Electrotransport and thermal properties of Tetrabutylammonium hydrogen sulfate

Tetrabutylammonium hydrogen sulfate, whose commonly used English abbreviation is TBAHS, is an organic ammonium salt that can serve a variety of functional roles, including as a surfactant, a catalyst, an emulsifier, a disinfectant, a bactericidal agent, and an antistatic agent. In addition to these diverse applications, Tetrabutylammonium hydrogen sulfate has been reported in the literature as an effective solvent for dissolving the ruthenium(II) complex known as fac-[Ru(CO)₂(H₂O)₃(C(O)C₂H₅)][CF₃SO₃]. Furthermore, Tetrabutylammonium hydrogen sulfate has also been employed as a co-catalyst in reaction media used for the production of alternating polyketones and propionic acid, demonstrating its utility beyond simple phase-transfer applications.

Figure1: Picture of Tetrabutylammonium hydrogen sulfate

Physicochemical properties

Under standard ambient conditions, Tetrabutylammonium hydrogen sulfate appears as white crystalline solid. Tetrabutylammonium hydrogen sulfate exhibits a melting point in the range of 169–171 °C and is known to be hygroscopic in nature. Additionally, Tetrabutylammonium hydrogen sulfate is readily soluble in water but is also susceptible to oxidation under certain conditions. When Tetrabutylammonium hydrogen sulfate is allowed to react with strong bases, an equilibrium mixture containing quaternary ammonium hydroxides is generated as a result of the anion exchange process.

Electrotransport and thermal properties

The thermal properties and conductivity of the quaternary ammonium compound Bu₄NHSO₄ were investigated for the first time. Tetrabutylammonium hydrogen sulfate remains thermally stable up to 260 °C, beyond which it begins to undergo slow decomposition. It was further demonstrated that the enthalpy of melting for Tetrabutylammonium hydrogen sulfate is −48.4 J/g. The conductivity of Tetrabutylammonium hydrogen sulfate varies over a wide range, from as low as 10⁻⁸ S/cm at 60 °C to as high as 10⁻² S/cm when the compound is in its molten state. Specifically, the conductivity of the molten form of Tetrabutylammonium hydrogen sulfate (observed between 165 and 180 °C) exceeds 10⁻² S/cm, with an associated activation energy of 0.5 eV. In contrast, at lower temperatures ranging from 50 to 125 °C, the activation energy for conductivity is 0.8 eV. Among the most critical factors influencing the conductive behavior of Tetrabutylammonium hydrogen sulfate are its structural characteristics, the nature of the bonding interactions between sulfate tetrahedra, and the energy of the hydrogen bonds present within the crystal lattice. The crystal structure of Tetrabutylammonium hydrogen sulfate consists of sulfate tetrahedra that are linked together in pairs via strong hydrogen bonds, forming isolated dimers with distances that are significant enough to allow for potential proton transfer. This particular structural arrangement consequently hinders the formation of efficient conductivity pathways within the material. [1]

Preparation methods

The synthesis of Tetrabutylammonium hydrogen sulfate can be accomplished primarily through two distinct Technological process. In the first approach, Tetrabutylammonium hydrogen sulfate is prepared by reacting iodobutane with tributylamine in acetonitrile, followed by refluxing the resulting product with methyl hydrogen sulfate, after which the desired compound is obtained through purification by distillation. The second method involves using tetrabutylammonium bromide as the starting material; Tetrabutylammonium hydrogen sulfate is then produced by exchanging the bromide ions with hydrogen sulfate ions via a reaction with concentrated sulfuric acid in dichloroethane, followed by crystallization and drying. As one of the most widely utilized phase-transfer catalysts available, Tetrabutylammonium hydrogen sulfate finds extensive application in a broad range of synthetic transformations, including the preparation of methylenedicarboxylic acid esters, the synthesis of ethers, the N-alkylation of indoles, and the dehalogenation of alkyl halides to form alkynes. Moreover, in the field of analytical chemistry, Tetrabutylammonium hydrogen sulfate is frequently employed as a component of the mobile phase in high-performance liquid chromatography (HPLC), where it facilitates the separation of various pharmaceutical compounds.

Effects as an electrolyte additive

Tetrabutylammonium hydrogen sulfate is an ion-pairing reagent that possesses properties similar to those of ionic liquids. Ionic liquids belong to a new branch of salts with unique characteristics, and they have ever-increasing applications in electrochemical systems, particularly in lithium-ion batteries. For the first time, the effects of Tetrabutylammonium hydrogen sulfate as an electrolyte additive in battery electrolyte were investigated on the hydrogen and oxygen evolution overpotential as well as on the anodic layer formation on the lead–antimony–tin grid alloy of a lead-acid battery. These investigations were carried out using cyclic voltammetry and linear sweep voltammetry in an aqueous sulfuric acid solution. Tetrabutylammonium hydrogen sulfate was also examined for its influence on the grid surface morphology after the cyclic redox reaction, which was studied using scanning electron microscopy. The results show that increasing the concentration of Tetrabutylammonium hydrogen sulfate in the electrolyte leads to an increase in both hydrogen and oxygen evolution overpotential, and consequently, the crystalline structure of the PbSO₄ layer is altered. Furthermore, cyclic voltammograms recorded on a carbon–PbO paste electrode indicate that in the presence of Tetrabutylammonium hydrogen sulfate in the electrolyte, the oxidation and reduction peak currents increase intensively, while the peak potentials for the oxidation and reduction of PbO are dependent on the concentration of Tetrabutylammonium hydrogen sulfate. [2]

Reference

[1] Ponomareva V G, Bagryantseva I N, Uvarov N F. Electrotransport and thermal properties of tetrabutylammonium hydrogen sulfate[J]. Ionics, 2021, 27: 2067-2071.

[2] Rezaei B, Taki M. Effects of tetrabutylammonium hydrogen sulfate as an electrolyte additive on the electrochemical behavior of lead acid battery[J]. Journal of Solid State Electrochemistry, 2008, 12(12): 1663-1671.