2-DIMENSIONAL STRUCTURES 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 cluster s of amphiphilic alcohols (CnH2n+1OH, n=23, 30, 31, and C19H39CO2(CH2) (n)OH, n=9, 10) on pure water at a temperature of 5 degrees C have bee n determined to near-atomic resolution in order to understand the rela tive abilities of these monolayers to induce ice nucleation. The struc tures were solved making use of grazing incidence synchrotron X-ray di ffraction (GID) data, complemented by lattice energy calculations. The GID data of the different monolayers within each of the two series (C nH2n+1OH, C19H39CO2CnH2nOH) are very similar. The molecules pack in a rectangular unit cell of average dimensions a=5.0 Angstrom, b=7.5 Angs trom for the normal alcohols CnH2n+1OH and a=5.7 Angstrom, b=7.5 Angst rom for C(19)H(39)CO(2)CnH(2n)OH. The plane group symmetry is essentia lly p1g1 for the normal alcohols The two-dimensional (2D) crystal stru ctures of self-aggregated clusters of amphiphilic alcohols (CnH2n+1OH, n=23, 30, 31, and C19H39CO2(CH2)(n)OH, n=9, 10) on pure water at a te mperature of 5 degrees C have been determined to near-atomic resolutio n in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazi ng incidence synchrotron X-ray diffraction (GID) data, complemented by lattice energy calculations. The GID data of the different monolayers within eech of the two series (CnH2+1OH, C19H39CO2C(n)H(2n)OH) are ve ry similar. The molecules pack in a rectangular unit cell of average d imensions a=5.0 Angstrom, b=7.5 Angstrom for the normal alcohols CnH2n +1OH and a=5.7 Angstrom, b=7.5 Angstrom for C19H39CO2CnH2nOH. The plan e group symmetry is essentially p1g1 for the normal alcohols CnH2+1OH and essentially pllg for the other group. The molecular chains are til ted from the vertical by an average angle of 9 degrees, in the directi on of the b axis, for CnH2n+1OH and by 29 degrees, in the direction of the a axis, for the other molecular type. The mclecular chains relate d by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of CnH2n+1OH permitted a reliable est imate of 0.07 Angstrom(2) for the molecular mean-squared motion parall el to the water surface. The absolute orientations of the molecules C1 9H39CO2CnH2nOH were determined by lattice energy calculations. The ani sotropic coherence lengths of the crystallites of C19H39CO2CnH2nOH der ived from the widths of the two observed Bragg peaks have been correla ted with the binding energies of molecules in different directions.CnH 2+1OH and essentially p11g for the other group. The molecular chains a re tilted from the vertical by an average angle of 90, in the directio n of the b axis, for CnH2n+1OH, and by 29 degrees, in the direction of the a axis, for the other molecular type. The molecular chains relate d by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of C,CnH(2+1)OH permitted a reliable estimate of 0.07 Angstrom(2) for the molecular mean-squared motion par allel to the water:r surface. The absolute orientations of the molecul es C19H39CO2CnH2nOH were determined by lattice energy calculations. Th e anisotropic coherence lengths of the crystallites of C19H39CO2CnH2nO H derived from the widths of the two observed Bragg peaks have been co rrelated with the binding energies of molecules in different direction s.