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The aim of this study was to investigate the impact of the addition of different amounts of magnesium aluminum silicate (MAS) to polymeric films based on quaternary polymethacrylates (QPMs, here Eudragit RS and RL). MAS contains negatively charged SiO(-) groups, while QPM contains positively charged quaternary ammonium groups. The basic idea is to be able to provide desired water and drug permeability by simply varying the amount of added MAS. Thin, free films of varying composition were prepared by casting and exposed to 0.1M HCl and pH 6.8 phosphate buffer. The water uptake kinetics and water vapor permeability of the systems were determined gravimetrically. The transport of propranolol HCl, acetaminophen, methyl-, ethyl- and propylparaben across thin films was studied using side-by-side diffusion cells. A numerical solution of Fick's second law of diffusion was applied to determine the apparent compound diffusion coefficients, partition coefficients between the bulk fluids and the films as well as the apparent film permeability for these compounds. The addition of MAS resulted in denser inner film structures, at least partially due to ionic interactions between the positively charged quaternary ammonium groups and the negatively charged SiO(-) groups. This resulted in lower water uptake, reduced water vapor permeability and decreasing apparent compound diffusivities. In contrast, the affinity of the investigated drugs and parabens to the films substantially increased upon MAS addition. The obtained new knowledge can be helpful for the development of novel coating materials (based on QPM-MAS blends) for controlled-release dosage forms., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The aim of this study was to use a mechanistically realistic mathematical model based on Fick's second law to quantitatively predict the release profiles from solid lipid extrudates consisting of a ternary matrix. Diprophylline was studied as a freely water-soluble model drug, glycerol tristearate as a matrix former and polyethylene glycol or crospovidone as a pore former (blend ratio: 50:45:5%w/w/w). The choice of these ratios is based on former studies. Strains with a diameter of 0.6, 1, 1.5, 2.7 and 3.5mm were prepared using a twin-screw extruder at 65 °C and cut into cylinders of varying lengths. Drug release in demineralised water was measured using the USP 32 basket apparatus. Based on SEM pictures of extrudates before and after exposure to the release medium as well as on DSC measurements and visual observations, an analytical solution of Fick's second law of diffusion was identified in order to quantify the resulting diprophylline release kinetics from the systems. Fitting the model to one set of experimentally determined diprophylline release kinetics from PEG containing extrudates allowed determining the apparent diffusion coefficient of this drug (or water) in this lipid matrix. Knowing this value, the impact of the dimensions of the cylinders on drug release could be quantitatively predicted. Importantly, these theoretical predictions could be confirmed by independent experimental results. Thus, diffusion is the dominant mass transport mechanism controlling drug release in this type of advanced drug delivery systems. In contrast, theoretical predictions of the impact of the device dimensions in the case of crospovidone containing extrudates significantly underestimated the real diprophylline release rates. This could be attributed to the disintegration of this type of dosage forms when exceeding a specific minimal device diameter. Thus, mathematical modelling can potentially significantly speed up the development of solid lipid extrudates, but care has to be taken that none of the assumptions the mathematical theory is based on is violated., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Using first principles molecular dynamics simulations in the isobaric-isothermal ensemble (T = 300 K, p = 1 atm) with the Becke-Lee-Yang-Parr exchange/correlation functional and a dispersion correction due to Grimme, the hydrogen bonding networks of pure liquid water, methanol, and hydrogen fluoride are probed. Although an accurate density is found for water with this level of electronic structure theory, the average liquid densities for both hydrogen fluoride and methanol are overpredicted by 50 and 25%, respectively. The radial distribution functions indicate somewhat overstructured liquid phases for all three compounds. The number of hydrogen bonds per molecule in water is about twice as high as for methanol and hydrogen fluoride, though the ratio of cohesive energy over number of hydrogen bonds is lower for water. An analysis of the hydrogen-bonded aggregates revealed the presence of mostly linear chains in both hydrogen fluoride and methanol, with a few stable rings and chains spanning the simulation box in the case of hydrogen fluoride. Only an extremely small fraction of smaller clusters was found for water, indicating that its hydrogen bond network is significantly more extensive. A special form of water with on average about two hydrogen bonds per molecule yields a hydrogen-bonding environment significantly different from the other two compounds.

Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C(18) stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile., (Copyright © 2011 Elsevier B.V. All rights reserved.)

A textile polyester vascular graft was modified with methyl-β-cyclodextrin (MeβCD) to obtain a new implant capable of releasing antibiotics directly in situ at the site of operation over a prolonged period and thereby prevent post-operative infections. We investigated the influence of the curing parameters (time and temperature) that allow control of the degree of functionalization (DF) of the support by MeβCD. The inclusion of ciprofloxacin (CFX) in the MeβCD cavity was observed in solution by two-dimensional (1)H NMR spectroscopy. The amount of CFX loaded on the modified graft increased with DF. Depending on the release medium (water, phosphate-buffered saline, or human plasma) and the DF of the prostheses, different kinetic profiles of release of CFX were obtained. The sustained release of CFX in human plasma was shown by microbiological assays that indicated prolonged antimicrobial activity against Staphylococcus aureus and Escherichia coli. Viability tests demonstrated the non-toxicity of MeβCD to an epithelial cell line (HPMEC), although a decrease in endothelial cell number was observed on the functionalized prosthesis, probably due to the roughness of the coating and also to the nature of the MeβCD polymer present on the surface of the fibers., (Copyright © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

The aim of this study was to evaluate the moisture-protective ability of different polymeric coatings. Free films and film-coated tablets (with cores containing freeze-dried garlic powder) were prepared using aqueous solutions/dispersions of hydroxypropyl methylcellulose (HPMC), Opadry AMB [a poly(vinylalcohol)-based formulation] and Eudragit E PO [a poly(methacrylate-methylmethacrylate)]. The water content of the systems upon open storage at 75% relative humidity (RH) and 22 degrees C (room temperature) was followed gravimetrically. Furthermore, polymer powders, free films and coated tablets were analyzed by differential scanning calorimetry (DSC) and dynamic vapor sorption (DVS). The type of polymer strongly affected the resulting water uptake kinetics of the free films and coated tablets. DSC analysis revealed whether or not significant physical changes occurred in the coatings during storage, and whether the water vapor permeability was water concentration dependent. Using DVS analysis the critical glass transition RH of Opadry AMB powder and Opadry AMB-coated tablets at 25 degrees C could be determined: 44.0% and 72.9% RH. Storage below these threshold values significantly reduces water penetration. Thus, DVS and DSC measurements can provide valuable information on the nature of polymers used for moisture protection.

Molecular simulations for a ternary nucleation system (water/n-nonane/1-butanol) demonstrate a more complex nucleation mechanism than previously thought, where critical nuclei with different compositions are present even for a given vapour-phase composition; the spatial distribution in these critical nuclei is heterogeneous and dumbbell and onion motifs are found; in the former, water and nonane nano-droplets are connected through a butanol handle, while in the latter a water core is surrounded by a nonane corona with an interfacial butanol shell.

Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble (T = 323 K and p = 10 atm) were carried out to probe structural properties of an isolated n-octadecane chain solvated in water, methanol, water-rich, or methanol-rich mixtures and, for comparison, of an isolated chain in the gas phase and for neat liquid n-octadecane. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to represent n-octadecane and methanol and the TIP-4P model was used for water. In all six environments, broad conformational distributions are observed and the n-octadecane chains are found to predominantly adopt extended, but not all-trans conformations. In addition, a small fraction of more collapsed conformations in which the chain ends approach each other is observed for aqueous hydration, the water-rich solvent mixture and the gas phase, but the simulation data do not support a simple two-state picture with folded and unfolded basins of attraction. For chains in these three "poor" solvent environments, the dihedral angles near the center of the chain show an enhancement of the gauche population. The ensemble of water-solvated chains with end-to-end contacts is preferentially found in a U-shaped conformation rather than a more globular state. An analysis of the local solvation structures in the water-methanol mixtures shows, as expected, an enrichment of the methyl group of methanol near the methylene and methyl segments of the n-octadecane chain. Interestingly, these local bead fractions are enhanced by factors of 2.5 and 1.5 for methyl and methylene segments reflecting the more hydrophobic nature of the former segments.

A dramatic transition in the mechanical properties of water is observed at the nanometer scale. For a water meniscus formed between two hydrophilic surfaces in the attractive region, with < or = 1 nm interfacial separation, the measured viscosity is 7 orders of magnitude greater than that of bulk water at room temperature. Grand canonical Monte Carlo simulations reveal enhancement in the tetrahedral structure and in the number of hydrogen bonds to the surfaces as a source for the high viscosity; this results from a cooperative effect of hydrogen bonding of water molecules to both hydrophilic surfaces.

This research addresses a comprehensive particle-based simulation study of the structural, dynamic, and electronic properties of the liquid-vapor interface of water utilizing both ab initio (based on density functional theory) and empirical (fixed charge and polarizable) models. Numerous properties such as interfacial width, hydrogen bond populations, dipole moments, and correlation times will be characterized with identical schemes to draw useful conclusions on the strengths and weakness of the proposed models for interfacial water. Our findings indicate that all models considered in this study yield similar results for the radial distribution functions, hydrogen bond populations, and orientational relaxation times. Significant differences in the models appear when examining both the dipole moments and surface relaxation near the aqueous liquid-vapor interface. Here, the ab initio interaction potential predicts a significant decrease in the molecular dipole moment and expansion in the oxygen-oxygen distance as one approaches the interface in accordance with recent experiments. All classical polarizable interaction potentials show a less dramatic drop in the molecular dipole moment, and all empirical interaction potentials studied yield an oxygen-oxygen contraction as the interface is approached.

Octanol-water partition coefficients are extraordinarily successful for correlating and predicting numerous processes of pharmacological and environmental importance. The amphiphilic nature of the 1-octanol molecules allows the octanol phase to mimic the complexities of many different environments ranging from biomembranes to soil. However, the structural details of the octanol phase and whether its structure is altered upon water saturation are not yet fully understood. Configurational-bias Monte Carlo simulations in the Gibbs ensemble demonstrate that a diverse spectrum of hydrogen-bonded aggregates exists in dry and wet 1-octanol, and that water saturation substantially alters the 1-octanol environment from predominantly linear aggregates in dry octanol to larger cylindrical micelles with water cores in wet octanol. These simulation results are able to reconcile the conflicting views (chain-like or water-centered aggregates vs spherical micelles) of the 1-octanol structure inferred from thermodynamic arguments, spectroscopic measurements, and diffraction experiments. Calculated partition constants quantify the influence of water saturation on the solubility characteristics of the dry and wet octanol phases.

The dramatic loss of retention in reversed-phase liquid chromatography when switching to 100% aqueous solvent and stopping flow (depressurizing) has long intrigued separation scientists. Recent experimental evidence suggests that the observed loss of retention is due to the loss of pore wetting with subsequent loss of solvent penetration in the porous matrix. One of the prevalent explanations of this phenomenon has been that the bonded phase chains, typically octadecyl silane bound to porous silica, would undergo significant conformational changes, viz. collapse, under pure aqueous conditions. As a definitive means toward elucidating the conformation of bonded-phase chains under pure aqueous conditions, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for a system of dimethyl octadecyl silane of intermediate coverage bound to the (111) face of beta-cristobalite and immersed in pure water. The results of two sets of simulations, which were started from two entirely different starting configurations as a validity check toward reaching the same equilibrium distribution of states, show that chains are neither clustering together nor laying on the surface but rather have a broad distribution of orientations and of conformational states. The interface between the bonded and solvent phases is rough on a molecular level, and clusters of water molecules are sometimes found to adsorb at the silica surface. This computational study lends further evidence that the driving force for the loss of retention when switching to pure aqueous conditions and depressurizing is not the collapse of bonded-phase chains.

Non-degradable microparticles based on ammonio methacrylate copolymers (Eudragit RS:RL 4:1 blends) containing the hydrophilic drug propranolol HCl and/or the lipophilic drug nifedipine were prepared with an oil-in-water (O/W) and a water-in-oil-in-water (W/O/W) solvent evaporation technique. Both drugs were successfully incorporated separately as well as simultaneously. In all cases, the resulting release rate(s) of the drug(s) was/were found to be controlled over periods of at least 8 h. To elucidate the underlying mass transport mechanisms, the microparticles were thoroughly characterized by X-ray powder diffractometry, differential scanning calorimetry, particle size analysis, and determination of the actual drug loading(s). Analytical solutions of Fick's second law of diffusion considering non-steady state conditions were used to describe the release of propranolol HCl. Interestingly, the resistance for drug release within the unstirred liquid boundary layers on the surfaces of the microparticles was found to be negligible compared to the diffusional resistance within the polymeric devices. Importantly, the mathematical theories could be used to normalize the experimentally determined in vitro drug release with respect to the microparticle size. Thus, the effect of the type of preparation method (O/W vs. W/O/W) and device composition (polymer blend plus one drug only vs. polymer blend plus drug combination) on the diffusional resistance within the microparticles could be studied. In addition, further insight into the occurring mass transport processes was gained. For example, the time-dependent evolution of the drug concentration profiles within the microparticles upon exposure to the release medium could be calculated. An interesting practical application of the mathematical theories is the possibility to predict the effect of different formulation parameters on the resulting drug release patterns, e.g. the effect of the microparticle size.

Adsorption and ordering at the vapor-liquid interfaces of mutually saturated water/1-butanol solutions at a temperature of 298.15 K were investigated using configurational-bias Monte Carlo simulations in the Gibbs ensemble and compared to the surface properties of neat water and 1-butanol liquids. A dense 1-butanol monolayer is observed at the surface of the water-rich phase, which results in a substantial decrease of its surface tension. In contrast, there is no enrichment of water molecules at the surface of the butanol-rich phase, and its surface tension is not significantly changed. Analysis of the interfacial structures reveals that these systems exhibit orientational ordering and composition heterogeneity. Analysis of the hydrogen-bonding distributions suggests that the formation of the 1-butanol monolayer is driven by an excellent match between water and the primary alcohol; that is, additional hydrogen bonds are formed between the excess free hydrogens of surface water and the excess hydrogen-bond acceptor sites of 1-butanol.

We have used the classical molecular dynamics technique to simulate the ordering of a water film adsorbed on an atomic model of a tip of a scanning tunneling microscope approaching a planar metal surface. For this purpose, we have developed a classical model for the water–substrate interactions that solely depends on the coordinates of the particles and does not require the definition of geometrically smooth boundary surfaces or image planes. The model includes both an electrostatic induction for the metal atoms (determined by means of an extended Lagrangian technique) and a site-specific treatment of the water–metal chemisorption. As a validation of the model we have investigated the structure of water monolayers on metal substrates of various topology [the (111), (110), and (100) crystallographic faces] and composition (Pt, Ag, Cu, and Ni), and compared the results to experiments. The modeling of the electrostatic induction is compatible with a finite external potential imposed on the metal. This feature is used to investigate the structural rearrangements of the water bilayer between the pair of scanning tunneling microscope electrodes in response to an applied external voltage difference. We find significant asymmetry in the dependence on the sign of the applied voltage. Another result of the calculation is an estimate of the perturbation to the work function caused by the wetting film. For the conditions typical for operation of a scanning tunneling microscope probe, the change in the work function is found to be comparable to the applied voltage (a few hundred millivolts). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Gibbs ensemble Monte Carlo (GEMC) method is a versatile approach for the prediction of fluid phase equilibria from particle-based simulations. For a one-component system, a GEMC simulation utilises two separate simulation boxes for the vapour and the liquid phases and a significant fraction of the computational effort is expended on special trial moves that transfer (swap) particles and exchange volume between the two boxes. The user needs to specify the frequency of swap and volume moves and the overall volume that controls the phase ratio. In this study, the efficiency of GEMC simulation protocols that yield three different frequencies of accepted swap and volume moves and three different phase ratios is assessed for the computation of the saturated vapour pressure and liquid density ofn-octane and water at three reduced temperatures. Differences in the simulation efficiency of up to an order of magnitude are observed, and recommendations are made for suitable GEMC simulation protocols. [ABSTRACT FROM PUBLISHER]

Abstract: The effects of stationary phase and solute chain length are probed by carrying out Monte Carlo simulations of dimethyl triacontyl (C30), dimethyl octadecyl (C18), dimethyl octyl (C8), and trimethyl (C1) silane grafted, and bare silica stationary phases in contact with a water/methanol mobile phase and by examining the retention of solutes from 1 to 14 carbons in length. Fairly small differences in structure are observed when comparing the C30, C18, C8 systems and the retention mechanism of nonpolar alkane solutes shows contribution from both partitioning and adsorption on all three of these stationary phases. Unlike in the other systems, the mobile phase solvent is highly structured at its interface with the C1 and bare silica phases, the former being enriched in methanol and the latter in water. Alkane solutes are unretained at the bare silica surface while alcohol solutes are only slightly enriched at the silica surface due to hydrogen bonding with surface silanols and surface bound solvent. With regard to solute size, it appears that the retention mechanism is not affected by the chain length of the solute. [Copyright &y& Elsevier]

Abstract: Configurational-bias Monte Carlo simulations in the isobaric–isothermal ensemble and in the canonical and isobaric–isothermal versions of the Gibbs ensemble were carried out to investigate the thermophysical properties of a 2-methoxy-1-(2-methoxypropoxy)propane/2-methoxy-1-((1-methoxypropan-2-yl)oxy)propane mixture and the mutual solubility of this dipropylene glycol dimethyl ether (DPGDME) mixture and water. The TraPPE-UA and TIP4P force fields were used for the DPGDME isomers and water, respectively. The TraPPE-UA force field yields satisfactory predictions for the specific density of the liquid phase and for the normal boiling point of the DPGDME mixture. Preliminary simulation data for the mutual solubility of the DPGDME mixture and water at T =298K pointed to an underprediction of the miscibility gap and, hence, simulations were carried out for variations of the TraPPE-UA DPGDME model with different partial charges for the ether moiety. Both the organic and the aqueous phase exhibit pronounced microheterogeneity and the persistence of very stable and large aggregates poses significant sampling challenges. The trajectories of independent simulations started from different initial compositions indicate either (i) that equilibrium has not yet been achieved, (ii) that composition fluctuations are sampled insufficiently over the current trajectory lengths, or (iii) that the system size is too small. Based on these preliminary simulation data, cautious estimates of the mutual solubilities at temperatures from 283 to 353K are made using a model with modified charges. These predictions yield mean signed deviations from the experimental benchmark data for the mutual solubilities of +5% and −4% for the organic and aqueous phases, respectively. [Copyright &y& Elsevier]

Gibbs ensemble Monte Carlo simulations were run with an efficient mixed-basis electronic structure method to explore the phase equilibria of water from first principles using Kohn-Sham density functional theory. The Perdew-Burke-Ernzerhof exchange/correlation density functional gives a higher critical temperature (700 K) and boiling point (480 K) than experiment, although good agreement is found for the saturated liquid densities. A systematic increase in the size of the basis set for the Becke-Lee-Yang-Parr exchange/correlation density functional from a double-ζ to quadruple-ζ split valence leads to further deviations from experiment on the saturated liquid and vapor densities, while the intermediate basis set gives the best results for the heat of vaporization at T = 423 K. Analysis of the liquid structure for all simulations shows changes that can partially be explained by the different densities at a given temperature, and both density functionals show a similar temperature dependence of the liquid structure. [ABSTRACT FROM AUTHOR]

Various driving forces have been suggested to explain retention and selectivity in reversed-phase liquid chromatography (RPLC). To provide molecular-level information on the retention mechanism in RPLC, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for model systems consisting of three phases: an n-hexadecane retentive phase, a mobile phase with varying water-methanol composition, and a helium vapor phase as reference state. liquid n-hexadecane functions as a model of a hydrophobic stationary phase, and a wealth of experimental data exists for this system. Gibbs free energies for solute transfers from gas to retentive phase, from gas to mobile phase, and from mobile to retentive phase were determined for a series of short linear alkanes and primary alcohols. Although the magnitude of the incremental Gibbs free energy of transfer for a methylene segment is always larger for the gas- to retentive-phase transfer than the gas- to mobile-phase transfer, it is found that the partitioning of alkanes and alkyl tail groups is mostly affected by the changes in the aqueous mobile phase that occur when methanol modifiers are added. In contrast, the partitioning of the alcohol headgroup is sensitive to changes in both the n-hexade-cane and the mobile phases. In particular, it is found that hydrogen-bonded aggregates of methanol are present in the n-hexadecane phase for higher methanol concentrations in the mobile phase. These aggregates strongly increase alcohol partitioning into the retentive phase. The simulation data clearly demonstrate that due to modification of the retentive-phase hydrocarbons by solvent components, neither the solvophobic theory of RPLC, advocated by Horvath and co-workers, nor the lipophilic theory of RPLC, advocated by Carr and co-workers, can adequately describe the separation mechanism of the hexadecane model system of a retentive phase studied here nor the more complex situation present in actual RPLC systems. [ABSTRACT FROM AUTHOR]

A combination of the aggregation-volume-bias and configurational-bias Monte Carlo algorithms and the umbrella sampling technique was applied to investigate two different binary vapor-liquid nucleation systems: water/ethanol and water/n-nonane. The simulations are able to reproduce the different nonideal nucleation behavior observed experimentally for these two systems, i.e., the mutual enhancement of nucleation rates for water/ethanol mixtures and the two-pathway nucleation for water/n-nonane mixtures. Structural analysis provides microscopic explanations for the observed nucleation behavior. In particular, the simulations show a large and size-dependent surface enrichment of ethanol in the water/ethanol droplets, which confirms the previous experimental interpretation for this system. The immiscibility observed even for small water/n-nonane clusters causes the two-pathway nucleation mechanism. [ABSTRACT FROM AUTHOR]

Studies the partitioning of normal alkane and primary alcohol solutes between water, neat or water-saturated 1-octanol, and helium vapor phases. Configurational-bias Monte Carlo simulations in the Gibbs ensemble using the optimized potentials for liquid simulations; Comparison of the partitioning between a helium vapor and dryor wet 1-octanol phases.

Obtaining knowledge of the mutual miscibility of alkanes, surfactants, and water over a wide range of temperatures and pressures is of great relevance for the energy industry. Molecular simulations using advanced sampling techniques and transferable force fields can serve as a predictive method to study a variety of phase equilibria at different temperatures, pressures, and overall compositions. In this work, configurational-bias Monte Carlo simulations in the NpT Gibbs ensemble with the Transferable Potentials for Phase Equilibria (TraPPE) force field are applied to investigate the binary mixture of n -dodecane and ethanol at pressures of 0.1 and 100 MPa and the ternary mixture of these compounds with water at one state point. The simulations qualitatively reproduce the increase of the upper critical solution temperature (UCST) with increasing pressure, but the miscibility gap is overestimated at both pressures and the UCSTs are shifted up by about 50 K compared to the experimental values. The TraPPE model correctly shows a gradual decrease for the ethanol solubility in the n -dodecane-rich phase upon addition of water to the mixture. [ABSTRACT FROM AUTHOR]

Abstract Monte Carlo simulations are performed to predict the interfacial tension (IFT) for water/ n -dodecane, water/toluene, and water/(50 wt% n -dodecane + 50 wt% toluene) mixtures at a pressure of 1.83 MPa and temperatures ranging from 383.15 to 443.15 K. Simulations for the binary mixtures are performed in the N 1 N 2 p N A T ensemble and those for the ternary mixtures are performed in the N 1 μ 2 μ 3 p N A T ensemble. In order to control the composition of the organic phase for ternary mixture simulations, separate reservoirs for toluene and n -dodecane molecules are utilized, and the minimum and maximum numbers of these molecules in the interfacial box are monitored. Identity switch moves are applied to facilitate the sampling of the spatial distributions of the organic molecules. Calculations of the IFT for water/ n -decane and water/benzene mixtures are used to benchmark the force fields and to develop mixing rules. The combination of the TraPPE–UA force field and the TIP4P/2005 water model with Lorentz-Berthelot combining rules yields fairly accurate predictions at moderate temperatures for water/ n -decane, but appears to slightly overestimate the IFT at elevated temperatures. For the water/benzene mixtures, the TraPPE–EH//TIP4P/2005 combination significantly overestimates the IFT over the entire temperature range. Subsequent modifications of the energetic Lennard-Jones cross-interaction parameters yield good agreement between experimental and simulated IFT data of water/ n -decane and water/benzene. Using a preliminary set of modified cross-interaction parameters for the TraPPE–UA/EH//TIP4P/2005 combination, our predictions for the Industrial Fluid Properties Simulation Challenge yield satisfactory agreement with the benchmark data (mean signed errors of − 2.2 , − 5.1 , and − 0.1 dyn/cm for water/ n -dodecane, water/toluene, and water/(50 wt% n -dodecane + 50 wt% toluene), respectively). A new set of modified cross-interaction parameters developed later based on longer simulation trajectories and a larger set of experimental data is found to reduce the mean signed errors to − 0.1 , − 0.6 , and + 2.5 dyn/cm, respectively, for these three systems. Although a significant enrichment of toluene molecules at the interface is found for the ternary mixture, this enrichment is not sufficient to reproduce the experimental IFT data. [ABSTRACT FROM AUTHOR]

Abstract: Gibbs ensemble Monte Carlo simulations were carried out to investigate the properties of a frozen-electron-density (or Kim–Gordon, KG) model of water along the vapor–liquid coexistence curve. Because of its theoretical basis, such a KG model provides for seamless coupling to Kohn–Sham density functional theory in mixed quantum mechanics/molecular mechanics implementations. The Gibbs ensemble simulations indicate limited transferability of a KG model to other state points. Specifically, a KG model that was parameterized by Barker and Sprik to the properties of liquid water at 300K, yields saturated vapor pressures and a critical temperature that are significantly under- and overestimated, respectively. [Copyright &y& Elsevier]