Electrostatic stability, measurement

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

Many investigators of steric stabilization have measured colloidal stability without taking the effort to find out whether the stability actually resulted from electrostatic stabilization. In many published articles it has been concluded that steric stabilization had been attained and further study showed this was not the case. One such example is a recent paper on "steric" stabilization by an additive of the same type used in this work. (12) The published photograph shows the silica particles in oil stabilized at interparticle separations several times the distances provided by the adsorbed films no electrical measurements had been made, but it they had, this particular dispersant would have provided about -200 mV of zeta-potential and given excellent electrostatic repulsion. The reader should be wary of any claims of steric stabilization unless the electrostatic contribution has been measured. 

The composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

One striking result [64] involves making a small change in regularity when one GC pair in a strand is simply inverted to CG, the local defect CG pair drops 0.6 eV below its previous position—that drop is 15 times the calculated bandwidth. This electrostatic stabilization means that the defect level is far from the conducting delocalized states, and corresponds to Anderson-type localization. With Anderson locahzation of this depth, the conductance is expected to decay exponentially. Indeed, exponential decay of conductance has been discussed in a number of measurements both on A-DNA and on poly(GC) sequences [65-67]. 

Enolization of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl- 1-methylpyridinium ion (94) have been measured13811 and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate.13811 For (94), qh = 1.9 x 102 M-1 s 1, about 1.1 x 106 times that for a typical methyl ketone such as acetone. Thermodynamically, it is >108 times more acidic (pAa values of 11.1 vs 19.3). These increases in kinetic and thermodynamic acidity are derived from through-bond and through-space effects, and the implications for enzymatic catalytic sites with proximal, protonatable nitrogen are discussed. The results for (94) suggest a pAa value of 8.8 for (95), a value that cannot be measured directly due to competing hydrolysis. 

This is the dependence expected for purely electrostatic stabilization of polyelectrolyte layers. Electro-steric interactions involve steric contributions [Eq. (3)] and electrostatic contributions [Eq. (4)]. For high ionic strength as in the interior of the polyelectrolyte shell the measured shear modulus should exhibit a characteristic G (r) e Kr-dependence which is apparent in a semi-logarithmic presentation of the data as in Fig. 13. The measured shear moduli are plotted as a function of the reduced distance r/2Rm which is... 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

Ferrimagnetic nanoparticles of magnetite (Fc304) in diamagnetic matrices have been studied. Nanoparticles have been obtained by alkaline precipitation of the mixture of Fe(II) and F(III) salts in a water medium [10]. Concentration of nanoparticles was 50 mg/ml (1 vol.%). The particles were stabilized by phosphate-citrate buffer (pH = 4.0) (method of electrostatic stabilization). Nanoparticle sizes have been determined by photon correlation spectrometry. Measurements were carried out at real time correlator (Photocor-SP). The viscosity of ferrofluids was 1.01 cP, and average diffusion coefficient of nanoparticles was 2.5 10 cm /s. The size distribution of nanoparticles was found to be log-normal with mean diameter of nanoparticles 17 nm and standard deviation 11 nm. 

Latex stability. Effect of pH. The pH of the latices were adjusted to about b before doing any of the previously described stability measurements. The effect of pH on the stability is described in Figure 8. A sharp drop in stability occurs below pH 2. This is possibly due to deionization of the sulphate group of the emuisifier, indicating an electrostatic repulsion to have been operative. 

EnoUzation of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl-1-methylpyridinium ion (94) have been measured and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate. For (94), = 1-9 x 10 M s , about 1.1 x 10 times that for... 

Electrokinetics. Bottle tests and centrifugation may be somewhat crude, but they do offer a relative measure of emulsion stability that combines, to some extent, all of the factors that affect stability. Electrokinetic measurements are somewhat more elegant because they allow direct measurement of the degree of electrostatic stability in an emulsion system. The zeta potential, or relative magnitude of the electric charge on the surface, is... 

A wide variety of electrokinetic measurements or instruments can be used to quantify the electrostatic stability of the dispersed phase. These measurements will only be summarized here. 

Figure 11. Schematic representation of the electrophoretic mobility (A) measurement showing the major components. In an applied electric field, emulsion droplets move according to their surface charge. These charges can electrostatically stabilize an emulsion system by preventing the droplets from coming into contact and coalescing. The motion of the droplets is visually observed, and the electrophoretic mobilities of a number of particles are measured to determine zeta potential. The sedimentation potential (B) is also illustrated.

The so-called potential can be taken as a first estimate for the surface potential. The potential is the electrostatic potential at the hydrodynamic shear plane close to the particle surface. It can be determined from electrophoretic mobility measurements of the particles in an electric field (see for example Ref. [23]). The potential is zero when the charge within the shear plane is zero. This is the case as the surface charge plus the charge due to adsorbed ions other than hydrogen (for example AIOH2 in the case of alumina suspensions) is zero. This point is the iso-electro-point (i.e.p.) of the material in the dispersion medium. The suspension pH with respect to the i.e.p. is an important criterion for a first judgement of possible electrostatic stability. 

A suspension is a dispersion of particles within a solvent (usually a low-molar-mass liquid). Thermodynamics (Brownian motion and collisions) favours the clumping of small particles, and this can be increased by flow. However, particles over 1 pm tend to settle under gravity, unless stability measures have been considered (matching the density of the particle to that of the medium, increasing the Brownian/gravitational force ratio, electrostatic stabilization, steric stabilization). Other complications can occur in the dynamics of suspensions, such as particle migration across streamlines, particle inertial effects and wall slip (Larson, 1999). 

Ottewill and coworkers (Cain et al., 1978) have used a different geometry from those cited above to measure the distance dependence of steric repulsion. This was an extension of the apparatus first devised by Roberts and Tabor (1971) to measure the interactions between electrostatically stabilized macroscopic... 

The stability of emulsion paints will be affected by the charge surrounding the latex pigment and extender particles. Flocculation will occur if the electric double layer is insufficient to maintain interparticle separation. Therefore, zeta potentials of the individual particles in the paint can give a measure of the tendency to flocculate and hence of the long-term stability of the paint. However, emulsion paints cannot rely solely on electrostatic stabilization because they need to withstand dilution with hard water. This means that some steric stabilization will invariably be used. 

Zeta potential (f) analysis can be used to measure the relative surface charges of nanoparticles such as polyplexes. It helps define a range of stability for colloids, when steric stabilization does not take place (only electrostatic stabilization). Zeta potential, as well as dynamic light scattering (DLS) are useful methods for determining if various fractions are present in solution (with different surface charge or size, respectively). 

---