The structures of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane

Abstract

Molecular-dynamics simulations are used to gain insights on recent sum-frequency spectroscopy and polarised neutron reflectometry measurements of the structure of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane. Simulations were carried out under quiescent and high-shear conditions. Mass-density profiles, molecular-height and molecular-orientation probability distribution functions, and in-layer radial distribution functions were calculated. The simulation results show that at high surface coverage, the film thickness is about 15 Å, and that the molecules are mainly pointing upwards from the surface at an angle of 40-50°. The results are compared critically against published experimental results, and the agreement is found to be good. The in-layer ordering of the hexadecylamine head-group atoms is found to be dictated by the crystalline structure of the iron-oxide surface, but this influence rapidly diminishes along the molecular backbone. The tail-group atoms show almost no positional ordering. Finally, an example calculation of the kinetic friction coefficient under high-shear conditions is presented. The lateral (friction) force is measured as a function of the normal (applied) force, and the kinetic friction coefficient is determined to be about 0.09, which is typical for this kind of system.

Molecular Dynamics Simulations of Glycerol Monooleate Confined between Mica Surfaces

“…Figure 8 shows that at shear ratesġ ⇠ 10 10 s 1 , the friction coefficient µ ' 0.06-0.08. Even at lower shear rates (ġ ⇠ 10 9 s 1 ) the friction coefficients of stearic acid and oleic acid in squalane (µ ' 0.25) 31 and hexadecylamine in dodecane (µ ' 0.08) 32 are higher. At least one contributing factor here is that n-heptane and toluene have lower viscosities than dodecane and squalane.…”

“…From earlier work on bulk GMO in n-heptane or toluene, it is known that the diameters of GMO RMs are in the range 25-30 Å. 32 In addition, the xy two-dimensional radial distribution functions in similar systems…”

“…This approximation has been tested explicitly in simulations of similar systems, and found to be accurate. 31,32 3 Results and discussion Figure 2 shows some simulations snapshots of GMO in n-heptane under static conditions, and with low and high shear rates. In Figure 2(a)-(c) the GMO was included as a RM (system H+RM).…”

Friction Modifier Additives

“…Relatively little attention was generally paid to the way the head group bonded to the substrate. More recently simulations of specific OFMs have been carried out both to study how their packing on surfaces influences friction [103] and also how monolayers behave when separated by a high pressure/high shear rate film of base oil [104]. A limitation of all work to date is that because of the very small time steps needed to model molecular motion accurately and the relatively slow rate of molecular diffusion [105], it is not yet possible to simulate the process of self-assembly to form an adsorbed film or the equilibrium between solution and surface film.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Doig et al [232] also used NEMD simulations to gain insights into sum-frequency spectroscopy (SFS) and polarised neutron reflectometry (PNR) experiments [233] of the structure of hexadecylamine films adsorbed on iron oxide surfaces in dodecane and hexadecane. At the highest surface coverage investigated in the NEMD simulations (1.8 nm −2 ), the film thickness was about 15-20 Å, and the molecules had an average tilt angle from the surface normal of 40°-50°, in agreement with experiment.…”