The monolayer isotherms of heneicosanoic acid show a variety of features in the range 1–8 °C; such features are often assumed to be solid-fluid or fluid-fluid transitions. Using x-ray diffraction (and searching for in-plane and off-plane peaks) we find (a) two distinct distorted-hexagonal (DH) solids at high pressures, both with essentially vertical chains; (b) two distinct tilted-DH solids at intermediate pressures; and (c) an expanded solid at the lowest pressures. All ‘‘plateaus’’ in these isotherms are in-plane structural transitions; all ‘‘kinks’’ are solid-solid tilting transitions.

• Received 7 September 1989

DOI:https://doi.org/10.1103/PhysRevLett.65.191