The Journal of Physical Chemistry B最新文献

The formation and transport of ionic charges in formic acid-water (HCOOH-H₂O) mixtures with initial water mole fractions ranging from X_H2Oⁱ = 0 to 1 were investigated using ¹³C and ¹H NMR, FTIR spectroscopy, viscosity, conductivity, and pH measurements. The maximum molar concentration of ions (H₃O⁺ and HCOO^-), along with the relative differences between theoretical and experimental densities, spin-lattice relaxation times (T₁), activation energies (E_a), viscosity (η), and conductivity (σ), were identified within the range of X_H2Oⁱ ≈ 0.5-0.7. These results indicate that pure formic acid (FA) solutions predominantly consist of cyclic dimers at room temperature. As the water mole fraction increases up to 0.6, a structural shift occurs from cyclic dimers to a mixture of linear and cyclic dimers, driven by the formation of strong hydrogen bonds. Beyond a water mole fraction of 0.6, the structure transitions to linear dimers, with FA molecules behaving as free entities in the water. Furthermore, the acidity was found to increase approximately 2-fold with every 0.1 increment in water mole fraction. These findings are critical for understanding the kinetics of formic acid anions in body fluids, the structure of the hydrogen bonding network, and ionization energies.

In this work, trivalent rare earth (RE)-rich borate glasses (30 La₂O₃-70 B₂O₃, 50 La₂O₃-50 B₂O₃, 60 La₂O₃-40 B₂O₃, and 50 Y₂O₃-50 B₂O₃) were modeled using ab initio molecular dynamics (AIMD) simulations through the melt-quenching route. It was found that the AIMD-derived structures reproduced the experimental structure factors and ¹¹B solid-state nuclear magnetic resonance data. Isolated borate units (monomers, dimers, and trimers) terminated with nonbridging oxygen were found in the structures. Polymer units containing four or more boron atoms were identified with and without three-membered boron rings (3-rings). Increasing the proportion of La₂O₃ in La₂O₃-B₂O₃ glasses resulted in an increased number of isolated units, indicating that La³⁺ acts as a network modifier, breaking the borate glass network. The formation of these units via the melt-quenching process was detected by labeling boron species at each AIMD step from 1500 to 300 K. Representation with transition matrices clarified the specific reaction routes, leading to the formation of isolated boron units in solid glass. A key finding is the stabilization of polymer units involving 3-ring formation. The formation of isolated units is achieved through the reaction of polymers without 3-rings. The RE coordination structure was thoroughly analyzed from the perspective of shape and symmetry. Reference structures derived from the solution of the Thomson problem were compared to the AIMD-derived coordination structures and crystalline LaBO₃ and YBO₃. The results highlight the specificity of the Y coordination structure with 3-rings in YBO₃, which is not observed in RE borate glasses. The analytical approaches and interpretations used in this study provide insights into the diverse coordination structures of glasses containing heavy elements other than REs.

Cholesteric liquid crystals (CLCs) are compelling responsive materials with applications in next-generation sensing, imaging, and display technologies. While electric fields and surface treatments have been used to manipulate the molecular organization and, subsequently, the optical properties of CLCs, their response to controlled fluid flow has remained largely unexplored. Here, we investigate the influence of microfluidic flow on the structure of thermotropic CLCs that can exhibit structural coloration. We demonstrate that the shear forces that arise from microfluidic flow align the helical axis of CLCs; alignment is a prerequisite for harnessing the promising photonic properties of CLCs. Moreover, we show that microfluidic flow can generate non-equilibrium structures exhibiting photonic band gaps that are inaccessible in the stationary cholesteric phase. Our findings have implications for the use of CLCs in applications involving flow processing such as additive manufacturing.

Graphene has been widely studied as an ideal material for the adsorption and separation. In this work, we used molecular dynamics simulations to investigate the evolution of diffusion and local structure of CH₄, CO₂, SO₂, and H₂O mixtures into double-layers graphene under seven different interlayer spacings and four different CO₂ concentrations. The results showed that the adsorption of CH₄ and CO₂ molecules on the graphene surface weakened with increased interlayer spacing. The diffusion capacities of CH₄ and CO₂ in the mixed system were significantly improved by increasing the interlayer spacing. In interlayer spacings ranging from 5 to 10 nm, the diffusion capacities of each component varied significantly in the order CH₄ > CO₂ ≫ H₂O > SO₂. Compared with CH₄ and CO₂, the local structures of SO₂ and H₂O were more affected by the interlayer spacing. Larger interlayer spacings or higher CO₂ concentrations were advantageous for the formation of stronger hydrogen bond structures between H₂O molecules. When the CO₂ concentrations were between 10% and 20% and the interlayer spacing of graphene was 8 nm, the graphene structure exhibited the best adsorption and separation effects on CH₄ and other components.

Thermoplastic starch (TPS) is an excellent film-forming material, and the addition of fillers, such as tetramethylammonium-montmorillonite (TMA-MMT) clay, has significantly expanded its use in packaging applications. We first used an all-atom (AA) simulation to predict several macroscopic (Young's modulus, glass transition temperature, density) and microscopic (conformation along 1-4 and 1-6 glycosidic linkages, composite morphology) properties of TPS melt and TPS-TMA-MMT composite. The interplay of polymer-surface (weakly repulsive), plasticizer-surface (attractive), and polymer-plasticizer (weakly attractive) interactions leads to conformational and dynamics properties distinct from those in systems with either attractive or repulsive polymer-surface interactions. A subset of AA properties was used to parametrize the MARTINI-2 coarse-grained (CG) force field (FF) for the melt and composite systems. The missing bonded parameters of amylose and amylopectin and the bead types for 1-4 and 1-6 linked α-D glucose were determined using two-body excess entropy, density, and bond and angle distributions in the AA TPS melt. This new MARTINI-2 CG model was also compared with the MARTINI-3 model for the TPS melt. However, the requirement of a polarizable water model necessitates the use of MARTINI-2 FF for the composite system. This liquid-liquid partitioning-based FF shows freezing and compaction of polymer chains near the clay surface, further accentuated by lowering of dispersive interactions between pairs of high-covalent-coordination ring units of TPS polymers and the montmorillonite sheet. A rescaling of the effective dispersive component of TPS-MMT cross interactions was used to optimize the MARTINI-2 FF for the composite system with structural (chain size distribution), thermodynamic (chain conformational entropy and density), and dynamic (self-diffusion coefficient) properties obtained from long AA simulations forming the constraints for optimization. The obtained CG FF parameters provided excellent estimates for several other properties of the melt and composite systems not used in parameter estimation, thus establishing the robustness of the developed model.

Due to the importance of the understanding of dissolution behavior of phosphate-based bioglasses (PBGs) in different biomedical applications, binary sodium and calcium phosphate glasses have been simulated for the first time using a newly developed ReaxFF force field and a standard melt-quench method with the LAMMPS classical molecular dynamics software. The partial radial distribution function of P-O within the first coordination shell indicated two distinct peaks corresponding to phosphorus bonding to NBO and BO, respectively, at distances consistent with those observed experimentally and a P-O coordination number of 4.0. Angular distribution functions were consistent with the experimental data. The calculated network connectivities are in good agreement with experimental data, and the detailed Q_n distributions are broader than would be expected.

Alchemical free energy (AFE) calculations can predict binding affinity changes as a function of structural modifications and have become powerful tools for lead optimization and drug discovery. Central to the setup and performance of AFE calculations is the manner of mapping alchemical transformations, known as the topology model. Single, dual, and hybrid topology models have been used with various AFE methods in the field. In recent works, λ-dynamics (λD) free energy calculations, specifically, have preferred the use of a hybrid multiple topology (HMT) for sampling multiple ligand perturbations. In this work, we evaluate a new topology method called ligand overlay (LO) for use with λD-based calculations, including the recently introduced λ-dynamics with a bias-updated Gibbs sampling (LaDyBUGS) approach. LO is a full multiple topology model that allows entire ligands to be sampled and restrained within a λ-dynamics framework. Relative binding free energies were computed with HMT or LO topology models with LaDyBUGS for 45 ligands across five protein benchmark systems. An overall Pearson R correlation of 0.98 and mean unsigned error of 0.32 kcal/mol were observed, suggesting that LO is a viable alternative topology model for λD-based calculations. We discuss the merits of using an HMT or LO model for future ligand studies with λD or LaDyBUGS calculations.

In this study, we investigate the quaternary ammonium-based ionic liquid (QAIL), methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, [N₁₈₈₈][TFSI], utilizing small angle neutron scattering (SANS) measurements and polarizable molecular dynamics (MD) simulations to characterize the short- and long-range liquid structure. Scattering structure factors show signatures of three length scales in reciprocal space indicative of alternating polarity (k ∼ 0.44 Å^–1), charge (k ∼ 0.75 Å^–1), and neighboring or adjacent (k ∼ 1.46 Å^–1) domains. Excellent agreement between simulation and experimental scattering structure factors validates various simulation analyses that provide detailed atomistic characterization of the different length scale correlations. The first solvation shell structure is illustrated by obtaining radial, angular, dihedral, and combined distribution functions, where two dominant spatial motifs, N⁺···N^– and N⁺···O^–, compete for optimal packing around the polar head of the [N₁₈₈₈]⁺ cation. Intermediate and long-range structures are governed by the balance between local electroneutrality and octyl chain networking, respectively. By computing the charge-correlation structure factor, S_ZZ, and the spatial extent of the octyl chain network using graph theory, the bulk-phase structure of [N₁₈₈₈][TFSI] is characterized in terms of electrostatic screening and apolar domain formation length scales.

We investigate the self-assembly and dynamics of double hydrophilic block copolymers (DHBCs) composed of densely grafted poly[oligo(ethylene glycol) methacrylate] (POEGMA) and poly(vinyl benzyl trimethylammonium chloride) (PVBTMAC) parent blocks by means of calorimetry, small- and wide-angle X-ray scattering (SAXS/WAXS), and dielectric spectroscopy. A weak segregation strength is evident from X-ray measurements, implying a disordered state and reflecting the inherent miscibility between the host homopolymers. The presence of intermixed POEGMA/PVBTMAC nanodomains results in homogeneous molecular dynamics, as evidenced through isothermal dielectric and temperature-modulated DSC measurements. The intermixed process undergoes a glass transition at a temperature approximately 40 K higher than the vitrification of bulk POEGMA segments, and it shifts to an even higher temperature by increasing the content of the hard block. At temperatures below the intermixed glass transition temperature, the confined POEGMA segments between the glassy intermixed regions contribute to a segmental process featuring (i) reduced glass transition temperature (T_g), (ii) reduced dielectric strength, (iii) broader distribution of relaxation times, and (iv) reduced fragility compared to the POEGMA homopolymer. We also observe two glass transition temperatures of dry PVBTMAC, which we attribute to the backbone and side chain segmental relaxation. To the best of our knowledge, this is the first time in the literature that these glass transitions of dry PVBTMAC have been reported. Finally, this study shows that excellent mixing of the two homopolymers is obtained, and this implies that different properties of this copolymer system can be tailored by adjusting the concentration of each homopolymer.

Complementary X-ray absorption fine structure (XAFS) and Raman spectroscopy studies were conducted on various UCl₃ concentrations in alkali chloride salt compositions. The samples were 5 mol % UCl₃ in LiCl (S1), 5 mol % UCl₃ in KCl (S2), 5 mol % UCl₃ in LiCl–KCl eutectic (S4), 50 mol % UCl₃ in KCl (S5), and 20 mol % UCl₃ in KCl (S6) molar concentrations. Samples were heated to 800 °C and allowed to cool to room temperature with measurements performed at selected temperatures; the highest temperatures showed the most stability and will be primarily referenced for conclusions. The processing and interpretation of the Raman and extended X-ray absorption fine structure (EXAFS) peaks revealed several uranium–oxygen bond lengths and symmetries in the samples before, during, and after heating. Based on published thermodynamic data of similar systems, X-ray absorption fine structure spectroscopy, and identification of Raman peaks, a β variation of α-U₃O₈, typical at room temperature, is the suspected dominant phase of all samples at high temperatures (800 °C). In the existing literature, this β structure of U₃O₈ was synthesized by slow cooling of uranium oxides from 1350 °C. This paper suggests the rapid formation of the compound due to the decomposition of the uranium chlorides or oxychlorides at increasing temperatures and O₂ reaction kinetics.