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

Relatively stable Langmuir films of mono-, di-, and tetra-n-octyl adducts O f 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 (pi) and a surface potential change (AV) were simultaneous ly 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 (Ao) is l arger the more n-octyl chains are attached to the C-60 cage. Aggregated mul tilayer films are formed for the toluene and tetrahydrofuran spread solutio ns while liquid monolayer films for the chloroform solutions. Remarkably, r elatively stable liquid monolayer films of pristine C-60 are obtained also from the chloroform solutions. For relatively concentrated chloroform solut ions, plateaus are developed in the pi -A adduct isotherms. Isotherms for b oth concentrated and diluted chloroform solutions show reversible compressi on and expansion with virtually no hysteresis. Comparison of the estimated and determined A(0) values indicates that all adduct molecules are horizont ally oriented in the monolayer films. Unexpectedly, large mean dipole momen t components normal to the water-air interface, determined from inflection points of the AV-A isotherms, are presumably due to preferential orientatio n of water molecules adjacent to the interface of the water subphase and th e floating adduct film. (C) 2001 Elsevier Science B.V. All rights reserved.