Interactions across liquid thin films

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Abstract

The paper gives an overview on interactions between two opposing interfaces in liquids. The interfaces are either solid (two silica surfaces) or fluid (in foam films or in membrane stacks). The liquids are either simple ones (water or salty water) or contain polyelectrolytes. In the latter case, the confinement leads to a structuring of the polyelectrolytes which leads to oscillatory forces in addition to DLVO forces. The effect of interface properties on the interactions is studied by modification of interfaces. In case of solid interfaces, they are modified by the adsorption of polyelectrolyte multilayers, while fluid interfaces are varied by changing the amphiphile/polymer combination. With regard to the studied problem, different methods are used to study the interactions.

Introduction

Thin liquid films play an important role in many macroscopic colloidal systems which consist of a dispersed and a continuous phase. For instance, in an aqueous foam air bubbles are dispersed in surfactant solutions, or in suspensions particles are dispersed in a liquid. Another example is vesicles in solutions. In all examples, the stability of the macroscopic system is dominated by the stability of the thin films separating the compartments of the dispersed phase, which is strongly related to the interactions between the opposing interfaces. It is evident, that the study of these thin films is of great interest for technical applications. For fundamental science thin films are suitable model systems for studies of interfacial forces which dominates properties of all colloidal systems. This explains the high attraction of these films for studies in colloidal science since many decades.

In order to study forces between particles in a colloidal suspension films between solid interfaces are used as a model system. A suitable technique to study forces between two solid interfaces is a colloidal probe atomic force microscope (CP-AFM). In the experiments presented in this paper, the solid interfaces were coated by layer-by-layer adsorption of polyelectrolytes, a method, which was introduced by Decher et al. [1] and Decher and Schmitt [2]. It is one of the most widely used surface modification techniques [3], [4], [5], [6], [7], [8]. For many polyelectrolytes, physisorption onto a charged surface is irreversible and results in surface charge changes after each adsorption step [9]. Thus, after the first adsorption step the surface can serve as a substrate for the adsorption of an oppositely charged polyelectrolyte and so on until the desired number of layers is reached. The polyelectrolyte layers have a well-defined thickness in the nanometer range, since adsorption is limited by the electrostatic self-repulsion of the polyelectrolyte blocks. The chapter of Schönhoff et al. in this issue describes the structure and hydration of polyelectrolyte multilayers more in detail [10].

Despite the broad variety of applications, studies on the interactions between surfaces that are coated with polyelectrolyte multilayers are scarce [11], [12], [13], [14]. But, knowledge about this matter is of high importance for the control of stability of colloidal dispersions. Because of the influence of the substrate, the thickness increase per adsorbed cycle of the initial layers (the so-called precursor regime) is different from the one in the “true” multilayer regime, which is typically reached after six adsorbed layers [12], [15]. The paper compares surface force data on both the precursor and the multilayer regime for the most widely used system, PAH/PSS. Interaction between silica substrates coated with multilayers both with and without a precursor layer of polyethyleneimine (PEI) are studied. PEI is known to lead to better multilayer growth [13], [16] and show strong affinity towards silica surfaces [17].

In order to study forces between foam bubbles, free-standing foam films (air/liquid/air) are used as a model system. In the presented studies, the forces were measured with a thin film pressure balance (TFPB). Foam films are of interest with respect to two aspects: (1) the film can be considered as the building block of a foam so that its properties affect the behavior of the whole macroscopic foam [18]. In this context, it is the molecular structure at and near the film surfaces rather than the structure of the film core which is important. (2) Secondly, the free-standing film presents a cavity which allows to study the effect of geometrical confinement on the structuring of polymers. Several years ago, it was found that thin films made of semi-dilute polyelectrolyte solutions exhibit a stratification process and that the forces between film surfaces are oscillatory [19]. These oscillatory forces were attributed to the layering of polymer coils, by analogy to forces in thin films of micellar solutions [20]. However, it was demonstrated then that the oscillations were rather related to the period of the semi-dilute network formed by the polymer chains [21], [22], [23]. Later on, experiments were performed with other polyelectrolytes, both with the TFPB and with CP-AFM. In combination with X-ray and neutron scattering the results fully confirmed the picture of a network-like structure [24], [25].

In the present paper, especially the influence of the rigidity of the polymer backbone and the degree of polymer charge on the polyelectrolyte structuring is studied.

Since long time, phospholipid and surfactant membranes have been studied as simple self-assembled model systems for more complex biological counterparts [26]. The physics and chemistry of lipid membranes and of lamellar surfactant systems is also important for many applications in the pharmaceutical, food and cosmetic industries. At the same time, interacting fluid membranes exhibit complex phase behavior, self-assembly properties and a variety of long-range non-covalent interactions which are of interest as low-dimensional colloidal model systems. Moreover, fundamental properties of interacting and fluctuating interfaces can be studied in these systems. Hereby, the focus is on structural analysis of such soft interfaces by X-ray and neutron scattering. Typical properties which are of interest are elasticity and the membrane charge density that can vary by specific ion condensation.

Section snippets

Methods for force measurements

The following techniques are described for measuring the force between two opposing surfaces.

Interactions between solid interfaces

Force–distance curves can be qualitatively divided into three characteristic shapes, displayed in Fig. 3. Fig. 3a represents the pure repulsion case (R): both approaching and retracting part show repulsion between probe and sample—there is no adhesion. In some cases, a weak adhesion appears in the retracting part, while the approaching part remains repulsive (Fig. 3b). This type of curves will be referred to as “repulsion and adhesion” (RA). Finally, “strong adhesion” (A) events were also

Molecular packing between interfaces in complex fluids

The confinement of micelles, liquid crystals, particles, macromolecules and even solvent molecules between two planar surfaces might lead to oscillatory forces, which cannot be described by the DLVO theory. In the following section, studies on entrapped polyelectrolytes solutions are presented. A common observation is that the chain distance is not affected by the confinement. But flexible and stiff polyelectrolytes behave differently with respect to their counterion distribution.

Acknowledgements

The work reviewed in this article was mainly performed within the German-French Collaborative Research Group Complex Fluids: from 3 to 2 dimensions. This program is jointly funded by the Deutsche Forschungsgemein-schaft (DFG, Germany), Project numbers Fi 235/14-1 to -4, the Centre National de la Recherche Scientifique (CNRS, France) and the Centre Energie Atomique (CEA, France).

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  • Cited by (0)

    1

    Present address: Laboratoire de Physique de l’état condensé, Univerité du Maine, F-72000 Le Mans, France.

    2

    Present address: University of Bayreuth, Faculty of Chemistry, Department of Physical Chemistry II, Universitätsstr. 30, D-95440 Bayreuth, Germany.

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