Mixtures solution microcalorimetry

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

In flow microcalorimetry the solid is placed in the microcalorimeter cell between, say two filters, and is successively brought in contact with solvent and mixtures (or solutions) of various compositions that flow through the cell or percolate over a small column of the adsorbent ). 

Isothermal microcalorimetry has also been used to determine the crystallinity of mixtures of amorphous and crystalline antibiotics [63]. DSC could not be used for this process since the samples decomposed prior to melting and an accurate quantification of the heat of fusion could not be determined. In contrast to studies carried out by Hogan et al. [ 64 ], in this case, it was shown that the heat of solution was not dependent on residual water content. The importance of initial water content is greatest when dealing with hydratable ionic species, since sodium and quaternary ammonium salts have very high heats of hydration. Therefore, before performing any analysis one must care to identify the extent of residual solvents or water present, as well as their effects on the heats of solution in the chosen system. 

Food scientists now realize that the simplistic view of such a mechanism is untenable to describe the behaviour of proteins in mixtures of components like foods. Furthermore, previous studies by protein chemists concerned isolated proteins, usually in aqueous solution at low concentrations and at pH values so far away from the isoelectric point as to discourage protein interactions and aggregation. For this reason, scanning microcalorimeters have been developed that have a detection limit low enough to study 1% w/w protein solutions [149], This, of course, up raises the question of the influence of concentration on DSC data and, therefore, the applicability of results obtained from one system to the other. It has been proved that water concentration plays an important role in the DSC response. Thus, it seems unlikely that conclusions based on dilute protein solutions examined by microcalorimetry can be directly transferable to much of the work done on food proteins at higher concentrations. 

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