Chiral and herringbone symmetry breaking in water-surface monolayers

Pattern Formation in Phase-Separated Langmuir and Langmuir Monolayer Films

Mixed monolayer films comprising hydrogenated and fluorinated surfactants can undergo phase separation to produce interfaces with diverse structures at the micrometer and nanometer scales. This review discusses our progress over the past decade to probe the relationship that exists between the molecular structure of the surfactants that comprise the films and the overall patterns formed in the monolayers. We review two main classes of mixed perfluorocarbon-hydrocarbon surfactant systems, including fatty acids and a recently developed family of EDTA-based gemini surfactants. In addition to summarizing the state-of-the-art of this field, the key scientific questions and relationships that require further elucidation are discussed, along with directions for continuing research into this fascinating area of research.

Molecular-Level Structure and Packing in Phase-Separated Arachidic Acid-Perfluorotetradecanoic Acid Monolayer Films

Synchrotron-based X-ray scattering measurements of phase-separated surfactant monolayers at the air-water interface provide molecular-level structural information about the packing and ordering of film components. In this work, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements were used to collect crystallographic structural information for binary mixed monolayers of arachidic acid (AA, C₁₉H₃₉COOH) with perfluorotetradecanoic acid (PA, C₁₃F₂₇COOH), a system that has previously been investigated using a variety of thermodynamic and micron-scale structural characterization methods. GIXD measurements at surface pressures of π = 5, 15, and 30 mN/m indicated that AA in pure and mixed films forms a rectangular lattice at π = 5 and 15 mN/m but a hexagonal lattice at π = 30 mN/m. PA formed hexagonal lattices under all conditions, with films being highly ordered and crystalline (as determined by Bragg peak width) at even the lowest surface pressures investigated. Phase separation occurred for all mixed monolayer film compositions and surface pressures, manifesting as diffraction peaks characteristic of the individual components appearing at different in-plane scattering vector q_xy. For both pure and mixed films, the molecular tilt angle of the AA hydrocarbon chain toward the nearest-neighbor was substantial at low pressures but decreased with increasing pressure. The PA fluorocarbon chain showed negligible molecular tilt under all conditions, and was oriented normal to the subphase surface regardless of surface pressure or the presence of AA in the films. In all cases, the two components in the mixed film behaved entirely independently of film composition, which is exactly the expected result for a fully phase-separated, immiscible system. XR measurements of film thickness at the air-water interface supported these results; overall film thickness approached the calculated ideal surfactant tail lengths with increasing surface pressure, indicating nearly normal oriented surfactants. The overall surfactant packing and crystallographic features of the mixed monolayers are discussed in terms of the lipophobic nature of the perfluorinated surfactant as well as in context of thermodynamic miscibility and domain structure formation reported elsewhere in the literature for these mixed monolayer systems.

Self-assembly of organic monolayers below the freezing threshold

One measure that arctic fish and amphibians use to minimize damage to cellular membranes during cooling and freezing processes is the production of cryo-protective substances. We have mimicked this biological "trick" by using the surface of a cryo-protectant as a liquid subphase for the preparation of organic membranes. Following this innovative approach, quasi two-dimensional amphiphilic monolayers were cooled to -40 degrees C at a liquid/gas interface. To date, the low temperature region of the generic phase diagram for alkane chain molecules has been only "virtually" accessible by tuning the molecular chain length. By extending the temperature range well below the freezing point of water, we gained new insights into membrane stability, morphology, and reorganization at low temperatures. Upon cooling relaxed monolayers at a surface pressure of 4.5 mN/m, we find a transition from a mesophase with tilted chains at ambient temperature toward a crystalline phase with upright chains at low temperatures. Structure factor calculations reveal that the chain alignment in the crystalline phase differs from the classical herringbone configuration.

Herringbone ordering and lattice distortions in a planar-molecule model for langmuir monolayers

A model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out.

Phase Transition in Langmuir Films of Octadecylmalonic Acid

The phase transitions observed in the surface pressure-molecular area (pi-A) isotherms of monolayers of the dicarboxylic compound octadecylmalonic acid (OMA) at the air/water interface were investigated using molecular dynamics simulation in the molecular area ranging from 20 to 50 A2/molecule in a triangular lattice. Potential energy, torsion angles, and hydration of the head groups under periodic boundary conditions have been used for the study. Hydration was done by a stacked array of water molecules relaxing under these boundary conditions. Twelve simulations were performed at T = 300 K to study the conformation of the packed molecular system. Bonded and nonbonded interaction potentials were taken from Biosym force fields. The transitions seen in the pi-A curves are explained from the breaks in the plots of potential energy at various concentrations, related to a liquid expanded-to-liquid condensed (LE/LC) transition, and classified under the first order type. Electron diffraction patterns of these films transferred onto transmission electron microscopy (TEM) grids were studied by high-resolution imaging and showed a clear transition from the LE to the LC phase. Copyright 1998 Academic Press.