Effect of Fluorination of the Hydrophilic Heads on Morphology and Molecular Structure of Langmuir Monolayers of Long-Chain Ethers

Abstract

Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD) were used to compare morphology and molecular structure of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water. When spread at a molecular area of 70 Å2/molecule both substances form islands of condensed phase and suspension of small 2D microcrystals attaching at the borderlines under compression. The DEE islands and the DEE monolayer at low surface pressure exhibit strong in-plane optical anisotropy, while the DFEE islands and the compact DFEE film are isotropic. The anisotropy of the DEE monolayer disappears at 10 mN/m, and above this surface pressure the monolayers with fluorinated and nonfluorinated heads are visually the same. The GIXD analysis identifies a L2‘ phase with NNN molecular tilt and NN lattice distortion in the DEE islands and DEE monolayer at low surface pressure. The maximum tilt angle at zero compression is 22.6°. The islands and the compact DFEE film consist of closely packed upright molecules forming an S phase. At high surface pressure both monolayers have the same upright molecular arrangement with centered rectangular lattice and practically the same unit cell parameters and lattice distortion. Under such conditions the DFEE and DEE molecules occupy the same area of the water surface Axy = A0 = 19.1 Å2. Since the van der Waals radius of the CF3 group is by 16% larger as compared to that of the CH3 group, the same A0 value of the two films means a significantly thinner hydration shell of the fluorinated head. Substitution of the terminal CH3 group in the DEE head by the CF3 group in the DFEE head removes the low-pressure tilted phase and causes formation of the condensed phase of upright fluorinated molecules even at no compression. The decreased headgroup hydration of DFEE could be responsible for the upright versus tilted orientation of the DFEE and DEE molecules at low surface pressure. We suppose that the mechanism of the continuous tilting transition causing erection of the DEE chains is related to an increasing dehydration of the headgroups under compression.