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G P Johari

Publications and source records attributed to G P Johari.

At least 19 recordsLinked to original sources
PubMed2007

Kinetics and thermodynamics of sucrose hydrolysis from real-time enthalpy and heat capacity measurements.

We report a real time study of the enthalpy release and heat capacity during the course of HCl-catalyzed hydrolysis of sucrose to fructose and glucose. Measurements were performed during both isothermal conditions and during slow heating and then cooling at a controlled rate. The reaction rate constant of the first-order kinetics follows an Arrhenius relation with activation energy of 109.2 kJ/mol of sucrose. On hydrolysis, the enthalpy decreases by 14.4 kJ/mol of sucrose at 310 K, and the heat capacity, Cp, increases by 61 J mol-1 K-1 of sucrose in the solution. The enthalpy of hydrolysis decreases with increase in the temperature and DeltaCp on hydrolysis increases. The effects are attributed to change in the configurational and vibrational partition functions as one covalent bond in sucrose breaks to form two molecules, which then individually form additional hydrogen bonds and alter the water's structure in the solution. Cp of the solution increases with temperature less rapidly before sucrose hydrolysis than after it. This may reflect an increase in the configurational contribution to Cp as the hydrogen bond population changes.

Algorithms↗
PubMed2007

Spontaneous liquifaction of isomerizable molecular crystals.

A lattice vacancy raises the energy of the neighboring (flexible) molecule in a crystal, which may be enough to isomerize it to a tautomer that does not fit the lattice site, thus creating a liquidlike local region embedding the vacancy. Similar regions may appear elsewhere in the lattice and the regions may ultimately merge. Thus a crystal may spontaneously liquefy over a period of hours to years at a temperature below its normal melting point. Simultaneous heat capacity and heat absorption measurements of several such molecular crystals show that they spontaneously liquefy at a temperature far below their reputed melting point, according to a non-exponential rate kinetics and a temperature dependent rate constant, and do not crystallize on cooling.

Journal Article↗
PubMed2006

On the nonlinear variation of dc conductivity with dielectric relaxation time.

The long-known observations that dc conductivity sigma(dc) of an ultraviscous liquid varies nonlinearly with the dielectric relaxation time tau, and the slope of the log sigma(dc) against log tau plot deviates from -1 are currently seen as two of the violations of the Debye-Stokes-Einstein equation. Here we provide a formalism using a zeroth order Bjerrum description for ion association to show that in addition to its variation with temperature T and pressure P, impurity ion population varies with a liquid's equilibrium dielectric permittivity. Inclusion of this electrostatic effect modifies the Debye-Stokes-Einstein equation to log(sigma(dc)tau)=constant+log alpha, where alpha is the T and P-dependent degree of ionic dissociation of an electrolytic impurity. Variation of a liquid's shear modulus with T and P would add to the nonlinearity of sigma(dc)-tau relation, as would a nonequivalence of the shear and dielectric relaxation times, proton transfer along the hydrogen bonds, or occurrence of another chemical process. This is illustrated by using the data for ultraviscous acetaminophen-aspirin liquid.

Acetaminophen↗
PubMed2006

Thermal conductivity of a polymerizing liquid.

Thermal conductivity kappa of seven polymerizing liquids has been measured in real time at different temperatures, and calorimetry and dielectric spectroscopy of one liquid are performed to help interpret the results. As a covalently bonded linear chain or a network structure in the liquid grows, kappa of the Debye equation initially increases with the polymerization time t(polym) as the molecular weight, density, and sound velocity increase, as on cooling a liquid. The measured kappa reaches a maximum and then decreases, thus showing a peak at a certain t(polym) and finally becomes constant, which is not the true behavior of steady state kappa. The dielectric relaxation time of the covalently bonded structure at the t(polym) for the kappa peak is less than 5 s and the extent of polymerization is below the vitrification plateau value. The peak height increases when the pulse time for kappa measurement is increased. An increase in the liquid's temperature shifts the kappa peak to a shorter t(polym). Liquid compositions polymerizing rapidly show a similar shift, and those polymerizing slowly or whose viscosity does not reach a high enough value show a small kappa peak or none. The kappa peak may be an artifact of the time dependence of heat capacity during the pulse time used for the kappa measurement, as proposed for glasses and supercooled liquids, similar to the changes in other properties observed as an artifact of kinetic freezing/unfreezing. For a polymerizing liquid, the peak may additionally arise when the rate of increase in the elastic modulus becomes equal to the rate of decrease in equilibrium Cp. In either case, its appearance does not distinguish the Brownian motions' slowing on polymerization from that on cooling or compressing a liquid.

Journal Article↗
PubMed2006

Glass behaviour: Poisson's ratio and liquid's fragility.

The lack of a reliable theory of glass physics has led to the pursuit of correlations between various glass or viscous liquid parameters, one of which is the slope m of the plot of log(viscosity) against Tg/T, extrapolated at the glass-transition temperature, Tg, also termed 'fragility'. Novikov and Sokolov conclude that the value of m for a liquid varies linearly with the ratio of the instantaneous bulk and shear moduli, K infinity/G infinity, of its glass according to the relation m=29(K infinity/G infinity -0.41). Because of the obvious importance of the elastic properties of a glass, we have investigated the basis for this relation and find that its premise is flawed because of the unjustifiable preference for an empirical variation of m with elastic properties, and because of the selected use of glasses. When more glasses are considered in the same way, m does not seem to be linearly related to K infinity/G infinity.

Comment↗
PubMed2006

Dielectric relaxation and elasticity during polymerization.

A molecular kinetics-elasticity relation has been investigated by using real time dielectric spectroscopy of a diepoxide-triamine liquid mixture polymerizing at 298 K. As the liquid polymerized, the dielectric relaxation time tau increased linearly with the exponential of the known value of the instantaneous shear modulus G(infinity), in agreement with the elastic model for viscous flow but without the effect of temperature. Thus the structure-dependent effect on the Brownian motions are separated from the temperature-dependent effect. In this time-dependent process, increase in G(infinity) may be compensated by an increase in T, thereby keeping G(infinity) and tau constant. In the potential energy landscape paradigm, a polymerizing liquid's state point, like a normal liquid's on cooling, continuously shifts to deeper and lower energy minima of higher curvature, but the shift occurs irreversibly to other parts of the total energy landscape, thus adding a reaction coordinate to the landscape. A minimum in the energy landscape corresponding to a structure formed by polymerization may be identical to a minimum in another landscape corresponding to another structure.

Journal Article↗
PubMed2006

Structural relaxation of acetaminophen glass.

PURPOSE: The aim is to determine the structural stability of acetaminophen glass with time and temperature change, and to examine the merits of adapting the structural relaxation models of the glassy state for pharmaceuticals. METHODS: Differential scanning calorimetry technique has been used to study the acetaminophen glass after keeping the samples for various periods at fixed temperatures and after keeping at various temperatures for fixed periods. RESULTS: A general formalism for thermodynamic changes during storage in a temperature fluctuating environment is given and the kinetics of the enthalpy and entropy decrease determined. At a fixed temperature, the decrease occurs according to a non-exponential kinetics. For the same storage time, but at different temperatures, the enthalpy and entropy decrease rises to a maximum value at a certain temperature and then declines. The peak appears at the temperature at which the internally equilibrated state of the sample is reached for a fixed storage time. The change in the normalized heat capacity during the heating of acetaminophen has been analysed in terms of a non-exponential, non-linear enthalpy relaxation model. CONCLUSION: A single set of parameters that fit the data for unannealed acetaminophen glass does not fit the calorimetric data for annealed glass. Since acetaminophen molecules form intermolecular hydrogen-bonds in the crystal state and likely to form such bonds more easily in the disordered state, effect of such bonds on structural relaxation is likely to be significant.

Acetaminophen↗
PubMed2006

Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt.

We report a thermodynamic study of the formation of tetrahydrofuran clathrate hydrate by explosive crystallization of water-deficient, near stoichiometric, and water-rich solutions, as well as of the heat capacity, C(p), of (i) supercooled tetrahydrofuran-H2O solutions and of the clathrate hydrate, (ii) tetrathydrofuran (THF) liquid, and (iii) supercooled water and the ice formed on its explosive crystallization. In explosive freezing of supercooled solutions at a temperature below 257 K, THF clathrate hydrate formed first. The nucleation temperature depends on the cooling rate, and excess water freezes on further cooling. The clathrate hydrate melts reversibly at 277 K and C(p) increases by 770 J/mol K on melting. The enthalpy of melting is 99.5 kJ/mol and entropy is 358 J/mol K. Molar C(p) of the empty host lattice is less than that of the ice, which is inconsistent with the known lower phonon frequency of H2O in the clathrate lattice. Analysis shows that C(p) of THF and ice are not additive in the clathrate. C(p) of the supercooled THF-H2O solutions is the same as that of water at 247 K, but less at lower temperatures and more at higher temperatures. The difference tends to become constant at 283 K. The results are discussed in terms of the hydrogen-bonding changes between THF and H2O.

Journal Article↗
PubMed2006

Evolution of vibrational properties during a macromolecule's growth.

The elastic constants and vibrational contributions to thermal properties of three polymerizing liquids were investigated by using the available hypersonic velocity measured by Brillouin light scattering in real time. During the addition polymerization to a molecular network structure, Poisson's ratio upsilon(Poisson) decreases approximately according to exp[-(kt(polym))]n, where both k and n are composition dependent. The Debye frequency increases and the corresponding heat capacity, energy, and entropy approaching a limiting value. upsilon(Poisson) of the vitrified polymer continues to decrease but much more slowly, indicating its continued slow polymerization and structural relaxation with time. In the potential energy landscape interpretation, a polymerizing liquid's state point continuously shifts to another landscape's more curved, deeper minima.

Journal Article↗
PubMed2006

Kinetics of spontaneous change in the localized motions of D-sorbitol glass.

The dielectric relaxation spectra of D-sorbitol glass have been studied in real time during annealing at 221.1 K, which is 47 K below its T(g) of 268 K. As the glass structurally relaxes during annealing, features of the Johari-Goldstein (JG) relaxation change with time: (i) the relaxation strength decreases, (ii) the relaxation peak at 48 Hz shifts to a higher frequency, and (iii) the relaxation spectra become narrower. All seem to follow the relation p proportional, variant exp[-(kt)(n)], where p is the magnitude of a property, k the rate constant, and t the time. The parameter n may well be less than 1, but this could not be ascertained. It is proposed that shift of the relaxation peak to a higher frequency and narrowing of the relaxation spectra occur when local, loosely packed regions of molecules in the glass structure collapse nonuniformly and the relaxation time of some of the molecules in the collapsed state becomes too long to contribute to the JG-relaxation spectra. Consequently, the half width of the spectra decreases, and the relaxation peak shifts to a higher frequency. Molecules whose diffusion becomes too slow after the local regions' collapse would contribute to the alpha-relaxation spectra and thus the net relaxation strength would increase on structural relaxation. It is argued that these findings conflict with the NMR-based conclusions that motion of all molecules in the glass and supercooled liquid contributes to the faster relaxation process.

Journal Article↗
PubMed2006

Orientation polarization from faster motions in the ultraviscous and glassy diethyl phthalate and its entropy.

Dielectric spectra of the beta relaxation in glassy and ultraviscous liquid diethyl phthalate show that its relaxation strength Delta epsilon(beta), the distribution of times, and the relaxation rate are more sensitive to temperature T in the ultraviscous liquid than in the glassy state. The Delta epsilon(beta) against temperature plot has an elbow-shaped break near T(g) of approximately 181 K, which is remarkably similar to that observed in the entropy, enthalpy, and volume against temperature plots, and in the plot of Delta epsilon(beta) against the liquid's entropy minus its 0 K value. The ratio of Delta epsilon(beta) to diethyl phthalate's entropy, after subtracting the 0 K value, is 1.08 x 10(-3) mol K/J in the glassy state at 120.4 K, which decreases slowly to 0.81 x 10(-3) mol K/J at 176 K near T(g) and thereafter rapidly increases to 1.57 x 10(-3) mol K/J at 190 K. Variation in Delta epsilon(beta) parallels the variation of the entropy. A change in the activation energy of the beta process at T>T(g) indicates that its rate is also determined by the structure of the ultraviscous liquid. Features of beta relaxation are consistent with localized motions of molecules and may not involve small-angle motions of all molecules.

Journal Article↗
PubMed2006

Dynamic heat capacity and relaxation time of ultraviscous melt and glassy acetaminophen.

The real and imaginary components, C'(p) and C''(p), of the complex heat capacity, C*(p)=C'(p)-iC"(p) of supercooled, ultraviscous melt of acetaminophen have been measured at different temperatures during cooling through its vitrification range and during heating through its glass-softening range by using a modulation frequency of 3.3 mHz. From these data, the distribution of relaxation time parameter, beta, and a characteristic (calorimetric or configurational) relaxation time, tau(cal), have been determined. A constant value of 0.65 for beta fits the data, and tau(cal) varies with the temperature according to the Vogel-Fulcher-Tammann equation, tau(cal) = 10(-12.95) exp[1813/(T - 240.5)]. This relation differs significantly from the one deduced by others in which the configurational entropy theory was used to deduce tau(cal). The C'(p) and C''(p) values measured during the cooling of its ultraviscous melt and during the heating of its glassy state show a small hysteresis only at low temperatures. These investigations also provide a comparison of calorimetric and dielectric relaxation times in ultraviscous acetaminophen and highlight the role of faster modes of relaxation at low temperatures in organic, molecular glasses that can help in a better understanding of the crystal nucleation process in glasses at T below their T(g).

Acetaminophen↗
PubMed2006

Effect of pressure on molecular and ionic motions in ultraviscous acetaminophen-aspirin mixture.

Effect of pressure and temperature on molecular motions and dc conductivity in ultraviscous, 50 wt% acetaminophen-aspirin melt has been studied by dielectric relaxation spectroscopy. The spectra obtained over the pressure range, 5-300 MPa and temperature range, 295-320 K show a distribution of relaxation times greater than found in pure acetaminophen. The equilibrium dielectric permittivity and relaxation time, tau, of the melt increase with increase in pressure and decrease in temperature and the dc conductivity, sigma(dc), decreases. The pressure and temperature variation of the limiting high frequency permittivity shows significant contribution from infrared polarization. The volumes of activation for sigma(dc) and tau vary with both the pressure and temperature, indicating that there is also a structural effect that determine sigma(dc). This effect would add a non-linear term to the Debye-Stokes-Einstein equation for variation of sigma(dc) with tau. The ultraviscous liquid remains stable with time, and with change in temperature and pressure, suggesting that a more stable glassy state of a pharmaceutical may be obtained by mixing a second component.

Acetaminophen↗
PubMed2005

Heat capacity of water in nanopores.

Heat capacity of controlled amounts of water in Vycor's 2 nm radius pores has been determined in real time during the course of water's isothermal nanoconfinement from bulk state at 358 K, by using temperature-modulated calorimetry. As water transfers from bulk to nanopores via the vapor phase, its heat capacity per molecule increases asymptotically toward a limiting value of 1.4 times the heat capacity of bulk water for 1.8 wt % water in Vycor and 1.04 times for 10.0 wt %. The observations indicate that vibrational and configurational contributions to the heat capacity are highest when the amount of water is insufficient to completely cover the pore wall, and they decrease as more water is present in the nanopores and water clusters form. The heat capacity of water in completely filled nanopores approaches the value for bulk water, thus indicating that the heat capacity varies with the water molecules' position in the nanopores.

Journal Article↗
PubMed2005

Position-dependent energy of molecules in nano-confined water.

Real time decrease in the energy (or enthalpy) measured during confinement of controlled amounts of water in 2 nm radius pores of Vycor shows that exothermic transfer of bulk water to nanopores via the vapour-phase occurred in two stages. In the first stage, at saturation pressure, H2O molecules from the vapour rapidly accumulated in the nanopore channels near the Vycor surface. In the second, at vapour pressure below saturation, the accumulation rate abruptly decreased and water (slowly) diffused and redistributed in the nanopore channels until the vapour pressure equilibrium was attained. The energy decrease per H2O molecule was highest, 14.5 kJ mol(-1), at low amounts when the pore-wall was incompletely covered by H2O. This value approached zero at higher amounts when pores were gradually filled. The results show that the vibrational and configurational contributions to the energy of H2O molecules depend upon their position in the nanopore and these contributions approach their bulk water values at high water concentration, but do not attain those values for completely filled pores.

Biophysical Phenomena↗
PubMed2005

Endothermic freezing on heating and exothermic melting on cooling.

Generally, a liquid freezes exothermally on cooling and a crystal melts endothermally on heating. Here we report an opposite occurrence--a liquid's endothermic freezing on heating and the resulting crystal's exothermic melting on cooling at ambient pressures. C(p) decreases on freezing and increases on melting, and the equilibrium temperature meets the thermodynamic requirement. Melting on cooling takes longer than freezing on heating. A rapidly cooled crystal state becomes kinetically frozen, evocative of a nonergodic state. Both C(p) and enthalpy relax like those of glasses, though the viscosity is only a few centipoise. The crystal state belongs to energy minima higher than those of the melt, which has consequences for the use of potential-energy landscape, or inherent structures, for a thermodynamic description of a material.

Journal Article↗