Repulsion electrostatic-induced

Double-layer forces are commonly used to induce repulsive interactions in colloidal systems. However, the range of electrostatic forces is strongly reduced by increasing the ionic strength of the continuous phase. Also, electrostatic effects are strong only in polar solvents, which is a severe restriction. An alternative way to create long-range repulsion is to adsorb macromolecules at the interface between the dispersed and the continuous phase. Polymer chains may be densely adsorbed on surfaces where they form loops and tails with a very broad distribution of sizes... 

Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already... 

Colloidal systems are often governed by van der Waals interactions. This class of dipole, induced dipole, and dispersion interactions causes a ubiquitous attractive potential between the particles and the container walls and between the particles themselves, which must be balanced to produce a stable colloid. Stabilization of the suspension can be achieved using functional surface groups on the particles. They can induce repulsive electrostatic and steric interactions, which counterbalance the attractive potential. A self-contained description of electrostatically stabilized colloids in polar solvents was first given in the classical DLVO theory by Derjaguin, Landau, Verwey and Overbeek [26, 30]. 

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

Several years ago Makino et al. 86) studied the influence of anions on the conformation of poly-[L-methionine-S-methylsulfonium] salts in solution. They found that especially perchlorate will induce a-helix formation whereas Cl- and Br do not. Since then several authors 87 92) have found a similar a-helix inducing effect in the case of poly-L-lysine (Lys) and other BPAA at low pH-values where the polymer molecules usually attain an extended conformation due to the electrostatic repulsion of the ammonium groups. Therefore, the a-helix inducing effect is obviously an... 

Equation (6.20) and the semiquantitative trends it conveys, can be rationalized not only on the basis of lateral coadsorbate interactions (section 4.5.9.2) and rigorous quantum mechanical calculations on clusters89 (which have shown that 80% of the repulsive O2 - O interaction is indeed an electrostatic (Stark) through-the-vacuum interaction) but also by considering the band structure of a transition metal (Fig. 6.14) and the changes induced by varying O (or EF) on the chemisorption of a molecule such as CO which exhibits both electron acceptor and electron donor characteristics. This example has been adapted from some rigorous recent quantum mechanical calculations of Koper and van Santen.98... 

Best approach toward a general solution of all problems of induced stability appears to be a two-pronged surface treatment involving electrostatic and steric protection. In order to increase repulsion energy, zeta should be increased and to enable the particles to resist compression to a distance of separation less than that at E, a bulky molecule should be attached firmly to the surface. Some systems do not accept both steric and ionic protection but for those that do, the combination shows most promise. Er should not be increased without some assurance that the particles will not be subjected to drastic compressive forces. 

Unlike the sand used above, the adsorption of HPAM on SiC at 20g/l NaCl is significant even in the absence of Ca2+ (Figure 5). This is mainly due to the lower charge density of SiC, hence the weaker electrostatic repulsion. The higher affinity of HPAM for SiC may also explain the attainment of maximum adsorption at lower Ca2+ level, and may also be the reason that the higher interaction of HPAM with Ca2+ can induce an adsorption level higher than that of PAM. 

Node and co-workers have found that the Diels-Alder reaction of nitroalkenes with 1-methoxy-3-trimethylsilyloxy-1,3-butadiene (Danishefsky s dienes) exhibit abnormal exo-selectivity. Electrostatic repulsion between the nitro and the silyloxy group of the diene induces this abnormal exo-selectivity (Eq. 8.10).17 This selective reaction has been used for the asymmetric synthesis of various natural products as shown in Scheme 8.6. 

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