Solubility of the A, B, and C polymorphs of stearic acid in decane, methanol, and butanone

Abstract

The solublllty of the A, B, and C polymorphs of stearlc acid Is measured In decane, butanone, and methanol. The experlments were carried out between 6 and 38, -2 and 39, and 8 and 33 OC for decane, methanol, and butanone, respectlvely. The enthalpies and entroples of dlssolutlon for decane and butanone agree well with those derlved from fuslon data. The enthalples check with those dlrectly determlned by calorlmetry. The entroples for the polymorphic transltlons B-C and A-C are found as 4.6 f 0.5 and 1.3 f 0.7 kJ mol-', respectlvely. The stablllty reglons of the polymorphs are deduced wlthln 2 K. The B and C polymorphs coexlst at 29 OC; B Is stable below thls temperature whereas C I s stable above It. The A polymorph Is unstable at all temperatures. As an example, at 25 OC, the solublllty of the B polymorph In 100 g of solvent Is 0.698, 1.608, and 4.575 g for decane, methanol, and butanone, respectlvely. Introductlon I n some respects stearic acid, CH3(CH,),&OOH, is an interesting substance to investigate the solubility of different polymorphic modifications: (i) the different modifications can be separately crystallized; (ii) the heats of polymorphic transformation are sufficiently high to give differences in solubility; (iii) the transitions between the polymorphs occur well below the melting point but are kinetically hindered, so that all polymorphs can be observed at the same temperature.Stearic acid mainly crystallizes from solution in three polymorphic modifications A, B, and C ( 7 ) . The B and C polymorphs are easily crystallized in lozenge-shaped, thin crystals, whereas the A polymorph forms needlelike crystals and is difficult to crystallize. Thus, we drew our attention to the B and C polymorphs, while only a few experiments were carried out with the A polymorph. Bailey et al.(2) first pointed out the differences in solubility of the stearic acid polymorphs. I n a following paper ( 3 ) they pointed out that the C polymorph is stable above 40 OC and the B polymorph below 40 OC. Below 10 OC they suspect the E form to be stable. This latter polymorph has nearly the same macroscopic crystallographic features as the B one. However, its occurrence conditions are not welldefined. No quantitative information about the differences in solubility was given (2, 3 ) .Solubilities for stearic acid are quite abundantly given in the literature. The data are nearly completely accumulated by Singleton (4). However, a closer examination shows a varying accuracy, which is often insufficient. Furthermore, the difference in solubility of the polymorphs is not quantitatively clarified. Since exact solubility data are needed for experiments concerning the crystallization of the B and C forms of stearic acid, we undertook careful measurements of the polymorph-specific solubility and selected decane, methanol, and butanone as solvents. ?On leave from the Institut fur Physikalische und Theoretische Chemie der TU, 0-3300 Braunschweig, West Germany. *On leave from the Faculty of Applied Biological Science, Hiroshima University, 2-17...

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