(3-Aminopropyl)triethoxysilane as a Model Silylamine Reversible Ionic Liquid Used as a Reversible Electrolyte

Ionic liquids can serve as stable electrolytes in solutions and can be particularly useful when using non-aqueous systems where electrolyte solubility can be a concern.  Separation of the electrochemical product from the electrolyte, ionic liquid-based or otherwise, can add challenges to the overall process.  Use of a switchable electrolyte offers the possibility to reduce or eliminate these separation challenges while maintaining the high conductivity properties required for electrochemical systems.  Reversible ionic liquids (RevILs) are a type of switchable solvent that could be used as a switchable electrolyte.  Switchable solvents are solvents that can have dramatic step-changes in their properties with the introduction of an external stimulus such as light, heat, or carbon dioxide.  RevIL systems can exist in two states: 1) a molecular liquids where it behaves like a traditional solvent with non-ionic and non-viscous properties, and 2) an ionic or reversible ionic liquid state with ionic and viscous properties that is formed in the presence of an external stimulus, commonly CO₂.  Silylamines are RevIL systems when they are switched in the presence of CO₂ and returned to the molecular liquid with heat.  The RevIL state is in the form of an ammonium-carbamate ion pair.

Triethoxysilylpropylamine (TEtoxySA) has been investigated as a reversible electrolyte.  When in the pure RevIL state, the conductivity is prohibitively low to be used as both the solvent and electrolyte.  If the RevIL is diluted in an organic solvent such as methanol, the conductivity of the solution increases by three orders of magnitude and reaches an electrochemically-suitable level.  Several reasons likely contribute to the change in conductivity.  The pure RevIL is very viscous and the anion and cation are of relatively same size, making mass transfer of the ions difficult.  The anion and cation also have strong affinities for each other.  In the RevIL-methanol solutions, viscosity drops significantly.  Also, the anion and cation become diluted, reducing the interaction with each other.  Another possible explanation for the increase in conductivity is that the ammonium cation could transfer its charge to the methanol, resulting in a smaller charge carrier through the system.  The properties of TEtoxySA as a reversible surfactant including conductivity and electrochemical window will be presented along with the phenomena associated with increases in conductivity.

Figure 1:  Conductivity of TEtoxySA RevIL-methanol mixtures.