Abstract
Molecular layering of liquids in nanometer-scale confinement is demonstrated for typical lubricant constituents such as polyalphaolefins (PAO) and an ester by means of atomic force microscopy. Layering is observed in force vs. distance curves for poly-(1-decene) tetramers (PAO6) and undecamers (PAO40) and for a 2-ethylhexyl monoester on graphite, mica, and polished steel surfaces and is compared to the layering of hexadecane and 1-hexadecene. On graphite surfaces, the confined molecules are oriented parallel to the surfaces for all liquids, resulting in layers with a thickness comparable to the diameter of the alkyl chains. On mica, confined hexadecane molecules also lie parallel to the surface, while the molecules in the first layer of 1-hexadecene and PAOs take a more upright orientation. Confinement on the oxidized polished steel surfaces results in a molecular layering which most often resembles the layering on graphite and differs significantly from layering on the ionic oxide mica.
Similar content being viewed by others
References
Gao, J.P., Luedtke, W.D., Landman, U.: Structure and solvation forces in confined films: linear and branched alkanes. J. Chem. Phys. 106(10), 4309 (1997)
Christenson, H.K., Gruen, D.W.R., Horn, R.G., Israelachvili, J.N.: Structuring in liquid alkanes between solid-surfaces—force measurements and mean-field theory. J. Chem. Phys. 87(3), 1834 (1987)
Zhu, Y.X., Granick, S.: Superlubricity: a paradox about confined fluids resolved. Phys. Rev. Lett. 93(9), 096101 (2004)
Bureau, L.: Rate effects on layering of a confined linear alkane. Phys. Rev. Lett. 99(22), 225503 (2007)
Nakada, T.: Atomic force microscopic study of subsurface ordering and structural transforms in n-alcohol on mica and graphite. Jpn. J. Appl. Phys. Part 2-Lett. 35(1A), L52 (1996)
Franz, V., Butt, H.J.: Confined liquids: solvation forces in liquid alcohols between solid surfaces. J. Phys. Chem. B 106(7), 1703 (2002)
Gosvami, N.N., Sinha, S.K., Hofbauer, W., O’Shea, S.J.: Solvation and squeeze out of hexadecane on graphite. J. Chem. Phys. 126(21), 214708 (2007)
Hayes, R., Warr, G.G., Atkin, R.: Structure and nanostructure in ionic liquids. Chem. Rev. 115(13), 6357 (2015)
Vanalsten, J., Granick, S.: Molecular tribometry of ultrathin liquid-films. Phys. Rev. Lett. 61(22), 2570 (1988)
Krass, M.D., Gosvami, N.N., Carpick, R.W., Müser, M.H., Bennewitz, R.: Dynamic shear force microscopy of viscosity in nanometer-confined hexadecane layers. J. Phys. 28(13), 134004 (2016)
Rudnick, L., Shubkin, R.: Synthetic lubricants and high- performance functional fluids, revised and expanded. In: Rudnick, L., Shubkin, R. (eds.) Chemical Industries, pp. 3–52. CRC Press, Boca Raton (1999)
Green, C., Lioe, H., Cleveland, J., Proksch, R., Mulvaney, P., Sader, J.: Normal and torsional spring constants of atomic force microscope cantilevers. Rev. Sci. Instrum. 75(6), 1988 (2004)
Hoth, J., Hausen, F., Mueser, M.H., Bennewitz, R.: Force microscopy of layering and friction in an ionic liquid. J. Phys. 26(28), 284110 (2014)
Xu, R.G., Xiang, Y., Leng, Y.S.: Computational simulations of solvation force and squeezing out of dodecane chain molecules in an atomic force microscope. J. Chem. Phys. 147(5), 054705 (2017)
Kong, L.T., Denniston, C., Muser, M.H.: The crucial role of chemical detail for slip-boundary conditions: molecular dynamics simulations of linear oligomers between sliding aluminum surfaces. Model. Simul. Mater. Sci. Eng. 18(3), 034004 (2010)
Cui, S.T., Cummings, P.T., Cochran, H.D.: Effect of branches on the structure of narrowly confined alkane fluids: n-hexadecane and 2,6,11,15-tetramethylhexadecane. J. Chem. Phys. 114(14), 6464 (2001)
Gosvami, N.N., Sinha, S.K., O’Shea, S.J.: Squeeze-out of branched alkanes on graphite. Phys. Rev. Lett. 100(7), 076101 (2008)
Lim, R.Y.H., O’Shea, S.J.: Discrete solvation layering in confined binary liquids. Langmuir 20(12), 4916 (2004)
Gobbo, C., Beurroies, I., de Ridder, D., Eelkema, R., Marrink, S.J., De Feyter, S., van Esch, J.H., de Vries, A.H.: Martini model for physisorption of organic molecules on graphite. J. Phys. Chem. C 117(30), 15623 (2013)
Gosvami, N.N., O’Shea, S.J.: Nanoscale trapping and squeeze-out of confined alkane monolayers. Langmuir 31(47), 12960 (2015)
McGuiggan, P.M.: The boundary lubrication properties of model esters. Tribol. Lett. 11(1), 49 (2001)
Crossley, J.: Dielectric-relaxation of 1-alkenes. J. Chem. Phys. 58(12), 5315 (1973)
Cooper, P.K., Wear, C.J., Li, H., Atkin, R.: Ionic liquid lubrication of stainless steel: friction is inversely correlated with interfacial liquid nanostructure. ACS Sustain. Chem. Eng. 5(12), 11737 (2017)
Liu, P.Z., Lu, J., Yu, H.L., Ren, N., Lockwood, F.E., Wang, Q.J.: Lubricant shear thinning behavior correlated with variation of radius of gyration via molecular dynamics simulations. J. Chem. Phys. 147(8), 084904 (2017)
Lahtela, M., Pakkanen, T.A., Nissfolk, F.: Molecular modeling of poly-alpha-olefin synthetic oils. J. Phys. Chem. 99(25), 10267 (1995)
Guangteng, G., Spikes, H.A.: The control of friction by molecular fractionation of base fluid mixtures at metal surfaces. Tribol. Trans. 40(3), 461 (1997)
Acknowledgements
The authors thank Stefan Brück for NMR analysis, Yuliya Silina and Claudia Fink-Straube for the gel chromatography, and Eduard Arzt for his continuous support of the project.
Author information
Authors and Affiliations
Corresponding author
Additional information
Marc-Dominik Krass and Günther Krämer have contributed equally to this work.
Rights and permissions
About this article
Cite this article
Krass, MD., Krämer, G., Dellwo, U. et al. Molecular Layering in Nanometer-Confined Lubricants. Tribol Lett 66, 87 (2018). https://doi.org/10.1007/s11249-018-1041-y
Received:
Accepted:
Published:
DOI: https://doi.org/10.1007/s11249-018-1041-y