Natural organic matter (NOM, or humic substance) has a known tendency to form colloidal aggregates in aqueous environments, with the composition and concentration of cationic species in solution, pH, temperature, and the composition of the NOM itself playing important roles. Strong interaction of carboxylic groups of NOM with dissolved metal cations is thought to be the leading chemical interaction in NOM supramolecular aggregation.Computational molecular dynamics (MD) study of the interactions of Na + , Mg 2+ , and Ca 2+ with the carboxylic groups of a model NOM fragment and acetate anions in aqueous solutions provides new quantitative insight into the structure, energetics, and dynamics of the interactions of carboxylic groups with metal cations, their association and the effects of cations on the colloidal aggregation of NOM molecules. Potentials of mean force and the equilibrium constants describing overall ion association and the distribution of metal cations between contact ion pairs and solvent separated ions pairs were computed from free MD simulations and restrained umbrella sampling calculations. The results provide insight into the local structural environments of metal-carboxylate association and the dynamics of exchange among these sites. All three cations prefer contact ion pair to solvent separated ion pair coordination, and Na + and Ca 2+ show a strong preference for bidentate contact ion pair formation. The average residence time of a Ca 2+ ion in a contact ion pair with the carboxylic groups is of the order of 0.5 ns, whereas the corresponding residence time of a Na + ion is only between 0.02 and 0.05 ns. The average residence times of a Ca 2+ ion in a bidentate coordinated contact ion pair vs. a monodentate coordinated contact ion pair are about 0.5 ns and about 0.08 ns, respectively. On the 10-ns time scale of our simulations, aggregation of the NOM molecules occurs in the presence of Ca 2+ but not Na + or Mg 2+ . These results agree with previous experimental observations and are explained 3 by both Ca 2+ ion-bridging between NOM molecules and decreased repulsion between the NOM molecules due to the reduced net charge of the NOM-metal complexes. Simulations on a larger scale are needed to further explore the relative importance of the different aggregation mechanisms and the stability of NOM aggregates.4

fects of Ca2+ on supramolecular aggregation of natural organic matter in aqueous solutions: A comparison of molecular modeling approaches. Geoderma, Elsevier, 2011, 169, pp.27-32

The 6s−npj (n = 6−9) electric-dipole matrix elements and 6s−ndj (n = 5−7) electric-quadrupole matrix elements in Ba + are calculated using the relativistic all-order method. The resulting values are used to evaluate ground state dipole and quadrupole polarizabilities. In addition, the electricdipole 6pj − 5d j ′ matrix elements and magnetic-dipole 5d 5/2 − 5d 3/2 matrix element are calculated using the same method in order to determine the lifetimes of the 6p 1/2 , 6p 3/2 , 5d 3/2 , and 5d 5/2 levels. The accuracy of the 6s − 5dj matrix elements is investigated in detail in order to estimate the uncertainties in the quadrupole polarizability and 5dj lifetime values. The lifetimes of the 5d states in Ba + are extremely long making precise experiments very difficult. Our final results for dipole and quadrupole ground state polarizabilities are αE1 = 124.15 a 3 0 and αE2 = 4182(34) a 5 0 , respectively. The resulting lifetime values are τ6p 1/2 = 7.83 ns, τ6p 3/2 = 6.27 ns, τ 5d 3/2 = 81.5(1.2) s, and τ 5d 5/2 = 30.3(4) s. The extensive comparison with other theoretical and experimental values is carried out for both lifetimes and polarizabilities.

We measure the hyperfine splitting of the 9S_{1/2} level of 210Fr, and find a magnetic dipole hyperfine constant A=622.25(36) MHz. The theoretical value, obtained using the relativistic all-order method from the electronic wave function at the nucleus, allows us to extract a nuclear magnetic moment of 4.38(5)micro_{N} for this isotope, which represents a factor of 2 improvement in precision over previous measurements. The same method can be applied to other rare isotopes and elements.

We present results of the first-principles calculation of Cs dipole static polarizabilities for the N s (N = 6 − 12), N pj (N = 6 − 10), and N dj (N = 5 − 10) states using the relativistic all-order method. In our implementation of the all-order method, single and double excitations of Dirac-Fock wave functions are included to all orders in perturbation theory. Additional calculations are carried out for the dominant terms and the uncertainties of our final values are estimated for all states. A comprehensive review of the existing theoretical and experimental studies of the Cs polarizabilities is also carried out. Our results are compared with other values where they are available. These calculations provide a theoretical benchmark for a large number of Cs polarizabilities. * Electronic address: usafrono@nd.edu; On leave from ISAN, Troitsk, Russia In 2005, Gould and Miller [3] wrote a comprehensive review of the experimental methods to determine the static electric-dipole polarizabilities. Miller and Bederson's earlier review from 1988 [4] concentrated on the bulk polarizability measurements and the atomic beam methods. Average bulk ground state static polarizabilities are measured by determining the dielectric constant of an atomic or molecular gas. The bulk dynamic polarizabilities are determined by measuring the refractive index of the gas, see [4]. The bulk methods are very accurate, but their limitation lies in the need to deal with atoms or molecules that are stable and gaseous at room temperature and the fact that the effect of the excited states can not be accounted for.In 1974, Molof et al.[5] used the E-H-gradient balance technique to measure the static electricdipole polarizabilities of alkali-metal atoms. They obtained the value (59.6 ± 1.2) 10 −24 cm 3 for electric-dipole polarizability of the ground state of cesium. Hall and Zorn [6] measured the value (63.3 ± 4.6) 10 −24 cm 3 for the electric-dipole polarizability of the ground state of cesium. They used the deflection of a velocity-selected atomic beam in inhomogeneous electric field. The technique is based on the fact that the deflection experienced by atoms moving through a region with known transverse electric field gradient is proportional to the dipole polarizability of the atoms. An important detail of this technique is that the precision with which the velocity of the atoms is known puts a limitation on the precision of the experiment. The short interaction time in the case of high velocity which leads to small deflection of the beam places another limitation on the accuracy of this method.In 1995, Ekstrom et al.[7] designed an atomic interference experiment that allowed them to measure the ground state energy shifts with spectroscopic precision and determine the ground state dipole polarizability. In 2003, Amini and Gould [8] designed an experiment that avoids the problems associated with the measuring the deflection of a thermal beam in transverse electric-field gradient. They measure the effect of the electric-field gradient on the lo...

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The endothermic decomposition of ammonium carbamate has been proposed as a novel heat sink mechanism for aircraft thermal management [

In this worii, results from atomistic molecular dynamics studies investigating the effect of swfactant concentration on the transport properties of bulk surfactant aqueous solutions, focusing on the anionic swfactant sodium dodecyl sulfate (SDS), are reported. The surfactant self-diffusion and the thermal conductivity of bulk aqueous SDS solutions were computed at a range of concentrations at room and boiling temperatures. Additionally, MP2f (Akin-Ojo et al., 2008, "Developing Ab Initio Quality Force Fields From Condensed Phase Quantum-MechanicslMolecular-Mechanics Calculations Through the Adaptive Force Matching Method," J. Phys. Chem., 129, p. 064108), one of a new generation water potentials is assessed for its suitability in reproducing the transport and thermal properties of bulk water. The thermal conductivity of MP2f water model was found to be: 0.64 Wl(m-K) at 298K and 0.66 Wl(m-K) at 373 K, in much better agreement with the experimental values compared to both the rigid and the flexible TIP3P water model.