S. Karaborni

Computer simulations of vapor-liquid phase equilibria of n-alkanes

For petrochemical applications knowledge of the critical properties of the n‐alkanes is of interest even at temperatures where these molecules are thermally unstable. Computer simulations can determine the vapor–liquid coexistence curve of a large number of n‐alkanes ranging from pentane (C5) through octatetracontane (C48). We have compared the predicted phase diagrams of various models with experimental data. Models which give nearly identical properties of liquid alkanes at standard conditions may have critical temperatures that differ by more than 100 K. A new n‐alkane model has been developed by us that gives a good description of the phase behavior over a large temperature range. For modeling vapor–liquid coexistence a relatively simple united atom model was sufficient to obtain a very good agreement with experimental data; thus it appears not necessary to take the hydrogen atoms explicitly into account. The model developed in this work has been used to determine the critical properties of the long‐chain alkanes for which experiments turned out to be difficult and contradictory. We found that for the long‐chain alkanes (C8–C48) the critical density decreases as a function of the carbon number. These simulations were made possible by the use of a recently developed simulation technique, which is a combination of the Gibbs‐ensemble technique and the configurational‐bias Monte Carlo method. Compared with the conventional Gibbs‐ensemble technique, this method is several orders of magnitude more efficient for pentane and up to a hundred orders of magnitude for octatetracontane. This recent development makes it possible to perform routinely phase equilibrium calculations of complex molecules.

The swelling of clays: molecular simulations of the hydration of montmorillonite.

The swelling of clay minerals on contact with an aqueous solution can produce strong adverse effects in the exploration and production of gas and oil. Molecular dynamics and Monte Carlo simulations were used to study the mechanism of swelling of sodium-montmorillonite. The simulations showed that the abundant clay mineral has four stable states at basal spacings of 9.7, 12.0, 15.5, and 18.3 angstroms, respectively. The amount of swelling and the locations of the stable states of sodium-montmorillonite are in good quantitative agreement with the experimental data.

Synthesis, surface properties and oil solubilisation capacity of cationic gemini surfactants

Abstract The critical micelle concentration (CMC) and the surface tension at the CMC have been determined for the gemini surfactants alkanediyl-α,ω- bis (dimethylalkylammonium bromide) by means of dynamic surface tension measurements. For the same number of carbon atoms in the hydrophobic chain per hydrophilic head group, geminis have CMC values well below those of conventional single-chain cationic or anionic surfactants. Surface tension values at the CMC do not differ much from those observed for conventional surfactants; The propensity of gemini micelles for oil solubilisation is significantly better than that of conventional surfactants; this is true on a molar basis as well as on a weight basis. Geminis also show enhanced selectivity for aromatic compounds over paraffinic compounds. Some geminis show unusual viscoelastic behaviour at concentrations where this is not observed for conventional surfactants.

Molecular dynamics simulations of Langmuir monolayers: A study of structure and thermodynamics

Molecular dynamics (MD) simulations have been performed on Langmuir monolayers of single chain surfactants at the air–water interface using a new anisotropic united atom model (AUA) for chain–chain interactions and a dipolar potential for head–head repulsions. Water–surfactant interactions are modeled using an external potential that does not fix the head group positions. The forces of the skeletal chains involved intramolecular effects of angle bending, and rotation among quartets of adjacent segments. Several molecular dynamics simulations have been performed on monolayers with densities ranging from 18 to 30 A2/molecule. The results show two transitions in the monolayer. The first phase transition is a melting from a triangular lattice state maintained by the carbon chains to a fluidlike state with chain diffusion and lattice defects. The second transition is characterized by a change in molecular conformation, but with no change in lattice defects.

Molecular dynamics simulations of model oil/water/surfactant systems

Molecular dynamics simulations have been performed on amphiphilic molecules, oil and water to investigate surfactant behavior in water-like and oil-like solvents. Using a simple model for water, oil and surfactant molecules on a powerful parallel computer, we were able to simulate the adsorption of surfactants at the water/oil interface and the self-assembly of surfactant molecules into micelles. Simulations on various classes of surfactant molecules with different headgroup sizes and interactions show a strong dependence of the dynamics and morphology of surfactant aggregates on the surfactant structure. In the presence of an oil droplet, micelles induce the transfer of oil molecules from the oil droplet to the micelles by means of three mechanisms.

Tilt transitions in Langmuir monolayers of long-chain molecules

Molecular dynamics simulations have been used to investigate tilt transitions in a monolayer model of amphiphilic molecules at an air–water interface. Eight simulations were performed at 300 K on monolayers in the density range of 18.5–25 A2/molecule. The model amphiphilic molecules contained 19 pseudoatoms, each representing a methyl or a methylene group, and a head group representing a carboxylate group. Amphiphile–amphiphile interactions were modeled using a new anisotropic united atom model that accounts implicitly for the presence of hydrogen atoms in alkanes; water–amphiphile interactions were modeled using two external potentials that do not constrain the head groups to the interface, allow methylene segments to enter the water, and provide a finite size interface of the same order of magnitude as the size of the experimental water–air interface. The tilt behavior of the monolayer was monitored as a function of molecular area. Tilt angle results and structure factor analysis point to the occurrence...

Erratum: “Computer simulations of vapor–liquid phase equilibria of n-alkanes” [J. Chem. Phys. 102, 2126 (1995)]

in the analytical tail corrections used to estimate the in-teractions beyond a spherical potential truncation.The results reported in Ref. 1 for the OPLS and de Pablomodels are not affected by this error, because a sphericaltruncation at 11.5 A without tail corrections was used for theOPLS model and the minimum image convention was em-ployed for the de Pablo model. However, the numerical re-sults reported for the Toxvaerd model and the new modelproposed in Ref. 1 are in error. The values of the Lennard-Jones parameter sare 3.527 A ~Toxvaerd model! and 3.93 A~new model!. Thus, in the former case the tail correctionswere underestimated by a factor of 44 and in the latter by 61.These large factors mean that the results reported in Ref. 1correspond in practice to calculations performed without tailcorrections.We have repeated the calculations for n-pentane,n-octane, and n-dodecane for the new model using propervalues for the tail corrections. The corrected critical pointsand coexistence densities are reported in Tables I and II. Thecritical temperatures are shifted to higher values ~by4to6%!, but the critical densities and pressures are affected to alesser extent. The coexistence curves for the Toxvaerd modelshould shift in a similar way to higher temperatures, i.e.,improving the agreement with the experimental data. Thequalitative results, such as the chain-length dependencies ofthe critical properties, are not affected by the error.

Computer simulations of surfactant structures.

Computer simulations are continuing to enable significant progress to be made in research concerning the structure, dynamics and rheology of surfactant structures and how these parameters relate to the topics of surfactant self-assembly, micelles, amphiphilic monolayers, bilayers and oil solubilization. The best insight into self-assembly and oil solubilization has come from idealized models. For monolayers, quantitative agreement with experimental data has been achieved in predicting tilt transition, tilt angle and direction. A level of sophistication of simulations has been attained whereby perfect agreement with experiments is pursued on complex issues such as the ordering of the chain backbones in various monolayer phases. For bilayers, most relevant time scales may still be just outside the reach of standard molecular dynamics simulations; nevertheless, ingenious computational techniques that go beyond standard molecular dynamics and Monte Carlo simulations have enabled much faster progress than previously estimated.

Exploring the Formation of Multiple Layer Hydrates for a Complex Pharmaceutical Compound

The pharmaceutical compound A, 3-{2-oxo-3-[3-(5,6,7,8-tetrahydro[1,8]naphthyridin-2-yl)propyl]imidazolidin-1-yl}-3(S)-(6-methoxypyridin-3-yl)propionic acid, is known to exist in five different crystalline forms that differ in the hydration state ranging from the anhydrous desolvate over hemihydrate, dihydrate, and tetrahydrate forms to the pentahydrate. The formation of the higher hydrates and the concomitant lattice expansion leads to undesirable tablet cracking at higher humidities. In this work, particle-based simulation techniques are used to explore the hydrate formation of compound A as a function of humidity. It is found that a simulation strategy employing Monte Carlo simulations in the isobaric-isothermal and Gibbs ensembles and transferable force fields, which are not parametrized against any experimental data for compound A, is able to yield satisfactory crystal structures for the anhydrate and pentahydrate and to predict the existence of all five hydrates.

Simulating Complex Fluids

Abstract In this Article, a review is given on the progress of simulating complex fluids. Two approaches are used to deal with the special requirements of simulations of complex fluids. Molecular dynamics on massively parallel computers allow long simulations on very large systems. This makes it possible to simulate the self-assembly of micelles and the solubilization of a droplet of oil. For problems in which dynamics is not essential, it is shown that a novel method, configurational-bias Monte Carlo, can be used to simulate efficiently systems containing chain molecules. The use of this method is illustrated by a calculation of the vapour-liquid curve of an alkane as long as octatetracontane C48.