  A model of rods with heads of variable size, which are confined to a planar
surface, is used to study the influence of the head group size on tilted phases
in Langmuir monolayers. Simple free energy considerations as well as exact zero
temperature calculations indicate that molecules with small head groups tilt
towards next nearest neighbors, and molecules with larger head groups towards
nearest neighbors. This provides a possible explanation for recent experimental
results, and for details of the generic phase diagram for fatty acid
monolayers.
