A Mechanism for Supercooling in Organic Liquids
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t. Res. Soc. Symp. Proc. Vol. 398 ©1996 Materials Research Society
the melting temperature of the chemical being tested. The temperature of the bath is slowly lowered until spontaneous freezing of the chemical occurs or the low temperature limit of the bath is reached (-30'C). The melting temperature of each chemical is obtained from published data. The degree of supercooling is calculated as the difference between the melting temperature and the spontaneous freezing temperature. The second method used to determine the degree of supercooling of selected chemicals is by differential scanning calorimetry (DSC). TA Instruments DSC test equipment and 0.25 inch x 0.25 inch x 0.012 inch glass slides (reference and sample pans) are used. A DSC sample having a weight of 2.5 ± 0.5 mg is used. The sample is heated to a temperature above its melting point and
A
B
C
P
O /0
CSCII
0
0
D
Diphenyl ether
••
E
Benzophenone
••
F
C
C
H H
0
Diphenylmethane
2-Benzoylpyridine
G
NH
C
N
IH
0
Di-2-Pyridyl ketone
4-Bromobenzophenone HH
~K
11 0 4-Methoxy-4'-methylbenzophenone
H
N
H
Benzalphthalide
-^-% S
N
B
P
L
0 N-Benzoylpiperidine
0
H
Br
Diphenylboron bromide
4-Methylbenzophenone
NH
N
M
0
HH.H
KH
JH
C-H
0I
0
CH
3-Benzoylpyridine
II
Br
H4--
Phenyl sulfoxide
Diphenylamine
s
R
0 c
/,, /, H H HH Benzyl sulfoxide
N
0
.•er
j
Diphenyl carbonate •
0\
/0
C-C HHHH Ethylene carbonate
S\
Figure 1. Experimental materials and their chemical structure. 88
/S
C-C HHHH Ethylene trithiocarbonate
then cooled at 20C/min. until spontaneous freezing occurs or a temperature of -100'C is reached. The sample is then heated at 5°C/min. to a temperature above its melting point and the cycle is repeated. The degree of supercooling is calculated as the difference between the melting temperature and the spontaneous freezing temperature. EXPERIMENTAL RESULTS Some organic liquids supercool more than others before spontaneous freezing occurs. Table I lists organic chemicals having a variety of structures (see Figure 1) along with their melting temperature, spontaneous freezing temperature, and degree of supercooling (spontaneous freezing data obtained by cooling the liquids in sealed glass tubes). Table II lists selected organic chemicals along with their melting and spontaneous freezing temperature, and their degree of supercooling (spontaneous freezing data obtained by DSC). The organic chemicals with a spontaneous freezing temperature of 147°C when cooled on a DSC slide. Diphenyl carbonate (Figure I C) has a structure differing from that of benzophenone in that an oxygen atom is positioned between each phenyl group and the carbon atom. With this structure, each phenyl - oxygen group rotates on the single bond to the carbon atom. The formation of a permanent pocket is unlikely due to the ability of this molecule to reconfigure its three-dimensional shape by rotation on the oxygen - carbon bond axis. Therefore, the oxygen atom projection has no permanent pocket to fit into. The ability to re
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