(226f) Molecular Modeling of the Adsorption Behavior of Normal Alkanes on Silica Surfaces

We present molecular dynamics (MD) simulations of normal alkanes (C₁₀ and C₁₀₀) adsorbed on the cleaved, hydroxylated, and reconstructed dense silica surfaces using the COMPASS force field. The cleaved silica surface was created by cleaving the bulk α-quartz crystal structure along the (001) plane, while the reconstructed dense silica surface with a slab thickness of 27 atomic layers was adopted from a previous work by Goumans et al.¹ The hydroxylated α-quartz (001) and β-cristobalite (111) surfaces were used as the model hydroxylated silica surfaces with (HWH) and without hydrogen bond (HWOH), respectively. Prior to MD simulations, the silica surfaces were optimized using periodic density functional theory calculations with the Perdew-Wang of generalized gradient approximation for the exchange-correlation functional and the double numerical with polarization basis set as implemented in the DMol3 code. In our MD simulations, it has been observed that the first adsorption layer containing alkane segments tends to orient parallelly to the silica surfaces, regardless of chain length and surface structure. Nevertheless, the calculated concentration profiles of carbon atoms above the surfaces show that the number of carbons in the first adsorbed layer varies with the surface structure, and it appears to follow the order of dense > cleaved > HWH ≈ HWOH with an exception of C₁₀₀ adsorbed on the HWOH surface. Accordingly, the calculated adsorption energies for both C₁₀ and C₁₀₀ have the same order of dense > cleaved > HWH ≈ HWOH. In addition, the simulation results show the adsorption energy varies little with chain length. Therefore, our simulations suggest that the affinity of silica surface for normal alkane is affect by the existence of OH group, but changes trivially with the orientation of OH group and the size of alkane.

(1) Goumans, T. P. M.; Wander, A.; Brown, W. A.; Catlow, C. R. A. Phys. Chem. Chem. Phys. 2007, 9, 2146.