CRYSTAL1,IZATION THROUGH MEMBRANES BY JAMES H. WALTOK, JR.
In his “Studien iiber die Bildung und Uniwandlung fester K6rper” Ostwald’ has shown that upon adding very minute fragments of the solid phase to a super-saturated solution or an under-cooled liquid crystallization takes place immediately. Carefully performed experiments with super-saturated solutions of sodium chlorate, sodium potassium tartrate, barium chloride, borax, potassium alum and under-cooled salol, thymol, and sodium thiosulphate showed that there is a limit to the size of the crystal which will induce crystallization. This limit is very low, in the case of salol a crystal of 10-7 gram causes the under-cooled liquid to crystallize while a crystal of IO-^ gram causes no formation of crystals. IThile reviewing Ostwald’s work it occurred to the author to make the following experiment. Divide a supersaturated solution intu two parts by means of filter paper, and by introducing a crystal into one part allow crystallization t o take place. The object of this experiment mas to see whether or not the crystals which form in the pores of the filter paper are of sufficient size to cause crystallization to take place in the other part of the solution. To test this idea experiments were carried out with a supersaturated solution of sodium sulphate. This solution was of such a concentration that when crystallization took place the whole mass became solidified. A number of tubes were made by cutting off the closed end of an ordinary fiveinch test tube. Over the flanged end of the tubes were tied various kinds of filter paper-ordinary qualitative, quantitative and hardened. In this and in subsequent experiments care was taken to have the filter paper extend at least an inch from the flanged end of the tube. The paper was tied securely by means of strong linen thread. The tubes were Zeit. phys. Chem.,
22,
289 (189’;)
Crystallization through Membranes
491
then placed in a beaker containing the supersaturated solution, the lower end of the tube standing about a quarter of an inch below the surface of the liquid. Since the filter paper extended about an inch above the surface of the liquid in the beaker, danger of leakage taking place between the paper and the glass was prevented. Experiment 1.-~4 few cul-)iccentimeters of the supersaturated liquid were poured into the tube and after standing a few minutes a crystal of sodium sulphate was placed in the tube. Crystallization took place and in each case crystals formed in the beaker, starting from the lower side of the paper, thus showing conclusively that the crystallization had been transmitted through the filter paper. Since the crystallization took place through the filter paper with such ease, experiments were made in which this substance was replaced by some of the membranes which are ordinarily used in dialysis. For this purpose ordinary parchment paper, collodion and gold beaters’ skin were used. The parchment paper was the ordinary grade supplied by dealers. The collodion films were prepared according t o the method described by Bigelow.’ 4 solution of collodion was poured upon a surface of mercury which was about five inches in diameter. When the film was sufficiently hard t o be removed it was placed in distilled water and preserved for the experiment. In some of the experiments performed the membranes had been prepared but twenty minutes, in others they were several days old. The results obtained, however, were the same. The gold beaters’ skin was very thin, and was of the grade supplied by dealers in chemical apparatus. In these experiments great difficulty was experienced in preventing premature crystallization of the supersaturated solutions. This is especially true after one has worked with the same substance for two or three days. The air becomes filled with the crystal dust to such an extent that a super-
’ Jour.
Am. Chem. SOC., 29, 1 6 7 j (1907)
#
492
James H . Walton, J r .
saturated solution can be preserved in an open vessel for only a very few minutes. The spreading of dust will be prevented in a large measure if care is taken after working with the solutions, not to rub the hands on a towel but t o wash them thoroughly in running water. Pipettes and similar apparatus used in working with the aqueous solutions should be kept in a dish of water to prevent evaporation of liquid which may have been left on the surface with subsequent spreading of dust. The solutions are best prepared and preserved in ordinary 2 5 0 cc. flasks, the necks of which have been sealed by drawing them out in a flame. This permits the outside of the flask to be washed free from any dust. To remove the solution it is best to take the flask into a different room from that in which the solution was prepared, open it by breaking off the sealed tip and remove the liquid by means of a pipette. The flask is best closed by means of a cap made of absorbent cotton. The method of procedure in these experiments was as follows: The supersaturated solution was placed in a testtube about four inches long, and the membrane tied securely over the mouth of the tube. In the case of undercooled liquids a few crj-stals were placed in the tube, they were melted and the tube closed with the membrane. The tube was then inverted and allowed to rest upon the bottom of a small beaker containing enough of the supersaturated solution to fill it to the depth of about one quarter of an inch. &4fterstanding for a few minutes the liquid in the beaker mas inoculated with a crystal. Experiment 2.-A supersaturated solution of sodium sulphate was used, the following results being obtained. : Gold beaters’ skin. Crystallization took place in the tube in less than one minute after the solution in the beaker was inoculated. Parch.ment paper. Crystallization in the tube in about one minute. Collodion. In several experiments the length of time necessary for crystallization varied from 15 to 30 minutes.
Crystallization through. Membranes
493
In order to see whether or not the phenomenon noted in this experiment is specific, or whether the crystals of other supersaturated solutions can be transmitted through membranes, the following experiments were tried : Experiment 3.--A supersaturated solution of borax was prepared and an experiment similar to the above was carried out, using the same kinds of membranes. The results obtained were similar to those of Experiment 2 , the times necessary for the appearance of crystals inside the tubes being as follows: Gold beaters' skin... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Collodion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parchment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I ~
3 minutes IO IO
''
Experiment +--At high temperatures sodium acetate is very soluble in water and upon cooling forms a supersaturated solution. Upon making experiments similar to the foregoing, using this salt, it was found that the transmission of crystals occurred with the three different membranes in less than one minute after the liquids in the beakers had been inoculated. Besides the three membranes mentioned above, a rubber membrane of approximately 0.005 inch thickness, similar to that employed by Kahlenberg' in his experiments on osmosis, was also used with a supersaturated solution of sodium acetate. KO crystals separated on the inside of the tube, even after it had stood for several days, the outside of the rubber being in contact with sodium acetate crystals during the whole time. The transmission of crystals through rubber was further tested by using a supersaturated solution of potassium alum, this substance, as was shown by Ostwald, being extremely sensitive to the slightest trace of the solid phase. In fact it is so sensitive that the preparation of a supersaturated solution was more difficult than any other solution used in these experiments. This was accomplished by preparing a Jour. Phys. Chem ,
IO,
141 (1906).
494
James
H . Waltoiz, J r .
saturated solution in a flask, heating it to boiling and filtering it into a test tube. The tube was then closed with the rubber membrane, and the hot liquid moved back and forth to dissolve any particles which might adhere to the side of the tube. A solution prepared in this way was tested as in the case of the experiments with sodium acetate and the rubber membrane. It stood for several days without crystallizing. Further experiments with the rubber membrane were made as follows: A supersaturated solution was placed in a strong glass distilling flask of about 2 0 0 cc capacity, and a piece of rubber tied firmly over the mouth of the flask. -4 piece of rubber tube provided with a strong pinch clamp was placed over the side arm of the flask. By blowing into the flask the rubber over the neck could be blown out into a spherical form and on closing the clamp the rubber remained distended. From the normal thickness of the rubber and the diameter of the mouth of the flask it was possible to calculate the thickness of the expanded rubber. This was found t o be 0.0003-0.0004 inch. The flask was inverted so that the supersaturated solution could run down inside the rubber balloon and crystallization started on the outside of the balloon. Using a supersaturated solution of sodium acetate the following results were observed in eight experiments : In five cases crystals appeared to be transmitted through the membrane, the time varying from ten minutes to twelve hours. In the other cases crystallization did not occur. In one case undercooled sodium acetate prepared by melting the salt in its crystal water was used instead of the supersaturated solution, and stood for six days under conditions similar to the foregoing without crystallization taking place. In this experiment, as in some of the others it was noticed that if the rubber was pressed even very gently the crystals began to separate from the solution a t once. In two experiments with supersaturated potassium alum solutions crystallization also appeared t o be transmitted through the rubber, the time in one case being about thirty minutes, in the other the crystals separating during the night,
Crystallization through Ad embranes
495
In the above experiments with the thin membranes the rubber was stretched to the point just short of disruption. Several of the rubber membranes through which the crystallization had been transmitted were tested for holes by blowing them out and placing on one side a solution of ferric chloride, on the other potassium sulphocyanate solution. No coloration of the sulphocyanate occurred, even after standing for several hours. Similarly some of the membranes were tested with a solution of potassium iodide on one side and water on the other. The water was tested for iodine by acidifying, adding hydrogen peroxide, then starch solution. Here, as in the previous case, negative results were obtained. Experimeizt 6.--1 supersaturated solution of sodium thiosulphate was used, the results obtained being similar to those with the borax and sodium sulphate, only in this case it took longer for the crystals to appear inside the tube, as is shown below. Parchment. Collodion Gold beaters' skin
I 1
20 Ij
I
minutes "
to
2
hours in different experiments
minute
Experiment 7.-m7ith the exception of the alum, the salts used in the foregoing experiments were those of sodium. It was of interest t o see whether or not the salts of heavier metals would show a difference of behavior. Lead acetate easily forms a supersaturated solution and was used for this purpose. n'ith the gold beaters' skin the results were similar to those already obtained. In the case of the parchment and also the collodion several hours elapsed before crystals separated on the inside of the tube. It was noticed that the length of time of transmission increased considerably with the thickness of the collodion. It seemed probable that this was due to the fact that the collodion and parchment were saturated with water, and before the crystallization could occur in the tube it was necessary to replace this water with
496
James H . Walton, J r .
the supersaturated solution. This idea was tested as follows : Tubes covered with collodion membranes and containing supersaturated solutions of lead acetate were allowed to stand in beakers of supersaturated solution for fifteen hours, so that the membrane would have an opportunity t o become thoroughly saturated with the solution. At the end of that time the solution in the beakers was inoculated and inside of three minutes crystals appeared in the tubes. The extreme length of time necessary for transmission of the crystals was therefore due primarily to the slowness of diffusion of the lead acetate.
Undercooled Liquids
X large number of substances, particularly organic compounds, easily form undercooled liquids. It was interesting to see if their behavior towards membranes was similar to the supersaturated solutions. Experiment ??.-Water was cooled to -4' by placing it in a small U-tube arranged as shown in the figure. This was made by cutting off one arm of the LT-tube, placing a membrane over the short arm and slipping over the membrane a piece of tube with diameter slightly larger than that of the U-tube. The first experiment was made with gold beaters' skin. The tube was placed A in the bath and after its contents had cooled to the temperature of the surrounding liquid a fragment of ice was introduced into the tube a t A. The liquid crystallized a t once, and in less than one minute crystals appeared in the lower part of the U-tube, spreading from the bottom of the membrane in the direction of the arrow. The results Fig. were confirmed by repeating the experiment. Similar results were obtained when collodion was used. In five experiments the time in which the crystals appeared on the lower side of the collodion membrane varied from two to five minutes.
M
Crystallization through hfembranes
497
The experiments were repeated using a rubber membrane but no crystallization took place in the lower tube. Experiment 9.-Thymol was melted and then allowed to become Undercooled. X number of experiments were made with this substance, the method used being exactly the same as in the experiments with sodium acetate. The membranes used were collodion, which had been allowed to dry without wetting, gold beaters' skin and rubber. In each case the crystallization was transmitted through the membrane, although the time necessary varied considerably with the different membranes. Rubber.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Collodion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gold beaters' skin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
I j-30
IO
minutes minutes
The rubber membrane was removed after the experiment and washed with alcohol. Upon examination it was found t o be intact. Bn undercooled solution of salol gave similar results with rubber, the crystals appearing in twelve minutes. Acetophenone was used in addition to the above undercooled liquids, the time necessary for the appearance of crystals being as follo?i.s : -~
,
~~
Collodion Rubber
z minutes
I37
( (
Exfierinzent io.-Phenol melts at approximately 43 ', but may be undercooled to about z d " , at which temperature it solidifies. The range of temperatures through which the undercooling may take place may be widened by adding a little water to the phenol. To about j cc of the liquid phenol a few drops of water were added and experiments made with parchment and rubber membranes. ITTth the parchment Moore Zeit phys Chern ,
12, j l j
(1893)
498
James H . Walton, J r .
the crystals appeared in the tube in seven minutes, with the rubber no separation of crystals took place. In several experiments which were carried out with the rubber membranes the liquid in the tube remained unchanged for several days. As soon as the rubber was removed and crystals added the separation of crystals occurred, showing that the solution was still undercooled. This experiment was repeated, a few drops of benzene being added to the liquid phenol instead of the water. Crystals formed in the tube, the time in several experiments varying from two to seven minutes. Experiment 11.-Frog’s skin and the membrane which surrounds an egg were used with a supersaturated solution of sodium sulphate, the experiments being carried out similarly to those already described. The skin was removed from the frog and the experiment made within seven minutes from the time the frog was killed. The results were similar to those obtained when sodium sulphate and gold beaters’ skin were used. It made no difference whether the crystallization was started from the inside or the outside of the frog skin, the transmission of crystals appeared to take place equally well in either direction. The white and yolk of an egg were carefully removed, leaving the lower part of the membrane unbroken. X small portion of the shell was removed from the bottom of the egg. The egg was then filled with a supersaturated sodium sulphate solution and placed in a beaker containing some of the same solution. In different experiments crystallization was started in the egg and in the beaker respectively. In each case, however, the crystals appeared almost instantaneously on the opposite side of the membrane, showing that the crystals are transmitted equally well in either direction.
Summary and Conclusions The results obtained in these experiments are collected in the following tables :
Crystallization through Membranes
499
TABLEI -___
Supersaturated solutions __
Parchment
Collodion
I
Sodium sulphate.. . . Sodium acetate . . . . . Borax.. . . . . . . . . . . . . . Sodium thiosulphate Potassium alum.. . . . Lead a c e t a t e . . . . . . . .
1
'
15-30 min. I min. I min. I min. IO rnin. ' 3 min. 15 m.-2 hrs. I min. ~
-
1
~
i
min.' min. min. min.
I I ' IO 20
_ _ ~ ~ _ _ _ _ _ _ Gold Rubber beaters' o 005 i n c h e s skin thick
'
,
_-
-
1-3 hrs.
3 m.-14 hrs.1
min.
I
TABLE2 Undercooled liquids Parchment
IVater, . . . . . . . . . . . . . . Thymol . . . . . . . . . . . . . Salol . . . . . . . . . . . . . . . . Acetophenone.. . . . . . Phenol water., . . . Phenol T benzene..
+
-
-
-
7 min. -
Gold beaters' skin
Collodion
i
i I
min. 75 min.
,
-
I
-
!
2-j
min.
~
I IO
-
Rubber inches thick
0.005
min. no min. 1j-30 min. 1 2 min. 37 min.
L
no 2-7
min.
With the supersaturated aqueous solution it will be seen that wherever water alone can pass through the membrane it is also possible for crystallization to be transmitted. With the expanded rubber an apparent exception to this general rule has been found, but here it must be remembered that the membrane is under abnormal conditions, being subjected to a strain just short of the amount necessary to cause its disruption. Under these conditions the possibility of a crystal being mechanically forced through the membrane would appear far from remote. It would seem as if such a mechanical action would leave an opening which, for the time being, might be closed by a crystal. By washing, however, the crystal would be dissolved and a solution like ferric Time necessary in each case for transmission of crystals through the respective membranes.
j00
James H . W a l l o n , J r .
chloride or potassium iodide should be able to pass through this opening. It has already been shown that this is not the case. No definite conclusions as to the behavior of these extremely thin membranes can be made at this timp, the whole matter will be made the subject of further investigation. In the case of the undercooled organic liquids it is seen that crystallization takes place via the rubber membrane. These are all liquids which are similar in nature to rubber, they consequently would be expected to saturate the rubber and permit the transmission of crystallization in a manner analogous to the saturation and transmission of parchment or collodion by sodium acetate solution. This is also substantiated by the results obtained with phenol. A small amount of water added to the phenol prevents the saturation of the rubber membrane and consequently the transmission of the crystallization. If the water is replaced by benzene, however, the transmission occurs as with the salol and thymol. L’nzrerszty 01 Ii-zscoitsui
