The synthesis of single-crystalline hollandite-type manganese oxides Na 2-x Mn 8 O 16 nanorods by... more The synthesis of single-crystalline hollandite-type manganese oxides Na 2-x Mn 8 O 16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.
X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified in... more X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific potential of mean force are in good agreement with the data. arXiv:1006.4616v1 [cond-mat.soft]
Ion distributions at interfaces underlie diverse phenomena that are driven by electrostatic inter... more Ion distributions at interfaces underlie diverse phenomena that are driven by electrostatic interactions. We present x-ray reflectivity studies of the interface between the electrolyte solutions of water and 1,2-dichloroethane as a function of interfacial electric potential. The data demonstrates the formation of a dense ionic layer at the interface for high potentials (>280 mV). Predictions of the ion distributions by Gouy-Chapman theory, which treats the ions as point charges by only including electrostatic interactions, are inconsistent with these data at the higher applied potentials. We introduce a Poisson-Boltzmann equation that incorporates an ion-specific potential of mean force, obtained from Molecular Dynamics simulations and by fitting to the data. The potential of mean force (PMF) accounts for various ion and solvent interactions. Predictions from this variant of the Poisson-Boltzmann equation are in good agreement with our x-ray data.
The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsi... more The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vapor-like depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose super-hydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the sub-angstrom proximity of water to soft hydrophobic materials.
Ion distributions play a central role in various settings-from biology, where they mediate the el... more Ion distributions play a central role in various settings-from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength Γ of ion-ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameterfree density functional theory that includes ion-ion correlations and ion-solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.
The synthesis of single-crystalline hollandite-type manganese oxides Na 2-x Mn 8 O 16 nanorods by... more The synthesis of single-crystalline hollandite-type manganese oxides Na 2-x Mn 8 O 16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.
X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified in... more X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific potential of mean force are in good agreement with the data. arXiv:1006.4616v1 [cond-mat.soft]
Ion distributions at interfaces underlie diverse phenomena that are driven by electrostatic inter... more Ion distributions at interfaces underlie diverse phenomena that are driven by electrostatic interactions. We present x-ray reflectivity studies of the interface between the electrolyte solutions of water and 1,2-dichloroethane as a function of interfacial electric potential. The data demonstrates the formation of a dense ionic layer at the interface for high potentials (>280 mV). Predictions of the ion distributions by Gouy-Chapman theory, which treats the ions as point charges by only including electrostatic interactions, are inconsistent with these data at the higher applied potentials. We introduce a Poisson-Boltzmann equation that incorporates an ion-specific potential of mean force, obtained from Molecular Dynamics simulations and by fitting to the data. The potential of mean force (PMF) accounts for various ion and solvent interactions. Predictions from this variant of the Poisson-Boltzmann equation are in good agreement with our x-ray data.
The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsi... more The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vapor-like depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose super-hydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the sub-angstrom proximity of water to soft hydrophobic materials.
Ion distributions play a central role in various settings-from biology, where they mediate the el... more Ion distributions play a central role in various settings-from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength Γ of ion-ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameterfree density functional theory that includes ion-ion correlations and ion-solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.
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Papers by Binyang Hou