C. M. Ellis
Goldsmiths' College London, England
I
The 2-Butoxyethanol-Water System
I
Critical solution temperatures
I
and salting-out effects
The 2-butoxyethanol (butylcelloso1ve)water system (1-3) is extremely convenient for demonstrating and investigating certain liquid phase relationships. It shows binary loop behavior (see Fig. 1) withm a reasonable temperature range, and a simple procedure (open test-tube fitted with thermometer and stirrer) has been used successfully a t this College for several years for plotting the portion of the phase diagram below 100°C. Accurate determinations of the complete loop however require sealed tubes to prevent evaporation and results by this method are presented in Table 1.' For this, technical 2-butoxyethanol
tubes were held in a wire cradle rocked continuously in a heating bath (Niter beaker) containing dibutyl phthalate which was stirred mechanically. The bath could be heated or cooled very slowly using a rnicroburner. Assessment of the point of phase change from Table 1. 2-Butoxyethanol-Water -2-Butoxyetbanol-Mole fraction Weight %
Miscibility temperature ('C) Lower Upper
Each temperature represents the mem of six observations (three on heating, three on cooling) of phase change.
2 - BUTOXYETHANOL ( weipml
Figure 1.
%I
Phore diagram for 2-butoxyethanol-water.
(B.D.H.) was dried with Linde 4.4 molecular sieve and fractionally redistiied in purified nitrogen, bp 170.1°C at 758 mm, ng = 1.4194 (in agreement with Schneider and Wihelm ( 4 ) ) Water was purified by ion-exchange and redistilled in nitrogen. Mixtures were made up individually by weight and introduced into glass tubes about 10 cm long and 1 cm diameter with 1-mm walls which were then cooled and sealed. Calibrated thermometers, read to O.Ol°C with a lens, were used and the 1
EDITOR'S NOTE: Readers familiar with the now-ouhf-print
"Laboratory Manual of Elementary Physical Chemistry," by E. Mack and W. G. France, D. Van Nostrand Ca., he., Prineeton, N. J., 1934, will recall that this system was the object of experiment 17, pp. 137-140.
clear homogeneous solution to cloudy mixture (and vice versa) is subjective, but was found for this system to be highly reproducible except near the steep sides of the diagram. The simple open-tube procedure was compared with the sealed-tube results and found to be almost as accurate near the lower critical solution temperature (LCST), with an average standard deviation of 0.05'C. In common with many similar systems, the maximum (UCST) and minimum of Figure 1 are both rather flat and it is difficult to estimate exactly where the UCST and LCST compositions lie. The procedure of Cox and Herington (6) tested by them on 16 other systems (12 UCST, 4 LCST) may be used. This consists of plotting logla(xl/x~,where xl = mole fraction of component 1, and x2 = mole fraction of component 2, at a point on the curve temperature T, against the cube root of the difference between T and the appropriate critical solution temperature T,. Compositions corresponding to the two ends of a given tie-line of Figure 1 give points which lie on two almost equi-inclined straight lines which intersect at or very near to the abscissa giving the critical solution composition (Fig. 2). This construction, using the data of Table 1, indicates that the LCST is 48.8~ 0.l0C at 27.4 0.5% by weight 2-butoxyethanol and the UCST is 130.2 + 02°C at 29.7 0.5%, which is appreciably higher. These temperatures agree with those of Schneider and Wilhelm (4) who used high-purity mac
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Volume 44, Number 7, July 1967 / 405
elevation (or depression) of the CST, a is a constant characteristic of the third component (the salt), b is a constant characteristic of the bimary system (but different for elevation and depression), and C is the concentration of added salt in moles per 1000 g total weight of mixture. Plots of log AT versus log C gave excellent straight lines above C = 0.01, as is usually found, and results are compared in Table 2; values of constant a
-+
Table 2.
0
Values of Constants in
AT
= aCb
for Alkali Metal Halides
a b
LiCl
NaCl
KC1
RbCl
CsCl
KBr
K1
35.5 0.78
49.8 0.79
50.9 0.80
490 0.79
46.3 0.78
24.5 0.81
41.5 0.83
-0.
~i~~~~ 2.
L O F O ~ ~of O critical ~ solution comporition.
terial, so that the data of Table 1 are believed to be more accurate than those hitherto available (1-3). The system may also be used to demonstrate the marked effect of a third component. Timmermans' rules are usually followed, i.e., when the additive is soluble in both original components, such as 2-ethoxyethanol, the loop shrinks (LCST rises and UCST falls) but when it is soluble in only one component (e.g., alkali metal halides) the loop expands. The salting-out effects of several of the latter in lowering the LCST of the 2-butoxyethanol-water system are shown in Figure 3. Dried AR halides were used except for LiCl and RbCl, in which cases the purity was checked volumetrically. The results could be fitted to a general equation of the type deduced by Carrington, Hickson, and Patterson (6) and applied by them and by nowden and Purnell (7) to the phenol-water UCST rase. This equation is AT = aCa, where AT is the ELEVATl N
1
4 W
5
a C
2 3
give the order of salting-out efficiency, which is the opposite of that of the solubilities of the salts in water at the same temperature (in molarities, calculated from tables (S)),i.e., comparing cations: KC1 > NaCl > RbCl > CsCl > LiCI, and comparing anions: KC1 > KBr > KI. Salting-out behavior has long been used to test liquid purity (9-11), theories of it have been reviewed (1%14), and there is current interest in it (15). 2-Butoxyethanol behaves as a typically basic polar nonelectrolyte (14) in that when being salted-out it is much more sensitive to anions than to cations (Fig. 3) and the cation order is also characteristic. Large anions usually salt-in rather than salt-out and this is shown by potassium iodide and by sodium dodecyl sulfate (3). The 2-butoxyethanol molecule can presumably bond directly to the hydrogen atoms of four water molecules via its own two oxygen atoms and via its hydroxyl hydrogen atom to another water molecule (which itself could bridge two 2-butoxyethanol molecules). The phase composition at the broadest point of Figure 1 lea. 90°C)- corresponds closely to this arrangement, having a molecular ratio of 1:4.55, in equilibrium with a verv dilute aaueous uhase. molecular ratio 1:62. The oxygen atoms would confer basic function and in salting-out, competition for water of hydration would be relatively less for cations (which can bond to oxygen in a hydrating water molecule) compared with anions, which bond to hydrogen. This effect, together with differences in orientation and hence free energy of the hydrating water molecules can explain qualitatively the greater variation in the efficiency of salting-out by anions (14).
_J
0
Literature Cited
Y) _J
4
: 0
-5
o
Z w w Z
Q
z
-10
0
-15 Figure 3. Effect of alkali metal halides on LCST of 2-butoryethanolwater. (Curves for sodium chloride m d rubidium chloride not shown; they lie between those for potasrim and eoerivm chlorides.)
406 / Journal of Chemical Education
(1) COX,H. L., AND CRETCHER, L. H., J. Am. Chem. Soe., 48, 451 (1926). (2) POPPE, G., Bull. SOC.ehim. Belg., 44,640 (1935). N., Bull. Soc. chim. Belg., 65, 476 (1956). (3) CHAKHOVSKOY, (4) SCANEIDER, G., AND WILKELM,G., Zed. physik. Chem. (Frankfurt),20,219 (1959). E. F. G., Tram. Faraday Sac., (5) Cox, J. D., AND HERINGTON, 52,926 (1956). (6) CARRINOTON, J. H., HICKSON, L. R., AND PATTERSON, W. H., J . Chem. Sac., 127,2544 (1925). (7) BOWDEN,S. T.,AND PURNELL, J. H., J . C h m . Sac., 535 (1954). (8) STEPAEN, H., A m STEPHEN, T.,"Solubilities of Inorganic and Organic Compounds," Pergamon, London, 1963, Val. 1, Part 1.
(9) CRISMER, L., Bull. Sac. chim. Bely., 9 , 145 (1895); 10, 312 (1896); 18,1(1904); 20,294 (1906). (10) JONES,U. C., J. Chem. Sac., 1374 (1823). H. T., AND I\IRSHILLA. G., J . SOC.Chem. Znd., (11) TIZARD, 40, 20T (1921). (12) GROSS,P. IN., Chem. Rev., 13,81 (1973).
(13) LONG, F. A., AND MCDEVIT,W. F., Chem. Rev., 5 1 , 119 (1952). (14) DOBRY-DUCLAUX, A., Chemiker-Zeilung 76,805 (1952). (15) VINOGRADOV, E. E., AND SISHKICAEV,V. I., Zhu?. ~tmkt. Khim., 7 , 103 (1966).
Volume 44, Number
7, July 1967
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407
