Colloidal system electric double-layer properties

Oxides, especially those of silicon, aluminum, and iron, are abundant components of the earth s crust they participate in geochemical reactions and in many chemical processes in natural waters, and often occur as colloids in water and waste treatment systems. The properties of the phase boundary between a hydrous oxide surface and an electrolyte solution depend on the forces operating on ions and water molecules by the solid surface and on those of the electrolyte upon the solid surface. The presence of an electric charge on the surface of particles often is essential for their existence as colloids the electric double layer on their surface hinders the attachment of colloidal particles to each other, to other surfaces, and to filter grains. 

The existence of a double layer determines the properties of many systems in electrochemistry, in colloidal sciences, in biology, etc. [1-4]. Owing to their importance, electrical double layers have long been and remain a subject of intense research on both experimental and theoretical aspects. This is covered by some recent textbooks and review articles [3,5-10]. 

A characteristic feature of colloidal dispersions is the large area-to-volume ratio for the particles involved. At the interfaces between the dispersed phase and the dispersion medium characteristic surface properties, such as adsorption and electric double layer effects, are evident and play a very important part in determining the physical properties of the system as a whole. It is the material within a molecular layer or so of the interface which exerts by far the greatest influence on particle-particle and particle-dispersion medium interactions. 

The electrical double layer interaction is a key feature of a great many colloidal systems that are of technological significance. In the preparation and useful shelf-life of paints, and/or inks, or in the area of detergency where the properties of charged surface active compounds come into consideration the stabilizing influence of the edl interaction is important. Special mention... 

Electric double layers at phase boundaries pervade the entire realm of Interface and colloid science. Especially in aqueous systems, double layers tend to form spontaneously. Hence, special precautions have to be taken to ensure the absence of charges on the surfaces of particles. Insight into the properties of double layers is mandatory, in describing for Instance electrosorption, ion exchange, electrokinetics (chapter 4), charged monolayers (Volume III), colloid stability, polyelectrolytes and proteins, and micelle formation of ionic surfactants, topics that are intended to be treated in later Volumes. The present chapter is meant to Introduce the basic features. 

Most of the electrochemical phenomena occur in size regimes that are very small. The effects of size on diffusion kinetics, electrical double layer at the interface, elementary act of charge transfer and phase formation have recently been reviewed by Petrri and Tsirlina [12]. Mulvaney has given an excellent account of the double layers, optical and electrochemical properties associated with metal colloids [11]. Special emphasis has been given to the stability and charge transfer phenomenon in metal colloid systems. Willner has reviewed the area of nanoparticle-based functionalization of surfaces and their applications [6-8]. This chapter is devoted to electrochemistry with nanoparticles. One of the essential requirements for electrochemical studies is that the material should exhibit good conductivity. 

Many properties of disperse systems are related to the distribution of charges in the vicinity of the interface due to the adsorption of electrolytes. The adsorption of molecules is driven by the van der Waals attraction, while the driving force for the adsorption of electrolytes is the longer-range electrostatic (Coulomb) interaction. Because of this, the adsorption layers in the latter case are less compact than in the case of molecular adsorption (i.e., they are somewhat extended into the bulk of the solution), and the discontinuity surface acquires noticeable, and sometimes even macroscopic thickness. This diffuse nature of the ionized adsorption layer is responsible for such important features of disperse systems as the appearance of electrokinetic phenomena (see Chapter V) and colloid stability (Chapters VII, VIII). Another peculiar feature of the adsorption phenomena in electrolyte solutions is the competitive nature of the adsorption in addition to the solvent there are at least two types of ions (even three or four, if one considers the dissociation of the solvent) present in the system. Competition between these ions predetermines the structure of the discontinuity surface in such systems -i.e. the formation of spatial charge distribution, which is referred to as the electrical double layer (EDL). The structure and theory of the electrical double layer is described in detail in textbooks on electrochemistry. Below we will primarily focus on those features of the EDL, which are important in colloid... 

Although this book significantly differs from the earlier Colloid Chemistry textbook, it nevertheless focuses on the specifics of educational and research work carried out at the Colloid Chemistry Division at the Chemistry Department of MSU. Many results presented in this book represent the art developed in the laboratories of the Colloid Chemistry Division, in the Laboratory of Physical-Chemical Mechanics (headed by E.D. Shchukin since 1967) of the Institute of Physical Chemistry of the Russian Academy of Science, and in other research institutions and industrial laboratories under the guidance of the authors and with their direct participation. Special attention is devoted in the book to the broad capabilities that the use of surfactants offers for controlling the properties and behavior of disperse systems and various materials due to the specific physico-chemical interactions taking place at interfaces. At the same time the authors made every effort to avoid duplication of material traditionally covered in textbooks on physical chemistry, electrochemistry, polymer chemistry, etc. These include adsorption from the gas phase on solid surfaces (by microporous adsorbents), the structure of the dense part of the electrical double layer, electrocapillary phenomena, specific properties of polymer colloids, and some other areas. 

Surface energies of sohds, surface and interfadal tensions and the interfacial region, thermodynamics of colloidal systems, improved electrical double layer theory, adsorbed pol)mer layers and steric stabilization, relationships between surface energies and bulk properties... 

As interfacial properties are often decisive for the behavior of colloidal systems, the most relevant interfacial properties, that is, interfacial tension, curvature, mono-layer formation, wetting, and the electrical double layer at charged interfaces, are treated in Chapters 5 through 9, respectively. 

Many macroscopic phenomena of colloidal suspensions arc related to the light scattering and the Brownian motion of the single particles. Both properties depend largely on particle size they are, therefore, frequently employed for the characterisation of particle systems (cf. previous chapter). Additionally, the small size of colloids enhances the significance of the interface to the particles physical behaviour. The interfacial properties additionally affect the interaction between particles and are, thus, cmcial for the macroscopic suspension behaviour (e.g. stability). A particular characteristic of interfaces is the electric double layer (EDL), which camiot be ignored in most situations. Its formation and sttucture is closely related to dissolved ionic species and their interaction with the particle surface (e.g. adsorption, precipitation). Last but not least, the interfacial properties can be affected by the solubility behaviour of the particle phase. 

One such phenomenon is the low-frequency dielectric dispersion (LFDD) of suspensions. This is the denomination given to the frequency dependence of the permittivity of dispersed systems for applied electric field frequencies close to characteristic frequencies of relaxation (typically in the kHz to MHz range) in the electrical double layer. A significant effect has been reported of the properties of the medium, of the particle, and of their interface on the relaxation pattern of the permittivity, in particular number of relaxations observed, natural frequencies, and amplitudes of those relaxations. This explains the increased interest in the determination of the permittivity of colloidal systems during the last decade or so [19-26]. In this contribution, we will show results that clearly demonstrate that the presence of the SPs affects the amplitude and characteristic frequency of the LFDD far more than could be explained by simple considerations of accumulation of effects. 

The second part covers the structural aspects of different colloidal systems. Chapters 3 and 4, by Martin-Molina et al. and Haro-Pdrez et al., deal with electric double layers and effective interactions. Chapters 5 and 6, by Delgado et al. and Martinez-Pedrero et al., explore the structure of extremely bimodal suspensions and fllaments made up of miaosized magnetic particles. Chapters 7 and 8, by Puertas and Fuchs, and Hynninen et al., analyze the role played by the attractive interactions, confinement, and external fields on the structure of colloidal systems. Chapters 9 and 10, by Tromp and Maldonado-Valderrama et al., cover some structural aspects in food emulsions. This second part of the book finishes with Chapter 11, by de Vicente, which analyzes the rheological properties of structured fluids in order to establish a connection between measured material rheological functions and structural properties. 

The description of a colloid should include particle size, mobility, charge and their distributions, charge/mass ratio, electrical conductivity of the media, concentration and mobility of ionic species, the extent of a double layer, particle-particle and particle-substrate interaction forces and complete interfacial analysis. The application of classical characterization methods to nonaqueous colloids is limited and, for this reason, the techniques best suited to these systems will be reviewed. Characteristic results obtained with nonaqueous dispersions will be summarized. Physical aspects, such as space charge effects and electrohydrodynamics, will receive special attention while the relationships between chemical and physical properties will not be addressed. An application of nonaqueous colloids, the electrophoretic development of latent images, will also be discussed. 

The electrostatic stability of a colloidal system depends not only on the magnitude of the electrical surface charge density but also on the dielectric properties of the medium, on its ionic strength, on the valence of the ions in the double layer, on the size of the particles, and on the temperature of the system (only slightly). The total interaction potential between two spherical particles charged by a single type of ions at the surface can be determined using the DLVO equation ... 

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