Morphology of Crystals of α-Lactose Hydrate Grown from Aqueous Solution

When dissolved in aqueous solution, α-lactose, whether originally in the anhydrous or the monohydrated form, readily undergoes mutarotation to yield the β isomer. At equilibrium, which is reached in 6.5 h at 292 K and more rapidly with increasing temperatures, the latter is present to the extent of ca. 60% w/w. At saturation, α-lactose hydrate precipitates from this solution being considerably less soluble than the β form. The resulting crystals have a highly asymmetric, tomahawk shaped morphology characterized by large {01̄1} faces. It is confirmed that this shape arises from growth inhibition due to the incorporation of the β isomer in the {01̄1} sectors of the crystals coupled with a characteristic lack of growth in the [01̄0] direction. Growth under conditions that restrict the formation of the β isomer yields more symmetrically shaped, needlelike crystals. These show a gradual transition to the tomahawk shape as the concentration of the β isomer in solution and hence in the crystal is increased. An assessment is made of the degree of incorporation of the β isomer into the crystal, its distribution and its influence on the defect structure of the material.