J . Phys. Chem. 1989,93,4674-4677
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adsorption revealing differences in the number of ligands, as well as the differences in the Cu2+ species formed upon exposure to C H 3 0 H , C2HSOH,and C2H4. It is of interest to compare these results with the results obtained in a similar study carried out on Cu2+ in zeolite ZSM-5.8 As mentioned previously, both ZSM-5 and mordenite are channeltype zeolites. Figure 8 illustrates the difference between the two structures. ZSM-5 consists of nearly circular channels of 0.54 X 0.56 nm diameter that are interconnected by sinusoidal channels that are 0.52 X 0.55 nm in diameter.I0 The intersections create a large increase in the available free space. I n a pair of recent ESEM s t u d i e ~ , Cu2+ ~ , ~ was found to coordinate to six water molecules and four molecules of ammonia in all forms of ZSM-5 (H+, Na’, Na+-H+, K+, or Ca2+). Cupric ion was able to add an additional ligand of either methanol or ethanol in H-ZSM-5. While similar results were obtained in the present study, cupric ion coordinates to more adsorbate molecules in ZSM-5 than in
mordenite. The difference can be attributed to the essential difference in the two zeolite structures. The large complexes formed between Cu2+and the adsorbate molecules in ZSM-5 are thought to be accommodated at the channel intersections. Mordenite does not contain these intersections, and this is reflected in these results. Large complexes such as [CU(CH,OH),~’] or [ C U ( C H ~ O H ) ]were ~ ~ +not observed in this study but are formed readily in ZSM-5. These results support the earlier work done on zeolite ZSM-5 and further illustrate the importance of the channel intersections found in ZSM-5 for catalytic applications. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Technology Research Program. We thank Dr. Ravi Kumar Kukkadapu for many helpful discussions. Registry No. Cu, 7440-50-8; H20, 7732-18-5; NH,, 7664-41-7; H*, 12408-02-5; N a , 7440-23-5; K, 7440-09-7; Ca, 7440-70-2.
Vitrified Dilute Aqueous Solutions. 3. Plasticization of Water’s H-Bonded Network and the Glass Transition Temperature’s Minimum Klaus Hofer, Andreas Hallbrucker, Erwin Mayer,* Institut f u r Anorganische und Analytische Chemie. Universitat Innsbruck, A-6020 Innsbruck, Austria
and G . P. Johari Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada (Received: June 29, 1988; In Final Form: October 18, 1988)
The glass transition temperature, T,, of hyperquenched dilute (-0.4-6 mol %) binary aqueous solutions of LiCI, ethylene glycol, and ethanol and its composition dependence have been studied by differential scanning calorimetry. On the initial addition of the solutes, Tgdecreases. Further addition causes it to reach a minimum value and thereafter T, increases. Thus the initial addition of ionic or nonionic solutes weakens the H-bonded network against its resistance to relaxation. The minimum in the T, with increasing concentration is likely to be associated with the structural changes of the H-bonded network on the hydration of ions, stability of ion pairs, H bonding with alcohols, hydrophobic interactions, etc., but which of these processes predominate is not certain. Nevertheless, our results do make it possible to identify a new hydration number in the deeply supercooled state of water.
Introduction Micrometer-size water droplets on hyperquenching, or rapid cooling at rates > I O s K s-l, produce glassy or vitreous water,’-3 which has recently been shown by differential scanning calorimetry (DSC) to have a reversible glass-liquid transition with an onset temperature, or T,, of 136 f 1 K.4 Dilute aqueous solutions can be similarly completely vitrified by hyperquenching of micrometer-size droplets, and their glass transition and other properties can now be studied in the completely vitrified state for the first time. The infrared spectra of vitrified aqueous dilute alkali-metal nitrate and perchlorate solutions3 and the thermal behavior of hyperquenched NaCI-H20 and ethylene glycol (EG)-H20 glassesShave already shown several interesting features. We report a study of the T s of hyperquenched dilute aqueous solution glasses as a function of the concentration of one structure-forming electrolyte, and two H-bonding, one monohydroxy (ethanol) and the second a dihydroxy (EG) alcohol, and show that the initial addition of the solutes lowers the T, of glassy water, or plasticizes ( I ) Mayer, E. J. Appl. Phys. 1985, 58, 663. (2) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1987, 91, 503. (3) Mayer, E. J. Phys. Chem. 1986, 90, 4455. (4) Johari, G. P.; Hallbrucker, A.; Mayer, E. Nnture 1987, 330, 552. ( 5 ) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1988, 92, 2007.
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it, an effect similar to that of plasticization of polymer^.^ With increasing concentrations of the solute T, increases, and this plasticization seems to effectively vanish at or near the T, minimum. The new data for hyperquenched dilute solutions are then connected with the T, of the “glass-forming composition regions’’ of their concentrated solution^^-'^ and discussed in terms of the changes in the mobility of water in the H-bonded structure.
Experimental Section Dilute solutions were vitrified by a procedure described in earlier reports.’-5 Briefly, aerosol droplets were transferred through a ~~~~~
~
~
~
(6) Ferry, J. D. Viscoelnstic Properties of Polymers; Wiley: New York, 1980. (7) Angell, C. A,; Sare, E. J. J. Chem. Phys. 1968, 49, 4713; 1970, 52, 1058. (8) Angell, C. A,; Sare, E. J.; Donnella, J.; MacFarlane, D. R. J . Phj’s. Chem. 1981, 85, 1461. (9) Angell, C. A,; Tucker, J. C. J. Phys. Chem. 1980, 84, 268. (IO) Kanno, H. J . Phys. Chem. 1987, 91, 1967. ( 1 1) Luyet, B.; Rasmussen, D. H. Biodynnmica 1968, I O , 167. (12) Rasmussen, D. H.; MacKenzie, A. P. J. Phys. Chem. 1971, 75,967. (13) Boutron, P.; Kaufmann, A. Cryobiology 1979, 16, 83. (14) Angell, C. A. In Wafer,A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1982; Vol. 7, Chapter I , p 19, with references.
Q 1989 American Chemical Society
Vitrified Dilute Aqueous Solutions small opening (200 Fm diameter), with nitrogen as carrier gas, into a high-vacuum system, accelerated by supersonic flow, and deposited on a cryoplate at 77 K. Details of nebulization and aerosol formation are described in ref 5 . Dilute solutions were nebulized with an ultrasonic nebulizer operating at 3 MHz (Engstrom, Model N B 108), concentrated solutions either with a retouching air brush ( 7 2 0 mol % EG) or with a pneumatic nebulizer (>5 mol 7% LiCI, Pari, Model 360). A Perkin Elmer DSC-4 instrument computerized with the TADS system was used. Curvature was reduced with the SAZ function, which subtracts during scanning a base line obtained with empty cells. Water, cyclohexane, and n-heptane (Merck, Uvasol quality) were used for calibration; their transition temperatures were accurate to better than f l K. Handling of the hyperquenched deposits and transfer into the DSC-4 instrument are described in ref 2 and 5 . The solute concentration of the hyperquenched glasses was determined by comparing the shape of their melting endotherms with those of the standard solution^.^ The sample mass could be determined from a comparison of peak areas, after the solute concentration was known. The hyperquenched deposits were monitored for the crystalline ice impurity by using X-ray diffraction. Optimally quenched samples of vitrified pure liquid water consistently contained 5 f 1% crystalline, mainly cubic, ice;5 dilute solution deposits (e.g., 1 M solutions) were completely vitreous. This is a notable effect of small amounts of solutes on the stability against crystallization of the liquid or glass.
Results The procedure developed for hyperquenched liquid water4 was liquid transition in dilute also used here to reveal the glass solution glasses. The procedure eliminates the enthalpy relaxation, which for a hyperquenched glass appears as an exotherm at T
