Molecular dynamics simulation of interaction between supercritical CO2 fluid and modified silica surfaces

The structural and dynamical properties of the supercritical CO2 fluid confined in the slit nanopores with the hydroxylated and silylated amorphous silica surfaces have been studied using molecular dynamics (MD) simulation. The amorphous bulk silica was obtained by a melt-quench MD simulation technique and the modified silica surfaces were artificially created by the attachment of hydrogen (-OH model) and trimethysilane (-Si(CH3)(3) model) to the nonbridging oxygen atoms on the silica surfaces. The VdW interaction potential between the CO2 molecule and the hydroxylated silica surface was determined based on the ab initio quantum mechanics (QM) computation. The adsorption potential distributions Of CO2 on the two modified silica surfaces were examined in order to evaluate the different surface interaction characteristics. The density profiles, the radial distribution functions, as well as the interfacial dynamics properties (self-diffusion coefficients and residence time) for the confined supercritical CO2 fluid have been simulated. It is demonstrated that the hydroxylated silica surface gives a stronger confining effect on the supercritical CO2 fluid as compared with the silylated surface. The remarkable impact on the supercritical CO2 fluid from the hydroxylated silica surface can be attributed to the H-bonding interaction between CO2 molecules and surface silanol groups. The analysis of the vibrational density of states of the confined supercritical CO2 fluid reveals the phenomena of the spectral shifts and the Fermi resonance in compaison with the bands in unconfined supercritical CO2. This spectrum behavior is associated with the enhanced interaction from the functional groups on silica surfaces.