Santiago Lago

Calculation of the absolute hydration enthalpy and free energy of H+ and OH−

The hydration enthalpy and Gibbs free energy of proton and hydroxide are calculated by means of a combination of ab initio density functional theory and a polarizable continuum model within the self-consistent reaction field method. The ion–water cluster models here used include up to 13 water molecules solvating the ions. This allows the first and second solvation shells to be described explicitly from first principles. Vibrational contributions to the enthalpy and entropy have been taken into account. Our best model of the hydrated proton includes three molecules in the first hydration shell and nine molecules in the second shell. The calculated proton hydration enthalpy is ≈−1150 kJ/mol, which is in rather good agreement with the most recent results from cluster–ion solvation data. The hydration free energy of the proton has a larger error of ≈50–80 kJ/mol as compared to recently reported values. The calculated hydroxide hydration enthalpy, ≈−520 kJ/mol, and hydration free energy, ≈−400 kJ/mol, are con...

Vapour—liquid equilibrium of the square-well fluid of variable range via a hybrid simulation approach

The equilibrium between vapour and liquid in a square-well system has been determined by a hybrid simulation approach combining chemical potentials calculated via the Gibbs ensemble Monte Carlo technique with pressures calculated by the standard NVT Monte Carlo method. The phase equilibrium was determined from the thermodynamic conditions of equality of pressure and chemical potential between the two phases. The results of this hybrid approach were tested by independent NPT and μPT calculations and are shown to be of much higher accuracy than those of conventional GEMC simulations. The coexistence curves, vapour pressures and critical points were determined for SW systems of interaction ranges λ = 1.25, 1.5, 1.75 and 2. The new results show a systematic dependence on the range λ, in agreement with results from perturbation theory where previous work had shown more erratic behaviour.

Monte Carlo study of liquid crystal phases of hard and soft spherocylinders

We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: (a) hard spherocylinders with an attractive square-well (SWSC) and (b) purely repulsive soft spherocylinders (SRS), in both cases for a length-to-breadth ratio L*=5. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*=5 probing thermodynamic states within the isotropic (I), nematic (N), and smectic A (Sm A) regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder (HSC) fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressur...

A novel orientation-dependent potential model for prolate mesogens

An intermolecular potential is introduced for the study of molecular mesogenic fluids. The model combines distinct features of the well-known Gay-Berne and Kihara potentials by incorporating dispersive interactions dependent on the relative pair orientation to a spherocylinder molecular core. Results of a Monte Carlo simulation study focused on the liquid crystal phases exhibited by the model fluid are presented. For the chosen potential parameters, molecular aspect ratio L*=5 and temperatures T*=2, 3, and 5, isotropic, nematic, smectic-A, and hexatic phases are found. The location of the phase boundaries as well as the equation of state of the fluid and further thermodynamical and structural parameters are discussed and contrasted to the Kihara fluid. In comparison to this latter fluid, the model induces the formation of ordered liquid crystalline phases at lower packing fractions and it favors, in particular, the appearance of layered hexatic ordering as a consequence of the greater attractive interaction assigned to the parallel side-to-side molecular pair configurations. The results contribute to the evaluation of the role of specific interaction energies in the mesogenic behavior of prolate molecular liquids in dense environments.

Self-consistent effective interactions in charged colloidal suspensions

We use an integral equation scheme to obtain self-consistently the effective interaction between colloids in salt-free charged colloidal suspensions. The colloid–counterion direct correlation function (DCF) is obtained for the fixed colloid–colloid pair structure by solving the corresponding hypernetted-chain equation (HNC). This DCF is then used to formulate an effective colloid–colloid pair potential for which the one-component reference hypernetted-chain equation is solved. Both processes are iterated until self-consistency is achieved. Counterion–counterion correlations are considered linear and uncoupled from the rest of the correlations. The method is based on a similar treatment utilized in liquid metals [Phys. Rev. B 61, 11400 (2000)] and provides equivalent results to those obtained using the standard multicomponent HNC equation for mixtures of charged hard spheres. The theory proves rather accurate when compared with molecular dynamic simulations of charged hard and soft spheres for colloidal ch...

Optical Characterization of Asphaltenes at the Air−Water Interface

Asphaltenes extracted from Arabian light crude oil have been investigated at the air-water interface of a Langmuir trough by in situ optical techniques. Brewster angle microscopy (BAM) and reflection spectroscopy have been used to extract new information about the microscopic organization of the asphaltene films in terms of association phenomena and chromophore orientation, respectively. The use of different spreading concentrations in the range 0.1-15 mg mL-1 reveals significant changes in the behavior at the interface with more condensed isotherms above 2 mg mL-1. This break point is related to the nanoaggregate-to-cluster association threshold in organic solution widely accepted in the recent asphaltene literature. BAM images support this observation with very different morphologies for the two spreading concentrations employed, 0.1 and 4 mg mL-1, respectively. The study of intensity changes in the corresponding normalized reflection spectra also confirms the transition in the asphaltene interfacial behavior between these two spreading concentrations. Finally, this technique helps with understanding the changes observed in the asphaltene films during a set of compression-decompression cycles.

Vapour–liquid equilibrium of fluids composed by oblate molecules

Gibbs ensemble Monte Carlo simulations are performed to obtain the vapour–liquid equilibrium of oblate-like fluids interacting through the Kihara intermolecular potential. Results confirm the validity of a perturbation theory for Kihara fluids, whose accuracy for prolate fluids was tested some years ago. As in the case of hard ellipsoids, the symmetry of the phase diagram of oblate and prolate models is analysed. An interesting relation of Boyle temperature and critical parameters with molecular volume is found for the considered models. As a particular application, this relation allows the prediction of some thermodynamic properties of a new promising biofuel 2,5dimethylfuran.

Adsorption of DNA to octadecylamine monolayers at the air–water interface

In this work, surface properties of octadecylamine (ODA) monolayers in the presence of different concentrations of calf thymus DNA in the aqueous subphase covering a range of 2-8μM have been investigated. The increase of DNA concentration is accompanied by a marked increment in the expansion of the corresponding isotherms. In addition, there is a change in the profile of the isotherms ranging from an abrupt liquid-solid transition for the lipid monolayer on pure water to a slow condensation of the monolayer in a liquid state when DNA is added to the subphase, demonstrating the effective adsorption of the polynucleotide to the long chain amine monolayer. Additional phase transitions appear in the isotherms upon addition of sufficient amount of DNA, revealing the existence of specific processes such as folding or squeezing out of the DNA. This system is, however, highly reversible during compression-expansion cycles due to the strong interaction between the two components. These results are also supported by Brewster Angle Microscopy (BAM) images showing significant changes in the morphology of the film. Integral reflectivity of the BAM microscope has been used to study both isotherms themselves and the kinetic process of DNA inclusion into the lipid-like ODA monolayer. This parameter has been proven to be very effective for quantification of the monolayer processes showing high consistency with the compressibility and kinetics results.

Gibbs ensemble simulation of the vapour-liquid equilibrium of square well spherocylinders

Gibbs ensemble Monte Carlo simulations have been performed for systems of square-well spherocylinders of different length-to-breadth ratio. The results are used to test a recent perturbation theory proposed for this kind of system. In addition, the results are compared to similar simulations performed for a Kihara fluid of elongated molecules. An unexpected good agreement is found for the coexistence thermodynamic and structural properties of both model fluids, hence suggesting that the hard spherocylinder plus square-well interaction should be considered as a reference potential for a perturbative treatment of more complex fluid models.

Liquid–vapour equilibrium of multipolar square-well fluids.Gibbs ensemble simulations and equation of state

Simulation results for a system comprising a square well plus either a point dipole or a point quadrupole are presented. The properties obtained are the vapour–liquid equilibrium densities and the critical properties. Critical densities are not very sensitive to the values of dipole or quadrupole, while critical temperatures increase significantly when the multipole strength rises. A comparison with a perturbation theory for multipolar square-well systems is presented. Overall agreement between simulated and theoretical values is good when comparison is restricted to quadrupoles or dipoles corresponding to the most relevant real polar substances but is only moderate for the largest multipolar strengths considered.