Stabilization of Tilt Order by Chain Flexibility in Langmuir Monolayers

Langmuir monolayers are modeled as systems of short chains, which are confined to a planar surface at one end, but free to move within the plane. The phase behavior is calculated in a mean field approximation, which combines the self consistent field method with elements of classical density functional theory. It is shown that phases with tilt order are unstable in systems of stiff chains, but can be stabilized by chain conformational entropy in systems of sufficiently flexible chains. The chain entropy is also responsible for the appearance of an additional untilted phase, the liquid expanded phase. The region of stability of the different phases is discussed, and their microscopic structure is analyzed in some detail. PACS numbers: 64.75, 68.18, 68.35