Competing interactions and domain-shape instabilities in a monomolecular film at an air-water interface

Abstract

Domain-shape instabilites are investigated in a two-dimensional binary mixture of near-critical composition, a monomolecular film confined to an air-water interface and composed of a phospholipid and cholesterol. We apply the methodology of spectral analysis to the quantitative description of domainwall configurations. This permits us to identify an elliptic instability and a branching instability leading into a "melted" stripe phase in the vicinity of the consolute point. In between, a regime exists in which domain-wall fluctuations exhibit a capillary-wave spectrum. PACS numbers: 68.10.-m, 61.30.Cz, 64.60.-i, 77.80.DjCompeting interactions give rise to a variety of complex phases in condensed matter. x A realization of this principle is afforded by systems whose constituents simultaneously experience mutual magnetostatic or electrostatic repulsion and an attractive interaction of short range. This scenario has been invoked to account for the formation of domains in uniaxial ferromagnets, 2 " 5 in ferrofluids, 6 and in ferroelectrics 7 by predicting periodic modulations of the magnetization and polarization, respectively. The competition of demagnetizing and domain-wall contributions to the free energy also leads to instabilities of individual domain and droplet shapes and governs the dynamics of their fluctuations. 4,8 Recently, the formation of domains has been discovered during phase coexistence in monomolecular films of phospholipids confined to an air-water interface. 9 Theoretical analyses 10,11 assume the complete equivalence, at the level of mean-field theory, of this phenomenon in two-dimensional amphiphilic films to domain formation in the aforementioned magnetic systems. Specifically, they invoke the van der Waals attraction between aliphatic chains as the source of a domainwall energy which is balanced by a depolarizing term arising from repulsive electrostatic interactions between the polar moieties of these compounds. Consequently, the predicted phase diagram contains a coexistence region marked by the appearance of periodic intralayer density or composition modulations. 10 In contrast to the rich diversity of domain morphologies and patterns documented to date in amphiphilic monolayers, 912 one notes the relative scarcity of quantitative analysis. In this Letter we introduce the methodology of spectral analysis, based on digital imageprocessing techniques, to the quantitative description of domain-wall configurations and their excitations. Its application to the study of binary mixed monomolecular films containing cholesterol and dimyristoylphosphatidylcholine (DMPC) (Ref. 13) permits us to identify several distinct regimes of domain-shape stability which are encountered as a film of near-critical composition ap-proaches the upper consolute point terminating a fluidfluid coexistence region. 14 In particular, we document the existence of an elliptic and of a branching instability of the domain walls. The latter manifests itself in the immediate vicinity of the critical point and represents the pr...

Neutral Lipids Induce Critical Behavior in Interfacial Monolayers of Pulmonary Surfactant

“…Appropriate mixtures of dimyristoyl phosphatidylcholine (DMPC) and cholesterol (30)(31)(32) and DPPC-cholesterol (34,35) exhibit the same characteristics of a critical point, although at pressures well below the values at which remixing occurs here. Surfactant preparations, however, that contain the complete set of phospholipids are far from binary mixtures.…”

Investigations of monolayers by fluorescence microscopy

“…These domains were observed to undergo slow shape fluctuations which were image analyzed in terms of excited modes of a circular domain (Seul, 1990;Seul and Sammon, 1990;Seul et al, 1991). The shape distortions increased in amplitude on approaching the critical point, as expected for a line tension decreasing towards zero, and ultimately the morphology converted to a highly branched pattern.…”

Modelling of thermodynamical and dynamical properties of magnetic stripes on 2D hexagonal lattice

“…For example, electrostatic dipole interactions induce stripe-like phases in Langmuir multilayers in water-air interface [1]. Three types of competing O-Cu interactions, with repulsive long range forces caused by substrate mediated elasticity, are responsible for Cu-O stripes in partly O-covered Fe(110) [2].…”