Aqueous acidic solution - ionic liquid biphasic system: An original investigation by diffusion ordered spectroscopy nuclear magnetic resonance

Abstract

Transfers of solutes from an aqueous phase to an ionic liquid phase and vice versa has been well studied for years. On the other hand, influence of the transferred solutes on the structure of the ionic liquid phase is not yet evaluated. Therefore, we focus on the aqueous - ionic liquid biphasic system composed of two immiscible phase containing nitric or hydrochloric acid and the [C4mim][Tf2N] ionic liquid. They have been yet examined in terms of water and acid solubility in the ionic liquid rich phase. After mixing and separation of the two phases, a part of water, H3O+ and nitrates species were transferred into the [C4mim][Tf2N] ionic liquid phase, but not chloride anions. The presence of the solutes water, H3O+ and nitrates in [C4mim][Tf2N] ionic liquid phase could modify local structure of [C4mim][Tf2N] ionic liquid. So far, our goal is to evaluate this structural change using several tools. For this, one dimensional 1H, 19F and 15N Nuclear Magnetic Resonance, Nuclear Overhauser effect and Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance were carried out on the [C4mim][Tf2N] liquid ionic phase. We observed that some of water molecules were free and formed local microstructure as water pocket inside the ionic liquid frame. The others were hydrogen bonded to [Tf2N−] anions via theirs oxygen atoms from sulfonyl groups. The free water fraction decreased as the concentration of [HNO3]aq, init increased. In case of [C4mim][Tf2N] ionic liquid mixed with hydrochloric acid, the fraction of hydrogen bonded water molecules was more important than with nitric acid.