Effects of Methanol on Physical Properties of α-and β-Lactose

Crystalline α-lactose monohydrate and crystalline β-lactose were treated with absolute methanol at room temperature and at reflux temperature. Methanol also was added to fresh aqueous solutions of α- and β-lactose to initiate crystallization. For each of the dried products of these methanol treatments melting point, heat of fusion, and heat capacity (all three by differential scanning calorimetry), density, and crystalline habit were determined. The same measurements were made on the untreated crystalline materials as well as the stable, anhydrous α-lactose obtained by heat treatment. The anhydrous α-lactose αM produced by methanol treatment of crystalline α-monohydrate, was different from the stable anhydrous α-lactose αs formed by heat. The melting point of αM was lower by 5.8C, heat of fusion higher by 33%, heat capacity lower by .027cal g-1 deg-1, and density higher by .025g cm-3 . In addition, the various physical measurements offered evidence of several other distinct forms of lactose, a species of β-lactose, βM, prepared by refluxing in methanol, another β-lactose form βM′, crystallized by methanol, and another anhydrous α-lactose form, αM′, also crystallized by methanol. Each of these three forms had a unique melting point, heat capacity, and density.