Calorimetric absorption spectroscopy

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

An understanding of the complex physico-chemical phenomena associated with the formation and behavior of cementitious compounds is facilitated through the application of many different types of investigative methods. Techniques such as NMR, XRD, neutron activation analysis, atomic absorption spectroscopy, IR/UV spectroscopy, electron microscopy, surface area techniques, pore characterization, zeta potential, vis-cometry, thermal analysis, etc., have been used with some success. Of the thermal analysis techniques the Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TG), Differential Scanning Calorimetry (DSC), and Conduction Calorimetric methods are more popularly used than others. They are more adaptable, easier to use, and yield important results in a short span of time. In this chapter the application of these techniques will be highlighted and some of the work reported utilizing other related methods will also be mentioned with typical examples. 

We have found a direct calorimetric determination of both K and AH° to be capable of producing much more reliable thermodynamic data than infrared or ultraviolet-visible spectroscopy. Furthermore, things that react extensively have enthalpies but may not have convenient changes in absorption bands. The procedure for the simultaneous determination of K and AH° from calorimetric measurements has been described (20—22). 

Probing Ca++—Phosphate Binding. Although this question has been at the center of the current investigations, little experimentation was reported on direct evidence which could be furnished by IR spectroscopy. In a brief mention of IR absorption of DPL-uranyl nitrate in nujol paste, it was concluded that indeed Ca++ interacts with the lipid phosphate group (7). Subsequently, after thermodynamic analysis of calorimetric studies with DPL dispersions in aqueous electrolyte, the same laboratory suggested the absence of direct Ca++-phosphate interaction (8). Strangely, the authors of the second work (8) failed to provide explanations for the discrepancy between this and a previous report (7). The particular predicament implies that either one of the two experiments had to be... 

Calorimetric techniques have also proved to be useful tools for determining energetic parameters in the photopolymerization process. For instance, photoacoustic spectroscopy (PA) has been used by Chance et al.(8>9) to study the reaction kinetics of 4BCMU. In a PA experiment the heat evolved as a result of a photochemical reaction as well as that due to photon absorption is detected by a microphone located in the gas surrounding the sample or by a piezoelectric transducer attached to the sample. The information extracted from this experiment is the reaction enthalpy for photopolymerization (per chain initiation event). 

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