Eugénia A. Macedo

Calculation of phase equilibria for solutions of strong electrolytes in solvent—water mixtures

The Sander model for correlation and prediction of salt effects on vapor-liquid equilibria (VLE) combines a Debye—Huckel-type term with a modified UNIQUAC equation with concentration-dependent-parameters. The Debye—Huckel term has in this work been modified to ensure a more correct representation of the long-range forces. The modification has required a reestimation of previously published interaction parameters. It is shown that the modified model represents VLE for alcohol—water—salt mixtures with the same accuracy as the Sander model. The modified model has a broader range of applicability than the Sander model since the solvents acetone and 1-butanol have been added to water methanol ethanol, 1- and 2-propanol, and the ions Cu2+, Ba2+, Ni2+ Hg2+ Sr2+, F−, and I− have been included besides Li+, Na+, K+, Ca2+, Cl−, Br−, NO−3 and CH3COO−.

Unified approach to the self-diffusion coefficients of dense fluids over wide ranges of temperature and pressure—hard-sphere, square-well, Lennard–Jones and real substances

Abstract In this work a physically sound approach for calculation of the self-diffusion coefficient of model and real fluids is presented. After carrying out a systematic study focused on the equations and molecular dynamic data available in the literature concerning the self-diffusivities of the hard-sphere (HS), square-well (SW) and Lennard–Jones (LJ) fluids, new models are proposed to represent this transport property. First of all, a free-volume-based equation with only one parameter is used to reproduce the recently published data for the HS fluid. Then it is demonstrated that reasonable models for the SW and LJ systems are liable to arise from the HS model by just introducing an attractive contribution. Finally, it is shown that it is possible to describe the self-diffusivities of real substances in terms of the equations devised for the SW and LJ fluids, which give rise to two-parameter models. These parameters are the diameter and the energy of the considered potential function. Reliable representation is accomplished with the proposed LJ model, the results (AAD=5.45%) being well comprised by the experimental accuracy. The validation involved the largest database ever used (2514 data points).

Ionic liquids as alternative co-solvents for laccase: Study of enzyme activity and stability

The activity and stability of commercial laccase (DeniLite base) in three different water soluble ionic liquids (ILs) (1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate, [emim][[MDEGSO4], 1-ethyl-3-methylimidazolium ethylsulfate, [emim][EtSO4], and 1-ethyl-3-methylimidazolium methanesulfonate, [emim][MeSO3]) have been studied and compared to that in two organic solvents (acetonitrile and dimethyl sulfoxide). Initial enzyme activities were similar among the ILs if the same conditions were used. A high reduction on initial enzyme activity was found with acidic pH (5.0). The effect of pH and solvent concentration on enzyme stability were investigated in more detail for 1 week. The enzyme maintained a high stability at pH 9.0 for all ILs tested. [emim][MDEGSO4] was the most promising IL for laccase with an activity loss of about 10% after 7 days of incubation. The kinetic studies in the presence of ABTS as substrate allowed to calculate the Michaelis- Menten parameters. Good agreement was found between experimental data and calculated values using the Michaelis-Menten mechanism, with a total average relative deviation of 2.1%.

1-Octanol/water partition coefficients of n-Alkanes from molecular simulations of absolute solvation free energies

The 1-octanol/water partition coefficient is an important thermodynamic variable usually employed to understand and quantify the partitioning of solutes between aqueous and organic phases. It finds widespread use in many empirical correlations to evaluate the environmental fate of pollutants as well as in the design of pharmaceuticals. The experimental evaluation of 1-octanol/water partition coefficients is an expensive and time-consuming procedure, and thus, theoretical estimation methods are needed, particularly when a physical sample of the solute may not yet be available, such as in pharmaceutical screening. 1-Octanol/water partition coefficients can be obtained from Gibbs free energies of solvation of the solute in both the aqueous and the octanol phases. The accurate evaluation of free energy differences remains today a challenging problem in computational chemistry. In order to study the absolute solvation Gibbs free energies in 1-octanol, a solvent that can mimic many properties of important biological systems, free energy calculations for n-alkanes in the range C1-C8 were performed using molecular simulation techniques, following the thermodynamic integration approach. In the first part of this paper, we test different force fields by evaluating their performance in reproducing pure 1-octanol properties. It is concluded that all-atom force fields can provide good accuracy but at the cost of a higher computational time compared to that of the united-atom force fields. Recent versions of united-atom force fields, such as Gromos and TraPPE, provide satisfactory results and are, thus, useful alternatives to the more expensive all-atom models. In the second part of the paper, the Gibbs free energy of solvation in 1-octanol is calculated for several n-alkanes using three force fields to describe the solutes, namely Gromos, TraPPE, and OPLS-AA. Generally, the results obtained are in excellent agreement with the available experimental data and are of similar accuracy to commonly used QSPR models. Moreover, we have estimated the Gibbs free energy of hydration for the different compounds with the three force fields, reaching average deviations from experimental data of less than 0.2 kcal/mol for the case of the Gromos force field. Finally, we systematically compare different strategies to obtain the 1-octanol/water partition coefficient from the simulations. It is shown that a fully predictive method combining the Gromos force field in the aqueous phase and the OPLS-AA/TraPPE force field for the organic phase can give excellent predictions for n-alkanes up to C8 with an absolute average deviation of 0.1 log P units to the experimental data.

Application of statistical experimental methodology to optimize reactive dye decolourization by commercial laccase.

Three-level Box-Behnken factorial design with three factors (pH, temperature and enzyme concentration) combined with response surface methodology (RSM) was applied to optimize the dye degradation of reactive red 239 (RR239), reactive yellow 15 (RY15) and reactive blue 114 (RB114) dyes by commercial laccase. Mathematical models were developed for each dye showing the effect of each factor and their interactions on colour removal. The model predicted for RY15 that a decolourization above 90% (after 24h) could be obtained when the enzyme concentration, temperature and pH were set at 109.8U/L, 39.2 degrees C and 6.6, respectively; whilst for RB114 and RR239 the temperature and enzyme concentration did not affect the decolourization (>90%) in the considered range and optimum pH value was found at 5.5-7.0 and 7.0-7.5, respectively. These predicted values were also experimentally validated. Average final values of responses were in good agreement with calculated values, thus confirming the reliability of the models of RY15, RB114 and RR239 decolourization.

Solubilities of Biologically Active Phenolic Compounds: Measurements and Modeling

Aqueous solubilities of natural phenolic compounds from different families (hydroxyphenyl, polyphenol, hydroxybenzoic, and phenylpropenoic) were experimentally obtained. Measurements were performed on tyrosol and ellagic, protocatechuic, syringic, and o-coumaric acids, at five different temperatures (from 288.2 to 323.2 K), using the standard shake-flask method, followed by compositional analysis using UV spectrophotometry. To verify the accuracy of the spectrophotometric method, some data points were measured by gravimetry, and in general, the values obtained with the two methods are in good agreement (deviations lower than 11%). To adequately understand the solubilization process, melting properties of the pure phenolics were obtained by differential scanning calorimetry (DSC), and apparent acid dissociation constants were measured by potentiometry titration. The aqueous solubilities followed the expected general exponential trend. The melting temperatures did not follow the same solubility tendency, and for tyrosol and ellagic acid, not only the size and extent of hydrogen bonding, but also the energy associated with their crystal structures, determine the solubility. For these binary systems, acid dissociation is not important. Approaches for modeling the measured data were evaluated. These included an excess Gibbs energy equation, the modified UNIQUAC model, and the cubic-plus-association (CPA) equation of state. Particularly for the CPA approach, a new methodology that explicitly takes into account the number and nature of the associating sites and the prediction of the pure-component parameters from molecular structure is proposed. The results indicate that these are appropriate tools for representing the water solubilities of these molecules.

Thermodynamic properties of sugars in aqueous solutions: correlation and prediction using a modified UNIQUAC model

Abstract A modified UNIQUAC model with linear temperature-dependent parameters is proposed to describe the thermodynamics properties (water activity, osmotic coefficient, vapor pressure, boiling temperature, freezing point and solubility data) of three binary systems: D-glucose/water, D-fructose/water and sucrose/water. New UNIQUAC interaction parameters are presented, estimated using the above mentioned binary data, collected from the literature. Good predictions of vapor-liquid and solid-liquid equilibrium data for ternary and quaternary mixtures of D-glucose, D-fructose, sucrose and water components are obtained. This approach uses the symmetric convention for all the components, pure fused sugar at solution pressure and temperature being its reference state. Therefore, it allows straightforward calculations of solid-liquid equilibria of sugars in mixed solvents. This constitutes an important advance over the approach based on the knowledge of the dilution enthalpy data available in the literature.

Representation of salt solubility in mixed solvents: a comparison of thermodynamic models

Abstract A simple and accurate apparatus for the measurement of salt solubilities in mixed solvents by an analytical method is presented. Salt solubility data have been measured for the systems water/methanol/NaCl, water/methanol/KCl at 298.15 and 323.15 K and for the system water/ethanol/NaCl at 298.15, 323.15 and 348.15 K. To correlate the solubility data two models are proposed. One assumes that the salt in solution is in the molecular form and the orginal UNIQUAC model with linear temperature dependent parameters is used, the other takes into consideration the salt dissociation and the original UNIQUAC model is combined with a Pitzer-Debye-Huckel expression to take into account the long-range interaction forces. In order to simplify the solubility calculations a new formulation is presented involving the symmetric convention for all species. This makes possible the direct access to the solubility product of the salt in terms of its thermodynamic properties. Both models correlate satisfactorily the solubility data although both temperature and electrostatic effects are very important in this type of equilibrium calculations.

A modified UNIFAC model for the calculation of thermodynamic properties of aqueous and non-aqueous solutions containing sugars

A UNIFAC based model is proposed to describe experimental thermodynamic properties of aqueous and non-aqueous solutions containing common sugars, e.g., d -glucose, d -fructose and sucrose. A new group ‘OHring’ is introduced due to proximity effects. New temperature-independent UNIFAC interaction parameters are estimated based on the binary data available for both d -glucose and d -fructose and only on the solubility data for sucrose. The UNIFAC model used in this work is able to accurately predict the water activity, boiling temperature and freezing point of aqueous sucrose solutions. However, the predictions of the osmotic coefficients for the sucrose/water system present relative deviations up to 9%. Satisfactory predictions of vapor-liquid and solid-liquid equilibrium data for ternary and quaternary mixtures of d-fructose, sucrose and water components are also obtained. This approach uses the symmetric convention for all the components, i.e. the pure fused sugar at solution pressure and temperature is its reference state. Therefore, it allows to carry out in a straightforward way, calculations of solid-liquid equilibria of sugars in mixed solvents. It is shown that the new UNIFAC model is able to qualitatively predict the ternary solubility data of d -glucose d -xylose, d -mannose and sucrose in mixed solvent mixtures of ethanol/water, methanol/water and ethanol/methanol. A comparison with a modified UNIQUAC—essentially correlative model—and a UNIFAC—correlative and predictive model—approaches available in the literature is also given: the performance of the model proposed in this work is in general inferior to that of the UNIQUAC model, however it gives always a better agreement with the experimental data when compared with another UNIFAC based model available in the literature.

Solid–liquid equilibrium of α-lactose in ethanol/water

Solubilities of a-lactose in water‐ethanol solvent mixtures were obtained using an accurate analytical method, at 25, 40 and 60C in a concentration range from 0 to 100 sugar-free wt.% of water. Thea-lactose solubility is a strong function of the water concentration in the solvent mixture. It increases monotonically with temperature and with water concentration. The experimental data were correlated using a UNIQUAC-based model and were predicted using two different UNIFAC-based models.