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 pi-electron terminated model monolayers with a high sur face free energy. We report the en face self-assembly of monomeric PcCoDS m onolayers on dioctadecydiammonium bromide (DODAB). Direct surface force mea surements showed that the phthalocyanine overlayers increased the adhesion between the surfactant membranes in water nearly 100-fold. This increased a ttraction correlated with the dye-induced aggregation of DODAB vesicles. Si multaneous force and electronic absorbance measurements indicate that; the formation of strong adhesive contacts between the dye layers corresponds wi th phthalocyanine dimerization. Further, the adhesion increased in proporti on to the dye coverage, and, at the maximum dye coverage, it is at least as strong as hydrophobic;interactions that stabilize the membranes. The surfa ce free energy of PcCoDS/DODA membranes, determined from JKR analysis of th e contact area vs applied load, is 5.2 +/- 0.4 mN m(-1). Analysis of the in tersurface attraction using Lifschitz theory for multilayered systems sugge sts that the dispersion force contributes substantially to the dye interact ions. Such forces acting between assemblies of other aromatic compounds in water may similarly contribute to the stability of molecularly engineered m aterials.