Raman spectroscopy and molecular dynamics simulations of aqueous two-phase systems for the purification of phosphoric acid.

In this work, Raman spectroscopy and molecular dynamics simulations were used to elucidate key interactions between polyethylene glycol (PEG) and phosphoric acid (H3PO4) in aqueous two-phase systems for the extraction of phosphoric acid. Extensive molecular dynamics simulations were performed, and radial distribution functions as well as hydrogen bonds between PEG and other molecules were measured. Experimental data were used in combination with the slope method to infer PEG-H3PO4 interactions, and the interpretation is consistent with molecular simulation results. Based on our experimental and simulation results, we propose a solvation mechanism governed by hydrogen bonding interactions: at low concentrations of H3PO4 within the polymer-rich aqueous solution, entropy dominates and phosphoric acid molecules have weak interactions with PEG; as the concentration of phosphoric acid increases above a certain critical value, enthalpy dominates with PEG molecules interacting strongly with H3PO4 molecules via hydrogen bonds.

Determination of liquid-vapor equilibrium and critical properties of fatty acids for biodiesel production through molecular dynamics.

In recent years, biodiesel production has emerged as an option for renewable and green fuel generation due to the constant reduction of fossil fuel reservoirs. Biofuels as biodiesel also show valuable attributes, environmentally speaking, due to their low environmental impact, contributing to the achievement of sustainability. However, costs are not allowable for large-scale production. Thereby, several novel processes have been proposed (e.g., reactive distillation) to solve this issue. An inconvenience for the development of these processes is the little information in the literature about the critical properties of fatty acids, which are precursors of biodiesel. Determination of critical properties for fatty acids through experimentation is difficult. The reason is that fatty acids tend to self-associate (to dimerize) due to carboxylic groups presence through hydrogen bonds, and consequently, have higher boiling points than other compounds of similar molecular mass (e.g., hydrocarbons, esters). Therefore, alternative methods for this determination are required. One choice is the group-contribution method, which is based on the structure of the molecule; however, results can significantly vary among different group-contribution approaches. Another alternative (and the focus of this research) for the determination of these properties is molecular simulation techniques. In this work, the liquid-vapor equilibrium as a function of temperature and the surface tension of three pure fatty acids of long chain (linoleic, oleic, and palmitic acid) have been calculated. Simulations have been performed by molecular dynamics using the method of direct determination of phase coexistence with the software GROMACS; in which the transferable potentials for phase equilibria united atom forcefield (TraPPE-UA) have been implemented for these specific molecules. Orthobaric densities and surface tension values have been reported at temperatures near the critical point (from 650 K to 800 K). Critical properties (temperature, pressure, density) have been extrapolated from trajectories obtained in these simulations using scaling law relations. Critical properties for these compounds are not available experimentally, therefore, group contribution calculations from the literature were used as a reference. In this comparison, the palmitic acid properties calculated in this work, show the best agreement among the three substances investigated.

Directing the far-from-equilibrium assembly of nanoparticles in confined liquid crystals by hydrodynamic fields.

The assembly of nematic colloids relies on long-range elastic interactions that can be manipulated through external stimuli. Confinement and the presence of a hydrodynamic field alter the defect structures and the energetic interactions between the particles. In this work, the assembly landscape of nanoparticles embedded in a nematic liquid crystal confined in a nanochannel under a pressure-driven flow is determined. The dynamics of the liquid crystal tensor alignment field is determined through a Poisson-Bracket framework, namely the Stark-Lubensky equations, coupled with the zero-Reynolds momentum equations and the liquid crystal Landau-de Gennes free energy functional. A second order semi-implicit time integration and a three-dimensional Galerkin finite element method are used to resolve flow and nematic fields under several conditions. In general, the zero Reynolds flow displaces the defects around the particles in the upstream direction and renders the surface anchoring ineffective when the flow strength dominates over the nematic elasticity. More importantly, the potential of mean force for particle assembly is non-monotonic independent of surface anchoring. Our results show that the confinement length scale determines the repulsion/attraction transition between colloids, while the flow strength modifies the static defect structure surrounding the particles and determines the magnitude of the energetic barrier for successful assembly. In the attractive regime, the particles move at different rates through the nematic until one particle eventually catches up with the other. This process occurs against or along the direction of flow depending on the flow strength. Ultimately, these results provide a template for engineering and controlling the transport and assembly of nanoparticles under far-from equilibrium conditions in anisotropic media.

Hierarchical complex self-assembly in binary nanoparticle mixtures.

Hierarchical self-assembly of soft matter provides a powerful route to create complex materials with enhanced physical properties. The understanding of the fundamental processes leading to such organization can provide design rules to create new functional materials. In this work, we use a simple model of polymer-grafted nanoparticles to explore the self-assembly of binary mixtures. By using Monte Carlo simulations we study the interplay of composition, density and particle sizes on the self-organization of such nanoparticle systems. It is found that complex hierarchical organization can take place for conditions where one-component systems form simple lattices. In particular, a mixture where one component forms a structure with 18-fold symmetry in a sea of an apparent disordered phase of the second component is observed to emerge for certain parameter combinations.

Fluctuations and phase transitions of uniaxial and biaxial liquid crystals using a theoretically informed Monte Carlo and a Landau free energy density.

In this work, we explore fluctuations during phase transitions of uniaxial and biaxial liquid crystals using a phenomenological free energy functional. We rely on a continuum-level description of the liquid crystal ordering with a tensorial parameter and a temperature dependent Landau polynomial expansion of the tensor's invariants. The free energy functional, over a three-dimensional periodic domain, is integrated with a Gaussian quadrature and minimized with a theoretically informed Monte Carlo method. We reconstruct analytical phase diagrams, following Landau and Doi's notations, to verify that the free energy relaxation reaches the global minimum. Importantly, our relaxation method is able to follow the thermodynamic behavior provided by other non-phenomenological approaches; we predict the first order character of the isotropic-nematic transition, and we identify the uniaxial-biaxial transition as second order. Finally, we use a finite-size scaling method, using the nematic susceptibility, to calculate the transition temperatures for 4-Cyano-4'-pentylbiphenyl (5CB) and N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Our results show good agreement with experimental values, thereby validating our minimization method. Our approach is an alternative towards the relaxation of temperature dependent continuum-level free energy functionals, in any geometry, and can incorporate complicated elastic and surface energy densities.

Phase diagram of a square-well model in two dimensions.

The phase behavior of a two-dimensional square-well model of width 1.5σ, with emphasis on the low-temperature and/or high-density region, is studied using Monte Carlo simulation in the canonical and isothermal-isobaric ensembles, and discontinuous molecular-dynamics simulation in the canonical ensemble. Several properties, such as equations of state, Binder cumulant, order parameters, and correlation functions, were computed. Numerical evidence for vapor, liquid, hexatic, and triangular solid is given, and, in addition, a non-compact solid with square-lattice symmetry is obtained. The global phase diagram is traced out in detail (or sketched approximately whenever only inaccurate information could be obtained). The solid region of the phase diagram is explained using a simple mean-field model.

Liquid-vapor equilibrium and surface properties of short rigid chains with one long range attractive potential.

Liquid-vapor coexistence and interfacial properties of short lineal rigid vibrating chains with three tangent monomers in two and three dimensions are calculated. The effect of the range and position of a long ranged square well attractive potential is studied. Orthobaric densities, vapor pressures, surface tensions, and interfacial widths are reported. Two types of molecules are studied. Chains of three tangent hard sphere monomers and chains of three and five tangent hard sphere monomers interacting with a square well attractive potential with λ(∗) = λ∕σ = 1.5 in units of the hard core diameter σ. The results are reported in two and three dimensions. For both types of chains, a long ranged square well attractive potential is located at various positions in the chain to investigate its effect in the properties of the corresponding systems. Results for hard sphere chains are presented for a series of different sizes of λ(∗) between 2.5 and 5. For square well chains the position in the chain of the long ranged potential has no influence in the coexistence and interfacial properties. Critical temperatures increase monotonically with respect to λ(∗) and critical densities decrease systematically for both types of chains. When the long ranged potential is located in the middle monomer of the hard sphere chains no critical point is found for λ(∗) < 2.4. No critical point is found when the long ranged potential is located in one of the extremes of the hard sphere chains.

Effect of flexibility on liquid-vapor coexistence and surface properties of tangent linear vibrating square well chains in two and three dimensions.

The effect of flexibility on liquid-vapor and interfacial properties of tangent linear vibrating square well chains is studied. Surface tension, orthobaric densities, vapor pressures, and interfacial thicknesses are reported and analyzed using corresponding states principles. Discontinuous molecular dynamics simulations in two and three dimensions are performed on rigid tangent linear vibrating square well chains of different lengths. In the case of two dimensions, simulation results of completely flexible tangent linear vibrating square well chains are also reported. Properties are calculated for chains of 2-12 monomers. Rigidity is controlled by trapping the first and last monomer in the chain in a vibrating well at half of the distance of the whole chain. Critical property values are reported as obtained from orthobaric densities, surface tensions, and vapor pressures. For the fully flexible chains, the critical temperatures increase with chain length but the effect saturates. In contrast, the critical temperatures increase for the rigid chains until no more critical point is found.