The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols C_<i>n</i>H_2<i>n</i>+1 OH (<i>n</i> = 13–31) on Water and Their Role as Ice Nucleators

Abstract

Abstract A systematic analysis of grazing incidence synchrotron X‐ray diffraction data of uncompressed amphiphilic alcohols C n H 2n+1 OH ( n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5°C is presented. 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 coherence lengths were also observed. For n –31–18 the molecules are arranged in a rectangular cell ( a ≈ 5 Å, b increases from ca. 7.4 to ca. 8.2 Å) with plane symmetry p 1 g 1. For n &lt; 18 a tilted free‐rotator phase is probably adopted. The two glide‐related molecules in the unit cell form a herringbone arrangement in which the chain axes are parallel and separated by [( a + b )/2]. The molecular chains are tilted from the vertical in the b direction; the tilt angle increases from ca. 7° to ca. 21° over the range n = 31 to 19, and then drops to 12° for n = 13. There is a continuous increase in molecular cross‐sectional area from 18.4 to 20.3 Å 2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Å 2 for n = 31 to 0.30 Å 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. The various structural properties of the C n H 2n+2 OH ( n = 31–13) monolayer series, such as degree of crystallinity and coherence length, lattice dimensions, chain orientation, and molecular motion, may be correlated with the ice‐nucleating efficiency of these alcohol monolayers as a function of n .