Phase diagrams of nonionic foam films: construction by means of disjoining pressure versus thickness curves

The thickness h of foam films can be measured as a function of the disjoining pressure Pi using a thin film pressure balance. Experimental Pi-h curves of foam films stabilized with nonionic surfactants measured at various concentrations resemble the p-V(m) isotherms of real gases measured at various temperatures (p is the pressure and V(m) is the molar volume of the gas). This observation led us to adopt the van der Waals approach for describing real gases to thin foam films, where the thickness h takes the role of V(m) and the disjoining pressure Pi replaces the ordinary pressure p. Our analysis results in a phase diagram for a thin foam film with spinodal, binodal as well as a critical point. The thicker common black film corresponds to the gas phase and the compact Newton black film for which the two surfaces are in direct contact corresponds to the dense liquid. We show that the tuning parameter for the phase behavior of the film is the surface charge density, which means that Pi-h curves should not be referred to as isotherms. In addition to the equilibrium properties the driving force for the phase transition from a common black film to a Newton black film or vice versa is calculated. We discuss how this transition can be controlled experimentally.     

Foam films stabilized with dodecyl maltoside. 2. Film stability and gas permeability

The gas permeability and stability of foam films stabilized by n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were determined. The permeability coefficient (K, cm/s) and the mean film lifetime were measured as a function of the surfactant concentration. The films are less permeable than those stabilized by other surfactants at comparable conditions. The permeability coefficient decreases with increasing surfactant concentration. It does not show a remarkable dependence on the salt concentration. Stable Newton black foam films (NBFs) are formed above a surfactant concentration of 3.9 x 10(-)(6) M beta-C(12)G(2) in the presence of 0.2 M NaCl. The theory of nucleation hole formation in NBFs was applied to describe the observed dependencies of the permeability and film stability on the surfactant concentration. The theory gave satisfactory relation to the experiment.     

Computer simulation studies of Newton black films

We study via molecular dynamics simulations thin films (Newton black films, NBF) consisting of water coated with sodium dodecyl sulfate (SDS) surfactants. We analyze in detail the film properties (distribution of particles, pair correlation functions, roughness of the film, tilt angle of the hydrocarbon chain, electron density profiles, and mobility of water molecules) as a function of water content in the film core (i.e., film thickness, H). Our simulations indicate that water is part of the bilayer structure as solvation water. We estimate that around 2.25 water molecules per surfactant are part of this solvation structure. The structural analysis of the NBF shows that the headgroups exhibit a high degree of in-plane ordering. We find evidence for the existence of cavities in the monolayer, where only water is present. The basic structure of the monolayer is conserved down to water contents of the order of 4 water molecules per surfactant (H approximately equal to 11 A). The computed monolayer roughness for the present model is 2.5 A, in good agreement with the experimental data. We find that the roughness is very sensitive to the details of the interatomic potentials. Water mobility calculations emphasize the sluggish dynamics of very thin NBF. Diffusion coefficients of water in the lateral direction strongly decrease with film thickness. We find that the typical mean squared displacement of water in the direction normal to the bilayer is between 9 and 80 A2. Overall, our results indicate that the equilibrium SDS Newton black films studied in the X-ray experiments contain from 2 to 4 water molecules per surfactant.     

Molecular transfer of surfactant bilayers: widening the range of substrates

Handling nanometer-thick films and nano-objects remains a challenge. Applying self-assembly properties of surfactants to nanomaterials manipulation may be the key to the fast, easy, cost-effective growth of 2D and 3D nanostructures. Newton black films (NBFs) are self-assembled bilayers of surfactant, well-organized, but fragile objects. To render such films amenable to practical applications, it is necessary to find ways to transfer them onto solid substrates. A method developed recently to transfer NBFs onto a solid substrate while preserving their molecular organization (Benattar, J.-J.; Nedyalkov, M.; Lee, F. K.; Tsui, O. K. C. Angew. Chem., Int. Ed. 2006, 45, 4186) is broadened here to different surfaces. The method requires hydrophobic, planar, atomically smooth surfaces. This study presents the adhesion of a fluorinated NBF surfactant onto hydrophobically treated silica and silicon surfaces (with etching or silanization). The structures of the free-standing film, bare substrates, and transferred films are investigated using X-ray reflectivity. The homogeneity of the surfaces before and after bilayer deposition is examined by atomic force microscopy (AFM). Multiple transfers are tested and described for the future development of more complex architectures involving many surfactant layers and inserted nanosized objects.     

Method for direct measurement of the film tension of black foam films at various capillary pressures

A new experimental method to directly measure the film tension of black foam films is developed on the basis of the Laplace equation. The method allows the determination of the tension of curved (spherical) films with various radii and capillary pressures. Measurements with Newton black films from sodium dodecyl sulphate aqueous solution have been carried out. The results show that in the studied range of curvature radii (70 ÷ 360 m) the film tension does not depend on the curvature and the capillary pressure.     

Properties of foam films stabilized with tetraethylammonium perfluorooctane-sulfonate

Properties of single foam films prepared with tetraethylammonium perfluorooctane-sulfonate (TAPOS) were studied. Film thickness was measured as a function of NH₄Cl concentration in the film forming solution. The dependence of the film disjoining pressure versus the film thickness (disjoining pressure isotherms) and the mean lifetime of the films were studied. The dependence of the film thickness on the electrolyte concentration showed the presence of an electrostatic double layer at the film surfaces. The electrostatic double layer component of the disjoining pressure was screened at a NH₄Cl concentration higher than 0.2 M where Newton black films (NBFs) of 6 nm thickness were formed. These films are bilayers of amphiphile molecules and contain almost no free water. The disjoining pressure isotherms of the foam films formed with 0.001 M TAPOS were measured at two different NH₄Cl concentrations (0.005 and 0.0005 M). The Deryaguin-Landau-Verwey-Overbeek (DLVO) theory describes well the isotherms with an electrostatic double layer potential of ∼140 mV. The mean lifetime, a measure of the stability of the NBFs, was measured depending on surfactant concentrations. The observation of NBF was possible above a minimum TAPOS concentration of 9.4 × 10⁻⁵ M. Above this concentration, the lifetime increases exponentially. The dependence of the film lifetime on surfactant concentration is explained by the theory for NBF-rupture by nucleation mechanism of formation of microscopic holes.     

Free-standing films of fluorinated surfactants as 2D matrices for organizing detergent-solubilized membrane proteins

The possibility of organizing detergent-solubilized membrane proteins in a plane within the core of Newton black films (NBFs) formed from fluorinated surfactants has been investigated. Fluorinated surfactants have the interesting characteristics of being poorly miscible with detergents and highly surface-active. As a result, when a membrane protein-the transmembrane domain of OmpA (tOmpA)-solubilized by the nonionic detergent C8E4 (tetraethylene glycol monooctyl ether) was injected under a monolayer of fluorinated surfactant, C8E4 and tOmpA/C8E4 complexes remained confined to the subphase. Vertical, macroscopic NBFs were drawn, and their structure was investigated by means of X-ray reflectivity. Depending on experimental conditions, the protein was shown to organize into either one or two monolayers stabilized by two monolayers of fluorinated surfactant. Two different mechanisms of protein insertion were investigated: (i) attachment of polyhistidine-tagged tOmpA/C8E4 complexes to nickel-bearing polar groups born by a fluorinated surfactant and (ii) spontaneous diffusion into the surfactant films. Possible applications are discussed.     

Shear-induced coalescence of oil-in-water Pickering emulsions

Thermodynamic treatment of thin liquid films in Part III of this series was applied to foam films stabilized by sodium dodecyl sulfate. Miscibility of sodium chloride and sodium dodecyl sulfate in the adsorbed films at the film surfaces and transition between the black films were studied by measuring film thickness and contact angle. A discontinuous change in the thickness and a break on the contact angle vs. concentration curve appeared at the transition. Judging from the phase diagram of adsorption, sodium chloride and sodium dodecyl sulfate are a little miscible in the adsorbed films. The miscibility was ascribed to specific interaction between sodium ion and dodecyl sulfate ion in the adsorbed films. The miscibility in an adsorbed film was compared between the film surface and meniscus and between the common black and Newton black films.     

Adhesion induced mesoscale instability patterns in thin PDMS-metal bilayers

We show that the surface of a thin elastomer-metal (aluminum) hybrid bilayer becomes spontaneously patterned when brought in adhesive contact with a rigid surface. The self-organized surface patterns show three distinct morphological phases-columns, labyrinths, and holes-depending on the area of contact. The characteristic wavelength of these patterns is found to be 2.94+/-0.20 times the total film thickness, independent of the morphological phase and the surface properties of the contacting surface. Interestingly, the metal films 60-120 nm thick showed the same scaling, but the bilayers with thicker metal films were completely stable. This observation demonstrates for the first time a "hard" transition to the instability as the elastic stiffness of the film is varied. We also report a protocol for alignment of the instability patterns and for transferring the metal patterns to another surface.     

Effect of Lipid Phase State and Foam Film Type on the Properties of DMPG Stabilized Foams

The drainage and stability of DMPG (l-α-phosphatidyl-dl-glycerol dimyristoyl) foams were studied by a microconductivity method under conditions where three different foam film types could be formed—thin foam films (TFF), common black foam films (CBF), and Newton black foam films (NBF). Foaming properties were investigated at 20 and 28°C where DMPG is in the gel and liquid-crystalline states. Higher conductivity signals were observed at the higher temperature where DMPG was in the liquid-crystalline state, which is indicative of wetter or more stable foams under these conditions. This effect was observed independent of foam film type. However, for a given phase state, the type of foam films formed significantly influenced the stability and rate of drainage of the foam. Indeed, the water content of the foams, obtained under conditions for formation of different foam films, is ranked in the order TFF > CBF > NBF. When the temperature was increased to 28°C (i.e., in the liquid-crystalline state), CBF and NBF showed a slight decrease in film thickness and an increase in film lifetime and surface molecular diffusion coefficient in the adsorbed layer. It is likely that the fluidity of the interfacial layer is an important factor contributing to DMPG foam stabilization.     

Phase diagrams of nonionic foam films: new interpretation of disjoining pressure vs thickness curves

Recently we constructed phase diagrams for thin foam films stabilized by a nonionic surfactant. The idea was born by synopsis of various disjoining pressure (pi) versus thickness (h) curves of foam films resembling p-Vm isotherms of real gases. The new concept of interpreting the pi-h curves of foam films in terms of phase diagrams allows us to describe experimental observations much more precisely. Three logical consequences will be discussed here to illustrate the strength of this approach. First, the observation is explained that common black films (CBF) rupture or form a Newton black film (NBF) within a certain pressure range rather than at a defined pressure. Both observations can be rationalized by invoking a nucleation process of holes or of the thinner NBF, respectively, in close analogy to the vapor to liquid condensation. Second, the question whether the CBF to NBF transition is discrete or continuous is answered by analyzing under which conditions the supercritical state of a foam film can be reached. Third, the evidence of corresponding states is discussed.     

Interactions between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside

The forces acting between nonpolar surfaces coated with the nonionic surfactant n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were investigated at concentrations below and above the critical micelle concentration. The long-range and adhesive forces were measured with a bimorph surface force apparatus (MASIF). It was found that the effect of hydrodynamic interactions had to be taken into account for an accurate determination of the short-range static interactions. The results were compared with disjoining pressure versus thickness curves that were obtained earlier with a thin film pressure balance (TFPB). This comparison led to the conclusion that the charges observed at the air-water interface are not due to charged species present in the surfactant sample. In addition, it was observed that the stability of thin liquid films crucially depends on the surfactant's bulk concentration (c) and thus on the packing density in the adsorbed layer. The force barrier preventing removal of the surfactant layer from between two solid-liquid interfaces increases with increasing c, while for foam films it is the stability of the Newton black film that increases with c. Finally, the results obtained for beta-C(12)G(2) were compared with those obtained for the homologue n-decyl-beta-d-maltoside (beta-C(10)G(2)) as well as with those obtained for nonionic surfactants with polyoxyethylene moieties as polar groups.     

Thermodynamic studies on thin liquid films. II. Foam film stabilized by decyl methyl sulfoxide

Thermodynamic equations in Part I of this series were extended so as to be applicable to electrolyte mixtures and the resultant equations were applied to the experimental results of a NaCl-decyl methyl sulfoxide (DeMS) mixture. Film thickness and contact angle of the black foam film stabilized by DeMS were measured as a function of the total molality of NaCl and DeMS at constant mole fraction of DeMS in the mixture under constant disjoining pressure. Newton black film was observed only above a certain DeMS concentration and the phase transition between common black and Newton black films took place twice as NaCl concentration increased at constant DeMS concentration. The surface densities of NaCl and DeMS at the film surface and the differences in the surface densities between the adsorbed films at the film surface and bulk one coexisting at equilibrium were numerically evaluated by applying the thermodynamic equations to the film tension obtained from the contact angle. The film states and phase transitions were clarified in terms of the film thickness and surface densities.     

Fabrication and Characterization of Sensitive Polymer (Nano)Composites

We report on the fabrication process of a polymer sensing device based on a thin film composite obtained by spinning or casting a carbon fillers/thermoplastic polymer dispersion on a glass substrates. In this work we produce thin composite films characterized by different grade of dispersion of carbon black in polymer matrix (Poly(methyl-methacrylate), Poly(2 hydroxy-ethyl-methacrylate) and atactic poly(styrene)). We discuss, in particular the effect of the polymer and filler characteristics on the thin film morphology. Device material response towards different analytes will be also discussed. Size distribution of filler in polymer suspension and deposition method type strongly influence homogeneity and conductivity of corresponding polymer composite films and finally their sensor properties.     

Gas permeability in polymer- and surfactant-stabilized bubble films

The gas permeabilities of thin liquid films stabilized by poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-SDS (sodium dodecyl sulfate) mixtures are studied using the "diminishing bubble" method. The method consists of forming a microbubble on the surface of the polymer solution and measuring the shrinking rates of the bubble and the bubble film as the gas diffuses from the interior to the exterior of the bubble. PNIPAM-stabilized films exhibit variable thicknesses and homogeneities. Interestingly, despite these variable features, the gas permeability of the film is determined principally by the structure of the adsorbed polymer layer that provides an efficient gas barrier with a value of gas permeability coefficient that is comparable to that of an SDS Newton black film. In the presence of SDS, both the film homogeneity and the gas permeability coefficient increase. These changes are related to interactions of PNIPAM with SDS in the solution and at the interface, where coadsorption of the two species forms mixed layers that are stable but that are more porous to gas transfer. The mixed PNIPAM-SDS layers, studied previously for a single water-air interface by neutron reflectivity, are further characterized here in a vertical free-draining film using X-ray reflectivity.     

Foam films as instrumentation in the study of amphiphile self-assembly

A survey on recent experimental investigations of microscopic foam films containing self-assembled amphiphilic structures is presented. A specific advantage of the microscopic film techniques is that the fine control of system parameters allows the estimation of the consecutive changes of film properties for low surfactant content and extremely small concentration changes. This gives a unique possibility to reach amphiphile quantities when initial onset of self-assembly is to be observed. The film characteristics are investigated via microinterferometric method, which operates with the measuring cell of Scheludko-Exerowa. The experimental set is additionally improved by including video-recording and consecutive image analysis. The results show the following: (1) Unstable black patterns (dots and spots) are observed; they have very short lifetimes and the films, which contain them rupture quickly. (2) Several of the kinetic characteristics of the films display a sharp change within a narrow surfactant concentration range. The experiments are interpreted on the basis of the assumption that a series of smaller self-assembled aggregates (premicelles) with various geometries exist at the interfaces and inside the thin film. The proposed theoretical scheme puts forward a mechanism connecting the formation of unstable black patterns (dots and spots) with the reorganization and destruction of the existing surfactant assemblies both in the bulk of the film and on the interfaces. The results suggest that the observed unstable black formations may serve as indicators for the presence of surfactant structures in amphiphilic solutions and the microscopic foam-film techniques has a serious potential as a prospective instrumentation in the study of amphiphilic self-assemblies.     

Determination of molecular anisotropy in thin films of discotic assemblies using attenuated total reflectance UV-visible spectroscopy

We report here an investigation of absorbance anisotropy in highly ordered, single bilayer (ca. 5.6 nm) Langmuir-Blodgett (LB) thin films of discotic liquid-crystalline phthalocyanines, using a recently introduced broad-band attenuated total reflectance (ATR) spectroscopic technique, capable of measuring dichroism in such films in the UV--visible optical region down to absorbances of ca. 0.003 absorbance units. On the basis of the ATR measurements of LB-deposited films, a thorough treatment was established to determine the ensemble average of the Cartesian components and the associated optical anisotropy of transition dipoles in the molecular film. In an effort to recover order parameters of molecular orientation, those results were interpreted with a circular dipole model, which is the expected model for the isolated molecule based on symmetry properties. We measured a strong dipole component normal to the film plane that cannot be explained in terms of a truly circular model, indicating that the molecular transition dipoles were perturbed upon aggregation. The utility of the experimental approach was further demonstrated by (a) investigating the effect of substrate modifiers (methyl- and phenyl-terminated silanes) on the ordering within the phthalocyanine film and (b) the effect of water immersion and re-annealing of the thin film on molecular ordering and optical anisotropy.     

X-ray scattering by black foam films

The x-ray scattering by the three types of black foam films (common black, Newton black, and stratified black films) was experimentally studied. A special device in which flat black films with an area of ca. 2 cm² can be produced was developed and x-ray diffraction patterns were obtained by a vertical diffractometer. The three types of films differ significantly in their x-ray reflections, which proves that they have different structure. For common black films, the comparison of observed and calculated intensities lead to a model, which corresponds to the three-layer model. The Newton black films exhibit diffraction trace with only one highly asymetric peak and there is, as of yet, no unambigous interpretation. The patterns of the stratified black films have several pronounced sharp peaks corresponding to the areas of different films with a given thickness.