Two-dimensional structure of crystalline self-aggregates of amphiphilic alcohols at the air-water interface as studied by grazing incidence synchrotron x-ray diffraction and lattice energy calculations

The two-dimensional (2D) crystal structures of self-aggregated clusters of amphipbilic alcohols (C_nH_2n+1OH, n = 23, 30, 31, and C₁₉H₃₉CO₂(CH₂)_nOH, n = 9, 10) on pure water at a temperature of 5 °C have been determined to near-atomic resolution in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazing incidence synchrotron X-ray diffraction (GID) data, complemented by lattice energy calculations. The GID data of the different monolayers within each of the two series (CnH2n+1OH, C₁₉H₃₉CO₂C_nH_2nOH) are very similar. The molecules pack in a rectangular unit cell of average dimensions a = 5.0 Å, b = 7.5 Å for the normal alcohols C_nH_2n+1OH and a = 5.7 Å, b = 7.5 Å for C₁₉H₃₉CO₂C_nH_2nOH. The plane group symmetry is essentially p1g1 for the normal alcohols CnH2n+1OH and essentially p1g1 for the other group. The molecular chains are tilted from the vertical by an average angle of 9°, in the direction of the b axis, for C_nH2_n+1OH and by 29°, in the direction of the a axis, for the other molecular type. The molecular chains related by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of C_nH_2n+1OH permitted a reliable estimate of 0.07 Å2 for the molecular mean-squared motion parallel to the water surface. The a bsolute orientations of the molecules C₁₉H₃₉CO₂C_nH_2nOH were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of C₁₉H₃₉CO₂C_nH_2nOH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.