Interfacial surface charge, importance

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

Preparation of nanoparticles can be by a variety of different ways. The most important and frequently used is emulsion polymerization others include interfacial polymerization, solvent evaporation, and desolvation of natural proteins. The materials used to prepare nanoparticles are also numerous, but most commonly they are polymers such as poly-alklcyanoacrylate, polymethylmethacrylate, poly-butylcyanoacrylate, or are albumin or gelatin. Distribution patterns of the particles in the body can vary depending on their size, composition, and surface charge [83-85]. In particular, nanoparticles of polycyanoacrylate have been found to accumulate in certain tumors [86,87]. 

The above forms for the Lennard-Jones surface-water interaction potential have been used as models of hydrophobic surfaces such as pyrophyl1ite, graphite, or paraffin. If the intention of the study, however, is to understand interfacial processes at mineral surfaces representative of smectites or mica, explicit electrostatic interactions betweeen water molecules and localized charges at the surface become important. 

Light-induced transformations over fluorinated titania (TiOi/F) cannot be initiated either by =Ti— 0 (OHads), due to the lack of =Ti—OH groups, or by SET from a surface complexed substrate, due to the fluoride competition. In addition to these major effects, the adsorption of molecular oxygen can be affected also and the surface charge is dramatically decreased. The last effect may be important particularly for charged substrates and intermediates and for the possibility of interfacial electron transfer. 

In Chapter 1, we have discussed the potential and charge of hard particles, which colloidal particles play a fundamental role in their interfacial electric phenomena such as electrostatic interaction between them and their motion in an electric field [1 ]. In this chapter, we focus on the case where the particle core is covered by an ion-penetrable surface layer of polyelectrolytes, which we term a surface charge layer (or, simply, a surface layer). Polyelectrolyte-coated particles are often called soft particles [3-16]. It is shown that the Donnan potential plays an important role in determining the potential distribution across a surface charge layer. Soft particles serve as a model for biocolloids such as cells. In such cases, the electrical double layer is formed not only outside but also inside the surface charge layer Figure 4.1 shows schematic representation of ion and potential distributions around a hard surface (Fig. 4.1a) and a soft surface (Fig. 4.1b). 

Our modeling approach was first used to describe the EDL properties of well-characterized, crystalline oxides ( 1). It was shown that the model accounts for many of the experimentally observed phenomena reported in the literature, e.g. the effect of supporting electrolyte on the development of surface charge, estimates of differential capacity for oxide surfaces, and measurements of diffuse layer potential. It is important to note that a Nernstian dependence of surface potential (iIJq) as a function of pH was not assumed. The interfacial potentials (4>q9 4> 9 in Figure 1) are... 

Surface atoms, crystal imperfections and adsorbed molecules can give rise to localized energy levels located in the band gap. These so-called surface states play an important role as mediators of interfacial electron transfer. The surface states may or may not be occupied by an electron the surface-localized electrons give a contribution, as, to the surface charge ... 

The interfacial electric polarizability y, being an important dynamic characteristic of the particle surface charge, can be easily determined from the electro-optical effect dependence on the square of the electric field strength (Eq. 6). A significant increase in the particle dimensions as well as the low surface charge of the colloid-polymer complex complicate the electric polarizability determination near to the system s isoelectric point (Figure 2). The electric polarizabilities are calculated in this review only for polymer covered particles in stabilized suspensions. One way to obtain correct values... 

The impedance response displays significant dependence on the applied interfacial potential (Figure 10). It can be deduced from the data analysis (32) that the protein adsorbs at the interface. At more negative potentials the aqueous side of the interface has a negative surface charge (anions) and at more positive potentials it has a positive surface charge (cations). It is an important result that the curves of imaginary impedance vs. interfacial... 

Among the six interfacial variables discussed in this section, the surface charge density oo, the surface potential (fo, and the potential at the OHP fd (usually called the diffuse layer potential), are most important in characterizing interfacial properties. The three remaining variables (i.e., ap, /p, and Od) can be estimated using Eqs. (5), (7), and (8) if oo, and /rf are known exactly. ao can be determined experimentally by the potentiometric titration method, and detailed explanation of the potentiometric titration is given, for example, by Yates [10]. The estimate of fo for the ceramic powder/aqueous solution interface is discussed in the next section, yd is perhaps the most important interfacial electrochemical parameter since it is closely correlated with the kinetic stability of a given colloidal suspension and it can be conveniently determined (approximately) experimentally. 

It is well established that ultralow interfacial tension plays an important role in oil displacement processes [16,18]. The magnitude of interfacial tension can be affected by the surface concentration of surfactant, surface charge density, and solubilization of oil or brine. Experimentally, Shah et al. [23] demonstrated a direct correlation between interfacial tension and interfacial charge in various oil-water systems. Interfacial charge density is an important factor in lowering the interfacial tension. Figure 6 shows the interfacial tension and partition coefficient of surfactant as functions of salinity. The minimum interfacial tension occurs at the same salinity where the partition coefficient is near unity. The same correlation between interfacial tension and partition coefficient was observed by Baviere [24] for the paraffin oil-sodium alkylbenzene sulfonate-isopropyl alcohol-brine system. 

Where a is the surface charge density, e is the permittivity, n is the refractive index (1.333), N the number of water molecules, A=fi n + 2)/ 2kTeo) and B=ii tP + 2)/ 3V), fi is the dipole moment of a water molecule (6.023 x 10 cm), T = 293 K, k = Boltzmann constant (1.3807 X 10" V C K ), the superscript o refers to quantities outside the interfacial electric field,/is the free energy density = eo F/V, y is the distance from the interface, P is the stric-tion pressure, and v is the volume per water molecule. Results of this calculation are presented elsewhere [24], but the important point here is that in developing a KMC model of the electrolyte-metal interface, some care must be taken in how the adsorbed concentration of water molecules is to be considered— if the electrol)te is to be explicitly considered in order to predetermine the various rates for transitions, especially if they are affected by the number of nearest neighbors. 

Figure 6 shows the effect of surfactant concentration on interfacial tension and electrophoretic mobility of oil droplets (14). It is evident that the minimum in interfacial tension corresponds to a maximum in electrophoretic mobility and hence in zeta potential at the oil/brine interface. Similar to the electrocapillary effect observed in mercury/water systems, we believe that the high surface charge density at the oil/brine interface also contributes to lowering of the interfacial tension. This correlation was also observed for the effect of caustic concentration on the interfacial tension of several crude oils (Figure 7). Here also, the minimum interfacial tension and the maximum electrophoretic mobility occurred in the same range of caustic concentration (17). Similar correlation for the effect of salt concentration on the interfacial tension and electrophoretic mobility of a crude oil was also observed (18). Thus, we believe that surface charge density at the oil/brine interface is an important component of the ultralow interfacial tension.

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