Langmuir Monolayers with Fluorinated Groups in the Hydrophilic Head: 2. Morphology and Molecular Structure of Trifluoroethyl Behenate and Ethyl Behenate Monolayers

Abstract

Morphology and molecular structure of Langmuir monolayers of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) were investigated via Brewster angle microscopy and grazing incidence X-ray diffraction (GIXD) at the air−water interface. When spread at 60 Å2/molecule, both substances form islands of a condensed phase. At smaller areas, they organize in “archipelago structures” with compact sections and 2D suspensions of microcrystals floating in streams on the free water surface. The microcrystals that are close to the “coastlines” adhere to the compact monolayer. The latter contains dark areas (“lakes”) that are being closed under compression. At expansion, the compact monolayers disintegrate to the same archipelagos but the microcrystals are now located mainly along the coastlines. Such mechanism of growth and disintegration of the apparently compact TFEB and EB films suggests a polycrystalline structure. Different friction at the microcrystal boundaries could be responsible for different compressional moduli of the TFEB and EB monolayers. The presence of voids in the films could be the cause of the compression−expansion hysteresis of the dynamic π/A isotherms of TFEB and EB and the larger mean molecular areas of TFEB. The GIXD part of the study shows that the TFEB islands and microcrystals consist of closely packed vertically oriented molecules occupying 18.9 Å2 of the water surface. The islands of EB are also densely packed, but their molecules are tilted at 17.6 ± 0.3° toward next-nearest neighbors. Compression does not change the GIXD molecular area of the TFEB monolayer. It decreases the tilt angle of the EB molecules causing transition from the tilted L2‘ phase to the upright S or CS phase. Above 12.3 ± 1.2 mN/m, the EB molecules occupy Axy = 18.8 Å2. At high surface pressure, TFEB and EB monolayers have the same structure with centered rectangular unit cells and the same lattice parameters. The vertical hydrocarbon chains form a “herringbone” arrangement. Analysis of literature data for other Langmuir monolayers and phospholipid bilayers suggests that the structural difference at low surface pressure could be due to the increased hydrophobicity of the trifluoroethyl group as compared with the ethyl ester group. On the basis of some previous GIXD results for monolayers of fatty acids and their methyl and ethyl esters, we speculate that the −CH2CF3 group could be oriented toward air even in the solid condensed S or CS phase. Such a “hook” conformation would explain the negative surface potential found for the TFEB film. This scenario is supported by a molecular model of the TFEB monolayer that will be presented in part 3 of this study.