Solution microcalorimetry

Solution microcalorimetry is another thermal method for the determination of the difference in lattice energy of polymorphic solids. The difference in heat of solution of two polymorphs is also the difference in lattice energy (more precisely lattice enthalpy), provided of course, that both dissolution experiments are carried out in the same solvent (Guillory andErb 1985 Lindenbaum and McGraw 1985 Giron 1995). The actual value for A Hi is independent of the solvent, as demonstrated in Table 4.1 for the two polymorphs of sodium sulphathiazole. Note also that the calculated heats of transition are virtually identical in spite of the fact that the heat of solution (A//s) is endothermic in acetone and exothermic in dimethylformamide. 

Microcalorimeters have the ability of directly measuring the order of the reaction (n), the rate constant (k), the reaction enthalpy (Ar//), and the equilibrium constant (ATeq). " For example, solution microcalorimetry may be used to determine the free energy of dissolution of a solid compound, which is particularly important in pharmaceutical research for dissolution studies and in the determination of the relative thermodynamic stability of polymorphs. " The change in the Gibbs-Helmholtz free energy, AGsoi, on dissolution is... 

Microcalorimetry has proven to be a particularly useful tool to detect different levels of disorder in pharmaceutical materials. Gao and Rytting demonstrated the validity of solution microcalorimetry to measure changes in the crystallinity during processing of both the drug compound and the excipients. Other workers have used elevated vapor pressures to trigger crystallization of disordered materials in the calorimeter and have been able to use the measured energy output to directly quantify the levels of disorder crystallinity in their samples. 

Hallen, D., Wadso, I. Solution microcalorimetry Pure Appl. Chem. 1989, 61, 123-132. 

The thermochemical and thermophysical properties of the rare earth sesquioxides were critically evaluated in 1973 (Gschneidner etal. 1973). A systematic comparison of rare-earth and actinide sesquioxides was published in 1983 (Morss 1983). Thermodynamic properties of europium oxides were assessed by Rard (1985). Since then the enthalpies of formation of AmjOj and CfjOj were determined by solution microcalorimetry. The Afif [Am (aq)] has been redetermined even more recently so the Af//°[Y (aq)] has been corrected in table 4. Recently, the enthalpy of formation of YjOj was redetermined by combustion calorimetry (Lavut and Chelovskaya 1990) and independently by solution calorimetry (Morss et al. 1993). The latter determination took advantage of a determination of Afff [Y (aq)] that used very pure Y metal (Wang et al. 1988). Assessed values are listed in table 4. 

For all of the transuranium elements, the enthalpy of formation of aquo ions was the first thermochemical property of the elements to be determined. One reason was that the measurement of the heat of solution of metals was an appropriate step in the determination of enthalpies of formation of compounds. A more fundamental reason is that the enthalpy of formation of an aquo ion establishes a fundamental property of that ion and references all stability studies of compounds of that ion. Since solution microcalorimetry is more readily done than combustion microcalorimetry, milligram-scale enthalpy-of-formation studies of aquo ions have been possible whereas such studies of oxides and halides by combustion calorimetry have not. In fact, microcalorimetry studies of trans-uranics have been carried out for nearly 40 years, with barely an order of magnitude improvement in sensitivity in all that time ... 

Previously, Lindenbaum and McGraw have used solution microcalorimetry to study drug forms. Because different crystal forms have different structures, they will inevitably have different heats of solution. However, the difference between the heats of solution of two polymorphs in different solvents should remain the same (Table 8.6) if there is no solvate formation. This difference is the heat of transition between the forms at that temperature. 

Usually the acid-base properties of poly electrolyte are studied by potentiometric titrations. However it is well known, that understanding of polyelectrolyte properties in solution is based on the knowledge of the thermodynamic properties. Up to now, there is only a small number of microcalorimetry titrations of polyelectrolyte solutions published. Therefore we carried out potentiometric and microcalorimetric titrations of hydrochloric form of the linear and branched polyamines at 25°C and 65°C, to study the influence of the stmcture on the acid-base properties. 

Data gathered by microcalorimetry, specifically changes in the partial heat capacity, Cp(T), of a polymer solution at temperatures below and above... 

Fig. 16 Heat generated (Q) as a function of the amount of Ag03SCp3 added to solutions of 36 (filled circle), 38 (filled diamond), and 37 (filled square) in THF, as determined by isothermal titration microcalorimetry (T=28 °C, c=0.2 mm)...

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

Angberg M, Nystrom C, Cartensson S. Evaluation of heat-conduction microcalorimetry in pharmaceutical stability studies VII. Oxidation of ascorbic acid in aqueous solution. Int J Pharm 1993 90 19 33. 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

Mourtzis, N. Cordoyiannis, G. Nounesis, G. and Yannakopoulou, K. (2003) Single and Double Threading of Congo Red into y-Cyclodextrin. Solution Structures and Thermodynamic Parameters of 1 1 and 2 2 Adducts, as Obtained from NMR Spectroscopy and Microcalorimetry, Supramol. Chem. 15, 639-649. 

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. 

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