Surface pressure-area isotherms and Brewster angle microscopy images of mixed binary films of dimyristoylphosphatidylclioline (DMPC) and the diacetylene 10, 12-tricosadiynoic acid (TRCDA) were recorded at different temperatures and mole ratios to investigate the molecular interactions and cooperative properties of the films. The experiments revealed that segregation. on the one hand, and significant intermolecular interactions, on the other hand, both contribute to the thermodynamic properties of the phospholipids and the diacetylene assemblies. In particular, the data demonstrate that higher temperatures and greater percentage of DMPC promote repulsion between the liquid-condensed phospholipid monolayer and the TRCDA domains. In contrast, at high TRCDA mole ratios, film contraction occurred (lower molecular areas) due to TRCDA multilayer formation (at high temperature) or intermolecular affinities (at low temperature). (c) 2005 Elsevier Inc. All rights reserved.