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Werner, C. ; Lyklema, J.
Editorial overview

The present issue of “Current Opinion in Colloid and Interface Science” is the first one dealing with the theme “Electrokinetic phenomena”. This late appearance is in contrast with the fact that electrokinetics belong to the oldest and most basic aspects of colloid science, dating back to the first part of the 19th century. Apparently, the current interest in complex systems has pushed the attention for basic interfacial electrostatics to the background. However, there are several reasons to prevent electrokinetics from falling into oblivion. Some of these are:

• The field underwent several useful theoretical extensions, and interpretations can now be made with higher sophistication.

• More advanced experimental techniques allow obtaining data under more complex situations, for instance at much higher concentrations, than was formerly possible.

• For those complex systems where the origin of charges is difficult to establish, electrokinetics remains the sole option for obtaining at least part of the information.

• The fact remains that comparing electrokinetic data under different conditions is generally helpful in discriminating between electrostatic and non-electrostatic, or “chemical” interactions.

These, and several other considerations have led to the creation of the present issue of COCIS. It already contains a variety of themes so that it shows how rich the field is. Obviously, within the limit of nine review articles it cannot cover the entire field comprehensively. In fact, re-reading the present collection already suggests a number of extensions that could be considered for a future edition. It is planned that the appearance of thematic issues on electrokinetics will acquire routine character, reflecting the developments in the field.

Given the initiating nature of the present issue, it was decided to include papers of general interest, that would not routinely appear in the upcoming topical issues of COCIS. The first of these is the historical introduction by Wall [1]. In that review one can read how several notions, that are now generally accepted have matured over the years. Electrokinetics have substantially contributed to the genesis of the electric double layer concept. The basic question was, and continues to remain, how it is possible that an electroneutral entity like a charged particle with its compensating counterlayer can move in an applied electric field. It is also shown that Helmholtz and Smoluchowski already derived equations for the electrophoretic mobility long before the inception of the notion of double layers. Another paper that goes to the roots is that by Lyklema on the interpretation of zeta potentials [2]. For solid surfaces, excluding situations of overcharging, it is almost always observed that the electrokinetically active charge is lower than the surface charge. The traditional interpretation is that there is a stagnant (electrokinetically inactive) layer on the solid surface, separated from the movable part of the water at a so-called slip plane at some distance from the surface. Lyklema claims that stagnancy finds its origin in the stacking of water molecules adjacent to hard surfaces.

A second group of papers can be categorized as critically reviewing the applicability of certain techniques or system treatments. Bellini and Jiménez [3] discuss the problems occurring when non-spherical particles are studied by electrophoresis, dielectric relaxation or electro-optically. A number of new features emerge, such as torque-inducing hydrodynamic flows, with ensuing anomalous perpendicular particle alignment, and doubling of the dielectric relaxations. For more complicated particle shapes it is almost impossible to develop general theories. Delgado and Groose [4] review the field of dielectric dispersion in aqueous media. After a historical review the three types of relaxation (a, ß and d) are distinguished and mathematical elaborations are given where possible. In some cases only numerical solutions are possible. The experimental problem of electrode polarization, particularly complicating low-frequency measurements, is also addressed. The paper finishes with a preview of particles with soft surfaces, a theme recurring in the chapter by Duval and Gaboriaud [5].

A third group of contributions deals with the electrokinetics of charged particles, approaching surfaces, collectively known as electrodeposition. Prieve, Sides and Wirth [6] discuss the two-dimensional structures that can develop upon electrodeposition. One of the possibilities is the creation of hexagonal two-dimensional arrays of homodisperse particles. This finding is not only theoretically interesting (how come that normally arriving particles move tangentially upon approaching the surface?) but it also has eminent practical interest, for instance in the preparation of photonic materials. Adamczyk et al. [7] describe how electrokinetics can be invoked in studying electro-deposited layers of particles. They put emphasis on adsorbed globular proteins, treated here as a particularly interesting type of charged nanoparticles.

Likewise, interfaces formed out of biomolecules are the focus of the review by Duval and Gaboriaud [5]. These authors combine electrokinetics with additional physicochemical experiments to study the electrosurface properties of the, even more complex, soft and permeable interfaces of bacterial cell walls, viruses and yeast cells. This creates a link to the review by Zimmermann et al. [8], which comprehensively deals with electrokinetic studies of the, ubiquitous but often ignored, phenomenon of unsymmetrical electrolyte ion adsorption at the surfaces of various materials, including reconstituted lipid bilayer membranes. Finally, the review by Bazant and Squires [9] covers another recently explored peculiarity in the formation of interfacial charge: non-linear, induced-charge electrokinetic phenomena. These authors report on theoretical, computational and experimental elaborations, which are highly important for applications of electrokinetics in microfluidic technologies.

Together, this collection of reviews very well reflects the wide significance of electrokinetic phenomena. At the same time, one can think of an equally wide group of other electrokinetic themes, that in the present volume have not, or only marginally been addressed, including electrokinetics in apolar media, non-linear electrokinetics (at very high fields), dynamic measuring techniques, and a variety of applications, for instance in soils. We are confident future editions will cover this group of themes.

Although the contributions to this first topical issue of COCIS on electrokinetics cover a wide spectrum of sub-themes, all of them adhere to the recently published set of recommendations for the use of terms and symbols in the measurement and interpretation of electrokinetic phenomena [10]. We endorse this important effort to facilitate the communication in the field and expect others to follow these recommendations when working on electrokinetics in the future.

The editors appreciate all authors for their endeavor to obtain a valuable new type of volume of COCIS and very much hope the new topical issue will be well received and considered useful by the readership.

Quelle
Current Opinion in Colloid and Interface Science 15, 117-118

DOI
http://dx.doi.org/10.1016/j.cocis.2010.01.004

Erschienen am
April 2010
 
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