Hybrid membranes based on polybenzimidazole PBI-O-PhT (a polymer based on 3,3′,4,4′‑tetraaminodiphenyl oxide and 3,3‑bis(p‑carboxyphenyl)phthalide) were obtained by casting with silica particles with the surface modified by sulfonic groups. The morphology, thermal stability, proton conductivity under various conditions, and hydrogen permeability of the obtained membranes were investigated. It was shown that modification with small amounts of silica (2–5%) increases the conductivity and practically has no influence on the gas permeability of the hybrid membranes.
We reply to the comment on our paper by Budkov (2018 J. Phys.: Condens. Matter 30 344001).
A new method of solubility estimations, applied to sparingly dissolved compounds in supercritical carbon dioxide has been introduced in this paper. The method is based on determination of solubility contributions along the thermodynamic path consisting of sublimation and solvation processes. The contribution of the sublimation process is taken from the experiment, while the free energy of solvation is calculated from the classical density functional theory based on the fundamental measure theory. The parameterization of potential was performed using the Weeks-Chandler-Anderson procedure, where the Lennard-Jones parameters were obtained from the thermodynamics data of solute and solvent critical points. The introduced method can efficiently predict the pressure crossover on solubility curve of sparingly dissolved compounds in supercritical carbon dioxide.
We present a nonlocal statistical field theory of a diluted solution of dipolar particles which are capable of forming chain-like clusters in accordance with the ’head-to-tail’ mechanism. As in our previous study [Yu.A. Budkov 2018 J. Phys.: Condens. Matter 30 344001], we model dipolar particles as dimers comprised of oppositely charged point-like groups, separated by fluctuating distance. For the special case of the Yukawa-type distribution function of distance between the charged groups of dipolar particles we obtain an analytical expression for the electrostatic free energy of solution within the random phase approximation. We show that an increase in the association constant leads to a decrease in the absolute value of the electrostatic free energy of solution, preventing its phase separation which is in agreement with the former computer simulations and theoretical results. We obtain a non-linear integro-differential equation for the self-consistent field potential created by the fixed external charges in a solution medium, taking into account the association of dipolar particles. As a consequence of the derived self-consistent field equation, in regime of weak electrostatic interactions, we obtain an analytical expression for the electrostatic potential of the pointlike test ion, surrounded by the chain-like clusters of the dipolar particles. We show that in the mean-field approximation the association does not change the bulk dielectric permittivity of the solution, but increases the solvation radius of the point-like charge, relative to the theory of non-associating dipolar particles.
The microwave spectrum of the NH3–N2 van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 - 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH3–(ortho)-N2 nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH3–(para)-N2 spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole 14N nuclei of NH3–N2 was resolved for both transitions, and the quadrupole coupling constants associated with theNH3 andN2 subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C3 axis of NH3 and the N2 axis is about 66. The average van derWaals bond lengths are slightly different for (ortho)-NH3–(ortho)-N2 and (para)-NH3–(para)-N2 and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND3–N2, NHD2–N2, and NH2D–N2, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH3–N2 complex.
Recently, we have investigated the three-dimensional structures of CPT in complexes with the ground-state substrate analogs carrying hydrophobic and positively charged side chains: benzylsuccinic acid (BZS) and S-(2-guanidinoethylmercapto)succinic acid (GEMSA). That allowed us to identify Leu211 and Leu254 – earlier unknown determinants of substrate recognition by CPT as density of their interaction with substrates and localization of their side chains in the S1′-subsite were dependent on substrate’s structure. The role of Leu211 and Leu254 as structure determinants of hydrophobic selectivity of CPT was confirmed by site-directed mutagenesis. In the present work, we compare the structural organization of the CPT active site in complexes with transition state analogs of phenylalanine and arginine-containing substrates to evaluate structural basis of the enzyme’s catalytic activity to a broad range of substrates.
Carboxypeptidase B (EC 184.108.40.206) (CPB) is commonly used in the industrial insulin production and as a template for drug design. However, its ability to discriminate substrates with hydrophobic, hydrophilic, and charged side chains is not well understood. We report structure of CPB complex with a transition state analog N-sulfamoyl-L-phenylalanine solved at 1.74 angstrom. The study provided an insight into structural basis of CPB substrate specificity. Ligand binding is affected by structure-depended conformational changes of Asp255 in S1'-subsite, interactions with Asn144 and Arg145 in C-terminal binding subsite, and Glu270 in the catalytic center. Side chain of the non-specific substrate analog SPhe in comparison with that of specific substrate analog SArg (reported earlier) not only loses favorable electrostatic interactions and two hydrogen bonds with Asp255 and three fixed water molecules, but is forced to be in the unfavorable hydrophilic environment. Thus, Ser207, Gly253, Tyr248, and Asp255 residues play major role in the substrate recognition by S1'-subsite.
The carbon coated nanoflower-like Li4Ti5O12/C composites were prepared via hydrothermal method followed by surface modification using sucrose or polyvinylidene fluoride (PVDF) as carbon sources. X-ray diffraction, SEM, TEM, Raman spectroscopy, TGA, and the electrochemical measurements were used for the materials characterization. Such modification leads to the formation of a high-conductive carbon coating. In the case of polyvinylidene fluoride use, fluorination of Li4Ti5O12 surface takes place also. As a result, electrochemical performance of the obtained composites is improved. In the potential range of 1–3 V, Li4Ti5O12, Li4Ti5O12/CPVDF, and Li4Ti5O12/Csucrose exhibit, respectively, the discharge capacities of 142.5, 154.3, and 170.4 mAh/g at a current of 20 mA/g and 57.2, 82.1, and 89.3mAh/g at a current of 3200 mA/g. When cycled in a potential range of 0.01–3 V, the discharge capacity of Li4Ti5O12/CPVDF increases up to 252 mAh/g at 20 mA/g.
In the present study the properties of novel cation-exchange membranes based on UV-oxidized polymethylpentene (PMP) with grafted sulfonated polystyrene are described. A correlation between the composition of the grafted copolymer (grafting degree, cross-linking degree) and transport properties (Na+-conductivity, permselectivity, diffusion permeability) of resulted membranes are discussed. It is shown that with increasing of grafting degree (GD) and lowering of cross-linking degree (CD) the concentration of functional groups in the inner solution and permselectivity decrease, while ionic conductivity increases. The obtained membranes have the GD ranging from 29 to 120% and CD from 0 to 5%. The best membranes have ionic surface resistance of 0.3–0.6 Ω cm2 in 0.5 M NaCl, apparent cation transport numbers of 0.870–0.998 and NaCl diffusion permeability of 3.3 · 10−8–5.5 · 10−7 cm2 s−1, as well as satisfactory mechanical performance. A comparison of transport properties (conductivity and cation transport number) of the obtained membranes with a properties of number of available samples was made. It is noted that some of the obtained samples are at the level of the best perfluorinated homogeneous membranes in terms of the ratio of conductivity and cation transport numbers. High ionic conductivity and permselectivity make the prepared membranes promising candidates for possible applications in electrodialysis, dialysis, reverse electrodialysis, Red-Ox flow batteries and other membrane processes.