Effect of the Incorporation of EuCl ₃ as a Fluorescent Probe into Ionic Liquids on Rheological and Electrical Properties for Tribological Applications

Fluorescence optical microscopy tracking the film thickness as a function of fluorescence intensity under the influence of an external electric field could be a very promising technique. Due to the insufficient fluorescence properties of ionic liquids, the incorporation of fluorescent probes is necessary. This research work investigates the incorporation of europium-(III) chloride (EuCl₃) as a fluorescent probe into the ionic liquid 1-hexyl-3-methylimidazolium chloride (HMIMCl) and its impact on the rheological and electrical properties of the resulting ionic liquid complex for future tribological use. The study showed that the incorporation of EuCl₃ imparts fluorescence properties to HMIMCl, characterized by two emission peaks at 590 and 612 nm, which correspond to two specific electronic transitions of the europium ion, namely, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂. Temperature-dependent current-voltage characteristics were measured, which showed increased conductivity correlated with reduced viscosity. Rheological analysis demonstrated that the addition of EuCl₃ increased the dynamic viscosity of the ionic liquid while remaining within the range of common low viscosity tribological oils. This research highlights the potential of fluorescent ionic liquids in tribological applications and paves the way for the development of smart, environmentally friendly lubricants, which would be able to react to an electric field. The findings contribute valuable insights into the design and optimization of smart fluids that can adapt their properties in response to external stimuli.