Exothermic Complex Research Articles

Substantial solubility and fluorescence property changes upon inclusion of rutaecarpine in cucurbit[7]uril: Kinetics and thermodynamics of binding

The effect of acid and cucurbit[7]uril (CB7) additions on the solubility and fluorescent properties of rutaecarpine (Rut), a pharmaceutically important plant alkaloid, was studied in aqueous solution. Approximately 550-fold water-solubility enhancement was achieved upon inclusion complex formation with CB7. The cationic protonated alkaloid (RutH+) was encapsulated in CB7 with a binding constant more than three orders of magnitude larger than that of the uncharged Rut. The stabilization of RutH+ by the host macrocycle led to a pKa increase of ∼3.3 units compared with that found in water. The substantial increase in fluorescence quantum yield upon RutH+–CB7 production could be exploited for the sensitive visualization of pH differences in the range of 4.5–6.5. The highly exothermic complexation was accompanied by considerable entropy diminution, partly because of the critical role of hydrogen bonding between the constituents. The rate constants of the creation and dissociation of RutH+–CB7 were 2.5 × 106 M−1 s−1 and 0.94 s−1 at 283 K, respectively.

Ionic Liquid Assisted Exothermic Complexation of Trivalent Lanthanides with Fluorinated β Diketone: Multitechnique Approach with Theoretical Insight.

The complexation of the betadiketone,1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (HFOD) was studied with trivalent lanthanide ions, viz. Nd3+, La3+, and Eu3+ in several methylimidazolium-based ionic liquids (Cnmim•NTf2, where, n = 4,6,8). In C6mim•NTf2, predominant formation of ML2+ and ML4- species was evidenced from the UV-vis absorption (Nd3+) as well as luminescence (Eu3+) spectral studies with log β2 ≈ 5.88 ± 0.04, log β4 ≈ 10.95 ± 0.06. The formation constants followed the trend C4mim•NTf2 > C6mim•NTf2 > C8mim•NTf2. The asymmetry factors for the ML2+ and ML4- species were found to be 1.2 and 1.59, respectively. The ML4- complex was found to have one primary coordination sphere water molecule with enhanced covalency between Eu3+ and O from HFOD (Judd Offelt constants Ω2 and Ω4 ≈ 17.2 and 2.35) compared to Eu3+aq, yet comparable to other β diketones. Complexation-induced temperature increase was confirmed by calorimetric measurements, indicating the exothermic complexation reaction (ΔHcomplexation ≈ -13.7 kJ mol-1), which is also spontaneous in nature (ΔG ≈ -68.1 kJ mol-1), with an enhancement in the entropy values. Due to complexation, the shifts in the peak positions (1686.66 cm-1, 1633.53 cm-1) associated with β diketone/ketone functional groups were evidenced. Density functional theory (DFT) calculation was performed to optimize the structural parameters including bond distance, bond angles, and energetics associated with the complexation.

Enhanced bioaccesibility of selective PDE4 inhibitor N-(3,4-dihydro-2h-1,5-benzodioxepin-7-yl) pyridine-4-carboxamide using polyamidoamine dendrimer

The aim of this study was to improve the aqueous solubility of highly hydrophobic but selective PDE4 inhibitor N-(3,4-dihydro-2h-1,5-benzodioxepin-7-yl) pyridine-4-carboxamide by associating it with polyamidoamine dendrimer. The PAMAM dendrimer restraining ethylenediamine core synthesized by a divergent approach was utilized for encapsulation. The solubility of conjugates was evaluated on the basis of concentration and generation of the dendrimer, pH of the solution, and temperature. The phase solubility diagram confirmed an increase in aqueous solubility of drug with increase in dendrimer concentration with respect to pH in the order 9.0 > 7.0 > 4.0. Moreover, values of thermodynamic parameters such as negative value of ΔHo, positive value of ΔSo and ΔGo reflects an exothermic complexation, presence of number of particles and spontaneity of the complexation. Overall, investigations validate the enhancement in solubility of drug after complexation with polyamidoamine dendrimer which further confirmed promising bioactivity of the drug-dendrimer conjugates.

Removal of ammonium nitrogen (NH4+-N) by Cu-loaded amino-functionalized adsorbents

Cu-loaded amino-functionalized SiO2 and weak-base-anion-exchange resin were prepared as advanced adsorbents for NH4+ removal and their adsorption ability and mechanisms were insightfully studied. Ammonium adsorption went through endothermic deprotonation followed by exothermic complexation (ΔH = −68.03 kJ/mol) to the Cu2+ ions. Such chemisorption followed a second-order kinetics and the Langmuir isotherm. The adsorption quickly reached an equilibrium within 1.0–60 min with an extraordinarily high rate constant of 0.01–2.86 g/mg-min. Besides, a high adsorption capacity of 9.30–21.37 mg/g was obtained. The adsorption ability effectively increased with increasing initial pH values, and the final pH was maintained at 5.3–5.8 due to the buffering effect in the –NH/CuCl2/NH3 system. Coexistence of 1.0–5.0 mM chloride-based salts (NaCl, KCl, CaCl2, MgCl2) promoted the NH4+ adsorption by 1.5–1.9 times. However, over amounts of salts inhibited the adsorption. The used adsorbent was successfully recovered by soaking with 0.15 M NaCl solution and 94.37–99.97% adsorption ability was preserved within four recovery cycles.

A Demonstration of Le Chatelier's Principle on the Nanoscale.

Photothermal desorption of molecules from plasmonic nanoparticles is an example of a light-triggered molecular release due to heating of the system. However, this phenomenon ought to work only if the molecule–nanoparticle interaction is exothermic in nature. In this study, we compare protein adsorption behavior onto gold nanoparticles for both endothermic and exothermic complexation reactions, and demonstrate that Le Chatelier’s principle can be applied to predict protein adsorption or desorption on nanomaterial surfaces. Polyelectrolyte-wrapped gold nanorods were used as adsorption platforms for two different proteins, which we were able to adsorb/desorb from the nanorod surface depending on the thermodynamics of their interactions. Furthermore, we show that the behaviors hold up under more complex biological environments such as fetal bovine serum.

Physicochemical, thermodynamic and analytical studies on binary and ternary inclusion complexes of bosentan with hydroxypropyl-β-cyclodextrin

The interactions of poorly aqueous soluble endothelin receptor antagonist bosentan (BOS) with hydroxypropyl-β-cyclodextrin (HPβCD) were assessed in presence and absence of an amino acid l-arginine (ARG), to improve its physicochemical properties. Initially, the phase solubility studies conducted in distilled water followed by thermodynamic investigations demonstrated an AL type of solubility profile and an enthalpy driven exothermic complexation process, respectively, in all cases. The analytical evidences for the formation of lyophilized binary and ternary complexes in solid state were generated and confirmed by differential scanning calorimetry (DSC), Fourier transformation infrared spectroscopy (FTIR), X-ray powder diffractometry (XPRD) and scanning electron microscopy (SEM). The solubility and dissolution of binary and ternary complexes were significantly improved upon complexation as compared to BOS alone, supported by decreased logP values of the complexes. However, the complexation efficiency of ternary system was found to be higher than binary, justifying the addition of ARG as an auxiliary substance to reduce the workable amount of HPβCD during formulation.

Aqueous solubility of calcium citrate and interconversion between the tetrahydrate and the hexahydrate as a balance between endothermic dissolution and exothermic complex formation

Aqueous solubility of calcium citrate tetrahydrate was found to decrease with increasing temperature, while solubility of hexahydrate increased with a transition temperature at 51.6 °C. Excess citrate increased calcium citrate solubility but decreased the calcium ion activity of the saturated solution with an initial solubility overshooting to form supersaturated solutions indicating binding of calcium to citrate with an association constant of 3.6 ± 0.1 × 104, ΔHº = −5.07 ± 0.04 kJ mol−1, ΔSº = 70.3 ± 0.3 J mol−1 K−1 at 25 °C. Dissolution of the tetrahydrate and hexahydrate was found to have ΔHº = 27 ± 9 kJ mol−1, ΔSº = −218 ± 30 J mol−1 K−1 and ΔHº = 57 ± 7 kJ mol−1, ΔSº = −126 ± 24 J mol−1 K−1, respectively, as determined from the temperature dependence of solubility corrected for complex formation. The exothermic complex formation results in inverse solubility only for the tetrahydrate with its moderate endothermic dissolution, which also precipitates at ambient temperature rather than the less soluble hexahydrate.

Reactivity of concentrated ammonium nitrate solutions in the presence of organic impurities

Assessment of mixtures reactivity under certain conditions is essential for safe and conscious designing and improving manufacturing processes. One commonly used compound, the reactivity of which is very important, is ammonium nitrate and its solutions. The biggest influence on the reactivity of ammonium nitrate has the temperature and presence of other compounds, including impurities, that catalyze reactions leading to degradation. One of such impurities are organic compounds which impact on the ammonium nitrate has not yet been clearly described. The aim of the research was to investigate the influence of selected organic impurities on the reactivity of aqueous solutions containing up to 50 wt % ammonium nitrate, buffered with 15 wt % diammonium phosphate. The measurements were carried out using differential scanning calorimetry. Solution without organic compounds showed no reactivity up to 240 C. The addition of 8 wt % ammonium oxalate did not increase the reactivity of ammonium nitrate. It was observed an endothermic decomposition reaction of oxalic acid, beginning at 185 C. During the measurement of a solution containing 6 wt % phenidone, it was registered an exothermic complex reaction, beginning at 189 C and showing relatively small thermal effect. Addition of 6 wt % pyrogallol caused a significant increase in reactivity of ammonium nitrate solution. Strongly exothermic reaction beginning at 192 C was observed. Spectrophotometric measurement showed a clear nitrate concentration loss in the sample after calorimetric measurement. The results show that the nitrate ion selectively oxidizes certain functional groups in organic compounds.

Inclusion complex formation of β-cyclodextrin and Naproxen: a study on exothermic complex formation by differential scanning calorimetry

Inclusion complex formation between β-cyclodextrin and Naproxen was investigated using differential scanning calorimetry (DSC) as a function of the β-cyclodextrin-to-Naproxen molar ratio, ranging from 0:5:1 to 5:1. When these mixtures are heated above the melting temperature of Naproxen, an exothermic peak is observed at a temperature slightly higher than the melting peak of Naproxen. This peak, which has not been previously reported, has been interpreted as an exothermic energy of inclusion complex formation. The magnitude of this complex formation peak was found to be dependent upon the composition of the β-cyclodextrin and Naproxen mixture and increased in magnitude to a maximum value at a β-cyclodextrin:Naproxen molar ratio of 2:1. In addition, Naproxen recrystallization and re-melting peaks seen in the cooling and re-heating scans, respectively, decreased in magnitude with increasing molar ratio and totally disappeared for the mixture with 5:1 of β-cyclodextrin to Naproxen ratio indicative of complete inclusion of Naproxen in the cyclodextrin cavities. Complete inclusion was further reflected by the disappearance of key Naproxen peaks in Fourier transform infrared spectra of samples recovered from DSC experiments. The large excess of β-cyclodextrin needed to fully complex the Naproxen was found to be due to slow kinetics. Increasing the hold time after the initial melting led to inclusion efficiencies up to 95 % even for the 2:1 mixture. These experiments suggest that ratios of β-cyclodextrin:Naproxen 2:1 or greater facilitate the process by increasing the presence of cyclodextrin molecules in the close proximity of the drug molecules and lead to high inclusion efficiencies.

Experimental Design of Topological Curves to Safely Optimize Highly Exothermic Complex Reacting Systems

Strongly exothermic solution homopolymerizations are a class of chain reactions particularly difficult to be optimized from both a safety and a productivity viewpoint. Particularly, lots of side undesired reactions (e.g., backbiting, propagation of tertiary radicals, chain transfer to monomer or solvent, etc.), which affect the selectivity with respect to the desired product, and relevant mass and heat transfer problems, due to the increasing system viscosity, take place during such syntheses. Under these unavoidable operating conditions, it is difficult to employ theoretical procedures that are able to safely optimize the analyzed process, because the development of a reliable mathematical model is often not affordable or too time-consuming. In this work, it is shown that the topological criterion theory and its related optimization procedure can be used to optimize experimentally (through a dedicated set of isoperibolic reaction calorimetry tests) a complex reacting system even if its reaction scheme an...

Neural Network Based Modelling and Control in Batch Reactor

The use of neural networks (NNs) in all aspects of process engineering activities, such as modelling, design, optimization and control has considerably increased in recent years (Mujtaba and Hussain, 2001). In this work, three different types of nonlinear control strategies are developed and implemented in batch reactors using NN techniques. These are generic model control (GMC), direct inverse model control (DIC) and internal model control (IMC) strategies. Within the control strategies, NNs have been used as dynamic estimator, dynamic model (forward model) and control (inverse model). An exothermic complex reaction scheme in a batch reactor is considered to explain all these control strategies and their robustness. A dynamic optimization problem with a simple model is solved a priori to obtain optimal operation policy in terms of the reactor temperature with an objective to maximize the desired product in a given batch time. The resulting optimal temperature policy is used as set-point in the control study. All types of controllers performed well in tracking the optimal temperature profile and achieving target conversion to the desired product. However, the NNs used in DIC and IMC controllers need training beyond the nominal operating condition to cope with uncertainties better.

Pentagonal Model of Membrane Channel of Na,K-ATPase. Computer Simulations of Structure and Electrostatic Properties

Computational modeling of the membrane channel of a sodium pump (Na,K-ATPase) is performed and the role of selected amino acids in binding of sodium ions is discussed. The channel is build as a pentameric 10-helix bundle. The transmembrane a-helices are determined from hydropathy calculations. The spatial arrangement of transmembrane a-helices is chosen according to the size of a pore, intersegment loops geometry, and orientation hydrophobicities of transmembrane segments. The latter property provides the numerical estimate of the distribution of the hydrophobic properties at the helical wheels. The model system involves the peptide part and 150 water molecules that soak the pore. The channel structure is submitted to geometry minimization and molecular dynamics relaxation. The relative stability of the channel states with the negatively charged acidic residues belonging to the pore interior decrease in the order Glu-334 > Asp-810 > Glu-785 > Asp-814. The estimated binding energies of 1-3 Na+ ions with the channel with the ionized Glu-334 and Glu-785 amino acids are in the range allowing the exothermic complexation.

Modeling and comparison of acetophenone hydrogenation in trickle-bed and slurry airlift reactors

The performance behavior of a trickle-bed and a slurry airlift reactor has been studied, in case of an exothermic complex multistep reaction. Liquid-phase hydrogenation of acetophenone over a Rh/C catalyst has been performed using both continuous reactors. Mathematical models have been developed and simulations compared with experimental data. Modeling the airlift reactor is easy because it has been operated under conditions where there are no significant mass transfer limitations. Conversely, the trickle-bed model incorporates wall heat transfer, external and internal mass transfer resistances coupled to Langmuir-Hinshel-wood-type kinetics, and shows that: (1) available correlations for gas-liquid transfer are not satisfactory, (2) partial wetting of the catalyst is most probably an important process, (3) internal diffusion limitations are important. Finally, the trickle-bed and airlift reactor performances have been rigourously compared in terms of productivities and yields, and the effect of various parameters discussed. Differences observed concerning the yields have been attributed to the influence of back mixing in airlift reactor and pore diffusional effects in the case of a trickle-bed. The choice of an optimal technology remains an open problem especially when exothermic and complex reactions are to be carried out.

Selectivity and Yield of Exothermic Consecutive Reactions in Catalytically Active Porous Membrane Reactor.

This paper analyzes the performance of a membrane reactor, in which the inside porous structure is impregnated with catalysts, under nonisothermal conditions for exothermic complex consecutive reactions. The mathematical model based on an extended Fick’s model, which incorporates both diffusive and convective fluxes inside the membrane pores, was developed in the form of nonlinear differential equations to be solved numerically. The selectivity and yield of the desired intermediate product were calculated as a function of critical dimensionless parameter group’s such as the mass and heat Peclet groups and the Thiele modulus. It was clarified with the calculated and experimental results that the selectivity and yield of intermediates are significantly affected by convective flow inside the macro-pores.

Safe design of industrial multitubular fixed bed reactors based on parametric sensitivity studies

The parametric sensitivity of a multitubular fixed bed catalytic reactor processing a highly exothermic complex reaction is examined here. The study combines two distinct models in order to foresee any risk in operation. For the multitubular reactor a cell model is used which takes into account the interaction between coolant flow on the shell side and the reacting gas in the tubes. The generalized sensitivity region is delimited by the maximum of the sensitivity of the particle temperature hot spot to different inlet parameters. The sensitivity functions are calculated along with the state variables adopting a two dimensional heterogeneous model.This strategy highlighted non uniformities in the sensitivity region boundaries for tubes in different rows of the bundle and proved to be in good agreement with other methods to foresee runaway. Moreover, it confirmed that coolant flow rate and its temperature have a strong influence on operation safety in either co- or counter-current coolant flow regimes.

Thermodynamics of complexation of 18-crown-6 with sodium, potassium, rubidium, caesium and ammonium ions in N,N-dimethylformamide

The Complexation of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with sodium, potassium, rubidium, caesium and ammonium ions has been studied by titration calorimetry in N,N-dimethylformamide (DMF) containing 0.1 mol dm–3(C2H5)4NCIO4 as a constant ionic medium at 25 °C. The calorimetric titration data were well explained in terms of the formation of the [M(18-crown-6)]+(M = Na+, K+, Rb+, Cs+, NH+4) and [Cs(18-crown-6)2]+ complexes, and their formation constants, enthalpies and entropies were determined. The stability of the [M(18-crown-6)]+(M = Na+, K+, Rb+, Cs+, NH+4) complexes is much more pronounced in DMF than in water. The formation entropies are similar for the sodium and potassium complexes in DMF and water, but are smaller for the rubidium, caesium and ammonium complexes in DMF than for the corresponding complexes in water. The stabilization of the [M(18-crown-6)]+ complexes is thus due to the more exothermic complexation in DMF than in water. Because the solvation of the univalent cations is expected to be enhanced to a greater extent in DMF than in water owing to the relatively large electron-pair donating ability of DMF, the more exothermic complexation in DMF than in water is ascribed to the different solvation of 18-crown-6 in the two solvents, i.e. 18-crown-6 is much more stabilized in water by forming hydrogen bonds with water molecules.