Phase behavior and molecular packing of octadecyl phenols and their methyl ethers at the air/water interface

Noncovalent molecular interactions, such as hydrogen bonding and van der Waals forces, play an important role in self-assembling to supramolecular structures. To study these forces, we chose monolayers at the air/water interface to limit the possible arrangements of the interacting molecules. Furthermore, monolayers provide useful tools to understand and study interactions between molecules in a controlled and fundamental way. The phase behavior and molecular packing of the phenols 1-(4-hydroxyphenyl)-octadecane (5a), 1-(3,4-dihydroxyphenyl)-octadecane (6), and 1-(2,3,4-trihydroxyphenyl)- octadecane (3) and their methyl ethers in monolayers at the air/water interface have been examined by π/A isotherms, Brewster angle microscopy (BAM), grazing incidence X-ray diffraction (GIXD) measurements, and density functional theory (DFT) calculations. The phenols are synthesized by Friedel-Crafts acylation of methoxybenzenes, hydrogenation of the resulting aryl ketones, and cleavage of the aryl methyl ethers. In the π/A isotherms and in BAM, the phenols show patches of the solid condensed phase at large molecular areas and the monolayers collapse at high pressures. Furthermore, the dimensions of the unit cell obtained by GIXD measurements are compatible with an arrangement of the phenyl rings that allows one aryl ring to interact with four adjacent phenyl rings in an edge-to-face arrangement, which leads to a significant binding energy. The experimental data are in good agreement with DFT calculations of 2D crystalline benzene and p-cresol arrangements. The enhanced monolayer stability of phenol 5a can be explained by hydrogen bonds of the hydroxyl group with water and van der Waals forces between the alkyl chains and aryl-aryl interactions.