Pattern Formation in Langmuir Monolayers Due to Long-Range Electrostatic Interactions

Abstract

A distinctive characteristic of Langmuir monolayers that bears important consequences for the physics of structure formation within membranes is the uniaxial orientation of the constituent dipolar molecules, brought about by the symmetry break which is induced by the surface of the aqueous substrate. The association of oriented molecular dipoles with the interface leads to the formation of image dipoles within the polarizeable medium – the subphase – such that the effective dipole orientation of every of the individual molecules is strictly normal to the surface, even within molecularly disordered phases. As a result, dipole-dipole repulsions play an eminently important role for the molecular interactions within the system – independent of the state of phase (while the dipole area density does of course depend on the state of phase) – and control the morphogenesis of the phase boundaries in their interplay with the one-dimensional (1D) line tension between coexisting phases. The physics of these phenomena is only now being explored and is particularly exciting for systems within a three-phase coexistence region where complete or partial wetting, as well as dewetting between the coexisting phases may be experimentally observed by applying fluorescence microscopy to the monolayer films. It is revealed that the wetting behavior depends sensitively on the details of the electrostatic interactions, in that the apparent contact angles observed at three-phase contact points depends on the sizes of the coexisting phases. This is in sharp contrast to the physics of wetting in conventional 3D systems where the contact angle is a materials property, independent of the local details. In 3D systems, this leads to Young’s equation – which has been established more than two centuries ago.

We report recent progress in the understanding of this unusual and rather unexpected behavior of a quasi-2D system by reviewing recent experimental results from optical microscopy on equilibrium phase shapes, non-equilibrium phenomena – such as relaxation of the shapes after distortions inferred by Laser tweezers or local impulse heating – and rheological properties of the system. The theoretical analysis of the underlying molecular interactions leads to a comprehension of the observed phenomena and reveals microscopic properties of the system in quantitative terms. In view of the recently proposed “lipid raft” hypothesis, a particularly fascinating implication of our results is the possibility that biochemical reactions which depend on complex interactions between membrane-bound proteins might be controlled by the non-conventional physics of the 2D system: As an electrogenic event – such as ion transfer across the membrane – changes the electrostatic properties of the membrane surface it might concurrently infer wetting between 2D phases and thus lead to the conjunction of membrane areas that were originally separated within the plane. If two reactants (e.g., membrane-bound enzymes) are dissolved in distinct phases, such a colloidal reorganization might rearrange the micro-evironment to bring them into close vicinity – and thus trigger the biochemical reaction.