Physical and Electrolytic Properties of Dialkoxyethanes
Authors: Nambu Noritoshi 
Published: December 2020

1,2-Dimethoxyethane (DME, ethylene glycol dimethyl ether) and 1,2-diethoxyethane (DEE, ethylene glycol diethyl ether) are dialkyl ethers of ethylene glycol. DME shows a high boiling point as compared to diethyl ether and tetrahydrofuran. DME is also used as a reagent in organometallic chemistry and as a solvent in some electrolyte solutions for lithium batteries. The solubility of lithium salts in DEE is much less than that in DME. Partial fluorination of DEE can improve the solubility of lithium salts. In contrast, the polyfluorinated and perfluorinated organic solvents show very low polarity. These solvents are the so-called fluorous media and are often immiscible with organic solvents as well as water. We report the physical and electrochemical properties of fluorinated DEEs: monofluorinated, trifluorinated, and tetrafluorinated DEEs (EFEE, ETFEE, and FETFEE, respectively).

The relative permittivity reflects the ease of progress of dielectric polarization. The relative permittivities of FETFEE, EFEE, and EFTEE decreased linearly with an increase in temperature. The relative permittivities of FETFEE, EFEE, and EFTEE decreased linearly with an increase in temperature. The relative permittivity of FETFEE was higher than those of ETFEE and EFEE: FETFEE > ETFEE ≈ EFEE > DEE. The viscosity is the property which opposes the relative motion of the adjacent layers of the liquid. It is regarded as the internal friction and affects electrolytic conductivity. The viscosity (η) of FETFEE was also higher than those of EFEE and ETFEE: FETFEE > EFEE > ETFEE > DEE. The kinematic viscositsiy (v) of ETFEE was as low as that of DEE above 323 K. The plots of log10(η/mPas) vs. T
−1 and the plots of log10(v
/ 10−2 m2 s−1) vs. T
−1 gave straight lines.

The conductivity of an electrolyte solution is a key factor determining the internal resistance and rate performance of lithium batteries. The conductivities (κ) of electrolyte solutions in ETFEE and FETFEE increased with an increase in temperature (T). The viscosities of the fluorinated DEEs decreased rapidly with an increase in temperature and approached that of DEE at high temperatures. Interestingly, the conductivity of LiPF6 solution in FETFEE was higher than that in ETFEE: EFEE > FETFEE > ETFEE. The partial fluorination can increase the ability of the solvent molecules to form hydrogen bonds. The number of hydrogen atoms that are bound to the same terminal carbon atom as fluorine atoms are as given below: FETFEE (2), EFEE (2), ETFEE (0), and DEE (0). The attraction between FETFEE 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 hydrogen bonding leads to the high acceptability of an electron-pair of a donor atom from a solute. This effect may result in the increased solvation of PF6
− ions and, consequently, in the higher degree of ionic dissociation. The fluoromethyl group (CH2F-) may serve as an anion-attracting group. In contrast, the polyfluorination can decrease the electron-pair donability of an oxygen atom of the C-O-C moiety. The trifluoromethyl group (CF3-) is a strong electron-withdrawing substituent. The anodic stability of FETFEE was higher than that of EFEE: FETFEE > ETFEE > EFEE. The finding suggests that the electron-pair donability decreases in the inverse order: FETFEE < ETFEE < EFEE. Fluorine is the most electronegative of all the elements. Most conventional measures for the electronic effect of substituents are provided as Taft constants (σ*) for substituents attached to aliphatic chains. The Taft constants of a trifluorometheyl group (CF3-: Taft σ* = 2.61, σ(calcd.) = 2.60) and a fluorometheyl group (CH2F-: Taft σ* = 1.10, σ(calcd.) = 1.17) are substantially larger than that of a metheyl group (CH3-: Taft σ* = 0, σ(calcd.) = −0.07). The value of 1.10 is comparable with a formyl group (-CHO: Taft σ* = 1.1, σ(calcd.) = 1.09).

The product of conductivity and viscosity (κη) observed for FETFEE, ETFEE, and EFEE was slightly decreasing at high temperatures. The plots of log10(η/mPas) vs. T
−1 gave straight lines. In contrast, the plots of log10(κ/mScm−1) vs. T
−1 displayed upward curvature. The decrease in the κη and the convex curve show that the conductivities of LiPF6 solutions in FETFEE, ETFEE, and EFEE are not inversely proportional to the viscosities at elevated temperatures.