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Advances in Colloid and Interface Science (v.114-115, #)
Zeta potentials in the flotation of oxide and silicate minerals by D.W. Fuerstenau; Pradip (pp. 9-26).
Adsorption of collectors and modifying reagents in the flotation of oxide and silicate minerals is controlled by the electrical double layer at the mineral–water interface. In systems where the collector is physically adsorbed, flotation with anionic or cationic collectors depends on the mineral surface being charged oppositely. Adjusting the pH of the system can enhance or prevent the flotation of a mineral. Thus, the point of zero charge (PZC) of the mineral is the most important property of a mineral in such systems. The length of the hydrocarbon chain of the collector is important because of chain–chain association enhances the adsorption once the surfactant ions aggregate to form hemimicelles at the surface. Strongly chemisorbing collectors are able to induce flotation even when collector and the mineral surface are charged similarly, but raising the pH sufficiently above the PZC can repel chemisorbing collectors from the mineral surface. Zeta potentials can be used to delineate interfacial phenomena in these various systems.
Hetero-interaction between Gouy–Stern double layers: Charge and potential regulation by Johannes Lyklema; Jérôme F.L. Duval (pp. 27-45).
This issue of Advances is devoted to the memory of Hans-Joachim Schulze who, before his untimely death, made substantial contributions in the domains of flotation, wetting and particle interaction, often under adverse working conditions. Many of his publications involve hetero-interaction between different oxides or between solids and air bubbles. In the present paper, the theory for the electric interaction between charged interfaces is reviewed and extended by considering regulation on the Gouy-Stern level, including hetero-interaction. We start from a generalization of the phenomenon of regulation, distinguishing electrostatic, chemical and entropic contributions to the regulation capacity. The analysis is carried out in the mean field approximation for flat double layers. Against this background we review and discuss the existing literature. It is shown that for an appropriate account it is mandatory to include Stern layers, to which not only electrical but also chemical and entropic contributions to the regulation capacity can be attributed. It is demonstrated that interaction at constant diffuse double layer potential or -charge works only at rather large distance of separation. At shorter distance, substantial deviations from this behaviour are found, with a trend that interaction at (about) constant potential is the better obeyed the higher the corresponding regulation capacities are.A lattice model is elaborated to visualize the profiles of the (surface, Stern and diffuse) charges and the corresponding potential changes upon overlap, including the cases where strong induction leads to charge-inversion of the layer with the larger regulation capacity. A number of elaborations is presented, but the potentialities of the model are virtually unlimited.
Amphiphilic nanostructures in thin liquid films by Elena Mileva; Plamen Tchoukov; Dotchi Exerowa (pp. 47-52).
A survey on recent experimental investigations of microscopic foam films containing self-assembled amphiphilic nanostructures is presented. The film characteristics are investigated via microinterferometric method, which operates with the measuring cell of Scheludko–Exerowa. The results show the following: (1) Unstable black patterns (dots and spots) are observed; they have very short lifetimes and the films which contain them rupture quickly. (2) Drainage times of the films display sharp changes within the studied surfactant concentration range. (3) The peculiarities in the film drainage properties are in accordance with the specific run of the adsorption isotherms of the initial surfactant solutions. The data are interpreted based on the assumption that a series of smaller self-assembled aggregates (premicelles) exist in amphiphilic solutions. The results show that the microscopic foam film technique has a serious potential as a prospective instrumentation for the study of amphiphilic self-assemblies in surfactant solutions.
Keywords: Amphiphilic nanostructures; Self-assembly; Thin liquid films
On the nature of Athabasca Oil Sands by Jan Czarnecki; Boryan Radoev; Laurier L. Schramm; Radomir Slavchev (pp. 53-60).
The existence of a thin aqueous film, separating bitumen (a form of heavy oil) from inorganic solids in Athabasca Oil Sands, is analysed based on “first principles�. There is a general consensus in the literature on the hydrophilic character of the solids in oil sands. However, a review of the references cited in support of the solids being encapsulated in thin water envelopes produced a surprising lack of evidence. A theoretical analysis indicates that a water film separating clean, hydrophilic quartz and bitumen is stable under most conditions, and unstable for acidic oil sand ores. The existence of water-wet solids in the Athabasca Oil Sands remains a reasonable yet unproven postulate. It could therefore be dangerous to accept the water-wet solids postulate and then use it to interpret other phenomena.
Keywords: Oil sands structure; Thin liquid films; Wetting films; Asymmetric liquid films; Stability of bitumen/water/silica films
Interfacial rheology of adsorbed layers with surface reaction: On the origin of the dilatational surface viscosity by Ivan B. Ivanov; Krassimir D. Danov; Kavssery P. Ananthapadmanabhan; Alex Lips (pp. 61-92).
A theoretical study of the phenomena, occurring in an adsorbed layer, subject to small dilatational perturbations was carried out. Two main processes, provoked by the perturbations (surface reaction and surfactant transport onto the surface) were considered. The reaction was described by means of the reaction coordinate. The derived general rheological equation for insoluble surfactants, gave as limiting cases Voight and Maxwell type equations for fast and slow reactions, respectively. Expressions for all characteristics of the process (surface elasticity, reaction elasticity, reaction relaxation time and dilatational surface viscosity) were obtained. The obtained generalized rheological equation for reactions involving soluble surfactants is a dynamic analog of Gibbs adsorption isotherms for a multi-component system with surface reaction, since similarly to Gibbs equation it relates the surface stress only to surface variables. It gives as limiting cases generalized forms for soluble surfactants of Voight and Maxwell equations. All new rheological equations were analyzed for deformations with constant rate and periodic oscillations and they were applied to three simple surface reactions (monomolecular with one product, dimerization and association). The mass transfer was analyzed initially in the absence of surface reaction. In this system the surface stress is purely elastic, but it was shown that if the adsorption perturbation is small, regardless of the type of surface perturbation and the mechanism of adsorption, the process of mass transport always obeys a Maxwell type rheological equation. For all considered processes surface viscosities were defined, but they were called “apparent�, because they stem from diffusion, rather than from interaction between the surfactant molecules and they depend not only on surface parameters, but also on the geometry of the system. The often used in the literature correlations between the lifetime of emulsions and foams and the imaginary elasticity were analyzed. It was shown that this approach lacks serious scientific foundations and could lead to erroneous conclusions. Finally, the problem for the coupling of the surfactant diffusion with the chemical reaction was analyzed and it was demonstrated on a simple example how it could be tackled.
Keywords: Interfacial rheology; Surface reaction; Insoluble and soluble surfactants; Surface viscosity; Surface elasticity; Maxwell and Voight laws; Role of diffusion; Oscillations and continuous deformation of surfaces
Electrostatics of heterogeneous monolayers. Constitutive equations and mathematical models by B. Radoev â?Ž; T. Boev; M. Avramov (pp. 93-101).
A new approach for modelling the electrochemistry of complex heterogeneous systems, composed of various bulk, surface and line phases, is proposed. A key step in it is the generalization of standard electrostatic boundary conditions, considered as 2D, 1D Poisson– Boltzmann equations. This viewpoint is applied to experimentally studied systems of two co-existing (e.g. liquid expanded, liquid condensed) monolayer phases. Another important feature of the given model is that, at electrochemical equilibrium, like with bulk double layers, surface double layers emerge on both sides of the contact line dividing the surface phases. A general mathematical formulation of the problem is proposed in respect of the electric potential as distributed in the complex heterogeneous system. Comments are also provided on the methods of solving the problem. The example illustrates a simplified system of two semi-infinite surface phases (monolayers) having equivalent parameters and divided by a contact line. The paper discusses the expressions obtained for the electrical potential, electrostatic energy, etc. and their dependence on the parameters of the system. Special focus is taken of the work of contact line formation.
Keywords: Monolayers; Surface electrostatics; Work of new phase formation
The effect of line tension on the shape of liquid menisci near stripwise heterogeneous walls by M. Hoorfar; A. Amirfazli; J.A. Gaydos; A.W. Neumann (pp. 103-118).
Most attempts for the measurement of line tension in solid–liquid–vapor systems are based on the drop size dependence of contact angles of sessile drops on smooth, homogeneous solid surfaces. Despite being a well-defined thermodynamic quantity, there are still significant discrepancies in both the magnitude and sign of line tension reported in different experimental arrangements and theoretical studies. In order to broaden the scope of experimental studies of line tension, a non-axisymmetric system, i.e., a stripwise heterogeneous wall arrangement, was considered. A numerical scheme has been developed to solve simultaneously both the modified Young equation and the Laplace equation of capillarity for such a stripwise wall and to generate a series of theoretical contact lines for non-zero line tension values. These theoretical curves can be compared to experimental profiles to determine line tension. The preliminary comparison of these curves with an experimental curve suggests that the line tension value is on the order of 10−6 (J/m), in agreement with values obtained from drop size dependence of contact angle studies. The comparison also shows that line tension in such systems cannot be as low as 10−10 (J/m), i.e., the order of the magnitude obtained from some theoretical studies and experimental approaches.
Keywords: Line tension; Stripwise heterogeneous wall; Minimal surface; Conformal mapping; Newton Raphson
Hydrodynamic fragmentation of nanoparticle aggregates at orthokinetic coagulation by Stanislav Dukhin; Chao Zhu; Rajesh N. Dave; Qun Yu (pp. 119-131).
Hydrodynamic forces on a doublet of large particles or aggregates during sedimentation cause the non-inertial fragmentation of the doublet, if the doublet Reynolds number and Stokes number are small. In lio-dispersed systems, this non-inertial fragmentation is known to promote the wet classification of large particles whereas the small particles are aggregated (in the secondary minimum) and hence cannot be separated despite the electrostatic repulsion. In aero-dispersed systems with negligible electrostatic repulsion, it is possible to have a narrower separation between two interacting particles so that the attractive surface forces such as van der Waals force could increase in orders of magnitude. As a result, the doublet fragmentation by the aerodynamic detaching force becomes very difficult or even impossible in sedimentation at a small Re. However, this study shows that, when extending above analysis for the interacting fractal aggregates of nanoparticles in a suspension, it is still possible to have aggregate (doublet of two nano-aggregates) fragmentation by the aerodynamic detaching force because the surface forces for nanoparticle contact between two aggregates may be in orders of magnitude smaller than that for micron-sized particles. Even with multiple contacts between two interacting nanoparticle aggregates, this prediction of aggregate fragmentation may still be valid because the contacts may break step by step due to the aggregate rolling along each other caused by a short-range aerodynamic interaction during their differential settling.The aerodynamics of aero-dispersed nanoparticle aggregates is analogous to the hydrodynamics of lio-dispersed solid particles. Therefore the hydrodynamic fragmentation model may be used to partially interpret the stability of nanoparticle fluidization process. Our model indicates that, without continued doublet fragmentation of nano-aggregates, the fluidized nanoparticle suspension would be de-fluidized within minutes via the cascading aggregation. However, the experimental evidence of a sustained operation of fluidization of nanoparticle aggregates over a very long time period without changing the size of nanoparticle aggregates indicates that there is a dynamic balance between the aggregation and fragmentation of nano-aggregates in nanoparticle fluidization. The prediction of critical size of nanoparticle aggregates caused by fragmentation in stabilized fluidization agrees with our in situ measurements.
Keywords: Nanoparticle aggregate; Aggregate sedimentation; Nanoparticle fluidization; Non-inertial aggregate fragmentation; Fractal aggregate
Critical thickness of microscopic thin liquid films by Emil D. Manev; Anh V. Nguyen (pp. 133-146).
This paper outlines the progress achieved during the four decades of research on the spontaneous destruction of the thinning microscopic liquid films through rupture or black spot formation at the so-called critical thickness. Although most of both experimental and theoretical results are primarily related to the foam films that form between gas bubbles, in many respects they can be principally generalized for emulsion films, as well as the wetting films confined between a bubble and a solid surface. The paper focuses on the validation, application and extension of the theory of the phenomenon. The experimental results are analysed with respect to the frequently observed deviations from the widely used model of a planar circular film with tangentially immobile surfaces. The applicability of the new theory of accelerated drainage due to spatial variation in thickness is expressed. The effects of surface tension, surface mobility, variation of the film size, and spatial thickness heterogeneity on the critical thickness are compared.
Keywords: Thin liquid film; Film stability; Critical thickness; Surfactant
Association-induced polymer bridging by poly(ethylene oxide)–cofactor flocculation systems by Theo G.M. van de Ven (pp. 147-157).
Poly(ethylene oxide) (PEO) is used in papermaking, water purification and mineral flotation as a flocculating agent. Despite the fact that PEO does not adsorb on cellulose and bleached lignin, and only poorly on calcium carbonate, it can nevertheless be used to deposit colloidal pigments on pulp fibers. PEO by itself is inefficient, but it can be made to work in combination with another compound, usually referred to as a cofactor or an enhancer. The cofactor associates with PEO and the association complex acts as an efficient bridging agent. There are two classes of cofactor: those who cause PEO to cluster and those that do not. In general PEO association-clusters are more efficient flocculating agents than non-clustered PEO. The PEO dissolution procedure has an important effect on the PEO flocculation efficiency. Before reaching thermodynamic equilibrium, PEO is in an entangled state. Cofactors that cluster PEO are likely to maintain PEO in an entangled state and, like the association-clusters, these entanglements are more efficient flocculation agents than well-dispersed PEO. Salt also affects the PEO–cofactor association. For most cofactors, salt is needed to induce the association. Calcium ions can act as bridging agents between association complexes, especially those containing carboxyl groups, thus promoting the formation of association-clusters.
Resonant diffusion on modulated surfaces by Roumen Tsekov; Elitsa Evstatieva (pp. 159-164).
Some theoretical methods for description of the diffusion on modulated surfaces are reviewed. A general formula for calculation of the diffusion coefficient of a particle moving in the field of a periodic potential is developed, which takes into account both the potential barrier effect and the dependence of the friction coefficient on the potential. The application of the theory to the diffusion of dimers on solid surfaces reveals a non-monotonous dependence of the diffusion coefficient on the ratio between the dimer and solid spatial parameters. This resonant dependence sometimes can be suppressed by the rotations and vibrations on the dimer. In the present theory the latter are described via an adiabatic approach.
Keywords: Brownian motion; Resonant diffusion; Modulated surfaces; Diffusion coefficient; Dimers
Direct measurements of particle–bubble interactions by Graeme Gillies; Michael Kappl; Hans-Jürgen Butt (pp. 165-172).
The essence of mineral flotation is the formation of strong particle–bubble aggregates. Hydrodynamics and DLVO forces hinder the particle–bubble approach, however the presence of a hydrophobic surface results in hydrophobic attraction and strong aggregate formation. In this article we review literature where the colloid probe technique has been used to investigate flotation, primarily particle–bubble interactions in aqueous solution. In some instance particles–bubble interactions have used to quantify the hydrophobic force, whilst hydrophobic forces are crucial for particle–bubble attachment, studies of hydrophobic forces are not the main focus of this article.
Keywords: Interaction force; Colloid probe technique; AFM; Flotation; Surface force
Polyelectrolyte-mediated surface interactions by Per M. Claesson; Evgeni Poptoshev; Eva Blomberg; Andra Dedinaite (pp. 173-187).
The current understanding of interactions between surfaces coated with polyelectrolytes is reviewed. Experimental data obtained with various surface force techniques are reported and compared with theoretical predictions. The majority of the studies concerned with interactions between polyelectrolyte-coated surfaces deal with polyelectrolytes adsorbed to oppositely charged surfaces, and this is also the main focus of this review. However, we also consider polyelectrolytes adsorbed to uncharged surfaces and to similarly charged surfaces, areas where theoretical predictions are available, but relevant experimental data are mostly lacking. We also devote sections to interactions between polyelectrolyte brush-layers and to interactions due to non-adsorbing polyelectrolytes. Here, a sufficient amount of both theoretical and experimental studies are reported to allow us to comment on the agreement between theory and experiments. A topic of particular interest is the presence of trapped non-equilibrium states that often is encountered in experiments, but difficult to treat theoretically.
Keywords: Surface forces; Polyelectrolyte; Steric interaction; Double-layer force; Electrosteric force; Structural force; Depletion attraction; Polyelectrolyte brush; Flocculation; Stabilization; Colloidal stability; Adsorption; Non-equilibrium state; Surface force apparatus; SFA; MASIF; Atomic force microscope; AFM
Adsorption on hydrophobized surfaces: Clusters and self-organization by Gerhard Lagaly; Imre Dékany (pp. 189-204).
The arrangement of liquid molecules on surfaces bristling with alkyl chains is deduced from adsorption studies, X-ray powder diffraction data, and microcalorimetric measurements of swelling-type layered materials, especially clay minerals. Small polar molecules such as water, ethanol, formamide, dimethylsulfoxide, and aromatic compounds are clustered between the alkyl chains pointing away from the surface. The energetic contribution related to the movement of the alkyl chains from direct contact with the surface atoms into upright positions is decisive. The importance of the interactions between the liquid molecules on the structure of the adsorption layer is clearly indicated by the changes of the adsorption layer thickness by salt addition. Thermodynamic data are obtained from surface excess adsorption isotherms from binary liquid mixtures combined with microcalorimetric measurements. Long-chain adsorptives such as long-chain alcohols interact with the surface alkyl chains by forming stable bimolecular films. These films undergo a series of higher-order phase transitions into kink- and gauche-block structures as the consequence of rotational isomerization of the alkyl chains. Such transitions are considered elementary processes in self-assembling films (layer-by-layer deposition, fuzzy films, Langmuir–Blodgett technique), and lipid membranes.
Keywords: Adsorption; Clay minerals; Hydrophobization; Kinks; Layered compounds; Self-assembling
Influence of surface active substances on bubble motion and collision with various interfaces by K. Malysa; M. Krasowska; M. Krzan (pp. 205-225).
Bubble motion as a function of distance from a point of its detachment and phenomena occurring during the bubble approach and collision with liquid/gas and liquid/solid interfaces in pure water and solutions of various surface active substances are described and discussed. It is showed that presence of surface active substance has a profound influence on values of the terminal velocity and profiles of the local velocity. At low solutions concentrations there are three distinct stages in the bubble motion: (i) a rapid acceleration, (ii) a maximum velocity value followed by its monotonic decrease, and (iii) attainment of the terminal velocity, while at high concentrations (and in pure water) there are only stages (i) and (iii). It is showed that the bubble terminal velocity decreases rapidly at low surfactant concentration, but there can be found some characteristic concentrations (adsorption coverage's) above which the velocity almost stopped to decrease. Immobilization of the bubble surface resulting from adsorption of the surface active substances (surface tension gradients inducement) causes over twofold lowering of the bubble velocity. Presence of the maximum on the local velocity profiles is an indication that a stationary non-uniform distribution of adsorption coverage (needed for immobilization the bubble interface) was not established there.When the rising bubble arrives at liquid/gas interface or liquid/solid interface there can be formed either foam or wetting film or three-phase contact (TPC). It is showed that prior to the foam and/or wetting film formation the bubble colliding with the interfaces can bounce backward and simultaneously its shape pulsates rapidly with a frequency over 1000 Hz. It is rather unexpected that even in the case of the free surface the bubble's shape and consequently its surface area can vary so rapidly. It shows straightforward that on such a rapidly distorted interface the adsorption coverage can be very different from that at equilibrium. This fact should be taken into account more appropriately in the discussion of the mechanism of formation and stabilization of various dispersed systems (e.g. foams, emulsions).Bubble collision with solids and formation of the three-phase contact is a necessary condition for flotation separation. It is rather common understanding that immediate attachment should occur in the case of hydrophobic surface, while there should be no attachment in the case of the hydrophilic ones. It is reported that even in the case of such hydrophobic solid surface as Teflon, the bubble attachment did not need to occur at first collision and in distilled water the bubble can bounce a few times without attachment. Presence of frother facilitates the bubble attachment to hydrophobic solid surface. Time scale of the TPC formation is very short, of an order of single ms. It was observed that presence of a micro-bubble at the solid surface facilitated drastically an attachment of the colliding bubble. Roughness of Teflon surface increases probability of the bubble attachment–most probably–as a result of higher probability of micro- and/or nano-bubbles presence at the solid surface.
Heterocoagulation of chalcopyrite and pyrite minerals in flotation separation by Timothy K. Mitchell; Anh V. Nguyen; Geoffrey M. Evans (pp. 227-237).
Heterocoagulation between various fine mineral particles contained within a mineral suspension with different structural and surface chemistry can interfere with the ability of the flotation processes to selectively separate the minerals involved. This paper examines the interactions between chalcopyrite (a copper mineral) and pyrite (an iron mineral often bearing gold) as they approach each other in suspensions with added chemicals, and relates the results to the experimental data for the flotation recovery and selectivity. The heterocoagulation was experimentally studied using the electrophoretic light scattering (ELS) technique and was modelled by incorporating colloidal forces, including the van der Waals, electrostatic double layer and hydrophobic forces. The ELS results indicated that pyrite has a positive zeta potential ( ζ) up to its isoelectric point (IEP) at ∼ pH 2.2, while chalcopyrite has a positive ζ up to its IEP at ∼pH 5.5. This produces heterocoagulation of chalcopyrite with pyrite between pH 2.2 and pH 5.5. The heterocoagulation was confirmed by the ELS spectra measured with a ZetaPlus instrument from Brookhaven and by small-scale flotation experiments.
Keywords: Heterocoagulation; Electrophoretic light scattering; Flotation; Chalcopyrite; Pyrite
Experimental techniques for studying the structure of foams and froths by R.J. Pugh (pp. 239-251).
Several techniques are described in this review to study the structure and the stability of froths and foams. Image analysis proved useful for detecting structure changes in 2-D foams and has enabled the drainage process and the gradients in bubble size distribution to be determined. However, studies on 3-D foams require more complex techniques such as Mutiple-Light Scattering Methods, Microphones and Optical Tomography. Under dynamic foaming conditions, the Foam Scan Column enables the water content of foams to be determined by conductivity analysis. It is clear that the same factors, which play a role in foam stability (film thickness, elasticity, etc.) also have a decisive influence on the stability of isolated froth or foam films. Therefore, the experimental thin film balance (developed by the Bulgarian Researchers) to study thinning of microfilms formed by a concave liquid drop suspended in a short vertical capillary tube has proved useful. Direct measurement of the thickness of the aqueous microfilm is determined by a micro-reflectance method and can give fundamental information on drainage and thin film stability. It is also important to consider the influence of the mineral particles on the stability of the froth and it have been shown that particles of well defined size and hydrophobicity can be introduced into the thin film enabling stabilization/destabilization mechanisms to be proposed. It has also been shown that the dynamic and static stability can be increased by a reduction in particle size and an increase in particle concentration.
Effect of interface modification on forces in foam films and wetting films by Regine v. Klitzing (pp. 253-266).
The paper reviews the effect of the surface composition on forces within aqueous foam and wetting films. In both types of films the charge of the air/water interface is varied by different surfactants. In wetting films the charge and the hydrophobicity of the solid substrate is changed by polymer coatings. The addition of polymers to foam films leads to the formation of surface active polymer/surfactant complexes or to depletion near the interfaces. The dissolution of polyelectrolytes within the film bulk can lead to structural forces. The selection of studies is made with respect to two questions: (1) What is the reason for charges at the air/water interface and (2) what is the mechanism of long-range hydrophobic interaction?
Keywords: Foam film; Wetting film; Disjoining pressure; Hydrophobic forces; DLVO forces; Structural forces; Polyelectrolyte multilayers
Ion redistribution and meniscus stability at Langmuir monolayer deposition by V.I. Kovalchuk; D. Vollhardt (pp. 267-279).
Considering the deposition of charged Langmuir monolayers, it is necessary to take into account electrostatic interactions between the monolayer and the substrate surface. These interactions depend on the nature of ionizable surface groups and on the ionic composition of the subphase (e.g. pH, multivalent counterions, indifferent electrolytes). The non-uniformity of electric, concentration and hydrodynamic velocity fields leads to the formation of ion concentration profiles near the three-phase contact line during the deposition. This effect is similar to the effect of concentration polarization in membranes and electrode systems. The ions redistribution results in change of the monolayer ionization, adhesion work, dynamic contact angle, and, therefore, in the change of morphology, composition and structure of the deposited monolayer. With increasing withdrawal velocity the meniscus can become unstable because of concentration polarization in the solution.
Selectivity and dynamic behavior of glass electrodes by H.-H. Kohler; C. Haider; S. Woelki (pp. 281-290).
Two types of models are presently used to describe the potential of a glass electrode: the ion exchange type, which is based on the equilibrium binding ratio of the glass surface, and the permeability type, based on the permeability ratio of the ions within the glass matrix. It is shown that the permeability type models can be derived from first physical principles while the exchange type models are questionable from the physical point of view. The derivation of the potential–concentration behavior shows that the steady-state characteristic of the electrode potential is determined by the selectivity properties of the dry glass layer. The rapidity of the potential response of the glass electrode can be traced back to the fact that, under certain constraints regarding the diffusion coefficients, the potential difference across this layer only depends on the boundary concentrations. The potential drift, in contrast, is determined by ion transport in the hydrated surface layers of the electrode. The theoretical predictions of single- and multi-layer permeability models are compared with experimental data for a sodium selective electrode.
Novel amphiphiles with preorganized functionalities—formation of Langmuir-films and efficiency in mineral flotation by P.U. Müller; C.C. Akpo; K.W. Stöckelhuber; E. Weber (pp. 291-302).
Based on the principle of supramolecular preorganization, three different types of new oligofunctional surfactants have been designed and prepared differing in both the degree of conformational flexibility and the hydrophilic–lipophilic balance of their structures due to the chosen building blocks including rather rigid and shape-persistent backbones or a dendritic subunit.Surface-active properties of the oligofunctional surfactants involving critical micellization concentration, surface tension at cmc, effectiveness of surface tension reduction, and the efficiency of adsorption were determined by use of the surface tension isotherms, respectively. In particular the amino-acid-based amphiphiles show remarkable surface-active properties with the adsorption at the air/water interface and also the aggregation to micelles starting at very low concentration. By analysis of the surface pressure–surface area isotherms interesting information on the packing behavior and orientation of the amphiphilic molecules in relation to the molecular structure could be obtained. Moreover, limiting area and collapse pressure of the Langmuir-films were determined. Morphological observation of the dynamic process of monolayer compression at the air/water interface was carried out by Brewster angle microscopy illustrating several phase states visualized as snap shots.Furthermore, the effectiveness of the new surfactants acting as collecting agents in the process of flotation using the model mineral fluorite was studied. A surprisingly high recovery of the mineral was obtained for the surfactants with constricted flexibility. The flotation plots clearly manifest that bundling of surfactant subunits by tethering to an aromatic core or by dendritic branching is profitable, indicating that effects of preorganization of the oligofunctional surfactant molecules are important.
Keywords: Preorganization; Oligofunctional surfactants; Mineral flotation; Langmuir-films; Brewster angle microscopy
Dilational rheology of adsorbed surfactant layers—role of the intrinsic two-dimensional compressibility by V.I. Kovalchuk; R. Miller; V.B. Fainerman; G. Loglio (pp. 303-312).
The frequency and concentration dependencies of the dilational elasticity and viscosity of adsorbed surfactant layers is yet not quantitatively understood. Especially at a surface coverage above 50% the measured high frequency limits of the elasticity pass typically a maximum or reach a plateau while classical models expect an exponential increase to very high values. The consideration of an intrinsic two-dimensional compressibility allows a much better description of experimental data. There are various surfactant systems discussed in literature following this new model. Although the impact of these interfacial characteristics on the dilational rheology is significant, the adsorption isotherm and equation of state change only slightly.
Keywords: Dilational rheology; Oscillating bubble method; Surfactant adsorption layers; Thermodynamic model; Diffusional exchange of matter; Surface layer compressibility