Double layer components, interactions

Ninham and coworkers [12,13] very recently proffered an alternative explanation. They proposed that the shortcoming of traditional DLYO theory in predicting particle stability has arisen from its ab initio decomposition of forces into van der Waals and double-layer components. More specifically, they suggested that not accounting for dispersive interactions between colloid surfaces and dissolved ions is primarily responsible for the reported discrepancies in the traditional colloidal-stability modeling approach. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

To these components of the traditional double layer free energy one should add an energetic contribution due to the supplementary ion interactions ... 

The DLVO theory [1,2], which describes the interaction in colloidal dispersions, is widely used now when studying behavior of colloidal systems. According to the theory, the pair interaction potential of a couple of macroscopic particles is calculated on the basis of additivity of the repulsive and attractive components. For truly electrostatic systems, a repulsive part is due to the electrostatic interaction of likely charged macroscopic objects. If colloidal particles are immersed into an electrolyte solution, this repulsive, Coulombic interaction is shielded by counterions, which are forming the diffuse layer around particles. A significant interaction occurs only when two double layers are sufficiently overlapping during approach of those particles. 

How was the theoretical DLVO curve in Figure 1.12 obtained The DLVO model [18, 19] postulates that the appropriate thermodynamic potential energy of interaction between two parallel flat plates can be described in terms of two components a repulsive term VR, resulting from the overlap of electrical double layers, and an attractive van der Waals interaction, VA. It also assumes that these interactions are additive, so that the total potential energy can be written as... 

At higher pressure some positive component outweighs the double layer repulsion. It might be speculated that this strong repulsion is due to steric interactions between the hydrophilic PEO brushes and (at least) three surface force components must be considered, i.e. n = n vw + n ei + rrst. 

Colloidal Si02 particles in an aqueous suspension provide a solid-liquid interface. The silanol groups on the particle surface are ionized at a pH 6. Consequently, the surface of the particle is negatively charged and a diffuse electrical double layer is produced in the vicinity of the solid interface (17,18). Because of the negative charges on the particles they repel one another and their agglomeration is prevented. The particles can be used to exert electrostatic repulsive and attractive interactions with the components involved in photosensitized reactions. 

Even though the components of the total interaction potential between such complex adsorbents as solid carbons and a wide range of adsorbates can be grouped in many different ways 315,316], it is convenient and meaningful to consider only the London dispersion (induced dipole) forces and the electrostatic (double-layer) forces [620,621,76,77]. 

Surfactant Effects on Microbial Membranes and Proteins. Two major factors in the consideration of surfactant toxicity or inhibition of microbial processes are the disruption of cellular membranes b) interaction with lipid structural components and reaction of the surfactant with the enzymes and other proteins essential to the proper functioning of the bacterial cell (61). The basic structural unit of virtually all biological membranes is the phospholipid bilayer (62, 63). Phospholipids are amphiphilic and resemble the simpler nonbiological molecules of commercially available surfactants (i.e., they contain a strongly hydrophilic head group, whereas two hydrocarbon chains constitute their hydrophobic moieties). Phospholipid molecules form micellar double layers. Biological membranes also contain membrane-associated proteins that may be involved in transport mechanisms across cell membranes. 

It is customarily assumed that the overall particle-particle interaction can be quantified by a net surface force, which is the sum of a number of independent forces. The most often considered force components are those due to the electrodynamic or van der Waals interactions, the electrostatic double-layer interaction, and other non-DLVO interactions. The first two interactions form the basis of the celebrated Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on colloid stability and coagulation. The non-DLVO forces are usually determined by subtracting the DLVO forces from the experimental data. Therefore, precise prediction of DLVO forces is also critical to the determination of the non-DLVO forces. The surface force apparatus and atomic force microscopy (AFM) have been used to successfully quantify these interaction forces and have revealed important information about the surface force components. This chapter focuses on improved predictions for DLVO forces between colloid and nano-sized particles. The force data obtained with AFM tips are used to illustrate limits of the renowned Derjaguin approximation when applied to surfaces with nano-sized radii of curvature. 

The main analyser chamber contains a spectrometer and ports for different excitation sources. The preparation chamber is needed for sample preparation e.g. cleavage). The electrolyte components are allowed to react with the interface in the adsorption chamber, where temperature control is used to stabilise the interface-adsorbate interaction. Water, halogen and alkali species are allowed to interact with electrode material to investigate structure and potential distribution of the electrochemical double layer (Sass, 1983 Bange et al, 1987 Sass et al., 1990). laegermann (1996) gives a comprehensive review of the semicondnctor/electrolyte interface within the vacnnm science approach. 

The molecular component of the disjoining pressure, IIm(/i), is negative (repulsive). It is caused by the London-van der Waals dispersion forces. The ion-electrostatic component, IIe(/i), is positive (attractive). It arises from overlapping of double layers at the surface of charge-dipole interaction. At last, the structural component, IIs(/i), is also positive (attractive). It arises from the short-range elastic interaction of closed adsorption layers. 

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