黑膜厚度与膜表面张力关系的研究

用微干涉测量技术直接测定楔压等温线,研究了电解质浓度对阳离子表面活性剂TTAB在浓度大于cmc时形成黑膜厚度的影响及膜表面张力与溶液表面张力之间的差别.结果显示,黑膜厚度取决于楔压和电解质浓度,随着楔压的增加,液膜厚度减少至一定程度后几乎保持不变,表明黑膜类型的转化是阶跃式的,而电解质屏蔽了液膜两个表面电荷层间的排斥作用,故电解质浓度增加,液膜厚度变小.由楔压等温线得出的膜表面张力的结果说明一般黑膜的表面张力与溶液的表面张力并无明显差别.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 2. Diffuse Adsorption Layers

Contribution of the image forces of dipole molecules diffusely distributed over the film thickness to disjoining pressure is considered. Relation for the dependence of this contribution on film thickness is derived. The approach to calculating the constants of dispersion interaction of solute molecules with film-confining phases is developed, and the values of these constants are calculated for a number of systems containing alcohol solutions in hydrocarbons.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 3. A Formation of Adsorption Monolayers with Dipole Orientation Normal to the Surface

Long-range surface forces arising in solution films upon the formation of adsorption monolayers are considered. It is shown that, for a number of systems and at a small interlayer thickness, these forces can reach the values comparable with the molecular component of disjoining pressure for the considered case when dipole moments are oriented normal to the interfaces.     

Investigation of the isotherms of the disjoining pressure of wetting films of binary nonionic solutions by the ellipsometric method

The paper deals with an experimental investigation into the influence of the second component on the thicknesses of the wetting films of a nonionic solvent. A technique has been developed for the production of pure, smooth, thin glass substrates for wetting liquid films.

The use of these glass substrates enabled us to exclude the influence on the experimental results of such noncontrollable factors as roughness and pollution of the substrate surface. The isotherms of the disjoining pressure of wetting films of a number of two-component mixtures of nonionic liquids on glass substrates were experimentally determined. The film thicknesses were measured by an ellipsometric method; the disjoining pressure for the film was preset by adjusting the pressure of solvent vapours. The results obtained demonstrate a qualitative agreement with the theory of the adsorption component of disjoining pressure developed by Derjaguin and Churaev.

It is also shown that even very small additions of a polar substance to a nonpolar solvent may cause a marked change in the thickness of films. In addition to adopting the theory of the adsorption component of disjoining pressure, certain assumptions are made about the formation of the structural component resulting from the addition of a polar component to quantitatively describe the results obtained. The contribution of the adsorption and structural components of disjoining pressure to the stability of films of solution is estimated.     

Influence of NaCl on the behavior of PEO-PPO-PEO triblock copolymers in solution, at interfaces, and in asymmetric liquid films

The solution behavior of the polymeric surfactant Pluronic F127 (PEO(99)PPO(65)PEO(99)) and its adsorption behavior on aqueous-silica and aqueous-air interfaces, as well as the disjoining pressure isotherms of asymmetric films (silica/aqueous film/air) containing F127, are studied. The interfacial properties of adsorbed F127 layers (the adsorbed amount Gamma and the thickness h) as well as the aqueous wetting film properties [film thickness (h) and refractive indexes] were studied via ellipsometry. The solution properties of F127 were investigated using surface tensiometry and light scattering. The interactions between the air-water and silica-water interfaces were measured with a thin film pressure balance technique (TFB) and interpreted in terms of disjoining pressure as a function of the film thickness. The relations between the behaviors of the asymmetric films, adsorption at aqueous air, and aqueous silica interfaces and the solution behavior of the polymeric surfactant are discussed. Special attention is paid to the influence of the concentrations of F127 and NaCl. Addition of electrolyte lowers the critical micelle concentration, diminishes adsorption on silica, and increases the thickness of the asymmetric film.     

Thin film drainage: hydrodynamic and disjoining pressures determined from experimental measurements of the shape of a fluid drop approaching a solid wall

Accurate measurements of the shape of a mercury drop separated from a smooth flat solid surface by a thin aqueous film reported recently by Connor and Horn (Faraday Discuss. 2003, 123, 193-206) have been analyzed to calculate the excess pressure in the film. The analysis is based on calculating the local curvature of the mercury/aqueous interface, and relating it via the Young-Laplace equation to the pressure drop across the interface, which is the difference between the aqueous film pressure and the known internal pressure of the mercury drop. For drop shapes measured under quiescent conditions, the only contribution to film pressure is the disjoining pressure arising from double-layer forces acting between the mercury and mica surfaces. Under dynamic conditions, hydrodynamic pressure is also present, and this is calculated by subtracting the disjoining pressure from the total film pressure. The data, which were measured to investigate the thin film drainage during approach of a fluid drop to a solid wall, show a classical dimpling of the mercury drop when it approaches the mica surface. Four data sets are available, corresponding to different magnitudes and signs of disjoining pressure, obtained by controlling the surface potential of the mercury. The analysis shows that total film pressure does not vary greatly during the evolution of the dimple formed during the thin film drainage process, nor between the different data sets. The hydrodynamic pressure appears to adjust to the different disjoining pressures in such a way that the total film pressure is maintained approximately constant within the dimpled region.     

On the Theory of the Stability of Dipole Molecule Solution Interlayers in Apolar Solvents: 1. Thermodynamics of Solution Film

Thermodynamics of the film of solution with apolar solvent and polar solute is analyzed. The expression for calculating disjoining pressure in a film via the potential electric energy of the filed generated by the solute dipoles is derived. The effect of the image forces on the orientation of dipole molecules near the interfaces is considered.     

Equilibrium thin liquid films

The latest results are reviewed and a number of new concepts of the thermodynamics of thin films are formulated. Current definitions of disjoining pressure and their applications for introducing disjoining pressure into thermodynamics of phase equilibria, as well as the new thermodynamic definition of the thickness of thin film, are considered. New approaches to the rigorous definition of disjoining pressure in curved films and films with nonuniform thickness, including transition zones of wetting films, are analyzed. The modulus of Gibbs’ elasticity is derived for the case of a thin film. The role of the elasticity of this type in thin films and its correlation with traditional transverse (Derjaguin) elasticity related to the disjoining pressure are explained.     

Homogeneous nucleation at high supersaturation and heterogeneous nucleation on microscopic wettable particles: A hybrid thermodynamic/density-functional theory

Homogeneous nucleation at high supersaturation of vapor and heterogeneous nucleation on microscopic wettable particles are studied on the basis of Lennard-Jones model system. A hybrid classical thermodynamics and density-functional theory (DFT) approach is undertaken to treat the nucleation problems. Local-density approximation and weighted-density approximation are employed within the framework of DFT. Special attention is given to the disjoining pressure of small liquid droplets, which is dependent on the thickness of wetting film and radius of the wettable particle. Different contributions to the disjoining pressure are examined using both analytical estimations and numerical DFT calculation. It is shown that van der Waals interaction results in negative contribution to the disjoining pressure. The presence of wettable particles results in positive contribution to the disjoining pressure, which plays the key role in the heterogeneous nucleation. Several definitions of the surface tension of liquid droplets are discussed. Curvature dependence of the surface tension of small liquid droplets is computed. The important characteristics of nucleation, including the formation free energy of the droplet and nucleation barrier height, are obtained.     

Evaluation of DLVO theory with disjoining-pressure and film-conductance measurements of common-black films stabilized with sodium dodecyl sulfate

We develop a unique film holder combining a thin-film balance with AC impedance spectroscopy to measure disjoining pressure, film conductance, and film thickness simultaneously. Foam films stabilized by sodium dodecyl sulfate (SDS) are investigated with and without added sodium chloride (NaCl) electrolyte. Classical colloidal theory, Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, is tested rigorously over a wide range of solution conditions by comparing the surface charge densities fit to disjoining-pressure isotherms with those estimated independently from film-conductance and surface-tension data. Film-conductance measurements strongly suggest that the adsorbed anionic surfactant is partially complexed with counterions. Therefore, to reconcile the different values of charge densities calculated from surface tension and film conductance with those from disjoining pressure, we propose a simple ion-binding electrostatic model. The ion-complexation framework predicts increased ion complexing with increasing solution ionic strength, in agreement with surface-tension and film-conductance data. Unfortunately, it is not possible to describe similarly the trends of the measured disjoining-pressure isotherms because the diffuse-layer charge density increases, or equivalently, the ion complexation decreases with increasing ionic strength. Accordingly, the ion-binding extension of classical DLVO theory does not permit agreement between theory and independent experimental data from surface tension, disjoining pressure, and film conductance.     

Maximum disjoining pressure in protein stabilized concentrated oil-in-water emulsions

A model for the prediction of the equilibrium profile of film thickness and continuous phase liquid holdup profile in a concentrated oil-in-water (O/W) emulsion is proposed. This model is employed to infer the maximum disjoining pressure in a concentrated corn oil-in-water emulsion stabilized by bovine serum albumin (BSA) from the experimental measurements of different proportions of oil, polyhedral O/W foam, and aqueous layers at different centrifugal accelerations. The inferred maximum disjoining pressures were found to be higher at higher concentrations of BSA, lower ionic strengths as well as at pH values farther away from pI. The predicted variations of disjoining pressure with film thickness for a concentrated O/W emulsion stabilized by BSA exhibited two maxima due to steric and electrostatic interactions, respectively. The experimental maximum disjoining pressures for toluene-in-water emulsion stabilized by BSA were found to be about two to three times the predicted maxima due to steric interactions but were two to three orders of magnitude higher than the maxima due to electrostatic interactions, thus indicating that steric interaction is the dominant stabilizing mechanism. The discrepancy between the experimental and predicted maximum disjoining pressures is believed to be mainly due to lack of information with regard to the thickness of the adsorbed protein layer at the oil—water interface.     

Structural and topographical characteristics of adsorbed WPI and monoglyceride mixed monolayers at the air-water interface

In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.     

Autophobic dewetting of Z-tetraol perfluoropolyether lubricant films on the amorphous nitrogenated carbon surface

The thermodynamic stability of thin films of the perfluoropolyether (PFPE) Z-Tetraol, as a function of molecular weight, on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image the autophobic dewetting of the Z-Tetraol films. Film dewetting results when the PFPE film thickness applied to the CNx surface exceeds a critical value. This critical dewetting thickness is identified as the monolayer thickness of the adsorbed PFPE film via measurements of the changes in the surface energy as a function of lubricant film thickness. The observed dewetting coincides with the film thickness at which the disjoining pressure goes to zero. The critical dewetting thickness is dependent on the PFPE molecular weight.     

Dewetting Revisited: New Asymptotics of the Film Stability Diagram and the Metastable Regime of Nucleation and Growth of Dry Zones

Stability properties of a nonwetting film are discussed. Assuming a general form of the disjoining pressure, accurate asymptotic formulas for the upper thickness range of the film instability/metastability are derived. This analysis is applied to two particular cases: a nonionic liquid film with the (m, n) power form of the disjoining pressure and an ionic liquid film with an exponentially decaying electrostatic part of the disjoining pressure. The metastable regime of dewetting is considered, and an expression for the critical radius of a hole is derived. A new Fokker-Planck kinetic model of metastable dewetting, applicable at early stages of the process, is developed. It yields a relationship between the number of viable holes (per unit area and unit time) moving in steady-state regime to the supercritical part of the "embryo size space" and the equilibrium number of "critical" holes determined from thermodynamics. The dynamics of metastable dewetting is quantitatively described in terms of the surface fraction of holes in the film. Continuous dynamic models of the metastable dewetting applicable in the entire range of times have to include the thermal noise, as proposed by V. S. Mitlin (1994, Colloids Surf. A 89, 97). Copyright 2000 Academic Press.     

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.     

Modern state of the investigation of long-range surface forces

Development of the concept of surface long-range forces and, in particular, the equilibrium disjoining pressure of liquid and gaseous interlayers has been set forth. Considered are the molecular, adsorption, electrical, structural, and electronic components of disjoining pressure. The contribution of the disjoining pressure to the hydrodynamics of thin layers is considered. The first theory of the frost heaving of soils has been formulated. Stated are the investigations of surface forces, in particular, in the processes of the formation of new interfaces and arising phenomena of the emission of electrons, ions, photons, and neutrons.     

Nonflat equilibrium liquid shapes on flat surfaces

The hydrostatic pressure in thin liquid layers differs from the pressure in the ambient air. This difference is caused by the actions of surface forces and capillary pressure. The manifestation of the surface force action is the disjoining pressure, which has a very special S-shaped form in the case of partial wetting (aqueous thin films and thin films of aqueous electrolyte and surfactant solutions, both free films and films on solid substrates). In thin flat liquid films the disjoining pressure acts alone and determines their thickness. However, if the film surface is curved then both the disjoining and the capillary pressures act simultaneously. In the case of partial wetting their simultaneous action results in the existence of nonflat equilibrium liquid shapes. It is shown that in the case of S-shaped disjoining pressure isotherm microdrops, microdepressions, and equilibrium periodic films exist on flat solid substrates. Criteria are found for both the existence and the stability of these nonflat equilibrium liquid shapes. It is shown that a transition from thick films to thinner films can go via intermediate nonflat states, microdepressions and periodic films, which both can be more stable than flat films within some range of hydrostatic pressure. Experimental investigations of shapes of the predicted nonflat layers can open new possibilities of determination of disjoining pressure in the range of thickness in which flat films are unstable.     

Interaction between a stationary mercury electrode and a hydrogen bubble in an aqueous sodium dodecyl sulphate solution

The stability and rupture of thin liquid films formed from an aqueous solution of Na₂SO₄ (0.05 mol dm⁻³) in the presence of 5 × 10⁻⁵ mol dm⁻³ sodium dodecyl sulphate between a stationary mercury electrode and a hydrogen bubble has been investigated as a function of electrode potential. The electrostatic component of disjoining pressure has been calculated using the results of capacity measurement for the mercury-solution interface. Special attention has been paid to films formed on positively charged mercury surfaces. In this case, despite the positive electrode polarization, the outer Helmholtz plane potential is found to be negative due to the high surface activity of the dodecyl sulphate anion. The van der Waals component of disjoining pressure has been calculated on the basis of a double sheath model of the two interacting surfaces, taking into consideration the orientation of the adsorbed surfactant layers at the interfaces. Calculations of the total disjoining pressure can explain film stability at negative mercury potentials, but do not explain film rupture when the polarization of the mercury is positive. The existence of a hydrophobic attractive interaction is postulated in the latter case.