T H E VELOCITY O F T H E CRYSTALLIZATION OF UNDERCOOLED WATER BY JAMES H. WALTON, JR., AND ROY C. JUDD
Although the velocity of crystallization of many undercooled liquids has been measured, comparatively little work has been done on the speed with which undercooled water changes to the solid phase. Turmlirzl measured the velocity with which this substance crystallizes, using thin-walled tubes of I 8 mm internal diameter. These tubes were filled with water and placed in a bath which was then cooled by exposure in a cold room. When a crystal of ice was brought in contact with the undercooled liquid, crystallization took place. The velocity could easily be determined by measuring the time necessary for the advancing surface of the solid phase to travel a given distance. Turmlirz's results are given in Table I, in which t is the number of degrees of undercooling, and V is the velocity of linear crystallization expressed in mm per second TABLEI _____________ ___
t .-__-I
-0.74 -1.12
, I
1
-1.40 -1.54 -I .62
1
-2.00
1
-2.40 -2.54 -2.67
~
1
-___
v
-
I
t
-
V
I
0.37 I .44 2.20
2.76 2.92 3.32 4.49 5 *24 5.58
-2.71 -2 * go -3.20 -3.49 -3 64 -4.14 -4.20 -4.60 9
-
i
'
1
'
5.77 7.06 7.47 10.23 11.28 16.93 18.15
I -
22.07
Because of spontaneous crystallization, Turmlirz was unable to make measurements below -4.6 '. In the light of the interesting and important work of Tammann,2 Marc, Sitzungsber. Wiener Akad., 103 IIa, 226 (1894). Kristallisieren und Schmelzen, p. 131 (1903). 3Zeit. phys. Chem., 61, 385 (1908);67, 470; 68, 104 (1909);73, 685 (1910);75, 7 1 0 (1911).
Crystallization of Undercooled Water
723
Freundlich,l and others, on this subject it seemed desirable to determine the speed of crystallization of water with special reference to factors affecting it, particularly the influence of dissolved substances. Experimental The usual method for measuring the speed of crystallization is to undercool the substance in a tube, and at a definite temperature to cause crystallization by introducing a fragment of the solid phase. The time necessary for the crystals to fill a certain length of the tube gives the linear velocity of crystallization. To undercool water more than two or three degrees, special precautions must be taken. The water must be pure, free from dust and air, and covered with a layer of oil. Paraffine oil may be used for this purpose, although Turmlirz used turpentine in his experiments. The volume of water used should be as small as possible. Tammann2 has shown that spontaneous crystallization is accompanied by the formation of crystalline nuclei throughout the. solution. The greater the volume of liquid used the greater is the chance of these nuclei being formed. The experiments of the authors show that with the same volume of water, undercooling is more easily carried out in a flask than in a tube; the water can be cooled to a lower temperature, and there seems to be less danger of spontaneous crystallization. In determining the velocity of crystallization of undercooled water, the following method was used: A soft-glass tube, similar to that shown in Fig. I , was filled to the level A
Fig.
I
with freshly boiled conductivity water. Paraffine oil that had been carefully washed was then placed in each arm of the 1
Zed. phys. Chem., 75, 245 (1910). Kristallisieren und Schmelzen, p. 148 (1903).
724
James H . Walton, Jr., and Roy C. Judd
.
tube so that there was a layer about an inch deep on the surface of the water. The tube was then placed through an opening in the cover of the thermostat described in the paper preceding this, and allowed to come to the temperature of the bath. This took at least three minutes. At the end of this time crystallization was started in the tube by scratching the side of the tube just below the layer of oil by means of a piece of wire. The time necessary for the crystalline surface to travel from B to C ( I meter in distance) was measured with a stop watch. Several measurements were made with the same tube full of water, and it was found that this gave just the same results as were obtained by filling the tubes each time an experiment was carried out. Results obtained by using tubes in which the water had been standing for several weeks, however, diverged greatly from those obtained with freshly filled tubes. This was doubtless due to the glass dissolved by the water on standing. An unsuccessful attempt was made to measure the velocity of crystallization in tubes with very thin walls. Spontaneous crystallization occurred much more frequently than with thickwalled tubes that permitted the water to cool slowly. The tubes used in the first experiments had a bore of 7 mm and an outside diameter of 12 mm. The velocity of crystallization was measured to -go. Below this temperature the results were uncertain, owing to spontaneous crystallization taking place before the tube was cooled to the temperature of the bath. The results obtained are given in Table I1 and shown graphically in Pig. 2, Curve I, in which the velocity is expressed as centimeters per minute. .Experiments with tubes of different bores and different thickness of walls were carried out for the purpose of showing the influence of these factors on the speed of crystallization of water. Tables I11 and IV give the dikmeter of the tube and the results obtained. These results are also shown graphically in Fig. 2, Curves I1 and 111. Tammann has shown that for undercooled liquids the velocity of crystallization increases with the degree of undercooling until a maximum is reached below which temperature
Crystallization of Undercooled Water
725
TABLE I1 (CURVEI) Velocity of crystallization of water Outside diameter of tube 12 mm Inside diameter of tube 7 mm Temp.
Time for I meter
-2.00
194.0 186.0
-3.61
124.4
Average
Velocity of linear crystallization cm per minute
190.0
31.6
124.0
48.4
84.2
71.4
56.0
107.I
52.3
114.7
22.5
266.7
19.45
308.0,
14.45
415 * 2
11.7
513* o
124.7 123.2 122.I
-4.67
-5.86
-6.18 -7. IO -7.50 -8.19 -8.38
127.0 127.2 I22 .o 85 .o 86.3 83 .o 85.1 82.6 82.5 85.2 56.I 54.0 53.9 56.8 57.0 57.0 57.0 53.1 51.4 22.0
23 .o 22.5 19.0 19.9 14.6 14.3 I2 .o 10.3 11.5 12.7 12.0
-9.07
8.8 8.8 8.7
8.77
684.0
726
James H . Walton, Jr., and Roy C. Judd
there is, at least for several degrees, no relation between the speed of crystallization and the temperature of the bath. Continued cooling of the bath results in decreasing the speed of crystallization. This phenomenon is explained as follows : At temperatures in the neighborhood of the freezing point of the solution, the velocity of crystallization is retarded by the heat of crystallization, which heats the layer of liquid that is in contact with the growing crystalline surface. The velocity with which this layer is cooled by the outside bath is one of the factors in determining the speed of crystalliza-
Fig.
2
tion. The greater the number of degrees the liquid is undercooled, the more quickly will this layer take on the temperature of the outside bath, consequently the less will the effect of the heat of crystallization be felt. This will result in the speed of crystallization increasing with the undercooling, up t o a point at which the above effect will just neutralize the heat of crystallization. The layer will then have the temperature of the melting point of the substance, and this point will be the maximum velocity of crystallization. From the above consideration it is evident that the size of the bore and the thickness of the walls of the crystallizing tubes are important factors in the velocity of crystallization,
Crystallization of Undercooled Water
727
TABLE I11 (CURVE11) Velbcity of crystallization of water Outside diameter of tube 14mm Inside diameter of tube 1 1 mm
' Velocity of linear crystallization I
Temp.
1 1
Time for I meter
16.0
' *
cm per minute
25.3 16.4
-7.65
-9
Average
15.70
25.5
235.3
15.9
377.8
548.4
~
I
92 12.50
Temp.
10.90
1
I
I0.jO
I
10.94
i
Time for I meter
I
Average
~
4 .IO
-6.60
-7.58
-8.58
41.6 42.1 41.4 42 .o 41.5 39.5 40.7 39.1 38.1 31.o 31 .o 30.8 31.I 25.9 26.0 25 .o
I
-1-
39.6
31.o 25.6
1
Velocity of linear crystallization cm per minute _.
728
James H . Walton, Jr., and R o y C. Judd
inasmuch as they determine the rate of the cooling of the crystallizing liquid. The results obtained in these experiments, using tubes of different diameters, is thus explained. This the underdoes not, however, explain the fact that a t -6' cooled water represented by Curve I11 crystallizes faster the reverse is true. than that in tube I, while a t -8' An inspection of Curve I shows that the point of maximum crystallization lies below the temperatures at which these experiments were carried out. Many attempts were made to determine the velocity of crystallization a t -15 O , but the liquid always crystallized spontaneously before the tube was cooled to the temperature of the bath. Crystallizing tubes were prepared by filling with pure boiling water, evacuating, and then sealing, but they could not be undercooled any lower than those prepared in the manner already described. These experiments are being continued, the particular line of investigation being the effect of dissolved substances upon the velocity of crystallization of water.
Summary I . The linear velocity of crystallization of undercooled water has been determined to -9'. 2. The thickness of the wall of the tube used in these determinations is an important factor in determining the speed of crystallization. 3. Because of spontaneous crystallization it was impossible to determine the temperature of the maximum velocity .of crystallization of water.
Department of Chemistry University of Wisconsin
