Errors, microcalorimetry

In principle, to carry out immersion microcalorimetry, one simply needs a powder, a liquid and a microcalorimeter. Nevertheless, it was early realized that the heat effects involved are small and the sources of errors and uncertainties numerous. Many attempts have been made to improve immersion microcalorimetric techniques. Before commenting on this type of experiment, we describe the equipment and procedure which has been found by Rouquerol and co-workers to be of particular value for energy of immersion measurements (Partyka et al., 1979). 

Therefore AH° may be calculated from the surface tension-log C plots at various temperatures. Unfortunately, the errors in locating the cmc (which in many cases is not a sharp point) leads to a large error in the value of AH°. A more accurate and direct method of obtaining AH° is microcalorimetry. As an illustration, the thermodynamic parameters, AG°, AH° and TAS° for octylhexaoxyethylene glycol monoether (CgE ) are given in Table 3.2. 

From Equation (17) an analysis can be made if the quantity of ML can be determined as a function of ligand added at each aliquot. However considerable errors may arise as this quantity must be assayed, after each aliquot of ligand is added. However non-invasive techniques such as isothermal microcalorimetry calorimetry can be used to directly determine the quantity of ML without disturbing the system. For a calorimetric analysis an estimation of the enthalpy change for the macromolecule-ligand interaction must be made. An assumption can be made that at the start of the study, the initial aliquot of ligand added to the system, if sufficiently small, will completely bind to the macromolecule (as initially the macromolecule will be in excess). The enthalpy change associated with this interaction can then be used to calculate the A// for the interaction (Equation (3)). 

Base line 3 is drawn extrapolating to a value of AH (a= 1) of —7.85 kcal mole obtained by potentiometric measurements at different temperatures on the model compound mentioned above. This extrapolation leads to 2i AH of 1.07 kcal residue" The values of AH obtained by microcalorimetry have to be compared with the value of 1.16 kcal residue" obtained by the temperature dependence of potentiometric data. Taking into account the errors connected with the potentiometric method, the errors of the calorimetric one (essentially due to the uncertain choice of the baseline) and the possible difference between AH and AHc, the agreement between the two sets of data is rather good indeed. This fact is most gratifying in view of the eventually large, unexplained. 

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