The two-dimensional (2D) crystal structures 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 temperature of 5 degrees 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, C19H39CO2CnH2nOH) are very similar. The molecules pack in a rectangular unit cell of average dimensions a=5.0 Angstrom, b=7.5 Angstrom for the normal alcohols CnH2n+1OH and a=5.7 Angstrom, b=7.5 Angstrom for C(19)H(39)CO(2)CnH(2n)OH. The plane group symmetry is essentially p1g1 for the normal alcohols The two-dimensional (2D) crystal structures 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 temperature of 5 degrees 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 eech of the two series (CnH2+1OH, C19H39CO2C(n)H(2n)OH) are very similar. The molecules pack in a rectangular unit cell of average dimensions 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 plane group symmetry is essentially p1g1 for the normal alcohols CnH2+1OH and essentially pllg for the other group. The molecular chains are tilted from the vertical by an average angle of 9 degrees, in the direction 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 related by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of CnH2n+1OH permitted a reliable estimate of 0.07 Angstrom(2) for the molecular mean-squared motion parallel to the water surface. The absolute orientations of the molecules C19H39CO2CnH2nOH were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of C19H39CO2CnH2nOH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.CnH2+1OH and essentially p11g for the other group. The molecular chains are tilted from the vertical by an average angle of 90, in the direction 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 related 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 parallel to the water:r surface. The absolute orientations of the molecules C19H39CO2CnH2nOH were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of C19H39CO2CnH2nOH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.