Cosolvency solvent polarity

A subsequent picosecond electronic absorption spectroscopic study of TPE excited with 266- or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously. However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S lp becomes shorter as the polarity is increased this was taken to be evidence of the zwitterionic, polar nature of TPE S lp and the stabilization of S lp relative to what is considered to be a nonpolar Sop, namely, the transition state structure for the thermal cis-trans isomerization. Although perhaps counterinmitive to the role of a solvent in the stabilization of a polar species, the decrease in the S lp lifetime with an increase in solvent polarity is understood in terms of internal conversion from to So, which should increase in rate as the S -So energy gap decreases with increasing solvent polarity. Along with the solvent-dependent hfetime of S lp, it was noted that the TPE 5ip absorption band near 425 nm is located where the two subchromophores— the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic 5ip should be expected to absorb hght. A picosecond transient absorption study on TPE in supercritical fluids with cosolvents provided additional evidence for charge separation in 5ip. 

In most, if not all, of the transition metal-catalyzed organometallic reactions including the Pd- or Ni-catalyzed cross-coupling, solvents are not used merely to dissolve and dilute reactants and reagents. They often serve as cocatalysts or promoters and even reactants in a limited number of cases. More than 20 solvents have been used for the Pd- or Ni-catalyzed cross-coupling (Table 2). In the absence of any specific information, it is not unreasonable to choose first THF. Frequently, it is desirable to use one or more cosolvents typically for an increased level of solvent polarity and/or electron-donating ability. One of the most frequently used solvents for this purpose is DMF, but some others, such as NMP, pyridine and NMI (A-methylimidazole), have also been used frequently. In some extreme cases,... 

With respect to selecting cosolvents, one should consider drug polarity and solvent polarity. Usually the solvents include glycerol, propylene glycol, and ethanol. Other solubilization techniques such as complexation and surfactants can also be used to enhance the solubility of the drug [36]. However, the solubilization techniques used in preclinical testing may not be same as the final formulation used in clinical studies and marketing. 

It should be mentioned that an equation analogous to Eq. (7-30a) has been successfully applied to salt effects on reaction rates arising from variations in solvent polarity on the addition of electrolytes (ionophores) [197] cf. also Eq. (5-99) in Section 5.4.5. For electrolyte solutions, the added salt can be treated as a more polar cosolvent [197]. 

Alternatively, Rubino and Yalkowsky found that a was a linear function of cosolvent polarity for a given solute. This is illustrated in Fig. 3 for the three lipophilic compounds phenytoin, diazepam, and benzocaine. Thus, knowledge of the solubility of a given drug in water and at least two cosolvents would permit cr to be estimated for other cosolvents by interpolation using an index of the desired cosolvent polarity. These studies permit the use of Eq. (4) as a means to rationally choose or eliminate solvents for formulation studies based on limited experimental solubility data and commonly obtained indexes of solute and solvent polarity. 

Eqs. (11)-(13) illustrate the concept that acceleration or deceleration of a reaction rate upon a change in solvent polarity depends on the charge of the reactants. For example, if 8i is water and E2 is an ethanol-water mixture, two oppositely charged ions will demonstrate a reduced stability in the ethanol-water system, whereas two similarly charged reactants will demonstrate an enhanced stability. This can be understood by realizing that water provides greater insulation to ions and polar species as compared to a cosolvent-water mixture. Thus, addition of cosolvents... 

Decreasing the solvent polarity is an effective strategy to decrease the amount of MgX-LG exchange and reduction reactions. Two potential explanations are a decrease in the solubility of MgX2-THF , or a shift in the Schlenk equilibrium (see Chapter 13). Subsequently, there are no Lewis acid sites in solution that could catalyze these competing reactions. For example the addition of toluene as a cosolvent can take a reaction that predominately undergoes reduction [80%, see Eq. (50)] to yield the nucleophilic substitution product [75% Eq. (61) 151]. 

The RESS process relies on the solvent properties of carbon dioxide. Because CO2 is a nonpolar molecule, this process will be mainly efficient and interesting for micronizing nonpolar molecules. For this reason, a preliminary study on the solubility of the compounds with pressure and temperature is necessary. As usual, the solvent polarity can be modified and enhanced by adding to the supercritical CO2, small quantities of an organic cosolvent. This is primarily because the solvent power of an SCF is strongly dependent on its density, which can be adjusted by small variations of pressure and temperature (11). 

Efforts have been made to develop hydrocarbon systems for CO2, as they could present significant advantages over high-cost fluorocarbon or siloxane counterparts. Recent advances are covered in section 3 of this article. Solubility of hydrocarbon materials in CO2 may be achieved by the addition of a polar cosolvent to CO2 to improve solvent polarity. For instance, AOT was shown by Ihara et ah to be completely soluble in CO2 with edianol as a co-solvent (59). Along similar lines, Hutton et aL in 1999, formed w/c microemulsions with 0.03 M AOT and 15 mol % ethanol or 10 mol % pentanol (60,61). 

In recent years, the extrapolation procedure developed separately by Yasuda and Shedlovsky has been reported [20,21,99] to provide pXa values from partially aqueous solutions that can sometimes closely approximate the values found in purely aqueous solutions. The Yasuda-Shedlovsky equation [Eq. (12)] can be used to correlate apparent pKg values in different solvent polarities (psl a) to approximate the aqueous pK value by extrapolation as a reciprocal function of the dielectric constant (e) of each of the cosolvent mixtures ... 

Solute/Cosolvent/Solvent Systems. Solubilities of solids may be modified by adding a small concentration of a nonpolar hydrocarbon (e.g. propane, octane) or a polar molecule (e.g. acetone, methanol). CO2 has a small polarizability and no dipole moment, so additives increase the polarizability of the solvent (i.e. the refractive index of Equation 7) and the dielectric constant (Equation 3). Polar cosolvent molecules also interact with functional groups on the solutes. Cosolvents may increase solubilities up to an order of magnitude although the enhancement is dependent on cosolvent concentration. Unfortunately, relatively few fundamental cosolvent studies are published at this time. 

Rates of aquation of [Co(NH3)5Br], and of its chromium(III) analog, have been determined in methanol-, ethanol-, isopropanol-, and r-butyl-alco-hol-water mixtures at various temperatures. The well-known compensation effect is apparent for all these cosolvents. In the aqueous methanol and aqueous ethanol series of solvent mixtures there is correlation of rate constants with solvent ionising power. A correlation of rate constants with solvent polarity was established in a much more limited investigation of aquation of a-cis-[Co(edda)(OH2)Cl], studied in water and in 20% methanol, ethanol, and acetone. The variation of rate constants, and of the activation parameters and A5, with solvent composition was claimed to indicate an mechanism. 

Poly(vinyhdene chloride) also dissolves readily in certain solvent mixtures (82). One component must be a sulfoxide or A/,Al-diaIk5lamide. Effective cosolvents are less polar and have cycHc stmctures. They include aUphatic and aromatic hydrocarbons, ethers, sulfides, and ketones. Acidic or hydrogen-bonding solvents have an opposite effect, rendering the polar aprotic component less effective. Both hydrocarbons and strong hydrogen-bonding solvents are nonsolvents for PVDC. 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

There is increasing interest in copolymer systems, which, due to their chemical heterogeneity, may require very complex eluent systems in order to dissolve the sample and ensure that the separation ensues hy a pure size exclusion mechanism. In these examples, the PLgel is also compatible with eluent systems containing mixed solvents of different polarity (including water as a cosolvent up to 10% hy volume) and in organic solvents modified with acids or bases (e.g., acetic or formic acid, triethanolamine) as it is stable in the pH range of 1-14. 

Moreover, with a change of solvent, a new tautomeric form can arise owing to formation of intermolecular hydrogen bonds in place of the previously existent intramolecular hydrogen bonds. This situation is characteristic, for example, for pyrimidine derivatives 49, for which the use of polar (DMSO, DMF, MeOH, HMPT) solvents or specifically solvating cosolvents (S) (e.g., a small amount of water or A-methylpyrrolidinone) leads to the appearance of ylidene tautomer 49b with the p-quinonoid disposition of the double bonds (Scheme 18) [88KGS521 90UK457]. 

Hydrolysis of substrates is performed in water, buffered aqueous solutions or biphasic mixtures of water and an organic solvent. Hydrolases tolerate low levels of polar organic solvents such as DMSO, DMF, and acetone in aqueous media. These cosolvents help to dissolve hydrophobic substrates. Although most hydrolases require soluble substrates, lipases display weak activity on soluble compounds in aqueous solutions. Their activity markedly increases when the substrate reaches the critical micellar concentration where it forms a second phase. This interfacial activation at the lipid-water interface has been explained by the presence of a... 

Hay et al.w,n have prepared high-molecular-weight a Bisphenol-A-derived poly(formal) (6) using a phase-transfer catalyst in DCM. A Bisphenol-AF-derived poly(formal) (7) is also synthesized by solution polycondensation of Bisphenol AF (1) with DCM in highly polar cosolvents in the presence of potassium hydroyxide (Scheme 3).12 Aprotic polar solvents such as A/W-dimethylformamide... 

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