| 1. |
- Jansson, Maria, et al.
(författare)
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The effect of the relative permittivity on the tactoid formation in nanoplatelet systems. A combined computer simulation, SAXS, and osmotic pressure study
- 2018
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797. ; 513, s. 575-584
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Tidskriftsartikel (refereegranskat)abstract
- The structural properties, and the intracrystalline swelling of Na+-, and Ca2+-montmorillonite (Na-, and Ca-mmt) have been investigated as an effect of decreasing the relative permittivity of the solvent, i.e. from water to ethanol (EtOH), utilizing the experimental techniques; small angle X-ray scattering (SAXS) and osmotic pressure measurements. The experimental data were compared with the continuum model, utilizing coarse-grained molecular dynamics bulk simulations, Monte Carlo simulations of two infinite parallel surfaces corresponding to two clay platelets, and the strong coupling theory. It was found that it is possible to tune the electrostatic interactions to obtain a transition from a repulsive to an attractive system for the Na-mmt by increasing the EtOH concentration, i.e. the Bjerrum length increases, and hence, the attractive ion-ion correlation forces are enhanced. A qualitative agreement was observed between the simulations and the experimental results. Moreover, a non-monotonic behavior of the intracrystalline swelling of Ca-mmt as a function of EtOH concentration was captured experimentally, where an increase in the osmotic pressure, and hence, an increase in the d-spacing was found at low concentrations, indicating that repulsive short-ranged interactions dominate in the system. Theoretically, the non-monotonic behavior could not be captured with the continuum model, probably due to the limitation that the electrostatic interactions solely enters the Hamiltonian via the Bjerrum length.
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| 2. |
- Phan Xuan, Tuan, 1984, et al.
(författare)
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Aggregation behavior of aqueous cellulose nanocrystals: the effect of inorganic salts
- 2016
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 23:6, s. 3653-3663
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Tidskriftsartikel (refereegranskat)abstract
- Natural anisotropic building-blocks such as cellulose nanocrystals (CNCs) have attracted considerable attention due to their biodegradability and nanometer-size. In this work the colloidal behavior of CNCs, obtained from sulfuric acid hydrolysis of microcrystalline cellulose, has been studied in presence of salts of different valences. The influence on the colloidal stability and nature of aggregates has been investigated for monovalent salts (LiCl, NaCl, KCl, CsCl), divalent salts (CaCl2 and MgCl2), and a trivalent salt (AlCl3), both experimentally by means of turbidity and small angle X-ray scattering (SAXS) measurements, as well as by Monte Carlo simulations using a simple coarse-grained model. For the entire salt series, a critical aggregation concentration (CAC) could be determined by turbidity measurements, as a result of the reduction of effective Coulomb repulsions due to the presence of sulfate groups on the CNC surface. The CACs also followed the Schulze-Hardy law, i.e. the critical aggregation concentration decreased with increasing counterion valence. For the monovalent ions, the CACs followed the trend Li+ > Na+ > K+ > Cs+, which could be rationalized in terms of matching affinities between the cation and the sulfate groups present at the surface of CNCs. From the SAXS measurements it was shown that the density of the aggregates increased with increasing salt concentration and ion valence. In addition, these findings were rationalized by means of simulation, which showed a good correlation with experimental data. The combination of the experimental techniques and the simulations offered insight into interaction-aggregation relationship of CNC suspensions, which is of importance for their structural design applications.
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| 3. |
- Thuresson, Axel, et al.
(författare)
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Flocculated Laponite-PEG/PEO dispersions with monovalent salt, a SAXS and simulation study.
- 2016
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 1095-7103 .- 0021-9797. ; 466, s. 330-342
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Tidskriftsartikel (refereegranskat)abstract
- It is well-known that clay can form lamellar structures i.e. tactoids, and recently it has been shown that the tactoid formation is dependent on the platelet diameter. To the authors knowledge, no tactoid formation has been observed for montmorillonite platelets with a diameter less than 60nm. In this study, small angle X-ray scattering in combination with coarse-grained modeling and molecular dynamics simulations have been utilized to study the sediment of Laponite-polyethylene glycol/polyethylene oxide (PEG/PEO) at elevated salt concentrations (150mM-1M). Laponite consists of platelets with a diameter of 25nm and it is known to have a relatively monodisperse size-distribution. At pH 10, the face of the platelets has a strong negative charge, whereas the rim is slightly positive. Here we show that it is possible to induce tactoids for Laponite if two constraints are fulfilled: (1) addition of high amount of salt such as NaCl, and (2) addition of a neutral polymer such as PEG. The role of the salt is to screen the repulsive interactions between the platelets and the role of the polymer is to bridge the platelets together: hence the loss in configurational entropy of the polymer is counteracted by the gain in attractive polymer-platelet interaction. As the concentration of NaCl and/or PEG increases, the Bragg peak becomes sharper, which is an indication of that larger tactoids are formed. Comparison between Laponite and montmorillonite shows that the interlayer distance between the platelets increases linearly with an increased Debye screening length for both type of clays, whereas the structure peaks of Laponite are broader compared to the montmorillonite. We argue that the main reason to the latter is due to the size of the platelets: (i) smaller platelets are less rotationally restricted and (ii) the effect of positive edge charges is larger when the platelets are smaller, which results in more irregular aggregates. In absence of the polymer, montmorillonite form tactoids above ∼0.3MNaCl whereas Laponite does not. Even though the model used is simple, we find qualitative agreement between experiments and simulations, which verifies that the underlying physics for tactoid formation is captured.
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| 4. |
- Thuresson, Axel, et al.
(författare)
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Flocculated Laponite-PEG/PEO Dispersions with Multivalent Salt : A SAXS, Cryo-TEM, and Computer Simulation Study
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:13, s. 7387-7396
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study is to scrutinize the mechanism behind aggregation, i.e., tactoid formation of nanostructures with the shape of a platelet. For that purpose, the clay minerals Laponite and montmorillonite have been used as model systems. More specifically, we are interested in the role of: the platelet size, the electrostatic interactions, and adsorbing polymers. Our hypothesis is that the presence of PEG is crucial for tactoid formation if the system is constituted by small nanometric platelets. For this purpose, SAXS, USAXS, Cryo-TEM, and coarse-grained molecular dynamics simulations have been used to study how the formation and the morphology of the tactoids are affected by the platelet size. The simulations indicate that ion-ion correlations are not enough to induce large tactoids solely if the platelets are small and the absolute charge is too low, i.e., in the size and charge range of Laponite. When a polymer is introduced into the system, the tactoid size grows, and the results can be explained by weak attractive electrostatic correlation forces and polymer bridging. It is shown that when the salt concentration increases the long-ranged electrostatic repulsion is screened, and a free energy minimum appears at short distances due to the ion-ion correlation effects. When a strongly adsorbing polymer is introduced into the system, a second free energy minimum appears at a slightly larger separation. The latter dominates if the polymer is relatively long and/or the polymer concentration is high enough. (Graph Presented).
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| 5. |
- Thuresson, Axel, et al.
(författare)
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Temperature Response of Charged Colloidal Particles by Mixing Counterions Utilizing Ca2+/Na+ Montmorillonite as Model System
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:14, s. 7951-7958
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Tidskriftsartikel (refereegranskat)abstract
- The osmotic pressure and the aggregation of charged colloids as a function of temperature have been investigated using Monte Carlo and molecular dynamics simulations for different ratios of monovalent and divalent counterions. In the simulations the water is treated as a temperature-dependent dielectric continuum, and only the electrostatic interactions are considered. It was found that the temperature response can be controlled, i.e., the osmotic pressure can increase, decrease, or be kept constant, as a function of temperature depending on the monovalent/divalent counterion ratio. The increase in osmotic pressure with temperature, which occurs at low enough surface charge density and/or low fraction of divalent ions, can be understood from the DLVO theory. The origin of the opposite behavior can be explained by the enhanced attractive electrostatic ion-ion correlation interactions with temperature. The constraint is that the absolute value of the surface charge density of the colloids must be above a certain threshold, i.e., high enough such that the attractive ion-ion correlations can dominate the interaction regarding the divalent ions. The current conclusions are supported by the microstructural characterization of Ca2+/Na+-montmorillonite clay using small-angle X-ray scattering. A qualitative agreement is observed between the simulations and the experimental data.
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