Mass transport and structure of liquid n-alkane mixtures in the vicinity of α-quartz substrates

Abstract

The molecular-scale structure and in-plane self-diffusion coefficients of several binary n-alkane mixtures in the vicinity of the (001) crystal surface of an α-quartz substrate terminated with H-groups have been examined in this study. The molar fraction of hexane has almost negligible influence on molecular structure of hexane–hexadecane liquid mixture near the α-quartz substrate, mainly because of strong molecular interaction between the α-quartz substrate and the alkane molecules. The in-plane self-diffusion coefficients of alkane molecules in their mixture in the vicinity of α-quartz have been studied. The self-diffusion coefficient becomes considerably lower in the solid–liquid interface region than in the bulk liquid region. The self-diffusion coefficients of hexane and hexadecane in the mixture are higher in the solid–liquid interface region as well as in the bulk liquid region with an increase in the molar fraction of hexane, primarily due to the increase in total free volume with an increase in molar fraction of hexane. The self-diffusion coefficient of alkane species in the mixture exhibits anisotropic characteristics in the solid–liquid interface region, which is mostly caused by the topology of the α-quartz substrate surface.