Two-stage Dissociation Research Articles

“Liquid-like” Water in Clathrates Induced by Host–Guest Hydrogen Bonding

Clathrate structures are sustained by a host lattice formed by hydrogen-bonding water molecules, which encapsulates guest molecules. Up to now, all water molecules in the host lattice are considered ice-like crystallized. Here, we discovered the occurrence of “liquid-like” water molecules and the resulting defects in (polar) tetrahydrofuran clathrates by liquid-state 1H NMR experiments. The liquid-like water molecules start occurring at 271 K, well below the apparent dissociation point (277 K) of the clathrate matrix, via extracting water molecules from the host lattice by host–guest H-bonding. We found an intriguing two-stage dissociation of the clathrate: Partial dissociation at 271 K converting one-third of water molecules into liquid-like followed by complete dissociation at 277 K. The clathrate structure is molecularly heterogeneous in the region between 271 K and 277 K. No liquid-like water exists in (nonpolar) cyclopentane clathrates. This work uncovers the essentiality of host–guest interaction for clathrate structures and the ability to tune their stability using polar molecules.

CHARACTERIZATION OF CERTAIN PHYSICAL PROPERTIES ON MODIFIED KAOLINITE-COUPLED CATALYSTS PREPARED WITH AG AND ZN IONS

This study used kaolinite clay as a carrier and exchanged Ag+ and Zn2+ onto the kaolinite via ion exchange. The coupled kaolinite-Ag/Zn catalysts at the nanolevel were formed after surface modification. Various physicochemical experiments, such as Fourier-transform infrared spectroscopy (FT-IR) analysis, x-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) analysis, thermal analysis, and potentiometric titration analysis were conducted to further discuss the physicochemical properties of the kaolinite-Ag/Zn catalysts. 
 First, a function group analysis on the kaolinite clay conducted using a Fourier-transform infrared spectrometer showed many significant and complex wave crests in the following wavenumber sections of the fingerprint area: 415~600 cm-1 and 750~1170 cm-1; these represent strong bonds between impure silicates (Si-O) and silicates (O-Si-O) within the silicate mineral. The mineral structures of the kaolinite-Ag/Zn catalysts remained intact after being sintered at 350oC. In terms of the x-ray diffraction analysis, following high-temperature sintering a dehydration reaction occurred in the kaolinite-Ag/Zn catalysts. The spacing of layers was thereby narrowed to 0.7170 nm. Moreover, a TEM observation of the kaolinite-Ag/Zn catalysts after high-temperature sintering showed that the sizes of Ag+ and Zn2+ on the kaolinite-Ag/Zn catalysts were between 25 and 60 nm, respectively. This proves that with ion exchange, the kaolinite-Ag/Zn catalysts are at the nanolevel. Regarding thermal analysis, the kaolinite-Ag/Zn catalysts had endothermic peaks at 464oC, 527oC and 888oC, respectively and exothermic peaks at 585oC and 921oC, respectively. As for TGA weight loss, two pyrolysis temperatures were found: The first pyrolysis occurred at 464oC, where the weight loss was caused by oxidative pyrolysis. The second pyrolysis occurred at 775oC, where the TGA weight loss resulted from decomposition of the remaining organic (carbon-containing) compounds. Lastly, regarding potentiometric titration, there were two end points of the titration for the kaolinite-Ag/Zn catalysts, indicating that there are two different acidic function groups. The titration curves of these said end points were obviously affected by two ions, and these two end points of the titration were quite obvious. The intensities of the two acidic function groups were similar; consequently, there is an obvious two-stage dissociation in the titration curves.

A Two-Stage Dissociation System for Multilayer Imaging of Cancer Biomarker-Synergic Networks in Single Cells.

The monitoring of cancer biomarkers is crucial to the early detection of cancer. However, a limiting factor in biomarker analysis is the ability to obtain the multilayered information of various biomarker molecules located at different parts of cells from the plasma membrane to the cytoplasm. A two-stage dissociation nanoparticle system based on multifunctionalized polydopamine-coated gold nanoparticles (Au@PDA NPs) is reported, which allows for the two-stage imaging of cancer biomarkers in single cells. We demonstrate the feasibility of this strategy on sialic acids (SAs), p53 protein, and microRNA-21 (miRNA-21) in MCF-7 breast cancer cells by two custom-built probes. Furthermore, the multicolor fluorescence information extracted is used for the monitoring of biomarker expression changes under different drug combinations, which allows us to investigate the complex interactions between various cancer biomarkers and to describe the cancer biomarker-synergic networks in single cells.

Methane hydrates formation and dissociation in nano silica suspension

Methane gas hydrate (MH) formation/dissociation were investigated in nano silica and water system in a non-stirred reactor. The MH yield was significantly improved in the presence of nano silica. The overall methane hydrate conversion (≥80%) was achieved by optimizing the nano silica to water ratio. Two-stage dissociation was observed after a rapid de-pressurization. Methane gas intake of this system was steady for multiple freezing–thawing cycles signifying improved stability for methane gas storage.

Two-stage dissociation in MgSiO 3 post-perovskite

The fate of MgSiO 3 post-perovskite under TPa pressures is key information for understanding and modeling interiors of super-Earths-type exoplanets and solar giants' cores. Here, we report a dissociation of MgSiO 3 post-perovskite into CsCl-type MgO and P2 1/c-type MgSi 2O 5 at ~ 0.9 TPa obtained by first principles calculations. P2 1/c-type MgSi 2O 5 should dissociate further into CsCl-type MgO and Fe 2P-type SiO 2 at ~ 2.1 TPa. The first dissociation should occur in all solar giants and heavy super-Earths, while the second one should occur only in Jupiter and larger exoplanets. Both dissociations are endothermic and have large negative Clapeyron slopes. If the first dissociation should occur in the middle of a silicate mantle, it could promote mantle layering. We provide essential thermodynamic properties of P2 1/c-type MgSi 2O 5 for modeling interiors of super-Earths.

Effect of Hydrophobic Side Chain on Poly(carboxyl acid) Dissociation and Surfactant Binding

Copolymers consisting of acrylic acid (AA) and 12-acryloyldodecanoic acid (ADA) [poly(ADA-co-AA)] were synthesized, and their dissociation behaviors and binding characteristics with ionic surfactant, dodecylpyridinium chloride, were investigated. The dissociation is strongly suppressed in the presence of ADA, and when the ADA molar fraction (FADA) is 0.5, the copolymer exhibits a distinct two-stage dissociation associated with AA and ADA. The binding process is also changed by the presence of ADA. When the ADA content is low, the surfactants bind cooperatively to form a stoichiometric complex. An increase in ADA content brings about noncooperative and nonstoichiometric complexion. The initiation constant (K0) for an isolated surfactant binding to poly(ADA(FADA = 1.0)) is more than 3 orders of magnitude larger than that of poly(AA(FADA = 0)), indicating that the initiation of the binding is associated not only with the electrostatic interaction but also with the hydrophobic interaction between the alkyl ch...

Osmotic pressure measurements of dissociating bovine hemoglobin

While the dissociation of hemoglobin, in general, has been studied extensively using various techniques under both alkaline and acidic conditions, the dissociation under alkaline conditions using membrane osmometry has not been previously reported. An equilibrium mathematical model describing the two-stage dissociation is presented in this paper and agrees with the experimental osmotic pressure measurements of bovine hemoglobin at pH's above 7.4. Results of this study show that the pH dependency under alkaline conditions at 37°C can be regressed to give approximate values of the dissociation constants for both the dissociation of the tetramer to the dimer and the dimer to the monomer as a function of pH. The data also indicate that, in the presence of the borate buffer used in these experiments, the liberation of approximately one proton occurs for the dissociation of the tetramer into the dimers; approximately two protons are liberated for the dissociation of the dimer into the monomer. The dissociation constants reported here for bovine hemoglobin are in reasonable agreement with those obtained under similar conditions using other techniques.

Analysis of the dependence on temperature of the acid dissociation constants for mono-carboxylic acids in water in terms of a two-stage mechanism

The dependence on temperature of acid dissociation constants in water are examined for four systems. The final equation is based on a two-stage dissociation where the enthalpy terms are assumed to be independent of temperature. The relatively small overall enthalpy change for dissociation is shown to be a consequence of the enthalpy changes for the individual stages. The dependence of the overall heat-capacity quantity on temperature is marked, showing a minimum at a characteristic temperature. For acetic acid, the calculated dependence is in qualitative agreement with calorimetric data.