Disjoining Pressure Measurements Research Articles

Thickness and Disjoining Pressure Measurements of Foam Films Stabilized by Surfactant Adsorbed Films

Thickness and Disjoining Pressure Measurements of Foam Films Stabilized by Surfactant Adsorbed Films

Stabilisation of dispersions using a graft copolymer of hydrophobically modified polyfructose

This paper describes the use of a graft copolymer of hydrophobically modified inulin (polyfructose) (INUTEC SP1) for the stabilisation of dispersions. It starts with a section on the adsorption and conformation of the polymer at the interface. The results show a high affinity isotherm with irreversible adsorption. This is due to the multi-point attachment of the graft copolymer to the surface by several alkyl chains that are grafted to the polyyfructose backbone. The adsorbed layer thickness Δ of the graft copolymer on model polystyrene particles with radius R was determined using rheological measurements. The relative viscosity-core volume fraction π of the latex are compared with the theoretical value calculated using the Dougherty-Krieger equation allowing one to obtain the hydrodynamic thickness Δ. The latter was found to be 9.2nm indicating strong hydration of the polyfructose loops and tails by water molecules. Evidence for this strong hydration was obtained using cloud point measurements. The graft copolymer INUTEC SP1 was then used to prepare emulsions of paraffin oil or silicone oil-in-water emulsions. The latter remained stable with no coalescence for more than 1year both at room temperature and at 50°C and also in the presence of high electrolyte concentrations (2moldm−3 NaCl). This high stability of the emulsion is due to the multi-point attachment of the graft copolymer with several alkyl chains to the oil, leaving the strongly hydrated loops and tails of polyfructose dangling in solution, thus ensuring effective steric stabilisation. The high stability of the emulsion was confirmed using disjoining pressure measurements which indicated the presence of a Newton black film that showed no rupture up to a disjoining pressure of 4.5×104Pa. INUTEC SP1 was then used for the preparation of polystyrene (PS) and polymethylmethacrylate (PMMA) latex by emulsion polymerisation. The stability of the resulting latexes was determine by measuring the critical coagulation concentration (CCC) using CaCl2. By post addition of INUTEC SP1 to the latex dispersion, the CCC is significantly increased indicating effective steric stabilisation. The latter was confirmed using atomic force microscopy (AFM) whereby the interaction force between a hydrophobic glass sphere and glass plate, both coated with the graft copolymer, was measured both in water and in the presence of Na2SO4 (up to 1.5moldm−3). In all cases repulsive interaction was obtained indicating the effectiveness of INUTEC SP1 for latex stabilisation. INUTEC SP1 was also used for stabilisation of nano-dispersions (with diameters in the range 20–200nm). The high ratio of adsorbed layer thickness to particle or droplet size (d/R) results in a very shallow attractive minimum in the energy-distance curve. This indicates absence of any weak or strong flocculation. INUTEC SP1 could significantly reduce Ostwald ripening of nano-emulsions by strong adsorption at the oil/water interface and enhancement of the Gibbs elasticity.

Effect of oppositely charged hydrophobic additives (alkanoates) on the stability of C14TAB foam films

The impact of the hydrophobicity of oppositely charged additives on the stability of C14TAB foam films has been studied using a thin film pressure balance (TFPB). Disjoining pressure isotherms of 10−4M C14TAB solutions containing negatively charged sodium alkanoates CnOONa with increasing chain length from n=4 to n=12 are measured. The results of disjoining pressure measurements are presented as maximum possible pressure Πmax applied to a stable film as a function of the alkanoate concentration. Furthermore, the surface properties of the mixed systems are described to gain a general understanding about the surface characteristics. Since C14TAB and alkanoates are oppositely charged, the question arises whether charge reversal at the film surfaces occurs and how it depends on the chain length of the alkanoate. The presented results indicate that C14TAB/alkanoate systems can be divided into three groups. Depending on the chain length, alkanoates act as organic salts (n=4), co-surfactant (n=6) or anionic surfactant (8≤n≤12). In case of alkanoates acting as anionic surfactant, the simulation of the isotherms by solving the non-linearized Poisson–Boltzmann equation under assumption of constant potential actually indicates that charge reversal occurs close to the nominal IEP. The film stabilities are markedly affected by the hydrophobicity and the amount of alkanoate.

Identification of Mobile Species in Cationic Polymer Lubricant Layer on Silicon Oxide from AFM and XPS Analyses

The nanoscale spreading of a cationic polymer lubricant (CPL) film consisting of polydimethylsiloxane with quaternary ammonium salt side chains on a SiO(2) surface was studied with the disjoining pressure measurements using atomic force microscopy. CPL shows a monotonic decrease in disjoining pressure as the film thickness increases from 1.3 to 4.5 nm, which suggests stable spreading in this thickness range. Comparing the spreading rates calculated from disjoining pressure and the viscosity of CLP to the self-healing time after tribo-contacts revealed that the ionic form may not be the main mobile species. The X-ray photoelectron spectroscopy analysis found that the CPL film on SiO(2) has about 30% of the quaternary ammonium salts (cationic groups) reduced to tertiary amines (neutral groups). The reduced CPL polymer has much lower viscosity than the original CPL polymer and yields a spreading rate consistent with that measured at the macroscale. Thus, the mobile component in the CPL/SiO(2) film responsible for self-healing is concluded to be the reduced tertiary amine components of CPL.

Negative charges at the air/water interface and their consequences for aqueous wetting films containing surfactants

Negative charges at the air/water interface have been proven previously by the authors via disjoining pressure measurements of wetting films. In the present study the surface charges of wetting film are modified in order to control the film stability. Studies at different pH values support the assumption that the negative charges are caused by an excess of hydroxide ions. Addition of different types of salt anions seems to lead to an increase in surface charges with decreasing hydration shell. Both water-soluble and water-insoluble amphiphiles replaces the charges. The air/water surface remains negatively charged in case of non-ionic surfactant (dodecyl-beta maltoside, C12G2) and anionic ampihiphiles (stearic acid and SDS). The charge will be reversed in case of cationic amphiphiles. Under all of those conditions, films against likely charged solid surfaces are stable and unstable against oppositely charged solid substrates. For this purpose, the charge of silicon wafers was precoated with polyelectrolytes before. At concentrations well below the CMC cationic surfactants are not able to reverse the charges at the air/water interface. Respective wetting films are less stable and show a partial dewetting even on likely charged surfaces. These results lead to the conclusion that the films are stabilised by electrostatic repulsion. Potential adsorption of water-soluble surfactants on oppositely charged surfaces is also taken into account and is studied by contact-angle measurements.

Measurement of disjoining pressure of Z-type perfluoropolyether lubricants on Si and SiN x surfaces

An AFM based measurement of the disjoining pressure ( Π) is described. For the measurement of nano-thin fluids on hard substrates it was shown that specific experimental requirements are placed on the cantilever stiffness and probe radius. The disjoining pressure of Fomblin Z03 was then measured on a Si surface and found to be consistent with literature values. The disjoining pressures ( Π) of 25 Å Zdol on SiN x films deposited under a flowing-gas of N 2 was measured and a sharp decrease in Π was observed with increasing N 2 concentration (in the gas) that was assigned to the decrease in the polarizability and/or the density of polarizable species in the surface.

Disjoining pressure measurements using a microfabricated groove for a molecularly thin polymer liquid film on a solid surface.

A method for measuring disjoining pressure of a molecularly thin liquid film on a solid surface by using a microfabricated groove has been developed. The shape of the meniscus of a thin film in the microgroove was measured with an atomic force microscope, and the disjoining pressure was obtained from the capillary pressure obtained from the measured curvature of the meniscus. Our method is applicable to a film with a thickness greater than the diameter of gyration in the polymer molecule. Moreover, the method can detect the changes in the disjoining pressure caused by ultraviolet light irradiation, and it is effective in investigating the intermolecular interaction between a thin film and a solid surface.

Stability of Foam Films and Surface Rheology: An Oscillating Bubble Study at Low Frequencies

The dilational surface elasticity epsilon and the dilational surface viscosity eta of the two nonionic surfactants n-dodecyl-beta-d-maltoside (beta-C12G2) and tetraethyleneglycol-monodecyl ether (C10E4) were studied using the oscillating drop method. The experiments were carried out at different concentrations and frequencies with an accessible frequency range of 0.005-0.2 Hz. The results are discussed in the light of previous disjoining pressure measurements that demonstrated that the stability of thin liquid films cannot be explained solely by the magnitude of the surface forces. Indeed, a comparison of the results obtained for beta-C12G2 with those obtained for C10E4 reveals a correlation between the stability of the films and the surface dilational elasticity of the respective monolayers.

Interfacial Films of Poly(ethylene oxide)−Poly(butylene oxide) Block Copolymers Characterized by Disjoining Pressure Measurements, in Situ Ellipsometry, and Surface Tension Measurements

Interactions between two air−water interfaces stabilized by poly(ethylene oxide)−poly(butylene oxide) block copolymers were investigated by employing the microinterferometric thin-film balance technique. A series of three block copolymers, two linear diblock copolymers, B8E41 and E106B16, and one linear triblock copolymer, E21B8E21, were investigated. In particular the range of the steric forces operating across the thin foam films was quantified. As expected, higher concentrations were required of the triblock copolymer than for the diblocks for the formation of a stable foam film. Despite the low polydispersity of the sample (Mw/Mn = 1.03), effects due to polydispersity on the concentration dependence of the disjoining pressure isotherm were noted. Furthermore, the adsorption at the air−water interface was determined by surface tension measurements. It was noted that long equilibrium times are required to obtain (quasi) equilibrium surface tension data. Time-resolved ellipsometry was used to follow the adsorption rates and determine the amounts adsorbed in situ at hydrophobic solid surfaces. The adsorbed amount at the solid−liquid interface determined by ellipsometry was similar to that at the air−water interface estimated from the surface tension isotherm provided polydispersity effects on the surface tension isotherm were taken into account. Moreover, interactions in foam films formed from mixtures of the diblock copolymer, B8E41, and a cationic surfactant, hexadecyltrimethylammonium bromide, were studied. Both electrostatic double-layer forces and, at shorter range, steric forces were present, demonstrating the formation of a mixed layer at the interface.

Surface Charging Mechanism and Disjoining Pressure of Electrolytic Soap Films

We present a theory developed to describe the diffuse double layer of electrolytic soap films. The hydrophobic nature of the soap molecules is modeled, in an effective field theory for the system, by the introduction of a free energetic potential favoring the presence of the soap molecules at the film surface. This potential can take various forms, depending on how accurately we wish to model the geometry of the system. The resulting theory is analyzed and leads to the estimation of the electrostatic interactions at the mean field level. A whole range of physical observables is predicted including the electrostatic component of the disjoining pressure isotherm and the Gibbs adsorption isotherm of the system. We compare the theory with accurate disjoining pressure measurements (using X-ray specular reflectivity) carried out on systems composed of sodium dodecyl sulphate (SDS) with two different monovalent electrolytes on a wide range of salinity. The theory appears to account particularly well for the experimental data. A comparison with the DLVO theory is given. Finally, we discuss the limitations of the theory and its modification due to the inclusion of dispersion forces.

Anionic Polyelectrolyte-Cationic Surfactant Interactions in Aqueous Solutions and Foam Films Stability

The objective of this work is to study polymer/surfactant interactions in aqueous solution and at the air/water interface. These interations are involved in many physicochemical phenomena, such as colloidal stabilization and wettability which are of major importance in oil application as for exemple drilling muds. More precisely, we have attempted to characterize interactions between a non surface active anionic copolymer (acrylamide/acrylamide sulfonate) and an oppositely charged cationic surfactant (C12 TAB). Our results show a synergestic surface tension lowering (coadsorption) at extremely low surfactant concentrations (10 to the power of (-3) to 10 to the power of (-1) CMC). At higher concentrations, namely above the so called Critical Aggregation Concentration (CAC), polymer-surfactant complexes form in the bulk and the macromolecules precipitate out of the solution. Foam films made from these mixed solutions are stable while C12TAB films are unstable. Disjoining pressure measurements on mixed films with surfactant concentration two orders of magnitude below the CAC show the existence of long range repulsive forces and a discrete film thickness transition. At the CAC, we obtain mixed films with gel-like networks that are strongly affected by the film thinning rate.

Disjoining Pressure Measurements for Foam Films Stabilized by a Nonionic Sugar-Based Surfactant

Disjoining pressure isotherms for foam films made from a nonionic surfactant, octyl β-glucoside, are measured at different surfactant concentrations, ionic strengths, and solution pH values. Below the cmc an electrostatic double-layer repulsion is present and dominates the long-range interaction. The decay length of the forces agrees with the expected Debye length and the measured long-range interactions are consistent with solutions to the nonlinear Poisson−Boltzmann equation using constant charge conditions. The deduced surface charge densities increase with pH and ionic strength but decrease with increasing surfactant concentration. At, or just above, the cmc, surfactant covers the interface and suppresses the charge sufficiently to induce a transition from a common black film to a Newton black film. Ultimately, the film stability is determined by both surface forces and elasticity. Combining both, via an overall film tension, leads to a general expression for the film elasticity.

Conformational Transitions of Mixed Monolayers of Phospholipids and Polyethylene Oxide Lipopolymers and Interaction Forces with Solid Surfaces

We studied conformational changes in monolayers of lipopolymers consisting of poly(ethylene oxide) head groups coupled to L-α-distearoylphosphatidylethanolamine (DSPE-EO n ; n = 45, 110) and of mixtures of these lipopolymers with L-α-dimyristoylphosphatidylethanolamine (DMPE) with the aim to deposit composite polymer-lipid films on solid substrates. Film balance experiments performed with pure DSPE-EOn over areas ranging from 0.5 to 60 nm 2 per molecule revealed two conformational changes which are interpreted as a transition from a pancake-like to a mushroom-like conformation and a mushroom-to-brush transition, as predicted by the Alexander-de Gennes theory of grafted polymers. The mixtures of DSPE-EO n (≤ 10 mol %) with DMPE exhibit in general lateral phase separation as demonstrated by film-balance and microfluorescence studies. The very complex behavior is determined (1) by the well-known tendency of two-dimensional polymer solutions for phase segregation at very low polymer molar fractions, (2) by the interaction of the polymer head groups, and (3) by the chain melting transition of DMPE. A homogeneous phase was found for the case of 10 mol % DSPE-EO 45 in DMPE in the lateral pressure range between about 3 and 5 mN/m. This is interpreted in terms of a pancake-like state with the lipopolymer head groups starting to overlap. In this region the monolayer is below the DMPE main transition. Finally we studied the interaction of DSPE-EO 45 monolayers with Si/SiO 2 wafers by ellipsometry. The thickness of the lipopolymer monolayers was determined as a function of the relative humidity of the atmosphere surrounding the sample in a hydration chamber connected to a film balance. The measurements yield film thickness vs disjoining pressure curves. These distance vs disjoining pressure measurements show interaction regimes which are determined by steric and electrostatic forces, respectively, in analogy to recent force apparatus measurements.

Disjoining pressure and stratification in asymmetric thin-liquid films

We directly measure, for the first time, disjoining pressure isotherms for asymmetric oil/aqueous surfactant/gas (i.e., pseudoemulsion) films using a modified version of the porous-plate technique first developed by Mysels in conjunction with thin-film interferometry. Dynamic film-thinning experiments are also performed on individual foam and pseudoemulsion films. At SDS surfactant concentrations above the critical micelle concentration (CMC) (0.1 M SDS), the pesudoemulsion films exhibit the same step-wise layer thinning observed in foam films under similar conditions. Further, we conduct dynamic thinning experiments on solid/liquid/gas systems and show that aqueous 0.2 M CTAB films sandwiched between glass and air also display discrete thinning transitions. All of these stratification transitions arise from oscillations in the disjoining pressure isotherm, generated by amphiphilic structuring within the film. For 0.1 M SDS dedecane/air pesudoemuslion films, the slope and peak height of the disjoining-pressure oscillations increase with each subsequent amphiphilic layer as film thickness decreases. Magnitudes of the structural forces are low (<100 Pa) but the length scale of the oscillations is large (∼10 nm) and rathe far reaching (∼50 nm). Moreover, for 0.1 M SDS solutions, the capillary pressures associated with film rupture are significantly lower for pseudoemulsion films (∼0.1 kPa) when compared to foam films (∼15 kPa) at equivalent conditions. Taken together, our dynamic thinning and equilibrium disjoining pressure measurements indicate that stratification in 0.1 M SDS films has little effect on both kinetic and thermodynamic films stability.

Direct measurement of disjoining pressure in black foam films II. Films from nonionic surfactants

Experiments are reported with foam films from two different polyoxyethylenic nonionic surfactants of relatively short and long polyoxyethylene chains. The direct measurement of the film disjoining pressure was combined with measurements of the film thickness dependence on the electrolyte concentration. The results obtained are discussed in connection with: (i) the type of black films observed, (ii) the influence of the POE chain length on the film properties, (iii) the agreement between the disjoining pressure isotherms obtained and the theoretical ones, (iv) the relationship between the charge and potential of the interacting diffuse double layers.

The extent of vicinal water: Implications from the density of water in silica pores

In this paper the densities of water confined to the pores of silica gels and porous glasses are reported. These densities are consistent with a model for vicinal (interfacial) water reported earlier by the senior author (F.M.E.). From these average pore densities the density of water is calculated as a function of distance from the silica surface at 10, 20, 30, 40°C. The results, at all studied temperatures, suggest that water is structurally modified by propinquity to surfaces to distances up to 3–5 nm from the surface. The results of the present experiments are consistent with the thermodynamic properties of water in clays as reported by Low ( Soil Sci. Soc. Amer. J. 43, 652 (1979); J. Colloid Interface Sci. 95, 45 (1983); J. Colloid Interface Sci. 96, 229 (1983) ), the results of disjoining pressure measurements by Peschel et al. ( Z. Naturforsch. 26a, 707 (1971); Naturwissenschaften 11, 1 (1969); J. Colloid Interface Sci. 34, 505 (1970); Colloid Polym. Sci. 260, 444 (1982) ) and the properties of intracellular water as discussed by Clegg ( in “Cell Associated Water” (W. Drost-Hansen and J. S. Clegg, Eds.), Academic Press, New York, 1979; in “Biophysics of Water” (F. Frank, Ed.), Wiley, New York, 1982); Amer. J. Physiol. 246, R133 (1984); Cell Biophys. 6, 153 (1984) .

Direct measurement of disjoining pressure in black foam films. I. Films from an ionic surfactant

Disjoining pressure isotherms for microscopic foam films were measured directly, using an apparatus where the film disjoining pressure was balanced by