  We formulate a simple effective model to describe molecular interactions in a
lipid monolayer. The model represents lipid molecules in terms of
two-dimensional anisotropic particles on the plane of the monolayer. These
particles interact through forces that are believed to be relevant for the
understanding of fundamental properties of the monolayer: van der Waals
interactions originating from lipid chain interaction, and dipolar forces
between the dipole groups of the molecular heads. Thermodynamic and phase
behaviour properties of the model are explored using density-functional theory.
Interfacial properties, such as the line tension and the structure of the
region between ordered and disordered coexisting regions, are also calculated.
The line tension turns out to be highly anisotropic, mainly as a result of the
lipid chain tilt, and to a lesser extent of dipolar interactions perpendicular
to the monolayer. The role of the two dipolar components, parallel and
perpendicular to the monolayer, is assessed by comparing with computer
simulation results for lipid monolayers.
