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| 2. |
- Abbas, Zareen, 1962, et al.
(författare)
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Activity Coefficients of Concentrated Salt Solutions: A Monte Carlo Investigation
- 2019
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Ingår i: Journal of Solution Chemistry. - : Springer Science and Business Media LLC. - 0095-9782 .- 1572-8927. ; 48:8-9
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Tidskriftsartikel (refereegranskat)abstract
- Monte Carlo (MC) simulations were used to calculate single ion and mean ionic activity coefficients and water activity in concentrated electrolytes and at elevated temperatures. By using a concentration dependent dielectric constant, the applicability range of the MC method was extended to 3 mol·L−1 or beyond, depending on the salt. The calculated activity coefficients were fitted to experimental data by adjusting only one parameter, i.e., the cation radius. Fitted ionic radii obtained by such a procedure indicate the extent of cation–anion interaction in a salt solution. For example, the fitted radii of Li+ and Na+ in LiClO3 and NaClO3 indicate that Li+ is strongly hydrated and has a weak interaction with the ClO3− ion whereas Na+ forms ion pairs and loses its hydration. The single ion activity coefficients for protons and chloride ions in HCl were calculated by MC simulations and compared with experimental values obtained by ion selective electrodes. The calculated single ion activity coefficients for protons and chloride ions are much lower and higher, respectively, than the experimental values. However, the mean activity coefficients of HCl obtained by the MC simulations, ion selective electrodes and vapor pressure measurements are in good agreement. In the case of NaCl and KCl the calculated single ion activity coefficients of Na+, K+, and Cl− are much closer to the values obtained by ion selective electrodes. The results in HCl indicate that the hydrated proton is large and includes the chloride ion within the hydration shell, i.e., the apparent size of the chloride ion is negligible.
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| 3. |
- Abbas, Zareen, 1962, et al.
(författare)
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From restricted towards realistic models of salt solutions: Corrected Debye–Hückel theory and Monte Carlo simulations
- 2007
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Ingår i: Journal of Mathematical Fluid Mechanics. - : Elsevier BV. - 1422-6952 .- 1422-6928 .- 0378-3812. ; 260:2, s. 233-247
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Tidskriftsartikel (refereegranskat)abstract
- The properties of bulk salt solutions over wide concentration ranges are explored by a combination of simple physical theory and Monte Carlo (MC) simulations. The corrected Debye–Hückel (CDH) theory which incorporates ion size effects in a linear response approximation is extended to yield free energy and other thermodynamic properties by integration of the chemical potential over concentration. Charging integration which is usually used to obtain an electrostatic contribution of total free energy of electrolytes is avoided in this new direct approach. MC simulations are performed with a modified Widom particle insertion method, which also provides directly the ionic activity coefficients. The validity of the CDH theory is tested by comparison with the MC simulation data for 1:1, 2:1, 2:2 and 3:1 restricted primitive model (RPM) electrolytes over a wide concentration range and at various ion sizes. Mean ionic activity and osmotic coefficients calculated by the CDH theory in RPM approximation of electrolyte are fitted to experimental data by adjusting only a mean ionic diameter. Good fits up to 1 molal (m) concentration are obtained for a large number of salt solutions. MC simulations data for unrestricted primitive model (UPM) of 1:1 and 2:1 electrolytes are also fitted to the experimental data by varying the cation radius while keeping the anion radius fixed at a crystallographic value. The success of this approach is found to be salt specific. For example good fits up to 2 and 3.5 m concentrations were obtained for LiCl and LiBr, respectively. However in the case of less dissociated salts such as NaCl and KI the experimental data could only be fitted up to one molal concentration. Possibility of extending the applicability range of the CDH theory to concentrations >2 m is explored by including a concentration dependent dielectric constant as measured in experiments. Mean ionic activity coefficients for a number of salts could successfully be fitted up to 3 m concentration by adjusting only a mean ionic diameter. Difficulties encountered in simultaneously fitting the mean ionic activity and osmotic coefficients at salt concentrations >2 m are discussed.
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| 4. |
- Abbas, Zareen, 1962, et al.
(författare)
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Monte Carlo simulation of the dissociation constants of CO2 in 0 to 1 molal sodium chloride between 0 and 25 °C
- 2013
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Ingår i: Marine Chemistry. - : Elsevier BV. - 0304-4203. ; 150:1, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Stoichiometric dissociation constants of the carbon dioxide system in NaCl solution between 0 and 1 mol and 0 to 25 °C were estimated by Monte Carlo (MC) simulations, and compared with Pitzer calculations and experimental measurements. The MC simulations used experimentally determined dielectric constants of water at different temperatures, and optimal agreement with the experimental data and Pitzer calculations was achieved by adjusting the ionic radii. This simple procedure resulted in effective ionic radii which were further used to simulate the activity coefficients of salt mixtures. The first and second stoichiometric dissociation constants of carbonic acid in NaCl solution (pK1⁎ and pK2⁎) were estimated from the MC-derived activity coefficients of mixed salts in NaCl. The MC results are in good agreement with the experimental data as well as with the Pitzer calculations. This study shows that Monte Carlo simulations in the temperature and ionic strength range relevant to seawater can provide pK values of the same quality as Pitzer calculations, and constitutes the first step in developing a temperature-dependent MC model for seawater. While MC calculations require greater computing resources, the number of parameters derived by fitting to thermodynamic data is substantially smaller than for Pitzer calculations.
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| 5. |
- Abbas, Zareen, 1962, et al.
(författare)
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Monte Carlo Simulations of Salt Solutions: Exploring the Validity of Primitive Models
- 2009
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 113:17, s. 5905-5916
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Tidskriftsartikel (refereegranskat)abstract
- An extensive series of Monte Carlo (MC) simulations were performed in order to explore the validity of simple primitive models of electrolyte solutions and in particular the effect of ion size asymmetry on the bulk thermodynamic properties of real salt solutions. Ionic activity and osmotic coefficients were calculated for 1:1, 2:1, and 3:1 electrolytes by using the unrestricted primitive model (UPM); i.e., ions are considered as charged hard spheres of different sizes dissolved in a dielectric continuum. Mean ionic activity and osmotic coefficients calculated by the MC simulations were fitted simultaneously to the experimental data by adjusting only the cation radius while keeping the anion radius fixed at its crystallographic value. Ionic radii were further optimized by systematically varying the cation and anion radii at a fixed sum of ionic radii. The success of this approach is found to be highly salt specific. For example, experimental data (mean ionic activity and osmotic coefficients) of salts which are usually considered as dissociated such as HCl, HBr, LiCl, LiBr, LiClO4, and KOH were successfully fitted up to 1.9, 2.5, 1.9, 3, 2.5, and 4.5 M concentrations, respectively. In the case of partially dissociated salts such as NaCl, the successful fits were only obtained in a more restricted concentration range. Consistent sets of the best fitted cation radii were obtained for acids, alkali, and alkaline earth halides. A list of recommended ionic radii is also provided. The reliability of the optimized ionic radii was further tested in simulations of the osmotic coefficients of LiCl−NaCl−KCl salt mixtures. A very good agreement between the simulated and experimental data was obtained up to ionic strength of 4.5 M.
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| 7. |
- Abbas, Zareen, 1962, et al.
(författare)
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Size-Dependent Surface Charging of Nanoparticles
- 2008
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Ingår i: J of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:15, s. 5715-5723
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Tidskriftsartikel (refereegranskat)abstract
- Experimental interest in the possible curvature dependence of particle charging in electrolyte solutions is subjected to theoretical analysis. The corrected Debye-Hückel theory of surface complexation (CDH-SC) and Monte Carlo (MC) simulation are applied to investigate the dependence of surface charging of metal oxide nanoparticles on their size. Surface charge density versus pH curves for spherical metal oxide nanoparticles in the size range of 1-100 nm are calculated at various concentrations of a background electrolyte. The surface charge density of a nanoparticle is found to be highly size-dependent. As the particle diameter drops to below 10 nm there is considerable increase in the surface charge density as compared with the limiting values seen for particles larger than 20 nm. This increase in the surface charge density is due to the enhanced screening efficiency of the electrolyte solution around small nanoparticles, which is most prominent for particles of diameters less than 5 nm. For example, the surface charge densities calculated for 2 nm particles at 0.1 M concentration are very close to the values obtained for 100 nm particles at 1 M concentration. These predictions of the dependence of surface charge density on particle size by the CDH-SC theory are in very good agreement with the corresponding results obtained by the MC simulations. A shift in the pH value of the point of zero charge toward higher pH values is also seen with a decreasing particle size.
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| 8. |
- Abbas, Zareen, 1962, et al.
(författare)
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Synthesis, characterization and particle size distribution of TiO2 colloidal nanoparticles
- 2011
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757. ; 384:1-3, s. 254-261
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Tidskriftsartikel (refereegranskat)abstract
- Nanoparticles of controlled size, well defined shape, pure phase and of clean surfaces are ideal model systems to investigate surface/interfacial reactions. In this study we have explored the possibility of synthesizing TiO2 nanoparticles in the size range of 7–20 nm under well controlled experimental conditions. A simple method based on the hydrolysis of TiCl4 was used to obtain particles having surfaces free from organics. Stable dispersions of TiO2 nanoparticles of various sizes were obtained by optimizing the reaction/dialysis time and temperature. The synthesized TiO2 particles were found to be predominantly of anatase phase and narrow particle size distributions were obtained. The TiO2 particles were characterized with respect to their phase, size and shape by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Particle size distribution in a colloidal dispersion was obtained by the electrospray scanning mobility particle sizer (ES-SMPS) method and compared with an average particle size determined from dynamic light scattering (DLS). The average particle sizes obtained by the DLS and ES-SMPS methods were in good agreement, while a primary particle size of 4 nm was found in X-ray diffraction irrespective of the particle size in solution. Early stages of the nucleation process were monitored by the ES-SMPS method. These results show that small particles of 4–5 nm are initially formed and it is highly likely that large particles are formed due to aggregation of primary particles.
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| 9. |
- Babaahmadi, Arezou, 1985, et al.
(författare)
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A study of the accelerated ageing process of cementitious materials
- 2011
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Ingår i: Advances in Construction Materials through Science and Engineering. ; RILEM PRO 79
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Konferensbidrag (refereegranskat)abstract
- This paper presents part of a study in a research project called “Ageing of the cementitious materials for storage of nuclear waste”. The goal is to establish a mechanistic model for predicting longevity of concrete in storage facilities for nuclear waste. As deposition of low and intermediate level radioactive waste (LILW) needs up to 100 000 years, it is necessary to analyze the service life of cementitious materials in this time perspective. Laboratory methods are, therefore, needed for accelerating the ageing process without making any influencing distortion in the properties of the materials. Chemical and electrochemical acceleration approaches were tried in this study. Chemical acceleration involves leaching of samples under conditions of actual groundwater. Electrochemical migration is also applied to accelerate leaching process. Chemical and XRD analysis is carried out to identify the compositions before and after the acceleration tests. The results indicate a possibility to produce aged specimens for further mechanical tests and other physical tests, such as porosity and diffusivity.
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| 10. |
- Babaahmadi, Arezou, 1985, et al.
(författare)
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Ageing of cementitious materials for storage of nuclear waste
- 2011
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Ingår i: Nordic Concrete Research. ; Publication No. 43, s. 429-432
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Konferensbidrag (refereegranskat)abstract
- This paper presents an on-going research project dealing with ageing process of cementitious materials in a perspective of hundreds and thousands years. As it is risky to use empirical models for extrapolation of performance data from relatively short term experiments, a mechanism-based (chemo-mechanical coupled) model for safer prediction of longevity of concrete in storage facilities for nuclear waste is needed. The project work involves thermodynamic modeling, development of accelerated aging tests, physical, chemical and mineralogical characterization of young and aged cementitious materials, including mechanical, transport (diffusivity) properties, binding (adsorption) capacities and surface complexation (charging) behavior.
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