Mixed Solvent Conditions Research Articles

2D sheet to 1D rod-like morphology regulated self-assembled ZnCo2O4 microstructures under mixed solvent conditions for battery-type supercapacitors

This study examines the effects of the mixed solvent's characteristics on the nucleation and crystal growth processes and the surface morphologies of zinc cobaltite (ZnCo2O4; ZCO) microstructures. We discovered that the number of interaction sites in ZCO microstructures significantly increases when the solvent changes. This influences the material's high crystallinity, phase purity, morphology, and significantly influences its electrochemical conductivity. One with deionized (DI) water and ethanol mixed solvent and the other with DI water and ethylene glycol (EG) to produce mesoporous ZCO with rod-like and sheet-like microstructures, respectively. The ZCO-R delivered a high specific capacity of 468.5 C g−1 (130.13 mAh g−1) at a current density of 1 A g−1 compared to the ZCO-S (351 C g−1 / 97.5 mAh g−1). Additionally, the ZCO-R microstructures showed good rate performance (82% of its initial capacity) and cyclic stability (90% of capacity retention after 5000 cycles) due to their higher surface area and mesopore features. Hence, utilizing mixed solvent conditions to design morphology-controlled electrode materials is a promising approach to enhancing the electrochemical performance of battery-type supercapacitors.

Droplet-based continuous flow synthesis of biologically active Bis(indolyl)methanes and Tris(indolyl)methanes

Bis(indolyl)methanes and Tris(indolyl)methanes are known as an important class of heterocyclic compounds because they could be employed as anticancer drugs, blood sugar stabilizers, and anti-viral agents. Herein, they were prepared using single-phase and droplet-based continuous flow methods. Reactions were optimized under mixed solvent conditions of H2O and dimethylformamide, and process stability and yields were examined. Excellent process stability and yields were achieved in considerably shortened reaction times when employing droplet-based continuous flow synthesis.

DFT Study To Explore the Importance of Ring Size and Effect of Solvents on the Keto-Enol Tautomerization Process of α- and β-Cyclodiones.

We have explored the effect of ring size on keto–enol tautomerization of α- and β-cyclodiones using the M062X-SMDaq/6-31+G(d,p)//M062X/6-31+G(d,p) level of theory. The calculated results show that the activation free energy barrier for the keto–enol tautomerization process of α-cyclopropanedione (1) is 54.9 kcal/mol, which is lower compared to that of the other cyclic diketo systems studied here. The four-membered α- and β-cyclobutanedione (2 and 6) do not favor keto–enol tautomerization unlike other studied cyclic systems because of the ring strain developed in the transition-state geometries and their corresponding products. Water-assisted keto–enol tautomerization with one molecule reveals that the free energy activation barriers reduce almost half compared to those for the uncatalyzed systems. The two-water-assisted process is favorable as the activation free energy barriers lowered by ∼10 kcal/mol compared to those of the one-water-assisted process. The ion-pair formation seems to govern the lowering of activation barriers of α- and β-cyclodiones with two water molecules during the keto–enol tautomerization process, which however also overcomes the favorable aromatization in the three-membered ring system. The free energy activation barriers calculated with the M062X-SMDaq/6-31+G(d,p) level predicted that the keto–enol tautomerization process for the α-cyclodiones follows the following trend: 2 > 3 > 4 > 5 > 1. Water-assisted tautomerization of α-cyclodiones also predicted 1-W and 1-2W as the most favored processes; however, 5-W and 5-2W were found to be disfavored in this case. The β-cyclodione systems also showed similar trends as obtained with α-diketone systems. The influence of bulk solvent on the keto–enol tautomerization process favors the formation of the enol form in a more polar solvent medium even under mixed solvent conditions in acetonitrile and hexane at M062X-SMDacetonitrile/6-31+G(d,p) and M062X-SMDhexane/6-31+G(d,p) levels of theory.

Modulating Enzyme Activity by Altering Protein Dynamics with Solvent.

Optimal enzyme activity depends on a number of factors, including structure and dynamics. The role of enzyme structure is well recognized; however, the linkage between protein dynamics and enzyme activity has given rise to a contentious debate. We have developed an approach that uses an aqueous mixture of organic solvent to control the functionally relevant enzyme dynamics (without changing the structure), which in turn modulates the enzyme activity. Using this approach, we predicted that the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) from Escherichia coli in aqueous mixtures of isopropanol (IPA) with water will decrease by ∼3 fold at 20% (v/v) IPA concentration. Stopped-flow kinetic measurements find that the pH-independent khydride rate decreases by 2.2 fold. X-ray crystallographic enzyme structures show no noticeable differences, while computational studies indicate that the transition state and electrostatic effects were identical for water and mixed solvent conditions; quasi-elastic neutron scattering studies show that the dynamical enzyme motions are suppressed. Our approach provides a unique avenue to modulating enzyme activity through changes in enzyme dynamics. Further it provides vital insights that show the altered motions of DHFR cause significant changes in the enzyme's ability to access its functionally relevant conformational substates, explaining the decreased khydride rate. This approach has important implications for obtaining fundamental insights into the role of rate-limiting dynamics in catalysis and as well as for enzyme engineering.

Controllable one-pot synthesis of uniform colloidal TiO2 particles in a mixed solvent solution for photocatalysis.

This study reports on the controllable synthesis of uniform colloidal titanium dioxide (TiO2) particles and their photocatalytic applications toward rhodamine B (RhB) degradation. The monodispersed TiO2 particles were synthesized under mixed solvent conditions by sol–gel chemistry in a one-pot process. Varying the ratio of solvent composition, the concentration of surfactant and TiO2 precursor was used to control the particle diameter, degree of monodispersity and morphology. The modification of the calcination temperature affected the crystallinity and crystalline phase of the colloidal TiO2 particles. When uniform, amorphous TiO2 particles were calcined at an optimal temperature (500 °C), the final sample exhibited beneficial characteristics such as high anatase crystallinity with a mixed phase of anatase and rutile and relatively high surface area. The photocatalytic efficiency of the uniform TiO2 sample with high anatase crystallinity with mixed phase and high surface area was dramatically enhanced towards RhB degradation under UV–vis irradiation. We systemically discuss the relationship between the synthetic parameters in our synthesis and the properties of the final TiO2 products, as well as the crystalline properties and performance enhancement of TiO2 photocatalysts calcined at different temperatures.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts.

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO2 drive our efforts to discover ways of converting excess CO2 into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO2 requires a fundamental understanding of how to generate metal hydrides that selectively donate H- to CO2, rather than recombining with H+ to liberate H2. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO2. Of particular interest are the nitride- and carbide-containing clusters: [Fe4N(CO)12]- and its derivatives and [Fe4C(CO)12]2-. In both aqueous and mixed solvent conditions, [Fe4N(CO)12]- forms a reduced hydride intermediate, [H-Fe4N(CO)12]-, through stepwise electron and proton transfers. This hydride selectively reacts with CO2 and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe4N(CO)12]- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO2; only H2 is observed in this case. In contrast, isoelectronic [Fe4C(CO)12]2- features a concerted proton-electron transfer mechanism to form [H-Fe4C(CO)12]2-, which is selective for H2 production even in the presence of CO2, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe4N(CO)12]- and its derivative [H-Fe4N(CO)11(PPh3)]- have hydricities modest enough to avoid H2 production but strong enough to make formate. [H-Fe4C(CO)12]2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H+, only H2 is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol-1 where C-H bond formation may be favored in water or MeCN, respectively.

Spectroscopic Analysis of Binary Mixed-Solvent-Polyimide Precursor Systems with the Preferential Solvation Model for Determining Solute-Centric Kamlet-Taft Solvatochromic Parameters.

Hydrogen bond donor/acceptor mixed-solvent systems for solutes that exhibit strong specific interactions are not readily characterized with methods that depend on solvatochromic parameters. In this work, the reaction of two monomers, 4,4′-oxidianiline (ODA) and pyromellitic dianhydride (PMDA), to form the common engineering plastic precursor, poly(amic acid) (PAA), are studied for the tetrahydrofuran (THF) mixed-solvent systems (THF-methanol, THF-ethanol, THF-water) with spectroscopy. Solute-centric (SC) Kamlet–Taft solvatochromic (K-T) parameters for the solvent environment around the monomer are determined using a proposed model that incorporates spectroscopically determined local composition (X(L)) around the ODA monomer and the preferential solvation model. For the example reaction to occur under homogeneous conditions, mixed-solvent conditions need have HBA-rich local compositions (0.30 < X(HBA)(L) < 0.83), high solute-centric basicity (β(SC) > 0.60), high solute-centric polarity, (π(SC)* > 0.63), and low solute-centric acidity (α(SC) < 0.63). The method developed allows characterization of mixed-solvent effects and can be readily extended to other systems that have strong specific interactions.

Optimization of the labeling yield by determination of the Cu+–acetonitrile complex constant in Cu+-catalyzed nucleophilic exchange reactions in mixed solvent conditions

To apply the Cu+-assisted nucleophilic exchange based radioiodination of aromatic compounds for more lipophilic compounds the reaction is carried out in mixed solvent conditions. Due to its physicochemical properties acetonitrile is an attractive solvent. Although acetonitrile forms complexes with Cu+ decreasing the labeling yield. This article describes a method for the determination of the complex constant at labeling temperature based on a Lineweaver–Burk approach, relating the reaction rate constant and the concentration of precursor in presence of different amounts of acetonitrile. The method also allows to calculate the adjusted amount of copper salt in order to obtain the same high labeling yield as obtained in absence of acetonitrile.

Understanding the alpha-helix to coil transition in polypeptides using network rigidity: predicting heat and cold denaturation in mixed solvent conditions.

Thermodynamic stability in polypeptides is described using a novel Distance Constraint Model (DCM). Here, microscopic interactions are represented as constraints. A topological arrangement of constraints define a mechanical framework. Each constraint in the framework is associated with an enthalpic and entropic contribution. All accessible topological arrangements of distance constraints form an ensemble of mechanical frameworks, each representing a microstate of the polypeptide. A partition function is calculated exactly using a transfer matrix approach, where in many respects the DCM is similar to the Lifson-Roig model. The crucial difference is that the effect of network rigidity is explicitly calculated for each mechanical framework in the ensemble. Network rigidity is a mechanical interaction that provides a mechanism for long-range molecular cooperativity and enables a proper treatment of the nonadditivity of a microscopic free energy decomposition. Accounting for (1) helix <--> coil conformation changes along the backbone similar to the Lifson-Roig model, (2) i to i + 4 hydrogen-bond formation <--> breaking similar to the Zimm-Bragg model, and (3) structured <--> unstructured solvent interaction (hydration effects), a six-parameter DCM describes normal and inverted helix-coil transitions in polypeptides. Under suitable mixed solvent conditions heat and cold denaturation is predicted. Model parameters are fitted to experimental data showing different degrees of cold denaturation in monomeric polypeptides in aqueous hexafluoroisopropanol (HFIP) solution at various HFIP concentrations. By assuming a linear HFIP concentration dependence (up to 6% by mole fraction) on model parameters, all essential experimentally observed features are captured.

Optical spectroscopic investigations of model beta-sheet hairpins in aqueous solution.

In this contribution we report optical spectroscopic data on a series of designed beta hairpins previously shown by NMR to contain a substantial population of beta-sheet structure. These models contain a designed hydrophobic cluster and a (D)Pro-Gly sequence to promote formation of a turn geometry. FTIR, electronic and vibrational CD (ECD and VCD) spectra for these small peptides are comparable to expected bandshapes for peptides of high beta-sheet content. The (D)Pro-Gly sequence provides a better turn motif than Asn-Gly as measured by its beta-sheet spectral characteristics. IR and VCD spectra are in qualitative agreement with theoretical simulations based on transfer of parameters from ab initio quantum mechanical force field and intensity computations for the turn and strands. These calculations provide assignments for some distinguishing modes in both IR and VCD spectra. Increased sheet structure can be induced in these hairpins by use of mixed solvent conditions. Thermal denaturation studies reveal that these hairpins undergo very broad unfolding transitions. Guanidine hydrochloride unfolding transitions for the selected hairpin models are similarly broad. However, the "end-states" of temperature and chaotropic denaturation are spectroscopically differentiable.

Reactivity of creatinine with alkaline 3,5-dinitrobenzoate: A new fluorescent kidney function test

Objectives: To develop a new quantitative fluorometric assay for creatinine with enhanced specificity and inherent fluorescence sensitivity. Design and Methods: Creatinine is reacted with 3,5-dinitrobenzoate under alkaline reaction conditions to produce a fluorophore. Alternatively, 3,5-dinitrobenzoyl chloride or methyl-3,5-dinitrobenzoate may be employed as reagents. Fluorophore development was evaluated for Group IA bases under aqueous and mixed solvent conditions. Results: The chemical reaction between creatinine and 3,5-dinitrobenzoate performed under highly alkaline conditions produced a fluorescent product with excitation and emission maxima near 410 and 475 nm, respectively. The reaction proceeds rapidly in the presence of group IA bases under both aqueous and mixed solvent conditions, in particular with water/dimethylsulfoxide and water/ 1,4-butanediol solvent systems containing 1.0 M LiOH. A linear calibration curve was obtained when the fluorescence intensity measured in microamperes was plotted versus creatinine concentration in the range of 5–50 μmol/L. The detection limit for creatinine was well below 1 μmol/L. Conclusions: A simple, sensitive and highly specific fluorometric method for the determination creatinine has been developed.