The Influence of {Ba2+}:{SO42-} Solution Stoichiometry on BaSO4 Crystal Nucleation and Growth in Aqueous Solutions

The impact of solution stoichiometry, upon formation of BaSO4 crystals in 0.02 M NaCl suspensions, on the development of particle size was investigated using Dynamic Light Scattering (DLS). Measurements were performed on a set of suspensions prepared with predefined initial supersaturation (Ωbarite = {Ba2+}{SO42-}/Ksp = 1000) and dissolved ion activity stoichiometries (raq = {Ba2+}:{SO42-} = 0.01, 0.1, 1, 10 and 100), at a pH of 5.5 to 6.0, and ambient temperature and pressure. At this Ωbarite and set of raq , the average apparent hydrodynamic particle size of the largest population present in all suspensions grew from ~ 200 nm to ~ 700 nm within 10 to 15 minutes. This was independently confirmed by TEM imaging. Additional DLS measurements conducted at the same conditions in flow confirmed that the BaSO4 formation kinetics were very fast for our specifically chosen conditions. The DLS flow measurements, monitoring the first minute of BaSO4 formation, showed strong signs of aggregation of prenucleation clusters forming particles with a size in the range of 200 – 300 nm for every raq. The estimated initial bulk growth rates from batch DLS results show that BaSO4 crystals formed fastest at near stoichiometric conditions and more slowly at non-stoichiometric conditions. Moreover, at extreme SO4-limiting conditions barite formation was slower compared to Ba-limiting conditions. Our results show that DLS can be used to investigate nucleation and growth at carefully selected experimental and analytical conditions. Additional SEM imaging on formed BaSO4 crystals for a range of initial conditions of Ωbarite (i.e. 31, 200, 1000 and 6000), raq (0.01, 0.1, 1, 10 and 100) and different background electrolytes (i.e. NaCl, KCl, NaNO3 , MgSO4 and SrCl2) confirms that {Ba2+}:{SO42-} impacts the growth rate significantly in different directions for the different background electrolytes at the different Ωbarite-values. Furthermore, the BaSO4 crystal morphology varies with raq and the type of background electrolyte. The combined DLS, TEM and SEM results imply that solution stoichiometry should be considered when optimizing antiscalant efficiency to regulate BaSO4 (scale) formation processes

Peak-ring magnetism: Rock and mineral magnetic properties of the Chicxulub impact crater

The Chicxulub impact event at ca. 66 Ma left in its wake the only complex crater on Earth with a preserved peak ring, characterized by a well-developed magnetic anomaly low. To date, little is known about its magnetic properties. The joint Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drill core M0077A revealed that the peak ring consists of uplifted and strongly deformed granitoid basement rocks overlain by a 130-m-thick impact melt and suevite layer. Pre- and postimpact hydrothermal systems affected this basement with maximum temperatures up to 450 °C. We used microscopy, mineral chemistry, temperature-dependent magnetic susceptibility, and hysteresis properties to characterize the magnetic mineralogy of pre-, syn-, and postimpact rocks. Compared to its amount of pure, stoichiometric shocked magnetite, the granitoid basement shows low magnetic susceptibility, which is in line with earlier experimental studies indicating that shock reduces magnetic susceptibility. Cation-substituted magnetite with varying compositions in the melt rocks carries a higher induced and remanent magnetization compared to the basement. In the granitoid basement, magnetite was partially oxidized to hematite by a pre-impact hydrothermal event, but at lithological contacts with high-temperature impact melt rock, this hematite was locally retransformed back to magnetite. Elsewhere in the granitoid basement, the temperature reached in the hydrothermal system was too low for hematite retransformation. It was also too low to anneal all the lattice defects in the shocked magnetite, which likely occurs above 540 °C. The presence of shocked magnetite in the granitoid basement well explains the magnetic anomaly low due to its unusually low induced magnetization

Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization

An exact analytical expression for the magnetic field of a cylinder of finite length with a uniform, transverse magnetization is derived. Together with known expressions for the magnetic field due to longitudinal magnetization, the calculation of magnetic fields for cylinders with an arbitrary magnetization direction is possible. The expression for transverse magnetization is validated successfully against the well-known limits of an infinitely long cylinder, the field on the axis of the cylinder and in the far field limit. Comparison with a numerical finite-element method displays good agreement, making the advantage of an analytical method over grid-based methods evident

Exact expression for the magnetic field of a finite cylinder with arbitrary uniform magnetization

An exact analytical expression for the magnetic field of a cylinder of finite length with a uniform, transverse magnetization is derived. Together with known expressions for the magnetic field due to longitudinal magnetization, the calculation of magnetic fields for cylinders with an arbitrary magnetization direction is possible. The expression for transverse magnetization is validated successfully against the well-known limits of an infinitely long cylinder, the field on the axis of the cylinder and in the far field limit. Comparison with a numerical finite-element method displays good agreement, making the advantage of an analytical method over grid-based methods evident

Interactions between amphoteric surfaces with strongly overlapping double layers

The entropic repulsion between strongly overlapping electrical double-layers from two parallel amphoteric plates is described via the Donnan equilibrium in the limit of zero electric field. The plates feature charge-regulation and the inter-plate solution is in equilibrium with a reservoir of a monovalent electrolyte solution. A finite electric potential and disjoining pressure is found at contact between the plates, due to a complete discharging of the plates. For low potentials, the decay of potential and pressure is fully governed by a characteristic length scale and the contact potential. Additionally, for large separations we find a universal inverse square decay of disjoining pressure, irrespective of the contact potential. The results of the Donnan theory show quantitative agreement with self-consistent field computations that solve the full Poisson equation

Controlling CaCO3 particle size with {Ca2+}:{CO32-} ratios in aqueous environments.

The effect of stoichiometry on the new formation and subsequent growth of CaCO3 was investigated over a large range of solution stoichiometries (10-4 < raq < 104, where raq = {Ca2+}:{CO32-}) at various, initially constant degrees of supersaturation (30 < ωcal < 200, where ωcal = {Ca2+}{CO32-}/Ksp), pH of 10.5 ± 0.27, and ambient temperature and pressure. At raq = 1 and ωcal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1-10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At raq â 1 (ωcal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on raq. Complementary molecular dynamics (MD) simulations confirm that raq affects CaCO3 nanoparticle formation substantially. At raq = 1 and ωcal ≫ 1000, the largest nanoparticle in the system had a 21-68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides ωcal, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO3 in geological, industrial, and geo-engineering settings

Depletion-induced chiral chain formation of magnetic spheres

Experimental evidence is presented for the spontaneous formation of chiral configurations in bulk dispersions of magnetized colloids that interact by a combination of anisotropic dipolar interactions and isotropic depletion attractions. The colloids are superparamagnetic silica spheres, magnetized and aligned by a carefully tuned uniform external magnetic field; isotropic attractions are induced by using poly(ethylene oxide) polymers as depleting agents. At specific polymer concen-trations, sphere chains wind around each other to form helical structures–of the type that previously have only been observed in simulations on small sets of unconfined dipolar spheres with additional isotropic interactions

Depletion-induced chiral chain formation of magnetic spheres

Experimental evidence is presented for the spontaneous formation of chiral configurations in bulk dispersions of magnetized colloids that interact by a combination of anisotropic dipolar interactions and isotropic depletion attractions. The colloids are superparamagnetic silica spheres, magnetized and aligned by a carefully tuned uniform external magnetic field; isotropic attractions are induced by using poly(ethylene oxide) polymers as depleting agents. At specific polymer concen-trations, sphere chains wind around each other to form helical structures–of the type that previously have only been observed in simulations on small sets of unconfined dipolar spheres with additional isotropic interactions