Polarized infrared study on the structure of two-dimensional nanoclusters of partially fluorinated long-chain fatty acid salts at ambient and elevated temperatures

Abstract

Atomic force microscopy (AFM) images of the single monolayer of five partially fluorinated fatty acid salts [CF3(CF2)m(CH2)nCOO−]2Cd2+ [(m,n)=(7,10), (7,16), (7,22), (5,22), and (3,22)] transferred from aqueous Cd2+ subphase to solid substrate revealed that the molecules had assembled into monodispersed two-dimensional clusters of tens of nanometers, whose morphology developed systematically with (m,n). Polarized infrared measurements detected a corresponding change with (m,n) in the –(CF2)–m and –(CH2)–n orientation angles. It is found that the van der Waals interaction between the C–C–C trans zig–zag planes of adjacent hydrocarbon segments is the driving force for the cluster formation, while the overlapping interaction between the fluorocarbon tails of neighbor salt molecules is responsible for cluster compactness. Grazing incidence reflection absorption spectra of the (m,n)=(7,10) single monolayer recorded during temperature elevation from 25 to 150 °C show that heating has caused the hydrocarbon chain and the terminal C–CF3 axis to be gradually randomly oriented, while the carboxylate C2 symmetry axis and the fluorocarbon long axis to realign toward the substrate normal. It is discovered that the single monolayer has thermal memory and cyclic heating-cooling treatment can render an excellent thermal stability of 120 °C to the –(CH2)–10 and CF3(CF2)–7 orientations. The different thermal behavior of the corresponding five-layer Y-type LB films, reported by Naselli, Swalen, and Rabolt [J. Chem. Phys. 90, 3855 (1989)] was explained as due to the longitudinal interaction between the headgroups of adjacent layers.