CRYSTALLIZATION THROUGH MEMBRANES B Y JAMES H. WALTON, JR.
In a previous communication1it was shown by the author that if a supersaturated solution or an undercooled liquid is divided by a membrane and a crystal is added t o one part, crystals will form in the other part, the crystallization being transmitted via the membrane. This was found to hold true with aqueous supersaturated solutions of sodium sulphate, acetate, tetraborate, thiosulphate, potassium alum and lead acetate with parchment, collodion and gold-beater’s skin. Similarly, undercooled water and thymol were used with collodion and gold-beater’s skin. An undercooled liquid of a type entirely different from any of those employed can be made by melting phosphorus in water to which a few crystals of urea have been added t o facilitate the undercooling. A few experiments were carried out with this substance. The apparatus used is shown in Fig. I . The membrane is fitted over the lower end of B, and this is slipped into A, which is n;ade of a glass tube with a bore slightly larger than B. The phosphorus was melted in a beaker containing about 2 0 0 cc of water to which about I gram of solid urea had been added. The hot liquid was then poured into a separatory funnel, and the clear liquid phosphorus with a little of the urea solution was run into each of the arms A and B. Fig. I On cooling to room temperature the phosphorus still remained liquid. Two series of experiments were made in which gold-beater’s skin and sheet rubber (0.005 in. thick) respectively, were used as membranes. In neither case, however, was the crystallization transmitted through the membrane. The use of undercooled phosphorus is particularly interesting, differing as it does from some of the other Jour. Phys. Chem.,
13, 490
(1909).
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46
James
H . Walton, J r .
substances used in the matter of speed of crystallization.' Phosphorus crystallizes with the velocity of 60,000 mm per minute, while salol, one of the substances used in the experiments already referred to, crystallizes a t the rate of 4 mm in one minute. This experiment confirms the conclusion already drawn in the preceding paper, viz.-that the crystallization through membranes depends primarily upon the nature of membrane and the dissolved substance.
Supersaturated Aqueous Solutions with Rubber Membranes In the first paper on this subject, reference was made to some experiments which tended to show that if very thin rubber were substituted for the parchment and collodion membranes, crystallization would be transmitted through it. At that time it was not possible to definitely establish this point, which is obviously of great importance-involving as it does the question of whether actual contact with the solid phase is necessary for the separation of crystals from a supersaturated solution. A series of very carefully performed experiments was carried out in order to decide this question. The apparatus used is shown in Fig. 2, which is simply a 2 5 0 cc distilling
Fig. 2
flask connected with a manometer. About 2 5 cc of the supersaturated solution was placed in A; it was then boiled for a Friedlander and G. Tamrnann: Zeit. phys. Chem., 24,
152
(1897).
Crystallization through Membranes
47
few minutes so that the steam would condense on the neck of the flask and wash down any solid particles which might by a any chance be lodged there. The stopper C-carrying glass tube, B over the end of which was tied a piece of sheet rubber, was then inserted in the neck of the flask, and the liquid allowed t o cool to the room temperature, about 23'. The system was then connected with the manometer E, and suction applied at D. This caused the rubber on the tube B to become expanded, its thickness being reduced t o 0.00030.0004 in. The manometer permits the pressure t o be regulated, so that the rubber will not be disrupted by too great a pressure. By means of a pinch cock, D was then closed and the apparatus allowed to stand for an hour. Any leaks in the membrane could be detected by the change in the manometer reading during this time. The tube B was then partially filled with a supersaturated solution of the same concentration as that in the flask, and after a few minutes was inoculated. Experiments were carried out with the following substances : Experiment I .-A supersaturated solution of sodium acetate made by heating 300 grams of the pure crystallized salt with 50 cc of water was used, the flask being filled in the manner described. The solution inside the expanded rubber was inoculated. On standing for 1-2 hours no separation of crystals occurred in the flask. Duplicates were carried out in this experiment. Experiment 2.-An experiment similar to the above was carried out with a supersaturated solution of potassium alum. The solution used was of no definite concentration. In its preparation a sample of alum consisting of crystals about the size of a pea, was used. About 30 grams were placed in the flask, enough distilled water added to just cover them, and the water heated until the crystals were dissolved, forming a solution supersaturated at room temperature. The rest of the experiment was carried out in a manner similar t o that just described. In one experiment no change was noticed after two hours, at the end of which time the rubber film
I
48
James H . Walton, J r .
broke. In a duplicate no crystallization was transmitted through the membrane even after twelve hours. Experiment 3.-A supersaturated solution of lead acetate was prepared by heating 2 0 0 grams of the crystals with 7 5 cc of water. Four experiments were conducted-the details being similar t o the experiments just described. After fifteen hours no transmission of crystallization had occurred. These experiments indicate the necessity of actual contact of the supersaturated solution with the solid phase before crystallization can occur. They show that with these salts whenever crystallization is transmitted through membranes it is brought about b y the formation of the solid phase in the pores of the membrane.
Experiments with Solutions Which Were Only Slightly Supersaturated I n all of the experiments made with aqueous sufiersaturated solution it was found that whenever water could pass through a membrane crystallization could be transmitted through the membrane. As a result of these experiments, therefore, it might seem reasonable to state the above as a generalization. It must be remembered, however, that the experiments referred t o were made with solutions which were very strongly supersaturated, and before any such generalization can be made it is necessary t o make similar experiments with solutions which are only slightly supersaturated. To prove this point eyperiments were carried out as follows : A saturated solution of a given salt was prepared .and kept a t 30' C. It could be supersaturated and the degree of supersaturation controlled by either of the following methods : I . By lowering the temperature of the bath any desired amount . 2. By measuring out a definite quantity of the saturated solution, raising its temperature fifteen or twenty degrees, dissolving a weighed quantity of the salt, and cooling the solution in the bath to 30' again.
Crystallization through Membranes
49
Method 2 was used in these experiments. The supersaturated solution was divided by a membrane, one-half was inoculated, and the other half examined from time t o time to see whether or not crystals had formed. The following experiments were made : A saturated solution of potassium alum was prepared by saturating at 35 ', then placing in a bath a t 30' and shaking for several hours in contact with the solid phase. Twenty-five cc portions of the solution were removed by means of a pipette and placed in beakers containing weighed quantities of the alum. These beakers stood in a bath which was kept at a temperature of 45-50' C. After the alum had dissolved portions of the solution were removed and placed in each of the tubes A and B-shown in Fig. 3, which is simply a large test tube closed with a rubber stopper carrying a smaller tube which has its lower end covered with a membrane. When parchment was used as a membrane considerable difficulty was experienced in fastening it over A in such a manner B that it would be perfectly tight. This was accomC plished by slipping a piece of rubber tubing over the tube, tying the wet parchment over the end in the usual manner, and allowing to dry. A cylinder of glass-C-of diameter large enough to Fig. 3 leave a space of about 3 mm between it and t h e ' outer wall of A was slipped over the tube. Molten paraffine was poured into this space, and a suction was applied at the upper end of A to draw the paraffine into all the crevices. The portion of parchment at the immediate lower end of A was kept wet in order to prevent its becoming saturated with paraffine, and thus made water-proof. This makes a very tight joint. Instead of paraffine, sealing wax can be used. Collodion membranes can easily be fastened over the end of the tube by means of liquid collodion, After introducing the solutions into A and B, they were closed, and shaken so that the hot solution (at 50° C) would dissolve from the stopper or sides of the tube any particles
James H . Walton, J r
SO
of the solid phase. The cell was then shaken for 1-2 hours in a thermostat which was kept a t a temperature of 30') to see if spontaneous crystallization would take place. It was then allowed to stand for 2-3 hours and examined for crystals in A or B. If no crystals had separated A was inoculated,and the cell allowed to stay in the bath with frequent shakings, and B examined for crystals from time to time. By holding to the light the separation of even a fraction of a gram of crystalline matter can readily be observed. The above precautions in filling the cell with warm solution and allowing to stand before inoculating, are absolutely necessary owing Co the fact that these supersaturated solutions are so sensitive to the slightest amount of the solid phase. In working in a laboratory in which these substances have been ground, weighed out, etc., it is almost impossible t o keep the surfaces of the cell free from the solid phase but by the above precautions these difficulties can be readily controlled. Experiment 4.-Supersaturated solutions of potassium alum were used with membranes of parchment and inoculated as described above. The degree of supersaturation and the results obtained are recorded in Table I .
TABLEI Results of experiment 4, using supersaturated solutions of potassium alum. Temperature 30' in all cases Degree of supersaturation' Grams 1.2 2 .o
4.0
1 , ~
4.0 8.0
8.0 2.0 2.0
1
Membrane used
Parchment Parchment Parchment Parchment Parchment Parchment Collodion Collodion
1 1 1
I
Transmission of crystals through membrane
No No No No Yes Yes Yes Yes
I
Length of time in thermostat
3 days 3 days 24 hours 4 days 30 min. 30 min. I2 I
hours hour
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51
It is evident that there is a minimum limit to supersaturation beyond which crystallization is not transmitted through membranes. With collodion and a supersaturated solution of potassium alum crystallization was transmitted through the membrane. It is of interest to note that with the same concentration, using parchment, no crystals separated in B. Experiment 5 .--Experiments similar to 4 were carried out with solutions of sodium acetate supersaturated to various degrees. The results are shown in Table 2. With parchment, as in the case of the alum solution, there is a minimum limit of supersaturation beyond which crystallization will not pass through the membrane. TABLE2 Results of experiment 5 , using supersaturated solutions of sodium acetate. Temperature 30° in all cases -~ _--__--___--_-___ ____________ I
saturarionGrams I .2 2 .O
4.0 2 .O 2 .O
1
I
used
I
~
Parchment Parchment Collodion Collodion
thrdugh membrane
No
1 1
No No
1
in thermostat
24 hours 30 rnin. 3 days
7 days
4.0 4.0
With sodium acetate and collodion membranes it is evident that this limit is higher than with the parchment. That sodium acetate of greater supersaturation does pass through collodion was shown in experiment 3 in the preceding paper. To be perfectly sure that these collodion membranes which did not transmit crystallization were still permeable to solutions of salts, they were tested with potassium iodide solution. This was found t o pass through immediately-a Expressed as grams of substance in to saturate the solution a t 30'.
IOO cc
beyond the amount necessary
52
James H . Walton, J r
test for the iodide being obtained in the solution on the other side of the membrane. Moreover, in experiments in which crystallization did not pass through-on cooling the liquid in the cell a few degrees, the crystals separated in B a t once.
Conclusion Owing to the complex nature of membranes in general, it is not possible to state the direct cause of this lower limit at which crystals may be transmitted through membrane. It seems to the author, however, that the existence of this lower limit points to the fact that the solution within the membrane has a lower concentration than the solution which surrounds the membrane. According to this theory, with a strongly supersaturated solution crystals would be transmitted through the membrane. With a solution whose degree of supersaturation was equal to (or less than) the difference between the concentration of the solution in the membrane and that surrounding the membrane, no transmission of crystallization could occur. Such a lowering of the concentration of the solution within the membrane could be explained by the formation of a compound of membrane and solute. I n the membrane we would have, not only supersaturated solution, but also this compound (membrane + solute) in equilibrium with solution in the pores, and it is easy to conceive of a case in which the solution is supersaturated to such a slight extent that the amount taken up by the membrane leaves in the capillaries of the membrane a solution just saturated or even less than saturated. Such a solution in the membrane would of course not permit the transmission of crystals. Summary The results of the foregoing experiments may be summarized as follows : I. The crystallization of under cooled phosphorus cannot be transmitted through rubber nor gold beater’s skin. 2 . In the case of supersaturated aqueous solutions the transmission of crystallization through membranes occurs only
Crystallization through Membranes
53
in cases in which the membranes are permeable t o water. This has been demonstrated by supersaturated solutions of potassium alum, sodium acetate and lead acetate, with rubber in. thick. In these cases no transmembranes o.ooo~-o.ooo~ mission took place, even after several hours. Contact with the solid phase is necessary. A new method is described by which rubber membranes of the above thickness may easily be obtained. 3. A minimum limit of supersaturation exists, below which crystallization will no longer be transmitted through membranes which are permeable to water. This phenomenon will be the subject of further investigation. University o j Wisconsin
