Surface properties of Langmuir films of mono-, di-, and tetra- n-octyl adducts of C 60 at the water–air interface

Abstract

Relatively stable Langmuir films of mono-, di-, and tetra- n-octyl adducts of C 60 were prepared at the water–air interface. The adducts were obtained by selective bulk electrosynthesis at controlled potential. For the films, surface pressure ( π) and a surface potential change (Δ V) were simultaneously measured as a function of surface area per molecule ( A) during the film compression. The spreading properties of the adducts strongly depend on the number of n-octyl chains linked to the C 60 cage and nature of the spread solution. The determined zero-pressure limiting area per molecule ( A 0) is larger the more n-octyl chains are attached to the C 60 cage. Aggregated multilayer films are formed for the toluene and tetrahydrofuran spread solutions while liquid monolayer films for the chloroform solutions. Remarkably, relatively stable liquid monolayer films of pristine C 60 are obtained also from the chloroform solutions. For relatively concentrated chloroform solutions, plateaus are developed in the π– A adduct isotherms. Isotherms for both concentrated and diluted chloroform solutions show reversible compression and expansion with virtually no hysteresis. Comparison of the estimated and determined A 0 values indicates that all adduct molecules are horizontally oriented in the monolayer films. Unexpectedly, large mean dipole moment components normal to the water–air interface, determined from inflection points of the Δ V– A isotherms, are presumably due to preferential orientation of water molecules adjacent to the interface of the water subphase and the floating adduct film.