Tammann Equation (tammann + equation)

Distribution by Scientific Domains


Selected Abstracts


Dielectric relaxation and crystallization of ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidine

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 5 2007
G.P. Johari
Abstract Molecular relaxation in ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidine has been studied by dielectric spectroscopy. The asymmetric relaxation spectra is characterized by the Kohlrausch distribution parameter of 0.46,±,0.02 for aspirin to 0.67,±,0.02 for progesterone. The dielectric relaxation time varies with the temperature, T, according to the Vogel,Fulcher,Tammann Equation, log10(,0),=,AVFT,+,[BVFT/(T,,,T0)], where AVFT, BVFT, and T0 are empirical constants. The extrapolated ,0 at calorimetric glass-softening temperature is close to the value expected. The equilibrium permittivity, ,0, is lowest for ibuprofen which indicates an antiparallel orientation of dipoles in its liquid's hydrogen-bonded structure. A decrease in ,0 with time shows that ultraviscous aspirin, progesterone, and quinidine begin to cold-crystallize at a relatively lower temperature than ibuprofen. ,0 of the cold-crystallized phases are, 4.7 for aspirin at 290 K, 2.55 for ibuprofen at 287 K, 2.6 for progesterone at 320 K, and 3.2 for quinidine at 375 K. It is argued that hydrogen-bonding, the Kohlrausch parameter, extent of localized motions and the long-range diffusion times all determine the physical and chemical stability of an amorphous pharmaceutical during storage. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 1159,1175, 2007 [source]


Dielectric study of equimolar acetaminophen,aspirin, acetaminophen,quinidine, and benzoic acid,progesterone molecular alloys in the glass and ultraviscous states and their relevance to solubility and stability

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 3 2010
G.P. Johari
Abstract Equimolar mixtures of acetaminophen,aspirin, acetaminophen,quinidine, and benzoic acid,progesterone have been vitrified and dielectric properties of their glassy and ultraviscous alloys have been studied. For 20,K/min heating rate, their Tgs are 266, 330, and 263,K, respectively. The relaxation has an asymmetric distribution of times, and the distribution parameter increases with increase in temperature. The dielectric relaxation time varies with T according to the Vogel,Fulcher,Tammann equation, log10(,0),=,AVFT,+,[BVFT/(T,,,T0)], where AVFT, BVFT, and T0 are empirical constants. The equilibrium permittivity is highest for the aspirin,acetaminophen and lowest for the benzoic acid-progesterone alloy, indicating a substantial interpharmaceutical hydrogen bonding that makes the alloy more stable against crystallization than the pure components. The benzoic acid,progesterone alloy is thermodynamically the most nonideal. It showed cold crystallization on heating, which is attributed to its relatively greater magnitude of the JG relaxation in relation to its ,-relaxation. It is argued that the difference between the free energy of an alloy and the pure components would have an effect on the solubility. Studies of solution thermodynamics of a glassy molecular alloy may be useful for optimizing choice of components and composition to form molecular alloys and to impact drug delivery. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 1358,1374, 2010 [source]


Dielectric studies of molecular motions in amorphous solid and ultraviscous acetaminophen

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 10 2005
G.P. Johari
Abstract The dielectric permittivity and loss spectra of glassy and ultraviscous states of acetaminophen have been measured over the frequency range 10 Hz,0.4 MHz. The relaxation spectra show an asymmetric distribution of times expressed in terms of the Kohlrausch exponent, ,, which remains constant at 0.79,±,0.02 over the 305,341 K range. The dielectric relaxation time increases on cooling according to the Vogel,Fulcher,Tammann equation. However, the values of the parameters are considerably different from the values deduced from earlier work by other researchers using the heat capacity of ultraviscous acetaminophen and relating it to its molecular mobility. The calorimetric glass softening temperature of 296 K obtained from differential scanning calorimetry is close to the value measured from dielectric relaxation. The equilibrium permittivity of ultraviscous acetaminophen decreases on heating like that of a normal dipolar liquid, as anticipated from the Curie law. But, its value decreases rapidly with time when it begins to crystallize. The equilibrium permittivity of this crystal phase is ,3.1 at 300 K and increases with temperature, which indicates a partial, orientational-disordering of its structure. The results show limitations of the procedures used in the modeling of the kinetics of molecular motions, that is, estimating physical stability, using thermodynamic considerations based on thermal analyses of the amorphous solid phase of acetaminophen. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:2207,2223, 2005 [source]


Temperature Dependence of Interactions Between Stable Piperidine-1-yloxyl Derivatives and a Semicrystalline Ionic Liquid,

CHEMPHYSCHEM, Issue 10 2010
Veronika Strehmel Dr.
Abstract The stable 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives with hydrogen-bond-forming (-OH, -OSO3H), anionic (-OSO3, bearing K+ or [K(18-crown-6)]+ as counter ion), or cationic (-N+(CH3)3 bearing I,, BF4,, PF6, or N,(SO2CF3)2 as counter ion) substituents are investigated in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide over a wide temperature range. The temperature dependence of the viscosity of the ionic liquid is well described by the Vogel,Fulcher,Tammann equation. Interestingly, the temperature dependence of the rotational correlation time of the spin probes substituted with either a hydrogen-bond-forming group or an ionic substituent can be described using the Stokes,Einstein equation. In contrast, the temperature dependence of the rotational correlation time of the spin probe without an additional substituent at the 4-position to the nitroxyl group does not follow this trend. The activation energy for the mobility of the unsubstituted spin probe, determined from an Arrhenius plot of the spin-probe mobility in the ionic liquid above the melting temperature, is comparable with the activation energy for the viscous flow of the ionic liquid, but is higher for spin probes bearing an additional substituent at the 4-position. Quantum chemical calculations of the spin probes using the 6-31G+d method give information about the rotational volume of the spin probes and the spin density at the nitrogen atom of the radical structure as a function of the substituent at the spin probes in the presence and absence of a counter ion. The results of these calculations help in understanding the effect of the additional substituent on the experimentally determined isotropic hyperfine coupling constant. [source]