## 3 Results and Discussion
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \(\mathbf{\alpha^{*}}\) [GPa\({}^{-1}\)] 25 \({}^{\circ}\)C & \(\mathbf{\alpha^{*}}\) [GPa\({}^{-1}\)] 50 \({}^{\circ}\)C & \(\mathbf{\alpha_{film}}\) [GPa\({}^{-1}\)] 25 \({}^{\circ}\)C & \(\mathbf{\alpha_{film}}\) [GPa\({}^{-1}\)] 50 \({}^{\circ}\)C \\ \hline \([\)Choline\(][\)[Proline] & 7.7 & 5.8 & 8.0 & 6.1 \\ \([\)C\({}_{4}\)C\({}_{1}\)im\(]\)BF\({}_{4}\) & 9.9 & 7.8 & 10.1 & 8.2 \\ \([\)C\({}_{4}\)C\({}_{1}\)im\(]\)PF\({}_{6}\) & 13.2 & 10.5 & 13.5 & 10.9 \\ \([\)C\({}_{4}\)C\({}_{1}\)im\(]\)Tf\({}_{2}\)N & 10.7 & 8.6 & 11.1 & 9.1 \\ \([\)C\({}_{6}\)C\({}_{1}\)im\(]\)Pf\({}_{6}\) & 13.7 & 11.1 & 14.1 & 11.6 \\ \([\)C\({}_{6}\)C\({}_{1}\)im\(]\)Tf\({}_{2}\)N & 9.5 & 8.6 & 9.5 & 8.5 \\ \([\)C\({}_{8}\)C\({}_{1}\)im\(]\)B\({}_{4}\) & 11.1 & 9.0 & 11.7 & 9.6 \\ \([\)C\({}_{8}\)C\({}_{1}\)im\(]\)Pf\({}_{6}\) & 13.3 & 11.1 & 13.9 & 11.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of pressure-viscosity coefficients. Figure 4: Viscosity pressure curves for two Isomers of Dodecane @ 40 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{l c c c} \hline \hline T [\({}^{\circ}\)C] & 25 & 50 & 90 \\ \(\alpha_{0}\) [GPa\({}^{-1}\)] & 8.92 & 6.76 & 4.73 \\ \(\alpha^{*}\) [GPa\({}^{-1}\)] & 7.65 & 5.81 & 4.08 \\ \(\alpha_{film}\) [GPa\({}^{-1}\)] & 7.96 & 6.05 & 4.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pressure-viscosity coefficients for [Choline] [Proline]. is that the film thickness generated will not be as large, since it is dependent on the viscosity of the lubricant in the inlet region of the contact. The second effect is that the effective viscosity inside of the high pressure region of the contact will be lower, which in turn will lead to a lower coefficient of friction in full film conditions. However, since a lower pressure-viscosity coefficient leads to lower film thicknesses, the transition from full film to mixed and boundary lubrication will occur at higher entrainment speeds or for lower roughness levels, which in turn will increase friction and wear. Furthermore, a low pressure-viscosity coefficient is often connected to a low limiting shear stress of the lubricant, which is also beneficial for low friction coefficients [37]. The values for \(\alpha^{*}\) are vastly different for those derived from viscosity in Table 3 and those derived from film thickness. This is a well-known issue for EHL [49]. The classical EHL film thickness formulas often require values of the pressure-viscosity coefficients that cannot be measured in a viscometer with the usual definitions. This issue can only be resolved when EHL employs real viscosity in calculations. The present investigation shows that [Choline] [Proline] is a very hygroscopic IL. A sample of the IL was left in open atmosphere at 25 \({}^{\circ}\)C and a relative humidity of approximately 15 % for 180 h and the water content was measured at certain intervals. The results can be seen in Figure 5. The water content in the IL increased to almost 10 wt % in 24 h and then leveled out at a level slightly below 15 wt %. Since this IL was shown to be highly hygroscopic it was decided that the film thickness and friction tests would be performed at the saturated level of water content of slightly below 15 wt %. The viscosity of the water containing IL and the reference paraffin oil was measured in a Bohlin CVO 100 rheometer at 25, 40 and 60 \({}^{\circ}\)C. The results are shown in Table 5. Here the refractive indexes used in the optical film thickness measurements are also presented. Figure 6 shows the central film thickness of [Choline] [Proline] containing slightly less than 15 wt % water and the paraffin oil at 25 and 60 \({}^{\circ}\)C at 0.5 GPa of pressure. The film thickness for the IL is only slightly higher than for the paraffin oil despite the IL having significantly higher viscosity.