Feedback between crystal growth rates and surface roughness

Abstract

This article analyzes a collection of recent research on a recently discovered feedback mechanism between the crystal growth rate and the surface features of the crystal, and the ramifications the mechanism has on crystallization in general and also on the design and analysis of industrial crystallizers. It has been found that growth under high supersaturations degrades the crystal surface, causing a roughening that is probably due to imperfect incorporation of growth clusters into the surface of the crystal. The effect becomes more pronounced under higher growth rate conditions, and for higher residence times under such conditions. The mechanism only occurs if the supersaturation is increased above a critical level known as the macroscopic roughening transition, although this level is typically quite low; for sucrose it has been measured as a relative supersaturation between 2.5 and 3.9% which is in the range used in many industrial crystallizations. The mechanism appears to be associated with the surface energy of the crystal, with the surface of high surface energy crystals being degraded at lower values of the supersaturation, and such crystals also have larger reductions in growth rate after the roughening has occurred. The mechanism also appears related to growth rate dispersion (GRD), since it gives a mechanism for variation in growth rates in batches of crystals, and also because GRD also appears to be more significant in species having a higher value of the surface energy. The mechanism also causes an increase in the impurity incorporation in the crystal, thus leading to reduced product crystal purities. The mechanism for the impurity incorporation is probably due to enhanced adsorption of impurity molecules due to large crystal surface areas (due to the roughening) and also larger numbers of adsorption sites. The mechanism also complicates measurement of crystal growth kinetics, and thus makes efficient design of industrial crystallization units more difficult.