Nonpolar solvents, dispersion stability

A key assumption implicit in the model is that isomerization and propagation take place only within the ion-counterion-monomer complex and that the ion-counterion pair is unreactive. Results of the (concurrent) composition and rate studies support this assumption. This is expected since the reactions of the ions are generally believed to proceed through charge-dispersed transition states. Formation of the ion-counterion-monomer complex provides intermediates which approach the energy level and charge dispersion of the transition states. In nonpolar solvent, stabilization of the ion-counterion pair provides the driving force for the formation of the complex. 

The initial decrease in optical rotation found in aqueous solutions of /3-lactoglobulin and ovalbumin is not, however, sufficient to differentiate globular proteins from simpler synthetic polypeptides in their transition behavior, for neither ribonuclease nor human serum albumin appear to exhibit it. The specific rotation of ribonuclease in water-2-chloroethanol mixtures becomes steadily less levorotatory as the proportion of nonpolar solvent increases (Weber and Tanford, 1959). In the case of human serum albumin Bresler (1958) and Bresler el al. (1959) find that only progre.ssive increases in specific rotation occur as the concentration of 2-chloroethanol is increased and that this change is accompanied by a steady rise in viscosity and the corresponding axial ratios characteristic of the formation of rodlike particles. If these proteins do have some initial helical content in water, as can be argued from their optical rotatory dispersion, then it appears that hydrophobic forces are not required for the stability of these regions. 

In the highly polar solvent, acetic acid, the reaction is completely nonstereospecific. The product distribution is consistent with reaction of ds-stilbene with bromine to form an intermediate carbocation, followed by reaction of bromide on either face of the planar intermediate to give 4-21 and 4-22. In the nonpolar solvent, carbon tetrachloride, the product is exclusively the dl product 4-21 that would result from anti addition of bromide to a bridged bromonium ion. In the polar solvent, the localized charge on an intermediate planar carbocation would be more stabilized than the bromonium ion by solvent interactions, because the charge on the bridged bromonium ion is more dispersed. 

Induced dipole-induced dipole and dipole-induced dipole interactions (van der Waals forces or dispersion forces) are the weakest of solvent-solute interactions and are predominant in solutions of nonpolar solutes in nonpolar or polar solvents and/or polar molecules in nonpolar solvents. These interactions, which are closely related to the polarizabilities of the solute and solvent, account for the small shifts to lower frequency of the absorption spectra of molecules upon going from the gas phase to solutions in nonpolar media. Presumably, the excited states of the nonpolar aromatic hydrocarbons and other conjugated hydrocarbons are more polarizable than the ground state, leading to stabilization of the excited states relative to the ground state and shifts of the absorption bands to the red. 

Fig 2). This is shown by the more compact sediments. Alumina with an amphoteric surface was stabilized by dispersants and strongly acid or strongly basic solvents by a combined steric and electrostatic mechanism. Hexane, which is a nonpolar solvent with no hydrogen bonding capability, is shown to be a poor solvent for AHAS, which is a highly polar dispersant. Hexane would introduce a low potential as well hence the stability had to rely solely on the steric stabilization contribution. When the adsorbate AHAS is not highly soluble in the medium, the dispersant is ineffective as a steric stabilizer. Despite the high dielectric constant of EtOH, it is shown to be an ineffective medium with LNA. EtOH as a weakly acid solvent introduced nearly a zero potential... 

Microemulsions [191, 192] are transparent, optically isotropic and thermodynamically stable liquids. They contain dispersion of polar and nonpolar solvent, usually water or aqueous solutions and oils. Adding surfactants stabilizes droplets of 1-100 nm in size. Due to amphiphilic properties of the surface active substances containing lipophilic groups and one or two lyophobic C-H chains mainly collected at the interface of two liquid phases, they cannot be mixed under normal conditions. Unlike traditional macroemulsion, which is kinetically stabilized only by the external mechanical energy supply, nano-domains in the microemulsions are formed spontaneously. Their size depends on the microemulsion composition, temperature and elastic properties of the separating film of surfactant. In particular, in the case of water-oil microemulsions with spherical nanosized micelles of water dispersed in oil, water droplets can be used as nanoreactors and templates for the solid nanoparticles fabrication. Since the reaction is initiated by the spatially restricted water and micelle, heterogeneous nucleation and crystal growth can be controlled. 

Chains attached to colloidal surfaces provide powerful forces for stabilization. Colloidal particles that normally coagulate from a solvent dispersion can thus be stabilized by adding a small amount of polymer to the dispersion. Such polymer additives are sometimes known as protective colloids, leading to steric stabilization. Both synthetic polymers and biopolymers such as proteins and gelatin are commonly used in both nonpolar and polar solvents. Industrially they are used in paints, toners, emulsions, suspensions, cosmetics, pharmaceuticals, processed foods, and lubricants. 

Stable dispersion of cellulose microcrystals in nonpolar solvents by using surfactants as stabilizing agents [157]... 

In addition to solvation free energies, Gao has studied the effects of solvent on the conformational equilibria of different molecules and on the free energy profiles for various reactions, including nucleophilic substitution and pericyclic reactions. Another area he has studied is the effect of solvent on solute molecules in their excited states and, in particular, the shifts that are observed in their spectra in different solvents. The calculations showed that, whereas the blue shifts of acetone arising from stabilization of the ground state in polar solvents could be successfully reproduced, the hybrid potentials could not account for the red shifts which arise in some nonpolar solvents. This is a limitation of the model as the red shifts are caused by changes in the dispersion interactions between the two states and the solvent. ... 

Steric stabilization of dispersions is very important in many industrial applications. In particular in nonpolar solvents, the adsorption or grafting of a polymer onto the stuface of particles is the only effective way to establish dispersion stability and prevent flocculation caused by the attractive van der Waals forces because electrostatic interactions are virtually absent in nonpolar solvents. 

Blau and co-workers prepared a CNT-polymer hybrid by suspended SWNTs in organic solvents poly (p-phenylenevinylene-co-2,5-dioctyloxy-m-phenylenevinylene) to wrap the copolymer around the CNTs. The electrical properties of these hybrids were improved relative to those of the individual components. A noncovalent method has been used to functionalize SWNTs by encapsulating SWNTs within cross-linked and amphiphilic poly(styrene)-blocfe-poly(actylic add) copolymer micelles (Figure 19). This encapsulation significantly enhanced the dispersion of SWNTs in a wide variety of polar and nonpolar solvents and polymer matrices because the copolymer shell was permanently fixed. Thus, encapsulated SWNTs may be stabilized with respect to typical polymer processing and recovery from the polymer matrix. 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

Figure 7.20 shows the settling rates of alumina suspensions as a function of surfactant adsorption. It can be observed that the suspensions are stabilized by the adsorption of the anionic, cationic, and nonionic surfactants (although to different extent) and the maximum stability corresponds to the onset of the plateau in the adsorption isotherm. The adsorption, as discussed earlier, takes place with the polar group of the surfactant interacting with the surface hydroxyls and the hydrocarbon tails of the amphipathic surfactant molecules sticking out into the solvent phase. Thus, the stability produced by surfactant adsorption can be attributed to a hydrophobic modification of the surface that disperses well in the nonpolar liquids. 

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