Molecular Organization via Ionic Interactions at Interfaces. 1. Monolayers and LB Films of Cyclic Bisbipyridinium Tetracations and Dimyristoylphosphatidic Acid

Abstract

Organization of nonamphiphilic tetracation cyclobis(paraquat-p-phenylene), BBP4+, at the lipid/water interface using cospreading and adsorption techniques has been investigated. Dimyristoylphosphatidic acid (DMPA) was used as the anchor lipid. The cospreading technique involves the spreading of a mixed solution of the lipid and BBP4+ while the adsorption technique utilizes diffusion-controlled adsorption of BBP4+ at the lipid/water interface. Surface pressure (π)-, surface potential (Δ V)-, and surface reflection (ΔR)-area isotherms along with UV spectroscopy techniques were used for characterization of the organization parameters. The π-A isotherms of the cospread DMPA/BBP4+ monolayers show expansion at low π and converge toward the DMPA isotherms at high π. The cospread monolayer isotherms show maximum expansion at a 1:1 DMPA:BBP4+ molar ratio. This type of behavior has usually been interpreted in terms of penetration (at low π) into and squeezing out (at high π) of the adsorbate from the hydrophobic part of the lipid monolayer. On the basis of conclusive evidence from ΔV and ΔR measurements, we propose a model which explains the expansion and convergence of isotherms without invoking penetration of adsorbate into the lipid monolayer. This model also anticipates the lipid:adsorbate ratio at which maximum expansion of isotherms and the formation of a compact Langmuir monolayer of adsorbate is expected and observed. The cospread monolayers were found to be stable (no loss of BBP4+) with time and could be transferred to quartz substrates using the vertical dipping method. The relative merits of the cospreading and adsorption techniques are discussed. Adsorption kinetics of BBP4+ at the DMPA/water interface are rationalized in terms of the Nernst diffusion layer model. The diffusion coefficient for BBP4+ was calculated to be 3.75 × 10−6 cm2/s, giving a Nernst layer thickness of 0.033 cm. © 1993, American Chemical Society. All rights reserved.