Polar solvents, dispersion stability

It can be observed that with an increase in solvent polarity the dispersion stability displays a maximum, which corresponds to a minimum in the normalized setthng rate the normalization is done to account for differences in the density and the viscosity of the solvents. The normalized setthng rate equals the observed settling rate times the solvent viscosity/(particle density minus the solvent density). The maximum stabihty in this case is observed in moderately polar solvents (20 < e < 45). Bare particles suspended in a hquid medium are in constant Brownian motion and can flocculate rapidly on colhsion if the 1 is larger than about 15 kT. Stabilization can usually be achieved by decreasing the van der Waals attractive forces. The potential energy due to the van... 

We will see in Chapter 11 that compounds other than alkyl halides can undergo SnI reactions. As long as the compound undergoing an SnI reaction is neutral, increasing the polarity of the solvent will increase the rate of the SnI reaction because the polar solvent win stabilize the dispersed charges on the transition state more than it will stabilize the relatively neutral reactant (Figure 9.10). 

The zeta potential is more important in aqueous (and other very polar solvent) dispersions, where electric forces (and stabilization) are important. The zeta potential is of less importance for non-aqueous (organic) dispersions, where steric stabilization is often a more effective stabilization method. 

Monosized polystyrene particles in the size range of 2-10 /am have been obtained by dispersion polymerization of styrene in polar solvents such as ethyl alcohol or mixtures of alcohol with water in the presence of a suitable steric stabilizer (59-62). Dispersion polymerization may be looked upon as a special type of precipitation polymerization and was originally meant to be an alternative to emulsion polymerization. The components of a dispersion polymerization include monomers, initiator, steric stabilizer, and the dispersion medium... 

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

This model was used in dispersion polymerization to predict the size of polymer particles stabilized through grafting on hydrophilic polymers such as PVPo. It provides a reasonable description of, for example, PVPo-stabilized polymerization of styrene in polar solvents. The present model does not apply to other types of dispersion polymerization where grafted comb or block copolymer stabilizers are active. The key controlling parameters in this model are the availability of graft and the minimum and maximum coverage, Qmin and Qmax. 

In general, both and k will decrease with increasing solvent polarity. The transition-state theory of chemical reactions suggests that this is because the initial state (monomer plus ion or ion pair) is more polar than the activated complex in which the monomer is associated with the cation, and the charge is dispersed over a larger volume. More polar solvents will tend, then, to stabilize the initial slate at the expense of the transition complex and reduce k and kp. 

It has been found that for pigments in solvents, a high dielectric constant leads to a more dispersed system. In general, polar liquids disperse polar solids and non-polar liquids disperse non-polar solids. For polar solids suspended in non-polar liquids, it is possible to use the difference in polarity to anchor a stabilizing molecule to the powder surface. The effectiveness is characterized by the heat of wetting, which can be determined by calorimetry. 

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. 

This is a very fast reaction that, under some conditions, can even exceed rates of H abstraction (309). Cyclization of LO to epoxides is the dominant reaction in aprotic solvents (including neat lipids), when lipids are at low concentration (275) or highly dispersed on a surface (315, 316), at room temperature (147, 308, 317), and at low oxygen pressures (275, 278) and the reaction accelerates with increasing polarity of the aprotic solvent (308-310). However, the stability of LO is reduced considerably in polar solvents (309, 310). Although epoxyallylic radicals from cyclization have been observed in pulse radiolysis studies of LO in aqueous solutions (308), H abstraction and scission reactions are much faster. This pattern can be seen in the change of cyclic products yields when oxidation was conducted in different solvents (Table 8). The change in competition over time is also apparent. 

In propagation, where the active centre is a fully developed free ion or ion pair, as opposed to a covalent species, the tremsition state will usually involve some degree of charge dispersion. Solvents of high polarity will not stabilize the transition state as much as the initial state. Such solvents, therefore, increase the activation barrier and reduce the rate of propagation. 

The effect of the solvent on the abundance of the conformers of 2-meth-oxyoxane is demonstrated in Table XIV, where molar fractions of the axial form are compared with available experimental data already shown in Table VI. For a number of solvents, the agreement is remarkably good. Although the results indicate a decreased abundance of the a form of 2-methoxyoxane with increase in the dielectric constant of the solvent, the dependence is not a simple one. The calculations also reproduce such subtle factors as the pronounced effect of chloroform when compared with other solvents of similar polarity and, conversely, a relatively weak effect of dimethyl sulfoxide in comparison to less-polar solvents. The analysis of the role of individual solvation energy terms in the total energy suggests that the conformationally most important term is the contribution of electrostatic interactions that stabilize the ap conformations. Conversely, the dispersion term shows only a shght conformational dependence. 

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. 

Micellar dispersions, which contain micelles along with individual surfactant molecules, are the typical examples of lyophilic colloidal systems. Micelles are the associates of surfactant molecules with the degree of association, represented by aggregation number, i.e. the number of molecules in associate, of 20 to 100 and even more [1,13,14]. When such micelles are formed in a polar solvent (e.g. water), the hydrocarbon chains of surfactant molecules combine into a compact hydrocarbon core, while the hydrated polar groups facing aqueous phase make the hydrophilic shell. Due to the hydrophilic nature of the outer shell that screens hydrocarbon core from contact with water, the surface tension at the micelle - dispersion medium interface is lowered to the values othermodynamic stability of micellar systems with respect to macroscopic surfactant phases. 

Microemulsion droplets can be considered as akin to micelles, in that there is a lipid-like region with polar or ionic groups in contact with water. There seems to be no real difference between w/o microemulsions and reverse micelles in apolar solvents. In both systems the interior is water surrounded by surfactant and organic solvent, and the aggregates behave as aqueous dispersions stabilized by surfactant and cosurfactant. 

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