Self-consistent field theory for the adsorption of alkanes on solid surfaces

The adsorption of alkanes is investigated with a self-consistent field (SCF) lattice theory for inhomogeneous systems. The lateral interaction parameter has been derived from alkane liquid-vapor equilibria, and adsorption isotherms are calculated as a function of the segment surface affinity and chain flexibility. Both homogeneous (X₈) and heterogeneous (YX₆Y) octamers are considered. Experimental adsorption isotherms of alkanes on both silica and methylated silica are linear until close to saturation. These isotherms can only be explained by the SCF theory if it is assumed that the chains are semiflexible and that the affinity of end and middle segments for the surface is different. From the calculations it can be concluded that the affinity of end segments for the bare silica surface is considerably higher than that of the middle segments. For the alkylated surface this difference is much smaller. At room temperature an alkylated surface is wetted by the YX₆Y chains, but at somewhat higher temperatures partial wetting is to be expected. Also this result is in qualitative agreement with experimental evidence.