Water solvent parameters

Table 7 Activation Parameters for the Resin Catalyzed Hydrolysis of Some Esters in Water-Solvent Mixtures...

The micellar surface appears to be less polar than water, based largely on shifts in fluorescence or charge-transfer spectra (Section 1). Although it may not be reasonable to apply bulk solvent parameters such as Z or dielectric constant to submicroscopic species such as micelles, the spectral and kinetic evidence are self-consistent. An additional point is that these reactions have... 

The Aspen NRTL-SAC solvent database was identified from the list of solvents presented in the pharmaceutical based International Committee on Harmonization s guidelines for residual solvents in API [28], Hexane, Acetonitrile and Water were selected as the basis for the X, Y and Z segments respectively, the binary interaction parameters for the segments together with molecular descriptors in terms of X,Y and Z segments were then regressed from experimental vapour-liquid and liquid-liquid equilibrium data from the Dechema database. The list of solvent parameters that were used in the case study are given in Table 13. 

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

Figure 6.16 shows a snapshot of the carbon-Nafion-water-solvent (CNWS) blend. The final micro structure was analyzed in terms of density map profiles, RDFs, pore size distributions, and pore shapes. The interaction parameters of the carbon particles were selected to mimic the properties of VULCAN-type C/Pt particles. 

As an example, consider phenol as the solute and water and toluene as two solvents. The parameters for phenol are A = 5.7, A = -12.9, A = -18.3, and A5 = 0.0091, whereas Aq is unspecified, but a negative quantity. With the solvent parameters from Tables 2.1 and 2.3, the standard Gibbs energy of solvation of phenol in water becomes Aq + 3.39, and in toluene Ao -1- 4.11 kJ mol". It is seen that As i,Gb is lower in water than in toluene, so that the transfer of phenol from water to toluene entails an increase in AjoItGb. The consequence of this is that phenol prefers water over toluene, since work would be required to make this transfer. It should be remembered that the standard Gibbs energies of solvation refer to the state of infinite dilution of the solute (solute-solute... 

Solvents can cause enzyme inactivation (decrease the number of active enzyme molecules). The exact mechanisms are not so well known, but it is clear that solvent polarity plays an important role. Several solvent parameters have been used to try to rationalise the influence of solvents on enzymes. The parameter which has been used most for this purpose is the log P value, which is defined as the logarithm of the partition coefficient of a substance in the standard 1-octanol/water two-phase system (Table 9.4). Log P values can be determined experimentally by measuring the partitioning of the solvent between octanol and water. Alternatively, log P values... 

The distribution ratios obtained were compared to the corresponding distribution ratios between water and Toctanol. The authors also calculated solvent parameters of [C4Cilm][PFg] (Section 9.3). It was shown that phenolate-ion associates with [C4CiIm][PFg] more strongly than other ions. The authors also mention the possibility of extraction of amino acids into [C4CiIm][PPg] in the presence of crown ether dibenzo-18-crown-6, though at rather moderate efficiency. 

Solvent effects on enzymatic reactions have been most thoroughly studied for esterification reactions. It has been observed that those reactions are favorably carried out in relatively hydrophobic solvents, while the equilibrium position is less favorable for esterification in more hydrophilic solvents. Correlations between equilibrium constants and solvent parameters have been evaluated. It was shown that the solubility of water in the solvent (Sw/0) gave better correlation with esterification equilibrium constants than log P and other simple solvent descriptors [61]. 

Since X depends on the mobility of the ion, it is a function of the ion type and of the solution parameters (solvent, solute concentration, temperature, etc.). At very low solute concentrations, X reaches a steady limiting value, A,0, that is often tabulated for the common ions in water solvent at specific temperatures. A table of A,0 values is given in Table 8.1. The A,0 values from such a table can be used to estimate the conductivity of a completely dissociated salt solution by application in Equation 8.10. For finite salt concentrations, the calculated values are indicative, but not exact. Onsager, Fuoss, and others [2,3] have developed equations by which quite accurate values of A, can be calculated from A,0 values under certain circumstances. 

Computational studies showed that the nature of the reactive species in the oxidation of trimethylamine, iodide ion, and dimethyl sulfide with lumiflavin is a C4 a-hydroperoxide complexed with water. The other two species, C4 a-hydroperoxide and C4 a-peroxide, yielded higher activation energies.237 Kinetic and spectroscopic studies on the effect of basic solvents, ethers, esters, and amides, on the oxidation of thianthrene-5-oxide with substituted peroxybenzoic acids indicated the involvement of the basic solvent in the transition state of the reactions. A solvent parameter, Xtc, based on the ratio of the trans to the cis form of thianthrene-5,10-dioxide, has been introduced.238... 

Fig. 10. CMC versus water content of apolar solvent. Parameter [H20]/[surfactant] = water initially attached to the surfactant.

In addition to test kits used in EPA-approved screening methods, a variety of other test kits are available from several manufacturers, for example, immunoassay test kits for BTEX in soil and water and for chlorinated solvents in water colorimetric kits for the detection of lead kits for a wide range of water quality parameter manufactured by Hach Company. 

Perhaps the most spectacular success of explanations based on solvation of ground states, published to date, is the dissection of activation parameters for solvolysis of t-butyl chloride in mixtures of ethanol and water, first discussed by Winstein and Fainberg (1957). The complex variation of AH and AS (Fig. 21) has been shown to be due almost entirely to ground state solvation effects, at least for the solvents ethanol—40% ethanol/water studied by Arnett et al. (1965). For 90%, 80%, 70%, 60%, 50% and 40% ethanol/water the parameter AH1 for solvation of the transition state (by transfer from the gas phase) was calculated to be linearly proportional to the corresponding value of AS, as expected from the behaviour of simple salts. The point for pure ethanol did not fall on the calculated line, and this was attributed to nucleophilic solvent assistance. The variation in AG, AH and AS (Fig. 21) can be reproduced remarkably well using ethane and the zwitterionic a-amino acid, glycine, as model compounds (Abraham et al., 1975 see also Abraham, 1974 Abraham and Abraham, 1974). 

The computational details for the QM/MM simulation were almost common to those for the simulation described in Section 17.5.1. The major change was made in the setup of the real-space grids of the QM cell. The number of grid points for each axis was increased to 64 from 32 where the grid spacing h was set at h = 0.152 A. The thermodynamic condition of the MM water solvent was set at T = 300 K, p = 1.0g/cm3. The LJ parameters in AMBER95 force field were employed... 

For the same large set of compounds used in deriving Eq. [21], the gas-water distribution coefficient, (unitless), for a set of 423 compounds at 298 K was correlated with the set of TLSER descriptors resulting in Eq. [27]. (The subscript 2 refers to solvent parameters.)... 

This kind of procedure, i. e. empirical estimation of solvent polarity with the aid of actual chemical or physical reference processes, is very common in chemistry. The well-known Hammett equation for the calculation of substituent effects on reaction rates and chemical equilibria, was introduced in 1937 by Hammett using the ionization of meta-ox /iflra-substituted benzoic acids in water at 25 °C as a reference process in much the same way [10]. Usually, the functional relationships between substituent or solvent parameters and various substituent- or solvent-dependent processes take the form of a linear Gibbs energy relationship, frequently still referred to as a linear free-energy (LFE) relationship [11-15, 125-127]. 

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