Non-equilibrium electric surface phenomena

Non-equilibrium aspects of traditional electrokinetic phenomena (electrophoresis, electroosmosis, streaming potential, sedimentation potential), electrostatic interaction of particles and new electrokinetic phenomena are considered. The significance of non-equilibrium electric surface phenomena for many major areas of modern colloid science (characterization of colloids, membrane science, transport phenomena and separation, particle interaction and coagulation) is established.The study of non-equilibrium electric surface phenomena is connected with the validation of the standard electrokinetic model (SEM), the development of a non-standard model and the development of an extensive programme of disperse system characterization based on integrated electrokinetic investigations. Experimental and theoretical studies of systems with a smooth, non-porous impermeable surface (mica in Anderson's experiments, and quartz microcapillaries with a molecule-smooth surface in Churaev's experiments) have shown that usually there are no significant difficulties in interpreting electrokinetic investigations despite the possible anomaly in the water structure near the surface and the possibility of maximum shear stress (yield stress), i.e. the anomalous viscosity and decreased dissolving power with respect to ions. However, systems which do not satisfy the conditions of the SEM are widely distributed, owing to the porosity, roughness or permeability of the boundary layer of the surface of the solid body which simultaneously belongs to the solid and liquid phases. In this layer, enclosed between the outer Helmholtz plane and the slipping plane, the motion of the liquid strongly slows down and the tangential flow of ions is characterized purely by the mobility which is close to the normal. Thus, a general property of a non-standard electrokinetic model is the presence of an anomalous (additional) surface conductivity in excess of the surface conductivity determined according to Bikerman's equation based on the ζ -potential alone.Confidence in modelling the electrokinetic phenomena has grown with the development of methods for modifying the surface such that its properties approach those of the SEM (Bijsterbosch and co-workers; Saville and co-workers).Extension of the particle characterization concept requires the measurement of both the mobile charge and the electrokinetic charge and from this an estimate of the thickness of the additional conductivity zone can be made. With the additional measurement of a titratable charge, it is possible to estimate the ion distribution between the dense and diffuse parts of the double layer (DL) and to estimate the decreased mobility of ions in the Stern layer or in the immobilized part of the DL.Quantitative laws governing the interaction of particles and corresponding to the non-standard model substantially differ from the traditional laws described by the DVLO theory as applied to the SEM. This is also true for adsorption properties which are characterized without sufficient reason by means of the ζ-potential. Therefore both the development of models of interaction and adsorption of ions, allowing for the non-standard electrokinetic model, and the extension of the particle characterization programme to integrated investigations of electric surface phenomena are required.Further generalization of the theory of electrokinetic phenomena is achieved. In addition to the surface charge another variety of surface force can be the origin of the electrokinetic phenomena.     

The history of electrokinetic phenomena

Electrokinetic phenomena comprise the phenomena where a liquid moves tangentially to a charged surface. Well-known phenomena of this kind are electrophoresis, electro-osmosis, streaming potential and sedimentation potential. A historical review is given here, starting with their discovery by F.F. Reuss in 1808 and continuing with the early investigators including G. Wiedemann, G.Quincke, E. Dorn and U. Saxén. It is also discussed how electrokinetic phenomena gave rise to the concept electrical double layer in colloid science. The development of the theory starting with H. Helmholtz, continuing with M. Smoluchowski is described. Extension of the theory including relaxation and surface conduction is included. Finally the history of other kinds of electrokinetic phenomena such as electroacoustics and diffusiophoresis is treated.     

Dynamic Light Scattering Based Microelectrophoresis: Main Prospects and Limitations

Microelectrophoresis based on the dynamic light scattering (DLS) effect has been a major tool for assessing and controlling the conditions for stability of colloidal systems. However, both the DLS methods for characterization of the hydrodynamic size of dispersed submicron particles and the theory behind the electrokinetic phenomena are associated with fundamental and practical approximations that limit their sensitivity and information output. Some of these fundamental limitations, including the spherical approximation of DLS measurements and an inability of microelectrophoretic analyses of colloidal systems to detect discrete charges and differ between differently charged particle surfaces due to rotational diffusion and particle orientation averaging, are revisited in this work. Along with that, the main prospects of these two analytical methods are mentioned. A detailed review of the role of zeta potential in processes of biochemical nature is given too. It is argued that although zeta potential has been used as one of the main parameters in controlling the stability of colloidal dispersions, its application potentials are much broader. Manipulating surface charges of interacting species in designing complex soft matter morphologies using the concept of zeta potential, intensively investigated recently, is given as one of the examples. Branching out from the field of colloid chemistry, DLS and zeta potential analyses are now increasingly finding application in drug delivery, biotechnologies, physical chemistry of nanoscale phenomena and other research fields that stand on the frontier of the contemporary science. Coupling the DLS-based microelectrophoretic systems with complementary characterization methods is mentioned as one of the prosperous paths for increasing the information output of these two analytical techniques.     

Synthetic electrically driven colloids: A platform for understanding collective behavior in soft matter

The collective motion of synthetic active colloids is an emerging area of research in soft matter physics and is important both as a platform for fundamental studies ranging from non-equilibrium statistical mechanics to the basic principles of self-organization, emergent phenomena, and assembly underlying life, as well as applications in biomedicine and metamaterials. The potentially transformative nature of the field over the next decade and beyond is a topic of critical research importance. Electrokinetic active colloids represent an extremely flexible platform for the investigation and modulation of collective behavior in active matter. Here, we review progress in the past five years in electrokinetic active systems and related topics in active matter with important fundamental research and applicative potential to be investigated using electrokinetic systems.     

Effect of hydration of polyamide membranes on the surface electrokinetic parameters: surface characterization by x-ray photoelectronic spectroscopy and atomic force microscopy

The surface and the solid/liquid interface of two polyamide membranes, one experimental (B0) and one commercial (NF45), have been characterized by X-ray photoelectronic spectroscopy (XPS), atomic force microscopy (AFM), and zeta potential, respectively. The surface roughness, determined by AFM data analysis, is different for the two membranes, and results show that the commercial NF45 membrane presents a much lower roughness than the experimental B0 membrane. XPS data indicate that the surface of membrane NF45 is similar to that of pure polyamide, while membrane B0 contains a considerable amount of impurities. The homogeneity in depth of both membranes was also studied by determining the composition profile at different analysis angles. Streaming potential along the membrane surface or tangential streaming potential (TSP) measurements with NaCl solutions at different concentrations were carried out with both membranes to determine the zeta potential and the electrokinetic surface charge density, and a correlation between membrane surface and interface parameters is made. Some differences in atomic concentrations of membrane surface elements and X-ray photoelectronic spectra of the samples used in TSP measurements and after a drying process at 90 degrees C for 24 h can be observed when they are compared with those for fresh membranes. Electrokinetic parameters for membrane NF45 (TSP, zeta potential, and surface electrokinetic charge density) obtained from three different series of measurements strongly decrease as a result of membrane use, but for membrane B0 they are practically independent of the number of measurements. This difference in the electrokinetic behavior of the two membranes has been related to the hydration process of the surface for each sample studied by XPS and AFM.     

Combination of electrokinetic remediation with permeable reactive barriers to remove organic compounds from soils

In recent years, technologies suitable for the remediation of environmental contaminants from soil have received considerable attention. Among them, electrokinetic (EK) remediation and permeable reactive barriers (PRBs) must be highlighted because of their environmental compatibility, versatility, amenability, scale-up practice, and cost-effectiveness. For this reason, the first section is focused on the fundamentals of EK remediation and PRB for environmental remediation, together with the phenomena that occur in the soil and that permit its effectiveness. The second section discusses most important materials used as barriers and describes the application, as well as the recent progress. The outlooks of PRB–EK technologies for the upcoming future are finally concluded in this minireview.     

L-tryptophan transport through a hydrophobic liquid membrane using AOT micelles: dynamics of the process as revealed by small angle X-ray scattering

Hydrophobic liquid membranes have a high technological potential in many fields of separation science. The dynamics of these systems is very complex and still not fully understood. In this work we studied the effect of the incorporation of cationic and anionic L-tryptophan at pH 1.8 and 10.0, respectively, in Aerosol-OT reverse micelles performing small angle X-ray scattering experiments. The use of a synchrotron radiation source allowed efficient in situ data acquisition. Several insights on L-tryptophan transport dynamics through hydrophobic membranes containing AOT could be obtained from these SAXS experiments, such as amino acid site localization and changes in the reverse micelle sizes.     

Electrokinetic characteristics of initial porous glasses and those modified with titanium- and aluminum-oxide particles

The structural (volume porosity, structural resistance coefficient, and average pore radius) and electrokinetic (specific electrical conductivity, ion-transport numbers, and electrokinetic potential) characteristics of macroporous glass membranes obtained from two-phase sodium-borosilicate glasses with different times of thermal treatment have been studied in solutions of hydrochloric acid and potassium chloride. The properties of the initial membranes have been compared with the characteristics of the same membranes modified by filtering through them suspensions of aluminum- and titanium-oxide nanoparticles with different weight concentrations. It has been shown that, at low degrees of pore channel surface coverage with nanoparticles (<0.1), the structural parameters of the membranes remain almost unchanged. In addition, it has been found that the presence of positively charged nanoparticles on the negatively charged surface increases the surface conductivity and the absolute value of the electrokinetic potential.     

Asymmetrical nanopores in track membranes: Fabrication,the effect of nanopore shape and electric charge of pore walls,promising applications

The properties of asymmetrical nanopores prepared by chemical etching of tracks of accelerated heavy ions are studied. Procedures are developed for controlling the size and shape of pores within wide limits. The presence of charged functional groups on pore walls is an intrinsic property of track membranes, which makes them a convenient object for studying electrokinetic phenomena in nanocapillaries. In electrolyte solutions, the asymmetrical “track” membranes demonstrate the diode effect. Two methods for fabricating asymmetrical nanopores in polyethylene terephthalate films are proposed and introduced into practice. Specific features of both methods, their advantages and drawbacks are considered. In addition to the brief survey of available information on diode-like track membranes, the new results on the mechanism of pore formation and the peculiarities of their geometry and electrokinetic properties are discussed. The emerging and potential applications of track membranes with asymmetrical pores are discussed briefly.     

Colloido-chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 2. Equilibrium electrochemical characteristics of membranes

The regularities of variations in the electrokinetic potential and surface charge of nanoporous glass membranes with different compositions have been studied as depending on the type of an electrolyte (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium chlorides) and the structure of pore space. It has been shown that, in solutions containing specifically sorbed organic counterions, the range of positive values of electrokinetic potential arises due to the superequivalent absorption of counterions in the Stern layer. It has been found that the influence of the specific adsorption of counterions on the electrokinetic potential of porous glasses increases with the amount of secondary silica in the pore space. The effects of the counterion specificity, pore channel sizes, and composition of a porous glass on the value of the surface charge have been analyzed. The absolute value of the surface charge has been shown to significantly increase in the presence of organic counterions in comparison with inorganic ions throughout the examined range of background electrolyte concentrations.     

Liquid interfacial nanoarchitectonics: Molecular machines,organic semiconductors,nanocarbons, stem cells,and others

The concept of nanoarchitectonics has been proposed as an extensional development of nanotechnology through fusions with material science and the other fields. In nanoarchitectonics, nano-units of atoms, molecules, and nanomaterials are architected into construction of functional material systems. In order to assemble intended structures or hierarchical structures from nano-units, it is more useful to confine nano-units at the interface. In addition, nanoarchitectonics is expected to output functions by harmonizing many units in dynamic environments. However, the liquid interfaces still have lots of unexplored matters in nanoscale because supports by advanced apparatus and techniques in nanotechnology are not always available. Specifically, this review paper summarizes examples of research on molecular manipulation, molecular arrangement and assembly, materials synthesis, and life manipulation at the liquid interface. These examples demonstrate that the liquid interface enables the control of dynamic functions of various size regions, from molecular-level phenomena such as the control of molecular machines to techniques of living creature size such as the control of stem cell differentiation. Liquid interfaces are very useful environments for controlling dynamic functions for a wide range of targets and would have tremendous potential in terms of functional exploration. The great potential of nanoarchitectonics at the liquid interface and the challenges to be solved in the future are also discussed.     

The electrokinetic behavior of charged non-spherical colloids

The response of charged colloids to electric fields is determined by combined phenomena occurring first in the electric double layer to then develop into long-range perturbations of ion concentration, local fields, and solvent flows. When particles are non-spherical, the loss of symmetry affects the short- and long-ranged processes modifying their behavior as observed through their electrophoretic mobility, dielectric permittivity, and electro-optical response. Recent measurements and theoretical developments have revealed phenomena characteristic for non-spherical particles, such as the doubling of the relaxations in the dielectric spectra, the appearance of torque-inducing hydrodynamic flows, and the anomalous perpendicular alignment. In this article we discuss in a unifying frame the recent experimental and theoretical progresses about the electrokinetic behavior of charged non-spherical colloids.     

Lattice Boltzmann model of microfluidics in the presence of external forces

We present here a lattice Boltzmann model in the presence of external force fields to describe electrokinetic microfluidic phenomena and consider pressure as the only external force to drive liquid flow. Our results from a 9-bit square lattice Boltzmann model are in good agreement with recent experimental data in a pressure-driven microchannel flow that could not be fully described by electrokinetic theory. The differences between the predicted and the experimental Reynolds numbers from pressure gradients are well within 5%. Results suggest that the lattice Boltzmann model described here is an effective computational tool for predicting the more complex microfluidic systems that might be problematic using conventional methods.     

Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers: 1. The origins of charge

This paper combines new experimental data for electrokinetic characterization of hydrophobic polymers with a detailed discussion of the putative origins of charge at water-hydrophobe interfaces. Complexities in determining the origin of charge are discussed in the context of design and modeling challenges for electrokinetic actuation in hydrophobic microfluidic devices with aqueous working fluids. Measurements of interfacial charge are complicated by slip and interfacial water structuring phenomena (see Part 2, this issue). Despite these complexities, it is shown that (i) several hydrophobic materials, such as Teflon and Zeonor, have predictable electrokinetic properties and (ii) electrokinetic data for hydrophobic microfluidic systems is most consistent with the postulate that hydroxyl ion adsorption is the origin of charge.     

Interface-mediated oscillatory phenomena

Oscillatory transport processes which occur in the far from equilibrium region have assumed great significance from the viewpoint of science of complexity. Oscillatory phenomena in the chemical reaction systems have been subjected to intense investigations both from theoretical and experimental angles. In the present review an effort has been made to bring transport processes other than conventional chemical reactions into focus: transport processes mediated by solid-liquid and liquid-liquid interfaces have been discussed. Transport through membranes including liquid membranes, liquid-liquid interfaces and the recently reported hydrodynamic oscillator have been covered. Applications of these systems in areas such as fabrication of sensors, phase transfer catalysis and, of course, the obvious biological action, e.g. excitation of biomembranes and tissues, have been reviewed. Theoretical frameworks proposed to rationalize the phenomena have also been critically reviewed.     

Structural forces reflecting polyelectrolyte organization from bulk solutions and within surface complexes

The interactions between two macroscopic surfaces approaching one another underlies many of the phenomena observed in Colloid and Interface science. In Russia this gave rise to the branch of colloid science now referred to as Surface Forces. Important discoveries, such as the molecular organization of solvent molecules at an interface, have been unveiled by surface force measurements. More recently, forces and structures at macromolecular length scales have been uncovered. In particular, oscillatory force profiles have been detected from aqueous solutions containing polyelectrolytes. The force-structure relationship can reflect organization in the bulk solution or the internal structure of the adsorbed layer. Using a range of surface force techniques, combined with X-ray and neutron scattering results, we review the main features of these fascinating systems and provide an overview of how they relate to other systems such as micellar solutions, polymer-surfactant complexes and simple solvents.     

PHOTOSENSITIVE BILAYER MEMBRANES AS MODEL SYSTEMS FOR PHOTOBIOLOGICAL PROCESSES

Abstract— Photobiological processes such as photosynthesis, photomorphogenesis, photomovement, and photoreception are all associated with the membranous portions of cells. The unique properties of membrane surfaces are apparently required to achieve biologically relevant energy transduction and photocontrol phenomena and consequently the use of model membrane systems is suggested as an advantageous approach to elucidation of the important physical and chemical processes involved. Black lipid membrane (BLM) and liposome techniques are critically reviewed as preferred techniques for constructing and manipulating lipid bilayers. The lipid bilayer is considered to be the basic foundation for biological membrane models, and specific physical phenomena observed with the bilayers and their biological ramifications are analyzed. Light-stimulated polarization of the membrane and electron transfer across the bilayer are viewed as appropriate analogs of vision and photosynthesis, respectively. Bilayer-adsorbed dye experiments are the simplest systems explored that exhibit polarization and charge transfer across the membrane. Chloroplast extract BLM experiments are cited as an example of the light-stimulated transfer of electrons across the membrane under the influence of a preexisting redox gradient. Biliprotein (phycocyanin or phycoerythrin) on one side of the chloroplast extract membrane permits the direction of electron flow across the membrane so that a redox gradient is created in a manner truly analogous to photosynthesis. The potential for solar energy conversion from such membranes is explicitly considered utilizing a schematic photoelectrochemical cell. Model membranes containing bacterial rhodopsin and phytochrome represent examples of ionic gradients that result in biological energy transduction. Studies of membranes that exhibit transient photoeffects are considered potentially relevant for the elucidation of phototaxis. The analysis of many properties of photosensitive membranes is greatly aided by the use of appropriate theoretical models. It is apparent that there is a great potential for the application of photosensitive model membranes in many research areas involving complex photobiological phenomena and novel methods for solar energy conversion.