Two glass transitions in the tetraethylammonium chloride—water system: Evidence for a metastable liquid—liquid immiscibility at low temperatures

Two glass transitions in the tetraethylammonium chloride—water system: Evidence for a metastable liquid—liquid immiscibility at low temperatures

Volume 103, number 3 TWO GLASS TRANSITIONS IN THE TETRAETHYLAMMONIUM EVIDENCE FOR A METASTABLE LIQUID-LIQUID H. KANNO, K. SHIMADA 30 December 19&G ...

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Volume 103, number 3

TWO GLASS TRANSITIONS IN THE TETRAETHYLAMMONIUM EVIDENCE FOR A METASTABLE LIQUID-LIQUID

H. KANNO, K. SHIMADA

30 December 19&G

CHEMICAL PHYSICS LETTERS

CHLORIDE-WATER

SYSTEM:

IMMISCIBILITY AT LOW TEMPERATURES

and K. KATOH

Department of Chemistry. h?eisei University, Hino, Tokyo 191. Japan Received 8 September 1983

The glass-forming composition region of the tetraethylammonium chloride-uater system has been determined_ The existence of a low-temperature

liquid-liquid

immiscibility is established.

1. Introduction Liquid-liquid immiscibility is very common in silicate and borate melts [I-S] and the regions of the stable immiscibilities in a number of binary silicate and borate systems have been established [ 11. However, in spite of the structural and thermodynamic similarities between water and silicon dioxide [6-81, there has been no well-established liquid-liquid irrnniscibility in any aqueous simple electrolyte solution. Several attempts [9-l l] have been devoted to seek the possible liquid-liquid immiscibility in the LiCi-water system. Two positive reports [9,10] have been refuted by recent small-angle neutron scattering experiments on some LiCl + D20 solutions at low temperatures [l 11. In this paper, we report the observation of two glass transitions in glassy aqueous tetraethylammonium chloride solutions in the concentration range R = 11 16 (R = moles of water/moies of salt), implying that there should be a metastable liquid-liquid immiscibility dome in the (C2H5)4NC1-water system.

2. Experimental Solutions of tetraethylammonium chloride in various concentrations were prepared by dissolving the required amount of dried tetraethylammonium chloride in distilled water. The glass transition temperature (Tg) of each solution was measured by a conventional, simple 0 009-2614/83/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

DTA method. The details of the experiment were essentially the same as reported previously [ 13]_ The overall cooling rate of the sample solution contained in a 2 mm i.d. Pyrex glass cell was ~6 X 10’ K/min. The Tg value was obtained at a heating rate of ~5 K/min in the warmup DTA trace from liquid-nitrogen temperature using chromel-ahrmel thermocouples.

3. Results and discussion Two typical DTA traces are shown in fig_ 1 and the Tg data summarized in fig. 2. The DTA warmup trace

ofaglassyR = 10 solution shows a glass transition at -95S°C followed by a sharp crystallization peak, while that of a glassy R = 11 solution gives a first glass transition (Tg r) at -111.7”C followed by a shallow crystdlization peak and then a second glass transition (TO,) comes up at -82°C to end with a second large cry;Tallization peak. Though it is difficult to see clearly the second glass transition in the DTA trace shown in fig. 1, we could easily confirm that the downward CP inflection at -82°C is really the second glass transition in the following v. ay: just after recording the first glass transition and crystallization curves as usual in the first run, the sample was requenched to liquid-nitrogen temperature and then the second glass transition was clearly observed in the second DTA trace. There is a first-order-transition-like heat-absorbing peak at -61 .S”C just in front of the second crystalliza219

30 December 1983

CHEMICAL PHYSICS LE-iTERS

Volume 103, number 3

Tg

R = 10

-110

-100

/

-90

-80

-70

-60

-50

TEMPERATURE / "C Fig. l- DTA traces of glassy aqueous tetraethylammonium chloride solutions of R = 10 and 11. Tg and Tc represent a glass tramition and a crystallization temperature respectively.

tion peak in the DTA trace for the R = 10 solution. A remarkable point is that this heat-absorbing peak was observed only in the concentration range between R = 8 and -10. At the moment, no definite explanation can be given for its origin and as it is considered to be irrelevant to the present discussion, it will be a subject of future investigations. The observed drastic changes between the two DTA traces indicate that large structural changes occurred between R = -10 and 11. As shown in fig. 2, two glass transitions were observed from R = 103 to 16, above which glass formation-was only partially realized. The most probable mechanism by which the above experimental results may be interpreted is a metastable liquid-liquid immiscibility. As the temperature is lowered, the homogeneous solution splits into a water-rich phase and a salt-rich viscous phase due to thermodynamic instability. These two immiscible liquid phases are. readily quenched to the vitreous states in the concentra220

tion range R = 10.5-16. The glassy R = 10.5 solution gave the intermediate results between the homogeneous solution and the phase-separated solution: namely, the Tg of the homogeneous solution and the two glass transitions of the phase-separated solution were shnultaneously observed in the glassy R = 10.5 solution. Additional support for the liquid-liquid immiscibility is the observation of the second glass transition even in the R = 17 and 18 solutions which are already opaque in the quenched state at liquid-nitrogen temperature. The opaqueness is a good indicator of crystallization in the case of aqueous solution [9]. Non-observation of the first glass transition (Tgl) in the R = 17 or 18 solution was due to the crystallization of the water-rich -phase in the quenching process. From the Tg values, the concentration of (C,H&NCl in the second phase (the salt-rich phase) is estimated to be around R = 6.5. Thus it is only natural to observe the second glass transition in the warmup DTA traces of the quenched solu-

CHEMICAL

Volume 103, number 3

PHYSICS LETTERS

Q Q

e

” D

-100

.

1983

former, can partially explain the situation that the liquid-liquid immiscibility in an aqueous electrolyte solution is a rare case. in the silicate systems, both separated phases are nonally stable even in the hi°ree supercooled temperature region [ 1] . Recent studies of pure water [7] have revealed that resemblance of water to silicon dioxide is enhanced at low temperatures and high pressures, suggesting that a similar metastable liquid-liquid immiscibility in aqueous electrolyte solution may be more frequently encountered at high pressures. Finally, it is interesting to note that the liquidliquid immiscibility is also observed in the tetraethylammonium bromide-water system but not in other tetraalkylammonium halide-water systems (alkyl = methyl, propyl and butyl; halide = Cl and Br).

I

-90

30 December

t-”

-ll@

References [l] H. Rawson, Inorganic glass-forming systems (Academic Press, New York, 1967) chs. 7,8. [2] ;F. Levin and S. Block, J. Am. Ceram. Sot. 40 (1957)

-12c 6

8

10

12

14

16

18

R Fig. 2. Variation of Tg with solution composition. Two glass transitions were observed between R = 105 and 16.

tions ofR =, 17 and 18 since the concentration of the salt-rich phase is in the glass-forming composition region. On the other hand, the concentration of the water-rich phase diminishes with decrease in the concentration of the mother solution, as indicated by the lowering of Tgl with increase in R _ Unfcrtunately there are inadequate thermodynamic data available for drawing an immiscibility dome for this system [ 133. However, seeing the structural and thermodynamic similarities [6--81 of tetrahedral network liquids of water and silicon dioxide, the metastable immiscibility region in this system is expected to occupy the composition region between pure water and the first eutectic. The fact that pure water is expected to be glass forming only in an uitra-fast quenching rate @lo7 K/s) [14] while silicon dioxide is a typical &SS

]3] JJ. Rockett, W.R. Foster and R.G. Ferguson, J. Am. Ceram. Sot. 48 (1965) 329. [4] W. Vogel and H.G. Byhan. Sihkattechnik 15 (1954) 212. 239,324. [5] Y. Maria. D-H. Wanington and R.W. Douglass. Phys. Chem. Glasses 8 (1967) 19. [6] B. Kamb, in: Water and aqueous solutions: structure, thermodynamics and transport processes, ed. R-X_ Home (Wiley, New York, 1972) ch. 1; J. Wong and C.A. Angel. Glass: structure by spectroscopy (Dekker, New York, 1976) ch. 2. [7] CA. AngeB end H. Kanno,Scisnce 193 (1976) 1121: H. Kanno and CA. AngelJ. J. Chem. Phys. 73 (1980) 1940. [8] I. Kushiro, Earth Planet. Sci. Letters 41 (1978) 87: SK. Sharma, D. Virgo and I. Kushiro. J. NonCryst. Solids 33 (1979) 235. [9] C.A. AngeB and E.J. Sam. J. Chem. Phys. 49 (1968) 47 13. t101 S.Y. Hsich, R.W. Gammon, P.B. Macedo and G.J. Xfontrose, J. Chem. Phys. 56 (1972) 1663. [ill J. Dupuy. J.F. Jai. C. Ferradow, P. Chieux, A.F. Wright. R. CaJemczuk and C-A. Angefl, Nature 296 (1982) 138. tw Y. Akama and H. Kanno, BulL Chem. Sot. Japan 55 (1982) 3308. 1131 W.-Y. Wen. in: Water and aqueous solutions: structure, thermodynamics and transport processes, ed. R.A. Home (Wiley, New York, 1972) ch. 15. ]J41 D.R. Uhhnann. J. Non-Cryst. Solids 7 (1972) 337.

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