Formation of partially demixed two-dimensional solid solutions from binary mixtures of n-alkanes with very different lengths

The interface layer formed between binary mixtures of n-alkanes and the graphite surface has been studied by scanning tunneling microscopy. These liquid mixtures are made of short alkane molecules (C17H36), which act as a solvent and longer molecules (C36H74) with a length ratio similar to2. At 296-298 K, the monolayers formed at the liquid/solid interface show an arrangement of domains characterized by parallel lamellae mainly made of parallel C36H74 molecules. The defects clearly visible in the middle axis of lamellae are attributed to pairs of C17H36 molecules adsorbed in place of one C36H74 molecule. Therefore, each domain in the monolayer can be described as a two-dimensional solid solution despite the large difference in lengths of involved alkanes. Qualitative information was inferred on both the packing of alkanes within the monolayer and the kinetics of the defects in the 2D solid solutions.