# Studying the Tribological Properties of Coffee Oil-Loaded Water-Based Green Lubricant
## 2 Materials and Methods
### Preparation of Lubricating Samples  
It was observed that bis(2-hydroxyethyl)ammonium oleate protic ionic liquid could facilitate the dispersion of coffee oil in water. The dispersions were carried out in the following way. A 50 mL conical glass flask containing appropriate amounts of deionised water, coffee oil, and PIL was immersed in an ultrasonic bath, heated to the temperature of 45 \({}^{\circ}\)C, and sonicated for 15 min. Further sonication did not result in better oil dispersion. While the amount of coffee oil dispersed in water was determined by the amount of PIL, several possible concentrations were investigated. Table 3 presents the composition of investigated lubricating samples and their abbreviations used throughout the manuscript. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Fatty Acid Composition** & **This Study** & **[PERSON] et al. [14]** & **[PERSON] et al. [21]** & **[PERSON] et al. [22]** \\ \hline
**Saturated fatty acids (\%):** & **51.83** & **37.61** & **43.03** & **86.01** \\ Butyric acid (C4:0) & 0.04 \(\pm\) 0.0009 & & & \\ Caprotic acid (C6:0) & 0.07 \(\pm\) 0.01 & & & \\ Caprylic acid (C8:0) & 0.18 \(\pm\) 0.05 & & & 0.09 \\ Capric acid (C10:0) & 0.24 \(\pm\) 0.07 & & & 0.01 \\ Undecylic acid (C11:0) & 0.04 \(\pm\) 0.0002 & & & \\ Lauric acid (C12:0) & 0.00 & & & 0.02 \\ Tricocaine acid (C13:0) & 0.67 \(\pm\) 0.15 & & & \\ Myristic acid (C14:0) & 0.07 \(\pm\) 0.13 & 3.82 & & 0.18 \\ Pentadecanoic acid (C15:0) & 0.03 \(\pm\) 0.015 & & & \\ Palmitic acid (C16:0) & 37.26 \(\pm\) 1.78 & 19.00 & 35.39 & 65.07 \\ Margaric acid (C17:0) & 0.14 \(\pm\) 0.045 & & & 0.11 \\ Stearic acid (C18:0) & 8.79 \(\pm\) 0.26 & 6.73 & 7.64 & 13.00 \\ Arachidic acid (C20:0) & 3.26 \(\pm\) 0.026 & 2.96 & & 5.47 \\ Heneicosylic acid (C21:0) & 0.1 \(\pm\) 0.033 & & & \\ Behenic acid (C22:0) & 0.8 \(\pm\) 0.05 & & & 1.35 \\ Tricosylic acid (C23:0) & 0.12 \(\pm\) 0.0013 & 5.11 & & 0.19 \\ Lignoceric acid (C24:0) & 0.02 \(\pm\) 0.001 & & & 0.52 \\
**Unsaturated fatty acids, \%:** & **48.17** & **62.27** & **45.63** & **13.99** \\
**Mononunsaturated fatty acids, \%:** & **10.19** & **33.56** & **9.78** & **8.98** \\ Myristoleic acid (C14:1 cis-9) & 0.14 \(\pm\) 0.05 & 20.00 & & \\ Palmitic acid (C16:1 cis-9) & 0.04 \(\pm\) 0.003 & & & \\ Oleic acid (C18:1) & 9.26 \(\pm\) 0.58 & 9.27 & 9.78 & 8.69 \\ Paullmitic acid (C20:1) & 0.33 \(\pm\) 0.006 & & & 0.29 \\ Erucic acid (C22:1) & 0.07 \(\pm\) 0.002 & & & \\ Nervonic acid (C24:1) & 0.35 \(\pm\) 0.025 & 4.29 & & \\
**Polyunsaturated fatty acids, \%:** & **37.98** & **28.71** & **35.85** & **5.01** \\ Linoleic acid (C18:2) & 36.59 \(\pm\) 1.95 & 28.71 & 35.85 & 4.72 \\ Linolenic acid (C18:3) & 1.15 \(\pm\) 0.045 & & & 0.04 \\ Eicosadinenoic acid (C20:2) & 0.03 \(\pm\) 0.0001 & & & \\ Dhomo-gamma-linolenic acid & 0.03 \(\pm\) 0.0002 & & & 0.15 \\ (C20:3 8 11,14) & 0.02 \(\pm\) 0.005 & & & 0.10 \\ Arachidonic acid (C20:4) & 0.02 \(\pm\) 0.005 & & & 0.10 \\ Eicosadineneoic acid (C20:5) & 0.11 \(\pm\) 0.0047 & & & \\ Brassic acid (C22:2) & 0.02 \(\pm\) 0.001 & & & \\ Docosahexaenoicenoic acid (C22:6) & 0.02 \(\pm\) 0.0033 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fatty acid composition of extracted coffee oil and its comparison with the results obtained by other researchers. The dispersion of coffee oil in water is strongly related to PIL concentration: more PIL could better disperse the oil. Figure 1 presents the appearance of prepared lubricating samples. The dispersions are light yellow, with increased saturation when the amount of additive increased. Clearly, 0.25 wt.% PIL is insufficient to disperse coffee oil, and a top layer of lower-density oil is formed. However, there was an exception when applying five wt.% of coffee oil. There was no distinguishable layer, but some oil-in-water droplets were observed. This phenomenon could be related to dispersions' viscosity and optimal counterpart ratio. The dispersions with one wt.% of PIL were much more homogeneous.