Interactions in lipid stabilised foam films☆
Graphical abstract
Introduction
Thin liquid free standing films or foam films as they have to be called according to the IUPAC recommendations [1] have been a fascinating object of studies for the last 60 years. These films attract the attention of children because of the charming play of colours during their thinning (Fig. 1), but they have also played an enormous role to prove such milestone theories in the mesoscopic and nanometre world like the theory of Derjaguin–Landau–Verwey–Overbeek (DLVO), which has been extensively used to predict the stability of lyophobic colloids [2], [3], [4], [5], [6] and the existence of a disjoining pressure (Π) between two surfaces in close proximity. A specific property of these films is a very large difference in size along the lateral and normal directions. Even though their area is in the macro-world and can be extended even up to square metres, their thickness can be tuned down to a few nanometres. This makes them a suitable tool to study the interactions between surfaces because of the comparatively easy handling, reproducible preparation and their symmetrical geometry.
Foam films are formed usually from solutions of surfactants [5], [6], [7]. Their thickness is conveniently measured using the microinterferometric method [5], [6], [7], [8], [9] (Fig. 1). The films consist of two adsorbed surfactant monolayers with a thickness h1 separated by a layer of the aqueous solution from which the film is created (Fig. 1b) with a thickness h2. The equivalent solution film thickness hw is the thickness of the film assumed to be a homogeneous leaf of surfactant solution. Indeed the film structure is more complicated (sandwich-like structure of layers with different refractive indices) where the thickness h is the real film thickness. The calculation of h and h2 can be easily obtained from the measured hw (for details see Part 6 Experimental techniques).
During the film formation, first a thick non-equilibrium film is formed which becomes thinner upon drainage of the solution due to the capillary pressure in the meniscus, gravity or action of the surface forces [5], [6], [7]. Finally, an equilibrium film is obtained with a uniform thickness depending on the thermodynamic conditions (pressure, surfactant and salt concentration, temperature, etc.). The very thin films appear black in reflected light and are called black films. Two types of black films exist (Fig. 1a). The thicker common black film (CBF) appears at lower electrolyte concentrations. It shows typical Newton rings in reflected light in the area of the meniscus which surrounds the film. The thickness of the CBF and its stability are controlled by the electrostatic double layer repulsion [5], [6], [7] in agreement with the classical DLVO theory. The electrostatic double layer repulsion is suppressed at higher electrolyte concentrations, and the equilibrium state in this case is a very thin Newton black film (NBF). Once this state is reached, the film thickness is independent on the electrolyte concentration and is only determined by the direct interaction of the surfactant adsorption layers by short-range surface forces.
The properties of foam films that have been explored in detail include equilibrium film thickness, contact angle film/meniscus, thinning rate, film stability, film elasticity, etc. [5], [6], [10]. Measurements on the film thickness allow establishing a direct relation of the strength of the interactions between the film surfaces and the distance between them. One of the most widespread techniques for foam film formation is the Scheludko–Exerowa glass ring cell technique [6], [7] (Fig. 1a, see also Part 6 Experimental techniques for details). A foam film is formed from a biconcave drop in a glass capillary. The cell allows the film thickness, contact angle with the surrounding meniscus or film stability to be measured.
The interaction between lipid bilayers in water has been intensively studied over the last decades [11], [12], [13], [14], [15], [16], [17]. Osmotic stress was applied to lamellar phases and the bilayer distance was measured using small-angle X-ray diffraction to evaluate the forces between two approaching lipid bilayers in aqueous solution [11], [15], [17]. Alternatively, the force and the distance were measured between lipid mono- or bilayers deposited on mica sheets using surface force apparatus [12]. Foam films stabilised by lipids offer another possibility to study the interactions between lipid monolayers [18], [19], [20], [21], [22], [23], [24], [25], [26]. Such studies deliver valuable information about the interactions between biological lipid membranes and especially their stability and permeability. Presenting inverse black lipid membrane, the foam films supply information about the properties of the self-organised lipid molecules in bilayers.
The present paper summarises the results of our studies on microscopic lipid stabilised foam films by measuring their thickness and contact angle. Different methods have been developed for the preparation of foam films stabilised by insoluble biosurfactants such as phospholipids [27], [28], [29]. The main part of the presented results concerns foam films prepared from dispersions of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) and some of its mixtures with the anionic lipid — 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG). The strength of the long range and short range surface forces between the lipid molecules as obtained from the thin foam films experiments is discussed. The van der Waals attractive force is calculated. The electrostatic repulsive force is estimated from experiments at different electrolyte concentrations (NaCl, CaCl2) or by modification of the electrostatic double layer surface potential by incorporating charged lipids in the lipid monolayers. The short range interactions are studied and modified by using small carbohydrates (fructose and sucrose), ethanol (EtOH) or dimethylsulfoxide (DMSO). Some results are compared with the structure of lipid monolayers deposited at the liquid/air interface (monolayers spread in Langmuir trough), which are one of most common biomembrane model systems [30], [31], [32], [33], [34]. The comparison between the film thickness and the free energy of film formation is used to estimate the contribution of the different components of the disjoining pressure to the total interaction in the film and their dependence on the composition of the film forming solution.
Section snippets
Forces acting between the foam film surfaces
A foam film is always surrounded by a bulk phase (meniscus). At equilibrium, the disjoining pressure (Π (h)) in the film equals the capillary pressure PC in the meniscus.where RC is the radius of the curvature of the meniscus which is equal to the radius of the glass capillary where the film is formed. σ is the surface tension of the solution from which the film is prepared.
The disjoining pressure has different contributions and it can be written as
Formation of lipid stabilised foam films
Phospholipids spontaneously form suspensions of vesicles in water solutions [34]. Depending on their treatment different vesicles can be formed. Small unilamelar vesicles (SUV) with a diameter of around 100 nm constituted from a single lipid bilayer are one of the most well-characterised forms of lipid dispersions in water [34]. Stable foam films can only be formed when the monolayers of surfactant which build the films are saturated and the adsorption density is the highest possible [6], [45].
van der Waals attraction
The long range surface forces operating in foam films are the van der Waals force and the electrostatic double layer force. The van der Waals force is always attractive in the case of symmetrical system of thin liquid layer between two semi-infinite gas (air in our case) phases [4]. This force is the origin of the negative component of the disjoining pressure ΠVW (Eq. (3)). It tends to decrease the film thickness and finally leads to its rupture. The Hamaker constant AH, respectively ΠVW can be
Short range interactions in lipid stabilised foam films
Phospholipid bilayers are major structural elements of biological membranes. Therefore, the study of the interaction of lipids with water and with water soluble solutes delivers information about the processes that occur at the membrane/water interface. An addition of certain solutes, such as small carbohydrates, dimethyl sulfoxide (DMSO) and ethanol, influences membrane stability. Soluble sugars stabilise liposomes [56], intact membranes [57] and whole cells [58] against irreversible
Conclusions
Phospholipid bilayers are the smallest components of barriers (i.e. membranes) separating living matter from its external environment. The lateral organisation of the lipid molecules in bilayers and the interactions between then normal to the lamella plane affect the stability and the transport processes through membranes. Foam films stabilised by lipids offer the possibility to study the interactions between lipid monolayers. Presenting an inverse black lipid membrane the foam films supply
Experimental techniques
The Scheludko–Exerowa glass ring experimental cell (Fig. 15) is a suitable tool for studying single foam films. The film is formed in a glass ring (1) with a radius R which is connected via a capillary to the body of the cell (2). A special piston made of Teflon (3) is used to suck the liquid from the glass capillary and thus forming the foam film. The glass ring is placed in a close glass vessel (4) which assures formation of saturated to water vapour atmosphere. This is an important condition
Acknowledgements
The Max-Planck Society is acknowledged for the financial support during the whole period of the project. Parts of the work were financed by the Deutsche Forschungsgemeinschaft through projects Sfb 312, Mu 1040/9-1, Mu 1040/4-1 and guest grant 436BUL17/1/99. R.K. was partially supported by the Dr. A. Kalojanoff Stiftung, München, Germany. The authors are grateful to Mrs. E. Rütze for technical assistance.
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This paper is dedicated to Prof. Helmuth Möhwald. All presented results were obtained at his Department of Interfaces at the Max-Planck Institute of Colloids and Interfaces in Golm/Potsdam, Germany.