Simple electrostatic adsorption

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

Nanoparticles have been used extensively for the immobilization of biomolecules [3]. In addition to their biocompatibility they can produce a unique microenvironment that provides improvement in the freedom of orientation for affinity binding with advantages over planar substrates, an increase in surface area for higher probe loading capacities, and enhanced diffusion of amplification agents. Modification of electrode surfaces with nanoparticles can be carried out by simple electrostatic adsorption or covalent attachments such... 

Above its lEP, the zeta potential of calcite is seen to change continuously with oleate concentration. Simple electrostatic adsorption under constant ionic strength will not lead to the observed changes in the zeta potential, thus implying that there is some specific kind of interaction taking place. It was found that the calcium... 

An even more realistic deposition mode for the ions of the second kind and most of the catalytic supports is the electrostatic adsorption through ion-pair formation at plane 2. The only difference between this deposition mode and the simple electrostatic adsorption is that the ions located at the front end of the diffuse part of the interface form ion pairs with the surface oxo/hydroxo-groups of opposite charge. The cations involved in the ion pairs retain their hydration sphere. The model related with this deposition mode is called basic Stem [32] and, as the Stem-Gouy—Chapmann model, it involves only two planes (the surface plane and the plane 2). 

The simple electrostatic adsorption (accumulation) in the diffuse part of the interface and at the plane 2 is rather the predominant mode in most cases when ammonia or halogen complexes are used for depositing noble metals (e.g. Pt(NH3)4 +, PtCls ) under conditions where these complexes remain intact in the impregnating solutions. 

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. 

Figure 3 Adsorption energy of monodentate-B adsorbed formic acid on ZnO(lOlO) as a function of coverage. Calculated values (solid lines) are compared to a simple electrostatic model (dashed line) based on the atomic charges, lxn coverages refer to surface cells extended in the (000T) direction, and nxl to extensions in the (1120) direction, nxn cells have been extended in both the (0001) and the (1120) directions.

The mechanism of particle capture by depth filtration is more complex than for screen filtration. Simple capture of particles by sieving at pore constructions in the interior of the membrane occurs, but adsorption of particles on the interior surface of the membrane is usually at least as important. Figure 2.34 shows four mechanisms that contribute to particle capture in depth membrane filters. The most obvious mechanism, simple sieving and capture of particles at constrictions in the membrane, is often a minor contributor to the total separation. The three other mechanisms, which capture particles by adsorption, are inertial capture, Brownian diffusion and electrostatic adsorption [53,54], In all cases, particles smaller than the diameter of the pore are captured by adsorption onto the internal surface of the membrane. 

Figure 2.34 Particle capture mechanism in filtration of liquid solutions by depth microfilters. Four capture mechanisms are shown simple sieving electrostatic adsorption inertial impaction and Brownian diffusion...

Adsorption may occur through simple electrostatic forces,1 e.g. alkali metals on tungsten, and silver or halide ions on silver halides. 

In most actual cases, other forces besides simple electrostatic attraction enter into the work of adsorption of the ions, so that this relation is only approximate. 

Values of the PZC at the Hg solution interface are shown as a function of electrolyte concentration in fig. 10.6. In the case of NaF, the PZC with respect to a constant reference electrode is independent of electrolyte concentration. However, in the cases of the other halides, the PZC moves to more negative potentials as the electrolyte concentration increases. The latter observation is considered to be evidence that the anion in the electrolyte is specifically adsorbed at the interface. Specific adsorption occurs when the local ionic concentration is greater than one would anticipate on the basis of simple electrostatic arguments. Anions such as Cl , Br , and 1 can form covalent bonds with mercury so that their interfacial concentration is higher than the bulk concentration at the PZC. Furthermore, the extent of specific adsorption increases with the atomic number of the halide ion, as can be seen from the increase in the negative potential shift. A more complete description of specific adsorption will be given later in this chapter. 

The reality is consequently more complex than the simple electrostatic model. The adsorption of surfactants onto surfaces is the result of various factors characteristics of the surfactant and of the surface, lateral interaction between the fatty chains of the adsorbed surfactant molecules, solvation of the surfactant and of the surface, etc. It is not the type of active ingredient-surface interaction that accounts for the deposition onto fabrics. 

It is a very simple methdology to investigate the adsorption specificity. In the cases where electrostatic adsorption or adsorption through hydrogen bonding takes place the surface groups remain practically intact and the pH does... 

In view of the above one may conclude that the joint use of the aforementioned simple methodologies enables one to achieve a quite clear distinction between electrostatic adsorption and adsorption through coordinative or hydrogen bonding. On the other hand, the distinction between the two latter... 

Figure 2 A very simple model of electrostatic adsorption on a negatively charged oxide surface with formation of a "double layer" (surface + diffuse layer). Small dosed drcles are cations, larger open drcles are anions, oq" surface charge density x distance from the surface into the solution k thickness of double layer < ) electric potential c ix) and c (x) local concentrations in cations and anions, respectively. The shaded area represents the excess of cations over anions in the diffuse layer, and therefore the amount of cations that are electrostatically adsorbed.

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is ... 

---