Hydration of humic and fulvic acids studied by DSC

Qualitative and quantitative aspects of hydration of four humic acids (HA) and three fulvic acids (FA) originating from different sources were investigated. DSC experiments at subambient temperatures were carried out in order to monitor differences in ice behavior originating from freezable water surrounding humic molecules. It was found that kinetic effects play a significant role in hydration processes of both HA and FA. In fact, the hydration took part over 21?days which was detected as a progressive decrease in ice melting enthalpy. Simultaneously, the peak shapes and positions changed indicating structural changes in the physical structure of the humic substances. In case of FA, the dependency of melting enthalpy on water concentration showed a linear trend resembling a complete hydration previously observed for water-soluble hydrophilic polymers. In contrast, the melting enthalpy of some HA increased in a step-like way with increasing water content, suggesting preservation of original hydrophobic scaffold during the hydration. The differences between the rather young FA and the rather old HA lead to the conclusion that water can play a significant role in processes of humification. We assume that separation of hydrophobic and hydrophilic domains and thus increase in nanoscale heterogeneity represents an important physical contribution to the overall humification process. It was also demonstrated that the higher content of oxygen in humic molecules is not the only indicator of higher water holding capacity. Instead the porosity of humic matrix seems to contribute as additional parameter into these processes.     

Hydration of cement minerals with various admixtures studied by differential thermal analysis and infrared spectrometry

A thermal analysis study of the effect of CaCl₂, CdI₂ and CrCl3 on the hydration reaction of 3CaO·SiO₂ (C₃S) was performed. The model suggested is derived from both the thermoanalytical studies and the infrared spectra.     

Hydration of tricalcium and dicalcium silicate mixtures studied using quasielastic neutron scattering

Quasielastic neutron scattering was used to study the hydration reaction of tricalcium and dicalcium silicate mixtures by following the fixation of hydrogen into the reaction products, and by applying hydration models to the data. The reaction kinetics were well-described by an Avrami-derived model for the nucleation and growth regime during early hydration times and a diffusion-limited model for later periods. This study showed that the hydration reaction is not a simple linear combination of the reactions for the individual components. Compressive strength tests correlated with the neutron scattering data, suggesting that the details of the interaction affect the microstructure and therefore the strength of the product. Results suggest that favorable reaction mechanics provide optimal strength when an 80-95% tricalcium silicate and 20-5% dicalcium silicate mixture is used.     

Hydration mechanism of polysaccharides: A comparative study

Water sorption was studied at 20 °C on films composed of different natural polymers. Three polysaccharides were investigated: chitosan, cellulose, and alginate. The major differences between these polymers, from a structural point of view, lay in the substitution of an OH group by an NH₂ function for chitosan and by an ionic COO^?Na⁺ group for alginate. An analysis of the experimental water sorption isotherms, expressed as the number of water molecules sorbed per repeating unit in the amorphous phase, associated with an analysis of the enthalpy profile related to the water sorption allowed us to propose a water sorption mechanism in two steps for all the polymers: water sorption on polymer‐specific sites in the first step and water clustering around the first sorbed water molecules in the second step. It was determined that two water molecules interacted with the polymer chains for cellulose and chitosan, whereas four water molecules were bonded to alginate chains. The specific sorption sites were identified as OH groups for cellulose, OH and NH₂ groups for chitosan, and ionic and OH groups for alginate. A systematic reduction of the half‐sorption time was observed in the activity range corresponding to this first sorption step, and it was explained by a water plasticization effect. On the other hand, an increase in the half‐sorption time was observed in the second sorption step, at a high activity (>0.8), for chitosan and alginate. A modelization associating the Guggenheim–Anderson–de Boer model and the clustering theory, applied to our systems, allowed us to relate the occurrence of this last phenomenon to the formation of water clusters containing more than two water molecules. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 48–58, 2005     

A study of the drying behaviour of various types of hydrated hydroxypropyl cellulose (HPC) and their mixtures with drugs of different solubility using DSC

Journal of Thermal Analysis and Calorimetry - The objective of this research was to interpret the drying behaviour of hydrated hydroxypropyl cellulose (HPC) types HF, MF, LF and their blends (1:1...     

Thermal decompositions of dialkyl peroxides studied by DSC

Studies on the thermal decompositions of diamyl peroxide (DAPO), dicumyl peroxide (DCPO), and tert-butyl cumyl peroxide (TBCP) were conducted by DSC. Heat of decomposition, exothermic onset point, and chemical kinetics were determined and compared to those data of di-tert-butyl peroxide (DTBP), a model compound for studying thermokinetics of organic peroxide and standardization of a calorimeter. Similarities and differences of decomposition mechanisms between these organic peroxides were proposed and verified. Kinetics on decomposition of uni-molecular reaction via these similar alkoxyl radials accompanying β C–C bond scission were discussed and compared to the results from ab initio calculations. The ranking of thermal stability on dialkyl peroxides is determined to be in the following sequence: DCPO < TBCP < DAPO < DTBP. This rate-determining step in thermal decomposition of dialkyl peroxides possessed an average eigenvalue of log A at about 13.1 ± 1.2. Activation energy on the thermal decomposition of these peroxides was calculated to be 139.5 ± 14.4 kJ mol^?1.     

Thermal transitions of gelatin evaluated using DSC sample pans of various seal integrities

Differential scanning calorimetry (DSC) has become a popular tool to investigate thermal transitions in food ingredients such as gelatin. Upon heating commercial gelatin samples beyond glass transition (T _g) and melting (T _m) temperatures, a relatively large endothermic transition (T _i) can be observed. We have observed that both the peak temperature and the enthalpy of the T _i transition are influenced by the integrity of the seal of the DSC pans used for the analysis. This study shows that escape of moisture from the DSC pan appears to be responsible for this effect. The effect of different types of DSC pans, as well as technique of sealing them on the T _i transition were evaluated using DSC, SDT, and TG–MS.     

Hydration of superoxide studied by molecular dynamics simulation

Molecular dynamics was used to study the hydration of superoxide (O). The Helmholtz free energy of hydration of O was estimated by the thermodynamic integration method. The diffusion of O and the water structure around O were also studied. Two water models were used in the calculations and the results were compared to experiments.     

Hydration of choline salts studied by NMR relaxation

The concentration dependence of T₁ for protons and deuterons in H₂O and D₂O solutions of three choline salts is anomalously high in dilute solutions. We suggest that this is due to formation of clathrate hydrate shells. Acetylcholine chloride does not show anomalous behaviour.     

Hydration of oligo(ethylene glycol) self-assembled monolayers studied using polarization modulation infrared spectroscopy

The interaction with water of protein-resistant monolayers (SAMs), self-assembled from (triethylene glycol) terminated thiol HS(CH2)11(OCH2CH2)3OMe solutions, was studied using in and ex situ polarization-modulated Fourier transform infrared spectroscopy. In particular, shifts in the position of the characteristic C-O-C stretching vibration were observed after the monolayers had been exposed to water. The shift in frequency increased when the SAM was observed in direct contact with a thin layer of water. It was found that the magnitude of the shift also depended on the surface coverage of the SAM. These findings suggest a rather strong interaction of oligo(ethylene glycol) SAMs with water and indicate the penetration of water into the upper region of the monolayer.     

Hydration of gas-phase ytterbium ion complexes studied by experiment and theory

Hydration of ytterbium (III) halide/hydroxide ions produced by electrospray ionization was studied in a quadrupole ion trap mass spectrometer and by density functional theory (DFT). Gas-phase YbX₂ ⁺ and YbX(OH)⁺ (X?=?OH, Cl, Br, or I) were found to coordinate from one to four water molecules, depending on the ion residence time in the trap. From the time dependence of the hydration steps, relative reaction rates were obtained. It was determined that the second hydration was faster than both the first and third hydrations, and the fourth hydration was the slowest; this ordering reflects a combination of insufficient degrees of freedom for cooling the hot monohydrate ion and decreasing binding energies with increasing hydration number. Hydration energetics and hydrate structures were computed using two approaches of DFT. The relativistic scalar ZORA approach was used with the PBE functional and all-electron TZ2P basis sets; the B3LYP functional was used with the Stuttgart relativistic small-core ANO/ECP basis sets. The parallel experimental and computational results illuminate fundamental aspects of hydration of f-element ion complexes. The experimental observations??kinetics and extent of hydration??are discussed in relationship to the computed structures and energetics of the hydrates. The absence of pentahydrates is in accord with the DFT results, which indicate that the lowest energy structures have the fifth water molecule in the second shell.     

Kinetics of crystallization of metallic glasses studied by non-isothermal and isothermal DSC

A method for describing the lengths of induction periods at linear-heating measurements, is employed for the study of induction periods in the crystallisation of metallic glasses. For Fe₇₅Si₁₅B₁₀ glass, close values of the related kinetic parameters were obtained from isothermal and nonisothermal measurements. On the basis of the results obtained, the absence of induction period in the first crystallisation step of Al₉₀Fe₇Nb₃ glass in the isothermal DSC measurement has been elucidated.     

Flash calcination of kaolinite studied by DSC,TG and MAS NMR

The structural changes occurring during the dehydroxylation of kaolinite have been followed using flash calcination to produce kinetically frozen calcines. The percentage of dehydroxylation was varied by changing the furnace residence time or temperature and/or heating speed. These calcination conditions affected the reaction kinetics, but the products depended only on the extent of dehydroxylation.Changes in the position and enthalpy of the endothermic transformation to metakaolinite and the high temperature exothermic reaction of metakaolinite have been followed using Differential Scanning Calorimetry of the flash calcines and related to Thermogravimetry. ²⁷Al magic-angle-spinning NMR spectra, at high magnetic fields and spin rates, enabled the reaction of kaolinite to be monitored and provides new information on the nature of the species formed in the course of dehydration.We thank the Science and Engineering Council for supporting our studies of mineral transformations and. R. Meinhold thanks N.Z. D.S.I.R. chemistry, and H Atakul thanks Istanbul Technical University, for leave to undertake these studies. We thank Bruker Analytische for access to NMR spectrometers in their Karlsruhe laboratory.     

Miscibility of polymer blends studied by a simplified method of data analysis using light heating modulated temperature DSC

A simple method of application of light heating modulated temperature DSC to a study of miscibility of polymer blends has been developed. In this method only the sample was measured and the standard materials were not used. The total heat flow, the complex heat capacity, the reversing and non-reversing heat flows were obtained as values measured from those quantities in hypothetical glassy state at T>T_g. The values of the hypothetical glassy state were calculated by extrapolation from T<T_g. The present method gives relative values but useful information can be obtained from the results. Some results from miscible and immiscible polymer blends are shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction of water with the regenerated cellulose membrane studied by DSC

One to three endothermal peaks atributted to melting of bulk and interfacial water were observed by DSC in the regenerated cellulose — water system. The profiles of thermal effects depend on water content, time of conditioning, film pretreatment and the conditions applied during the preceding freezing-thawing cycles. The occurrence might be deduced of melting-crystallisation processes. A large amount of non-freezable strongly bounded water was also detected. Although cellulose absorbs water quickly after immersion, the structural changes consisting on ordering of polymer fraction occur during further conditioning due to increase in strength of water binding. Using the membranes in the separation module at 90°C causes weakening of these bonds. Differences between interaction of particular cellulose films with water can be detected during the first, the second and the third heating. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydration of aprotic donor solvents studied by means of FTIR spectroscopy

The paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.     

Hydration of thermally denatured lysozyme studied by sorption calorimetry and differential scanning calorimetry

We have studied hydration (and dehydration) of thermally denatured hen egg lysozyme using sorption calorimetry. Two different procedures of thermal denaturation of lysozyme were used. In the first procedure the protein was denatured in an aqueous solution at 90 degrees C, in the other procedure a sample that contained 20% of water was denatured at 150 degrees C. The protein denatured at 90 degrees C showed very similar sorption behavior to that of the native protein. The lysozyme samples denatured at 150 degrees C were studied at several temperatures in the range of 25-60 degrees C. In the beginning of sorption, the sorption isotherms of native and denatured lysozyme are almost identical. At higher water contents, however, the denatured lysozyme can absorb a greater amount of water than the native protein due to the larger number of available sorption sites. Desorption experiments did not reveal a pronounced hysteresis in the sorption isotherm of denatured lysozyme (such hysteresis is typical for native lysozyme). Despite the unfolded structure, the denatured lysozyme binds less water than does the native lysozyme in the desorption experiments at water contents up to 34 wt %. Glass transitions in the denatured lysozyme were observed using both differential scanning calorimetry and sorption calorimetry. Partial molar enthalpy of mixing of water in the glassy state is strongly exothermic, which gives rise to a positive temperature dependence of the water activity. The changes of the free energy of the protein induced by the hydration stabilize the denatured form of lysozyme with respect to the native form.