A DETERMINATION OF THE PHASE-DIAGRAM OF RELAXED LANGMUIR MONOLAYERS OF BEHENIC ACID

Grazing incidence x-ray scattering (GIXS) and Brewster angle microscop y (BAM) are used to determine the pi-T phase diagram of Behenic acid m onolayers supported on the surface of water (pH=2.0) over the temperat ure range of 3 degrees C to 20.6 degrees C. The phase diagram is const ructed from measurements taken during isothermal compressions in which the surface pressure relaxed to a stable value at each surface densit y, and during temperature scans at fixed average surface density. The phase diagram is different than those previously reported for Behenic acid primarily because of the surface pressure relaxation. For tempera tures less than 12 degrees C the phase diagram exhibits similar phases and topology as the published diagrams, although the location of the phases in the pi-T plane is different. Temperature scans combined with the isotherms, and the Clausius-Clapeyron relation are used to determ ine three coexistence lines that meet in a triple point. Changes in en tropy across the phase boundaries are determined. Near room temperatur e (20.6 degrees C) only one phase is measured over the range of surfac e pressure from 0 dynes/cm to the collapse pressure in contrast to rep orted measurements on monolayers out of equilibrium (i.e., when the su rface pressure is not allowed to relax) that exhibit several phases ne ar room temperature at pressures higher than the collapse pressure. Di screpancies are observed between the average area per molecule (A(T)) and the area per molecule determined from measurements of the unit cel l (A(X)) in the close parked regions of the phase diagram. It is conje ctured that the pi A(X) plane is a better representation of the ordere d equilibrium monolayer phases than the pi-A(T) plane. Isotherms plott ed in the pi-A(X) plane are used to determine the compressibility of t he ordered phases and the nature of the phase transitions. (C) 1996 Am erican Institute of Physics.