Advances in Colloid and Interface Science (v.168, #1-2)
Editorial Board (IFC).
Obituary by Victor Starov; Ludmila Boinovich; Vladimir Sobolev (1-2).
Colloid particle and protein deposition — Electrokinetic studies by Z. Adamczyk; M. Nattich; M. Wasilewska; M. Zaucha (3-28).
Recent developments in the electrokinetic determination of particle, polyelectrolyte and protein deposition at solid/electrolyte interfaces, are reviewed. In the first section basic theoretical results are discussed enabling a quantitative interpretation of the streaming current/potential and microelectrophoretic measurements. Experimental results are presented, pertinent to electrokinetic characteristics of simple (homogeneous) surfaces such as mica, silica and various polymeric surfaces used in protein studies. The influence of the ionic strength, background electrolyte composition and pH is discussed, and the effective (electrokientic) charge of these interfaces is evaluated. In the next section, experimental data obtained by streaming potential measurements for colloid particle mono- and bilayers are presented and interpreted successfully in terms of available theoretical approaches. These results, obtained for model systems of monodisperse colloid particles are used as reference data for discussion of more complicated experiments performed for polyelectrolyte and protein covered surfaces. Results are discussed, obtained for cationic polyelectrolytes (PEI, PAH) and fibrinogen adsorbing on mica, interpreted quantitatively in terms of the theoretical approach postulating a heterogeneous 3D charge distribution. The Gouy–Chapman model, based on the continuous charge distribution proved inadequate. Interesting experimental data are also discussed, obtained by electrophoretic methods in the case of protein adsorption on colloid latex particles. In the last section, supplementary results on particle deposition on heterogeneous surfaces produced by controlled protein adsorption are discussed. Quantitative relationships between the amount of adsorbed protein, zeta potential of the interface and the particle coverage are specified. Possibility of evaluating the heterogeneity of protein charge distribution is pointed out. The anomalous deposition of colloid particles on protein molecules bearing the same sign of zeta potential, which contradicts classical DLVO theory, is interpreted in terms of the fluctuation theory. It is concluded that theoretical and experimental results obtained for model colloid systems and flat interfaces can be effectively used for interpretation of protein adsorption phenomena, studied by electrophoresis. In this way the universality of electrokinetic phenomena is underlined.Recent developments in the electrokinetics of colloid, polyelectrolyte and protein covered surfaces were reviewed. Anomalous deposition of proteins on surfaces bearing the same sign of zeta potential, contradicting classical DLVO theory, was interpreted in terms of the fluctuation theory. It was shown how experimental results obtained for model colloid systems and flat interfaces can be used for interpretation of protein adsorption, studied by electrophoresis.Display Omitted
Keywords: Colloid particle deposition; Deposition of colloids; Electrokinetic methods–protein adsorption; Polyelectrolyte adsorption and multilayers; Protein adsorption; Streaming potential measurements; Zeta potential of covered surfaces;
Experimental study of hybrid nematic wetting films by A.M. Cazabat; U. Delabre; C. Richard; Y. Yip Cheung Sang (29-39).
Liquid crystal layers, with thickness less than 1 μm, are deposited on isotropic – solid or liquid – substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as “hybrid nematics”. The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.Display Omitted► Nematics deposited on substrates with hybrid anchoring may show film coexistence. ► Thicker films may be striped due to the anisotropy of nematic elasticity. ► Available theoretical models account partly on the thicker films properties. ► Coexistence of molecular and mesoscopic thinner films occurs close to NI transition. ► The structure of mesoscopic thinner films is still largely unknown.
Keywords: Thin wetting films; Film coexistence; Nematic elasticity; Nematic order parameter; Instability patterns;
Disjoining pressure and capillarity in the constrained vapor bubble heat transfer system by Arya Chatterjee; Joel L. Plawsky; Peter C. Wayner (40-49).
Using the disjoining pressure concept in a seminal paper, Derjaguin, Nerpin and Churaev demonstrated that isothermal liquid flow in a very thin film on the walls of a capillary tube enhances the rate of evaporation of moisture by several times. The objective of this review is to present the evolution of the use of Churaev's seminal research in the development of the Constrained Vapor Bubble (CVB) heat transfer system. In this non-isothermal “wickless heat pipe”, liquid and vapor flow results from gradients in the intermolecular force field, which depend on the disjoining pressure, capillarity and temperature. A Kelvin–Clapeyron model allowed the use of the disjoining pressure to be expanded to describe non-isothermal heat, mass and momentum transport processes. The intermolecular force field described by the convenient disjoining pressure model is the boundary condition for “suction” and stability at the leading edge of the evaporating curved flow field. As demonstrated by the non-isothermal results, applications that depend on the characteristics of the evaporating meniscus are legion.
Keywords: Disjoining pressure; Capillarity; Liquid–vapor–solid contact line; Kelvin–Clapeyron model; Evaporating meniscus; Constrained vapor bubble;
The metastable states of foam films containing electrically charged micelles or particles: Experiment and quantitative interpretation by Krassimir D. Danov; Elka S. Basheva; Peter A. Kralchevsky; Kavssery P. Ananthapadmanabhan; Alex Lips (50-70).
The stepwise thinning (stratification) of liquid films containing electrically charged colloidal particles (in our case — surfactant micelles) is investigated. Most of the results are applicable also to films from nanoparticle suspensions. The aim is to achieve agreement between theory and experiment, and to better understand the physical reasons for this phenomenon. To test different theoretical approaches, we obtained experimental data for free foam films from micellar solutions of three ionic surfactants. The theoretical problem is reduced to the interpretation of the experimental concentration dependencies of the step height and of the final film thickness. The surface charges of films and micelles are calculated by means of the charge-regulation model, with a counterion-binding (Stern) constant determined from the fit of surface tension isotherms. The applicability of three models was tested: the Poisson–Boltzmann (PB) model; the jellium-approximation (JA), and the cell model (CM). The best agreement theory/experiment was obtained with the JA model without using any adjustable parameters. Two theoretical approaches are considered. First, in the energy approach the step height is identified with the effective diameter of the charged micelles, which represents an integral of the electrostatic-repulsion energy calculated by the JA model. Second, in the osmotic approach the step height is equal to the inverse cubic root of micelle number density in the bulk of solution. Both approaches are in good agreement with the experiment if the suspension of charged particles (micelles) represents a jellium, i.e. if the particle concentration is uniform despite the field of the electric double layers. The results lead to a convenient method for determining the aggregation number of ionic surfactant micelles from the experimental heights of the steps.Display Omitted► The step height in stratifying films coincides with the particle effective diameter. ► The latter is the diameter of a hard sphere with the same second virial coefficient. ► The step height coincides also with the mean interparticle distance in the bulk. ► This happens if the interparticle repulsion leads to a uniform jellium distribution. ► If salt is added, Boltzmann distribution is present, but the films do not stratify.
Keywords: Charged particles in liquid films; Stepwise thinning of foam films; Degree of micelle ionization; Aggregation number of ionic micelles;
Effect of gas type on foam film permeability and its implications for foam flow in porous media by R. Farajzadeh; R.M. Muruganathan; W.R. Rossen; R. Krastev (71-78).
The aim of this paper is to provide a perspective on the effect of gas type on the permeability of foam films stabilized by different types of surfactant and to present a critical overview of the tracer gas experiments, which is the common approach to determine the trapped fraction of foam in porous media. In these experiments some part of the gas is replaced by a “tracer gas” during the steady-state stage of the experiments and trapped fraction of foam is determined by fitting the effluent data to a capacitance mass-transfer model.We present the experimental results on the measurement of the gas permeability of foam films stabilized with five surfactants (non-ionic, anionic and cationic) and different salt concentrations. The salt concentrations assure formation of either common black (CBF) or Newton black films (NBF). The experiments are performed with different single gasses. The permeability of the CBF is in general higher than that of the NBF. This behavior is explained by the higher density of the surfactant molecules in the NBF compared to that of CBF. It is also observed that the permeability coefficient, K(cm/s), of CBF and NBF for non-ionic and cationic surfactants are similar and K is insensitive to film thickness. Compared to anionic surfactants, the films made by the non-ionic surfactant have much lower permeability while the films made by the cationic surfactant have larger permeability. This conclusion is valid for all gasses. For all types of surfactant the gas permeability of foam film is largely dependent on the dissolution of gas in the surfactant solution and increases with increasing gas solubility in the bulk liquid.The measured values of K are consistent with rapid diffusion of tracer gasses through trapped gas adjacent to flowing gas in porous media, and difficulties in interpreting the results of tracer-foam experiments with conventional capacitance models. The implications of the results for foam flow in porous media and factors leading to difficulties in the modeling of trapped fraction of foam are discussed in detail. To avoid complications in the interpretation of the results, the best tracer would be one with a permeability close to the permeability of the gas in the foam. This puts a lower limit on the effective diffusion coefficient for tracer in an experiment.Display Omitted► The permeability of foam films stabilized by three types of surfactants to different gases with different solubility in water was experimentally determined. ► The permeability of films stabilized by all types of surfactants differs for different gases. ► Film permeability is higher when solubility and diffusion coefficient of the gas in water are greater. ► The best tracer gas is one with a permeability close that of the gas in the foam.
Keywords: Foam film; Gas permeability; Monolayer; Surfactant; Gas–liquid interface; Gas type; Porous media; Tracer-gas experiment;
Electrostatic and steric interactions in oil-in-water emulsion films from Pluronic surfactants by G. Gotchev; T. Kolarov; Khr. Khristov; D. Exerowa (79-84).
Stabilization of oil-in-water emulsion films from PEO–PPO–PEO triblock copolymers is described in terms of interaction surface forces. Results on emulsion films from four Pluronic surfactants, namely F108, F68, P104 and P65 obtained with the Thin Film Pressure Balance Technique are summarized. It is found that film stabilization is due to DLVO (electrostatic) and non-DLVO (steric in origin) repulsive forces. The charging of the oil/water film interfaces is related to preferential adsorption of OH– ions. This is confirmed by pH-dependent measurements of the equivalent film thickness (h w) at both constant capillary pressure and ionic strength. With reducing pH in the acidic region, a critical value (pHcr,st) corresponding to an isoelectric state of the oil/water film surfaces is found where the electrostatic interaction in the films is eliminated. At pH ≤ pHcr,st, the emulsion films are stabilized only by steric forces due to interaction between the polymer adsorption layers. Disjoining pressure (Π) isotherms measured for emulsion films from all the four Pluronic surfactants used at pH < pHcr,st show a transition to a Newton black film with increasing Π. The experimental data before the NBF-transition in the disjoining pressure isotherms are fitted to the Alexander–de Gennes’ scaling theory for steric interaction between polymer brushes with the PEO-brush thickness as a free parameter. The NBF observed are stabilized most probably by short-range steric forces that may differ from the brush-to-brush interaction.Display Omitted
Keywords: Interaction forces; Oil-in-water emulsion films; PEO-PPO-PEO triblock copolymers; Steric forces; Electrostatic forces;
Coalescence map for bubbles in surfactant-free aqueous electrolyte solutions by Roger G. Horn; Lorena A. Del Castillo; Satomi Ohnishi (85-92).
Factors influencing bubble coalescence in surfactant-free aqueous electrolyte solutions are considered in this compilation of literature results. These factors include viscous and inertial thin film drainage, surface deformation, surface elasticity, mobility or otherwise of the air–water interface, and disjoining pressure. Several models from the literature are discussed, with particular attention paid to predictions of transitions between regions where behaviour is qualitatively different. The transitions are collated onto a single chart with salt concentration and bubble approach speed as the axes. This creates a map of the regions in which different mechanisms operate, giving an overall picture of bubble coalescence behaviour over a wide range of concentration and speed. Only mm-size bubbles in water and NaCl solutions are discussed in this initial effort at creating such a map. Data on bubble coalescence or non-coalescence are collected from the literature and plotted on the same map, generally aligning well with the predicted transitions and thus providing support for the theoretical reasoning that went into creating the coalescence map.Display Omitted
Keywords: Bubble coalescence; Thin film drainage; Surface elasticity; Air–water interface; Free liquid film;
Hofmeister effect on micellization, thin films and emulsion stability by Ivan B. Ivanov; Radomir I. Slavchov; Elka S. Basheva; Doroteya Sidzhakova; Stoyan I. Karakashev (93-104).
The Hofmeister effect on the critical micelle concentration (CMC), the thin liquid film electrostatic disjoining pressure (Πel) and the critical coalescence pressure of emulsion drops (P CR) were investigated. For CMC literature data were used, but Π and P CR were measured by us. The essence of the theoretical approach was to modify existing theories of CMC and Πel by using generalized Gouy equation and dimensionless surface potential (ΦS), involving the counterion specific adsorption energy (u 0). The computational procedure of u 0 does not involve any adjustable parameters. Linear dependences of ln(CMC), ΦS and P CR on u 0 were found in conformity with Hofmeister series. The experimental slopes of ln(CMC) and ΦS vs. –u 0 /k B T were negative and very close to the theoretical ones. A hypothesis was put forward for explanation of the positive slopes of P CR on u 0. The obtained results suggest that the counterion specific adsorption energy u 0 encompasses all major factors, involved in the Hofmeister effect for the studied phenomena. If this is confirmed by analysis of more phenomena, revealing Hofmeister effect, one could claim that u 0 is the factor controlling the Hofmeister effect and a powerful tool for its study.Display Omitted► The ion adsorption energy quantifies Hofmeister effect on micelles and thin films. ► CMC of ionic surfactants depends on the counterion and follows Hofmeister series. ► Corrin–Harkins eq. for micelles with two counterions and specific effects is deduced. ► Hofmeister effects on disjoining and critical coalescence pressure are observed.
Keywords: Ion specific effects; Hofmeister series; Adsorption of ions; CMC of ionic surfactants; Thin liquid films; Film trapping technique;
Foam, emulsion and wetting films stabilized by polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants by Khr. Khristov; H. Petkova; L. Alexandrova; M. Nedyalkov; D. Platikanov; D. Exerowa; J. Beetge (105-113).
This review explores three (A, B, C) polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants belonging to the group of star-like polymers. They have a similar structure, differing only in the number of polymeric branches (4, 6 and 9 in the mentioned order). The differences in these surfactants' ability to stabilize foam, o/w/o and w/o/w emulsion and wetting films are evaluated by a number of methods summarized in Section 2. Results from the studies indicate that differences in polymeric surfactants' molecular structure affect the properties exhibited at air/water, oil/water and water/solid interfaces, such as the value of surface tension, interfacial tension, critical micelle concentration, degree of hydrophobicity of solid surface, etc. Foam, emulsion and wetting films stabilized by such surfactants also show different behavior regarding some specific parameters, such as critical electrolyte concentration, surfactant concentration for obtaining a stable film, film thickness value, etc. These observations give reasons to believe that model studies can support a comprehensive understanding of how the change in polymeric surfactant structure can impact thin liquid films properties. This may enable a targeted design of the macromolecular architecture depending on the polymeric surfactants application purpose.Display Omitted
Keywords: Star-like polymers; Foam films; Emulsion films; Wetting films;
Concentration polarization effect at the deposition of charged Langmuir monolayers by V.I. Kovalchuk; E.K. Zholkovskiy; M.P. Bondarenko; V.M. Starov; D. Vollhardt (114-123).
The review summarizes the results of the recent studies of the electrokinetic relaxation process within the meniscus region during the deposition of charged Langmuir monolayers. Such electrokinetic relaxation is the consequence of the initial misbalance of partial ion fluxes within a small region near the contact line, where the diffuse parts of electric double layers, formed at the monolayer and the substrate surface, overlap. The concentration polarization within the solution near the three-phase contact line should lead to long-term relaxations of the meniscus after beginning and stopping the deposition process, to changes of the ionic composition within the deposited films, to change of the interaction of the monolayer with the substrate, and to dependence of the maximum deposition rate on the subphase composition.
Keywords: Langmuir–Blodgett films; Three-phase contact; Meniscus relaxation; Charged Langmuir monolayers; Electrokinetic mechanism; Concentration polarization; Pattern formation;
Surface force measurements on freely suspended liquid films by D. Langevin; C. Marquez-Beltran; J. Delacotte (124-134).
This paper reviews existing studies of freely suspended liquid films, focusing on the role of the forces between surfaces. The important role of kinetics phenomena is discussed. Examples of studies making use of solutions containing surfactants, proteins and particles are compared. The different aspects of film lifetime are discussed, from film formation to film rupture. A comparison with the few existing theories is also made.
Keywords: Thin films; Surface forces; Surface rheology;
The first step in layer-by-layer deposition: Electrostatics and/or non-electrostatics? by Johannes Lyklema; Louise Deschênes (135-148).
A critical discussion is presented on the properties and prerequisites of adsorbed polyelectrolytes that have to function as substrates for further layer-by-layer deposition. The central theme is discriminating between the roles of electrostatic and non-electrostatic interactions. In order to emphasize this feature we refrain from discussing practical problems sometimes incurred in polyelectrolyte adsorption like freezing-in of non-equilibrium situations, patchwise attachment, unclear chemistry and only consider solid substrates. Although it is in principle ambiguous to discriminate between coulombic and non-coulombic or “chemical” interactions, it will be shown that, as a rule, non-coulombic contributions to the interactions cannot be neglected. They are responsible for the familiar overcharging. For obtaining more insight, it is recommended to consider electrometric techniques such as electrokinetics, conductometry and potentiometry, in combination with other analytical techniques applied to well-defined systems, for which various parameters can be modulated in a systematic way.Display Omitted► Stacking of alternating layers of positive and negative polyelectrolytes on solid substrates can lead to multilayered systems. ► Many examples from the literature indicate that the binding force between the layers is not solely electrostatic. ► Non-electrostatic interactions can readily lead to overcharging, and hence to multiple stacking. ► Methods to assess the contribution of non-electrostatic contributions to the interactions are discussed. ► Many of these interactions can be observed in the interaction of the first polyelectrolyte layer with the substrate.
Keywords: Polyelectrolyte adsorption; Surface conduction; Layer-by-layer deposition; Polyelectrolyte multilayers; Countercharge distribution; Charge inversion;
The model of hydrophobic attraction in the framework of classical DLVO forces by Nataliya A. Mishchuk (149-166).
The present article focuses on the analysis of experimental data and interpreting of the influence of water depletion near hydrophobic particles and nanobubbles formed on their surface or in the space between them on van der Waals and electrostatic components of interparticle interaction. It is shown that the difference between simplified and more detailed models of DLVO forces explains the nature and main characteristics of hydrophobic attraction.The models of DLVO forces are developed for particle/particle and particle/bubble interaction at water depletion or nanobubbles on interface (a, b) and at presence of nanobubbles in the space between macrobodies (c, d). Display Omitted► Hydrophobic attraction can be described in the framework of classical DLVO forces. ► Water depletion near interface decreases local dielectric permittivity. ► Local changes in dielectric permittivity affect the van der Waals interaction. ► The structure of an electric double layer depends on local dielectric permittivity. ► Nanobubbles near hydrophobic surfaces change the van der Waals interaction.
Keywords: Adsorption layer; Dielectric dispersion; Nanobubbles; Nanosegments; Water depletion;
Dynamics of interfacial layers—Experimental feasibilities of adsorption kinetics and dilational rheology by N. Mucic; A. Javadi; N.M. Kovalchuk; E.V. Aksenenko; R. Miller (167-178).
Each experimental method has a certain range of application, and so do the instruments for measuring dynamic interfacial tension and dilational rheology. While the capillary pressure tensiometry provides data for the shortest adsorption times starting from milliseconds at liquid/gas and tens of milliseconds at liquid/liquid interfaces, the drop profile tensiometry allows measurements in a time window from seconds to many hours. Although both methods together cover a time range of about eight orders of magnitude (10− 3 s to 105 s), not all surfactants can be investigated with these techniques in the required concentration range. The same is true for studies of the dilational rheology. While drop profile tensiometry allows oscillations between 10− 3 Hz and 0.2 Hz, which can be complemented by measurements with capillary pressure oscillating drops and the capillary wave damping method (up to 103 Hz) these six orders of magnitude in frequency are often insufficient for a complete characterization of interfacial dilational relaxations of surfactant adsorption layers. The presented analysis provides a guide to select the most suitable experimental method for a given surfactant to be studied. The analysis is based on a diffusion controlled adsorption kinetics and a Langmuir adsorption model.The frequency dependence of the dilational rheology is a very sensitive characteristic for each surfactant. The maximum in the dilational viscosity is directly related to the characteristic frequency of this surfactant, and it depends on the surfactant bulk concentration.Display Omitted►The adsorption isotherm spans over two to four orders of magnitude of bulk concentration. ►The dynamic interfacial tension and the dilational rheology describe the adsorption dynamics. ►-To measure the dynamic interfacial tensions typically one experimental method is not sufficient. ► The bubble/drop profile analysis tensiometry and capillary pressure tensiometry are complement. ► Oscillating drops/bubbles based on shape analysis or capillary pressure measurements represent the best techniques for studies of the dilation rheology. ► On the basis of the Langmuir adsorption model the optimum selection of techniques can be proposed.
Keywords: Dynamic surface tension; Dilational rheology; Oscillating drop method; Maximum bubble pressure method; Drop profile analysis tensiometry; Capillary pressure tensiometry; Surfactants; Langmuir isotherm; Diffusion controlled adsorption;
Dilational surface visco-elasticity of polyelectrolyte/surfactant solutions: Formation of heterogeneous adsorption layers by B.A. Noskov; G. Loglio; R. Miller (179-197).
Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.Display Omitted► Dynamic surface properties of polyelectrolyte/surfactant solutions are reviewed. ► Concentration dependencies of the dynamic surface elasticity have some general features. ► Kinetic dependencies of the dynamic surface elasticity can have one or two local maxima. ► Results indicate nanoparticle formation in the surface layer. ► Scanning probe microscopy and ellipsometry confirm the conclusions.
Keywords: Polyelectrolyte/surfactant complexes; Dilational surface rheology; Dynamic surface elasticity; Dynamic surface tension; Relaxation processes in the surface layer; Nano-particles;
Dynamic aspects of small bubble and hydrophilic solid encounters by Luke Parkinson; John Ralston (198-209).
The capture of solid particles suspended in aqueous solution by rising gas bubbles involves hydrodynamic and physicochemical processes that are central to colloid science. Of the collision, attachment and aggregate stability aspects to the bubble-particle interaction, the crucial attachment process is least understood. This is especially true of hydrophilic solids. We review the current literature regarding each component of the bubble-particle attachment process, from the free-rise of a small, clean single bubble, to the collision, film drainage and interactions which dominate the attachment rate. There is a particular focus on recent studies which employ single, very small bubbles as analysis probes, enabling the dynamic bubble–hydrophilic particle interaction to be investigated, avoiding complications which arise from fluid inertia, deformation of the liquid–vapour interface and the possibility of surfactant contamination.Display Omitted► Focus on very small, 20-120 µm diameter bubble-hydrophilic solid encounters. ► New approaches for measuring dynamic bubble-solid interaction forces. ► Interactions over the force range of natural colloid interactions with negligible bubble deformation and inertia are reviewed.
Keywords: Bubble; Particle; Microbubble; Attachment; Film; Drainage;
Adhesion models: From single to multiple asperity contacts by Polina Prokopovich; Victor Starov (210-222).
This review presents a summary of the current adhesion models available to date, between real rough surfaces, starting from single asperity models and expanding to multiple asperity contacts. The focus is made on multi-asperity contact interactions. Both van der Waals and contact mechanics approaches have been considered and relevant adhesion models are reviewed and discussed. The influence of the meniscus forces on adhesion has been considered, along with a summary of the various meniscus models. The effect of surface geometry, its topography and environmental conditions on meniscus action are also discussed along with its integration into multi-asperity adhesion models.
Keywords: Adhesion; Meniscus force; Multi-asperity adhesion models; JKR; DMT;
Asymmetric gas mixture transport in composite membranes by V.I. Roldughin; V.M. Zhdanov (223-246).
The asymmetry effects in gas and electrolyte transport through composite membranes are considered. The interrelation between the kinetic theory and non-equilibrium thermodynamics description of gas mixture transport in channels is discussed. The kinetic expressions for transport and slip coefficients are given. The effect of surface forces on gas transport is discussed. A set of general equations related to gas mixture flows in capillaries and porous media is deduced. The nano-size effects in gas flows are outlined. The theoretical analysis of one-way flow effect and asymmetric separation properties of a two-layer porous membrane is given.
Keywords: Membrane; Asymmetric transport; Diffusion; Slip effects; Surface forces; Kinetic theory; Non-equilibrium thermodynamics; Gas mixture separation;
Multipurpose smart hydrogel systems by Yu Samchenko; Z. Ulberg; O. Korotych (247-262).
This paper represents the review of the last investigations in the field of smart polymeric hydrogels and our contribution to this matter. New hydrogel systems and nanocomposites based on acrylic monomers (acrylamide, acrylonitrile, acrylic acid, N-isopropylacrylamide etc.) with incorporated nanosized colloidal silver, hydroxyapatite and carbon nanotubes with a new set of properties have been obtained and examined. These systems can sharply change their characteristics when minor external physical (electric and magnetic fields, temperature etc.) or chemical (pH, ionic strength) stimuli are applied. Such stimulus-responsive polymeric systems are very promising from the standpoint of different medical applications, especially for the development of intelligent drug delivery systems. On the base of designed hydrogel iontophoretic transdermal therapeutic systems, endoprosthesis for the replacement of bone tissue and hydrogel burns coatings with immobilized mesenchymal cells were obtained and tested.Display Omitted
Keywords: Hydrogels; Smart polymeric systems; Thermosensitive; Swelling; Drug realese; Iontophoresis; Nanoreactors; Stem cells; Acrylamide; Acrylonitrile; Acrilic acid; NIPAAm;
Interparticle interactions in concentrated suspensions and their bulk (Rheological) properties by Tharwat Tadros (263-277).
The interparticle interactions in concentrated suspensions are described. Four main types of interactions can be distinguished: (i) “Hard-sphere” interactions whereby repulsive and attractive forces are screened. (ii) “Soft” or electrostatic interactions determined by double layer repulsion. (iii) Steric repulsion produced by interaction between adsorbed or grafted surfactant and polymer layers. (iv)and van der Waals attraction mainly due to London dispersion forces. Combination of these interaction energies results in three main energy-distance curves: (i) A DLVO type energy-distance curves produced by combination of double layer repulsion and van der Waals attraction. For a stable suspension the energy-distance curve shows a “barrier” (energy maximum) whose height must exceed 25 kT (where k is the Boltzmann constant and T is the absolute temperature). (ii) An energy-distance curve characterized by a shallow attractive minimum at twice the adsorbed layer thickness 2δ and when the interparticle-distance h becomes smaller than 2δ the energy shows a sharp increase with further decrease of h and this is the origin of steric stabilization. (iii) an energy-distance curve characterized by a shallow attractive minimum, an energy maximum of the DLVO type and a sharp increase in energy with further decrease of h due to steric repulsion. This is referred to as electrosteric repulsion. The flocculation of electrostatically and sterically stabilized suspensions is briefly described. A section is devoted to charge neutralization by polyelectrolytes and bridging flocculation by polymers. A distinction could be made between “dilute”, “concentrated” and “solid suspensions” in terms of the balance between the Brownian motion and interparticle interaction. The states of suspension on standing are described in terms of interaction forces and the effect of gravity.The bulk properties (rheology) of concentrated suspensions are described starting with the case of very dilute suspensions (the Einstein limit with volume fraction ϕ ≤ 0.01), moderately concentrated suspensions (0.2 > ϕ ≥ 0.1) taking into account the hydrodynamic interaction and concentrated suspensions (ϕ > 0.2) where semi-empirical theories are available. The rheological behavior of the above four main types of interactions is described starting with “hard-sphere” systems where the relative viscosity–volume fraction relationship could be described. The rheology of electrostatically stabilized suspensions was described with particular reference to the effect of electrolyte that controls the double layer extension. The rheology of sterically stabilized systems is described using model polystyrene suspensions with grafter poly(ethylene oxide) layers. Finally the rheology of flocculated suspensions was described and a distinction could be made between weakly and strongly flocculated systems.
Keywords: Interparticle interactions; Flocculation; Bridging; Dilute; Concentrated; Solid suspensions; States of suspensions; Rheology of suspensions;
Transport properties of long straight nano-channels in electrolyte solutions: A systematic approach by Andriy E. Yaroshchuk (278-291).
The principle of local thermodynamic equilibrium is systematically employed for obtaining various transport properties of long straight nano-channels. The concept of virtual solution is used to describe situations of non-negligible overlap of diffuse parts of electric double layers (EDLs) in nano-channels. Generic expressions for a variety of transport properties of long straight nano-channels are obtained in terms of quasi-equilibrium distribution coefficients of ions and functionals of quasi-equilibrium distribution of electrostatic potential. Further, the Poisson-Boltzmann approach is used to specify these expressions for long straight slit-like nano-channels. In the approximation of non-overlapped diffuse parts of double electric layers in nano-channels, simple analytical expressions are obtained for the apparent electrophoretic mobilities of (trace) analytes of arbitrary charge as well as for the salt reflection coefficient (osmotic pressure), salt diffusion permeability and electro-viscosity (electrokinetic energy conversion). The approximate solutions are compared with the results of rigorous solution of non-linearized Poisson-Boltzmann equation, and the accuracy of approximation is shown to be typically excellent when the nano-channel half-height exceeds ca.3 Debye screening lengths.Due to non-negligible electrostatic adsorption of ions by nano-channels, the apparent electrophoretic mobilities of counter-ionic analytes in nano-channels are smaller than in micro-channels whereas those of co-ionic analytes are larger. This dependence on the charge is useful for the separation of analytes of close electrophoretic mobilities.The osmotic pressure is shown to be positive, negative or pass through maxima as a function of applied salt-concentration difference within a fairly narrow range of ratios of nano-channel height to the Debye screening length. The diffusion permeability of charged nano-channels to single salts is demonstrated (for the first time) to be typically larger than that of neutral nano-channels of the same dimensions due to electrical facilitation of salt diffusion.
Keywords: Nano-channel; Capillary electrophoresis; Separation; Ion; Salt reflection coefficient; Salt diffusion permeability; Electro-viscosity; Electroosmosis;