Thermodynamics and X-ray studies of 2-alcohol monolayers: Two-dimensional phase diagrams

Abstract:: Monolayers of short chiral alcohols at the water surface provide a simple model for chiral interactions between molecules. This paper is focused on alcohols with two particular chain lengths, 2-tridecanol (2C13) and 2-tetradecanol (2C14). Thermodynamic and structural parameters were measured, varying the ratio of left and right enantiomers within monolayers. The evolution of melting temperatures and entropies is not consistent with the formation of a racemate. Grazing X-ray diffraction, revealing the molecular stacking, shows clearly that there is no chiral separation. In these two compounds molecules spontaneously self-assemble at the water surface as a solid solution. The chiral polar head-group is partially screened by the chain thermal disorder of the rotator phase. 2-tridecanol exhibits a hexagonal rotator phase stable in time and independent of temperature. Experiments performed close to the melting point show pre-transitional effects for mixtures different from 50/50. This broadening of the peak can be related to defects in 2D crystals, phenomena already observed for the melting of some 3D systems.