Matrix Effects in Differential Scanning Calorimetry

Abstract

Problems arise in differential scanning calorimetry (DSC) when large samples and high levels of amplification are used to enhance sensitivity. Artifact peaks are generated by the instrument, calorimetric sensitivity is reduced and DSC peaks are distorted as the transition interface passes through the specimen. These effects have been investigated by peak shape analysis for fusion, crystallisation and a polymorphic transition occurring in model DSC specimens. The results were used to develop two techniques for studying interactions in solid composites by means of heat-flux DSC. These novel techniques were applied to solid composites containing porphyrins and nickel(II) oxide. One of the techniques utilised an improved procedure for measuring the thermal conductivity (λ) of small samples. Values of λ were measured for several pure porphyrins and composites, and the results compared with calculated values from various mathematical models which predict transport properties. This allowed structural changes at the interface between the solid phases to be inferred. The occurrence of these changes was supported by electron microscopy. The second technique was designed to isolate relatively weak DSC peaks from background interference. A fully reacted composite specimen was used to generate a reference DSC curve, which was subtracted from the initial DSC scan. This compensated for the varying heat capacity and Neel transition in nickel(II) oxide, and allowed two types of event to be detected. One of these appeared to be a general feature for free-base porphyrins, corresponding to formation of a nickel-porphyrin complex and water. Reaction began at 0.5-0.6 of the fusion temperature (T/K) for the porphyrin, which Tammann's rule identifies as the region where lattice mobility becomes significant. The second type of event appeared as a very sharp exothermic peak, whose onset corresponded to decomposition of carbonate and hydroxide functions associated with nickel. Rapid acceleration of the process occurred at the Neel transition point in nickel(II) oxide. Both events are believed to represent interactions at the interface between the solid phases.