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J. Phys. Chem. 1903, 87, 4034-4037
the drop which becomes active as nucleators between 0 "C and T, i.e., n&T) = SotJ(t)dt. The stochastic hypothesis seems to fit best the results obtained.13 But then, the S shape need not necessarily indicate a heterogeneous nucleation process since a homogeneous nucleation process for which the nucleation rate J has not reached its stationary value also gives an Sshaped crystallization curve. In this case it is better to consider the memory and the precooling effects as the distinguishing features of the nucleation process. Precooling experiments on water emulsions inseminated with AgI are therefore now undertaken in order to see if the precooling effect is well correlated with a heterogeneous nucleation process. Considering a heterogeneous nucleation process, the problem which remains to be solved is how this process may occur within the emulsion maintained a t a fixed subzero temperature. It has already been shown that the
behavior described above is not specific to water emulsions. For instance, it has been observed in benzene emulsion^'^ as well and also in molten hydrate salts dispersed within the same carrier as water.20 A better knowledge of the structure of the w/o emulsions is needed, specially about the water-oil-surfactant interface. Some experimental work in this area has already been undertaken6 Acknowledgment. The authors are indebted to C. A. Angell, F. Franks, P. H. E. Meijer, B. Mutaftschiev, and D. H. Rasmussen for profitable discussions. Registry No. Water, 7732-18-5;AgI, 7783-96-2. (19) F. Broto, D. Clausse, and J. P. Dumas, Proceedings of the JournBes de CalorimBtrie et d'Analyse Thermique, Vol. VII, Besangon, May 1976. (20) B. Combes, L. Babin, and D. Clausse, SBminaire "Stockage thermique et sa modBlisation"La Bade, France, 1982. Published in Reu. G6nBrale Thermiq., special issue 254, 209-13 (1983).
Dielectric Study of Dispersed Ice Microcrystals by the Depolarization Thermocurrent Technique P. Pissls,' L. Apekls, C. Chrlstodoulldes, and G. Boudourls NaNonal Technical University, Physics Laboratory A, Zografou Campus, Athens 624, Greece (Received: August 23, 1982; In Final Form: November 30, 1982)
Dispersions of ice microcrystals obtained from the breakdown of water-in-oil emulsions were investigated by means of the depolarization thermocurrent (DTC) technique in the temperature range 85-250 K. Two predominant peaks were observed at temperatures of about 140 and 225 K. The low-temperature DTC peak at 140 K was studied extensively with different kinds of electrodes. Its position and shape were found to change in the course of time. The characteristics of the low-temperature DTC peak in dispersionsof ice microcrystals at advanced states of evolution with time are discussed in relation to those of the low-temperature DTC in macroscopic pure ice. Our results provided more evidence that the low-temperature DTC peak in dispersions of ice microcrystals is due to dipolar absorption in ice.
Introduction This paper deals with depolarization thermocurrent (DTC) measurements on dispersions of ice microcrystals in oil in the tempeature range 85-250 K. When waterin-oil (W/O) emulsions with water globules a few microns in diameter are cooled, the water globules supercool to an extent of about 40 OC.' From the breakdown of supercooling, dispersions of ice microcrystals are obtained, The dielectric study of such dispersions is of interest due to the practical interest in emulsions. On the other hand, the comparison of the dielectric behavior of dispersions of ice microcrystals with that of macroscopic ice samples (of a few mm3 or more) could give us information useful to the understanding of the dielectric behavior of ice a t the molecular level. In a previous paper2 we reported, for the first time, on DTC measurements in dispersions of ice microcrystals in oil in the temperature range 85-250 K. With Au electrodes, two predominant peaks were observed at temperatures of about 140 and 225 K. The low-temperature DTC peak at 140 K was studied extensively. We could show (1) J. Lachaise and M. Clausse, J . Phys. D,8, 1227 (1975). (2) P. Pissis, L. Apekis, C. Christodoulides, and G. Boudouris, Z. Nuturforsch. A , 37, 1000 (1982).
that this peak and the dielectric absorption observed in ice emulsions in the kilohertz frequency range by many investigators3+ using ac methods are due to the same relaxation mechanism. Moreover, based on our own and existing experimental results on ice emulsions, macroscopic pure ice, and macroscopic HF-doped ice and taking into account an explanation given by Johari and Whalley7 for the temperature dependence of the activation energy W of the dielectric absorption in macroscopic pure and HFdoped ice, we could explain for the first time, why, although the observed dielectric absorption in ice emulsions is generally attributed to dipolar absorption in ice, the emulsion activation energy is so much lower than that of macroscopic pure ice a t temperatures higher than about -50 "C: Due to supercooling breakdown the concentration of extrinsic physical defects in ice emulsions is very high, so that extrinsically generated orientational defects dominate over the intrinsically generated ones in the whole temperature range, with the result that the activation (3) I. D. Chapman, J . Phys. Chem., 72, 33 (1968). (4) G. Evrard in "Physics and Chemistry of Ice", Whalley, Jones, and Gold, Ed., Royal Society of Canada, Ottawa, 1973, p 199. (5) B. Lagourette, J . Phys., 37, 945 (1976). (6) C. Boned, B. Lagourette, and M. Clauase, J . Glaciol., 22, 145 (1979). ( 7 ) G. P.Johari and E. Whalley, J . Chem. Phys., 75, 1333 (1981).
0022-3654/83/2087-4034$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 27, 1983 4035
Dielectric Study of Dispersed Ice Microcrystals
TABLE I : Peak Temperature TM,Activation Energy W, and Contribution t o the Static Permittivity A e , of the Low-Temperature DTC Peak in Ice Emulsions and Macroscopic Ice ( @ = l.OO)a brass electrodes
gold electrodes TM, @
K
0.40b O.4Oc 0.65b l.OOb
142 121 145 126
w,
eV
TM, A€
6.5 0.31 6.2 0.23 0.33 14.3 0.24 1 7 4
K
140 119 144 121
w,
eV
Ae
0.31 3.7 0.22 3.4 0.32 8.8 0.25 3 1
1
Ill
