Molecular Dynamics Studies of Overbased Detergents on a Water Surface

Abstract

Molecular dynamics (MD) simulations are reported of model overbased detergent nanoparticles on a model water surface which mimic their behavior on a Langmuir trough or large water droplet in engine oil. The simulations predict that the structure of the nanoparticle on a water surface is different to when it is immersed in a bulk hydrophobic solvent. The surfactant tails are partly directed out of the water, while the carbonate core maximizes its extent of contact with the water. Umbrella sampling calculations of the potential of mean force between two particles showed that they are associated with varying degrees with a maximum binding free energy of ca. 10 kT for the salicylate stabilized particle, ca. 8 kT for a sulfurized alkyl phenate stabilized particle, and ca. 5 kT for a sulfonate stabilized particle. The differences in the strength of attraction depend on the proximity of nearest approach and the energy penalty associated with the disruption of the hydration shell of water molecules around the calcium carbonate core when the two particles approach. This is greatest for the sulfonate particle, which partially loses the surfactant ions to the solution, and least for the salicylate, which forms the weakest water "cage". The particles are separated by a water hydration layer, even at the point of closest approach.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…The function of many lubricant additives depends on chemical reactions, which are generally not considered in classical MD simulations. Despite this, classical MD simulations have given unique insights into the behaviour of a range of lubricant additives including detergents [207,208], dispersants [209,210], viscosity modifiers [91,92,211], anti-wear additives [212][213][214][215], and corrosion inhibitors [216][217][218]. The most widely studied class of lubricant additives with MD are friction modifiers [219], which are a wellsuited application of confined NEMD simulations.…”

Mixed Flow Reactor Experiments and Modeling of Sulfuric Acid Neutralization in Lube Oil for Large Two-Stroke Diesel Engines

“…Use of slow-steaming, combined with new engine designs and tunings for further fuel savings, led to increased operating cylinder pressure and lower liner surface temperature . These changes in operating conditions result in increased water and acid condensation on the cylinder lubrication (lube) oil film on the cast iron liner surfaces, − promoting a combination of corrosion and wear (cold corrosion) of the cylinder liners and piston rings and significantly reducing their lifetimes. ,− The condensing acid is mainly aqueous sulfuric acid (H 2 SO 4 ), originating from the oxidation of the sulfur-rich fuel oil burned in marine diesel engines. , Commercial lube oils are typically formulated with calcium carbonate (CaCO 3 ) overbased detergent additives, present as nanometer-sized reverse micelles, which neutralize condensed H 2 SO 4 in the lube oil film. − To counteract the increased condensation rate of H 2 SO 4 , it is necessary to increase the amount of CaCO 3 being fed to the cylinder liners, which is preferably done by increasing the concentration of CaCO 3 in the lube oil rather than by increasing the oil feed rate. ,, Under-lubrication may result in cold corrosion, which can occur within hours, whereas overlubrication is cost-intensive and may lead to bore-polish (creating a mirrorlike cylinder liner surface), which prevents formation of a coherent lube oil film. Both may eventually result in scuffing (direct metal to metal contact). , Actually, a certain degree of controlled corrosion is beneficial; if the cylinder liners are a little rough, they can better maintain a protective lube oil film. , Therefore, to limit and control cold corrosion in engines, the lubrication strategy must be optimized.…”

Pulling Simulations and Hydrogen Sorption Modelling on Carbon Nanotube Bundles

“…Pull codes can now be routinely called by most molecular dynamics programs. The method is increasingly being applied to better understand a range of molecular interactions and allow one to describe the free energy profile along a particular reaction coordinate [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48].…”