Influence of Fatty Acid Additives on the Dissolution Behavior of Adipic Acid Crystals

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Abstract
Adipic acid crystallized from water containing various concentrations of hexanoic, octanoic or undecanoic acid was examined using differential scanning calorimetry (DSC), density measurements and dissolution rates. Parallel decreases (and increases) in the enthalpy of fusion (Δhf), entropy of fusion (ΔSf) and melting point (Tm) were observed; these changes correspond to increases (and decreases) of both the crystal energy and the concentration of crystal imperfections, resulting from the presence of additives in the crystals. When the crystals were well wetted, the rate of dissolution was inversely related to ΔHf, Tm and ΔSf in agreement with the usual assumption that “impurity” defects increase the energy and consequently enhance the dissolution rate of the crystals. The initial dissolution rate of the crystals, however, decreased with decreasing ΔHf, Tm and ΔSf, suggesting a reduction of wettability which may be caused by an associated increase in the concentration of surface irregularities. DSC appears to be a more sensitive indicator of crystal imperfection than density for doped adipic acid crystals.