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

Atomic force microscopy (AFM) images of the single monolayer of five partia lly fluorinated fatty acid salts [CF3(CF2)(m)(CH2)(n)COO-](2)Cd2+ [(m,n)=(7 ,10), (7,16), (7,22), (5,22), and (3,22)] transferred from aqueous Cd2+ sub phase to solid substrate revealed that the molecules had assembled into mon odispersed two-dimensional clusters of tens of nanometers, whose morphology developed systematically with (m,n). Polarized infrared measurements detec ted a corresponding change with (m,n) in the -(CF2)-(m) and -(CH2)-(n) orie ntation 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 cluste r compactness. Grazing incidence reflection absorption spectra of the (m,n) =(7,10) single monolayer recorded during temperature elevation from 25 to 1 50 degrees C show that heating has caused the hydrocarbon chain and the ter minal C-CF3 axis to be gradually randomly oriented, while the carboxylate C -2 symmetry axis and the fluorocarbon long axis to realign toward the subst rate normal. It is discovered that the single monolayer has thermal memory and cyclic heating-cooling treatment can render an excellent thermal stabil ity of 120 degrees 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)] w as explained as due to the longitudinal interaction between the headgroups of adjacent layers. (C) 2000 American Institute of Physics. [S0021-9606(00) 70327-8].