THE STRUCTURAL-PROPERTIES OF UNCOMPRESSED CRYSTALLINE MONOLAYERS OF ALCOHOLS CNH2N-31) ON WATER AND THEIR ROLE AS ICE NUCLEATORS(1OH (N=13)

A systematic analysis of grazing incidence synchrotron X-ray diffracti on data of uncompressed amphiphilic alcohols CnH2n+1OH (n = 31, 30, 23 , 20, 19, 18, 16, 14, 13) on a water subphase at 5 degrees C is presen ted. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2-D crystalline material formed fell drastically; shorter crystalline coh erence lengths were also observed. For n = 31-18 the molecules are arr anged in a rectangular cell (a approximate to 5 Angstrom, b increases from ca. 7.4 to ca. 8.2 Angstrom) with plane symmetry p1g1. For n < 18 a tilted free-rotator phase is probably adopted. The two glide-relate d molecules in the unit cell form a herringbone arrangement in which t he chain axes are parallel and separated by \(a + b)/2\. The molecular chains are tilted from the vertical in the b direction; the tilt angl e increases from ca. 7 degrees to ca, 21 degrees over the range n = 31 to 19, and then drops to 12 degrees for n = 13. There is a continuous increase in molecular cross-sectional area from 18.4 to 20.3 Angstrom (2) and in the atomic displacement parameter parallel to the water sur face; it increases from 0.1 Angstrom(2) for n = 31 to 0.30 Angstrom(2) for n = 19. We explain the preference for chain tilt along the b axis , rather than a, in terms of hydrogen bonding to the water subphase. T he various structural properties of the CnH2n+1OH (n = 31-13) monolaye r series, such as degree of crystallinity and coherence length, lattic e dimensions, chain orientation, and molecular motion, may be correlat ed with the ice-nucleating efficiency of these alcohol monolayers as a function of n.