T H E CREEPING O F SOLUTIONS* BY E . ROGER WASHBURN
It is not an uncommon experience, in almost every chemical laboratory, to see the glass walls of beakers or crystallizing dishes, above the surfaces of evaporating solutions, lined or coated with a solid deposit of the material which had been in solution. This deposit is usually present on the walls not only throughout the space through which the solution has been lowered by evaporation but often several centimeters above the original level of the solution. I n fact the deposit often creeps to the top of the containing vessel and down on the outside, sometimes even spreading out on the table top around the dish. The progress of a typical experiment will be described in detail. A glass crystallizing dish was filled about one third full of an aqueous solution of sodium chloride. As the water evaporated, crystals of the salt appeared on the surface of the liquid, on the bottom of the dish, and on the sides at the point where the meniscus seems to become parallel to the vertical walls. As the evaporation continued, the deposit seemed to creep up the walls, over the top, and down on the outside. The solid deposit reached a thickness of almost a centimeter at the top of the dish. Soon some of the solution started to spread out on the table around the dish, in much the same manner that it would have done if the bottom of the dish had been cracked. The bottom was, however, perfectly intact. The aqueous solutions of a number of the readily soluble salts have been studied in this connection. Among others, there might be mentioned, barium chloride, sodium nitrate, potassium nitrate, copper sulphate and potassium dichromate. Each of these solutions showed the same tendency to creep on the glass walls, copper sulphate showing the phenomenon to an especially marked degree. Solutions in other solvents than water have also been found to display the same tendency to creep up the sides of dishes. Saphthalene in such solvents as ethyl alcohol, benzene and toluene shows a tendency to creep under ordinary laboratory conditions. Sulfur in carbon disulfide displays the phenomenon in a manner easily studied because of the rapid evaporation of the solvent. That solutions will climb on other materials than glass is to be expected and is of interest. Although it seems to be difficult to clean the surfaces of some metals or to prevent reactions between them and the concentrated salt solutions, it has been observed that the solutions of such salts as have been mentioned will creep on gold, silver and platinum. The creeping also takes place on porcelain. Sulfur in carbon disulfide shows a marked tendency to climb on copper. The fact that the copper becomes coated with a thin layer of black copper sulfide immediately upon being immersed in the solution does not prevent the climbing. * Contribution from the Chemistry Laboratxy of the Universitt 01 Sebraska.
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This phenomenon is so common that few stop to wonder at it, or to seek an explanation. I n fact a search of the literature failed to bring to light any reference to the subject or its causes. The results of a number of experiments have led to the following interpretation. The nature of the observations have been such that results of a quantitative nature cannot be presented, although it mag be that certain phases of the problem do deserve a more quantitative study. When an evaporating solution wets the walls with which it is in contact it will rise on them above the body of the liquid. As evaporation takes place crystallization will set in, usually at the surface, both on the body of the solution and on the film on the walls. The crystals on the surface of the film form a more or less compact crust at the point of first formation. This crust conforms to, but is not in actual contact with, the supporting walls. There is thus formed a capillary space bounded on one side by the crystal crust and on the other by the wall material. Solution rises through this capillary layer, wetting the walls and thus climbing to a new height; the crust grows, perhaps to each side as well as upward, and more liquid rises in its self-made capillary. I n cases where a solid crust is not formed there are also capillary layers between the crystals through which liquid may climb. We have been led to believe, however, that the greater part of the liquid rises through the capillary between the crust and the walls. Experimental facts which have led to this explanation may be summed up as follows. If the solution does not wet the walls, the crust does not form above the liquid level, and the solution does not climb noticeably on the walls. For example, solutions of salts do not climb in vessels coated with paraffin. The climbing does not occur if the walls are oily or greasy. The climbing of a solution, which may have started on clean walls, may be stopped by painting on the walls above the solution with hot paraffin. The crust will form up to the paraffin layer and stop, conforming in outline to the painted stripe. When evaporation is slow, or is prevented, little or no crust will form and grow. Saturated solution of salts, or of sulfur in carbon disulfide, do not form crusts in stoppered bottles. If a clean test tube be inverted into the evaporating solution of some salt the crust will form and the solution will climb on the outside but not on the inside of the tube. If the bottom be punched out of the tube, however, and a gentle current of air be blown in and out, the crust will form and the solution will climb on the inside. If the crust of crystals above the solution be gently pressed aside it will be observed that the crystals are separated from the glass by a liquid layer. The moisture can be seen or otherwise tested for, and the fact that the crust is loose from the solid wall is evident by the ease with which it is moved. Xfter sufficient time has elapsed for the liquid to evaporate entirely from the capillary layer, the crust nil1 crumble easily if it is thick; but it may adhere to the glass walls with considerable force if it is thin. That the solution must climb through a self-made capillary IS indicated by the following simple experiment. X saturated solution of sulfur in carbon
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disulfide was placed in a small bottle to the depth of about one centimeter. The walls above the solution were oily so that they were not wet by the solution. A narrow strip of copper was then placed in the bottle, with the bottom in the solution and the top resting against one side at the top of the bottle. The solution wet the metal and the crust soon began to form and to climb until it reached the top of the strip. KOclimbing was observed on the glass just above the solution. R h e n the solution of sulfur reached the top of the bottle on the copper strip it spread out on the glass which here was clean, completely covering the upper portion before the liquid in the bottle had all evaporated. Experiments of this type require several hours for completion, which would indicate that the solution must be well protected from evaporation until it reaches the top for, as is well known, carbon disulfide evaporates very rapidly. I n very favorable, and therefore rather uncommon, instances the crust climbs up and over the top of the walls and down on the outside far enough so that it will actually siphon the remaining liquid out of the dish through the capillary layer. This phenomenon is similar to the siphoning of a liquid out of a dish through a wick or other porous fabric. Of course, when the crust is once over the top of the wall, surface tension or wetting ceases to be the principal factor in causing the downward spread for the force of gravity readily accounts for the further growth. Attention should be called to the interesting and somewhat similar experiment mentioned by K. D. Bancroft,’ in connection with the apparently coherent layer of metal which rises in the liquid film several centimeters above the level of the liquid in which one has shaken aluminum powder or copper powder. Although this phenomenon is different from the one which has been described in that evaporation is not an essential factor i t is pertinent because it offers additional evidence that a liquid film may rise some distance on the walls above the main level of the liquid. The same phenomenon is shown by the tendency of very fine precipitates, such as barium sulfate, to creep up in the liquid layer on the walls of beakers above the solution and thus sometimes cause trouble in analytical work. It is doubtful whether any important information may be obtained by a quantitative study of the relative amounts of climbing on different surfaces for the following reasons. If the liquid does not wet the surface a t all, it will not climb a t all. If it does wet it, even a little; in other words if attraction or adhesion between the liquid and the solid is greater than the cohesions1 forces of the liquid, the liquid will climb and on evaporating will build its capillary through which it will rise a little higher. This process will continue until a height is reached, the same for all clean, wetted, surfaces, determined only by the surface tension of the solution and the diameter of the capillary space through which it climbs; providing that the capillary layers do not become blocked by crystals growing together and in actual contact with the glass. “Applied Colloid Chemistry,” pp. 104-105 (1926).
