Two-dimensional crystallization of phthalocyanine pigments at the air/water interface

Two-dimensional crystallization of highly planar phthalocyanine (Pc) pigments underneath the headgroups of a lipid Langmuir monolayer was observed and characterized by synchrotron X-ray diffraction at grazing angles of incidence (GID). The crystallization was achieved through spontaneous adsorption of positively charged, water-soluble Pc's to a spread dihexadecyl phosphate (DHDP) monolayer at the air/water interface. Analysis of the GID and rod profiles show that the lipid, pigment, and counterions form a complex in which the pigment plane is tilted with respect to the liquid surface; this is consistent with previous independent X-ray reflectivity investigations. In addition, the two-dimensional crystalline order of DHDP monolayers on pure H2O has been determined and an analysis of its structure both before and after complexation is presented.