| 1. |
- Jönsson, Bo, et al.
(författare)
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Controlling the cohesion of cement paste
- 2005
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 21:20, s. 9211-9221
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Tidskriftsartikel (refereegranskat)abstract
- (T)he main source of cohesion in cement paste is the nanoparticles of calcium silicate hydrate (C-S-H), which are formed upon the dissolution of the original tricalcium. silicate (C3S). The interaction between highly charged C-S-H particles in the presence of divalent calcium counterions is strongly attractive because of ion-ion correlations and a negligible entropic repulsion. Traditional double-layer theory based on the Poisson-Boltzmann. equation becomes qualitatively incorrect in these systems. Monte Carlo (MC) simulations in the framework of the primitive model of electrolyte solution is then an alternative, where ion-ion correlations are properly included. In addition to divalent calcium counterions, commercial Portland cement contains a variety of other ions (sodium, potassium, sulfate, etc.). The influence of high concentrations of these ionic additives as well as pH on the stability of the final concrete construction is investigated through MC simulations in a grand canonical ensemble. The results show that calcium ions have a strong physical affinity (in opposition to specific chemical adsorption) to the negatively charged silicate particles of interest (C-S-H, C3S). This gives concrete surprisingly robust properties, and the cement cohesion is unaffected by the addition of a large variety of additives provided that the calcium concentration and the C-S-H surface charge are high enough. This general phenomenon is also semiquantitatively reproduced from a simple analytical model. The simulations also predict that the affinity of divalent counterions for a highly and oppositely charged surface sometimes is high enough to cause a "charge reversal" of the apparent surface charge in agreement with electrophoretic measurements on both C3S and C-S-H particles.
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| 2. |
- Jönsson, Bo, et al.
(författare)
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Ion-ion correlations in liquid dispersions
- 2004
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Ingår i: Journal of Adhesion. - : Informa UK Limited. - 0021-8464. ; 80:5, s. 339-364
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Tidskriftsartikel (refereegranskat)abstract
- Ion-ion correlations play an important role in liquid dispersion with, strong eleetrostatic inlet-actions. Examples can be found in very diverse areas with the setting of cement paste as one extreme and the compactation, of DNA as another. One particularly spectacular effect of ion-ion correlations is that the traditional double layer repulsion sometimes can be converted into a net attraction. This typically takes place in the presence of multivalent counterions and/or in solutions with low dielectric permittivity. The attractive forces are driven by the energy while the repulsive are mainly of entropic origin, and the final outcome is a delicate balance of these contributions. Here we present two simple models, which give a conceptually simple description of this balance.
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| 3. |
- Jönsson, Bo, et al.
(författare)
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Onset of cohesion in cement paste
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:16, s. 6702-6709
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Tidskriftsartikel (refereegranskat)abstract
- It is generally agreed that the cohesion of cement paste occurs through the formation of a network of nanoparticles of a calcium-silicate-hydrate ("C-S-H"). However, the mechanism by which these particles develop this cohesion has not been established. Here we propose a dielectric continuum model which includes all ionic interactions within a dispersion of C-S-H particles. It takes into account all co-ions and counterions explicitly (with pure Coulomb interactions between ions and between ions and the surfaces) and makes no further assumptions concerning their hydration or their interactions with the surface sites. At high surface charge densities, the model shows that the surface charge of C-S-H particles is overcompensated by Ca2+ ions, giving a reversal of the apparent particle charge. Also, at high surface charge densities, the model predicts that the correlations of ions located around neighboring particles causes an attraction between the particle surfaces. This attraction has a range of approximately 3 nm and a magnitude of 1 nN, values that are in good agreement with recent AFM experiments. These predictions are stable with respect to small changes in surface-surface separation, hydrated ion radius, and dielectric constant of the solution. The model also describes the effect of changes in cement composition through the introduction of other ions, either monovalent (Na) or multivalent (aluminum or iron hydroxide).
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| 4. |
- Pegado, Luis, et al.
(författare)
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Ion-ion correlation attraction in a molecular solvent.
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:18
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Tidskriftsartikel (refereegranskat)abstract
- We have performed Monte Carlo simulations for two like-charged infinite plates with counterions and solvent in between. The solvent is described at the primitive model (PM) level and also as a Stockmayer fluid. Both descriptions share the same qualitative ion correlation picture. At sufficiently high coupling, the pressure between the plates shows an attractive minimum, which is lost or reduced upon decreasing counterion valency, surface charge density, or increasing the screening. The pressure profiles become more oscillatory with higher density, which masks the ion-ion correlation attraction. An interpretation in terms of interaction free energies rather than pressures clarifies the picture. Analyzing the different contributions to the pressure gives further support to the conclusions; the same components are responsible for the same effect in the PM and in the molecular solvent.
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| 5. |
- Pegado, Luis, et al.
(författare)
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Like-charge attraction in a slit system: pressure components for the primitive model and molecular solvent simulations
- 2008
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Ingår i: JOURNAL OF PHYSICS-CONDENSED MATTER. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:49
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Konferensbidrag (refereegranskat)abstract
- We have recently reported Monte Carlo simulations for a system of two infinite like-charged plates in a dipolar fluid solvent (Pegado et al 2008 J. Chem. Phys. at press). The pressure as a function of plate separation qualitatively reproduces the ion-ion correlation attraction picture seen in primitive model studies, where the solvent only enters the picture implicitly through its dielectric constant epsilon(r), scaling all charge-charge interactions. Here we analyse in detail the different components of the pressure between the two plates. This shows that, by changing any of the relevant parameters (counterion valency, surface charge density or dielectric screening), the appearance or increase of a pressure minimum is connected to the same components in both the primitive model and the dipolar solvent model. Decomposing the pressure is helpful in distinguishing between solvent depletion or packing effects and the coexisting correlation attraction. Although the pressure can be evaluated at any plane parallel to the surfaces, the analysis of the pressure at the midplane provides the best physical insight.
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| 6. |
- Pegado, Luis, et al.
(författare)
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The transition from a molecular to a continuum solvent in electrical double layers showing ion-ion correlation effects.
- 2011
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 13, s. 16324-16335
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Tidskriftsartikel (refereegranskat)abstract
- We analyze, using Monte Carlo simulations, how a dielectric medium, modeled as a Stockmayer fluid, modulates the force between two similarly charged surfaces. A major objective is to provide a basis for understanding the strengths and weaknesses of the primitive model. The system studied has uniformly charged walls separated by counterions and solvent, where the latter is kept at constant chemical potential as the separation between the walls is varied. For two different types of Stockmayer fluids, one with a "low" (e ~ 4.4) and one with a "high" (e ~20) relative dielectric permittivity, the size of the solvent molecules is varied systematically. As the size of the solvent molecules becomes smaller one approaches the continuum limit, where the primitive model should give an increasingly more accurate representation. We find that having an explicit description of the solvent gives rise to an oscillatory component in the force between the surfaces. The wavelength of the oscillations reflects the diameter of the solvent molecules. The smaller the solvent molecules the smaller are the amplitudes of the oscillations. On integrating the force curves to yield interaction free energies the oscillatory features become less apparent. For the smallest solvent size studied the interaction curves show clear similarities with those obtained from the primitive model. The qualitative effect of the dielectric screening is recovered. It is found that the deviations from the mean field description also appear for the molecular solvent. All this suggests that there are no major deviations due to the neglect of many-body contributions in the solvent-averaged potential of the primitive model. This also holds for the incompressibility assumption implicitly applied when using the primitive model.
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| 7. |
- Stenhammar, Joakim, et al.
(författare)
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An exact calculation of the van der Waals interaction between two spheres of classical dipolar fluid.
- 2010
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 114:42, s. 13372-13380
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Tidskriftsartikel (refereegranskat)abstract
- An exact treatment of the van der Waals interaction between two spherical dielectric bodies possessing purely classical degrees of freedom is presented. The spheres are described by multipole expansions of their fluctuating charge distributions, and the correlation between the fluctuations are taken into account using classical electrostatics and statistical mechanics. The presented approach avoids both the assumption of pairwise additivity of Hamaker theory and the implicit linear response assumption of Lifshitz theory. The resulting equations are solved numerically for D/a ≥ 0.01, where a is the radius of the spheres and D is their minimum separation, for a system with ε = 80, and the results are compared to the analytical Hamaker formula with a Hamaker constant calculated from Lifshitz theory.
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| 8. |
- Wennerström, Håkan, et al.
(författare)
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On Stern-Gerlach coincidence measurements and their application to Bell's theorem
- 2013
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Ingår i: Physics Essays. - Ottawa : Physics Essays Publication. - 0836-1398. ; 26:2, s. 174-180
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Tidskriftsartikel (refereegranskat)abstract
- We analyze a coincidence Stem-Gerlach measurement often discussed in connection with the derivation and illustration of Bell's theorem. The treatment is based on our recent analysis of the original Stern-Gerlach experiment [H. Wennerstrom and P.-O. Westlund, Phys. Chem. Chem. Phys. 14, 1677 (2012)1, where it is concluded that it is necessary to include a spin relaxation process to account for the experimental observations. We consider two limiting cases of a coincidence measurement using both an analytical and a numerical description. In one limit, relaxation effects are neglected. In this case, the correlation between the two spins present in the initial state is conserved during the passage through the magnets. However, at exit the z-coordinate along the magnetic field gradient is randomly distributed between the two extreme values. In the other limit, T-2 relaxation is assumed to be fast relative to the time of flight through the magnet. In this case, the z-coordinate takes one of two possible values as observed in the original Stern-Gerlach experiment. Due to the presence of a relaxation process involving transfer of angular momentum between particle and magnet, the initially entangled spin state changes character leading to a loss of correlation between the two spins. In the original derivations of Bell's theorem based on a coincidence Stem-Gerlach setup, one assumes both a perfect correlation between the spins and only two possible values for the z-coordinate on exit. According to the present calculations, one can satisfy either of these conditions but not both simultaneously. (C) 2013 Physics Essays Publication.
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| 9. |
- Wennerström, Håkan, et al.
(författare)
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The Stern-Gerlach experiment and the effects of spin relaxation
- 2012
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : RSC Publishing. - 1463-9076 .- 1463-9084. ; 14, s. 1677-1684
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Tidskriftsartikel (refereegranskat)abstract
- The classical Stern-Gerlach experiment is analyzed with an emphasis on the spin dynamics. The central question asked is whether there occurs a relaxation of the spin angular momentum during the time the particle passes through the Stern-Gerlach magnet. We examine in particular the transverse relaxation, involving angular momentum exchange between the spin of the particles and the spins of the magnet. A method is presented describing relaxation effects at an individual particle level. This leads to a stochastic equation of motion for the spins. This is coupled to a classical equation of motion for the particle translation. The experimental situation is then modeled through simulations of individual trajectories using two sets of parameter choices and three different sets of initial conditions. The two main conclusions are: (A) if the coupling between the magnet and the spin is solely described by the Zeeman interaction with the average magnetic field the simulations show a clear disagreement with the experimental observation of Stern and Gerlach. (B) If one, on the other hand, also allows for a T(2) relaxation time shorter than the passage time one can obtain a practically quantitative agreement with the experimental observations. These conclusions are at variance with the standard textbook explanation of the Stern-Gerlach experiment.
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| 10. |
- Westlund, Per-Olof, 1952-, et al.
(författare)
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Electron spin relaxation at low field
- 2010
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 12:1, s. 201-206
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Tidskriftsartikel (refereegranskat)abstract
- The low field ESR lineshape and the electron spin-lattice relaxation correlation function are calculated using the stochastic Liouville theory for an effective electron spin quantum number S = 1. When an axially symmetric permanent zero field splitting provides the dominant relaxation mechanism, and when it is much larger than the rotational diffusion constant, it is shown that both electron spin correlation functions S(0)S(t) (n = 0,1) are characterized by the same relaxation time tau(S) = (4D(R))(-1). This confirms the conjectures made by Schaefle and Sharp, J. Chem. Phys., 2004, 121, 5287 and by Fries and Belorizky, J. Chem. Phys., 2005, 123, 124510, based on numerical results using a different formalism. The stochastic Liouville approach also gives the paramagnetically enhanced nuclear spin relaxation time constants, T(1) and T(2), and the ESR lineshape function I(omega). In particular, the L-band (B(0) = 0.035 T) ESR spectrum of a low symmetry Ni(ii)-complex with a cylindrical ZFS tensor is shown to be detectable at sufficiently slowly reorientation of the complex. The analysis shows that the L-band spectrum becomes similar to the zero-field spectrum with a electron spin relaxation time tau(S) = (4D(R))(-1).
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