Molecular Dynamics Simulations of Glycerol Monooleate Confined between Mica Surfaces

The structure and frictional properties of glycerol monooleate (GMO) in organic solvents, with and without water impurity, confined and sheared between two mica surfaces are examined using molecular dynamics simulations. The structure of the fluid is characterized in various ways, and the differences between systems with nonaggregated GMO and with preformed GMO reverse micelles are examined. Preformed reverse micelles are metastable under static conditions in all systems. In <i>n</i>-heptane under shear conditions, with or without water, preformed GMO reverse micelles remain intact and adsorb onto one surface or another, becoming surface micelles. In dry toluene, preformed reverse micelles break apart under shear, while in the presence of water, the reverse micelles survive and become surface micelles. In all systems under static and shear conditions, nonaggregated GMO adsorbs onto both surfaces with roughly equal probability. Added water is strongly associated with the GMO, irrespective of shear or the form of the added GMO. In all cases, with increasing shear rate, the GMO molecules flatten on the surface, and the kinetic friction coefficient increases. Under low-shear conditions, the friction is insensitive to the form of the GMO added, whereas the presence of water is found to lead to a small reduction in friction. Under high-shear conditions, the presence of reverse micelles leads to a significant reduction in friction, whereas the presence of water increases the friction in <i>n</i>-heptane and decreases the friction in toluene

Solubility and Nucleation of Methyl Stearate as a Function of Crystallization Environment

Crystallization studies of methyl stearate from supersaturated dodecane, kerosene, and toluene solutions reveal strong evidence that solvent choice influences solubility and nucleation behavior. Solute solubility is less than ideal with toluene, kerosene, and dodecane, respectively, exhibiting the closest behavior to ideality, the latter consistent with the highest solvation. Polythermal crystallization studies using the Kashchiev–Borissova–Hammond–Roberts (KBHR) model [Kashchiev et al. J. Phys. Chem. B 2010, 114, 5441; Kashchiev et al. J. Cryst. Growth 2010, 312, 698; Camacho et al. CrystEngComm 2014, 16, 974] reveal a progressive nucleation (PN) mechanism with crystallite interfacial tension (γeff) values between 0.94 and 1.55 mJ/m2, between 1.21 and 1.91 mJ/m2, and between 1.18 and 1.88 mJ/m2 for dodecane, kerosene, and toluene, respectively. Nucleation rates at the critical undercooling lie between 4.56 × 1016 and 1.79 × 1017 nuclei/mL·s, with the highest rates associated with crystallization from kerosene solutions. Iso-supersaturation nucleation rates are the highest for dodecane ranging from 2.39 × 1017 to 3.63 × 1018 nuclei/mL·s. Nucleation in toluene appears to be hindered by its relatively higher interfacial tension, which is associated with nucleation rates about an order of magnitude less than those obtained for dodecane

The crystal morphology and growth rates of triclinic N-docosane crystallising from N-dodecane solutions

A detailed analysis of the crystal morphology of triclinic n-docosane (C22H46) is presented together with a preliminary assessment of the supersaturation-dependence of the growth rates for the predicted (hkl) faces. A methodology to index the experimentally observed crystal faces, based on a combined BFDH and zone axis methodology is defined. Analysis using this methodology yields the morphological indexation of n-docosane to be (001), (112), (102), (010), and (1 - 33) or (130) based on the expected triclinic crystal structure. Crystals of n-docosane growing from supersaturated n-dodecane (C12 H26) solutions, as studied using in-situ optical microscopy, at three different supersaturation (σ) levels 0.01, 0.02 and 0.05, reveal that the crystal morphology changes with increasing in supersaturation, evolving from a habit consistent with a triclinic crystal system to a habit that is perhaps more representative of an orthorhombic structure. Growth rates determined for the (112) and (102) faces as well as for those less dominant faces range between 0.51 and 9.85 mm/s, in good agreement with previously reported data for other organic molecules including n-alkanes

Molecular dynamics simulations of structure and friction in lubricants

Glycerol monooleate (GMO) is a common engine oil lubricant additive used to reduce friction and wear in engines. The aim of this thesis is to investigate the properties of GMO in bulk and under confined conditions with shear using molecular dynamics simulations. The self-assembly of GMO into reverse micelles (RMs) in toluene and n-heptane solvents is studied at a range of concentrations and subsequently with several common engine impurities over simulation timescales of 5-30 ns. The dimensional properties of the RMs are found to correspond well with experimentally studied SANS/SAXS measurements. Secondly, the properties of GMO confined between mica surfaces are studied under quiescent and shear conditions. Under shear, the performance of GMO as a friction modifier is studied and the structural and frictional properties are examined. In particular, the mass density and velocity profiles of the fluid are used to gain insights into the structure and dynamics of the confined GMO fluid films, under a variety of shear rates and surface separation. The data is found to fit excellently to the universal friction curve. Following this the effect of hydrolysing the GMO to oleic acid is studied in bulk and under shear, it is found that increasing oleic acid concentration typically reduces the propensity to self-assemble and under confinement increases the friction coefficient. And finally, a study on a range of other similar surfactants is conducted to investigate the effect of unsaturation, head group and chain length on the calculated friction coefficient and the structure of the surfactant films

Glycerol Monooleate Reverse Micelles in Nonpolar Solvents: Computer Simulations and Small-Angle Neutron Scattering

The formation of glycerol monooleate reverse micelles in <i>n</i>-heptane and toluene at room temperature is studied using molecular-dynamics simulations and small-angle neutron scattering. The glycerol monooleate concentrations under consideration are in the range of 5–20 wt %. Under these conditions, spontaneous reverse-micelle formation is observed on the simulation timescale (up to 30 ns). From simulations, the typical dimensions (semiaxes) of the equivalent ellipsoids with the same masses and moments of inertia are in the range of 15–23 Å, with instantaneous shapes that are slightly nonspherical. By analyzing the scattering form factors from simulation and experiment, the radii of gyration of the reverse micelles are determined to be approximately 15 Å. The number of glycerol monooleate molecules in a reverse micelle is smaller in toluene (∼20) than in <i>n</i>-heptane (∼30), but the overall dimensions are similar due to greater penetration of the toluene in to the reverse micelle. The effects of low concentrations (1 wt %) of water, acetic acid, and ethanol on the reverse-micelle dimensions are determined. The overall structural effects are small, but the distributions of the molecules within the reverse micelles are shown to be sensitive to the molecular polarity

Time and space resolved methods: general discussion

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Time and space resolved methods : General discussion

Jim De Yoreo presented some slides on in situ AFM, TEM, dynamic force spectroscopy (DFS) and optical spectroscopy investigations of nucleation in the calcium carbonate system: The free energy barrier to homogeneous nucleation of calcite calculated within the framework of classical nucleation theory (CNT) is prohibitive, even at concentrations exceeding the solubility limits of the amorphous phases. Consistent with this analysis, during nucleation in pure solutions, in our in situ TEM experiments we observed direct formation of all phases, including amorphous calcium carbonate (ACC), as well as the three predominant crystalline phases: calcite, vaterite, and aragonite, even under conditions in which ACC readily forms. In addition to direct formation pathways, we observed indirect pathways in which ACC transforms to aragonite and vaterite through nucleation within or on the precursors, rather than via dissolution and reprecipitation. We also observed aragonate transformation to calcite, but never recorded an instance in which ACC transforms into calcite, except via dissolution–reprecipitation reactions

Molecular self-assembly and clustering in nucleation processes: general discussion

crosscheck: This document is CrossCheck deposited related_article: http://dx.doi.org/10.1039/C4FD00215F related_article: http://dx.doi.org/10.1039/C4FD00217B related_article: http://dx.doi.org/10.1039/C4FD00275J related_article: http://dx.doi.org/10.1039/C4FD00225C related_article: http://dx.doi.org/10.1039/C4FD00269E related_article: http://dx.doi.org/10.1039/C4FD00262H related_article: http://dx.doi.org/10.1039/C5FD00037H copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal history: Advance Article published 15 June 2015; Version of Record published 29 June 2015status: publishe