STCDIES OX GL-ISS
I. The Transition between the Glassy and Liquid States in the Case of Some Simple Organic Conipounds BY GEORGE S. PARKS A S D HUGH
31. HUFFJIAS
This paper is the first of a series dealing with the nature of the glassy state. In it we are presenting some data which we have recently obtained i n a study of the transition between the glassy and liquid states in the cases of normal propyl alcohol and propylene glycol. For a more complete consideration of the phenomenon involved we have also reviewed some recent investigations on ethyl alcohol and glycerol. In the past a glass has been generally considered to be simply a n undercooled liquid. As a consequence, the transition between the glassy and liquid states has been imagined as gradual and continuous. Thus, for instance, Sernst’ has stated regarding a glass that “externally it has the properties of a solid, owing to great viscosity and considerable rigidity, produced by strong mutual action of the molecules. An amorphous body differs from a crystal, however, in its complete isotropy and absence of a melting point; on heating, it passes continuously from the amorphous to the usual liquid state, as its properties show steady change with rise of temperature, and no breaks anywhere.” Tammann’s view is essentially similar, as the following quotation* will indicate. “The viscosity of a liquid increases with increasing undercooling, and in a rather narrow temperature interval it increases very rapidly to values characteristic of solid crystals. X brittle glass is thus formed from an easily mobile liquid. This change in viscosity does not correspond to the behavior of the other properties, which in this temperature interval change relatively only slightly. The change in viscosity is a continuous one and no temperature can be chosen as the freezing-point, the point a t which the liquid becomes solid. Glasses are undercooled lzquzds.” By our own studies upon the transition of organic glasses into the corresponding liquids we have been led to a somewhat different conclusion. Khile there is no definite temperature, comparable to the melting point of a crystal, at which all properties undergo a sharp change, there is nevertheless a temperature interval, definite and reproducible, in which a number of properties change with a rapidity approaching that observed in the case of the melting process of a crystal. I n brief, there is a softening region instead of a melting point. The glass as it exists below this softening region differs so markedly from the liquid existing a b o x that it might well be considered as a different state of the substance. For this reason we have recently suggested the 1 ?
Sernst (translation by Tiaard): “Theoretical Chemistry,” 94 (191I ) . Tammann (Mehl): “The States of Aggregation,” 3 (1925).
STUDIES ON GLASS
I843
possibility of regarding glass as a fourth state of matter,’ distinct from both the liquid and crystalline states and yet showing to some extent characteristics of both these states. X promising alternative hypothesis involves the view that it is a colloid very much analogous to a jelly. According to this last explanation the softening region represents the temperature range through which the jelly changes into an associated liquid. h t any rate, whatever be the true explanation, we feel that the data which we consider in the following sections of this paper are indicative of a distinct and fundamental difference between the glassy and liquid states. Heat Capacity Data hIany organic compounds, especially those containing one or more hydroxyl groups such as the alcohols and sugars, can be converted from the liquid to the glassy condition by cooling. In some cases, that of ethyl alcohol for instance, the cooling process must be carried on rapidly by means of liquid air to prevent crystallization. In other cases the glass appears to be extremely stable and can be formed either by slowly cooling or rapidly oooling, as suits the desire of the investigator; propylene glycol is an instance of the latter type. When a simple alcohol glass, such as that of glycerol, is heated its heat capacity at first increases gradually with the temperature in much the same fashion as in the case of the crystalline form. Eventually, however, a temperature region is reached in which the specific heat of the material is almost doubled within an interval of only IO’. h maximum point is then attained and thence the heat capacity drops off slightly with increasing temperature to give the curve for the liquid state. We thus find two distinct, continuous specific heat-temperature curves, one for the glass essentially similar to that for the crystalline matcrial and one for the liquid. Between the two there is a transitional region existing over a temperature interval of IO’ to 20’. This phenomenon was first studied by Gibson, Parks and Latime? in the case of the glasses of ethyl and n-propyl alcohols. Later a similar behavior was observed separately by Simon3and by Gibson and Giauque‘ for glycerol glass. More recently Parks5 has repeated the work in the case of ethyl alcohol. In the present investigation we have obtained new results for npropyl alcohol and also fairly extensive data on propylene glycol. Thus we now have available specific heat data for four related compounds in the glassy state. dfaterzals-The n-propyl alcohol employed in the present investigation was prepared from Eastman’s “refined” product by treatment and distillation with lime i n the ordinary manner. The resulting material was carefully fractionated and the middle third was selected for use in the measurements. I(had a boiling range of 0.05’and a density of o.801o at 25”4O.
’ Parks and Huffman: Science, 64,364 (19261. 2
4 5
Gibson, Parks and Latimer: J. Am. Chem. Soc.. 42. 1547 (Igzoi, Simon: Ann. Physik, 68,260 (1922). Gibson and Giauque: J. Am. Chem. Soc.. 45,93 (19231. Parks: J. Am. Chem. Soc., 47,341 (~gzj).
GEORGE S . PARKS AND HUGH M. HUFFMAN
I844
The propylene glycol was a high-grade material, obtained from the Special Chemicals Company. It was carefully fractionated by distillation a t a pressure of 4 mm. and a temperature of about 73' C. The resulting middle fraction, which had a density of 1.0493at 25.8'/4', was used for the present work. Method-In principle, the method of Nernst was employed with an aneroid calorimeter in determining the "true" or instantaneous specific heats. A measured amount of heat was supplied by a n electric current to the substance contained in a copper calorimeter, which was suspended in a vacuum and surrounded by a silvered copper cylinder in order t o diminish the conduction and radiation of heat to and from the surroundings. h thermocouple in the center of the calorimeter measured the rise in temperature. The entire apparatus and details of experimental procedure have been fully described in other places.' I n view of the accuracy of the various measurements involved, the error in the experimental values thereby obtained is probably less than IY;,except in the transition interval. In this particular region the errom may be considerably larger than this value, owing to uncertainty in the attainment of thermal equilibrium and also to incipient crystallization in the case of the propyl alcohol. However, all cooling corrections were made in a uniform manner and hence the specific heat results should be comparable throughout, altho there may be mme uncertainty as to the absolute values.
TABLE I Specific Heats of Ethyl Alcohol Glass and Liquid Temp., OK.
C p per gram
86.8 87.5 88.1 90.8 91.9 95.8 97.5
0.264 0.266 0.263 0.296 0.301 0.420
100.5
0.443 0.423
105.j
0.399
Temp., "K.
C p per gram
0.415 0,455 0.468 0,496
110.2
160.0 200.0
240.0 2jo.o 27j.o
0.50;
0.543
290.0
0.j 7 2
298.0
0.j88
TABLEI1 Specific Heats of X-Propyl Alcohol Glass and Liquid Temp., "I
