Solute polarity index

A useful guide when using the polarity index is that a change in its value of 2 units corresponds to an approximate tenfold change in a solute s capacity factor. Thus, if k is 22 for the reverse-phase separation of a solute when using a mobile phase of water (P = 10.2), then switching to a 60 40 water-methanol mobile phase (P = 8.2) will decrease k to approximately 2.2. Note that the capacity factor decreases because we are switching from a more polar to a less polar mobile phase in a reverse-phase separation. 

Changing the mobile phase s polarity index, by changing the relative amounts of two solvents, provides a means of changing a solute s capacity factor. Such... 

Solvent strength determines the value, but not the selectivity. The mobile phase can be established by using the polarity index P proposed by Snyder. The highest values of P represent the strongest solute adsorbed in conventional TLC but represent the weakest for the separation in reversed phases. Sometimes aqueous polar mixtures cannot totally wet the chemically bonded layer. For this reason, checking... 

However, not withstanding the above objections, further discussion of the Snyder solvent triangle classification method is justified by its common use in many solvent optimization schemes in liquid chromatography. The polarity index, P, is given by the sum of the logarithms of the polar distribution constants for ethanol, dioxane and nltromethane and the selectivity parameters, X, as the ratio of the polar distribution constant for solute i to... 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

Grathwohl (1990) found a relationship between sorption capacity and the the atomic H/O ratio of NOM. Similarly, there is a good relationship between log Koc and the polarity index (PI) of SOM, defined as the (0+N)/C ratio (DePaolis and Kukkonen 1997 Rutherford et al. 1992 Xing 1997 Xing et al. 1994a). The effect of SOM polarity on sorption of organic compounds is consistent with the well-known theory of solvent polarity on solute solubility. In studying the influence of SOM composition... 

As the logarithm of 1-octanol-water partition coefficient (log P) describes the hydrophobicity of molecules and the retention of solutes in RP-HPLC depends on the hydrophobicity, a strong correlation can be expected between the log V value and the retention of solutes in RP-HPLC. Besides log P, a considerable number of physicochemical parameters have been tested for their capacity to predict retention in RP-HPLC. Thus, Snyder s polarity index, fraction of positively and negatively charged surface area, molecular bulkiness, nonpolar surface area, electron donor and acceptor capacity, various ster-ical parameters, and the energy of highest occupied molecular orbit have all been included in QSRR calculations. 

This polarity index measures the intermolecular attraction between a solute and a solvent, whereas the Hildebrand solubility parameter is defined for pure solvent. For example, ether is not very polar and has a Hildebrand value of 7.4—about the same as hexane, which has a value of 7.3. However, ether can accept protons in the form of hydrogen bonds to its nonbonding electron pairs, and consequently its polarity index is 2.8 compared to 0.1 for hexane. 

Prom the electron paramagnetic resonance (EPR) spectmm of the nitroxide side chain, four primary parameters are obtained 1) solvent accessibility, 2) mobility of the R1 side chain, 3) a polarity index for its immediate environment, and 4) the distance between R1 and another paramagnetic center in the protein. Solvent accessibility of the side chain is determined from the collision frequency of the nitroxide with paramagnetic reagents in solution. The mobility, polarity, and distances are deduced from the EPR spectral line shape. For regular secondary stmc-tures, accessibility, mobility, and polarity are periodic functions of sequence position. The period and the phase of the function reveal the type of secondary stmcture and its orientation within the protein, respectively (71, 74). In the case of membrane proteins, the topography of the secondary stmcture with respect to the membrane surface can also be described (75, 76). 

For the purpose of estimating the solubility of a solute it is necessary to have some measure of the polarity of a solute or a solvent. Based on Eqs. (1) and (2), a useful polarity index should be a measure of a material s intermolecular forces, Cn and C22-Table 1 contains a list of solvents that are typically used in liquid pharmaceutical formulations and three measures of solvent polarity. Each measure of solvent polarity, or polarity index, is based upon a different measure of a material s property. For example, dielectric constant is a measure of the electrical insulating properties of a solvent, solubility parameter is determined from the molar energy of vaporization, and... 

Effect of Solvent Strength on Retention Factors. Solvents that interact strongly with. solutes are often termed strong" solvents. Strong solvents are often, but not always, polar solvents. Solvent strength depends on the nature of the analyte and stationary phase. Several indexes have been developed fur quantitatively describing the polarity of solvents. The most useful of these for partition chromatography is the polarity index P which was developed by Snyder." This parameter is based on. solubility measurements for the substance in question in three solvents dioxane (a low ... 

The CS INDO program [1,2], modified by the incorporation of the solute-solvent interaction as described in section 2.1, was used to calculate molecular geometries, charge distributions and electronic absorption spectra of the merocyanines M1-M3 (Fig. 1) as a function of the solvent polarity index. ... 

The interaction index is somewhat similar to Snyder s polarity index, P, although significant differences are observable with polar compounds (Jandera et al., 1982). The interaction index defines the interactions between the solute and the mobile phase and the model demonstrates that there is a quadratic relationship between the log of the capacity ratio and the volume fraction of organic solvent in the eluent. A consequence of this model is that there is a linear relationship between the corrected log of the capacity ratio log k = (log k — log )/Fx and the interaction index, where is the phase ratio and is the molar volume of the solvent. The retention of a specific solute may therefore be predicted from the interaction index of the solute and specific physical parameters of the solvent. This model has been used to accurately determine the retention behaviour of solutes in both binary and ternary solvent systems (Jandera et al., 1982 Colin et al., 1983a). 

Perfluorinated solvents exhibit extremely low polarities, which can be quantified in many ways. As analyzed elsewhere, one of the best scales in terms of modeling the ability of a solvent to solvate or complex a solute or transition state involves the shift of the absorption maximum of a perfluoroheptyl-substituted dye. This dye was optimized to be soluble in both fluorocarbons and very polar solvents such as DMSO (dimethyl sulfoxide). Over 100 solvents have been assayed, and some of the resulting Ps or Spectral Polarity Index values are given in Table 3.3. 

A second useful treatment is that of Snyder (95), in which solvents are evaluated on the basis of a polarity index calculated from the solvent interaction with three test solutes, dioxane, ethanol, and nitromethane. Figure 16(12) shows an SEC chromatogram of an asphalt for the four solvents indicated. The results show significant decrease in association at 800 A as one goes from tetraline to benzonitrile. Although tetraline has the lowest dielectric constant and benzonitrile the highest, the order is reversed for THF (E = 7.25) and chloroform (E = 4.806). On the basis of Snyder s polarity parameter/ however, the order is THF (P = 4.2) chloroform (P = 4.4), and benzonitrile (P = 4.6), which agrees with the 800 A order. 

In order to develop a quantitative measure of the solvent s relative ability to intermolecularly interact with the solutes as proton acceptors, proton donors, and strong dipoles Snyder established a new semiempirical model [13,14] coupling the solvent s polarity index (P ) with the so-called corrected gas-liquid partition coefficients or solubility constants (fC ) of the selected test solutes ethanol (a model proton donor), dioxane (a model proton acceptor), and nitromethane (a model strong dipole). The main relationship of this approach is... 

To investigate the hydrophilic-hydrophobic balance of basket 40, we monitored its tendency to self-assemble in solution (namely, THF/water) by UV-visible spectroscopy, following a procedure already reported [51]. A set of physical descriptors were developed, namely, the aggregation polarity index (API), the... 

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