Modulated structures and competing interactions in amphiphilic monolayers

Abstract

We investigate the effects of electrostatic interactions on the phase behavior and structure of insoluble Langmuir monolayers at the liquid/air interface. Both for charged and neutral monolayers, the competition between such repulsive long-range and attractive short-range interactions of the monolayer tends to stabilize modulated phases. Phase diagrams are obtained in two limits: (i) close to the liquid-gas critical point and (ii) at low temperatures.Monolayers of insoluble amphiphilic molecules, such as surfactants, fatty acids, and phospholipids at the liquid/air interface (Langmuir monolayers), have been studied quite extensively over the last 60 years (for a general review, see refs. 1 and 2). They are of fundamental interest because of their variety of two-dimensional phase transitions. In addition, these monolayers are studied as simple models for biological membranes of phospholipids. Measurements of surface pressure versus area per molecule show that monolayers undergo a two-dimensional liquid-gas transition at very low surface pressure (<0.1 dyne/cm; 1 dyne = 10 4N) (3)(4)(5). At higher surface pressures, a peculiar "kink" in the isotherms is seen in many experiments. The origin of this singularity is not clear and is a matter of dispute [refs. 6 and 7; see, for instance, ref. 8 and ref. 9 (and references therein)]. It has been interpreted as either a second-order transition between two liquid phases (termed "liquid-condensed" and "liquid-expanded") or as a first-order liquid-solid transition under poorly controlled conditions: presence of impurities, undersaturated water vapor pressure, retention of the spreading solvent, or nonequilibrium determination of the isotherms (6, 7).Recently, other experimental techniques have been developed and applied to the study of structural properties of monolayers. Among others they include electric surface potential (10-12) and viscoelastic measurements (13), nonlinear optics (14), epifluorescence microscopy (15-18), and x-ray diffraction (19, 20) from a synchrotron source. The epifluorescence microscopy, for example, allows a direct visualization of monolayers on length scales of micrometers. In lipid monolayers, an organization of liquid-like and solidlike regions that repeat periodically is seen and is shown schematically in Fig. 1.In this paper, we address the interesting question of the physical origin of such modulated structures. We briefly present theoretical calculations (21-23) that explain such modulated structures as the result of a competition between dipolar and short-range interactions of the monolayer. We also draw the analogy with other systems (magnetic films and ferrofluids) where similar competition gives rise to modulated structure on different length scales.Many neutral amphiphilic molecules carry a permanent dipole. Dipole-dipole interaction between two dipoles vanes as r-3, where r is the distance between the two dipoles. In two dimensions, we can estimate the dipolar energy ofa monolayer, which has a concentration modulation with wav...

Anomalous pH-Dependent Enhancement of p-Methyl Benzoic Acid Sum-Frequency Intensities: Cooperative Surface Adsorption Effects

“…A clever solution to this problem was first proposed in classic Redlich–Kister (RK) mixing models of nonideal systems, where the small expansion parameter is the difference in the number densities, ρ A – – ρ HA , which is reasonably expected to be small near p K . A similar expansion using this difference was also exploited by Andelman et al ,, At the two extreme pH ranges, when the difference in number densities gets large, the prefactor ρ A – ρ HA in eq ensures that the excess free energy is negligible because ρ A – is small at low pH, and ρ HA is small at high pH. Thus, eq may in fact be used with confidence over the entire pH range in fitting data to a good approximation.…”

“…A clever solution to this problem was first proposed in classic Redlich−Kister 39 (RK) mixing models of nonideal systems, where the small expansion parameter is the difference in the number densities, ρ A − − ρ HA , which is reasonably expected to be small near pK. A similar expansion using this difference was also exploited by Andelman et al 41,42,44…”

Origin of similarity of phase diagrams in amphiphilic and colloidal systems with competing interactions

“…In order to describe microsegregation in systems with competing interactions, several authors extended functional (1) by including different repulsive terms. 8,9,31 Unfortunately, such an approach suffers from an inconsistent treatment of the attractive and repulsive parts of interactions; while the former is included in the coarse-grained functional, the latter has a microscopic form. The universal emergence of modulated phases was noted in these and other studies, but it remains unclear why the pattern formation in the presence of frustration should be the same as in amphiphilic systems.…”

Striped nanowires and nanorods from mixed SAMS

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