Tuning the nanostructure and tribological properties of a non-ionic deep eutectic solvent with water addition

Hypothesis:
The addition of water to a non-ionic N-oxide deep eutectic solvent (DES) composed of phenylacetic acid (PhAA) and N-dodecylmorpholine-N-oxide (MO-12) in a 1:1 M ratio (PhAA/MO-12) will promote interfacial nanostructure formation due to increased proton transfer and solvophobic interactions, leading to reduced friction.

Experiments:
The interfacial structure and friction of PhAA/MO-12 with water content up to 41.9 wt% were investigated at mica surfaces. Atomic force microscopy (AFM) was used to measure normal force-separation profiles, lateral images, and nanoscale friction.

Findings:
Conductivity increases over twentyfold with the addition of 23.6 wt% water. AFM force curves reveal that increasing water content in PhAA/MO-12 leads to a more pronounced interfacial structure with steps extending further into the bulk. High-resolution near-surface images show a well-defined sponge-like nanostructure at 23.6 wt% water, which is absent in the neat DES. The enhanced nanostructure is attributed to increased proton transfer from PhAA to MO-12 and segregation of polar and apolar domains driven by water-strengthened solvophobic interactions. Friction reduces up to 72 % for ≥7.0 wt% water compared to the neat DES, due to a more robust boundary layer facilitated by water.