Andrés Mejía

Pickering emulsions stabilized by amphiphilic nano-sheets

We demonstrate the fabrication of amphiphilic nano-sheets, which are either surface- or edge-modified plates with thickness at the atomic scale, one of the thinnest amphiphilic particles reported so far. They are obtained by exfoliation of functionalized layered crystals, the first time that laminar structures have been utilized to produce such particles. Stable emulsions were made utilizing these nano-sheets. The adsorption of the amphiphilic nano-sheets onto the oil-in-water interfaces and the reduction of surface tension between the PDMS and the amphiphilic nano-sheet suspensions were quantitatively characterized.

Phase and interface behaviors in type-I and type-V Lennard-Jones mixtures: theory and simulations.

Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.

Stabilization of Pickering foams by high-aspect-ratio nano-sheets

We developed Pickering foams highly stabilized by high-aspect-ratio (ξ = diameter/thickness) nano-sheets. The effects of particle aspect ratio, concentration, and hydrophobicity were also investigated. To our knowledge, our study provides the first experimental evidence of the effect of particle aspect ratio on particle-stabilized foams. The adsorption properties of these highly anisotropic nano-sheets are strongly affected by their small thickness and large lateral size (i.e., two-dimensional). These high-aspect-ratio nano-sheets were obtained by exfoliation of α-zirconium phosphate (ZrP) crystals with propylamine (C3H7NH2, PA). The hydrophobicity of the nano-sheets was tailored by adjusting the PA : ZrP molar ratio in the suspension. The morphology and stability of the foam depend on the nano-sheet aspect ratio and concentration as well as on the PA : ZrP molar ratio. Here, we found that using low and high aspect ratio nano-sheets having a high and an intermediate degree of hydrophobicity, respectively, is the successful formula to obtain high foam stability. The aqueous foams were characterized by optical and cross-polarized micrographs. Scanning electron microscopy (SEM) micrographs of dried foams revealed the adsorption of the PA–ZrP nano-sheets on the air–water interface. The foam stability was studied by measuring the foam and the water volume as a function of time to obtain the foam decay and water drainage rate, respectively. We also observed that the foams were stabilized by jammed layers of nano-sheets located in the bulk and at the air–water interface. These layers of particles prevent air diffusion between the bubbles, hence arresting Ostwald ripening and coalescence.

Comparison of United-Atom Potentials for the Simulation of Vapor–Liquid Equilibria and Interfacial Properties of Long-Chain n-Alkanes up to n-C100

Canonical ensemble molecular dynamics (MD) simulations are reported which compute both the vapor-liquid equilibrium properties (vapor pressure and liquid and vapor densities) and the interfacial properties (density profiles, interfacial tensions, entropy and enthalpy of surface formation) of four long-chained n-alkanes: n-decane (n-C(10)), n-eicosane (n-C(20)), n-hexacontane (n-C(60)), and n-decacontane (n-C(100)). Three of the most commonly employed united-atom (UA) force fields for alkanes (SKS: Smit, B.; Karaborni, S.; Siepmann, J. I. J. Chem. Phys. 1995,102, 2126-2140; J. Chem. Phys. 1998,109, 352; NERD: Nath, S. K.; Escobedo, F. A.; de Pablo, J. J. J. Chem. Phys. 1998, 108, 9905-9911; and TraPPE: Martin M. G.; Siepmann, J. I. J. Phys. Chem. B1998, 102, 2569-2577.) are critically appraised. The computed results have been compared to the available experimental data and those fitted using the square gradient theory (SGT). In the latter approach, the Lennard-Jones chain equation of state (EoS), appropriately parametrized for long hydrocarbons, is used to model the homogeneous bulk phase Helmholtz energy. The MD results for phase equilibria of n-decane and n-eicosane exhibit sensible agreement both to the experimental data and EoS correlation for all potentials tested, with the TraPPE potential showing the lowest deviations. However, as the molecular chain increases to n-hexacontane and n-decacontane, the reliability of the UA potentials decreases, showing notorious subpredictions of both saturated liquid density and vapor pressure. Based on the recommended data and EoS results for the heaviest hydrocarbons, it is possible to attest, that in this extreme, the TraPPE potential shows the lowest liquid density deviations. The low absolute values of the vapor pressure preclude the discrimination among the three UA potentials studied. On the other hand, interfacial properties are very sensitive to the type of UA potential thus allowing a differentiation of the potentials. Comparing the interfacial tension MD results to the available experimental data and SGT results, the TraPPE model exhibits the lowest deviations for all hydrocarbons.

Uniform discotic wax particles via electrospray emulsification

We present a novel colloidal discotic system: the formation and self-assembling of wax microdisks with a narrow size distribution. Uniform wax emulsions are first fabricated by electrospraying of melt alpha-eicosene. The size of the emulsions can be flexibly tailored by varying the flow rate of the discontinuous phase, its electric conductivity, and the applied voltage. The process of entrainment of wax droplets, vital for obtaining uniform emulsions, is facilitated by the reduction of air-water surface tension and the density of the continuous phase. Then uniform wax discotic particles are produced via phase transition, during which the formation of a layered structure of the rotator phase of wax converts the droplets, one by one, into oblate particles. The time span for the conversion from spherical emulsions to disk particles is linearly dependent on the size of droplets in the emulsion, indicating the growth of a rotator phase from surface to the center is the limiting step in the shape transition. Using polarized light microscopy, the self-assembling of wax disks is observed by increasing disk concentration and inducing depletion attraction among disks, where several phases, such as isotropic, condensed, columnar stacking, and self-assembly of columnar rods are present sequentially during solvent evaporation of a suspension drop.

Resolving Discrepancies in the Measurements of the Interfacial Tension for the CO2 + H2O Mixture by Computer Simulation.

Literature values regarding the pressure dependence of the interfacial tension of the system of carbon dioxide (CO2) + water (H2O) show an unexplained divergence and scatter at the transition between low-pressure gas-liquid equilibrium and the high-pressure liquid-liquid equilibrium. We employ the Statistical Associating Fluid Theory (SAFT) and canonical molecular dynamics simulations based on the corresponding coarse grained force field to map out the phase diagram of the mixture and the interfacial tension for this system. We showcase how at ambient temperatures a triple point (gas-liquid-liquid) is expected and detail the implications that the appearance of the third phase has on the interfacial tensions of the system.

Hydrothermal synthesis of layered α-zirconium phosphate disks: control of aspect ratio and polydispersity for nano-architecture

The crystals of layered compounds are normally in the shape of disks or plates. Systematic experiments revealed that regular-shaped α-zirconium phosphate crystalline disks with a size-to-thickness ratio from 1 to 50 and size polydispersity as low as 0.2 can be obtained through hydrothermal treatment in 3 M to 15 M phosphoric acid solutions. Transmission and scanning electron micrographs revealed that the growth of the disks is mediated by oriented attachment, which happened continuously throughout the hydrothermal treatment between various sized disks. Ostwald ripening is effective in improving the regularity of the shape of the disks, especially under prolonged hydrothermal treatment. Preferred attachment among the flat surfaces of the disks leads to the diverse developments of their size and thickness polydispersities.

Perfect wetting along a three-phase line: Theory and molecular dynamics simulations

Wetting behavior along a three-phase equilibrium has been obtained by density gradient theory (DGT) and molecular dynamics simulations for a type-II equal size Lennard-Jones mixture. In order to perform a consistent comparison between both methodologies, the molecular parameters of this type of mixture were defined from the global phase diagram of equal size Lennard-Jones mixtures. We have found excellent agreement between predictions from the DGT (coupled to a Lennard-Jones equation for the bulk phases) and simulations results for both the phase and interface behavior, in the whole temperature, pressure, and concentration ranges. For all conditions explored in this work, this type-II mixture shows a three-phase equilibrium composed by a bulk immiscible liquid phase (L1) and a bulk gas phase (G) separated by a second immiscible liquid phase (L2). A similar phase distribution is obtained from the interfacial concentration profile in the whole range of conditions used in this work. This type of structure is a clear evidence that L2 completely wets the GL1 interface. The wetting behavior is also confirmed by the values and evolution of the interfacial tensions. In summary, this kind of type-II mixture does not show wetting transitions and exhibits a permanent perfect wetting in all the thermodynamic conditions explored here.

Association and molecular chain length effects on interfacial behavior

The scope of this work is to analyze the influence of the molecular chain length and association effects on the properties of vapor–liquid interfaces. Calculations are based on the gradient theory applied to the Statistical Associated Fluid Theory (SAFT) equation of state (EOS), yielding thus an approach that predicts both phase equilibrium and interfacial properties. In addition, the theoretical structure of the SAFT-EOS includes the specific effects under consideration. The approach proposed here is coherent with the density functional theory, although it is more direct to apply, and predictions are in good agreement with experimental data. Results show that the interface thickness decreases, while the interfacial tension increases, as the molecular chain length and/or the association degree increases at isothermal conditions. Such a trend may be explained in terms of the distortion of the cohesion energy. Detailed examples are discussed for subcritical binary mixtures and predictions are confronted with experimental data. §Partial results of this paper were presented at the 17th iupac conference on chemical thermodynamics, Rostock, Germany on July 28 – August 2, 2002.

Isobaric Vapor-Liquid Equilibria and Densities for the Binary Systems Oxolane + Ethyl 1,1-Dimethylethyl Ether, Oxolane + 2-Propanol and Propan-2-One + Trichloromethane

Vapor-liquid equilibrium data have been determined at 50 kPa for the binary systems oxolane (THF) + ethyl 1,1-dimethylethyl ether (ETBE) and oxolane + 2-propanol, and at 94 kPa for the system propan-2-one + trichloromethane. Excess volumes have also been determined from density measurements at 298.15 K. The systems oxolane + ethyl 1,1-dimethylethyl ether and oxolane + 2-propanol exhibit slight to moderate positive deviations from ideal behavior and no azeotrope is present. The system propan-2-one + trichloromethane exhibits negative deviations from ideal behavior and presents an azeotrope. The excess volumes of the system oxolane + ethyl 1,1-dimethylethyl ether are negative over the whole mole fraction range while those of the system oxolane + 2-propanol are positive. Excess volumes of the system propan-2-one + trichloromethane, change from negative to positive as the concentration of propan-2-one increases. The activity coefficients and boiling points of the solutions were correlated with the mole fraction by the Wohl, Wilson, UNIQUAC, and NRTL equations, and predicted by the UNIFAC group contribution method. Excess volume data were correlated using the Redlich-Kister expansion. The chemical association theory was applied for explaining the equilibrium behavior of the systems oxolane + 2-propanol and propan-2-one + trichloromethane.