Thermal behavior of a bentonite

The mineralogical composition of the Kütahya calcium bentonite (CaB) from Turkey was obtained as mass% of 60% calcium rich smectite (CaS), 30% opal-CT (OCT), trace amount illite (I), and some non-clay impurities by using chemical analysis (CA), X-ray diffraction (XRD), and thermal analysis (TG-DTA) data. The crystallinity, porosity, and surface area of the samples heated between 25–1300°C for 2 h were examined by using XRD, TG, DTA and N₂-adsorption-desorption data. The position of the 001 reflection which is the most characteristic for CaS does not affect from heating between 25–600°C and then disappeared. The decrease in relative intensity (I/I ₀) from 1.0 to zero and the increase in full width at half-maximum peak height (FWHM) from 0.25 to 1.0° of the 001 reflection show that the crystallinity of the CaS decreased continuously by rising the heating temperature from 25 to 900°C and then collapsed. The most characteristic 101 reflection for opals intensifies greatly between 900 and 1100°C with the opal becoming more crystalline. The total water content of the natural bentonite after dried at 25, 105 and 150°C for 48 h were determined as 8.8, 5.0 and 2.5%, respectively. The mass loss occurs between 25 and 400°C over two steps with the maximum rate at 80 and 150°C, respectively. The exact distinction of the dehydration temperatures for the adsorbed water and interlayer water is seen almost impossible. The temperature interval, maximum rate temperature, and mass loss during dehydroxylation are 400–800°C, 670°C and 4.6–5.0%, respectively. The maximum rate temperatures for decrystallization and recrystallization are 980 and 1030°C, respectively. The changes in specific micropore volume (V _mi), specific mesopore volume (V _me), specific surface area (S) were discussed according to the dehydration and dehydroxylation of the CaS. The V _mi, V _me and S reach to their maxima at around 400°C with the values of 0.045, 0.115 cm³ g⁻¹ and 90 m² g⁻¹, respectively. The radii of mesopores for the bentonite heated at 400°C are distributed between 1–10 nm and intensified approximately at 1.5 nm.     

A model developed for acid dissolution thermodynamics of a Turkish bentonite

A model was proposed to calculate some thermodynamic parameters for the acid dissolution process of a bentonite containing a calcium-rich smectite as clay mineral along with quartz, opal and feldspar as impurities. The bentonite sample was treated with H₂SO₄ by applying dry method in the temperature range 50–150°C for 24 h. The acid content in the dry bentonite-sulphuric acid mixture was 45 mass%. The total content (x) of Al₂O₃, Fe₂O₃ and MgO remained in the undissolved sample after treatment was taken as an equilibrium parameter. An apparent equilibrium constant, K _a, was calculated for each temperature by assuming K _a=(x _m−x)/x where x _m is the total oxide content of the natural bentonite. Also, an apparent change in Gibbs free energy, ΔG _a^o, was calculated for each temperature by using the K _a value. The graphs of lnK _a vs. 1/T and ΔG _a^o vs. T were drawn and then the real change in both the enthalpy, ΔH ^o and the entropy, ΔS ^o, values were calculated from the slopes of the straight lines, respectively. Inversely, real ΔG ^o and K values were calculated from the real ΔH ^o and ΔS ^o values through ΔG ^o = −RT ln K = ΔH ^o − TΔS ^o equation. The best ΔH ^o and ΔS ^o fittings to this relation were found to be 65687 J mol⁻¹ and 164 J mol⁻¹K⁻¹, respectively.     

Kinetics and thermodynamics of rebaudioside A adsorption on a strongly acidic cation exchange resin

To explain the mechanism underlying the adsorption of stevia's polar component rebaudioside A in hydrophilic interaction liquid chromatography mode, the characteristics of rebaudioside A adsorption on various resins in an organic‐solvent‐rich system were studied. Among the tested resins, the strongly acidic cation resin FPC11 showed the best adsorption behavior for rebaudioside A. The factors affecting the adsorption kinetics of the resin for rebaudioside A are discussed. The results showed that the pseudo‐second‐order reaction model and intra‐particle diffusion model best described the adsorption kinetics of rebaudioside A on the resin. The adsorption rate was controlled by physical sorption, mainly via electron sharing or electron transfer between the adsorbent and the adsorbate. The adsorption process with multiple stages involved weak initial adsorption behavior. Thermodynamic studies showed that the adsorption of rebaudioside A on the resin was not an ideal monolayer adsorption, but mutual adsorption effects between the adsorbates. The adsorption was a spontaneous, entropy‐increasing endothermic process. The synergistic effect of hydrogen bonding and ion–dipole was a possible driving force.     

Study of The Thermal Decomposition of Some New Cyanine Dispersed Dyes

The structure of the studied compounds was confirmed by elemental analysis, IR and NMR. TG and DTG data were used to study the kinetics of the thermal decomposition process of some new cyanine dispersed dyes derived from 2-[(p-chloro)-styryl]-6-[5-(3-aryl-rhodanine)]-1,2-dihydropyridine. The reaction was found to follow first order mechanism. The activation energy E_a was calculated and used to calculate the thermodynamic parameters enthalpy change (H^*), entropy change (S^*), and free energy change (G^*), related to the thermal decomposition process. Two different calculation methods including Coats—Redfern and Chatterjee were applied to evaluate thekinetic and thermodynamic parameters. Quantum study was performed to support both the structure and the thermal data.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal behavior of a mineral mixture of sepiolite and dolomite

An industrial raw material taken from Beypazarı (Ankara, Turkey) region was heated at different temperatures in the 100–1100°C interval for 2 h. The volumetric percentage of particles having diameter below 2 μm in an aqueous suspension of the material held 24 h were determined as 85% by the particle size distribution analysis. The mineralogical composition of the material was obtained as mass% of 81% sepiolite, 15% dolomite, and 4% interparticle water by using the X-ray diffraction (XRD) and thermal analysis (TG, DTA) data. The temperature ranges were determined for the dehydrations of the interparticle water and the zeolitic water as 25–340°C, for the dehydration of the bound water as 340–580°C, and for the dehydroxylation of the hydroxyls as 800–833°C in the sepiolite. The zig-zag changes in the specific surface area (S/m² g⁻¹) and specific micro-and mesopore volume (V/cm³ g⁻¹) with the temperature increases were discussed according to the dehydrations and dehydroxylation of the sepiolite.     

Characterization and Improvement of Ion Exchange Capacities of Mexican Clinoptilolite-rich Tuffs

Clinoptilolite-rich tuffs from the states of Chihuahua and Oaxaca in Mexico were modified with NaCl solutions to improve their ion exchange properties using different conditioning processes. First, the elemental and sodium compositions were identified by both induced coupled plasma atomic emission spectroscopy and neutron activation, respectively. The clinoptilolite-rich tuffs were characterized by scanning electron microscopy and X-ray diffraction and the surface areas were also determined in both tuffs. Effective, internal and external cation exchange capacities were determined. It was found that the elemental composition was different in both zeolitic rocks. A sodium treatment using a 0.1 M NaCl solution and reflux thus, improves effective cation exchange capacities of the Mexican clinoptilolite-rich tuffs. The external and internal cation exchange capacities depend on the size of the particle, as well as the source of the zeolitic rocks. The quaternary ammonium salt (HDTMABr or BCDMACl) considered is important in those determinations.     

Thermal analysis of some organoclays

Thermogravimetric (TG) and differential thermal analysis (DTA) curves of methyltributylammonium smectite (MTBAS), methyltrioctylammonium smectite (MTOAS), and di(hydrogenatedtallow)dimethylammonium smectite (DHTDMAS), and also corresponding sodium smectite (NaS) and tetraalkylammonium chlorides (TAAC) were determined. The TAACs was decomposed exactly by heating up to 500°C. The adsorbed water content of 8.0% in the pure NaS was decreased down to 0.2% depending on the size of the non-polar alkyl groups in the tetraalkylammonium cations (TAA⁺). The thermal degradation of the organic partition nanophase formed between 2:1 layers of smectite occurs between 250–500°C. Activation energies (E) of the thermal degradations in the MTBAS, MTOAS and DHTDMAS are 13.4, 21.9, and 43.5 kJ mol⁻¹, respectively. The E value increases by increasing of the interlayer spacing along a curve depending on the size of the alkyl groups in the TAA⁺.     

Thermal characteristics in a nanometer scale

As examples of studies on thermal characteristics of materials with a nanometer scale two topics are discussed. One is heat capacity and thermal conductivity of small materials at low temperatures. It based upon the recent findings that heat capacity depends on the limited number of the phonon modes in low angular frequency region and the distinct characteristic is the appearance of quantized thermal conductance in heat transfer through a narrow wire with hundreds nm. The other is the thermophysical properties at the ordinary interface. The disordered structure appearing in the interfacial region with a width of a few nm is discussed, which is comparable to the phonon mean free path, should be taken into account to reveal the characteristic thermal behavior at room temperature. This revised version was published online in August 2006 with corrections to the Cover Date.     

氧化程度对氧化石墨结构与阳离子交换容量的影响

通过改进hummers法制备了不同氧化程度的氧化石墨。采用XRD、FTIR及XPS等对不同氧化程度氧化石墨样品的结构特征、含氧官能团种类与含量及阳离子交换容量进行表征。结果表明,石墨经氧化后结构层上键入羟基(C-OH)、环氧基(C-O-C)和羧基(-COOH)等含氧官能团;随氧化程度的增加,石墨结构逐渐全部转化为氧化石墨结构,C-O-C和-COOH的含量逐渐增大,而C-OH的含量先增大后略有减小,阳离子交换容量也表现为先增大后减小,对应的最大值分别为1.70、3.80和4.50 mmol·g^-1;氧化石墨碳平面上C-OH发生去质子化反应在层间产生H⁺,其他阳离子与之交换进入GO层间域,C-OH的含量是影响氧化石墨阳离子交换容量的主要因素,随C-OH含量的增加,氧化石墨样品的阳离子交换容量增大。     

氧化程度对氧化石墨结构与阳离子交换容量的影响

通过改进hummers法制备了不同氧化程度的氧化石墨。采用XRD、FTIR及XPS等对不同氧化程度氧化石墨样品的结构特征、含氧官能团种类与含量及阳离子交换容量进行表征。结果表明,石墨经氧化后结构层上键入羟基(C-OH)、环氧基(C-O-C)和羧基(-COOH)等含氧官能团;随氧化程度的增加,石墨结构逐渐全部转化为氧化石墨结构,C-O-C和-COOH的含量逐渐增大,而C-OH的含量先增大后略有减小,阳离子交换容量也表现为先增大后减小,对应的最大值分别为1.70、3.80和4.50 mmol·g-1;氧化石墨碳平面上C-OH发生去质子化反应在层间产生H+,其他阳离子与之交换进入GO层间域,C-OH的含量是影响氧化石墨阳离子交换容量的主要因素,随C-OH含量的增加,氧化石墨样品的阳离子交换容量增大。     

Thermodynamics of cation exchange on natural heulandite

Microcalorimetric measurements were carried out on the heat effects of ion exchange in a system containing natural heulandite and aqueous solutions of pairs of electrolytes at 303.15 K. The electrolytes studied were NaCl, KCl, RbCl, CsCl, and NH ₄ Cl. The ion exchange enthalpy decreases in the cation series: Na⁺ > NH₄ ⁺ > K⁺ > Rb⁺ > Cs⁺.Institute of the Chemistry of Carbon Materials, Siberian Branch, Russian Academy of Sciences, 650099 Kemerovo. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 754–756, March, 1992.     

The influence of exchangeable cation on thermal behaviour of ground vermiculite

Grinding and contact with water or salt solution increased the specific surface (ssa) but lowered the first dehydration effect (escaping up to 150°C) and increased the second dehydration effect (150 to 500°C). The dehydroxylation was moved to lower temperatures and was only ΔM(500-1100°C)=3.7±0.3 % as compared to 5.5% in the parent vermiculite (V). Except ΔM(20-150°C), the mass losses measured at the remaining T ranges, were consistent in the ground samples, thus the grinding for 2 min caused the homogenization of the crystal structure of vermiculite [ΔM(150-500°C)=7.6±0.7%]. DTA curves after grinding and cation exchange indicate an important exothermal peak at 795-870°C, its temperature depending on exchangeable cation. It indicates the formation of high temperature phases (enstatite, forsterite, spinel). The lowest temperature of the peak (795°C) was observed in V-gr-Li, here lithium silicate was formed. The highest peak temperature (870°C) was found in V-gr-K, where almost only forsterite developed. These exothermal peaks were very weak in unground V with various exchangeable cations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal differential diagnosis of mica mineral group

The following criteria can be used for differential diagnosis of mica mineral group: weight loss < 350°C; weight loss during dehydroxylation (500–1000°C); peak temperature of structural decomposition and formation of high temperature phases; course of dilatometric curves during dehydroxylation and structural decomposition interval (Fig. 1).Using the single criteria by stepwise comparing a complete thermal differentiation is possible between the members of mica mineral group.Dedicated to Dr. Robert Mackenzie on the occasion of his 75th birthday     

Thermal analysis of macromolecules

This review traces the development of thermal analysis over the last 50 years as it was experienced and contributed to by the author. The article touches upon the beginning of calorimetry and thermal analysis of polymers, the development of differential scanning calorimetry (DSC), single-run DSC, and other special instrumentations, up to the recent addition of modulation to calorimetry and superfast calorimetry. Many new words and phrases have been introduced to the field by the author and his students, leaving a trail of the varied interests over 50 years. It began with cold crystallization and more recently the terms oriented, intermediate phase, glass transitions of crystals, and decoupled chain segments were coined. In-between the following phenomena were named and studied: extended-chain crystals, irreversible thermodynamics of melting of polymer crystals, zero-entropy-production melting, dynamic differential thermal analysis (DDTA), the rule of constant increase of C _p per mobile bead within a molecule at the glass transition temperature, superheating of polymer crystals, melting kinetics, crystallization during polymerization, chin-folding principle, molecular nucleation, rigid amorphous phase, system of classifying molecules, macroconformations, amorphous defects, rules for the entropy of fusion based on molecular shape and flexibility, single-molecule single-crystals, systems for classifying phases and mesophases including condis phases, and the globally metastable semicrystalline polymers with reversible, local subsystems. This review is update of a publication written in 1995 and published under the same title in the J. Thermal Anal., 46 (1996) 643. Parts F and G are fully new, and Part G is the basis for my lecture: ‘The development of the idea of thermodynamic decoupling in macromolecules’.     

Thermal stability of Ag-exchanged clinoptilolite rich mineral

Thermal stability of clinoptilolite rich mineral from Western Anatolia, Turkey and its Ag-exchange forms was investigated. Parent mineral of different sizes were heated up to 1000°C with heating rate of 2 and 10°C min^-1 using differential thermal analyzer (DTA) and thermogravimetric analyzer (TG). Ag exchange was conducted both in conventional constant temperature waterbath and microwave at 40, 60 and 80°C. The exchanged minerals were then characteized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), DTA and TG. The particle size and heating rate do not have significant effect on the thermal behavior of the parent mineral and no structural changes were observed with Ag exchange, only decomposition temperature was lowered. It was finally concluded that, Ag-exchanged clinoptilolite rich minerals were less thermally stable compared to parent mineral that does not affect their use for possible applications.     

Thermal stability of binder for mineral wool insulations

The boundary of the ‘analytical’ thermal stability of a phenol-formaldehyde-urea binder was determined, assuming the lack of nitrogen losses as a criterion of thermal stability.     

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.     

Application of Thermal Analysis in Radiocarbon Dating

Thermal analysis combined with mass spectrometry was applied to radiocarbon dating procedures (age determination of carbon containing samples) to determine the optimal temperature range for the reduction of CO₂ over metallic cobalt of various particle sizes. Experiments were carried out to show the different catalytic activities of cobalt of particle sizes 1, 1–45 and 44 μm. The morphology of the cobalt samples and the deposited carbon were investigated. The quantification of CH₄ and CO formed during the reduction of CO₂ was done by means of pulse thermal analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption and characteristics of base-treated pillared clays

The effect of base treatment on the cation exchange capacity (CEC) of pillared clays and their adsorption isotherms for Cu²⁺, Cr³⁺ and Pb²⁺ have been investigated. Results indicate that although the CEC of pillared clays are only about 15% of that of the parent clays, a large fraction of the native clays CEC may be recovered by treatment with base. The fraction of the CEC recovered depends upon the base strength, its concentration, and the temperature. Contrary to previous suggestions the mechanism of recovery is related to the destruction of pillars which is accompanied by the loss of surface area. It is possible under conditions specified to prepare these base treated pillared clays as a new class of useful, regenerable adsorbent for heavy metal adsorption from aqueous solution.