A molecular dynamics study of CaCO₃ nanoparticles in a hydrophobic solvent with a stearate co-surfactant

Abstract

Stearates containing overbased detergent nanoparticles (NPs) are used as acid neutralising additives in automotive and marine engine oils. Molecular dynamics (MD) simulations of the self-assembly of calcium carbonate, calcium stearate as a co-surfactant and stabilising surfactants of such NPs in a model explicit molecular hydrophobic solvent have been carried out using a methodology described first by Bodnarchuk et al. [J. Phys. Chem. C, 2014, 118, 21092]. The cores and particles as a whole become more elongated with stearate, and the surfactant molecules are more spaced out in this geometry than in their stearate-free counterparts. The rod dimensions are found to be largely independent of the surfactant type for a given amount of CaCO3. The corresponding particles without stearate were more spherical, the precise shape depending to a greater extent on the chemical architecture of the surfactant molecule. The rod-shaped stearate containing nanoparticles penetrated a model water droplet to a greater depth than the corresponding near-spherical particle, which is possibly facilitated by the dissociation of nanoparticle surfactant molecules onto the surface of the water in this process. These simulations are the first to corroborate the nanoparticle-water penetration mechanism proposed previously by experimental groups investigating the NP acid neutralisation characteristics.

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.…”

Molecular Dynamics Studies of Overbased Detergents on a Water Surface

“…In the third stage, steered MD and umbrella sampling was used to calculate the free energy of association between two nanoparticles of the same surfactant type, using the same number of species in the OB particle and water as in simulations D to F. The number of each species in the simulations is given in Table . The model nanoparticles in the simulations were performed using the Coulombic annealing and re-forming procedure invented and described in refs − .…”

“…Resolution of this issue reduces to identifying the role that the water interface plays in determining the OB particle shape and also on the effective interaction energy as a function of separation between two OB particles. Our more recent molecular simulation studies have shown that even trace amounts of water interact strongly with the OB calcium carbonate core in the bulk oil. , Therefore, it might reasonably be expected that the effects of surrounding water in the LT experiment arrangement could be significant, although this issue has yet to be fully resolved. This is one of the objectives of this study.…”

“…Our more recent molecular simulation studies have shown that even trace amounts of water interact strongly with the OB calcium carbonate core in the bulk oil. 18,19 Therefore, it might reasonably be expected that the effects of surrounding water in the LT experiment arrangement could be significant, although this issue has yet to be fully resolved. This is one of the objectives of this study.…”

“…There is a body of evidence to support the nanoparticle nature of the overbased chemical additive, in which the individual particles are in the form of reverse micelles each just a few nanometers across. This comes from neutron scattering, − transmission electron microscopy (TEM), − molecular simulation, − and Langmuir trough measurements. ,, The nanoparticle composition of the product facilitates its uniform distribution on dissolution in the oil. The rate of acid neutralization can as a result be controlled more systematically by adjusting the chemistry.…”