ON THE DIMORPHISM OF DL-MALIC ACID

The two crystalline forms of dl-malic acid have been studied by means of X-ray powder diffraction and differential thermal analysis. The stable form I melts at 402 K (ΔfusHI = 250 J g-1). The metastable form II melts at 396 K (ΔfusHII = 225 J g-1). It can be concluded from structural and thermodynamic data that form I is the stable form and form II is probably a high-pressure modification. Furthermore, the high values of the entropies of fusion of both forms show that, in these phases, the molecules do not undergo large-amplitude motion. © 1990.