The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols CnH2n+1 OH (n = 13-31) on Water and Their Role as Ice Nucleators

A systematic analysis of grazing incidence synchrotron X‐ray diffraction data of uncompressed amphiphilic alcohols CnH2n+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 p1 g1. For n < 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 Å2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Å2 for n = 31 to 0.30 Å2 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 CnH2n+2OH (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. Pronounced structural changes are observed in the uncompressed monolayers of CnH2n+1OH on water at 5°C as a function of chain length (n = 13–31). A correlation is established between monolayer structure and the efficiency of the alcohols as ice nucleators. One important factor here is the degree to which the 2‐D lattice of the monolayer (left) matches that of hexagonal ice (right).