Solubility solute-solvent interactions

Solubility limits depend on the stabilization generated by solute-solvent interactions balanced against the destabilization that occurs when solvent-solvent interactions are dismpted by solute. Thus, we must examine intermolecular interactions involving water and alcohol molecules. 

In general, solubility depends on the relative magnitudes of three pairs of interactions, namely solute-solute, solvent-solvent and solute-solvent (Robb, 1983). For a substance to be soluble in a given liquid, the solute-solvent interactions must be greater than or equal to the other two interactions. 

The qualitative discussion of solubility has focussed so far on the attractive forces in solute-solvent interactions. However, where water is concerned, it is also important to consider the forces of repulsion due to the so-called hydrophobic interactions that may arise in certain cases (Franks, 1975). These hydrophobic interactions may be explained in terms of thermodynamic concepts. 

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

Supercritical solutions are characterized by very low solvent densities. As a result, they possess the interesting feature that solubility is determined more by solute-solute than solute-solvent interactions. Thus we were able to express the solubilities of naphthalene and a series of indole derivatives in four different supercritical solvents (C2H4, C2H6, C02 and the highly polar CHF3) in the same functional format, only the numerical coefficients varying from one to another.57 Solute-solvent interactions do occur,58 but solubility can be represented quite... 

The parameters a and p indicate the capacity of a solvent to donate or accept a hydrogen bond from a solute, i.e., the solvent s hydrogen bond acidity or basicity. % is intended to reflect van der Waals-type solute-solvent interactions (dipolar, dispersion, exchange-repulsion, etc.). Equation (43) was subsequently expanded to include a term representing the need to create a cavity for the solute (and thus to interrupt solvent-solvent interactions).188 For this purpose was used the Hildebrand solubility parameter, 5, which is defined as the square root of the solvent s energy of vaporization per unit volume.189 Thus Eq. (43) becomes,190... 

This model assumes the absence of specific solute-solvent interactions and is based upon a linear relationship between the free energy of solution and solute surface area. It assumes that the overall solubility is simply the sum of the solubilities in the individual solvent components. This model treats the cosolvent and the water as distinct entities and neglects any interaction between them [19,145,226,253,261]. 

This stoichiometric solubility product, or K, incorporates specific and nonspecific effects resulting from solute-solute and solute-solvent interactions. As noted in... 

BIOMINERALIZATION SOLUBILITY PRODUCT Solubility product constant, BIOMINERALIZATION Solute-solvent interactions,... 

Even though Hildebrand theory should not apply to solvent systems having considerable solvent-solvent or solute-solvent interactions, the solubility of compounds in co-solvent systems have been found to correlate with the Hildebrand parameter and dielectric constant of the solvent mixture. Often the solubility exhibits a maximum when plotting the solubility versus either the mixed solvent Hildebrand parameter or the solvent dielectric constant. When comparing different solvent systems of similar solvents, such as a series of alcohols and water, the maximum solubility occurs at approximately the same dielectric constant or Hildebrand parameter. This does not mean that the solubilities exhibit the same maximum solubility. 

Another type of nonideal SEC behavior, which will not be covered in this chapter, is related to the use of mixed mobile phases (multiple solvents). Because solute-solvent interactions play a critical role in controlling the hydrodynamic volume of a macromolecule, the use of mixed mobile phases may lead to deviations from ideal behavior. Depending on the solubility parameter differences of the solvents and the solubility parameter of the packing, the mobile phase composition within the pores of the packing may be different from that in the interstitial volume. As a result, the hydrodynamic volume of the polymer may change when it enters the packing leading to unexpected elution results. Preferential solvation of the polymer in mixed solvent systems may also lead to deviations from ideal behavior (11). 

The effectiveness of the method is most probably based on the fact that alkyl hypochlorite is formed at the oil/water interface where the cosurfactant alcohol resides. The oxidation that follows takes place either inside or on the surface of oil droplet. The rate of the reaction can result from a large hydrocarbon/water contact area permitting interaction between oil-soluble sulfide with interfacial cosurfactant that served as an intermediary. An extension ofthis procedure to mustard deactivation has also been proposed [20b]. Such systems could be also applied to the degradation of several environmentally contaminating materials The formation of microemulsions, micelles and vesicles is promoted by unfavourable interactions at the end sections of simple bilayer membranes. There is no simple theory of solute-solvent interactions. However, the formation of... 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

The effects of solute-solvent interactions play a greater role in the chromatography of polymers than in conventional TLC. Normally, solubility plays a very small role in the TLC behavior of monomeric compounds. For polymers, however, the effects of solute-solvent interaction are critical. Solvents with 8 s which match that of the polymer are good thermodynamic solvents, and should displace the polymer during development. 

The solvophobic model of liquid-phase nonideality takes into account solute—solvent interactions on the molecular level. In this view, all dissolved molecules expose microsurface area to the surrounding solvent and are acted on by the so-called solvophobic forces (41). These forces, which involve both enthalpy and entropy effects, are described generally by a branch of solution thermodynamics known as solvophobic theory. This general solution interaction approach takes into account the effect of the solvent on partitioning by considering two hypothetical steps. First, cavities in the solvent must be created to contain the partitioned species. Second, the partitioned species is placed in the cavities, where interactions can occur with the surrounding solvent. The idea of solvophobic forces has been used to estimate such diverse physical properties as absorbability, Henry s constant, and aqueous solubility (41—44). A principal drawback is calculational complexity and difficulty of finding values for the model input parameters. 

TheD term accounts for part of the effects of solution enthalpy. Enthalpy of mixing results when the solute-solvent interaction force is different from the solute-solute and the solvent-solvent interactions. Intermolecularforces can be further characterized as dispersion, dipolar, and hydrogen-bond forces. In the mobile order solubility approach, dispersion and dipolar forces were not separated. The effects of these two forces on solubility were expressed in terms of modiLed solubility parameters, S andSj. The relationship between solubility and solubility parameters can be derived in the... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

To explain the missing term, Kamlet et al. (1986) hypothesized that the solute-solute and the solute-solvent interactions are not properly sorted out bet 0ad 7r. To explain the insigniLcant relationship between and solubility, Kamlet and coworkers reasoned that alkanol solutes act only as hydrogen-bond acceptors in water. 

On the basis of excess Gibb s energy approach of VNfohl [56], Williams and Amidon [57-59] predicted solubilities in aqueous cosolvent systems. This approach is based on expressing the solubility of a compound in a binary solvent system as the sum of the solubilities of the compound in each pure solvent plus any interaction terms resulting from solvent-solvent to solvent-solute interactions. This approach predicted solubilities fairly well, but included some simplifying assumptions about solute-solvent interactions that may not be applicable to all systems. 

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