T H E DEVITRIFICATION TEMPERATURES OF SOLUTIONS OF A CARBOHYDRATE SERIES B. J. LUYET Department of Biology, St. Louis University, St. Louis, Missouri Received March 4, 1939
Several liquids, when cooled a t a velocity of the order of some hundred degrees per second, become vitreous. The glasses so obtained devitrify, that is, crystallize, when upon warming they reach certain temperatures called the devitrification temperatures. The latter are, therefore, the temperatures at which a body pctsses from the vitreous to the crystalline state (2). Unlike the melting or the boiling temperatures, they are not sharply defined points but extend over a range, devitrification being slow a t the lower end of the range and gradually faster a t the upper end. The devitrification temperatures should therefore be expressed in the form of temperature-time relations. By use of the method of immersion of thin layers in liquid air for rapid cooling, the vitreous state was obtained in various aqueous solutions: gelatin, albumin, gums, sugars, inorganic salts, etc. (1). The investigation of the devitrification temperatures of such solutions was then begun, and it was observed that there is apparently a relation between these temperatures and the structural complexity'of the solute. The present report is concerned with that relation as shown in sugar solutions. Vitreous sugar solutions are transparent; when they devitrify they become intensely opaque. In polarized light, between crossed Nicols, the vitreous substance is dark; after devitrification it reestablishes light. To determine the temperature-time curve, the change in the opacity of the material in ordinary light was used as a criterion of devitrification. A small drop of the sugar solution to be studied was enclosed between two glass plates 0.1 mm. thick, maintained a t a distance of 0.1 mm. from each other by two glass strips of that thickness interposed between the plates. This preparation, held in a clip, was first immersed in liquid air for vitrification; it was then withdrawn and, after 8 sec. had been allowed for the evaporation of the liquid air carried along, it was immersed in a constant-temperature bath of isopentane contained in a transparent (unsilvered) Dewar flask. Isopentane was used on account of its immiscibility with water. The increase in opacity was observed against a 100watt lamp placed beyond two pieces of ground glass a t a distance of 25 881
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B. J. LDYET
cm. from the preparation. Five arbitrary degrees of opacity were distinguished by comparing the devitrifying material with an opacity scale made by depositing layers of various thicknesses of white paint on a piece of glass. For establishing the fifth degree-the only one which is concerned in this paper-the opacity of a preparation frozen a t a few degrees below zero was taken. The temperature was measured by an alcohol thermometer graduated to - 50°C. in fifths of degrees and recalibrated a t 0°C. against an Anschutz thermometer (with P.T.R. certificate). The readings were corrected for incomplete immersion of the stem after the correction values had been established by comparison of the incompletely immersed thermometer with another of the same make, completely immersed and itself recalibrated a t 0°C. The duration of devitrification was measured with a stop watch. The results obtained with glucose, C ~ H B O ~at, 2 M concentration, with sucrose, CI2H~OlI,at 2 M concentration, with raffinose, C ~ B ~ Z O ~ S . 5HzO at 1 M concentration, and with dextrin, (CsH1oO&, at 2 M / x concentration are graphed in figure 1. The products used were of the purest grade prepared by the Pfanstiehl Chemical Co. These sugars were chosen on account of the number of their carbon atoms, which is six or a multiple of six, but they are not entirely comparable otherwise: raffinose crystallizes with five molecules of water, the others are anhydrous; glucose has a free aldehydic group, the polysacharrides have none; the number of hydroxyl groups does not vary proportionally to the number of carbon atoms; glucose seems to possess only a sixmembered ring, while sucrose probably has a five-membered ring and a six-membered ring; etc. I n each of the carbohydrate solutions that were studied-and the same is true of many other solutions-the physical properties related to crystallization changed from one range of concentrations to another, there being three well-observed ranges: (1) A high concentration range in which the solution did not freeze a t any temperature, for example, 3 M sucrose; devitrification was then impossible. (2) A low concentration, 2 M for sucrose, at which the solution could be vitrified and devitrified. On devitrification, the preparation took on an amber color when observed in transmitted light. The opacity described above consists, then, in a deepening and darkening of the originally transparent, then translucent, amber layer. This appearance of the preparation and the gradual increase in opacity, which can well be observed in these conditions, are, no doubt, due to the fine grain of the crystals. (3) An intermediate concentration, 2.4 M for sucrose, in which the pattern of the devitrifying material is different from that just described; the amber color is absent, the increase in opacity does not take place with the smooth gradation of the less concentrated solution, and the crystalline structure seems to consist of a coarser grain. The temperatures a t which this type of devitrification
883
DEVITRIFICATION OF CARBOHYDRATE SOLUTIONS
-I
oo
8
-I2O
- 14
0
- I 6' - I a0 X
--222O 00
E
A
I
A
b b
2
3
e
b
4
MIN.
5
FIG.1. Plot of time against devitrification temperature. Curve A, 2M/zdextrin; curve B, 1M raffinose; curve C (unbroken line), 2M sucrose; curve C (dashed line), 1M sucrose; curve D, 2M glucose. The circles a t the right-hand end of each curve indicate that the solution was not devitrified in 5 min.
occurs are lower than those of the former type; for sucrose they are more than 15°C. lower.
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B. J. L W E T
I n the present work only the devitrification temperatures of solutions of the lower concentration range are compared. An attempt was made to use for that purpose the same molecular concentration for all the sugars, but this was impossible. While a concentration of 2 M was convenient for glucose and sucrose, it could not be used for raffinose which, at that concentration, is in the upper range and does not freeze a t any temperature. On the other hand, a concentration of 1 M was convenient for TABLE 1 Time for devitrification of sucrose solutions at various temperatures TEMPIBATVBI
1
TIME FOB DEYITBIFICATION OF 1 M BUCBWE
‘C.
Iemnda
-28.4 -26.8 -27.6 -27.9 -28.3 -29.0 -29.4 -30.0 -30.4 -30.8 -31.1 -31.5 -31.7
11 16 23 23 30 38 49 63 83 120 175 250
.d.
8 13 20 17 26 32 42 48 71 88 95 185 235
400
TABLE 2 Devitrification temperatures i n a standard time of five minutes LIUQAB BOLDTION
1
CONCDNTBATION
1
TEYPID8ATUPD
I
mYBDB OF ATOMS
00.
Glucose.. . . . . . . . , . . . . . . . . . . . . . . Sucrose.. . . . . . . . . . . . . . . . . . . . . . . (Sucrose.. , . . . . . . . . . . . . . . . . . . , . Ra5nose. . . . . . . . . . . . . . . . , . . . . . Dextrin., . . . . . . . . . . . . . , , . . . . . .
2M 2M 1M 1M 2M/x
-40.6 -31.8 -31.4 -27.2 -9.4
raffinose and sucrose, but glucose could not be vitrified at that concentration, since it froze on being immersed in liquid air. So the following were selected for study: 2 M glucose, 2 M sucrose, 1 M rdlinose, and 2 M / x dextrin. To render the comparison between raffiose and sucrose possible, a determination of the devitrification temperatures of 1 M sucrose was also made (the dashed line on the graph). There is a very slight difference in devitrification temperatures for a
DEVITRIFICATION O F CARBOHYDRATE SOLUTIONS
885
considerable change in concentration within a given concentration range. By mounting two drops, one of 1.4 M and the other of 1.6 M sucrose, on the same preparation and observing simultaneously the change in opacity, no difference could be observed between them. The same procedure, however, allowed one to detect a difference of about 0.4"C. in the devitrification temperatures of 1 M and 2 M sucrose. The figures obtained for the devitrification times of 1 M and 2 M sucrose a t various temperatures are given in table 1. The difference in opacity between the two concentrations was mostly noticeable during the first stages of the darkening of the amber-colored preparation; it became less a t the last stages. The higher concentrations caused a depression of the devitrification temperatures, a fact which perhaps is to be attributed to the same cause as the depression of the freezing point. Taking 5 min. m a standard devitrification time, the devitrification temperatures of the sugar solutions studied range as shown in table 2. According to preliminary results, solutions of substances of high molecular weight, like the gums, agar agar, and gelatin, hale a devitrification temperature in the neighborhood of - lO'C., while solutions of substances of low molecular weight, such &s formaldehyde, ethylene glycol, and glycerol, devitrify between -80" and -60°C. The various radicals exert a definitely different influence on the devitrification temperatures, and this influence must be studied before any attempt a t establishing a quantitative relation of more general nature can be made. REFERENCES (1) LUYET,B. J.: Bull. Am. Physical SOC.12, No. 7 , 7 (1937); 14, No. 2, 29 (1939). (2) TAMMANN, G.:The States of Aggregation, p. 275. Tramlated by R. F. Mehl. D. Van Nostrand Co., New York (1925).
