| 1. |
- Jenkins, Samantha, 1967-, et al.
(author)
-
The role of hydrogen bonding in nanocolloidal amorphous silica particles in electrolyte solutions
- 2009
-
In: Journal of colloid and interface science. - : Elsevier BV. - 1095-7103 .- 0021-9797.
-
Journal article (peer-reviewed)abstract
- Explicit solvent (water) molecular dynamics simulations were undertaken containing three pairs of amorphous silica nanoparticles, having diameters of 2.0nm, 2.4nm and 2.8nm, respectively. Mean forces acting between the silica nanoparticles were calculated in a background electrolyte, i.e., NaCl at four different concentrations. Dependence of the inter-particle potential of mean force on the center of mass separation, silicon to sodium ratio (Si:Na(+)), background electrolyte concentration, number of hydrogen bonds directly linking pairs of silica nanoparticles and the density of charged surface sites, are calculated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The close relationship between the variation of the number of hydrogen bonds between the pairs of silica nanoparticles and the inter-particle potential of mean force indicates that the degree of inter-particle hydrogen bonding quantifies, for a given size of nanoparticle, the degree of nanoparticle 'stickiness'. Simulations also show that the number of hydrogen bonds between the charged surface (O(-)) sites and the surrounding water molecules increases with increase in charged sites, in agreement with the interaction behavior of silica nanoparticles usually seen in experiments.
|
|
| 2. |
- Jenkins, Samantha, 1967-, et al.
(author)
-
The mechanics of charge-shift bonds: A perspective from the electronic stress tensor
- 2011
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 510, s. 18-20
-
Journal article (peer-reviewed)abstract
- The mechanism of charge-shift bonding is discussed from the perspective of the electronic stress tensor. In charge-shift bonds, the tensile mode of the stress tensor (attraction of electrons to the nuclei) is stronger, relative to the compressive modes (attraction of electrons toward the bond axis) than in conventional covalent bonds. Similar bonding indicators based on the eigenvalues of the tensor of second derivatives of the electron density and bond metallicity measures also correlate with charge-shift binding. These indicators seem preferable to the electron-density Laplacian and the local energy density because they can distinguish between weak covalent bonds and charge-shift bonds.
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Jenkins, Samantha, 1967-, et al.
(author)
-
Molecular dynamics simulation of nanocolloidal amorphous silica particles : Part I.
- 2007
-
In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:22, s. 224711-
-
Journal article (peer-reviewed)abstract
- Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles in aqueous solution, with diameter of 4.4 nm and with four different background electrolyte concentrations, to extract the mean force acting between the two silica nanoparticles. Dependences of the interparticle forces on the separation and the background electrolyte concentration were demonstrated. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was investigated. A "patchy" double layer of adsorbed sodium counterions was observed. Dependences of the interparticle potential of mean force on the separation and the background electrolyte concentration were demonstrated. Direct evidence of the solvation forces is presented in terms of changes of the water ordering at the surfaces of the isolated and double nanoparticles. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was investigated in terms of quantifying the effects of the number of water molecules separately inside each pair of nanoparticles by defining an impermeability measure. A direct correlation was found between the impermeability (related to the silica surface "hairiness") and the disruption of water ordering. Differences in the impermeability between the two nanoparticles are attributed to differences in the calculated electric dipole moment.
|
|
| 6. |
- Jenkins, Samantha, 1967-, et al.
(author)
-
Molecular dynamics simulation of nanocolloidal amorphous silica particles: Part III
- 2009
-
In: The Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130
-
Journal article (peer-reviewed)abstract
- Explicit-solvent molecular dynamics simulations were applied to four pairs of amorphous silica nanoparticles, two pairs having a diameter of 2.0 nm and two pairs with diameter 3.2 nm. The silica nanoparticles were immersed in a background electrolyte consisting of Ca2+ and Cl− ions and water and mean forces acting between the pair of silica nanoparticles were extracted at four different background electrolyte concentrations. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. Dependence of the interparticle potential of mean force on the center-of-mass separation and the silicon to sodium ratio (5:1 and 20:1) is demonstrated. A Si:Na+ ratio of 5:1 gave more repulsive interparticle potentials and lower numbers of internanoparticle or “bridging” hydrogen bonds. Conversely a Si:Na+ ratio of 20:1 yielded more attractive potentials and higher numbers of bridging hydrogen bonds. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodium double layer on water ordering was observed. The number of water molecules trapped inside the nanoparticles was investigated, and at the highest background ionic concentrations were found to consistently behave in accordance with there being an osmotic pressure. This study highlights the effect of divalent (Ca2+) background ions on the interparticle potentials compared with previous work using monovalent (Na+) background ions. Mechanisms of coagulation and the stability of silica nanocolloids found from this work appear to be in agreement with findings from experiments described in the literature.
|
|
| 7. |
- Jenkins, Samantha, 1967-, et al.
(author)
-
Molecular dynamics simulations of nanocolloidal amorphous silica particles: Part II
- 2008
-
In: The Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128:16
-
Journal article (peer-reviewed)abstract
- Explicit molecular dynamics simulations were applied to a pair of amorphous silica nanoparticles with diameter of 3.2 nm immersed in a background electrolyte. Mean forces acting between the pair of silica nanoparticles were extracted at four different background electrolyte concentrations. The dependence of the interparticle potential of mean force on the separation and the silicon to sodium ratio, as well as on the background electrolyte concentration, are demonstrated. The pH was indirectly accounted for via the ratio of silicon to sodium used in the simulations. The nature of the interaction of the counterions with charged silica surface sites (deprotonated silanols) was also investigated. The effect of the sodium double layer on the water ordering was investigated for three Si:Na+ ratios. The number of water molecules trapped inside the nanoparticles was investigated as the Si:Na+ ratio was varied. Differences in this number between the two nanoparticles in the simulations are attributed to differences in the calculated electric dipole moment. The implications of the form of the potentials for aggregation are also discussed.
|
|
| 8. |
- Eshagh, Mehdi, et al.
(author)
-
Smoothing impact of isostatic crustal thickness models on local integral inversion of satellite gravity gradiometry data
- 2011
-
In: ACTA GEOPHYSICA. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 59:5, s. 891-906
-
Journal article (peer-reviewed)abstract
- The effects of topographic masses on satellite gradiometric data are large and in order to reduce the magnitude of these effects some compensation mechanisms should be considered. Here we use the isostatic hypotheses of Airy-Heiskanen and the recent Vening Meinesz-Moritz for compensating these effects and to smooth the data prior to their downward continuation to gravity anomaly. The second-order partial derivatives of extended Stokes' formula are used for the continuations over a topographically rough territory like Persia. The inversions are performed and compared based on two schemes of the remove-compute-restore technique and direct downward continuation. Numerical results show that the topographic-isostatic effect based on Vening Meinesz-Mortiz's hypothesis smoothes the data better than that based on Airy-Heiskanen's hypothesis. Also the quality of inversions of the smoothed data by this mechanism is twice better than that of the nonsmoothed ones.
|
|
| 9. |
- Guevara-Garca, Alfredo, et al.
(author)
-
Pointing the way to the products Comparison of the stress tensor and the second-derivative tensor of the electron density
- 2011
-
In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 134:23, s. 234106-
-
Journal article (peer-reviewed)abstract
- The eigenvectors of the electronic stress tensor can be used to identify where new bond paths form in a chemical reaction. In cases where the eigenvectors of the stress tensor are not available, the gradient-expansion- approximation suggests using the eigenvalues of the second derivative tensor of the electron density instead; this approximation can be made quantitatively accurate by scaling and shifting the second-derivative tensor, but it has a weaker physical basis and less predictive power for chemical reactivity than the stress tensor. These tools provide an extension of the quantum theory of atoms and molecules from the characterization of molecular electronic structure to the prediction of chemical reactivity. © 2011 American Institute of Physics.
|
|
| 10. |
- Hosseini, Vahid, 1987-, et al.
(author)
-
Fe and Cr phase separation in super and hyper duplex stainless steel plates and welds after very short aging times
- 2021
-
In: Materials & design. - : Elsevier Ltd. - 0264-1275 .- 1873-4197. ; 210
-
Journal article (peer-reviewed)abstract
- Fe and Cr phase separation in ferrite, causing 475°C-embrittlement, was studied after very short aging times in super duplex stainless steel (SDSS) and hyper duplex stainless steel (HDSS) plates and welds. Atom probe tomography showed that hot-rolled SDSS, experiencing significant metal working, had faster kinetics of phase separations compared to the SDSS and HDSS welds after 5 min aging at 475 °C. The surface of the 33-mm SDSS plate had faster Fe and Cr phase separation and larger toughness drop. A higher density of dislocations next to the austenite phase boundary in ferrite, detected by electron channeling contrast, can promote the phase separation at the surface of the plate with lower austenite spacing. The toughness dropped in HDSS welds after aging, but SDSS welds maintained their toughness. An inverse simulation method considering an initial sinusoidal nanometric Cr and Fe fluctuation showed that Ni increases the interdiffusion of Cr in the system, resulting a higher degree of phase separation in SDSS welds than the HDSS weld. Within the composition range of the studied SDSS and HDSS materials, the processing influences the Fe and Cr phase separation more than the variation in composition during short aging or typical fabrication times.
|
|
