An Experimental Study of the Stability and Dynamics of Langmuir Films of Fullerene Derivatives and Their Mixtures with Pentadecanoic Acid
The synthesis and behavior of the Langmuir monolayers of several fullerene (C60) derivatives is reported. True monolayers are found only for one derivative with a 12 C-atom chain substituent. The interfacial rheology of these monolayers has been studied by measuring the thermal capillary wave spectra. It is found that the elasticity and the dilational viscosity increase from diluted monolayers to an area per molecule of approximately 150 Å2, which roughly corresponds to the equilibrium distance for the intermolecular potential of the fullerene molecules. For more compressed monolayers both the elasticity and viscosity decrease. These results can be qualitatively explained in terms of the model described by Evans for monolayers of C60 (Evans, A. K. J. Phys. Chem. 1998, 102, 7016). The mixed Langmuir monolayers of x pentadecanoic acid + (1 - x) fullerene derivative have also been studied, for 0 ≤ x ≤ 1. True monolayers are found up to high values of x. The monolayers present a liquid expanded to liquid condensed transition up to values of x corresponding to one fullerene molecule per six of acid. This can be interpreted in terms of the ability of the mixture to maintain the hexagonal structure characteristic of the monolayer of pure pentadecanoic acid.