A Molecular Dynamics investigation of the influence of framework flexibility on self-diffusivity of ethane in Zn(tbip) frameworks

Configurational-Bias Monte Carlo simulations of the adsorption isotherm of ethane in Zn(tbip) (H(2)tbip = 5-tert-butyl isophthalic acid), a representative of metal-organic frameworks, show an inflection at a loading, q(i) = 6 molecules per unit cell. This inflection causes the inverse thermodynamic factor, 1/Gamma(i), to display a minimum at q(i) = 6, along with a maximum at q(i) approximate to 10. Molecular Dynamics (MD) simulations of the self-diffusivity D-i,D-self, taking account of the framework flexibility of Zn(tbip) show that the D-i,D-self - q(i) dependence follows that of 1/Gamma(i) - q(i), with a minimum at q(i) = 6. Remarkably, MD simulations assuming a rigid framework yield significantly lower values of the self-diffusivities, and show a monotonic decrease of D-i,D-self with q(i).