Evaluation of the Polarity in Binary Liquid Mixtures as a Function of Volume Fraction

Abstract

The variation of solvatochromic polarity parameters as a function of mole fraction data may not represent the realistic environment of the solvatochromic probe molecule. This is because of the fact that the solvation environment of the solvatochromic probe molecules is not only determined by the number of individual solvent molecules around it but also by the size of those individual solvent molecules. In this regard we wish to understand the variation of ET N polarity parameter of binary liquid mixtures as a fuction of volume fractions. Representation of the ET N parameter as a function of volume fraction data leads to a completely different dataset for binary liquid mixtures where water is used as one of the solvents. These mixtures usually display negative deviations from linearity for the ET N values, when ET N values are plotted as a function of mole fraction values of the solvents. On the other hand, if the ET N values are plotted as a function of volume fraction values of the solvents in binary liquid mixtures containing water, these deviations are minimized to a large extent and even reversed to the positive deviations in some of these mixtures. This study gives an insight into a long lasting problem of understanding the negative deviations of the ET N values from linearity observed in binary liquid mixtures of water, as these deviations are either minimized or reversed into positive deviations when ET N values are plotted versus volume fraction of solvents. This study opens up a whole new discussion about the solvatochromic data of the solvent mixtures, whether this data must be represented as a function of mole fraction values or as a function of volume fraction of the solvents. The representation of the solvatochromic data as a function of volume fraction of solvents might lead to new insights in studying the solute-solvent interactions in binary solvent mixtures.

Polarity of Organic Solvent/Water Mixtures Measured with Reichardt’s B30 and Related Solvatochromic Probes—A Critical Review

“…We are therefore in full agreement with the conclusions of [133], that the volume fraction gives better results in terms of linear correlation. For the correlation with E T (30), however, it makes no qualitative difference whether the mass or the volume fraction is used to determine N av,z .…”

“…Bentley [27] has shown that the volume fraction correlates better linearly with the static dielectric constant (ε r ) or the E T (30) values of alcohol/water mixtures than the mole fraction as a composition parameter of alcohol/water mixtures. The volume fraction has also been recommended in a recent publication to explain the E T (30) as a function of solvent composition more accurately than using the mole fraction [133]. Accordingly, for ethanol/water and methanol/water mixtures, the N av,w and N av,v quantities have been calculated and empirically tested as variables for correlation with E T (30) [62].…”

“…For the correlation with E T (30), however, it makes no qualitative difference whether the mass or the volume fraction is used to determine N av,z . Therefore, the motivation for using the volume fraction given in [133] should be reconsidered. Using the mass fraction would give similar results.…”

Solvatochromism in Mixtures of Hydrogen Bond Acceptor (HBA) Solvents with Water

“…However, in the analysis of solvatochromic data, the mole or volume fractions of mixtures are still mainly used as measures of the composition of mixtures. [14][15][16][17] The eqn. (6) gives the average molar concentration of the mixture in terms of a specific fraction z of the mixture.…”

“…It is therefore clear that it is of fundamental importance, since the choice of the measure of the quantitative composition of the mixture can lead to a different physical statement. [14][15][16][17] This important fundamental problem has been circumvented by many authors by using the quantity that best correlates with the model used in an appropriate relationship. Thus, in the past, the discrepancies were ignored and the mole fraction was used as a seemingly logical compromise because it is linear to the Gibbs free energy of mixing.…”