Thermal analyses of commercial magnesium stearate pseudopolymorphs

Two commercial magnesium stearate powders in two well-characterised structural states are investigated using DSC and coupled TGA-DTA under dry nitrogen flow. They consist of either a mixture of crystalline hydrates or a poorly crystallised so-called anhydrate. Following the degassing of unbound water, 1 or 3 weight-loss peaks are observed below about 100 °C, each associated with one heat loss peak at the same temperature. The present results and a review of graphical data from literature show that the so-called anhydrate always contains a significant amount of water. At the beginning of the dehydration process, the heat loss is the same as the standard heat of vaporisation of water and then gradually departs from it by positive values. The idea according to which the mixture of dihydrate and trihydrate loses water to form the anhydrate cannot be quantitatively reconciled with the present and other gravimetric results.     

Effect of pulverization on hydration kinetic behaviors of creatine anhydrate powders

The crystal orientation of creatine monohydrate varies significantly with tableting performance and pulverizing mechanism. Furthermore, the X-ray diffraction patterns of anhydrous forms of untreated creatine monohydrate and of pulverized creatine monohydrate exhibit different crystal orientations. However, hygroscopic forms of unpulverized creatine anhydrate and pulverized creatine anhydrate was exhibit the same diffraction peak pattern. The hygroscopicity of unpulverized and pulverized creatine anhydrate has been investigated by hydration kinetic methods using isothermal differential scanning calorimetry data. Testing of the hygroscopicity of unpulverized and pulverized creatine anhydrate at various levels of relative humidity (RH) at 25 °C revealed that the anhydrate was stable at less than 33% RH, but was transformed into the monohydrate at more than 52% RH. Hydration data of unpulverized and pulverized creatine anhydrate at 60% and 75% RH were calculated to determine hydration kinetics using various solid-state kinetic models. The hydration type of unpulverized and pulverized creatine anhydrate powder follows the zero-order mechanism (Polany–Winger equation) R1. The transition rate constant of pulverized creatine anhydrate, calculated from the slope of the straight line, was about 1.34–1.36 times higher than that of unpulverized creatine anhydrate.     

Determination of hydration kinetics of sulfaguanidine anhydrate in aqueous solution by calorimetry

A heat conduction microcalorimeter was used to evaluate the isothermal transition in water from anhydrate to monohydrate at 298 K. Sulfaguanidine (SGN) anhydrate was used as a model compound for the measurement of hydration kinetics in water. It is the well-known that SGN is very slightly soluble in water and capable of existing as the anhydrate or monohydrate form in the solid state. The transition rates of SGN anhydrate to monohydrate in tablets and granules were investigated. The hydration kinetics of tablets with controlled surface areas, obtained by coating the side with paraffin in aqueous solution, followed an apparent zero-order mechanism. On the other hand, the transition mechanism of the granules involved a phase boundary-controlled contracting interface reaction.     

Effect of grinding on dehydration of crystal water of theophylline

The effect of grinding on the dehydration of crystal water of theophylline has been studied. It was observed that the water content of theophylline hydrate decreased with increased grinding time. As the grinding time proceeded, the results of differential scanning calorimetry (DSC) indicated that crystal water of ground theophylline hydrate dehydrated in three steps at ca. 58, 44, and 17 degrees C, respectively. Powder X-ray diffraction study revealed that the crystal lattice of theophylline monohydrate collapsed by grinding, and part of the theophylline molecules subsequently rearranged the collapsed lattice to form theophylline anhydrate. The result of Fourier transformed infrared spectroscopy demonstrated that the hydrogen bonds between crystal water molecules and theophylline molecules were weakened or destroyed to some extent by grinding. It was supposed that crystal water in the ground theophylline hydrate might exist at least in three molecular states of different hydrogen-bonding. From DSC study, it was suggested that the ruptured hydrogen bonds of water molecules in the ground theophylline hydrate were strengthened after storage under 96.5% relative humidity at 30 degrees C.     

Total Solubility Determination of Mixtures Containing Ampicillin Anhydrate and Ampicillin Trihydrate

The dissolved concentrations of ampicillin anhydrate, ampicillin trihydrate, and their mixtures were experimentally determined. For 100% ampicillin anhydrate and 100% ampicillin trihydrate cases, the dissolved concentration (mg/mL) was equal to the total amount of 100% ampicillin anhydrate and 100% ampicillin trihydrate in 1 mL solution up to the solubility (10.8 mg/mL) of 100% ampicillin anhydrate and the solubility (5.4 mg/mL) of 100% ampicillin trihydrate. For an ampicillin anhydrate to ampicillin trihydrate mixture ratio of 75:25, the relationship between the dissolved concentration and the total amount of the mixture in 1 mL solution was the same as the 100% ampicillin anhydrate case. As for the ampicillin anhydrate to ampicillin trihydrate mixture ratios of 50:50 and 25:75, the total solubility of 10.8 mg/mL could be reached at greater amounts of the mixtures. A model of diffusion‐controlled dissolution rate mechanism with two experimental solubilities, 10.8 mg/mL for ampicillin anhydrate and 5.4 mg/mL for ampicillin trihydrate, was used to successfully interpret the solubility data.     

Rates and mechanisms of conversion of ice nanocrystals to hydrates of HCl and HBr: acid diffusion in the ionic hydrates

This FTIR study focuses on solid-state chemistry associated with formation and interconversion of the ionic HX (X = Cl, Br) hydrates. Kinetic data are reported for conversions of ice nanocrystal arrays exposed to the saturation pressure of the acids in the 110 approximately 125 K range. The product is amorphous acid dihydrate in the case of HBr, and amorphous monohydrate for HCl. The rate-determining step is identified as HX diffusion through the hydrate product crust toward the interfacial reaction zone, rather than diffusion through ice, as commonly believed. Slowing of the conversion process is thus observed with increasing thickness of the crust. The diffusion coefficient (D(e)) and activation energy values for HX diffusion through the hydrates were evaluated with the help of the shrinking-core model. Hydrate crystallization occurs as a separate step, upon heating above 130 K. Subsequently, rates of reversible transitions between crystal di- and monohydrates were observed upon exposure to acid vapor and acid evacuation. In conversion from di- to monohydrate, the rate slows after fast formation of several layers; subsequently, diffusion through the product crust appears to be the rate-controlling step. The activation energy for HBr diffusion through crystal dihydrate is found to be significantly higher than that for the amorphous analogue. Conjecture is offered for a molecular mechanism of HX transport through the crystal hydrate, based on (i) spectroscopic/computational evidence for the presence of molecular HX bonded to X(-) in each of the ionic hydrate phases and (ii) the relative E(a) values found for HBr and HCl diffusion. Monte Carlo modeling suggests acid transport to the reaction zone along boundaries between "nanocrystallites" generated by multiple hydrate nucleation events at the particle surfaces. The reverse conversion, of crystalline monohydrate particles to the dihydrate phase, as well as dihydrate to trihydrate, displays nearly constant rate throughout the particle conversion; suggesting desorption of HX from the particle surface as the rate-limiting factor. Like for D(e), the activation energies for desorption were found to be approximately 20% greater for HCl than HBr for related hydrate phases.     

Thermodynamic studies of the binding interactions of surfactin analogues to lipid vesicles Application of isothermal titration calorimetry

Isothermal titration calorimetry was applied for studying the binding interactions of cyclic and linear surfactins with different ionic charge (z= −2 and −3) and lipid chain length (n=14 and 18) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC) vesicles in 10mMTris buffer at pH8.5with 150mMNaCl at 25°C. Surfactin analogues interacted spontaneously (ΔG _D ^w→b < 0) with POPC vesicles. The binding reactions were endothermic (ΔH _D ^w→b > 0) and entropy-driven process (ΔS _D ^w→b > 0). Moreover, significant differences in the binding constant values (K) ranging from 6.6·10³ to 9.6·10⁴ M⁻¹ show that cyclic structure and the increase of lipid chain length are favourable on the surfactin binding affinity to POPC vesicles, whereas the rise of the number of negative charges has an opposite effect.     

Identification of Composite Cement Hydration Products by Means of X-Ray Diffraction

 In this paper the effect of limestone, fly ash, slag and natural pozzolana on the cement hydration products is studied. Four composite cements containing limestone, natural pozzolana from the Milos Island, slag and fly ash have been produced by intergrinding clinker (85%), the above main constituent (15%) and gypsum. The grinding process was designed in order to produce cements of the same 28d compressive strength. The hydrated products, formed after 1–28 days, were studied by means of X-ray diffraction. Unhydrated calcium silicate compounds of clinker and hydration products such as C*H, C*S*H and ettringite are clearly observed. Although there is not significant differentiation among samples hydrated for the same period of time, modifications of calcium aluminate hydrates as well as sulfoaluminate hydrates, are indicated by the XRD patterns. In samples of limestone cement, monocarboaluminate is formed in the first 24 hours and is still present after 28 days.     

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in π2 configurations of diatomic molecules

The CEPA-PNO method is used for calculating the energy difference ΔE _ST between the³∑⁻ and the¹Δ states of diatomic molecules in electronic π² configurations. An analysis of the contribution of electron correlation to ΔE _ST is performed in terms of physically understandable effects such as direct correlation, dynamic spin polarization, semiinternal and internal excitations. It is shown that these effects are of completely different importance for the molecules treated in this study: For C₂ the direct correlation between the two singly occupied π-orbitals is the dominant correlation contribution to ΔE _ST; for O₂, S₂, SO the internal excitation π _u ² → π _g ² is predominant, whereas for NH and PH there is a close competition between the direct correlation and the spin polarization of the underlying σ-orbitals. The basis set dependence of these effects is investigated, in particular for NH. Our final results reproduce experimental values of ΔE _ST within 0.05–0.10 eV.     

Ab-initio study on low-lying states of the TiSi molecule

 The electronic structure of the TiSi molecule was examined using two types of multireference single and double excitation configuration interactions with highly extended basis sets, one including valence correlation and the other including valence and core–valence correlation. A multireference coupled-pair approximation (MRCPA) was further applied to the latter. The calculations suggest a ⁵Δ ground state, and the lowest excited state is ³Π and is only slightly (0.12 eV as estimated by MRCPA) above the ground state. The spectroscopic constants of the low-lying ¹Δ, ³Δ, ¹Π, ⁵Π, and ⁷Σ⁺ states as well as the ⁵Δ ground state and the ³Π excited states were evaluated, and we found that the molecule has only a weak σ bond and that six of the eight valence electrons essentially do not contribute to the bonding. The bonding nature of TiSi in these states is discussed in comparison with the TiC molecule. Received: 7 October 2000 / Accepted: 8 January 2001 / Published online: 3 May 2001     

Dehydration of sodium carbonate monohydrate with indirect microwave heating

In this study, dehydration of sodium carbonate monohydrate (Na₂CO₃·H₂O) (SCM) in microwave (MW) field with silicon carbide (SiC) as an indirect heating medium was investigated. SCM samples containing up to 3% free moisture were placed in the microwave oven. The heating experiments showed that SCM is a poor microwave energy absorber for up to 6 min of irradiation at an 800 W of microwave power. The heat for SCM calcination is provided by SiC which absorbs microwave. The monohydrate is then converted to anhydrous sodium carbonate on the SiC plate by calcining, i.e. by removing the crystal water through heating of the monohydrate temperatures of over 120 °C. The calcination results in a solid phase recrystallization of the monohydrate into anhydrate. In the microwave irradiation process, dehydration of SCM in terms of indirect heating can be accelerated by increasing the microwave field power.     

Kinetic Analysis of Dissociation of Smithsonite from a Set of Non-isothermal Data Obtained at Different Heating Rates

The heats of hydration reactions for MgCl₂⋅4H₂O and MgCl₂⋅2H₂O include two parts, reaction enthalpy and adsorption heat of aqueous vapor on the surfaces of magnesium chloride hydrates. The hydration heat for the reactions MgCl₂⋅4H₂O+2H₂O→MgCl₂⋅6H₂O and MgCl₂⋅2H₂O+2H₂O→MgCl₂⋅4H₂O, measured by DSC-111, is –30.36 and –133.94 kJ mol^–1,respectively. The adsorption heat of these hydration processes, measured by head-on chromatography method, is –13.06 and –16.11 kJ mol^–1, respectively. The molar enthalpy change for the above two reactions is –16.64 and –118.09 kJ mol^–1, respectively. The comparison between the experimental data and the theoretical values for these hydration processes indicates that the results obtained in this study are quite reliable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectroscopic studies of model polar stratospheric cloud films

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid—nitric acid monohydrate (NAM), dihydrate (NAD) and trihydrate (NAT). Two distinct crystalline forms of the trihydrate were also observed. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to characterize laboratory surfaces on which measurements of heterogeneous reaction rates are performed. Our studies suggest that “water-rich NAT” is a two-phase system with separate ice and NAT crystalline regimes. Finally, we have used FTIR spectroscopy to determine the desorption kinetics for evaporation of model PSC films. Ice evaporation was found to follow zero-order desorption kinetics with a desorption barrier of 12±2 kcal/mol and a preexponential factor of 10^30.5±1.5 molec/cm²-s.     

Theoretical investigation of the intermolecular H-bonding and proton transfer in cytosine assisted by water and methanol

Abstract  

This investigation is devoted to study of the electronic structure of several H-bonded complexes of cytosine tautomers with water and methanol. The stability of these “supersystems” (aggregates) was estimated by calculating the bonding energies ΔE and ΔE _b. The energy barriers of water/methanol-assisted tautomeric conversions were calculated (intermolecular proton transfer), and the electronic structures of the transition states were studied. Each transition state of the proton-transfer reaction was determined as a first-order saddle point on the potential energy surface (full coordinate hyperspace). The crystal structure of the tetramer of cytosine monohydrate was also investigated. It was found that this structure is close to a conical intersection.     

Physical characterization of the polymorphic variations of magnesium stearate and magnesium palmitate hydrate species

The anhydrate, dihydrate, and trihydrate phases of chemically pure magnesium stearate and magnesium palmitate have been prepared and characterized as to their structural characteristics. The magnesium palmitate materials were obtained as significantly larger crystals than were the magnesium stearate materials, and the crystals of the dihydrate phase of either material were found to be the most fully developed. The crystal structures of all materials were judged to be very similar to each other, differing primarily in the magnitude of the long (001) crystal spacing. Thermal analysis studies revealed that the water of hydration contained within the dihydrate phases of either magnesium stearate or magnesium palmitate was more tightly bound than was the water of hydration within the corresponding trihydrate phases. These findings provide further support for the structural picture where the water contained in these lattice structures is present between the intermolecular planes.     

Calcite,vaterite and aragonite forming on cement hydration from liquid and gaseous phase

Cement hydration products were studied as influenced by the hydration conditions (hydration time in liquid phase; relative humidity, RH, in gaseous phase). The formation of calcium hydroxide (portlandite, P) and its transformation to calcium carbonates is mainly discussed here. More hydration products, including P, were formed in liquid phase (paste) than in water vapor (powder), due to the higher availability of water molecules. Full hydration was observed only in the paste hydrated for 6 month, otherwise the P content, estimated from its water escape, DM(400-800°C), increased after storage in water vapor of the prehydrated paste. All the three polymorphs of CaCO₃ (calcite, vaterite and aragonite) were found on prolonged contact with air of the hydrated powder (XRD, HRTEM). Their content was dependent on sequence of RH conditions on hydration: higher after water retention, WR, on lowering RH=1.0→0.95→0.5, than after water sorption, WS, on increasing RH in the inverse order. It increased also on wetting and drying, both of hydrated powder and paste. Ca was found to accumulate on the micro-surfaces of WR samples (SEM, TEM), whereas more Al was observed on WS samples and the crystallinity of hydration products was here higher (ED). Dissolution-diffusion-recrystallization was possible: small Al-ions concentrated at one end and the bigger Ca ions - at the other end of some needles (TEM). At 400-500°C the P in cement transforms in air into CaCO₃, which decomposes at 600-700°C. Thus the sensitivity to carbonation was estimated from ΔM(600-800°C). This value was similar in pastes hydrated for 1 month and in powder (WR). It was lower in powder WS and much lower in the paste (6 months). It increased pronouncedly when the prehydrated paste was stored in water vapor in WS. The nanocrystals of portlandite, vaterite and aragonite, embedded in the amorphous matrix, were observed by HRTEM in the hydrated powder. They may contribute to the cement strength. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reversed-phase HPLC separation of Δ;2 and Δ;3 isomers of 7-ADCA and cephalexin monohydrate

Summary High-performance liquid chromatography was applied for the separation and determination of the Δ² and Δ³ isomers of 7-aminodeacetoxycephalosporanic acid (7·ADCA) and cephalexin monohydrate in their mixtures. Separation was performed on a column containing bonded octadecyl silica phase. The effects of pH (in the range of 3.0–7.6) on the investigated compounds were studied. Two organic modifiers, acetonitrile and methanol, were used to improve and accelerate separation. The applied procedure was very reliable for quantitative determination. Excellent correlation with the data of microbiological assay was found. The procedure was very convenient even when other impurities were present in the sample. This is an advantage with respect to microbiological assays.     

Solvation of propanediol ions by water molecules in the gas phase

The thermochemical properties of protonated hydrates of 1,2- and 1,3-propanediols have been investigated using electrospray ionization-high pressure mass spectrometry. The binding enthalpies, entropies, and free energies of the stepwise hydration of protonated propanediols with one to three waters are reported. The observed negative entropy change [ΔΔS_1,3^o for the addition of the third water to 1,3-propanediol·H⁺(H₂O)₂ suggests a stable structure due to an increased number of hydrogen bonds and the loss of the intramolecular hydrogen bond in the water cluster ion. The thermochemical properties of two isomers of butanediol were also investigated in order to further elucidate the structures of the protonated propanediols.     

The rigid amorphous fraction in semicrystalline macromolecules

The first experimental evidence of the existence of the rigid amorphous fraction (RAF) was reported by Menczel and Wunderlich for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate) (PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC _p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated from the ΔC _p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional) amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling. Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions. After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration.     

Thermochemistry of Microhydration of Sodiated and Potassiated Monosaccharides

The thermochemical properties ΔH ^o _n , ΔS ^o _n , and ΔG ^o _n for the hydration of sodiated and potassiated monosaccharides (Ara = arabinose, Xyl = xylose, Rib = ribose, Glc = glucose, and Gal = galactose) have been experimentally studied in the gas phase at 10 mbar by equilibria measurements using an electrospray high-pressure mass spectrometer equipped with a pulsed ion beam reaction chamber. The hydration enthalpies for sodiated complexes were found to be between −46.4 and −57.7 kJ/mol for the first, and −42.7 and −52.3 kJ/mol for the second water molecule. For potassiated complexes, the water binding enthalpies were similar for all studied systems and varied between −48.5 and −52.7 kJ/mol. The thermochemical values for each system correspond to a mixture of the α and β anomeric forms of monosaccharide structures involved in their cationized complexes.