Surface-tension-induced liquid-crystal growth of monomeric lipid A-diphosphate in aqueous dispersions is reported as a function of concentration, (c), and temperature, (T), and at low ionic strength (10(-3) M). As the temperature was varied, a solid liquid transition was revealed in the surface layer at a fixed lipid A-diphosphate bulk concentration. Here, the development of different two-dimensional (2-d) faceted crystal morphologies was observed and, as growth proceeded, these faceted 2-d crystals became unstable. Selected area electron microscopy diffraction (SAED.) and X-ray diffraction (XRD) measurements of the faceted 2-d crystalline lipid A-diphosphate layers exhibited a pseudohexagonal molecular arrangement. The crystalline layer was a smectic F, SF, phase below the critical temperature, T-c, and a smectic I, S-I, phase above T-c (15 degrees C). Both phases could be described in terms of the same C-centered monoclinic unit cell. The in-plane order extended for a limited distance although the layers were coupled. The analysis of the SAED patterns revealed short-range order in the SF phase (5-15 degrees C), but long-range order in the S-I phase, for the temperature range 15-30 degrees C. The observed 2-d solid hexatic phase and the 2-d liquid hexatic phase had correlation lengths of 220 angstrom. This, the hexatic phase, displayed short-range in-plane positional order and quasi long-range, sixfold bond-orientational order. The S-I phase showed long-range order characteristics of a hexatic 2-d crystal. The two-, four-, or six-layer crystalline lipid A-diphosphate films exhibited 2-d hexatic order and 6n-fold bond-orientational order. These films did not evolve into the S-F phase, demonstrating that the two phases were thermodynamically distinct. A finite tilt angle of phi = 15 degrees was calculated for the lipid A-diphosphate molecule; the tilt was toward the small side of the rectangular 2-d lattice. The constraint of six close-packed acyl chains in two distinct phases with the same symmetry suggests that the S-F -> S-I transition was first-order.