TY - JOUR
T1 - Electrotunable Friction in Diluted Room Temperature Ionic Liquids
T2 - Implications for Nanotribology
AU - Pivnic, Karina
AU - Bresme, Fernando
AU - Kornyshev, Alexei A.
AU - Urbakh, Michael
N1 - Publisher Copyright:
©
PY - 2020/11/25
Y1 - 2020/11/25
N2 - Using nonequilibrium molecular dynamics (NEMD) simulations, we study the mechanism of electrotunable friction in the mixture of a room temperature ionic liquid (RTIL), BMIM PF6, and an organic solvent, acetonitrile. The dilution itself helps to reduce the viscosity and thereby reduce the viscous contribution to friction. At the same time, we find that under nanoscale confinement conditions, diluted RTIL solutions, of just ∼10% molar fraction, still feature a remarkable variation of the friction force with the electrode surface charge density, not weaker than had been earlier shown for nanoconfined pure RTILs. In both classes of systems the electrotunable friction response is due to accumulation of counterions at charged surfaces. For both diluted mixtures and pure RTILs, the friction force is minimal for uncharged surfaces and it increases with surface charge of either sign but only in the range of low and moderate surface charges (16-32 μC/cm2). At higher surface charges (43-55 μC/cm2), the effect is different: in the pure RTIL, the friction force continues to increase with the surface charge, while in the diluted RTIL mixture it features a maximum, with a reduction of friction with the increasing surface charge. This contrasting behavior is explained by the difference in the slip conditions found for the pure and the diluted RTIL solutions in contact with highly charged surfaces. Overall, we demonstrate that nanoscale films of diluted mixtures of RTIL provide lower friction forces than the pure RTIL films, preserving at the same time a significant electrotunable response when the liquids are confined between symmetrically charged surfaces. Nanoconfinement between asymmetrically charged surfaces leads to a reduction of friction compared to the symmetric case, with a concomitant decrease in the range of friction variation with the surface charge density. Our results highlight the potential of diluted RTIL mixtures as cost-effective electrotunable lubricants for future nanotribological applications.
AB - Using nonequilibrium molecular dynamics (NEMD) simulations, we study the mechanism of electrotunable friction in the mixture of a room temperature ionic liquid (RTIL), BMIM PF6, and an organic solvent, acetonitrile. The dilution itself helps to reduce the viscosity and thereby reduce the viscous contribution to friction. At the same time, we find that under nanoscale confinement conditions, diluted RTIL solutions, of just ∼10% molar fraction, still feature a remarkable variation of the friction force with the electrode surface charge density, not weaker than had been earlier shown for nanoconfined pure RTILs. In both classes of systems the electrotunable friction response is due to accumulation of counterions at charged surfaces. For both diluted mixtures and pure RTILs, the friction force is minimal for uncharged surfaces and it increases with surface charge of either sign but only in the range of low and moderate surface charges (16-32 μC/cm2). At higher surface charges (43-55 μC/cm2), the effect is different: in the pure RTIL, the friction force continues to increase with the surface charge, while in the diluted RTIL mixture it features a maximum, with a reduction of friction with the increasing surface charge. This contrasting behavior is explained by the difference in the slip conditions found for the pure and the diluted RTIL solutions in contact with highly charged surfaces. Overall, we demonstrate that nanoscale films of diluted mixtures of RTIL provide lower friction forces than the pure RTIL films, preserving at the same time a significant electrotunable response when the liquids are confined between symmetrically charged surfaces. Nanoconfinement between asymmetrically charged surfaces leads to a reduction of friction compared to the symmetric case, with a concomitant decrease in the range of friction variation with the surface charge density. Our results highlight the potential of diluted RTIL mixtures as cost-effective electrotunable lubricants for future nanotribological applications.
KW - electrotunable friction
KW - interfacial slippage
KW - ionic liquids
KW - molecular dynamics simulations
KW - nanotribology
KW - organic solvents
UR - http://www.scopus.com/inward/record.url?scp=85096135808&partnerID=8YFLogxK
U2 - 10.1021/acsanm.0c01946
DO - 10.1021/acsanm.0c01946
M3 - ???researchoutput.researchoutputtypes.contributiontojournal.article???
AN - SCOPUS:85096135808
SN - 2574-0970
VL - 3
SP - 10708
EP - 10719
JO - ACS Applied Nano Materials
JF - ACS Applied Nano Materials
IS - 11
ER -