Advances in Colloid and Interface Science (v.104, #1-3)

Boris Vladimirovich Derjaguin by N.V. Churaev; L.B. Boinovich (ix-xiii).

Foam and wetting films: electrostatic and steric stabilization by D. Exerowa; N.V. Churaev; T. Kolarov; N.E. Esipova; N. Panchev; Z.M. Zorin (1-24).
Foam films and wetting films on quartz, obtained from aqueous solutions of two different surfactants [cetyltrimethylammonium bromide (CTAB) and PEO–PPO–PEO triblock copolymer (F108)] with NaCl as a background electrolyte, are considered as convenient models to compare the properties of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methodology of assessment of foam and wetting films is used to allow precise determination of the film thickness. In the case of CTAB films, experimental data for the potential ϕ0 of the diffuse electric layer at the solution/air interface and the potential ϕ1 at the solution/quartz interface are used to analyze the stability of the films studied. A conclusion drawn is that electrostatic interaction forces stabilize both kinds of films studied. It is shown that with increasing CTAB concentration a charge reversal occurs at both the solution/air and solution/quartz interfaces that determines the stability/instability of the foam and wetting films. Concentration ranges where both types of films produce stable (equilibrium) films are found. There are also concentration ranges where the films either rupture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide formation of unstable (rupturing and metastable) and stable films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (>2×10−4 mol dm−3) that both cases render formation of stable equilibrium films. In the case of F108 films, the comparison of foam films and wetting films on quartz indicates film stability that is either electrostatic or steric in origin. On the basis of the effect of electrolyte concentration on film thickness, the transition from electrostatic to steric stabilization is demonstrated for both kinds of films. The critical electrolyte concentration at which this transition occurs is determined. Foam films are found to be always stable (equilibrium). Formation of either unstable (rupturing and metastable) or stable (equilibrium) wetting films on quartz is established depending on the solution composition. The effects are similar for both hydrophilic and hydrophobic quartz surfaces. The results obtained show certain similarity between foam and wetting films. In both cases, electrostatic forces below the critical electrolyte concentration, and above it steric forces govern film stability. Some specific properties of the wetting films are induced by the asymmetric boundary conditions as distinct from symmetric foam films.
Keywords: Foam film; Wetting film; Electrostatic repulsion; Steric repulsion;

Thin wetting films from aqueous solutions of polyoxyethylene–polyoxypropylene ABA-block copolymer+NaCl in different concentrations on silicon carbide substrate have been studied. The polymer adsorption at the silicon carbide/aqueous solution interface has been studied using the spectrophotometric method, based on the colored polymer–iodine complex. The area per molecule, the conformation of the adsorbed molecules and the thickness of the adsorption layer at the solid/liquid interface were determined. The three-layer model of the structure of thin liquid films on the flat SiC surface has been used for calculation of the film thickness from the data obtained by the microinterferometric method. The dependence of the film thickness on time has been determined. The influence of the electrolyte concentration on the equilibrium film thickness has been studied. It was found that for NaCl concentrations higher than 3×10−2 M the film thickness remained constant, i.e. the electrostatic disjoining pressure was fully suppressed. The disjoining pressure/thickness-isotherm has been obtained experimentally using the dynamic method. It has been interpreted by the superposition of a negative van der Waals component and a positive steric component of the disjoining pressure.
Keywords: Polyoxyethylen–polyoxypropylene ABA-block copolymer; Silicon carbide; Adsorption; Thin wetting film; Disjoining pressure; Steric stabilization;

The behavior of black foam films from aqueous dispersions of dimyristoylphosphatidyl-choline (DMPC) with addition of the soluble phospholipid dimyristoylphosphatidylglycerol (DMPG) has been studied in dynamic conditions. The dynamic contact angles θ and the gas permeability coefficient K have been measured using the diminishing bubble method. The DMPC vesicle suspension in water is obtained through sonication and DMPG is dissolved in it. Two solutions with different NaCl concentrations (0.1 M and 0.5 M) have been studied. The behavior of the dynamic contact angles is very different for DMPC black films with, and without DMPG, respectively. They follow very different time dependence during spontaneous or forced variations of the bubble size. The gas permeability coefficient is significantly reduced by the DMPG addition. The NaCl concentration also influences this specific behavior. It seems that the electrically charged DMPG anions, which determine a significant electrostatic disjoining pressure, play an important role for this specific behavior. The results are discussed in connection with data regarding the thickness and structure of these black foam films. Films from DMPC+DMPG solutions in ethanol plus water mixed solvent have been studied as well, but no quantitative results could be obtained.
Keywords: Dimyristoylphosphatidylcholine (DMPC); Dimyristoylphosphatidylglycerol (DMPG); Black foam film; Dynamic contact angle; Gas permeability;

Polyelectrolytes as adhesion modifiers by Per M. Claesson; Andra Dedinaite; Orlando J. Rojas (53-74).
Adsorbed layers of polyelectrolytes have been studied with atomic force microscopy (AFM) and the interferometric surface force apparatus (SFA). Particular emphasis was put on determining the effect of the polyelectrolyte charge density on surface topography, and the effect of the polyelectrolyte coating on the adhesive properties. The AFM was employed to image individual polymer chains at low adsorption densities and to characterize the layer topography and coverage at higher adsorption densities. The adhesive properties between two polyelectrolyte-coated surfaces in air were determined as a function of the number of contacts made at any given spot. The data provide evidence for formation of electrostatic bridges, particularly when highly charged polyelectrolytes are used. Further, material transport between the surfaces is observed when the polyelectrolyte is either highly charged or have a very low charge density. For intermediate charge densities we could not observe any indication of material transfer. The adhesion between one polyelectrolyte-coated surface and one bare surface was initially higher than that between two polyelectrolyte-coated surfaces. However, due to material transfer between the two surfaces the adhesion decreased significantly with the number of times that the surfaces were driven into contact. For the polyelectrolytes of the lowest charge density the results suggest that entanglement effects contribute to the adhesive interaction. The modification of the adhesion by polyelectrolytes in practical systems such as in the case of dry-strength additives to improve paper resistance is also considered.
Keywords: Adhesion; Polyelectrolytes; AFM imaging; Surface forces; Dry-strength additives; Paper strength;

The evidence for nanobubbles as the origin of the long-ranged attractions measured between hydrophobic surfaces immersed in water is reviewed by focusing upon several unique features of the force curves. Also covered is the morphology of nanobubbles as revealed by direct imaging with tapping mode atomic force microscopy. A discussion of the origin, thermodynamic stability and practical implications of nanobubbles is given.
Keywords: Nanobubble; Hydrophobic attraction; Surface force; Force measurement;

The image-charge forces appearing due to polarization of phases, confining the liquid film, by the electric field of polar solute molecules are analyzed. The main attention is focused on non-symmetric systems. The consideration of image-charge effects of different types indicates that the contribution from diffusely distributed solute molecules is weak and hardly affects film stability. It is shown that the correlation interaction between layers adsorbed on opposite interfaces is also not very essential for the system, at least for the gaseous adsorbed layer. The appreciable force found is due to the interaction between the relatively dense adsorbed layer and its images. The origin of a strong force in that case is associated with the variation of solute adsorption with film thickness. The calculated magnitude of image-charge forces on the basis of parameters typical for solutions in hydrocarbon solvents are large enough to dominate in the film force balance. Some experimental observations are discussed within the framework of the developed approach.
Keywords: Liquid films; Disjoining pressure; Image-charge effects; Adsorption;

Spreading of liquid drops over porous substrates by V.M. Starov; S.A. Zhdanov; S.R. Kosvintsev; V.D. Sobolev; M.G. Velarde (123-158).
The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) has been investigated in the case of both complete wetting (silicone oils of different viscosities) and partial wetting (aqueous SDS solutions of different concentrations). Nitrocellulose membranes of different porosity and different average pore size have been used as a model of thin porous layers, glass and metal filters have been used as a model of thick porous substrates. The first problem under investigation has been the spreading of small liquid drops over thin porous layers saturated with the same liquid. An evolution equation describing the drop spreading has been deduced, which showed that both an effective lubrication and the liquid exchange between the drop and the porous substrates are equally important. Spreading of silicone oils over different nitrocellulose microfiltration membranes was carried out. The experimental laws of the radius of spreading on time confirmed the theory predictions. The spreading of small liquid drops over thin dry porous layers has also been investigated from both theoretical and experimental points of view. The drop motion over a dry porous layer appears caused by the interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in a growth of the drop base, and (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and a growth of the wetted region inside the porous layer. As a result of these two competing processes the radius of the drop base goes through a maximum as time proceeds. A system of two differential equations has been derived to describe the time evolution of the radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters, one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate, and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments were used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters were determined. Experiments were carried out on the spreading of silicone oil drops over various dry nitrocellulose microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer was monitored. In agreement with our theory all experimental data fell on two universal curves if appropriate scales were used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer using a dimensionless time scale. Theory predicts that (a) the dynamic contact angle dependence on the dimensionless time should be a universal function, (b) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in our system. These predictions are in the good agreement with our experimental observations. In the case of spreading of liquid drops over thick porous substrates (complete wetting) the spreading process goes in two similar stages as in the case of thin porous substrates. In this case also both the drop base and the radii of the wetted area on the surface of the porous substrates were monitored. Spreading of oil drops (with a wide range of viscosities) on dry porous substrates having similar porosity and average pore size shows universal behavior as in the case of thin porous substrates. However, the spreading behavior on porous substrates having different average pore sizes deviates from the universal behavior. Yet, even in this case the dynamic contact angle remains constant over the duration of the second stage of spreading as in the case of spreading on thin porous substrates. Finally, experimental observations of the spreading of aqueous SDS solution over nitrocellulose membranes were carried out (case of partial wetting). The time evolution of the radii of both the drop base and the wetted area inside the porous substrate was monitored. The total duration of the spreading process was subdivided into three stages: in the first stage the drop base growths until a maximum value is reached. The contact angle rapidly decreases during this stage; in the second stage the radius of the drop base remains constant and the contact angle decreases linearly with time; finally in the third stage the drop base shrinks while the contact angle remains constant. The wetted area inside the porous substrate expands during the whole spreading process. Appropriate scales were used to have a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate, and the dynamic contact angle vs. the dimensionless time. Our experimental data show: the overall time of the spreading of drops of SDS solutions over dry thin porous substrates decreases with the increase of surfactant concentration; the difference between advancing and hydrodynamic receding contact angles decreases with the surfactant concentration increase; the constancy of the contact angle during the third stage of spreading has nothing to do with the hysteresis of contact angle, but determined by the hydrodynamics. Using independent spreading experiments of the same drops on a non-porous nitrocellulose substrate we have shown that the static receding contact angle is equal to zero, which supports our conclusion on the hydrodynamic nature of the hydrodynamic receding contact angle on porous substrates.
Keywords: Hydrodynamic; Substrates; Non-porous; Concentration;

Field-induced disturbance of the double layer electro-neutrality and non-linear electrophoresis by Vladimir Shilov; Sandor Barany; Constantino Grosse; Oksana Shramko (159-173).
The influence of applied electric field on the field-induced variation of the electrolyte concentration (concentration polarization) disturbs the electro-neutrality of the system, represented by a dispersed particle and its electric double layer in electrolyte solution. The manifestation of this electro-neutrality disturbance in the non-linear electrophoresis was considered in the framework of a procedure of successive approximations in powers of the applied field strength. Analytic expressions describing the component of the electrophoretic velocity proportional to the cubic power of the applied field strength (cubic electrophoresis) were obtained for arbitrary values of the surface conductivity (of Dukhin number). The model restrictions are spherical non-conducting particle with homogeneous surface and thin double layer.
Keywords: Non-linear electrophoresis; Electro-neutrality; Concentration polarization;

Effect of interfacial phenomena on dewetting in dropwise condensation by Shripad J. Gokhale; Joel L. Plawsky; Peter C. Wayner (175-190).
Image-analyzing interferometry was used to investigate the dynamics of the dewetting meniscus of a partially wetting fluid on a modified quartz surface during dropwise condensation. The vivid difference in the behavior of the retracting meniscus with respect to its variation in apparent contact angle and curvature after the merger of the drop with the meniscus was found to depend on the wettability of the surface. On the hydroxylated quartz surface, the meniscus shed mass during retraction. The dewetting velocity decreased with time. On a slightly hydrophobic quartz surface, the meniscus showed a curvature gradient in the axial direction during drop merger and that gradient decreased as the meniscus moved towards the corner. The dewetting of the meniscus is discussed using the interfacial concepts of spreading and the Kelvin–Clapeyron phase change model.
Keywords: Contact angle; Condensation; Dewetting; Spreading; Surface phenomenon;

The interaction forces between particles containing grafted or adsorbed polymer layers have been investigated using rheological and surface force measurements. Polystyrene latex dispersions with grafted poly(ethylene oxide) (PEO) chains (M=2000) were used for the rheological measurements. Results were also obtained for latex dispersions stabilised with adsorbed graft copolymers of poly(methyl methacrylate–methacrylic acid) with methoxy capped PEO chains (M=750). The relative viscosity η r –volume fraction ϕ curves for the latex dispersions with grafted PEO chains were established for three particle radii of 77.5, 306 and 502 nm. For comparison the η r ϕ curve was calculated using the Dougherty–Krieger equation. This allows one to obtain the adsorbed layer thickness δ as a function of ϕ. The results showed a decrease of δ with increase of ϕ, which was attributed to the interpenetration and/or compression of the PEO chains on increasing ϕ. Viscoelastic measurements as a function of ϕ showed a change from predominantly viscous to predominantly elastic response at a critical volume fraction, which indicated the onset of the strong steric repulsion when the polymer layers begin to overlap. A similar trend was obtained with the latex particles containing adsorbed graft copolymer layers. A scaling law was used to fit the elastic part of the logG′−logϕ curve (where G′ is the elastic modulus). This fit could be used to estimate the compressibility of the PEO chains. The correlation of the rheology of concentrated sterically stabilised dispersions with interparticle interactions was investigated by measuring the energy–distance curves for the graft copolymer that was adsorbed on smooth mica sheets. Using de Gennes scaling theory, it was possible to calculate the energy of interaction between the polymer layers. The high frequency modulus of the latex dispersions was obtained as a function of the volume fraction and the results were compared with those calculated from the potential of the mean force. The trends in the variation of the modulus with volume fraction were similar for the experimental rheological results and those calculated using the directly measured interaction forces. The above results demonstrated the powerful use of rheological measurements for studying the interaction between sterically stabilised dispersions in concentrated systems.
Keywords: Interaction forces; Grafted polymer layers; Adsorbed polymers;

Structuring and sedimentation stability of titanium phosphate nanoparticles in polymer solutions by O.Ya. Uryupina; N.V. Serebryakova; V.I. Roldughin (227-238).
The influence of dispersed phase nature on the sedimentation stability as well as coagulation structure of titanium phosphate (TP) nanoparticles in polymer suspensions has been investigated. Two systems are considered: (i) TP suspension in toluene/ethyl cellosolve mixed solution of ephoxy resin E-40 and (ii) multicomponent system, ferric oxide, talc and TP suspension in toluene/ethyl cellosolve solution of E-40, as a model of the practical varnish–paint systems. For the case of a polar solvent, a unique phenomenon, extremal dependence of the strength of model systems on the concentration TP, is detected and increase of sedimentation stability of TP organic suspensions with time of contact of TP with a solvent is observed. Data of FTIR-spectroscopy show that phenomena detected result from the formation adsorption shell possessing high structural viscosity on TP nanopaticles.
Keywords: Nanoparticles; Titanium phosphate; Sedimentation; Structuring; FTIR-spectroscopy;

Influence of magnetic field on the molecular orientation in epitropic mesophase of nitrobenzene by B.A. Altoiz; T.V. Naroditskaya; Yu.M. Popovskii (239-243).
The influence of magnetic field on the molecular orientation type in epitropic liquid crystalline (ELC) phase of nitrobenzene was investigated by transmitted light ellipsometry method in gap optical lightguide with variable thickness. The outcomes of the experiment allow us to suggest the structure reorganization in wall-adjacent ELC layer of nitrobenzene, similar to Freederick's transition in thermotropic LC.
Keywords: Epitropic liquid crystalline (ELC) phase; Magnetic field action; Wall-adjacent layer; Nitrobenzene; Gap optical lightguide;

Dilational surface viscoelasticity of polymer solutions by B.A Noskov; A.V Akentiev; A.Yu Bilibin; I.M Zorin; R Miller (245-271).
A review of recent results on the dilational surface viscoelastic properties of aqueous solutions of non-ionic polymers is given. In the frequency range from 0.001 up to 1000 Hz the methods of transverse and longitudinal surface waves and the oscillating barrier method were applied. Viscoelastic behavior of adsorbed polymer films significantly differs from the behavior of films formed by only conventional surfactants of low molecular weight. For example, the dynamic surface elasticity of the former systems is low and almost constant in a broad concentration range. One can observe the increase of the surface elasticity only at extremely low concentrations and/or in the range of semi-dilute solutions. If the surface stress relaxation in conventional surfactant solutions is usually determined by the diffusional exchange between the surface layer and the bulk phase, the relaxation processes in the polymer systems proceed mainly inside the surface layer. Possible mechanism of the latter relaxation is discussed.
Keywords: Polymer solutions; Polymer films; Dynamic surface properties; Capillary waves; Surface viscoelasticity;

The stability of polystyrene latexes prepared with 2,2′-azobis(N-2′-methylpropanoyl-2-amino-alkyl-1)-sulphonates as surface-active initiators (inisurfs) in emulsion polymerization is investigated. The experimentally observed stability during emulsifier-free emulsion polymerization depends on the alkyl chain length of the inisurfs. The calculated barrier height of the overall interaction energy fits nicely with the experimentally observed stability during the polymerization.
Keywords: Stability; Polymer dispersion; Surface-active initiator;

Two component model of orientationally ordered wall adjacent liquid layers by A.Yu. Popovskii; Yu.M. Popovskii; V.I. Mikhailenko (285-292).
Electronic-vibrational absorption spectra of anisole in isotropic bulk liquid phase and in orientationally ordered wall adjacent layers formed on quartz surface were measured. The spectra were registered at room (∼20 °C) temperature. The series of spectra measured for different thickness of the liquid interlayer and normalized by area manifests the existence of isobestic points. It determines the presence of different types of absorption centers there. Statistical analysis of obtained spectra shows that the number of these centers is equal to two; one of them is conditioned by a monomer and another one by the dimer. The thickness dependence of the relative concentration of monomers was investigated.
Keywords: Anisole; Electronic-vibrational spectra; Orientationally ordered wall adjacent layer; Isobestic points; Dimer;

Dielectric susceptibility of wall adjacent nitrobenzene layers by Yu.M Popovskii; A.A Goryuk (293-297).
Measurements of thickness dependence of dielectric susceptibility of nitrobenzene interlayers formed on conductive substrates were carried out. The obtained data were processed under the assumption of existence of dimers (with opposite directions of molecular dipole moments) in orientationally ordered wall-adjacent layers. The value of the orientationally ordered layer thickness coincides with the one determined by other methods.
Keywords: Nitrobenzene interlayers; Dielectric susceptibility; Dimer;

Shear viscoelastic properties of liquids and their boundary layers by Badma B Badmaev; Tuyana S Dembelova; Bair B Damdinov (299-305).
An acoustical resonance method with piezoquartz vibrator was used in the experimental determination of shear elasticity modulus and a tangent of mechanical loss angle of studied liquids and their boundary layers. It has been shown that liquid has an earlier unknown low frequency (approx. 100 kHz) viscoelastic relaxation process. The experimental results of investigation of low frequency shear elasticity of different class of liquids and their solutions have been presented. An experimental research of shear properties in dependence on shear deformation rate has been carried out. The possibility of the discovery of anomalous high viscosity of liquids has also been considered.
Keywords: Shear modulus; Liquids; Layers; Acoustic method; Viscosity;

Temperature dependence of shear elasticity of some liquids by Ochir R. Budaev; Marina N. Ivanova; Bair B. Damdinov (307-310).
The temperature dependencies of real and imaginary parts of complex shear modulus of liquids have been obtained. It has been shown that both real shear modulus G′ and imaginary shear modulus G″ decrease with increasing temperature. The experiments have been carried out by resonance method at a shear oscillation frequency of 74 kHz. Liquids of viscosity 0.02–0.03 Pa·s have been investigated.
Keywords: Liquid; Shear modulus; Temperature; Structure; Clusters;

Formulae for the van der Waals attraction energy and force between a spherical particle and an infinite cylinder are derived by the method of additive summation of the pair interactions described by the potential of the general form U m =α m ρ m . The formula of Rosenfeld and Wasan for the non-retarded vdW force between a sphere and a cylinder (m=6) is confirmed and the compact expression for the retarded force (m=7) is obtained. The comparison is given for the forces of the retarded vdW interaction between a sphere and a cylinder, another sphere, a row of spheres and a half-space. Also, the compact formulae for the energy of the vdW interaction of a point-like particle (atom, molecule) with a sphere and a cylinder are derived for the case of arbitrary m.
Keywords: Van der Waals interaction; Pair potential; Additive approach; Retardation effect;

Electrosurface properties of poly(ethylene terephtalate) track membranes by V.V Berezkin; V.I Volkov; O.A Kiseleva; N.V Mitrofanova; V.D Sobolev (325-331).
The electrosurface properties of track membranes made from poly(ethylene terephtalate) were determined by streaming potential (SP) and electron spin resonance (ESR) methods. The membrane pore diameters were in range from 20 to 200 nm. The pore electrical charge surface density values calculated with data obtained with methods mentioned differed dramatically. This difference was explained with the gel layer located at pore surface. The pore possesses a high large surface amassing a large electrical charge arising as a result of carboxylic groups dissociation. This charge cannot be detected with SP method due to high hydraulic resistance of the gel layer. In contrast to SP method, ESR detects all active centres parental of electrical charge at gel surface.
Keywords: Track membrane; Streaming potential; Electron spin resonance; Electrosurface properties; Gel layer;