The Journal of Physical Chemistry B最新文献

Understanding ion transport in polymer electrolytes isimportantfor elucidating the molecular origins of transport limitations andopportunities in nanostructured solid polymer electrolytes for lithiumbatteries. Here, we use all-atom molecular dynamics simulations toelucidate how cylindrical confinement regulates the structure anddynamics of poly(ethylene oxide) (PEO)-based electrolytes. Decreasingthe nanotube diameter drives pronounced polymer chain segregationand reorganizes the spatial distribution of coordinating oxygen sites.This confinement-induced structural rearrangement markedly suppressesion–segment codiffusion along short chain segments and inhibitsinterchain hopping, leading to a substantial decrease in Li⁺ mobility. However, upon decreasing nanotube diameter, Li⁺–EO coordination becomes stronger, which amplifies distinctcation–cation correlations; this competition between slowedion mobility and strengthened ion–ion correlations gives riseto a nonmonotonic dependence of ionic conductivity on nanotube diameter.To disentangle geometric confinement from specific polymer electrolyte–wallinteractions, we perform control simulations with hard walls, wherethe hard wall provides only steric exclusion; these results confirmthat the nonmonotonic trend cannot be explained by geometric confinementalone and instead requires the inclusion of specific polymer electrolyte–wallinteractions. Together, our results reveal the intricate couplingamong confinement geometry, interfacial interactions, polymer organization,and ion transport, providing an atomistic basis for understandingion transport in nanochannel-confined polymer electrolytes.

Understanding the molecular origin of the distinct transportbehaviorof Li⁺ and Na⁺ in polymer electrolytes remainsa central challenge for next-generation energy-storage materials.Here, cation transport in poly(ethylene oxide) (PEO) and poly(ethylenecarbonate) (PEC) electrolytes is investigated using atomistic moleculardynamics simulations combined with quantum mechanical calculationsof representative first-shell coordination motifs. Across 14 PEO-basedformulations, Na⁺ exhibits higher mobility than Li⁺, associated with a more diffuse and dynamically labile coordinationenvironment, whereas Li⁺ preferentially occupies compact,long-lived, high-coordination states. Four PEC-based systems assesshow matrix rigidity modulates the relationship between local coordinationenergetics and long-range transport. We introduce a population-weightedenergetic descriptor, ⟨E_int⟩,constructed from Boltzmann-averaged interaction energies and MD-derivedcoordination-state populations. The descriptor captures variationsin the cationic contribution to conductivity, σt⁺, within PEO, while PEC reveals the additional influence of polymerdynamics. Within the predominantly polymer-mediated regime examinedhere, anion participation can stabilize persistent coordination statesassociated with reduced cation mobility. A reduced representationbased on dominant coordination states retains the principal PEO transportrelationship, highlighting the importance of state-specific populationsand interaction energies rather than average coordination propertieswhile emphasizing limits to cross-family descriptor transferability.

The separation of lanthanide elements by solvent extractionistraditionally governed by differences in the thermodynamic stabilityof extracted complexes; however, ion-specific interfacial complexationcan play an important role in determining selectivity in kinetic separations.Nevertheless, the structural and chemical origins of such interfacialselectivity remain poorly understood. In this work, we combine sumfrequency generation (SFG) vibrational spectroscopy with X-ray scatteringto resolve interfacial structures of extractant–ion complexesat liquid surfaces. Measurements across pH and lanthanide concentrationregimes reveal how changes in aqueous conditions alter the molecularorganization of interfacial bis(2-ethylhexyl) phosphoric acid (HDEHP)–lanthanideion complexes. Across the lanthanide series, light lanthanide ions,e.g., those from La to Dy, tend to form relatively ordered monolayerswith HDEHP as characterized by strong CH and weak OH vibrational signalswith corresponding electron density profiles and surface ion concentrations.In contrast, heavy lanthanide ions, e.g., those from Er to Lu, tendto produce more disordered interfacial assemblies as reflected byweaker CH stretches, stronger OH signals that are accompanied by surprisinglylow lanthanide ion surface densities. These systematic, structural,and chemical differences suggest that HDEHP–heavy lanthanidecomplexes are more readily organized into increasingly centrosymmetricstructures that are poised for transport, explaining their apparentabsence at liquid–liquid interfaces at equilibrium. These resultsestablish interfacial organization as a key factor governing lanthanideextraction chemistry and point to kinetic handles to impart unconventionalselectivity.

Howdo solute concentration and molecular chemistry govern thestructural and mechanical response of lipid membranes? We addressthis using microsecond-scale molecular dynamics simulations of dipalmitoylphosphatidylcholine(DPPC) bilayers with chloroform (CHCl₃) and a homologousseries of alkanols (methanol, ethanol, octanol) over 0–50%concentrations. Increasing solute concentration enhances membraneoccupancy (f_mem) and membrane partitioning,with the relative membrane affinity following the order OcOH >CHCl₃ > EtOH > MeOH. The structural response dependsstrongly onsolute chemistry. Chloroform induces pronounced thinning and largefluctuations, consistent with deep, transient insertion. Methanolperturbs primarily the headgroup region, while ethanol shows intermediatebehavior with partial insertion. Octanol undergoes deep, lipid-likeincorporation and increases acyl-chain ordering while simultaneouslyenhancing thickness fluctuations and interleaflet interdigitation.Across all systems, increasing concentration reduces the area compressibilitymodulus and redistributes the lateral pressure profiles, indicatingconcentration-dependent membrane softening and structural reorganization.The unbiased equilibrium free-energy landscapes reveal solute- andconcentration-dependent changes in membrane-associated solute distributions.These profiles are used as qualitative descriptors of the sampledequilibrium free-energy landscape, while the partitioning fraction,partition coefficient, and free energy of partitioning provide theprimary quantitative measures of membrane affinity. These resultsdemonstrate that membrane response is governed by the interplay ofsolute partitioning, insertion depth, lipid organization, and mechanicalreorganization, rather than by chain ordering alone. The present simulationsidentify solute-induced structural reorganization and mechanical softeningas concentration-dependent responses of the DPPC bilayer, withoutdirect evidence for membrane melting, pore nucleation, or rupture.

The short-range to mid-range structure of inorganic glassesprofoundlyimpacts their physical properties. In this work, we have used multiplespectroscopic methods to examine the change in the network structureof amorphous silica with addition of Cl. Unlike analogous F-dopedsilicas, where the bonding of halogen leads to terminal Si–Fgroups, NMR and Raman data for Cl-doped v-SiO₂ indicatebridging between silica tetrahedra, preserving rather than reducingnetwork connectivity and contributing to their elastic stiffness andviscosity behaviors. These bridging Cl atoms make v-SiO₂ stiffer while having the same effect on density behavior per mol% dopant as F in v-SiO₂ and thus may be a desirable attributein tailoring v-SiO₂ for optical applications.

Onsager reciprocityis commonly regarded as a consequence of near-equilibriumlinear response. We show that, for chemically coupled systems satisfyingmicroscopic reversibility and Curie symmetry, the underlying processrate matrix remains exactly symmetric arbitrarily far from equilibrium.Considering two coupled reactions connected by a pair of symmetry-relatedtrajectory families, we derive an exact nonlinear flux relation inwhich thermodynamic driving enters only through nonlinear thermodynamicvariables, while the process rate matrix retains a symmetric structurefor arbitrarily large chemical potential differences. The trajectoryrate matrix is diagonal in the natural trajectory basis, and a Hadamardtransformation maps this exact trajectory description to the experimentallyobservable process basis, yielding a symmetric process rate matrixwith equal diagonal elements and reciprocal off-diagonal elements.In the linear-response limit, the full symmetric constitutive matrixcoincides with the differential response matrix, recovering the Onsagerreciprocal relations. Away from equilibrium the two are generallydistinct: the structural symmetry of the process representation persists,whereas the differential response matrix need not remain symmetric.We further derive exact ratcheting relations for the observable forwardand reverse rate constants, showing that thermodynamic affinitiesand intrinsic kinetic asymmetry play complementary roles in determiningdirectionality and chemical coupling. These results establish a generalsymmetry framework for chemically coupled systems that extends beyondconventional linear irreversible thermodynamics.

Ap3is a nonfluorescent second-harmonic-generation (SHG)-activedye used to stain cellular and artificial membranes. In this study,the interaction between Ap3 and a Langmuir monolayer of the phospholipid,1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC),at the air/aqueous interface was investigated by heterodyne-detectedvibrational sum frequency generation (HD-VSFG) spectroscopy. The second-ordersusceptibility (χ⁽²⁾) spectra at the air/aqueousAp3 solution interface in the presence of the DPPC monolayer stronglydepended on the visible probe wavelength, indicating that electronicresonances significantly contribute to the spectra. Two distinct VSFGbands at 1601 and 1638 cm^–1, as well as a largevibrationally nonresonant (VNR) contribution, were detected in theχ⁽²⁾ spectra, all of which are attributed to theAp3 molecules interacting with the DPPC monolayer at the interface.We analyzed the molecular orientation of Ap3 using both vibrationallyresonant and nonresonant SFG signals. The 1638 cm^–1 VSFG band indicated that the azobenzene moiety is tilted approximately55° from the interface normal, with the alkyl chains pointingtoward the air phase. Analysis of the VNR signal using a simplifiedmodel of the electronic response gave a tilt angle of approximately57°, in close agreement with the VSFG-band result.

In this work, we study the combined influence of pH-dependentsurfacecharge regulation and the viscoelectric effect on electroosmotic flow(EOF) in a charged microchannel. A semi-analytical model is developedto examine how the bulk pH, electrolyte concentration, and surfacedissociation characteristics affect the electric-double layer andtheir eventual impact on the underlying flow. Results of this endeavourshow that increasing the bulk pH initially enhances the electroosmoticflow by increasing the surface potential. However, at higher pH, thestronger electric field near the wall leads to a considerable increasein viscosity, which in turn, reduces the flow rate. The effect becomesmore pronounced at higher electrolyte concentrations, where the thinnerelectrical-double layer produces a stronger electric field near thewall. Besides, we show that the influence of the viscoelectric effectis weak under acidic conditions but becomes important as the solutionbecomes more basic. The average flow velocity deviates significantlyfrom the classical Helmholtz–Smoluchowski velocity at higherbulk pH, salt concentration, and viscoelectric coefficient. The sensitivityto bulk pH on electroosmotic flow, strongly depends on the salt concentration,and increases at higher pH as well. These findings demonstrate thatthe increase in electroosmotic flow expected from higher surface chargecan be partly or even substantially offset by viscoelectric effectmodulated viscosity enhancement. The inferences of this study providea clearer understanding of the coupled effects of pH-dependent surfacecharge and viscoelectricity on electroosmotic flow. Findings are seemedto be useful for the design of pH-responsive microfluidic and electrokineticsystems.

We present anotherstep in our efforts to expand far-field imagingbeyond the diffraction limit and beyond the limitations imposed byfluorescent labels. The model system we examine is based on the exploitationof entangled photon pairs at 810 nm generated by spontaneous parametricdown conversion (SPDC) of a 405 nm source to resolve the separationsof the line pairs of a series of two-dimensional objects. A ghost-imagingoptical system directs signal and idler photons along two paths, oneof which contains the objects. Images are constructed using a covarianceanalysis of the entangled pairs using typically 10⁶ framescollected with an EMCCD detector that was calibrated to account forthe number of entangled pairs of photons, which is necessary for anaccurate determination of the resolution and its comparison with theclassical method. Conservation of momentum requires that there isinversion symmetry between the transverse k vectors ofthe photons passing through the target and their entangled counterpartsdirected along the targetless path. Our covariance analysis not onlyaddresses this symmetry but also that the width of the covariancepeak is broader than a single pixel owing to the nature of SPDC andthe details of the optical arrangement. Consideration of this broadeningprovides significant improvement of the image quality. The Heisenberglimit (HL) is achieved within experimental error: using the same opticalalignment, the resolution obtained with entangled pairs of 810 nmphotons is, within experimental error, twice that obtained with classical810 nm photons. More importantly, the error bars of measurements with810 nm entangled pairs are quantified by the Cramér-Rao lowerbound (CRLB) and compared with those of the classical measurements.While other workers have done quantum imaging on nonpoint sources,to our knowledge we are the first group that not only has done thequantum imaging but also has comparedthe CRLB and the measured error of thequantum and classical measurements for a series of different-sizedobjects. (Similar comparisons have, however, been made for systemsthat can be approximated as point sources, e.g., determining the separationof two laser beams).