Optical and Direct Force Measurements of the Interactions between Monolayers of Aromatic Macrocycles on Surfactant Monolayers N. Lavrik and D. Leckband* Department of Chemical Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 Received April 19, 1999. In Final Form: July 22, 1999 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. Introduction During the past decade, several techniques have been used to investigate the forces between biomimetic materi- als. An approach to studying the molecular details of their interactions is to directly measure the forces between the surfaces of interest. 1-3 In particular, force-distance profiles can be measured with the surface force apparatus (SFA). 1,2,4-8 These profiles can be quantitatively analyzed in terms of the different contributing fundamental surface forces, such as van der Waals, electrostatic double layer, 6,7 hydration, 8,9 osmotic, 4 steric, 10 and hydrophobic interac- tions. 11 Direct force measurements of specific receptor- ligand binding events, 12-15 and nonspecific protein inter- actions 13,16 have also been carried out with the SFA technique. 3 However, the complexity of most noncovalent biological bonds makes it difficult to probe the strength of individual chemical contributions to typical receptor- ligand bonds. As a result, analyses of specific biochemical interactions typically report the net bond strength rather than the details of the physical interactions. 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