On the Phenomena of Liquid Evaporation during Crystallization

A. M. Kuzmin

What I am going to write about is not something new. Everyone who has had occasion to deal with the evaporation of liquid media has undoubtedly seen the patterns that accompany the process of evaporation. The author drew attention to them in connection with the study of crystallization phenomena from supersaturated solutions under conditions of natural and forced evaporation. Special investigations of the phenomena described below, which accompany evaporation, were carried out in a room where, over a long period of time, the temperature fluctuated within the range of 20–23°C.

Usually the process of evaporation is regarded as uniform, its rate depending on the humidity condition of the surrounding medium. However, experimental studies of the evaporation of water vapor from saturated aqueous solutions and of water vapor from the surface of distilled and ordinary drinking water poured into a vessel suggest rather that the evaporation process develops as an intermittent process and, by its nature, may be classed among periodically rhythmic phenomena of nature.

In studying the phenomenon of water evaporation from the surface of aqueous salt solutions—for example, common salt, potassium dichromate, alum, copper sulfate, and other salts—one may observe the following picture. Evaporation of the solvent from a saturated solution leads to the solution becoming somewhat supersaturated: at first very small crystals appear on the surface of the solution; some of them are deposited on the walls of the crystallizer within the meniscus, some, overcoming the forces of surface tension, sink down to the bottom, and some continue to be retained on the surface. After some time of the experiment, many rings form on the walls of the crystallizer; their number will increase if the process of evaporation and crystallization continues for an arbitrarily long time.

Looking closely at the pattern of the deposit obtained on the walls of the vessel, we clearly see that this deposit forms a system of parallel rings (Fig. 1), the width of which ranges from 1.75 to 0.5 cm. The rings themselves, regardless of their width, have one and the same structural arrangement. Thus, the upper part of a ring (a), for example in the case of potassium alum, is thickened as a result of the precipitation of a large number of very small crystals; downward, the number of crystals in the deposit decreases and the deposit (b) becomes whitish and more translucent. In the sa-

In the very lower part of the ring \((c)\) the number of crystals is small; the crystals are often plate-like and thin; therefore this part of the ring \((c)\) always appears transparent. In the case of a weakly supersaturated state of the solutions, which occurs at the beginning of the experiment, part of the ring \((c)\) may be almost devoid of precipitate; conversely, in the case of a relatively high supersaturation of the solution in region \((c)\), the number of precipitate crystals increases, the precipitate becomes distinctly whitish, and even white, but in terms of the density and number of crystals that have separated out it still differs noticeably from part \((a)\) of the ring. Thus, the structure of each ring, according to the character of the precipitate, can be expressed by the formula \((abc)\).

Each precipitate ring is sharply separated from its neighbors owing to the fact that the transition from \((c)\), with a relatively small content of crystals, to \((a)\), with a relatively large amount of salt precipitate, is abrupt, by a jump. The width of the rings ranges from 1.75 to 0.5 mm; in Fig. 1 it can be seen that there are 17 rings over 27 mm. An especially clear such pattern can be observed in the case of crystallization of colored salts, for example potassium bichromate. In this case the upper part of the ring \((a)\) is colored orange-yellow, red-orange-yellow, while part \((c)\) of the ring is colored yellow. Part \((a)\) is composed of an aggregate of very small tabular and prismatic crystals, forming a clearly visible ridge, whereas in \((c)\), compared with \((a)\), crystals are deposited parallel to the walls of the crystallizer,

Fig. 1. Salt evaporation rings above a supersaturated solution of alum in a beaker 85 mm high.

large thin-plate crystals. The transition from (a) to (c), as a rule, is gradual, which is no less clearly expressed in the gradual change of color from orange-red for (a) to light yellow for (c); the transition from (c) of one ring to (a) of another adjacent ring, situated below it, is usually abrupt.

Evaporation of vapors from the surface of distilled or drinking water essentially repeats the same picture as in the case of evaporation of water vapors from the surface of salt solutions. However, there are so few dissolved salts in drinking water, and still fewer in distilled water, that such an extremely dilute solution is not capable, even over the course of a long evaporation experiment, of forming a supersaturated solution. Therefore, to record the evaporation pattern one has to resort to other methods. Observations of evaporation from the surface of water were carried out in open vessels, cylinders 5–12 cm in diameter and 25–50 cm high. On the surface of the liquid

Fig. 2. Dust rings of evaporation of distilled water in a cylinder 5 cm in diameter; 2, A—diagram of the arrangement of the rings in a graduated cylinder; 2, Б—part of the cylinder within 5.05–8.1 cm. The structure of each of 10 rings is shown. Ingredients (a), (b), (c), characterizing the unequal density of the deposit on the cylinder walls.

dust is deposited. The latter, accumulating on the surface in the form of a film, during evaporation, upon tearing, was drawn toward the meniscus and deposited on the wall of the cylinder in the form of a dust ring, the sharpness of whose formation depends on the degree of wetting and on the ability of dust particles to adhere to the glass (Fig. 2).

As a result of similar experiments with the evaporation of water in a cylinder, a distinct system of parallel dust rings forms on its walls. Judging from Fig. 2, A, the width of the rings varies over a wide range, from 3 to 0.5 mm, more often within 1.3–0.8 mm. Each of the rings is constructed according to the same principle as in the case of evaporation of salt solutions. In each dust ring all three ingredients can be distinguished. Thus, the upper part (a) of each ring is represented by a compacted mass of dust particles adhering to the walls of the vessel. Downward, in the middle part of the ring, the number of dust particles gradually decreases, and at the base of the ring (c) their number is extremely small. Therefore this part of the ring appears almost empty. Such clarity in the structure of the dust rings is very clearly noticeable even to the unaided eye. Fig. 2, A was obtained over the course of 50 days.

Let us give one more example with dust-like rings obtained in cylinders 30–50 cm in diameter, the bottom of which is convex inward into the vessel (Fig. 3). At the bottom of these cylinders, water is present level with the protruding part and evaporates freely. As evaporation proceeds and the liquid level falls, a series of concentric rings forms around the protrusion. At the same time, identical dust rings, analogous to those described above, also form on the walls of the cylinder. In their structure, both types of rings in wide cylinders are similar, but the rings on the vessel walls are narrow and close together, whereas on the bottom of the cylinder around the convex part they are wide, and the characteristic rhythmic structure of each ring, determined by the formula (abc), is clearly expressed, i.e., the number of dust particles decreases from the sharply defined inner edge toward the outer one.

Fig. 3. Dust rings on the convex part of the cylinder.

The following lies at the basis of the theoretical explanation of the experimentally obtained pattern with rings during evaporation of liquid from the surface of aqueous salt solutions and distilled and drinking water and alcohol.

One of the main factors in the development of the evaporation process is the contradiction between the solution or water and their surrounding medium. If the “solution,” “water,” with respect to H₂O, may under the given \(P\) and \(T\) be considered completely saturated, then the medium surrounding them under the same conditions will be far from saturated. Hence, accordingly, H₂O particles will be torn away from the surface in the form of vapor, which above the evaporating surface will form temporary condensations in the form of a cloud of vapor. Such an assumption is quite probable if the evaporation rate at some moment exceeds the rate of diffusion of vapors into the surrounding medium. In this case the pressure of the surrounding medium above the surface of the evaporating liquid, initially equal to \(P\), will increase by an amount \(p\), equal to the vapor pressure above the liquid. With an increase in the total pressure \(P\) by the amount \(p\), the evaporation rate decreases to a minimum. However, the cloud of vapor, owing to the undersaturation of the surrounding medium, diffuses into it, as a result of which the pressure again falls and becomes equal to \(P\). It follows from this that at the beginning, when the pressure is equal to \(P\) of the surrounding medium,

evaporation proceeds more intensely, the pressure increases and becomes equal to \(P+p\), with the increase of which the rate of evaporation correspondingly decreases. After the first stage comes a second, then a third... Again an accelerated release of vapor begins, with the formation of a cloud; again the pressure increases to \(P+p\); again evaporation, from a maximum, gradually, but rapidly in time, decreases to a minimum, while the vapor cloud is then scattered by diffusion, as a result of which the pressure again becomes equal to the pressure of the surrounding medium, and so on.

Experimental investigations make it possible to refine only somewhat the schematic picture presented above of the development of the evaporation process. Thus, in the examples of aqueous solutions of salts that are in a state of saturation and have the composition \(q[(A.nH_2O)+mH_2O+Q]^1\), i.e., all the water of the solvent, as the formula shows, is completely bound with the salt dissolved in it. Upon evaporation of some amount of \(H_2O\) from the given solution, the latter becomes supersaturated.

Upon further investigation, the solution in some part of it becomes unstable and, owing to the removal of part of the solvent, precipitation of the solid phase \((A:nH_2O)\) occurs, along with a decrease in the volume of the solvent, which is expressed in a lowering of the level of its remaining part and in the corresponding gradual change in the position of the meniscus. The salt that has precipitated from the solution is deposited partly along the meniscus and forms a ring of precipitate. Investigation of the latter shows that in its upper part \((a)\) the greatest number of crystals is deposited, the number of which gradually, but rapidly, decreases toward the base of the ring \((c)\). This fact indicates that at the beginning of the formation of the ring, part \((a)\), relatively abundant—one may even say vigorous—evaporation occurs, accompanied at the same time by abundant deposition of salts in the ring; then the intensity of evaporation decreases as the pressure increases, and at the same time the quantity of salts deposited in zones \((b)\) and \((c)\) of the ring decreases. The picture, as indicated above, may continue indefinitely, so long as there is solution.

In salt solutions there is still one additional factor that manifests itself to a known extent in the process of evaporation. Thus, in order to obtain a saturated solution for the salt \(A.nH_2O^1\), it is necessary, under the given conditions, to have neither more nor less than \(mH_2O\) of solvent. In the successive stages of evaporation, the release of vapor in the amounts \(a_1H_2O\), \(a_2H_2O\), \(a_3H_2O\ldots\) leads to a decrease in the solvent and to the corresponding precipitation of the solid phase; but the further the evaporation process proceeds, the more supersaturated the solution becomes, the stronger the tendency of the dissolved substance to retain around itself as much solvent as possible. This leads to the result that, with increasing supersaturation, the amount of water that can evaporate decreases, and the distance between the rings of precipitate decreases.

As for the picture of evaporation from the surface of a liquid containing a very insignificant amount of salts, which over a long period of time cannot form a saturated solution, in this case as well the evaporation process develops according to the same scheme. Dust rings likewise make it possible to assert that at the beginning of each stage of evaporation, when above the surface of the liquid the pressure is equal to \(P\) of the surrounding medium, there is a relatively energetic increase in pressure, which becomes equal to \(P+p\), and at the same time there occurs a gradual slowing of evaporation to a minimum, and parallel with this also a gradual—

\(^1\) \([A\cdot nH_2O + mH_2O]+Q\) is a complex in which some salt \((A\cdot nH_2O)\) is in the dissolved state in \(H_2O\); \(n=0, 1, 2, 3\), \(m\) is the amount of solvent necessary for dissolving the salt, \(Q\) is the heat of dissolution, \(q\) is the number of complexes necessary to obtain a saturated solution.

a gradual decrease in the deposition of dust particles from a maximum to a minimum along the meniscus of the descending surface. Between one stage and another, when evaporation from the surface becomes minimal, dust particles accumulate on the surface of the liquid; they may form a discontinuous layer—a film—and in this case retard evaporation. Establishing a pressure over the liquid equal to \(P\) leads to a renewed contradiction between the “water” and the “medium,” which causes relatively vigorous evaporation of water vapor; this leads to rupture of the “dust film” and to the drawing of dust particles toward the meniscus and their deposition in a ring with the features mentioned above.

In conclusion, first of all, we note that the observed picture of evaporation in general does not depend on the size of the liquid surface, since evaporation from a unit area under identical physicochemical conditions will in all cases be of one and the same magnitude.

Secondly, the evaporation process will to a certain extent depend on the degree of humidity of the medium.

Thirdly, the process of evaporation of a liquid medium develops as an intermittently continuous one. Its discontinuity is well emphasized by the various rings of precipitate on the walls of vessels; the number of such rings may be arbitrarily large, depending on the height of the liquid column.

Each ring is formed over a definite interval of time, so that evaporation in time is a periodically repeated process. Each ring of precipitate, characterizing the intensity of evaporation over one or another period of time, has, as was noted above, a rhythmic (regular) structure, which may be expressed by the formula \((abc)\). At the same time the intermittently continuous process of evaporation is a periodically rhythmic process; as a whole it may be expressed by the formula... \((abc)\), \((abc)\), \((abc)\), \((abc)\)...

The rings themselves and the rings of salts that periodically form on the walls of vessels are only a means, a method that makes it possible to reveal the essence of the evaporation process. The width of the rings and their sharpness depend on many causes, for example, on the rate of evaporation, the hygroscopicity of the salt, the nature of the solution itself, and its ability to wet the walls of the crystallizer. A clear picture of the rings is observed in the initial stage of the experiment; later it is obscured to varying degrees. The rings of salts and dust deposited on the walls of the crystallizer themselves form a capillary system, which facilitates the penetration of solution, water, along the walls of the crystallizer above the level of the solution. This motion through the capillaries is intensified still more during evaporation, for evaporation proceeds not only from the surface of the solution, the water, but also from the surface of the walls of the crystallizer. This kind of motion of the solution through the system of capillaries leads to the deposition of new portions of salts, which, being deposited, obscure the earlier clear picture of the rings. The salt and dust rings thereby undergo various kinds of deformation in the form of ruptures of the rings, creep, and distortion of the sharpness of the picture.

Observations of the evaporation of water clearly indicate that evaporation is a periodically rhythmic process.