Electric charge effects

Scherer, P. G. and Seelig, J. (1989). Electric charge effects on phospholipid head-groups. Phosphatidylcholine in mixtures with cationic and anionic amphiphiles, Biochem., 28, 7720-7728. 

Electrical-charge effects can be further exploited by using charged membranes (as referred to above) to increase retention of all species with like polarity. It is important to mention that it may be possible to exploit electrostatic interactions even for solutes with similar isoelectrical points, due to different charge-pH profiles for the different species present. The membrane pore-size distribution also affects selectivity by altering the solute sieving coefficients locally. Narrow pore-size distributions, especially for electrically charged membranes, will impact very positively on membrane selectivity and overall performance. 

The assumption on the electric charge effect of excess electrons on the rate constant of their interfacial transfer is supported by an evident similarity of these semiconductor colloidal systems with metal colloids, for which effect of the charge of electrons captured by the particle is well known and agrees with the microelectrode theory . Moreover, kinetic curves similar to those we found for CdS colloids were observed previously for silver colloids in ref. [17], where the particles charge q was shown to decrease by the law... 

Electric charge effects—the van der Waals and electrostatic forces can also be a strong driver of collapse and shrinkage. 

In this section, we will not discuss all the peculiarities of the electric charge effect on the adsorption kinetics as the state of experiments is still discouraging and deserves much more attention in future. 

Interparticle forces include the van der Waals attractive forces, electrostatic r ul-sive forces arising firom surface chaiges on the particles, and entropic repulsive forces due to water-soluble polymera adsoibed/anchored to the particle surface and/or due to adsorbed surfactants. These interparticle forces become important as the interparticle distance, h, becomes smaller, and are significant factors at h < 10 nm. Both smaller particle diameter, d, and higher volume fracfion, < >, lead to decreasing values of h, as shown in Equation (13.8). As already seen in Section 13.2.4, the thickness of adsorbed surfactant layers can be an appreciable fraction of the interparticle distance. Adsorbed polymer layers can be of the order of 10 nm in thickness, and in systons with low ionic strength, electrical charge effects can extend much further. 

It might be expected that inclusion of small particles in a powder could decrease porosity if they fit into voids between larger particles. In practice, the opposite effect is usually observed. The explanation for this follows from consideration of the main factors that determine closeness of packing and flow properties of powders the size, shape, and surface properties of the particles. It is often found that the effects of surface properties outweigh the others because they govern the friction and adhesion between particles. As the size of particles decreases, the ratio of surface to volume increases, thus magnifying frichonal resistance. Other factors that may contribute to increased friction or stickiness are the presence of liquid films and electrical charge effects. 

Table 4.11 compares SIMS and SNMS (cfr. also Table 8.57 of ref. [110a]). Detection limits in the sub-ppm range are accessible under optimised analytical conditions. A lateral resolution of less than 100 nm and an in-depth resolution of a few nm can be achieved. One of the unique features of SNMS is the ease of analysis of insulators. This is at variance to SSMS, GD-MS and SIMS, which are handicapped by electrical charging effects. Laser SNMS is not strictly restricted to elemental analysis, but can also be applied to the characterisation of molecular surfaces. For an optimum yield of intact molecular ions and characteristic fragments it is necessary to optimise laser power density, wavelength, and pulse width [112],... 

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