Differential molar heat capacities to test ideal solubility estimations
S H Neau1, S V Bhandarkar, E W Hellmuth
1School of Pharmacy, University of Missouri, Kansas City 64110-2499, USA. shneau@pop.umkc.edu
Pharmaceutical Research
|May 1, 1997
Summary
Calculating ideal solubility requires heat capacity difference (ΔCp). Neither assuming ΔCp is zero nor using entropy of fusion (ΔSf) as ΔCp accurately predicted solubility for pharmaceutical compounds studied.
Area of Science:
- Physical Chemistry
- Pharmaceutical Sciences
- Thermodynamics
Background:
- Ideal solubility calculations are crucial in pharmaceutical development.
- Accurate prediction requires the difference in molar heat capacity (ΔCp) between solid and supercooled liquid states.
- ΔCp is often unknown, leading to simplified estimation methods.
Purpose of the Study:
- To evaluate the validity of two common assumptions for estimating ΔCp in ideal solubility calculations.
- To assess the accuracy of assuming ΔCp = 0 or ΔCp ≈ ΔSf.
- To investigate these assumptions for five structurally diverse pharmaceutical compounds near their melting points.
Main Methods:
- Differential scanning calorimetry (DSC) was used to determine solid and liquid heat capacities.
- Linear equations were fitted to heat capacity data.
- Extrapolation to the melting point yielded differential molar heat capacity (ΔCp).
Main Results:
- Linear heat capacity data were obtained for all compounds.
- Ideal solubility at 298 K was calculated and compared to estimates from both assumptions.
- For the compounds studied, ΔCp was significant and closer to ΔSf than to zero.
Conclusions:
- Neither the ΔCp = 0 nor the ΔCp ≈ ΔSf assumption accurately predicted ideal solubility.
- The study highlights limitations in simplified models for solubility prediction.
- Accurate ΔCp determination is essential for precise ideal solubility calculations in pharmaceutical contexts.
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