Asymmetric Growth of α-Resorcinol Crystals: Comparison of Growth from the Vapor Phase and from Aqueous Solution

Crystals of acentric α-resorcinol (space group Pna 2 1, point group mm 2), grown from the vapor phase, present a wide range of morphological forms. Increase in supersaturation (σ) yields, successively, prismatic (σ ∼ 0.001), columnar (σ ∼ 0.003), needlelike (σ ∼ 0.005), and, finally, hexagonal platelike (σ > 0.007) morphologies. This change in habit is achieved principally by extension in the [001̄] polar direction. No observable continuous growth occurs on the (011) and (01̄1) faces presenting in the [001] direction. Similar platelike morphologies and absence of growth in the [001] direction are reported for crystals grown from aqueous solutions at the highest supersaturations (σ > 0.01). Growth from aqueous solution at lower supersaturations could not be studied with the growth method used due to the low growth rates of all crystal faces under these conditions. Previous reports of apparently faster rates of growth along the [001] direction in α-resorcinol crystals grown from aqueous solution are shown to have resulted from the observation of facetting in the early stages of development of the nucleus or of damaged seeds. Once this facetting is completed on the faces propagating with a growth vector in the [001] direction [e.g., the (011), (01̄1), and contiguous {111} faces], the growth rates of these faces, and hence in the [001] direction, fall to zero or immeasurably low values. At the same time, the growth of faces presenting in the 〈 hk 0〉 and [001̄] directions continues normally. These close similarities in the development of crystals from both the vapor and the aqueous phases lead to the conclusion that the polar morphology of this and, potentially, of all similar acentric materials, is an intrinsic consequence of differences in normal growth (or lack of growth) of the faces at the opposite ends of the polar axis. It does not result primarily from solvent inhibition of growth, as is the current proposal.