Adsorption, oriented overgrowth and mixed crystal formation

Abstract

In this paper is considered the cause of strong adsorption of certain substances on specific crystal faces, and the relations between this phenomenon and the other two types of phenomena set down in the title—oriented over growth of different crystals on each other, and mixed crystal formation between two substances. A theory of adsorption is suggested, according to which this mode of association of two substances is related to the other two modes, oriented overgrowth and mixed crystal formation, very much more closely than is commonly realized. Present knowledge of strong adsorption on specific crystal faces is due to the study of habit modification by dissolved impurities. Urea nitrate grown from a solution containing methylene blue forms a good example; Gaubert found that the presence of methylene blue in the solution modifies the habit of the crystals, and at the same time it becomes included in the crystals, colouring them blue. The modification of habit in such cases appears to be caused by strong preferential adsorption of the impurity on certain crystal faces; the rate of growth of these faces (thickness of solid deposited in a given time) is reduced, and they are therefore prominent on the final crystal, since it is always the most slowly growing faces which predominate. The evidence for this view is that when the adsorbed substance is a coloured substance and can thus be seen in the crystal, as in the above example, it is seen to be located in pyramid-shaped regions opening out from the centre of the crystal to the faces whose rate of growth has been reduced. Other typical examples with coloured impurities are: urea nitrate in presence of picric acid, which affects different faces from those affected by methylene blue; Pb (NO 3 ) 2 in presence of methylene blue; and alum in the presence of diamine sky blue FF.