Dipole interactions in monomolecular layers

Dipole interaction energies have been calculated for two-dimensional arrays of point dipoles in a medium of unit dielectric constant for point dipole moments of 1.5 Debye spaced 4.81 A apart. The latter values were selected to correspond to single alkane chain amphipathic molecules in a hexagonal close-packed state. For horizontal dipole planar arrays the following values (kcal/mole) are reported: polarized hexagonal, -0.805; antipolarized square, -0.744; locally polarized hexagonal with holes, -0.598; and locally polarized hexagonal sites at centers of hexagons filled with dipoles in a triangularly polarized superlattice, -0.462. Calculations for a perpendicular dipole planar array showed excellent agreement with a previously reported value. Applications to gas-condensed phase interfacial dipole arrays are discussed with the emphasis placed on the air-water interface. It is concluded that dipole-dipole interactions are not normally a structure determining factor in close-packed monomolecular films but they may play a role in certain special cases such as with monolayers of zwitterionic phospholipids.