Absolute Orientation of Molecules of Amphiphilic Alcohols in Crystalline Monolayers at the Air−Water Interface

Abstract

The absolute orientations of the amphiphilic molecules α-hydroxy ω-bromo alcohols BrCnH2n OH, n = 21, 22, and the alkyl hydroxy esters CmH2m+1COO(CH2)nOH, m = 14, 15, n = 10, in crystalline monolayer forms on water have been determined, the former by grazing incidence X-ray diffraction (GIXD) and the latter by sum frequency generation (SFG). The assignment was made for the alkyl hydroxy esters by establishing the polar angle between the terminal CH3−C bond and the normal to the plane of the monolayer; for the bromo alcohols the assignment was made by a determination of the two-dimensional crystal structure via X-ray structure factor calculations. The SFG results are in agreement with reported GIXD and lattice energy analyses of the alkyl hydroxy esters m = 19, n = 9, 10. These studies have further revealed the absolute orientation of the alcohol C−OH bonds at the water surface, which in turn can be correlated with the ice-nucleating behavior of the monolayers on supercooled water drops in terms of the odd and even values of n.