Highly charged surface

An understanding of the chemical behaviour of the element can aid in the choice of appropriate techniques and methods, the application of which would not disrupt the interaction of the element with associated constituents. For example, in the study of aluminium some relevant information may include its amphoteric nature, its ability to form predominantly ionic complexes, its tendency to form hydroxides, and the stability of aluminium complexes formed with biological ligands. It is clear that in order to maintain the ionic interactions the pH, ionic strength and, of lesser importance, the ionic composition of the medium used for sample preparation should be similar to that found in vivo. In addition, highly charged surfaces should not come into contact with the sample. 

The strong interaction of dextran sulfates with cationic functions in porous support materials is exploited to create new highly charged surfaces for adsorption of proteins. It was revealed that new and strong ionic exchange resins are accessible by simple and rapid deposition of dextran sulfates on commercial DEAE- or MANAE-agarose. The material is characterised by an increased charge density on the porous surface of the support, which can perfectly bind protein material, as demonstrated in Fig. 15 [153]. 

In another set of experiments, directed evolution was used to stabilize a psychrophilic subtilisin, S41, isolated from the Antarctic bacterium TA41 (Miyazaki et al., 2000 Miyazaki and Arnold, 1999). S41 conserves the overall subtilisin fold and shares relatively high identity with other subtilisins (Fig. 10), but contains several features not found in the common mesophilic subtilisins, including a highly charged surface, several inserted surface loops, and decreased numbers of salt bridges and aromatic-aromatic interactions (Davail et al., 1994). In common with other enzymes from psychrophilic sources, S41 is less stable at high... 

Influence of Field Effect. Since electron transfer rates are directly related to the field, a judicious manipulation of the distance of a sensitizer and an electron acceptor (or donor) from a highly charged surface across the Stem layer (Figure 2, equation 7) is expected to result in altered efficiencies. This expectation has been realized In achieving effective charge separation under the Influence of a positive electric field, generated by DODAC vesicles (35). Rate constant for electron transfer from L-cystelne to the excited state of Ru(bpy) ... 

Finally, the expression (78) for q(z) can be used to modify the nonlinear Poisson-Boltzmann theory in order to consider a highly charged surface [59, 60]. In this case, for the profile (h z) the new term appears which exactly reproduces the last electrostatic term in the contact theorem (73). 

Blaakmeer J, Bohmer MR, Cohen Stuart MA, Fleer GJ. Adsorption of weak polyelectrolytes on highly charged surfaces. Poly(acrylic acid) on polystyrene latex with strong cationic groups. Macromolecules 1990 23 2301-2309. 

When a surfactant is adsorbed onto a solid surface, the resultant effect on the character of that surface will depend largely upon the dominant mechanism of adsorption. For a highly charged surface, if adsorption is a result of ion exchange, the electrical nature of the surface will not be altered significantly. If, on the other hand, ion pairing becomes important, the potential at the Stern layer will decrease until it is completely neutralized (see Fig. 9.5). In a dispersed system stabilized by electrostatic repulsion, such a reduction in surface potential will result in a loss of stability and eventual coagulation or flocculation of the particles (Chapter 10). 

The point of zero charge (pzc) is the pH value when the numbers of [—M—OHj] and [—M—0 ] are equal. At the pHpzc, abrasive particles show zero zeta potential. This insufficient surface charge between abrasive particles decreases the energy barrier for agglomeration, resulting in poor dispersion stability. Thus, it has to be formulated at pH < pHpzc or pH > pHpzc where abrasive particles have a highly charged surface. 

The potentials in water are lower than what is usually reported for mica (-80 mV (27-29)) and cellulose (10-20 mV (14,17,30)). However, in a previous study we also found that the surface charge of cellulose was lowered in the presence of a highly charged surface like mica (14). 

While Eq. [56] describing the potential is useful in predicting its value near a highly charged surface, this result can be extended to give an approximate description everywhere. The assumption of retaining only the contribution... 

For two highly charged surfaces with a = 0.01co/A in a 0.1 M 1 1 electrolyte (kd=0.1A ), we have Oave = 2.16 giving 2R>40A for a = 0.001 eo/A we have 2R > 20A or two Debye lengths. At separations much smaller than this, the assumptions of a continuum solvent and point ions are called into question. 

The analytical approximation for the potential profile near a highly charged surface obtained earlier, given by Eqs. [147] and [149] or Eqs. [152] and [154], is applicable to a spherical micelle with Zc again representing the highest counterion valence and the screening constant. The potential at the surface, Eq. [153], then becomes... 

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