Experimental three-dimensional description of the liquid hexadecane/graphite interface

Single layer graphene induces load-bearing molecular layering at the hexadecane-steel interface

The influence of a single layer graphene on the interface between a polished steel surface and the model lubricant hexadecane is explored by high-resolution force microscopy. Nanometer-scale friction is reduced by a factor of three on graphene compared to the steel substrate, with an ordered layer of hexadecane adsorbed on the graphene. Graphene furthermore induces a molecular ordering in the confined lubricant with an average range of 4-5 layers and with a strongly increased load-bearing capacity compared to the lubricant on the bare steel substrate.

Nanometer-Scale Distribution of a Lubricant Modifier on Iron Films: A Frequency-Modulation Atomic Force Microscopy Study Combined with a Friction Test

Liquid lubricants used in mechanical applications are low-vapor-pressure hydrocarbons modified with a small quantity of polar compounds. The polar modifiers adsorbed on the surface of sliding solids dominate the friction properties when the sliding surfaces are in close proximity. However, a few methods are available for the characterization of the adsorbed modifiers of a nanometer-scale thickness. In this study, we applied frequency-modulation atomic force microscopy to evaluate the vertical and lateral density distributions of the adsorbed modifier in a real lubricant, namely, poly-α-olefin (PAO) modified with an orthophosphoric acid oleyl ester. The liquid-induced force on the probing tip was mapped on a plane that was perpendicular to the lubricant-iron interface with a force sensitivity on the order of 10 pN. The PAO in the absence of the ester modifier was directly exposed to the film, which produced a few liquid layers parallel to the film surface with layer-to-layer distances of 0.6-0.7 nm. A monomolecular layer of the modifier was intermittently adsorbed with increasing ester concentration in the bulk lubricant, with complete coverage seen at 20 ppm. The C₁₈H₃₅ chains of the oleyl esters fluctuating in the lubricant produced a repulsive force on the tip, which monotonically decayed with the tip-to-surface distance. The dynamic friction coefficient of sliding steel-lubricant-steel interfaces, which was separately determined using a friction tester, was compared with the force map determined on the iron film immersed in the corresponding lubricant. The complete monomolecular layer of the ester modifier on the static lubricant-iron boundary is a requirement for achieving smooth and stable friction at the sliding interface.

Effect of temperature on the viscoelastic properties of nano-confined liquid mixtures

The behaviour of fluids confined in nanoscale gaps plays a central role in molecular science and nanofluidics, with applications ranging from biological function to multiscale printing, osmosis and filtration, lab-on-chip technology and friction reduction. Here atomic force microscopy is used to shear five different mixtures of hexadecane and squalane confined between the tip apex and atomically flat graphite. The shearing amplitudes are typically <2 nm, hence reflecting highly localised information at the interface. The evolution of each mixture's viscoelastic properties is studied as a function of temperature, between 20 °C and 100 °C. The results, complemented by sub-nanometre resolution images of the interface, show that spatial organisation of the liquid molecules at the surface of graphite largely dominates the measurements. Squalane presents a higher effective affinity for the surface by forming a robust self-assembled layer in all mixtures. This results in a step-like change of the viscous and elastic response of the confined liquid as the confining pressure increases. In contrast, measurements in pure hexadecane show a continuous and linear increase in the apparent viscosity with pressure at all temperatures. This is interpreted as a more fragile interfacial layer and images show that it can be completely removed at high temperatures. Depending on the mixture composition, measurements can be strongly location-dependent which suggests molecular clustering and nanoscale phase separation at the interface.

Mass transport and structure of liquid n-alkane mixtures in the vicinity of α-quartz substrates

The asymmetric self-diffusion characteristics of binary n-alkane mixtures in the first adsorption layer near α-quartz substrate were demonstrated and discussed.

Monte Carlo simulations of thin hydrocarbon films: composition heterogeneity and structure at the solid-liquid and liquid-vapor interfaces

The structural properties of 10 nm thick lubricant films consisting of binary and ternary n-alkane mixtures (8 ≤ n ≤ 12) adsorbed on a structureless metal substrate were studied for several temperatures and compositions using Monte Carlo simulations. Configurational-bias Monte Carlo identity switch moves are essential to sample the spatial distribution in these mixtures. Longer alkanes are found to preferentially adsorb onto the substrate while shorter alkanes are enriched at the liquid-vapor interface. This preferential adsorption is evident even when the two chains differ by only one methylene unit and the longer chain is the minor component. Enhanced composition heterogeneity and orientational ordering and fewer gauche defects are characteristic features of the first layer near the substrate.

Stability of the parallel layer during alkane spreading and the domain structures of the standing-up layer

The spreading of liquid alkanes over surfaces plays an important role in applications such as lubrication, painting, and printing. To make significant advances in these fields, it is essential to increase our understanding of the interactions between alkanes and surfaces. Long-chain alkanes form two typical adsorption structures on a surface--the parallel phase and the standing-up phase. The most thermodynamically stable structure is the parallel phase, in which the alkane molecules lie flat on the surface. If the temperature is slightly below the bulk melting point, then alkanes form a thermodynamically stable standing-up phase on top of an existing parallel layer. At lower temperatures, the standing-up phase becomes metastable. Using atomic force microscopy, we have found that the standing-up alkane layer consists of multiple domains, indicating that the standing-up layer forms through a multinucleation process during the liquid-solid transition on the surface. If, however, the temperature is above the melting point, then we have found that the standing-up layer shrinks to a droplet and leaves a residue on its original position. During the spreading of an alkane droplet, the parallel layer forms on the substrate surface surrounding the droplet by adsorption from the vapor, which precedes the arrival of the liquid. There has been uncertainty, however, as to whether the parallel layer moves with the liquid alkane or remains stationary during spreading. In this study, we used the residue left on the parallel layer as a landmark to monitor the movement of the parallel layer during the spreading of an alkane droplet. Using this landmark, we found that the parallel layer remained stationary on the substrate, indicating that the liquid alkane spreads on a stationary parallel layer surface. Therefore, this study reveals that the surface properties of the parallel layer--not the surface properties of the substrate--control the spreading and wetting of a liquid alkane.