STABLE SURFACE FILMS OF CRYSTALS

The author of the paper being reviewed*) describes a very curious phenomenon: regular, well-formed crystals of sodium and potassium chloride, obtained by crystallization from solution, do not dissolve completely when they are secondarily immersed in pure water for an extended time. After dissolution of the main mass of the crystal, there remains a shell-like, extremely thin film, reproducing the original contours of the crystal. These surface films can be observed only with the aid of an electron microscope, which requires placing the dry specimen in a vacuum. Since the capillary forces that arise during drying destroy the residual surface films, a special drying method is necessary in order to preserve them. It consists in gradually replacing the water containing the surface films first by acetone and then by ether, which has a lower surface tension. When the ether evaporates, the films are not destroyed. To establish precisely whether the surface films coincide with the surfaces of the original crystal, stereophotographs were made of the crystals before dissolution and of the residual films after dissolution.

The surface films are so thin that they cannot be photographed at normal incidence of the electron beam onto the film under study. As the angle of incidence of the beam is decreased, the visibility of the films improves. A study of the detailed structure of the films showed that they are formed by mutually intertwined threads, resembling a fabric. Dissolving

) N. Hast, Ark. f. Fysik, 4*, 535 (1952).

salt diffuses through these pores. Slow growth of the initial crystals favors the formation of films of only thin and almost structureless kind, whereas rapid crystallization gives thicker films of a fibrous structure. To clarify the nature of the films, the following experiments were carried out:

1. A drop of NaCl solution was allowed to evaporate freely on the specimen holder, which was then immediately immersed in water until the crystals that had fallen out had completely dissolved. The surface films were firmly held on the specimen holder. The water was then replaced by acetone, and the acetone by ether. The ether evaporated, and the surface films were photographed. In this case it is natural to suppose that the films are a layer of impurities—for example, molecules of oxygen, nitrogen, or dust particles from the air—adsorbed by the free surfaces of the crystals.

2. To eliminate the possibility of such contamination, the specimen holder was immersed in a saturated KCl solution at \(+50^\circ\). On cooling to \(+30^\circ\), crystals precipitated on the holder. The solution was then heated again until the crystals dissolved; the holder was removed and immersed in pure water, which was replaced by acetone and then by ether.

After evaporation of the ether, the same films as in experiment 1 could be observed. Consequently, airborne contamination is not always the source of the films.

1. A specimen holder with fine grains of marble or calcite adhering to it was immersed for 30 minutes in a 6% solution of \(\mathrm{HNO}_3\). The acid was then replaced by water, the water by acetone, etc. In the dry preparation, surface films could again be observed, stable to the action of the acid. The films of large crystals consisted of finer films, as if corresponding to the crystallites forming the large crystals.

2. Finely powdered rock salt, after three hours in water and subsequent drying, likewise revealed large films consisting of small cells—crystallites.

3. Crushed crystals of ice from distilled water leave surface films that resist both melting and dissolution in water. Condensate of water vapor, after freezing and melting, also forms such surface films.

In considering the causes of the formation of stable films, the author regards as most probable the assumption that even in the purest liquids or other substances there is a sufficient quantity of impurities to form these extremely thin films. Foreign ions become concentrated, i.e., form an ionic atmosphere, in the immediate vicinity of the crystal surface during the whole period of its growth. However, after the dissolution of the initial crystals, as a rule no impurities are observed inside them. Consequently, foreign ions coagulate during dissolution, forming surface films.

Another supposition is that these films are a constituent part of the normal process of crystal growth. Such a view contradicts existing theories, according to which ions crystallizing on the surface are precisely the most soluble.

Regardless of the interpretation of this phenomenon, surface films made of foreign ions or molecules, if they actually form during crystallization along grain (block) boundaries, should strongly influence the structure-sensitive properties of crystals.

A. Kh.