data for water at 17.7°was corrected to 25°u sing his temperature dependence of refractive index. These values were then used as standards for measuring the dispersion of formic acid and the 8 molar urea solution. In order to obtain the refractive index of the sodium bromide solutions at 25°the refractometer was first calibrated at 18°u sing water vs. sodium bromide. The sample temperature was then raised to 25°and the calibration (which is independent of temperature) used to determine the difference in refractive index of water and sodium bromide solutions.Solvents. The solvents used were of the best grade obtainable commercially and were not purified further except for the dioxane and ethylene dichloride which were distilled. The urea was recrystallized once from ethanol. Refractive indices were measured at 25°at the sodium D line with an Abbe refractometer and were: formic acid 1.3692, 8 molar urea
Computations of (89) may be simplified by the observation that the phase shifts ~I' do not depend on the orientation quantum numbers M L ', M s , M J , and A; as a result, when!, is squared, summed, averaged, and integrated as in (89) the answer is a sum over partial cross sections q (J, J', N') for each N'. The sums over NI'(1), N/2), K(1), K(2) have terms which depend on the phase shifts ~M[NI'(1), These coefficients are in turn sums of properly normalized Clebsch-Gordan coefficients, over the orientation quantum numbers M L '(I), M L '(2), M s (1), M S (2), M J , M/, for C9, and over Nl) or A(2) for C3. For fixed L= 1, S=!, J=!, J'=!, and N' (in general case, N'~2) there are 81 coefficients; we find, however, by calculation that only 13 of these are nonzero, and have a written program to compute them. Note that K~ 1 in every case.Intermolecular potential-energy functions for pairs of simple polyatomic molecules are calculated by assuming that the total interaction is the sum of all the interactions between atom pairs on the two molecules. The interaction parameters for the constituent atoms are evaluated from noble-gas and diatomic-gas data. The calculated potential-energy functions are tested by calculating second virial coefficients and comparing with experimental data. It is generally found that the calculated potential-energy functions are good approximations for the molecules considered.
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