Interferometric and X-ray investigation of the growth of long-chain fatty acid crystals I. Polymorphism and polytypism in palmitic acid crystals

Abstract

Two polymorphic forms of palmitic acid crystals grown from dilute solutions at approximately 0°C have been observed. These polymorphs grow as small rhomboidal (or related hexagonal shaped) crystal plates parallel to the (001), the acute angle between the adjacent edges, which are parallel to <110> directions, being 74° for one polymorph and 56° for the other. Hexagonal plates corresponding to these rhomboidal plates are derived by cutting them with a pair of edges parallel to the a - or b - axis. The lattice constants of these small single crystals, which are nearly 100μ in size and only a few microns thick, have been measured by the application of a fine focus X-ray tube of the Ehrenberg-Spears type, and a specially designed oscillation cylindrical camera of 1 cm radius. For the polymorph with edge angle 74° the constants are a =5·63 Å, b = 7·39 Å, c sin β = 40·20 Å, β = 62°2', the b -axis being a screw axis. For the other polymorph with the edge angle 56° the unit cell constants are a =9·68 Å, b = 5·05 Å, c sin β = 36·43 Å, β = 51°3', the b -axis not being k screw axis. The dimensions of the basal planes of the two monoclinic forms can be derived from the rectangular prism of the orthorhombic paraffin crystals by taking oblique sections at the appropriate angle β. Palmitic acid crystals grow by the screw dislocation mechanism showing growth spirals. The measurement of spiral heights by the application of multiple-beam internal interference fringes has been carried out for the polymorph with the edge angle 74°, and it is found that steps which are integral multiples (including unity) of the X-ray repeat distance for this polymorph are formed. In addition, steps which are half-integral multiples of this X-ray unit has been measured, The interlaced type of growth spiral has also been observed. These observations can be explained only by recognizing that in addition to polymorphism, these long-chain acids exhibit polytypism, which, however, has not yet been found by X-ray diffraction experiments. Several cases of the formation of large dislocations, their movement, macroslips and cleavage of extremely thin crystals have been illustrated. The birefringence of very thin crystals of the two polymorphs has been measured for different wave-lengths in the visible region by the use of multiple-beam fringes of equal chromatic order on doubly silvered crystals.