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

Monolayers of short chiral alcohols at the water surface provide a simple m odel for chiral interactions between molecules. This paper is focused on al cohols with two particular chain lengths, 2-tridecanol (2C13) and 2-tetrade canol (2C14). Thermodynamic and structural parameters were measured, varyin g 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 m olecules spontaneously self-assemble at the water surface as a solid soluti on. The chiral polar head-group is partially screened by the chain thermal disorder of the rotator phase. 2-tridecanol exhibits a hexagonal rotator ph ase stable in time and independent of temperature. Experiments performed cl ose to the melting point show pre-transitional effects for mixtures differe nt 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.