Specific ion effects on the electrokinetic properties of iron oxide nanoparticles: Experiments and simulations

We report experimental and simulation studies on ion specificity in aqueous colloidal suspensions of positively charged, bare magnetite nanoparticles. Magnetite has the largest saturation magnetization among iron oxides and relatively low toxicity, which explain why it has been used in multiple biomedical applications. Bare magnetite is hydrophilic and the sign of the surface charge can be changed by adjusting the pH, its isoelectric point being in the vicinity of pH = 7. Electrophoretic mobility of our nanoparticles in the presence of increasing concentrations of different anions showed that anions regarded as kosmotropic are more efficient in decreasing, and even reversing, the mobility of the particles. If the anions were ordered according to the extent to which they reduced the particle mobility, a classical Hofmeister series was obtained with the exception of thiocyanate, whose position was altered. Monte Carlo simulations were used to predict the diffuse potential of magnetite in the presence of the same anions. The simulations took into account the ion volume, and the electrostatic and dispersion forces among the ions and between the ions and the solid surface. Even though no fitting parameters were introduced and all input data were estimated using Lifshitz theory of van der Waals forces or obtained from the literature, the predicted diffusion potentials of different anions followed the same order as the mobility curves. The results suggest that ionic polarizabilities and ion sizes are to a great extent responsible for the specific ion effects on the electrokinetic potential of iron oxide particles.The authors thank the financial support from the following institutions: (i) ‘Ministerio de Economía y Competitividad, Plan Nacional de Investigación, Desarrollo e Innovación Tecnológica (I + D + i)’, Projects MAT2013-44429-R, MAT2012-36270-C04-04 and -02. (ii) ‘Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía’, Projects P09-FQM-4698, P10-FQM-5977, and P11-FQM-7074. (iii) European Regional Development Fund (ERDF)

Untersuchung der elektrophoretischen Mobilität blanker und funktionalisierter, sphärischer Kolloide mittels molekulardynamischer Simulationen

In this work, the electrophoretic mobility of colloids in salt solutions are studied by means of coarse-grained Molecular Dynamics simulations. Two different types of colloids are considered; bare colloids and polyelectrolyte-grafted colloids. A novel model for simulation of large bare colloids in the presence of explicit ions is developed. Comparison of the results with experimental data helps gain a better understanding of the mechanisms responsible for the interesting phenomenon of mobility reversal. Furthermore, a hitherto unknown electrokinetic behavior of polyelectrolyte-grafted colloids is found from simulations including full hydrodynamic interactions. The validity of the existing theories is verified via comparison with simulation results.Diese Arbeit beschäftigt sich mit Computersimulationen der Elektrophorese kolloidaler Teilchen in Salzlösungen. Zwei unterschiedliche Typen von Kolloiden sind betrachtet; nackte Kolliode und Polyelektrolyt-beschichtete Kolloide. Ein neues Modell ist entwickelt, das Simulationen von großen nackten Kolloiden in Gegenwart von expliziten Ionen ermöglicht. Dabei helfen Vergleiche mit unabhängigen Experimenten, die Simulationen mancher Phänomene, wie z.B. Mobilitätmkehr, besser zu verstehen. Außerdem, ist ein neues, bisher unbekanntes, elekrophoretisches Verhalten von Polyelektrolyt-beschichteten Kolloiden, durch Computersimulationen entdeckt, die eine vollständige Beschreibung der hydrodynamischen Wechselwirkungen einschließen. Dabei werden die Simulationsergebnisse mit bestehenden Theorien verglichen, deren Gültigkeit und Grenzen dadurch überprüft werden

Influence of Two Ground-Based Skidding Systems on Soil Compaction Under Different Slope and Gradient Conditions

Forest soils are sensitive to compaction by forest machinery. Forest operations such as harvesting and skidding have a high potential for soil compaction. This study was carried out in the Hyrcanian hardwood forests of Iran to measure the changes in bulk density (ρb) in the top 10 cm of soil following machine and animal skidding. The density change or compaction was induced by (i) a rubber-tired skidder in three skid trails: flat skid trail (STF), skid trail with transversal slope (STTS), and skid trail with longitudinal gradient (STLG) and (ii) a mule in two animal trails: flat animal trail (ATF) and animal trail with transversal slope (ATTS). Soil cores were collected pre- and post-skidding at random locations along the upslope and downslope tracks of each skid trail to determine bulk density and moisture content of the soil. Average soil bulk density in the tracks of machine skid trails was significantly greater than the soil density outside the tracks, but the increase in bulk density was not significant on the animal trails. An increase in soil density was considered significant if p < 0.05. A highly significant increase in soil bulk density (p < 0.01) occurred with machine skidding after the first 12 skidding cycles on the STF before stabilizing. In comparison, on STLG and STTS the increase of bulk density was greatest after the first six skidding cycles. Additional cycles did not cause a significant increase in soil density. While the number of skidding cycles to reach the steady compaction state varied between flat trails and those with a slope, the severity of compaction was the same between them. Concentrating machine operations will minimize the areal extent of compaction and the use of mules could further reduce the impact of the extraction of short logs

Computing the Electrophoretic Mobility of Large Spherical Colloids by Combining Explicit Ion Simulations with the Standard Electrokinetic Model

The electrophoretic mobility of large spherical colloids in different salt solutions of varying valency and concentration is studied via a combination approach of numerically solving the standard electrokinetic model with a ζ potential that has been obtained from explicit ion simulations of the restricted primitive model, thus going beyond the standard mean-field treatment. We compare our theoretical mobility curves to two distinct sets of experimental results and obtain good agreement for monovalent and divalent salt solutions. For the case of the trivalent La³⁺ salt, the experimentally obtained mobility reversal at high ionic strengths can be obtained only by adding an additional attractive interaction of 4<i>k</i>_B<i>T</i> to the potential between the colloid and La³⁺, hinting at the presence of a nonelectrostatic binding term for this ion. It is also shown that, contrary to intuition, charge inversion does not necessarily result in mobility reversal