SELF-CONSISTENT-FIELD THEORY FOR THE ADSORPTION OF ALKANES ON SOLID-SURFACES

The adsorption of alkanes is investigated with a self-consistent held (SCF) lattice theory for inhomogeneous systems. The lateral interactio n parameter has been derived from alkane liquid-vapor equilibria, and adsorption isotherms are calculated as a function of the segment surfa ce affinity and chain flexibility. Both homogeneous (X(8)) and heterog eneous (YX(6)Y) octamers are considered. Experimental adsorption isoth erms of alkanes on both silica and methylated silica are linear until dose to saturation. These isotherms can only be explained by the SCF t heory if it is assumed that the chains are semiflexible and that the a ffinity of end and middle segments for the surface is different. From the calculations it dan be concluded that the affinity of end segments for the bare silica surface is considerably higher than that of the m iddle segments. For the alkylated surface this difference is much smal ler. At room temperature an alkylated surface is wetted by the YX(6)Y chains, but at somewhat higher temperatures partial wetting is to be e xpected. Also this result is in qualitative agreement with experimenta l evidence.