Molecular simulation of diffusion and sorption of alkanes and alkane mixtures in poly[1-(trimethylsilyl)-1-propyne]

In the present study, we investigated the sorption and transport of pure and mixed alkanes and gases (H₂, CH₄, C₂H ₆, C₃H₈, i-C₄H ₁₀, and n-C₄H₁₀) in poly[1-(trimethylsilyl)1-propyne] (PTMSP) through the use of molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations. The bonded constants in the DREIDING II and COMPASS force fields have been parameterized for PTMSP from AMI. and GAUSSL4,N94 calculations of the dimer. The densities of amorphous cells generated from the two force fields agreed well with experimental data. Diffusion coefficients were calculated by means of the Einstein relationship using (NVT ensemble) MD simulation. There was good agreement between experimental and simulation diffusion coefficients for the gases and alkanes. MD trajectories indicated conformational changes of PTMSP that enable the formation of channels between adjacent holes of the free volume. The high mobility of the trimethylsilyl group studied by rotational time correlation analysis could facilitate channel formation. Sorption isotherms of alkanes in PTMSP were obtained by fixed-pressure GCMC simulation. The results for the pure-component isotherms were in qualitative agreement with experiment data. There was good agreement between simulation and experimental results for the solubility and dual-mode sorption parameters. Diffusion coefficients and solubilities obtained from transition-state theory (TST) simulation for light gases (H₂ and CH₄) agreed well with MD and GCMC simulations, whereas the agreement was not as good for larger condensable vapors.; High overall free volume and free volume distribution contributed to the high gas permeability of PTMSP. There was excellent correlation between In D and f_a, the accessible free volume fraction, for PTMSP and each of the four alkanes. Fractional free volume obtained by Voorintholt grid search method was consistent with the experimental values. TST results indicated that the larger the gas molecule, the larger was the free volume element in which gas resided and the longer was the residence time. The simulated diffraction pattern was in good agreement with experimental data and indicated a large interchain distance for PTMSP. It was found from simulation that the solvent casting during PTMSP membrane preparation did not make much difference in terms of the final membrane density and fractional free volume.; Four gas/vapor mixtures were investigated by MD and GCMC simulations—CH ₄/C₃H₈, CH₄/n-C ₄H₁₀, H₂/C₃H₈, and H ₂/n-C₄H₁₀. It was found from GCMC simulation that the larger gas molecule was more competitive to the sorption site for binary gas mixtures. Compared with pure gas permeability, the permeability of the permanent gas in binary mixture decreased more than that of the condensable vapor. This behavior was consistent with a blocking mechanism suggested for permanent gas/condensable vapor transport in PTMSP.