Vapour diffusion coefficients and collision parameters for cyclic molecules

Abstract

Measurements are reported on the inter-diffusion coefficients of benzene, thiophen, pyridine, piperidine and ietrahydrothiophen vapours in binary systems with hydrogen, nitrogen and oxygen. Previously published results on diffusion coefficients for cyclohexane and on the viscosities of the vapours together with new viscosity measurements for piperidine and tetrahydro-thiophen have been combined with the diffusion data to evaluate collision diameters for this family of cyclic molecules.Assuming collision diameters obtained from viscosity, a Lennard-Jones 12 : 6 potential has been applied to the measured diffusion coefficients to obtain a value of thc intcraction parameter €12 referring to interactions between unlike molecules. Comparjson of values of €12 with those calculaied from viscosity measurements using the usual combining rules for intermolecular potentials reveals discrepancies for all these binary systems.A more exact form of the combining rules has been derived and used to calculate €12. Agreement between parameters based on viscosity and diffusion is improved in the cases of the measurements with nitrogen and oxygen. These now yield self-consistent values for diffusion from which other collision processes may be calculated quite accurately. Even with the more exact combining rules, most of the cyclic molecules still show abnormally large diameters in collisions with hydrogen, described as the " hydrogen effect ".Measurements of the inter-diffusion coefficients of various hydrocarbons with hydrogen, nitrogen or oxygen have been previously reported,l covering a range of paraffin hydrocarbons from C5 to Clo and a number of unsaturated C6 hydrocarbons. The diffusion coefficients can be expressed in terms of kinetic theory as functions of the mass, temperature, a collision diameter cr, and an attraction potential E . Depending upon the particular form of interaction potential assumed to act between molecular centres, corresponding values of the collision diameter can be calculated. Gas kinetic theory indicates that the correct form of intermolecular potential should be one in which the same value of the collision diameter satisfies the equations for all the diffusion coefficients and also satisfies the equations for viscosity.From the previous work it has been found that the use of conventional intermolecular potentials, such as the rigid sphere or the Lennard-Jones 12 : 6 potential, cannot be made to satisfy the experimental results for viscosity and all the diffusion data. An outstanding discrepancy is that collision diameters calculated from diffusion measurements on hydrogen+ hydrocarbon systems usually appear to be anomalously large on the basis of the two-parameter 12 : 6 potential. This discrepancy has been termed the " hydrogen effect " and may be due to the small mass of the hydrogen and hence its anomalous energy transfer effects.The present research deals with a group of molecules, benzene, pyridine and thiophen and their fully hydrogenated derivatives whose cyclic structures are more compact than...

Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: Volume 2. Diffusivities of organic compounds, pressure-normalised mean free paths, and average Knudsen numbers for gas uptake calculations

“…(9) do not take into account the effects of different structures of isomers, which may have different collisional cross sections and thus different diffusion volumes. The measured (Cummings and Ubbelohde, 1953;Cummings et al, 1955;Hudson et al, 1960;Altshuller and Cohen, 1960;Nagata and Hasegawa, 1970) and estimated diffusivities of four isomers (cyclohexane, methyl cyclopentane, 1-hexene, and 2,3-dimethyl-2butene) of C 6 H 12 are listed in Table A1 in the Appendix, showing good agreement between measured and estimated values. This suggests that the effect of different isomers may be of minor importance.…”

“…Temperatures of tropospheric and stratospheric interest range from ∼ 200 to ∼ 300 K. However, most of the diffusivity measurements were only carried out at around room temperature. For those studies in which the effect of tempera- ture was investigated, they were usually performed at temperatures > 300 K. The measured diffusivities of 2-propanol (Gilliland, 1934;Lugg, 1968;Arnikar and Ghule, 1969;Nagata and Hasegawa, 1970), benzene (Lee and Wilke, 1954;Hudson et al, 1960;Altshuller and Cohen, 1960;Getzinger and Wilke, 1967;Lugg, 1968;Katan, 1969;Arnikar and Ghule, 1969;Nagata and Hasegawa, 1970), n-pentane (Lugg, 1968;Barr and Watts, 1972;Nagasaka, 1973), and ethyl formate (Lugg, 1968;Nagata and Hasegawa, 1970) are plotted as a function of temperature in Fig. 1, together with the estimated diffusivities (black curves) using Fuller's method.…”

Interaction energy in surface diffusion

“…If the surface diffusion of the adsorbed molecule is to be studied fundamentally, then it seems logical to investigate and analyze the gas-solid interaction energy for the system. There are some publications (2, 3,6,7,12,14) which deal with the calculation of the dispersion energy for both homogeneous and heterogeneous systems. However, no one has attempted to apply those techniques to the study of surface diffusion.…”

Prediction of second virial coefficients of mixtures from the principle of corresponding states

“…Values of the second virial coefficient B, of the gas mixtures were reported over a range of temperature and it was shown that these values lie quite close to those obtained by the use of their corresponding states equation (eqn. (4)), together with the combining rules ( 6) and ( 7) and the geometric mean rule for Tt2 (eqn. ( 8)).…”