Morphology of crystals of α-lactose hydrate grown from aqueous solution

When dissolved in aqueous solution, alpha -lactose, whether originally in the anhydrous or the monohydrated form, readily undergoes mutarotation to yield the beta 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, alpha -lactose hydrate precipitates from this solution being considerably less soluble than the beta form. The resulting crystals have a highly asymmetric, tomahawk shaped morphology characterized by large {0(1) over bar 1} in the {0(1) over bar 1} sectors of the crystals coupled with a characteristic lack of growth in the [0(1) over bar 0] direction;. Growth under conditions that restrict the formation of the beta isomer yields more symmetrically shaped, needlelike crystals. These show a gradual transition to the tomahawk shape as the concentration of the beta isomer in solution and hence in the crystal is increased. An assessment is made of the degree of incorporation of the beta isomer into the crystal, its distribution and its influence on the defect structure of the material.