Phase diagrams for monolayers of long-chain thioacetates

Brewster-angle microscopy has been employed to determine the phase diagrams of Langmuir monolayers of several long-chain thioacetates: S-hexadecyl acetate, S-octadecyl acetate, S-eicosyl acetate and S-docosyl acetate. The phases in S-octadecyl acetate were identified by carrying out a dilution study with octadecyl acetate. A composite thioacetate diagram could be established by superimposing the diagrams for the individual thioacetates with suitable shifts to account for the differences in chain length. Infrared reflection-absorption spectroscopic studies of monolayers of octadecyl acetate and S-octadecyl acetate show that the differences between their phase diagrams can be attributed, at least in part, to the extent of hydrogen bonding of the carbonyl group.