- Email: [email protected]
Electrokinetics
Current Opinion in Colloid & Interface Science 24 (2016) A1–A2
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial overview
Electrokinetics
We are happy to present herewith the third topical issue of Current Opinion in Colloid and Interface Science on electrokinetics. The preceding topical issues appeared in 2010 (Curr. Opin. Colloid Interface Sci. 15 (2010) 117–213) and in 2013 (Curr. Opin. Colloid Interface Sci. 18 (2013) 71–163), respectively. Re-reading these first two issues it becomes clear how wide the subject matter is and how diverse the sub-themes are, varying between biological and advanced technological developments, involving both theoretical and experimental advances and offering a broad range of applications. The present issue is no exception to this trend, and further extensions can be anticipated for future issues. Some of the included review articles may be useful for beginners in the area, whereas others are more advanced, requiring a basic understanding of the concepts of the field. The present issue contains nine contributions. Some are theoretical, others experimental, but most of them at least consider both aspects. Perhaps this is germane to electrokinetics, where even the definition of measured quantities requires some theoretical consideration. Even in the simplest case of electrophoresis of a perfectly smooth particle one has to ask what a ζ-potential really is. In many of the present papers such issues recur at a more advanced level. We start with the paper by Delgado, Carrique and Ruiz-Reina, dealing with the extension of both DC and AC electrokinetics of suspensions to solutions of very low salt concentration. The emphasis is on aqueous solutions, non-aqueous solutions having been addressed in the previous COCIS-ELKIN issue. By their very nature, the considered solutions always contain some charged species, like dissociation products of the water itself and products of CO2 dissolution within it. Theory is elaborated for surface conductivity and dielectric spectroscopy, in which also ion size effects are considered, so that a rather complete theoretical and experimental picture emerges. Applicability ranges are given for the various approximations. The paper by Duval, Werner and Zimmermann is a logical step of the recent development of electrokinetics of soft surfaces. They develop theory and contribute mobility and streaming current experiments of flat surfaces covered by charge-stratified polyelectrolyte layers, similar to those made by the now familiar layer-by-layer deposition method. The electrolyte concentration plays a dominant role and an interesting new feature emerged: for certain systems common intersection points were observed, i.e. points where electrokinetic characteristics as functions of the electrolyte concentration pass through the same point. Such points do not coincide with the isoelectric points. Phenomenologically they are identical to the common intersection points observed for the surface charge on oxidic materials as functions of the indifferent electrolyte concentration, although the systems and measured parameters are different.
http://dx.doi.org/10.1016/j.cocis.2016.08.002 1359-0294/© 2016 Published by Elsevier Ltd.
The, rather theoretical, contribution by Lopez-Garcia, Horno and Grosse also deals with dielectric and other electrokinetic properties of aqueous colloidal suspensions where, as in the paper by Delgado et al., attention is paid to ion size effects. In this case these have been incorporated through the Carnahan–Starling closure, which is an improvement as compared with the Bikerman model. The predictions may be useful when data at relatively high surface charges and surface potentials are considered. Keh reviews the present state of diffusiophoresis (motion caused by an external concentration gradient) adding diffusio-osmosis, also known as capillary osmosis (drag of the solute by an applied tangential concentration gradient). The treatment is essentially theoretical but applications can be found in transport and diffusion from and to interfaces, transport through channels and through pores and in the motion of two particles near a charged surface. In his contribution, Adachi reviews the sedimentation and electrophoresis of porous flocs and particles covered by a polyelectrolyte, obviously important practical issues. Essentially he translates the parameters in the Darcy equation in terms of processes occurring at different scales. This study has theoretical and experimental parts. In sedimentation the flock size determines the Stokes drag, whereas the permeability is dominated by the largest channels; these are also at the kernel of flock break-up. Ramos, Garcia-Sanchez and Morgan reviewed the electrokinetics of conducting microparticles, thereby adding new experimental data. In the paper by Sides and Prieve measurements of ζ-potentials from flows with axial symmetry are reviewed. Three of these methods can be distinguished: the spincoat, the rotating disk and the impinging jet method. All three originated to a large part in the group of the authors. The aim is eperimental, viz. measuring electrokinetic potentials, but theory is needed to describe the coupling of electric and hydrodynamic transport. The main purpose of the paper by Persat and Santiago is extending existing electrokinetics to the domain of microfluids, where diffusional transport becomes relatively more relevant in the domain of electrohydrodynamics. The work is mainly theoretical but various applications are reviewed, covering a discussion about the size scale where certain features become prominent. The last two contributions, i.e. those by Alkafeef and Al-Mam and by Lapcik are applications and indicate the wide spread of the field of electrokinetics. The former stems from the oil recovery industry and deals with the asphaltene nuisance. Asphaltenes are rather polar components occurring in crude oils, which tend to adsorb at the surfaces of consolidated sandstones and hence to block the oil flow. The adsorbates carry a bit of charge, but these charges can have a long range of
A2
Editorial overview
action because of the low dielectric permittivity of the medium. The paper describes how streaming current measurements can help to characterize the composition of the adsorbates. Lapčik shows how inverse gas chromatography can help to characterize rough surfaces before they are subjected to electrokinetic studies. He derives his experimental material mostly from pharmaceutical products.
Johannes Lyklema⁎ Carsten Werner ⁎ Corresponding author. E-mail address: [email protected].
Contents lists available at ScienceDirect
Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis
Editorial overview
Electrokinetics
We are happy to present herewith the third topical issue of Current Opinion in Colloid and Interface Science on electrokinetics. The preceding topical issues appeared in 2010 (Curr. Opin. Colloid Interface Sci. 15 (2010) 117–213) and in 2013 (Curr. Opin. Colloid Interface Sci. 18 (2013) 71–163), respectively. Re-reading these first two issues it becomes clear how wide the subject matter is and how diverse the sub-themes are, varying between biological and advanced technological developments, involving both theoretical and experimental advances and offering a broad range of applications. The present issue is no exception to this trend, and further extensions can be anticipated for future issues. Some of the included review articles may be useful for beginners in the area, whereas others are more advanced, requiring a basic understanding of the concepts of the field. The present issue contains nine contributions. Some are theoretical, others experimental, but most of them at least consider both aspects. Perhaps this is germane to electrokinetics, where even the definition of measured quantities requires some theoretical consideration. Even in the simplest case of electrophoresis of a perfectly smooth particle one has to ask what a ζ-potential really is. In many of the present papers such issues recur at a more advanced level. We start with the paper by Delgado, Carrique and Ruiz-Reina, dealing with the extension of both DC and AC electrokinetics of suspensions to solutions of very low salt concentration. The emphasis is on aqueous solutions, non-aqueous solutions having been addressed in the previous COCIS-ELKIN issue. By their very nature, the considered solutions always contain some charged species, like dissociation products of the water itself and products of CO2 dissolution within it. Theory is elaborated for surface conductivity and dielectric spectroscopy, in which also ion size effects are considered, so that a rather complete theoretical and experimental picture emerges. Applicability ranges are given for the various approximations. The paper by Duval, Werner and Zimmermann is a logical step of the recent development of electrokinetics of soft surfaces. They develop theory and contribute mobility and streaming current experiments of flat surfaces covered by charge-stratified polyelectrolyte layers, similar to those made by the now familiar layer-by-layer deposition method. The electrolyte concentration plays a dominant role and an interesting new feature emerged: for certain systems common intersection points were observed, i.e. points where electrokinetic characteristics as functions of the electrolyte concentration pass through the same point. Such points do not coincide with the isoelectric points. Phenomenologically they are identical to the common intersection points observed for the surface charge on oxidic materials as functions of the indifferent electrolyte concentration, although the systems and measured parameters are different.
http://dx.doi.org/10.1016/j.cocis.2016.08.002 1359-0294/© 2016 Published by Elsevier Ltd.
The, rather theoretical, contribution by Lopez-Garcia, Horno and Grosse also deals with dielectric and other electrokinetic properties of aqueous colloidal suspensions where, as in the paper by Delgado et al., attention is paid to ion size effects. In this case these have been incorporated through the Carnahan–Starling closure, which is an improvement as compared with the Bikerman model. The predictions may be useful when data at relatively high surface charges and surface potentials are considered. Keh reviews the present state of diffusiophoresis (motion caused by an external concentration gradient) adding diffusio-osmosis, also known as capillary osmosis (drag of the solute by an applied tangential concentration gradient). The treatment is essentially theoretical but applications can be found in transport and diffusion from and to interfaces, transport through channels and through pores and in the motion of two particles near a charged surface. In his contribution, Adachi reviews the sedimentation and electrophoresis of porous flocs and particles covered by a polyelectrolyte, obviously important practical issues. Essentially he translates the parameters in the Darcy equation in terms of processes occurring at different scales. This study has theoretical and experimental parts. In sedimentation the flock size determines the Stokes drag, whereas the permeability is dominated by the largest channels; these are also at the kernel of flock break-up. Ramos, Garcia-Sanchez and Morgan reviewed the electrokinetics of conducting microparticles, thereby adding new experimental data. In the paper by Sides and Prieve measurements of ζ-potentials from flows with axial symmetry are reviewed. Three of these methods can be distinguished: the spincoat, the rotating disk and the impinging jet method. All three originated to a large part in the group of the authors. The aim is eperimental, viz. measuring electrokinetic potentials, but theory is needed to describe the coupling of electric and hydrodynamic transport. The main purpose of the paper by Persat and Santiago is extending existing electrokinetics to the domain of microfluids, where diffusional transport becomes relatively more relevant in the domain of electrohydrodynamics. The work is mainly theoretical but various applications are reviewed, covering a discussion about the size scale where certain features become prominent. The last two contributions, i.e. those by Alkafeef and Al-Mam and by Lapcik are applications and indicate the wide spread of the field of electrokinetics. The former stems from the oil recovery industry and deals with the asphaltene nuisance. Asphaltenes are rather polar components occurring in crude oils, which tend to adsorb at the surfaces of consolidated sandstones and hence to block the oil flow. The adsorbates carry a bit of charge, but these charges can have a long range of
A2
Editorial overview
action because of the low dielectric permittivity of the medium. The paper describes how streaming current measurements can help to characterize the composition of the adsorbates. Lapčik shows how inverse gas chromatography can help to characterize rough surfaces before they are subjected to electrokinetic studies. He derives his experimental material mostly from pharmaceutical products.
Johannes Lyklema⁎ Carsten Werner ⁎ Corresponding author. E-mail address: [email protected].