Solubilization Solute effects

How can chemists measure the importance of solvents Solvents solubilize solutes. The general rule for organic or nonelectrolyte inorganic solutes, whether gaseous, liquid, or solid, is that they would show adequate solubilities. Expressions for the estimation of solubilities are presented in chapter 3. The effects on the rate of reactions will be discussed there, and comparisons made among classes of solvents. 

The irradiation of micellar solutions effects the phase behavior and the critical micelle concentration (CMC). Because radiation sterilization of biopharmaceutical products is a common routine it is important to investigate the influence of radiation on surfactants that are widely used in the pharmaceutical industry for formulations as wetting agents, emulsifiers, or solubilizers. In particular, in drug formulations... 

Ohbu et. al studied the effect of the degree of quaternary ammonium substitution of cationic cellulose ethers on thekr binding of sodium dodecyl sulfate (SDS) using equilibrium dialysis, dye solubilization, solution density and NMR spin-lattice relaxation measurements. Structures were proposed for different SDS/polymer compositions. The concentration of polymer they studied was such that, at its nominal CMC, SDS was present in stoichiometric excess over the polymer. 

The shape of the individual molecules in a solvent has a large influence on the solvent s ability to solubilize solutes. For example, molecules with their dipole along the long molecular axis can nicely solubilize an ion because several solvent molecules can approach the ion (Figure 3.2). However, when the dipole is along the short axis, solvation is not very effective because fewer molecules can approach the ion. 

Higuchi s analysis [24] predicts that substantial effects on dissolution rate will only be evident when the drug concentration in solution approaches or exceeds saturation solubility. The dissolution model used by Higuchi assumes that an equilibrium exists between the solid and the solution at the interface and that the rate is controlled by the diffusion of free and solubilized solute across the diffusion layer which has a thickness S. 

The variations in the composition of a biphasic mixture as a function of concentration and temperature have also revealed themselves in complex kinetic and (apparent) Arrhenius parameters. Examples of these effects are found in time-resolved studies of the dynamics of triplet excited state ketones [354, 355, 361, 362] and in kinetic studies of the allyl-p-dimethylamino-benzenesulphonate quaternization discussed above [357, 359, 363]. Prior to the development of a full understanding of the ramifications of phase separation phenomena, such complex behaviour was often misinterpreted and ascribed to the reaction of a homogeneously solubilized solute in the smectic phase [359, 361-363]. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The covalent character of mercury compounds and the corresponding abiUty to complex with various organic compounds explains the unusually wide solubihty characteristics. Mercury compounds are soluble in alcohols, ethyl ether, benzene, and other organic solvents. Moreover, small amounts of chemicals such as amines, ammonia (qv), and ammonium acetate can have a profound solubilizing effect (see COORDINATION COMPOUNDS). The solubihty of mercury and a wide variety of mercury salts and complexes in water and aqueous electrolyte solutions has been well outlined (5). 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Microemulsions or solubilized or transparent systems are very important ia the marketing of cosmetic products to enhance consumer appeal (32,41). As a rule, large quantities of hydrophilic surfactants are required to effect solubilization. Alternatively, a combination of a solvent and a surfactant can provide a practical solution. In modem clear mouthwash preparations, for example, the flavoring oils are solubilized in part by the solvent (alcohol) and in part by the surfactants. The nature of solubilized systems is not clear. Under normal circumstances, microemulsions are stable and form spontaneously. Formation of a microemulsion requires Httle or no agitation. Microemulsions may become cloudy on beating or cooling, but clarity at intermediate temperatures is restored automatically. 

FIG. 18 The effect of chain length on the pseudo rate constants (Km (100 90)) of n-hexadecane solubilization by olefinsulfonates/dobanol 45-3 solutions at 40°C. AOS, a-olefinsulfonate IOS a, aged I-olefinsulfonate IOS d, directly hydrolyzed I-olefin-sulfonate. 

Immiscible solvents like water and oil can be transformed by addition of solubilizers to single-phase solutions. Amphiphilic substances are known as effective solubilizers. Solubilization depends on the HLB of the components that ought to form a single phase and on the kind of solubilizer used. Phosphorus-containing surfactants with their variety of possible molecular structures are solubilizers that can be tailored to the task demanded. 

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