Optical and direct force measurements of the interactions between monolayers of aromatic macrocycles on surfactant monolayers

The water soluble anionic dye, cobalt phthalocyanine disulfonate (PcCoDS), was used to prepare π-electron terminated model monolayers with a high surface free energy. We report the en face self-assembly of monomeric PcCoDS monolayers on dioctadecydiammonium bromide (DODAB). Direct surface force measurements showed that the phthalocyanine overlayers increased the adhesion between the surfactant membranes in water nearly 100-fold. This increased attraction correlated with the dye-induced aggregation of DODAB vesicles. Simultaneous force and electronic absorbance measurements indicate that the formation of strong adhesive contacts between the dye layers corresponds with phthalocyanine dimerization. Further, the adhesion increased in proportion to the dye coverage, and, at the maximum dye coverage, it is at least as strong as hydrophobic interactions that stabilize the membranes. The surface free energy of PcCoDS/DODA membranes, determined from JKR analysis of the contact area vs applied load, is 5.2 ± 0.4 mN m^-1. Analysis of the intersurface attraction using Lifschitz theory for multilayered systems suggests that the dispersion force contributes substantially to the dye interactions. Such forces acting between assemblies of other aromatic compounds in water may similarly contribute to the stability of molecularly engineered materials.