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1. Using a pseudophase model to determine AO distributions in emulsions: Why dynamic equilibrium matters

   https://doi.org/10.1002/ejlt.201600277  

   Bravo‐Díaz, Carlos ; Romsted, Laurence S. ; Losada‐Barreiro, Sonia ; Paiva‐Martins, Fátima ( September 2017 , European Journal of Lipid Science and Technology)

   For decades, explorations with ground state, thermal reactions combined with pseudophase kinetic models and methods for interpreting the results have provided insights into the properties of the different regions of homogeneous association colloids. More recent successful determination of antioxidant (AO) distributions by this approach is providing new insights into AO efficiency in opaque, well‐mixed two‐phase intact emulsions and eliminating the need to separate the phases. The chemical probe reacts with AOs exclusively in the interfacial region of the emulsion, permitting simplification of the kinetic treatment, and determining its distribution between the oil, interfacial, and aqueous regions. AO distributions are obtained from the two partition constants,and, of the AOs between the oil‐interfacial and aqueous‐interfacial regions, respectively.andvalues are obtained by fitting the observed rate constant,k_obs, versus surfactant concentration profiles with an overall kinetic approach or model we call the “pseudophase chemical kinetic method.” However, because emulsions break up and reform, and reactants and other components diffuse at various time scales within and between the oil, interfacial, and water regions,k_obscould also depend on reactant diffusion coefficients. Here we demonstrate that reactant diffusion is generally orders of magnitude faster than most thermal reactions and reactant distributions between the multiple oil, aqueous, and interfacial droplets and regions of emulsions are in dynamic equilibrium throughout the multiphase systems during the time course of the reaction. Thus, kinetic probes are powerful tools for determining structure‐reactivity, for example, the HLB, relationships governing AO distributions and efficiencies in emulsions.

   Practical applications:The analysis presented here demonstrates that one of the basic assumptions of the pseudophase chemical kinetic model that we have developed has a solid foundation in the properties of emulsions. That is, we can determine the distributions of reactants between oil (O), interfacial (I), and aqueous (W) regions of the emulsions because the diffusivity coefficients of reactants within emulsions are orders of magnitude greater than the rate of the reaction between the antioxidant and the 4‐hexadecylbenzenediazonium probe. Consequently, we can use the same kinetic model in emulsions as we have used in homogeneous microemulsions. The method permits determination of the partition constants of many antioxidants between the O‐I and W‐I regions of the emulsions and from them their distributions. The method provides new insights into the relationships between antioxidant hydrophobic‐lipophilic balance (HLB) and its efficiency in emulsions and a natural explanation for the cut‐off effect observed with increasing antioxidant HLB.

   Interpreting chemical reactivity in emulsions requires that reactive componentsAandBbe in dynamic equilibrium, that separate second order rate constants,kdefined for the oil, interfacial, and aqueous regions, subscripts O, I, and W, and for simplicity, the volume of each region depends on the added volumes of oil, surfactant, and water.

    

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2. The lift force on a charged sphere that translates and rotates in an electrolyte

   https://doi.org/10.1002/elps.201900029  

   Khair, Aditya S. ; Balu, Bhavya ( March 2019 , ELECTROPHORESIS)

   The distortion of the charge cloud around a uniformly charged, dielectric, rigid sphere that translates and rotates in an unbounded binary, symmetric electrolyte at zero Reynolds number is examined. The zeta potential of the particle ζ is assumed small relative to the thermal voltage scale. It is assumed that the equilibrium structure of the cloud is slightly distorted, which requires that the Péclet numbers characterizing distortion due to particle translation,, and rotation,, are small compared to unity. Here,ais radius of the particle;Dis the ionic diffusion coefficient;and, whereUandΩare the rectilinear and angular velocities of the particle, respectively. Perturbation expansions for smallandare employed to calculate the nonequilibrium structure of the cloud, whence the force and torque on the particle are determined. In particular, we predict that the sphere experiences a force orthogonal to its directions of translation and rotation. This “lift” force arises from the nonlinear distortion of the cloud under the combined actions of particle translation and rotation. The lift force is given by. Here, ε is the permittivity of the electrolyte;is the Debye length; andis a negative function that decreases in magnitude with increasing. The lift force implies that an unconstrained particle would follow a curved path; an electrokinetic analog of the inertial Magnus effect. Finally, the implication of the lift force on cross‐streamline migration of an electrophoretic particle in shear flow is discussed.

    

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3. On the use of radio‐frequency offsets for improving double‐quantum homonuclear dipolar recoupling of half‐integer‐spin quadrupolar nuclei

   https://doi.org/10.1002/mrc.5142  

   Duong, Nghia Tuan ; Lee, Daniel ; Mentink‐Vigier, Frédéric ; Lafon, Olivier ; De Paëpe, Gaël ( May 2021 , Magnetic Resonance in Chemistry)

   Detecting proximities between nuclei is crucial for atomic‐scale structure determination with nuclear magnetic resonance (NMR) spectroscopy. Different from spin‐1/2 nuclei, the methodology for quadrupolar nuclei is limited for solids due to the complex spin dynamics under simultaneous magic‐angle spinning (MAS) and radio‐frequency irradiation. Herein, the performances of several homonuclear rotary recoupling (HORROR)‐based homonuclear dipolar recoupling sequences are evaluated for²⁷Al (spin‐5/2). It is shown numerically and experimentally on mesoporous alumina thatoutperforms the supercycled S₃sequence and its pure double‐quantum (DQ) (bracketed) version, [S₃], both in terms of DQ transfer efficiency and bandwidth. This result is surprising since the S₃sequence is among the best low‐power recoupling schemes for spin‐1/2. The superiority ofis thoroughly explained, and the crucial role of radio‐frequency offsets during its spin dynamics is highlighted. The analytical approximation of, derived in an offset‐toggling frame, clarifies the interplay between offset and DQ efficiency, namely, the benefits of off‐resonance irradiation and the trough in DQ efficiency forwhen the irradiation is central between two resonances, both for spin‐1/2 and half‐integer‐spin quadrupolar nuclei. Additionally, density matrix propagations show that thesequence, applied to quadrupolar nuclei subject to quadrupolar interaction much larger than radio‐frequency frequency field, can create single‐ and multiple‐quantum coherences for near on‐resonance irradiation. This significantly perturbs the creation of DQ coherences between central transitions of neighboring quadrupolar nuclei. This effect explains the DQ efficiency trough for near on‐resonance irradiation, in the case of both cross‐correlation and autocorrelation peaks. Overall, this work aids experimental acquisition of homonuclear dipolar correlation spectra of half‐integer‐spin quadrupolar nuclei and provides theoretical insights towards improving recoupling schemes at high magnetic field and fast MAS.

    

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4. Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

   https://doi.org/10.1029/2021JA029475  

   Workayehu, A. B. ; Vanhamäki, H. ; Aikio, A. T. ; Shepherd, S. G. ( October 2021 , Journal of Geophysical Research: Space Physics)

   We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF Bin NH (Bin SH) in most local seasons under both signs of IMF B. For Bin NH (Bin SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During Band Bin NH (Band Bin SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.180.09 in winter and 1.170.09 in autumn. During Band Bin NH (Band Bin SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.160.07 for FAC. We also find an explicit Beffect on auroral currents in a given hemisphere: on average Bin NH and Bin SH causes larger currents than vice versa. The explicit Beffect on divergence‐free current during IMF Bis in very good agreement with the Beffect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.

    

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5. On the variation of bi‐periodic waves in the transverse direction

   https://doi.org/10.1111/sapm.12446  

   Henderson, Diane M. ; Carter, John D. ; Catalano, Megan E. ( September 2021 , Studies in Applied Mathematics)

   Weakly nonlinear, bi‐periodic patterns of waves that propagate in the‐direction with amplitude variation in the‐direction are generated in a laboratory. The amplitude variation in the‐direction is studied within the framework of the vector (vNLSE) and scalar (sNLSE) nonlinear Schrödinger equations using the uniform‐amplitude, Stokes‐like solution of the vNLSE and the Jacobi elliptic sine function solution of the sNLSE. The wavetrains are generated using the Stokes‐like solution of vNLSE; however, a comparison of both predictions shows that while they both do a reasonably good job of predicting the observed amplitude variation in, the comparison with the elliptic function solution of the sNLSE has significantly less error when the ratio of‐wavenumber to the two‐dimensional wavenumber is less than about 0.25. For ratios between about 0.25 and 0.30 (the limit of the experiments), the two models have comparable errors. When the ratio is less than about 0.17, agreement with the vNLSE solution requires a third‐harmonic term in the‐direction, obtained from a Stokes‐type expansion of interacting, symmetric wavetrains. There is no evidence of instability growth in the‐direction, consistent with the work of Segur and colleagues, who showed that dissipation stabilizes the modulational instability. Finally, there is some extra amplitude variation in, which is examined via a qualitative stability calculation that allows symmetry breaking in that direction.

    

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