Langmuir Monolayers with Fluorinated Groups in the Hydrophilic Head. 1. Comparison of Trifluoroethyl Behenate and Ethyl Behenate Monolayers: Molecular Models, Mechanical Properties, Stability

Abstract

In a series of three related papers we compare mechanical properties and stability, morphology and structure, and electrostatic potential and ellipsometric thickness of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) Langmuir monolayers. The aim of these papers is to study the effect of fluorination of a methyl group in the hydrophilic head on monolayer properties and structure. In the present Part 1 we show that trifluoroethyl ester forms significantly more unstable films with higher compressibility (lower compressional modulus) than the unsubstituted ethyl ester. Both TFEB and EB surface pressure−area isotherms show compression−expansion hysteresis, but this hysteresis is larger for the fluorinated ester. The surface pressure−area loop of TFEB is shifted to larger molecular areas as compared to EB and gives larger limiting molecular areas at zero compression. This points to different volumes and/or conformations of the fluorinated and nonsubstituted hydrophilic heads. Maps of molecular lipophilicity and molecular electrostatic potential, based on semiempirical quantum mechanical models of the two molecules in vacuo, relate the observed differences in monolayer properties to decreased hydrophilicity of the trifluoroethyl group and a stronger electrostatic repulsion between the hydrocarbon chains of TFEB. Such a repulsion results from polarization of the CH2 groups adjacent to the heads that is more significant for the trifluoroethyl behenate molecule.