# Effect of the Incorporation of EuCl3 as a Fluorescent Probe into Ionic Liquids on Rheological and Electrical Properties for Tribological Applications

## TL;DR

This study explores adding EuCl3 to an ionic liquid to create a fluorescent material with useful properties for smart lubricants.

## Contribution

The novel use of EuCl3 as a fluorescent probe in ionic liquids for tribological applications is demonstrated.

## Key findings

- EuCl3 incorporation imparts fluorescence with emission peaks at 590 and 612 nm.
- Dynamic viscosity increases with EuCl3 addition but remains suitable for tribological use.
- Conductivity increases with temperature, correlating with reduced viscosity.

## Abstract

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.