Dielectric Properties of Binary Solvent Mixtures
Authors: Nambu Noritoshi 
Published: December 2020

Binary solvent mixtures exist as liquids over a wide range of temperatures and pressures. For the rational use of the solvent mixtures it is essential to know their physical properties as functions of their composition and the ambience. The excess functions of physical constants (X(sln)E) can be represented in the polynomial form of the molar fraction (x
2) of the cosolvent by the empirical Redlich-Kister equation.

The equation clearly satisfies the condition that X(sln)E is zero at x
2 = 0 and at x
2 = 1. The physical constants of solutions include relative static permittivity, refractive index, viscosity, molar volume, vaper pressure, etc. The relative permittivity reflects the ease with which dielectric polarization proceeds. The relative permittivity is a measure of the solvent’s ability to insulate opposite charges from each other and has a very significant effect on the strength of the interactions between ions especially in dilute solutions. Electrostatic attractions and repulsions between ions are smaller in solvents with higher relative permittivities.

We investigated the relative permittivities (ε
r(sln)) and excess relative permittivities (ε
r(sln)E) of binary mixtures of ethylene carbonate (EC) and a linear carbonate as a function of the composition at 25 ºC. The symbol x
2 denotes the molar fraction of the low-viscosity solvent: 2-fluoroethyl methyl carbonate (2FEMC), ethyl methyl carbonate (EMC), fluoromethyl methyl carbonate (FMMC), or dimethyl carbonate (DMC). EC is a cyclic carbonate, whereas 2FEMC, EMC, FMMC and DMC are linear carbonates. EC and EMC are commonly used as high-polarity and low-viscosity solvents, respectively, for lithium-ion batteries (LIB). Therefore, the relative permittivities of the binary mixtures decrease with an increase in x
2.

a
0 is a dominant factor governing ε
r(sln)E. a
0 may reflect the difference in the cooperative orientation polarization between the same and different types of dipoles. a
1 and a
2 may represent the difference in the contribution of the two components to the cooperative orientation polarization.

The equations of the composition dependence of ε
r(sln) and ε
r(sln)E fitted experimental data. The ε
r(sln) of the EC−2FEMC binary mixture was higher than that of the EC−EMC counterpart. The value of a
0 found in the EC−2FEMC binary solvent system (−49) was less negative than that in the EC−EMC binary solvent system (−51). These findings suggest that the amount of the cooperative orientation polarization of the EC−2FEMC binary mixture is larger than that found for the EC−EMC counterpart. The attraction between EC and 2FEMC molecules can be based on nonconventional weak intermolecular hydrogen bonding (CF−H···O or C−H···F−C). The weak hydrogen-bonding system does not exchange its proton and therefore it is no more a genuine hydrogen bond; it is an electrostatic attraction between positive charge on the hydrogen and negative charge on the organic fluorine or the organic oxygen. The characterization holds also for the EC−FMMC binary solvent system. The value of a
0 found in the EC−FMMC binary solvent system (−32) was also less negative than that in the EC−DMC binary solvent system (−41).

The discussion of the composition dependence of ε
r(sln) and ε
r(sln)E is analogous to a positive deviation from Raoult's law. The positive deviation is characterized by vapor pressures that are higher than those calculated for an ideal solution. The ideal solution would show straight lines in a vapor pressure vs. composition diagram. The escaping tendencies of the components in the solution are accordingly higher than the escaping tendencies in the individual pure liquids. The effect has been ascribed to cohesive forces between unlike components that are smaller than those within the pure liquids, resulting in a trend away from complete miscibility. If the two liquids constituting the mixture are both nonpolar or if both are moderately polar, the positive deviations from ideal behavior are not large. On the other hand, if one component is slightly polar while the other is highly polar, considerable positive deviations may occur.