M. Alba

Reduction in the surface energy of liquid interfaces at short length scales

Liquid–vapour interfaces, particularly those involving water, are common in both natural and artificial environments. They were first described as regions of continuous variation of density, caused by density fluctuations within the bulk phases. In contrast, the more recent capillary-wave model assumes a step-like local density profile across the liquid–vapour interface, whose width is the result of the propagation of thermally excited capillary waves. The model has been validated for length scales of tenths of micrometres and larger, but the structure of liquid surfaces on submicrometre length scales—where the capillary theory is expected to break down—remains poorly understood. Here we report grazing-incidence X-ray scattering experiments that allow for a complete determination of the free surface structure and surface energy for water and a range of organic liquids. We observe a large decrease of up to 75% in the surface energy of submicrometre waves that cannot be explained by capillary theory, but is in accord with the effects arising from the non-locality of attractive intermolecule interactions as predicted by a recent density functional theory. Our data, and the results of comparable measurements on liquid solutions, metallic alloys, surfactants, lipids and wetting films should thus provide a stringent test for any new theories that attempt to describe the structure of liquid interfaces with nanometre-scale resolution.

High-resolution x-ray scattering measurements: I. Surfaces

X-ray scattering investigations of surfaces and interfaces in soft-condensed matter are reviewed. Both high-resolution structural determinations in the direct space and investigations of fluctuations with long-range correlations requiring a high resolution in the Fourier space are discussed. All the scattering cross-sections for diffraction or diffuse scattering are derived within a unified frame, and the experimental aspects related to their measurement are discussed in detail. The general principles are illustrated by various examples of studies of the liquid-vapour interface, Langmuir and Langmuir-Blodgett films, wetting films, polymer films, liquid crystals and liquid-liquid interfaces.

Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

Abstract:The linear compressibility of two-dimensional fatty acid mesophases has been determined by grazing incidence X-ray diffraction. The unit cell parameters of the , , , S and phases of behenic acid and of the phase of myristic acid were determined as a function of surface pressure and temperature. Surface pressure versus molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for a two-dimensional isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10 m/N) are observed in the tilted phases. They are apparently independent on the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in the or phases, S phase, and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted phase and for one direction of the S and phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for the and phases and can be traced to subtle reorganizations upon untilting.

Structure and fluctuations of liquid surfaces and interfaces

Abstract X-ray scattering techniques are increasingly used for the study of liquid surfaces and interfaces. We give here scattering cross-sections for diffraction or diffuse scattering from liquid interfaces and we discuss the examples of the liquid–vapour interface and of films at a liquid–liquid interface. We show that the interfacial structure can be understood as resulting of the interplay between thermal fluctuations, van der Waals forces, and elastic properties.

Ordered interfaces for dual easy axes in liquid crystals

Using nCB films adsorbed on MoS 2 substrates studied by x-ray diffraction, optical microscopy and Scanning Tunneling Microscopy, we demonstrate that ordered interfaces with well-defined orientations of adsorbed dipoles induce planar anchoring locked along the adsorbed dipoles or the alkyl chains, which play the role of easy axes. For two alternating orientations of the adsorbed dipoles or dipoles and alkyl chains, bi-stability of anchoring can be obtained. The results are explained using the introduction of fourth order terms in the phenomenological anchoring potential, leading to the demonstration of first order anchoring transition in these systems. Using this phenomenological anchoring potential, we finally show how the nature of anchoring in presence of dual easy axes (inducing bi-stability or average orientation between the two easy axes) can be related to the microscopical nature of the interface. Introduction Understanding the interactions between liquid crystal (LC) and a solid substrate is of clear applied interest, the vast majority of LC displays relying on control of interfaces. However this concerns also fundamental problems like wetting phenomena and all phenomena of orientation of soft matter bulk induced by the presence of an interface. In LCs at interfaces, the so-called easy axes correspond to the favoured orientations of the LC director close to the interface. If one easy axis only is defined for one given interface, the bulk director orients along or close to this axis [1]. It is well known that, in anchoring phenomena, two major effects compete to impose the anchoring directions of a liquid crystal, first, the interactions between molecules and the interface, second, the substrate roughness whose role has been analyzed by Berreman [2]. The influence of adsorbed molecular functional groups at the interface is most often dominant with, for example in carbon substrates, a main influence of unsaturated carbon bonds orientation at the interface [3]. In common LC displays, there is one unique easy axis, but modifications of surfaces have allowed for the discovery of promising new anchoring-related properties. For instance, the first anchoring bi-stability has been established on rough surfaces, associated with electric ordo-polarization [4] and the competition between a stabilizing short-range term and a destabilizing long-range term induced by an external field, can induce a continuous variation of anchoring orientation [5]. More recently, surfaces with several easy axes have been studied extensively. It has been shown that control of a continuous variation of director pretilt, obtained in several systems [6, 7], is associated with the presence of two different easy axes, one perpendicular to the substrate (homeotropic) and one planar [7, 8]. Similar models can explain the continuous evolution of anchoring between two planar orientations observed on some crystalline substrates [9]. However, in the same time, two easy axes can also lead to anchoring bi-stability [10, 11] or discontinuous transitions of anchoring [9], which is not compatible with the model established to interpret observed control of pretilt. In order to be able to predict if bi-stability or continuous combination of the two easy axes occurs for one given system, it becomes necessary to understand the microscopic origin of the easy axes.

Adsorbed organic monolayers on crystalline substrate: the example of 8CB on MoS2

Abstract We show in this paper how the combination of scanning tunneling microscopy (STM) and X-ray diffraction under grazing incidence (GIXD) is fruitful for the study of ordered interfaces between thick organic films and crystals. In the case of 4-8-alkyl-4′-cyanobiphenyl (8CB) on molybdenum disulphide (MoS2), the commensurability of the adsorbed network has been determined, as well as the intracrystallographic cell structure. This latter one evidences the fine structure of the network characterized by the formation of pair associations in one lamella over two, which reflects the influence of microscopical interactions on the adsorbed structure.

Revealing the Structure of Focal Conics Cores and their Influence on the Evolution with Temperature: An X-ray Study of Ultra-thin 8CB Films

ABSTRACT We have studied by x-ray diffraction the deformations of thin smectic films induced by antagonistic anchorings, as a function of thickness and temperature. The structure of the cores is revealed for thicknesses of the order of 0.15 μm when the deformations are dominated by the presence of the focal conics cores. In a cylindrical geometry imposed by the unidirectional anchoring on the substrate, the cores are tube-shaped rotating grain boundaries (RGB). The spatial extension is of the order of 0.14 × 0.22 μm2, larger than the ones usually proposed in the literature. A drastic evolution of the x-ray scans with temperature is also revealed. It corresponds to a variation of the location of the RGB and is analyzed as the result of a weak variation of the 8CB/air surface tension with temperature.

Organic monolayers : Interface between 8CB liquid crystals and MoS2 monocrystal

Abstract Using scanning tunneling microscopy (STM) and grazing incidence X-ray diffraction (GIXD) we investigate the interface between the 8CB liquid crystal and the MoS2 monocrystalline surface from room temperature to 130°C. The interface is found to be formed by a dilated adsorbed monolayer of 8CB commensurate with the 4×16 superstructure of the MoS2 up to 110°C. Above this temperature, this monolayer undergoes a phase transition to an incommensurate structure leading to a more compact stacking of the molecules.