The Journal of
Physical Chemistry
Registered in U.S. Patent Office
0 Copyright, 1981, by the American Chemical Society
VOLUME 85, NUMBER 11
MAY 28, 1981
LETTERS Homogeneous Nucleation and Glass Transition Temperatures in Solutions of Li Salts in D 2 0 and H20. Doubly Unstable Glass Regions C. A. Angell," E. J. Sare, J. Donnella, and D. R. MacFarlane Department of Chemistry. Purdue University, West Lafayelte, Indiana 47907 (Received: January 28, 198 1; In Final Form: April 10, 1981)
Various lithium salt solutions in water and DzO have been studied to determine the regions of composition in which bulk samples can be vitrified during cooling, and also those regions in which even micron-size samples cannot be prevented from crystallizing. The respective glass transition temperatures and homogeneous nucleation temperatures have been determined. There is an interesting intermediate region where, at normal cooling rates, bulk samples fail to nucleate at the expected temperature due, evidently, to slow growth of critical fluctuations. In this region samples may be obtained as glasses which have a special instability toward crystallization. These are of much interest since they provide a means of studying the poorly understood homogeneous nucleation phenomenon in bulk samples under conditions in which the process can be promoted, and also arrested for study, at will.
Vitreous and viscous LiC1-HzO solutions have attracted considerable attention in recent years from workers with interests ranging from radiation chemistry to solution structure and Currently, such solutions are (1)G. V.Buxton, H. A. Gillis, and N. V. Classen, Chem. Phys. Lett., 32, 533 (1975). (2)G. V.Buxton and K. G. Kemsley, J. Chem. SOC.,Faraday Trans., 72, 1333 (1976). (3)C. T. Moynihan, N. Balitactac, L. Boone, and T. A. Litovitz, J. Chem. Phys., 55, 3013 (1971). ( 4 ) C. T. Moynihan, R. D. Bressel, and C. A. Angell, J. Chem. Phys., 55, 4414 (1971). (5)S.-Y. Hsich, R.W. Gammon, P. B. Macedo, and C. J. Montrose, J . Chem. Phys., 56, 1663 (1972). (6)N. Boden and M. Mortimer, J . Chem. SOC.,Faraday Trans., 73, 353 (1977). 0022-3654/81/2085-1461$01.25/0
being investigated as model systems for homogeneous nucleation mechanism studies using small-angle neutron scattering and conductimetric methods where the DzObased systems are to be preferred for technical reasons. It is desirable therefore that some basic vitrification and crystallization data for such systems be made available. Glass transition temperatures have been published for the system LiC1-Hz0,8 but interesting comparisons with D20-based systems made a t the same timeg have never been made available in the open literature. Recent studies of the crystallization of emulsified solutions performed in this laboratory now permit a general scheme of low-tem(7)E.J. Sutter and J. F. Harmon, J . Phys. Chem., 79, 1958 (1975). (8)C.A. Angell and E. J. Sare, J. Chem. Phys., 52, 1058 (1970). (9)E.J. Sare, Ph.D Thesis, Purdue University, 1970.
0 1981 American Chemical Society
1462
The Journal of Physical Chemistty, Vol. 85, No. 7 I, 198 I
Letters
t " " " " '
9, 250
Ti (H20)
b
:
L
..........
,.:
~
ilcl-
200 -
H20
x \
I4
-140
IO
8
7
8.
5
0
I
II
9 8 7
4 8 ,
5
0
10
I , , , ,
5
6 4
15
3
35
I
,
,
,
4
8
20
2 5 I
,
I
30
3
35
4 I
, ,
25
20
15
IO
R
-140
4
¶
1 1 1 1 1 1 1 , 1 1 1 1 ,
,
4
25
I--
150-
25 4
I
30
I
I
I
I
1
I
I
I
t
Mole P e r c e n t Sol1 I
601
0
,
5
10
15
20
25
30
IO
I5
20
25
M
Mole Percent Sol1
Figure 1. Variation of glass transition temperature with composition for several lithium salt solutions in H20and D20. Lower sections show variation with composition of the DTA signal magnitude at T, and indicate the changes in heat capacity per unit volume accompanying the glass transition.
perature crystallization and vitrification for these systems to be laid out.
Experimental Section The experimental aspects of this work, which are very simple, have been described before.8J0 They involve only simple differential thermal analysis (DTA) of (1)emulsified solutions during cooling to determine the homogeneous nucleation temperature, T h , and (2) bulk solutions during heating after preliminary vitrification to determine glass transition temperatures (for solutions which did not crystallize on cooling). For the nucleation temperature determinations, emulsions prepared by methods described previouslylOJl were cooled in a two-hole DTA block which was mounted in an insulated jacket immersed in liquid nitrogen, the insulation being such that steady cooling at approximately 15' m i d (10)H.Kanno and C. A. Angell, J. Phys. Chem., 81, 2639 (1977). (11)D.H.Rasmussen and A. P. MacKenzie, "Water Structure at the Water-Polymer Interface", H. H. G. Jellinek, Ed., Plenum, New York, 1972,pp 126-145.
(12)C. A. Angell and E. J. Sare, J. Chem. Phys., 49, 4713 (1968).
The Journal of Physical Chemistry, Vol. 85,
Letters
250r-----7 24 0
mole% MnCI,or
Ca(NOJ,;
1/2 (mole% LiCi )
Flgure 3. Th and T, dependence on composition for three aqueous salt solutions showing that, on an equivalent percent basis, MnCI, is the most effective salt for repressing the nucleation of ice. The reason for the invariance of T, below 6 % MnCI, or (LICI), is not properly understood at this time.
LiC1-H20 and LiC1-D20 these results are shown together with the values of the homogeneous nucleation temperatures Th in Figure 2. We show also the liquidus line for the equilibrium phase transition. Finally, to put the relation between Th and Tgfor aqueous LiCl solution in perspective, we compare them with previously reported data for Ca(N03)2 solution^,'^ and with new data for the nucleation temperature of MnC12 solutions, in Figure 3. The latter system was chosen for study because of the rather extensive region of composition-independent glass transition temperatures observed in an earlier study! The significance of this region is not fully understood at this time, but the coincidence of the break in the T plot with the intersection of the Tgand the extrapolate4 Th plots, is probably significant. Composition scales in all cases are given both as mole percent of dissolved salt and R value ( R = moles of water/moles of salt; molality M = 5 5 . 5 / R ) .
Discussion The most interesting aspect of this work lies in the relation between the Th and Tg at the dilute end of the glass-forming range. However, before focusing attention on this problem we comment briefly on the comparison of glass transition temperatures between the different systems reported in this study. The decrease in glass temperature at a given composition, as the anions are changed from acetate to bromate to chloride, is easily interpreted in terms of the different strengths of hydrogen bonding between water molecules and anions of different proton-attracting ability. The correlations between Tgand hydrogen-bonding-abilityindicating parameters, such as the Bronsted base association constant Kb, have been pointed out before. The differences between the LiC1-H20 and LiC1-D20 solution T 's seen also in Figure 1 are presumably to be interpreted in the same terms, since, due to differences in zero-point vibrational frequencies, the D20-C1- bond appears stronger than the H20-C1- bond. However, the Tgdifference is seen to (13) C. A. Angel1 and J. Donnella, J. Chem. Phys., 67, 4560 (1977).
No. 11, 1981 1463
vanish at compositions containing less than four water molecules per lithium ion, which is the composition of the first coordination shell of hydrated lithium ions, according to various structural studies made on this This suggests that the influence on T of anion binding effects is most important for secon% hydration shell molecules, though we must note that there is little observable tendency of the T gplots for different anion salts to approach one another as the water content falls below that necessary for a single hydration shell. The reason for the increase in glass temperatures when bromide ions are substituted for chloride ions at water contents less than the number necessary for the first hydration shell is by no means clear. Since Br- is a weaker base than C1-, the most likely explanation is to be sought in packing effects, where the larger size of the bromide ion relative to the hydrated lithium cation may be an important factor. We note that, at higher water contents, the order of Tp's reverses as would be expected from the basicity agrument. The changes in differential emf (A) which occur at the glass transition are shown in the bottom of this diagram. These displacements are determined primarily by the changes in heat capacity at Tgon a per unit volume basis, but thermal conductivities must also be involved since the thermocouple is sampling the changes in heat flow into the samples, Thus care is needed in interpretation of the A variations. Their relative values are consistent with the expectation of larger changes in heat capacity for the s a l b of complex anions (in which orientational degrees of freedom will be excited on gaining freedom to move in the supercooled liquid state). We turn now to consideration of the relation between homogeneous nucleation temperatures and glass transition temperatures exhibited in Figures 2 and 3. The importance of the relationships seen here relates particularly to the possibiltiy of experimental studies of the relatively little-understood phenomenon of homogeneous nucleation of crystals in supercooled liquids. It is a theoretical prediction,16J7and an experimental observation, that the probability of nucleation occurring is an extremely sharp function of decreasing temperature in the region of observable nucleation rates. For this reason it is possible with emulsion samples to determine the Th vs. temperature curve for binary systems quite reproducibly. However, this sharp function of temperature reflects the rapid increase in probability of occurrence of crystal-like configurations of critical size as part of the spectrum of order fluctuations characteristic of the liquid in its internal equilibrium state. As the changing composition depresses Th to lower and lower temperatures, the time scale on which the internal equilibrium distribution of fluctuations can be established starts to become comparable with the time scale for observation, and this introduces a new factor in the manifestation of the nucleation phenomenon, viz. the fluctuation growth and decay time for nucleus-like fluctuations. It is because this time is considerably longer than the relaxation time for achievement of the local amorphous packing equilibrium (due to the larger size and specific order of the nucleus-like fluctuations) that no crystallization phenomenon could be observed at our lowest cooling rate in the composition regions in Figure 3 shown by dashed porition of the Th (14) A. H. Narten, F. Vaslow, and H. A. Levy, J . Chem. Phys., 58,5017 (1973). (15) N. E. Hewish and J. E. Enderby, private communication. (16) D. Turnbull, Contemp. Phys., 10, 473 (1969). (17) D. R. Uhlrnann, J. Non-Cryst. Solids, 7, 337 (1972).
1464
J. Phys. Chem. 1981, 85, 1464-1466
plots, and instead glass formation (loss of local internal equilibrium) could be observed at a lower temperature. The form of the nucleation rate vs. temperature relation which results from the interplay of these two factors is shown against the consequent Th observations in Figure 2. The theory for curves of this type is described by Turnbull16 and Uhlmann.17 The important consequence of this is that, in special composition ranges, a supercooled liquid can be trapped in the vitreous state in a condition in which it is fundamentally unstable against growth of order fluctuations. This means that the phenomenon of nucleation can be observed, and the parameters determining the size and distribution of critical nuclei measured, during rewarming studies of the initially vitrified systems. The time scale for observation of these phenomena can be extended a t the experimentalist’s will depending on the choice of observation temperature. Futhermore, the development can be terminated for static observation a t any chosen time simply by recooling the sample by a few degrees. Smallangle neutron-scattering experiments conducted recently at the Institut Laue-Langevin, for instance, have indicated that nucleation may occur relatively abruptly after isothermal annealing periods of up to 48 h.lS However, such observations have been made without the advantage of any previously determined Th vs. Tgrelations. To our knowledge none of the extensive studies of glass devitrification have been performed with such relationships a t hand (indeed most have been performed under conditions of heterogeneous nucleation, T > Tg > Th), and it is hoped that their conscious application, using the degree of instability, Th-T, as a controlled variable, may result in new advances in this interesting and technologically important area. Advantage has now been taken of these observations by Chieux, Dupuy, and colleague^^^ and ourselvesz0to provide guidelines for small-angle neutron, (18) P. Labarbe, A. Wright, A. K. Bandyopadhyay, and J. Zarzycki, J. Non-Cryst. Solids, submitted for publication. (19) P. Chieux, J. Dupuy, R. Calemzuk, J. F. Jal, C. Feradou, and C. A. Angell, to be submitted for publication. (20) C. A. Angell and D. R. MacFarlane, to besubmitted for publication.
light-scattering, and, most recently, conductimetric studies of the nucleation process for ice and DzO ice in certain of these systems. The results of these investigations will be reported separately.
Concluding Remarks Glasses are always unstable with respect to the metastable liquid from which they form by structural arrest and will move spontaneously toward the metastable free energy surface given only a chance (i.e., the thermal energy, kT9) for diffusion to occur. By comparison, the rate a t which a “clean” glass will move from the metastable surface to the stable (crystalline state) free energy surface during such an “annealing” process is usually negligible. In the composition region in which Tglies below Th,however, the glasses can relax at comparable rates toward both the metastable liquid and the crystal free-energy surfaces, hence should be regarded as doubly unstable and incapable of regaining even the status of a metastable phase. Notwithstanding this lamentable dearth of thermodynamic respectability, such doubly unstable materials can be of great scientific interest. For instance, all the (splatquenched) metallic glasses, toward which so much attention has been directed in recent years, are undoubtedly in this condition, although their homogeneous nucleation temperatures are generally unknown. A “kinetic instability index” may be defined by the dimensionless quantity (Th - T,)/T,, which should approximately scale with the relaxation time ratio 7in/~wt1 (where “in” means “within the amorphous phase”, and “out” means “out of the amorphous phase into the crystalline phase”). As this ratio approaches unity it will no longer be possible to observe the glass transition of a successfully quenched “glassy” phase during the reheating process.
-
Acknowledgment. The authors are indebted to the Department of the Interior, Office of Saline Water (now Water Resources Technology), and the National Science Founation, under Grant No. 14-31-0001-3883and DMR8007053, respectively, for support of this work. (21) C. A. Angell, J. H. R. Clarke, and L. V. Woodcock, Adu. Chem. Phys., 48,398 (1981).
Evidence for Photodissociation of Water Vapor on Reduced SrTiO, (111) Surfaces in a High Vacuum Environment S. Ferrert and G. A. SomorJal” Materials and Molecular Research Division, Lawrence Berkeley L65oratory, and Department of Chemistry, Universify of California, Berkeley, California 94720 (Received February 25, 198 1)
We have observed D2 photoproduction from DzO vapor at a pressure of 1 X lo-’ torr on a reduced SrTiOB(111) crystal face at a temperature of 600 K. Electron energy loss spectroscopy data showed a decrease in the surface concentration of Ti3+species while the crystal face was being illuminated and the Dz photoproduced, indicating that the oxygen was being incorporated into the crystal lattice. In this Letter we report for the first time the photodecomposition of water adsorbed from the gas phase in high vacuum conditions on metal-free, reduced SrTi03 single crystals. In the high vacuum environment we could utilize ‘Departmentof Physics, University of Madrid, Madrid 34, Spain. 0022-3654f 8 1/2085- 1464$01.25f 0
electron spectroscopies while the reaction was taking place, allowing us to monitor the chemical changes which occurred a t the crystal surface. In this way the role of the Ti3+surface species in the photoreaction could be directly established. We have monitored D2 evolution into the gas phase (by a quadrupole mass spectrometer) from D 2 0 0 1981 American Chemlcal Soclety
