Alkane films on water: stability and wetting transitions

Types of surface forces determining the disjoining pressure isotherms of wetting films of low-molecular-weight alkanes on water surface are discussed. The van der Waals forces in alkane interlayers at different temperatures were calculated using a combination of exact equations of the Dzyaloshinsky—Lifshitz—Pitaevsky macroscopic theory and the multi-oscillator model for representation of the dielectric permittivity spectra of contacting bodies. Taking account of competitive action of the van der Waals and image forces allows one not only to reproduce specific features of wetting in the systems studied at different temperatures, but also to describe quantitatively the contact angles and the experimentally observed isotherms of polymolecular adsorption. The experimentally detected wetting transition in the water—pentane—vapor system was rationalized using the results of calculations mentioned above and the Derjaguin—Frumkin theory of wetting. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 256–266, February, 2008.     

Polymolecular adsorption and capillary condensation in narrow slit pores

Capillary condensation and polymolecular adsorption in narrow slits has been calculated, where the fields of surface forces overlap one another. The calculations were carried out on the basis of macroscopic theory of dispersion forces and the isotherms of lone adsorption layers at the free surface. It has been shown that under the effect of mutual attraction through a gap, polymolecular adsorption films lose their stability long before their thickness has approached the half-width of a flat slit. This results in hysteresis of the capillary condensation in an ensemble of plane-parallel slits.

In the case of systems having strong adsorbate-adsorbate interaction, there has been detected the existence of the lower limit of sizes of slit pores, wherein the capillary meniscus can coexist with adsorption films. With a slit width smaller than the critical one, the meniscus is likely to form a finite contact angle with “dry” surfaces of a slit. Thus an explanation has been given of the lower limit of the capillary condensation in an ensemble of flat-surface, slit pores. In the case of strong adsorbate-adsorbent interaction, the coexistence of meniscus with adsorption films within the scope of the approach used is possible in slits of any width.

The value of corrections for the surface forces effect to be entered in the calculations of slit pores dimensions has been analyzed on the basis of the capillary condensation data obtained.

In wedge-shaped slits there also exists, besides lower limit the upper limit of capillary hysteresis.     

Wetting and surface forces

In this review we discuss the fundamental role of surface forces, with a particular emphasis on the effect of the disjoining pressure, in establishing the wetting regime in the three phase systems with both plane and curved geometry. The special attention is given to the conditions of the formation of wetting/adsorption liquid films on the surface of poorly wetted substrate and the possibility of their thermodynamic equilibrium with bulk liquid. The calculations of contact angles on the basis of the isotherms of disjoining pressure and the difference in wettability of flat and highly curved surfaces are discussed. Mechanisms of wetting hysteresis, related to the action of surface forces, are considered.     

Isotherms of Capillary Condensation Influenced by Formation of Adsorption Films

Isotherms of capillary condensation are often used to determine the vapor sorption capacity of porous adsorbents as well as the pore size distribution by radii. In this paper, for calculating the volume of capillary condensate and of adsorption films in a porous body, an approach based on the theory of surface forces is used. Adsorption isotherms and disjoining pressure isotherms of wetting films are presented here in an exponential form discussed earlier. The calculations were made for straight cylindrical capillaries of different radii and slit pores of different width. The mechanisms of capillary condensation differ in cylindrical and slit pores. In cylindrical pores capillary condensation occurs due to capillary instability of curved wetting films on a capillary surface, when film thickness grows. In the case of slit pores, coalescence of wetting films formed on opposite slit surfaces proceeds under the action of attractive dispersion forces. Partial volumes of liquid in the state of both capillary condensate and adsorbed films are calculated dependent on the relative vapor pressure in a surrounding media. Copyright 2000 Academic Press.     

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.     

Surface forces in wetting films

A short review of various components of surface forces acting in a non-symmetrical system such as wetting films is presented here. Experimental results are compared with modified DLVO theory, which includes, besides dispersion and electrostatic, structural (solvation) forces caused by a change in liquid structure in conditions of confined geometry. The peculiarities of disjoining pressure isotherms and conditions of the film stability of non-polar and polar simple liquids, as well as of aqueous solutions of electrolytes and surfactants, are systematically considered from a historical perspective.     

Wetting films

Wetting films of nonpolar liquids are stabilized due to action of the repulsion dispersion forces. For aqueous films, it is necessary to takes additionally into account action of electrostatical and structural forces.Disjoining pressure isotherms of a thick methastable β -films of electrolyte and surfactant solutions can be quantitavely described on the basis of theory of long-range electrostatical forces. Thicknesses of thinner α-films of water formed as a result of vapour adsorption depend on the surface hydrophilicity and are controlled by the action of structural repulsion forces.     

Wetting and dewetting behaviour of hygroscopic liquids: Recent advancements

This review covers the most recent researches on wetting and dewetting phenomena related to the application of hygroscopic liquids in industry and technologies. Hygroscopic liquids actively absorb moisture from the surrounding air; therefore, they are used in the processes required for the moisturizing of surfaces or preventing the icing as well as control of evaporation rate and the humidity. The air humidity and wettability of substrates were shown to be crucial parameters affecting the wetting/dewetting kinetics of hygroscopic liquids. It is the adsorption of moisture on the hydrophilic surface that promotes the formation and spread of the precursor layer. The latter, in turn, is the droplet-spreading driver. The absorption of moisture by the liquid itself gives only a slight effect on wetting. The work devoted to the dewetting of hygroscopic liquids during evaporation, and the influence of thermal effects arising from contact with moist air on the wetting kinetics is also considered.     

Foam and wetting films from amphiphilic block copolymers: Isoelectric state and stability

Foam and wetting films from PEO-PPO-PEO triblock copolymers Synperonic P85 and F108 are studied under the identical conditions, using microinterference method. The range of background electrolyte concentration, where DLVO (electrostatic and van der Waals) forces and non-DLVO (steric) forces act in the films, is determined. From the dependence of the film thickness on pH, it is unambiguously shown that electrostatic interactions (i.e., the potential and surface charge) in the foam and wetting films caused by the presence of nonionic polymer surfactants arise due to the preferential adsorption of OH⁻ ions at the solution-air interface. The films obtained below the critical pH values are sterically stabilized; i.e., a decrease in pH induces a transition from electrostatic to steric stabilization. Three-layer models are designed for both types of films that allow to calculate electrostatic disjoining pressure Π_el. The values of ϕ₀ potential of the foam film are used to calculate Π_el in wetting films. A relation between the isoelectric state of foam and wetting films and their stability is found to exist in the range of pH corresponding to electrostatic stabilization. Metastable films, film rupture, or the transition to sterically stabilized films were also found. The text was submitted by the authors in English.     

New Prediction Model of Surface and Interfacial Energies Based on COSMO-UCE**

The nature and strength of intermolecular and surface forces are the key factors that influence the solvation, adhesion and wetting phenomena. The universal cohesive energy prediction equation based on conductor-like screening model (COSMO-UCE) was extended from like molecules (pure liquids) to unlike molecules (dissimilar liquids). A new molecular-thermodynamic model of interfacial tension (IFT) for liquid-liquid and solid-liquid systems was developed in this work, which can predict the surface free energy of solid materials and interfacial energy directly through cohesive energy calculations based on COSMO-UCE. The applications of this model in prediction of IFT for water-organic, solid (n-hexatriacontane, polytetrafluoroethylene (PTFE) and octadecyl-amine monolayer)-liquid systems have been verified extensively with successful results; which indicates that this is a straightforward and reliable model of surface and interfacial energies through predicting intermolecular interactions based on merely molecular structure (profiles of surface segment charge density), the dimensionless wetting coefficient R_A/C can characterize the wetting behavior (poor adhesive (non-wetting), wetting, spreading) of liquids on the surface of solid materials very well.     

Effect of hydrophobicity on the stability of the wetting films of water formed on gold surfaces

We have developed a methodology that can be used to determine disjoining pressures (Π) in both stable and unstable wetting films from the spatial and temporal profiles of dynamic wetting films. The results show that wetting films drain initially by the capillary pressure created by the changes in curvature at the air/water interface and subsequently by the disjoining pressure created by surface forces. The drainage rate of the film formed on a gold surface with a receding contact angle (θ(r)) of 17° decreases with film thickness due to a corresponding increase in positive Π, resulting in the formation of a stable film. The wetting film formed on a hydrophobic gold with θ(r)=81° drains much faster due to the presence of negative Π in the film, resulting in film rupture. Analysis of the experimental data using the Frumkin-Derjaguin isotherm suggests that short-range hydrophobic forces are responsible for film rupture and long-range hydrophobic forces accelerate film thinning.     

Wetting properties of polyacetylene films

The wetting characteristics of free-standing polyacetylene films were determined by using a standard series of wetting liquids. As-prepared films of cis-polyacetylene were found to contain a significant polar contribution to the surface free energy, which became nearly entirely dispersive upon thermal isomerization to the trans form. Both isomeric forms are characterized by a critical surface tension of wetting γ_c ≈ 51 mN/m, which is considerably higher than that normally obtained from organic polymers. These results have been interpreted with respect to surface oxidation and tested by examining an oxidized film.     

Wetting Films of the Aqueous Cationic Polyelectrolyte Solutions on Quartz

It was shown that the stability of the wetting films of aqueous cationic polyelectrolyte solutions on the flat quartz surface depends on solution concentration. At a low concentration, the films are stable owing to the electrostatic repulsive forces. At a high polyelectrolyte concentration, the films are unstable due to the hydrophobization of quartz and the appearance of the hydrophobic attractive forces in the films. In the intermediate concentration range, the films are metastable and their lifetime depends upon the competition between the electrostatic repulsive and hydrophobic attractive forces. Thus, the concentration of cationic polyelectrolyte substantially affects the wetting conditions of the quartz surface. This conclusion can also be extended to other solid surfaces negatively charged in aqueous solutions, which is inherent to the majority of natural materials.     

Wetting of CVD carbon films by polar and nonpolar liquids and implications for carbon nanopipes

The handling, dispersion, manipulation, and functionalization of carbon nanotubes and nanopipes often require the use of solvents. Therefore, a good understanding of the wetting properties of the carbon nanotubes is needed. Such knowledge is also essential for the design of nanotube-based nanofluidic devices, which hold the promise of revolutionizing chemical analysis, separation, drug delivery, filtration, and sensing. In this work, we investigated the wetting behavior of individual nanopipes produced by the chemical vapor deposition (CVD) of carbon in porous alumina templates and of thin carbon films produced by the same technique. The carbon pipes and films have the same chemistry and structure as determined by Raman and infrared spectroscopies and, when similarly treated, demonstrate the same qualitative wetting behavior, as determined by optical microscopy. Thus, measurements conducted on the carbon film surface are relevant to the nanopipes. In the case of the nanopipes, filling with various liquids was monitored. Contact angle experiments with both polar (water, glycerol, ethylene glycol, ethanol, tetra-hydro furan, and 2-propanol alcohol) and nonpolar liquids (cyclohexane, hexadecane, poly(dimethylsiloxane), and a fluoro-silicone) were conducted on films using the sessile drop method. The contact angles on the CVD carbon films ranged from 0 to 79 degrees. The exposure of the carbon films to a NaOH solution, typically used to dissolve the alumina template, led to a significant decrease of the contact angle, especially in the case of polar liquids.     

Combinatorial wetting in colour: an optofluidic nose

We present a colourimetric litmus test for simple differentiation of organic liquids based on wetting, which achieves chemical specificity without a significant sacrifice in portability or ease-of-use. Chemical specificity is derived from the combination of colourimetric wetting patterns produced by liquids in an array of inverse opal films, each having a graded wettability, but using different surface groups to define that gradient.     

Rupture of wetting films caused by nanobubbles

It is now widely accepted that nanometer sized bubbles, attached at a hydrophobic silica surface, can cause rupture of aqueous wetting films due to the so-called nucleation mechanism. But the knowledge of the existence of such nanobubbles does not give an answer to how the subprocesses of this rupture mechanism operate. The aim of this paper is to describe the steps of the rupture process in detail: (1) During drainage of the wetting film, the apex of the largest nanobubble comes to a distance from the wetting film surface, where surface forces are acting. (2) An aqueous "foam film" in nanoscale size is formed between the bubble and the wetting film surface; in this foam film different Derjaguin-Landau-Verwey-Overbeek (DLVO) forces are acting than in the surrounding wetting film. In the investigated system, hydrophobized silica/water/air, all DLVO forces in the wetting film are repulsive, whereas in the foam film the van der Waals force becomes attractive. (3) The surface forces over and around the apex of the nanobubble lead to a deformation of the film surfaces, which causes an additional capillary pressure in the foam film. An analysis of the pressure balance in the system shows that this additional capillary pressure can destabilize the foam film and leads to rupture of the foam film. (4) If the newly formed hole in the wetting film has a sufficient diameter, the whole wetting film is destabilized and the solid becomes dewetted. Experimental data of rupture thickness and lifetime of wetting films of pure electrolyte and surfactant solutions show that the stabilization of the foam film by surfactants has a crucial effect on the stability of the wetting film.     

Selective wetting of heterogeneous surfaces by solutions of surfactants

Selective wetting of dimethyldichlorosilane-modified glass plates by solutions of tetradecyltrimethylammonium bromide (TDTAB), a cationic surfactant, in p-xylene has been studied. When surfactant concentrations are lower than the critical micelle concentration (CMC), the contact angles under selective wetting conditions increase with increasing hydrophobic surface fraction. When surfactant concentrations are higher than CMC, contact angles are the same on all substrates studied. The adsorption of the surfactant on hydrophilic and hydrophobic regions of heterogeneous surfaces and the stability of wetting films are taken into account in interpreting the results.     

Tribological properties of self-assembled monolayers of catecholic imidazolium and the spin-coated films of ionic liquids

A novel compound of an imidazolium type of ionic liquid (IL) containing a biomimetic catecholic functional group normally seen in mussel adhesive proteins was synthesized. The IL can be immobilized on a silicon surface and a variety of other engineering material surfaces via the catecholic anchor, allowing the tribological protection of these substrates for engineering applications. The surface wetting and adhesive properties and the tribological property of the synthesized self-assembled monolayers (SAMs) are successfully modulated by altering the counteranions. The chemical composition and wettability of the IL SAMs were characterized by means of X-ray photoelectron spectroscopy (XPS) and contact angle (CA) measurements. The adhesive and friction forces were measured with an atomic force microscope (AFM) on the nanometer scale. IL composite films were prepared by spin coating thin IL films on top of the SAMs. The macrotribological properties of these IL composite films were investigated with a pin-on-disk tribometer. The results indicate that the presence of IL SAMs on a surface can improve the wettability of spin-coated ionic liquids and thus the film quality and the tribological properties. These films registered a reduced friction coefficient and a significantly enhanced durability and load-carrying capacity. The tribological properties of the composite films are better than those of pure IL films because the presence of the monolayers improves the adhesion and compatibility of spin-coated IL films with substrates.     

A study on the relationship between critical surface tension of wetting and oil agglomeration recovery of calcite

In this study, it was investigated that the relationship between the critical surface tension of wetting of calcite and agglomeration recovery depending on pH and amount of collector (Na oleate). For this purpose, effects of pH and the amount of collector on the agglomeration recovery were investigated and, also, zeta potential measurements and Fourier transform infrared spectrophotometer (FTIR) analyses have been carried out to determine the adsorption type of Na oleate on calcite surface. In this paper, the decisive rules could not put down as evidence of agglomeration success with the critical surface tension of wetting value as in the flotation, because there are different liquids as water and oil (bridging liquid) in the agglomeration system.