A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C20H42). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.
Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the glass transition
from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total
and reversing apparent heat capacities on heating and cooling through the glass transition region as a function of heating
rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate)
(PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries
the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to
the glass transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat
capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations
with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be
used to match model and experiment.
An adiabatic calorimetry was used for some investigations of equilibrium and non-equilibrium phase transitions. For one of
the substances studied (4,4′-di-n-heptyloxyazoxybenzene) it was possible to determine temperature dependence of an order parameter and number of clusters of
high temperature phase in a region of a phase transition. For another substance (liquid 3,4 dimethylpiridine) an anomaly on
the specific heat curves was interpreted as being responsible for a decay of molecules’ clusters.
Non-equilibrium phase transitions were investigated for some liquid crystal substances. The process of transformation between
metastable and stable phases was described quantitatively. The conclusions obtained concern the stability of metastable phases.
Authors:T. J. Snee, C. Barcons, H. Hernández, and J. M. Zaldívar
A simple esterification reaction is used to demonstrate standard procedures for determining the thermokinetic parameters of an exothermic reaction from adiabatic calorimetric data. The influence of variations in the heat capacity of the sample due to changes in temperature and concentration is explored. Shortcomings in the simple interpretation of adiabatic data are identified and isothermal heatflow calorimetry is used to reveal autocatalytic effects which were not apparent from the adiabatic experiments. A more rigourous interpretation of the adiabatic and isothermal data is outlined and used to predict the conditions which can lead to exothermic runaway in a batch reactor. Mathematical simulation of the conditions in a jacketed reactor is used to demonstrate the importance of developing reliable kinetic expressions before assessing the safety of a batch process.
Authors:J. Regis Botelho, A. Duarte Gondim, I. Garcia dos Santos, P. Dunstan, A. Souza, V. Fernandes, and A. Araújo
The standard molar enthalpy of formation of crystalline dialkyldithiocarbamate chelates, [Pd(S2CNR2)2], with R=CH3 and i-C3H7, was determined through reaction-solution calorimetry in 1,2-dichloroethane, at 298 K. Using the standard molar enthalpies
of formation of the gaseous chelates, the homolytic (52618 and 66610) and heterolytic (269318 and 295710 kJ mol-1) mean enthalpies of palladium-sulphur bond dissociation were calculated.
Results of the experimental thermodynamic investigations of the system Bi-Au2Bi obtained by using Oelsen calorimetry are presented in this paper. Activities, activity coefficients and partial molar quantities
for bismuth in the temperature interval 600-1000 K are determined. Based on the values of the thermodynamic parameters, negative
deviation from Raoult law was obtained for the investigated system. Also, bismuth activities and activity coefficients dependence
on composition and temperature is calculated by the nonlinear programming using gradient method.
of the calorimetric continuous titration experiment is presented. The proposed
method is basing on the collection of larger number of experimental data points
than could be obtained from the classical isothermal titration calorimetry
experiment. After the deconvolution procedure resulting in the correction
for the calorimeter time response the pure power effect signal could be obtained.
The collected data enable the detailed analysis of the closely populated 2:1
and 1:1 weak complexes.
Authors:E. Schmolz, R. Dewitz, B. Schricker, and I. Lamprecht
Two geographical subspecies of the honeybee Apis mellifera, the European bee A. m. carnica and the Egyptian bee A. m. lamarckii, were investigated by direct calorimetry. Maximum, mean and minimum heat production rates were determined for groups of 6 bees as a function of temperature and daytime. Smaller Egyptian subspecies showed significantly higher mass specific metabolic rates than the European one. Maximum and mean heat production rates decreased exponentially with growing temperatures while the minimum values remained constant.
Mukesh Chandra (extreme R) are also seen
It is giving me immense pleasure in introducing this special issue of the Journal of Thermal Analysis and Calorimetry based on the papers devoted to Indian Council of Chemists
Polyester yarn spun over a range of wind-up speeds has been characterized using differential scanning calorimetry. Samples prohibited from shrinking during the thermal analysis show slightly smaller heats of fusion and crystallization and reduced cold crystallization temperatures than samples that are free-to-shrink, particularly in the intermediate wind-up speed region. One critical observation made is that the cold crystallization temperature reaches a minimum at some relatively low overall molecular orientation (Δn ∼ 0.020) and fails to decrease further. A likely explanation is that crystallization continuously removes polymer from the melt in the spinline in regions where the local orientation reaches some critically high value.