Water Activity Coefficients Research Articles

Numerical simulation of evaporation and drying of a bi-component droplet

A mathematical model to describe the evaporation and drying behavior of a single quiescent spherical bi-component droplet containing evaporating liquid and dissolved solid substance is developed with primary focus on the application of this model to study the evaporation and drying of a droplet of polymer (Polyvinylpyrrolidone – PVP) in water and Mannitol in water. The water evaporation rate is computed using a convective two-film model with modifications to account for solid layer formation. The non-ideality effect induced by the presence of PVP or Mannitol on vapor pressure of water is accounted through the calculation of activity coefficient of water in either system. The problem under consideration is governed by the continuity (diffusion) and energy equations, which are solved using a finite difference method. The effect of drying conditions such as gas velocity, temperature, and relative humidity on evaporation and drying rate is studied. It is concluded that the present model effectively captures the initial stages of single droplet drying.

Sucrose–water mixture: From thermodynamics to solution structure

What is the structure of sucrose–water mixture? I employ an exact statistical thermodynamic theory (Kirkwood–Buff theory) to calculate information regarding sucrose–water, water–water and sucrose–sucrose interactions solely from volumetric and osmometric data. We found that (i) Long-ranged hydration structure, beyond the first hydration shell, influences thermodynamics; (ii) The inferred minimum of the activity coefficient of water at the high sucrose concentration is due to the increases in the self association of water. These findings from a rigorous theory are consistent with previous simulation studies.

Binary and ternary VLE of the 2-(diethylamino)-ethanol (DEEA)/3-(methylamino)-propylamine (MAPA)/water system

A mixed 2-(diethylamino)-ethanol (DEEA) and 3-(methylamino)-propylamine (MAPA) system could be an attractive alternative solvent to improve the performance of CO2 capture for low partial pressure cases. This solvent has the advantages of forming two liquid phases upon CO2 loading, one rich in CO2 and the other very low in CO2. Having a highly concentrated rich solvent improvements could be reached by reducing the sensible heat and improving the equilibrium sensitivity hence reducing the need for stripping steam. Also it is possible that the heat of absorption may change to the better.To better understand this system in designing the separation unit requires substantial work on characterization of the solvent. One important aspect is to provide equilibrium data. In this work new ebulliometric VLE data for the binary DEEA/H2O and DEEA/MAPA systems and the ternary DEEA/MAPA/H2O system are reported at different temperatures and concentrations. Results show that pure MAPA is more volatile than DEEA, but in aqueous solution MAPA was found to be less volatile. A mix of DEEA and MAPA in aqueous solution tends to lower the volatility thus makes the system more advantageous by reducing volatility. The activity coefficients for the species in the ternary aqueous system are found to be lower than the activity coefficients obtained from the corresponding binary aqueous mixtures.The UNIQUAC framework was implemented to represent the experimental data. The six UNIQUAC parameters were determined and were able to predict P-T-x-y, activity coefficient, excess enthalpy and freezing point depression for both the binary and ternary systems. However, a small inconsistency was observed between water activity coefficients determined from ebulliometer and freezing point depression measurements.

Prediction of hydrate formation conditions in the solutions containing electrolyte and alcohol inhibitors and their mixtures using UNIQUAC-NRF models

In this work, the prediction of hydrate formation conditions in the presence of the thermodynamic inhibitors is presented. Using the van der Waals–Platteeuw model, computation of the L–S–G equilibrium is carried out for hydrate formation conditions in the presence of methanol and electrolytes using the local composition models of the UNIQUAC-NRF and Electrolyte-UNIQUAC-NRF for nonelectrolyte and electrolyte systems, respectively. The model is applied to calculate water activity coefficient for prediction of the hydrate formation pressure of methane, ethane, H2S in the absence and presence of the electrolytes. The results of the present model are compared with the experiment and UNIFAC group contribution model. Also both UNIQUAC-NRF and UNIFAC activity coefficient models are used to predict the hydrate formation conditions in the presence of alcohols. The results show very good agreement with the experiment.

Sorption and permeation of acetic acid–water mixtures by pervaporation using copolymer membrane

AbstractPoly (acrylonitrile‐co‐methyl acrylate) copolymer designated as PANMA was used for making pervaporation membrane. This membrane was used for separation of acetic acid–water mixtures over the concentration range of 80–99.5 wt% acetic acid in water. Interaction parameters based on Flory–Huggins lattice model and engaged species induced clustering (ENSIC) model was used to explain swelling of the membranes. Coupling in sorption was explained in terms of activity coefficient of water and acid in feed and membrane using Flory–Huggins model and also by interpolating ENSIC parameters. Flow coupling in pervaporation was also determined from phenomenological deviation coefficients. Intrinsic membrane properties like partial permeability and membrane selectivity of the solvents were also determined. Diffusion coefficient and plasticization coefficient of the solvents were obtained using a modified solution–diffusion model. The copolymer membrane showed high flux and water selectivity for highly concentrated acid. Thus, at 30°C temperature 1–20 wt% water in feed was concentrated to 82–84 wt% water in permeate and for 0.95 wt% water in feed, the membrane showed thickness normalized flux and water selectivity of 1.71 kg m−2 h−1 mμ and 409, respectively. OLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Separation of isopropyl alcohol–water mixtures by pervaporation using copolymer membrane: Analysis of sorption and permeation

Membrane made from copolymer of acrylonitrile and methyl acrylate designated as PANMA was used for pervaporative dehydration of isopropyl alcohol (IPA) over the concentration range of 75–100wt% IPA in water. Sorption of the membrane was evaluated in terms of thermodynamic interaction parameters based on Flory–Huggins lattice model. Engaged species induced clustering (ENSIC) model was also used to explain swelling of the membranes. Coupling in sorption was explained by activity coefficient of water and IPA in feed and membrane using Flory–Hugging thermodynamics and also by interpolating ENSIC parameters. Intrinsic membrane properties like partial permeability and membrane selectivity was also determined. Total solvent volume fraction (TSVF) model was used to determine diffusion coefficient and plasticization coefficient of the solvents. The membrane was found to show very high water selectivity (3540) and good flux (3.3kg/m2hμm) for highly concentrated (99.8wt%) IPA at 30°C temperature.

Influence of Carbonation and Chemical Activity of Water on Coupled Moisture-Ion Transport in Cementitious Materials

The influence of the chemical activity effects on moisture transport in the presence of ions in cementitious materials is studied. Wetting-drying tests are carried out on various hardened concretes (sound and fully carbonated) with water. Moisture profiles are experimentally assessed by gamma-ray attenuation. A multispecies and multiphase numerical model, in which the Lin and Lee approach is applied to compute the chemical activity coefficients of water in presence of ions, is used to simulate the transport of ions and moisture. The drying kinetics is analyzed to assess the intrinsic permeability of the material by inverse analysis. Experimental profiles are compared to the simulations to illustrate the relevance of the modeling.

Experimental study and thermodynamic modelling of methane clathrate hydrate dissociation conditions in silica gel porous media in the presence of methanol aqueous solution

In this work, the phase equilibria of clathrate hydrates of methane in the presence of pure water and 0.035 mass fraction of methanol aqueous solution in confined silica gel pores with (10 and 15)nm mean diameters are measured and reported. A thermodynamic model is also developed for prediction of the obtained experimental hydrate dissociation data. The Valderrama–Patel–Teja (VPT-EoS) equation of state (EoS) accompanied with the non-density dependent (NDD) mixing rules coupled with a previously developed activity model are applied to evaluate the fugacity of the species present and the activity coefficient of water in methanol aqueous solution. Acceptable agreement between the reported data and the predicted results using the proposed model and an existing method reported in the literature demonstrates the reliability of the presented model.

Water activity coefficients in aqueous amino acid solutions by molecular dynamics simulation: 1. Force field development

New force fields for molecular dynamics (MD) simulation of aqueous zwitterionic amino acid simulations were developed. These were especially designed to calculate activity coefficient of water in amino acid solutions with high accuracy. For example, aqueous solutions of the following amino acids were considered: glycine, alanine, α-aminobutyric acid, α-aminovalerianic acid, valine and leucine. The force fields were obtained by quantum chemical calculations using B3LYP/6-31G and MP2/6-311(d,p) model theories in combination with the Merz–Kollmann–Singh scheme. To further increase the accuracy of the force field, a polarised continuum was considered in all quantum chemical calculations. Water activity coefficients obtained from MD using different all-purpose literature force fields, namely, OPLS, AMBER ff03 and GROMOS 53A6 as well as experimental data are compared with the results utilising the new force field. The new force field is shown to give better results compared with experimental data than existing force fields.

Activity Coefficients of Complex Molecules by Molecular Simulation and Gibbs-Duhem Integration

Activity coefficients of solvents and solutes in different aqueous solutions of alcohols and polymers are determined by molecular dynamic simulations. These data are often not accessible by simulation due to unacceptably high computational demands. Therefore, we applied a combination of two methods: water activity coefficients were determined directly via Overlapping Distribution Method, while counter-component activity coefficients were calculated indirectly by Gibbs-Duhem integration of the respective water activities. Results are in good agreement with experimental data. The method can easily be applied to determine activity coefficients of very complex components in water or other simple solvents.

The Use of the Multiobjective Function-De Algorithm in the Application of Extended UNIQUAC for CO2-MEA-H2O System

The extended UNIQUAC model has been used to simulate the absorption of carbon dioxide (CO 2 ) absorption by methylethylamine (MEA), using differential evolution algorithm method to regress UNIQUAC parameters. The model was successfully applied to correlate simultaneously total pressure versus CO 2 concentration and MEA more fraction, water activity coefficient and the excess properties. Good results were obtained compared to other regression methods such as Levenberg-Marquardt (LM).

Thermodynamic studies of the ionic liquid 1-hexyl-3-methylimidazolium chloride [C 6mim][Cl] in polyethylene glycol aqueous solutions

Vapour pressure osmometery (VPO) measurements at T = 308.15 K for {[C 6mim][Cl] + water}, {[C 6mim][Cl] + (0.005, 0.0155, and 0.0263) mol · kg −1 PEG2000 + water} and {[C 6mim][Cl] + (0.0017, 0.0052, and 0.0088) mol · kg −1 PEG6000 + water} systems and isopiestic measurements at T = 298.15 K for {[C 6mim][Cl] + PEG2000 + water} and {[C 6mim][Cl] + PEG6000 + water} systems have been carried out. The VPO measurements were carried out at very low concentrations of PEG and from which the values of the water activities, osmotic coefficients, vapour pressure and activity coefficients were obtained. The data obtained from the VPO method show that over the whole concentration range of the ionic liquid (IL), the activity coefficients of [C 6mim][Cl] in the presence of PEG2000 are increased. Although, at high IL concentrations, the values of the activity coefficient of [C 6mim][Cl] in the presence of PEG6000 are also increased, however for low concentrations of IL the values of the activity coefficient of [C 6mim][Cl] in pure water are larger than those in aqueous PEG6000 solutions. For a known IL concentration, the values of water activity coefficient for the binary {[C 6mim][Cl] + water} system are larger than those for the ternary {[C 6mim][Cl] + PEG + water} systems and decrease by increasing the concentration of PEG or decreasing the molar mass of PEG. The constant water activity lines of the all ternary systems obtained from the isopiestic method show positive deviation from the linear isopiestic relation (Zdanovskii–Stokes–Robinson rule) derived using the semi-ideal hydration model. The results have been interpreted in terms of the solute–water and solute–solute interactions.

Thermodynamic model for predicting phase equilibria of simple clathrate hydrates of refrigerants

In this communication, a thermodynamic model is presented for the study of the phase equilibria of clathrate hydrates of simple refrigerants. The van der Waals–Platteeuw solid solution theory is used to model the hydrate phase while it is assumed that vapor phase is an ideal gas of refrigerant ignoring its water content and the aqueous phase is considered as pure water (activity coefficient=1) ignoring aqueous solubility. The results through this model are successfully compared with the experimental data reported in the literature for clathrate hydrates of four refrigerants namely C 2H 2F 4 (1, 1, 1, 2-tetrafluoroethane or HFC-134 a or R-134 a), C 2H 4F 2 (1, 1-difluoroethane or HFC-152 a or R-152 a), CH 2F 2 (difluoromethane or HFC-32 or R-32), and C 2H 3Cl 2F (1,1-dichloro-1-fluoroethane or HCFC-141 b or R-141 b).

Phase Diagrams and Water Activities of Aqueous Ammonium Salts of Malonic Acid

Malonic acid has been observed in the free troposphere and as a component of tropospheric aerosol, among other dicarboxylic acids. These aerosols can uptake ammonia, which partially or completely neutralizes the acids. Therefore, the impact of ammoniated dicarboxylic acids on the phases that can exist in aerosols at atmospheric temperatures needs investigation. To that end, the low temperature, solid/liquid phase diagrams of ammonium hydrogen malonate/water, ammonium malonate/water, and triammonium hydrogen malonate/water have been investigated with differential scanning calorimetry and infrared spectroscopy of thin films. Results show that the order of increasing solubility is triammonium hydrogen malonate, ammonium hydrogen malonate, malonic acid, and ammonium malonate. We have also determined a hydrate may form in the ammonium malonate system and decompose below 240 K. We report water activities at the ice melting points for each system up to the respective eutectic concentrations, and find for a given mole fraction of water, increasing ammonium content leads to decreasing water activity coefficients.

Water Activity Coefficient and Proton Mobility in Hydrated Acidic Polymers

Water vapor sorption and proton conductivity data of composite and cross-linked sulfonated aromatic polymers, sulfonated (poly ether ether ketone) (SPEEK) and sulfonated (poly phenyl sulfone) (SPPSU), determined at various temperatures and relative humidities, are used to determine effective water activity coefficients and proton mobilities in these ionomers. The data are challenged with classical physico-chemical laws for electrolyte solutions. The concentration dependence of water activity coefficients in hydrated aromatic polymers is related to an internal pressure exerted by the polymer, which is of the order of the tensile strength of the membranes. The strong concentration dependence of proton mobility is attributed to the particular environment that protons experience inside the nanometric hydrated channels with immobilization of protons near sulfonate groups situated on the channel boundaries. At high proton concentration, the percolation threshold of hydrated channels is observed in SPEEK, which presents hydrated nanochannels with higher tortuosity than Nafion or SPPSU. In SPEEK–silylated poly-phenyl-sulfone (SiPPSU) composites, this percolation threshold is not observed, pointing to special interface paths for conduction.

Thermodynamics of aqueous solutions of methyldiethanolamine and diisopropanolamine

The vapour–liquid equilibrium (VLE) of the systems of water + methyldiethanolamine (MDEA) and water + diisopropanolamine (DIPA) was measured at several temperatures with a static total pressure apparatus. The solid–liquid equilibrium (SLE) of the same systems was measured at low amine concentrations by means of two experimental methods: a visual method and a Differential Scanning Calorimeter (DSC). The activity coefficients of water + MDEA were modelled with the NRTL equations. The model parameters were regressed from VLE, SLE and excess enthalpy data from this work and from the literature. The model developed in this work was compared with models found in the literature. The NRTL equations were also used to model the activity coefficients of the system of water + DIPA. The model parameters were fitted from the VLE and SLE data measured in this work.

Compatible solutes: Thermodynamic properties and biological impact of ectoines and prolines

Compatible solutes like ectoine and its derivatives are deployed by halophile organisms as osmolytes to sustain the high salt concentration in the environment. This work investigates the relation of the thermodynamic properties of compatible solutes and their impact as osmolytes. The ectoines considered in this work are ectoine, hydroxyectoine, and homoectoine. Besides solution densities (15–45 °C) and solubilities in water (3–80 °C), component activity coefficients in the aqueous solutions were determined in the temperature range between 0 and 50 °C. The latter is important for adjusting a certain water activity and therewith a respective osmotic pressure within a cell. The characteristic effect of ectoines is compared to that of prolines, as well as to that of incompatible solutes as salts and urea. The experimental results show that the influence on the activity (coefficient) of water is quite different for compatible and incompatible solutes: whereas compatible solutes cause decreasing water activity coefficients, incompatible solutes lead to an increase in water activity coefficients. Based on this quantity, the paper discusses the impact of various osmolytes on biological systems and contributes to the explanation why some osmolytes are more often and at other temperatures used than others. Moreover, it was found that the anti-stress effect of an osmolyte is weakened in the presence of a salt. Finally, it is shown that the thermodynamic properties of compatible solutes can be modeled and even predicted using the thermodynamic model PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory).

Measurement and Prediction of Vapor−Liquid Equilibrium of Aqueous 1-Ethyl-3-methylimidazolium-Based Ionic Liquid Systems

Isothermal vapor−liquid equilibria were determined for two binary ionic liquid + water systems, from 323.3 to 368.2 K, and water mole fractions greater than 0.5. The ionic liquids used were 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium trifluoroacetate. In addition, predictive thermodynamic modeling of the vapor−liquid equilibrium was performed by correlating the nonrandom two-liquid,(39) universal quasi-chemical,(40) and electrolyte-NRTL(41) models to previously measured excess enthalpy and infinite dilution activity coefficient data from the literature. For each of the systems studied at least two of the models provided adequate predictions of vapor pressure and water activity coefficients. The good predictions of vapor−liquid equilibria by these common activity coefficient models lead us to favor excess enthalpy and infinite dilution activity coefficient data over vapor−liquid equilibria data, since predictions of excess enthalpies from vapor−liquid equilibria are not satisfactory.

Widening the gap between measurement and modelling of secondary organic aerosol properties?

Abstract. The link between measured sub-saturated hygroscopicity and cloud activation potential of secondary organic aerosol particles produced by the chamber photo-oxidation of α-pinene in the presence or absence of ammonium sulphate seed aerosol was investigated using two models of varying complexity. A simple single hygroscopicity parameter model and a more complex model (incorporating surface effects) were used to assess the detail required to predict the cloud condensation nucleus (CCN) activity from the sub-saturated water uptake. Sub-saturated water uptake measured by three hygroscopicity tandem differential mobility analyser (HTDMA) instruments was used to determine the water activity for use in the models. The predicted CCN activity was compared to the measured CCN activation potential using a continuous flow CCN counter. Reconciliation using the more complex model formulation with measured cloud activation could be achieved widely different assumed surface tension behavior of the growing droplet; this was entirely determined by the instrument used as the source of water activity data. This unreliable derivation of the water activity as a function of solute concentration from sub-saturated hygroscopicity data indicates a limitation in the use of such data in predicting cloud condensation nucleus behavior of particles with a significant organic fraction. Similarly, the ability of the simpler single parameter model to predict cloud activation behaviour was dependent on the instrument used to measure sub-saturated hygroscopicity and the relative humidity used to provide the model input. However, agreement was observed for inorganic salt solution particles, which were measured by all instruments in agreement with theory. The difference in HTDMA data from validated and extensively used instruments means that it cannot be stated with certainty the detail required to predict the CCN activity from sub-saturated hygroscopicity. In order to narrow the gap between measurements of hygroscopic growth and CCN activity the processes involved must be understood and the instrumentation extensively quality assured. It is impossible to say from the results presented here due to the differences in HTDMA data whether: i) Surface tension suppression occurs ii) Bulk to surface partitioning is important iii) The water activity coefficient changes significantly as a function of the solute concentration.

Water activities, activity coefficients and solubility in the binary and ternary aqueous solutions [formula omitted] with [formula omitted

In this investigation, the ternary aqueous solutions of lithium chloride with alkaline earth metal { yLiCl+(1− y)Y Cl 2}(aq) with Y Mg 2+; Ca 2+; and Ba 2+ have been studied using the hygrometric method at 298.15 K. The water activities are measured at total molalities from 0.20 mol kg −1 to saturation for different ionic-strength fractions y of LiCl with y = 0.20 , 0.50 and 0.80. The obtained data allow the deduction of osmotic coefficients. From these measurements, the mixing ionic parameters θ LiY and ψ LiY Cl are determined and used to predict the solute activity coefficients in the mixtures. Also, we have completed measurements of water activities from 6 mol kg −1 to saturation of the pure electrolyte solutions of CaCl 2(aq) using the hygrometric method. Few studies have extended their measurements to saturation (11.00 mol kg −1 for CaCl 2⋅2H 2O). These results are compared with those obtained by other techniques. The values of the pure electrolyte and mixing ionic-interaction parameters, which give the best fit of the activity data in binary, ternary solutions and solubility data are tabulated. Thermodynamic characteristics, solubility products K s p ∘ and the standard molar Gibbs energy of formation Δ G f m 0 of the crystallizing solids are given. The predicted solubility isotherms are compared with the literature data. The behaviour of aqueous electrolyte solutions of alkaline-earth with lithium of chlorides { yLiCl+(1− y)Y Cl 2}(aq) with Y Mg 2+; Ca 2+ or Ba 2+, is correlated and the results show that the ionic interactions in these solutions are essentially related to the nature of solute combined with the solute LiCl.