591
ANIONEXCHAKGE OF METALCOMPLEXES
Anion Exchange of Metal Complexes. XVII. 1 The Selective Swelling of the Exchanger in Mixed Aqueous-Organic Solvents by Y. Marcus Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem, Israel
and J. Naveh Israel Atomic Energy Commission, Nuclear Research Center, Neges, Israel
(Received August 5 , 1968)
The sn-elling of Dowex-1 anion exchangers of cross-linking4, 8, and 16% DVB was measured in several organic solvents and their aqueous mixtures for the chloride and perchlorate ionic forms. For the pure solvents and chloride form resins swelling decreases in the order water > formamide > methanol > ethanol > acetonitrile > l-propanol > 2-propanol > acetone > dioxane. The perchlorate form shows altogether less swelling, and relatively high swelling by formamide and dimethylformamide. In mixed aqueous-organic solvents preferential swelling by the alcohols is found at the lowest mole fractions and by water over most of the composition range. The amides and methanol show rather little preference for swelling by either water or the organic solvent. Most of the observed phenomena can be explained by the effects of the organic solvent on the structure of water outside the resin, and by preferential ion-solvation effects.
Introduction The selective sorption and elution of metal complexes on anion exchangers from mixed aqueous-organic solvents has been shown in recent years to be an extremely powerful tool for metal-ion separations,2 even more than the now classical separation in aqueous solutions.3 In an earlier publication in this series’ it was pointed out that, whereas in aqueous solutions thc net reaction occurring, whatever the actual mechanism, is the exchange of complex metal anions for the counter anionsin the resin, in anhydrous organic solvents of low dielectric constant the net reaction is the distribution of the uncharged metal complex species between the two phases. These two extreme types of behavior give rise to the experinientally useful method of “combined ion exchange and solvent extraction” based on the use of mixed aqueous-organic solvent^.^ In order to understand the reactions occurring in such mixtures, and hence to be able to predict the behavior of yet untested systems, it is important to have full information on the swelling behavior of the resin and its selectivity for water and the solvent, on the manner in which these properties are affected by electrolytes, and on their invasion into the resin from mixed solvents. These factors are subject to study in our laboratories now, and this paper reports on systems with no added electrolytes. Whereas selectivities of cation exchangers for both ions and solvents have been reported by numerous authorsJ5the behavior of anion exchangers has been the subject of only a few reportsS6 As will be shown later in the Discussion, there are great similarities between the modes of behavior shown by these two kinds of resin, and the information available for cation ex-
changers js therefore relevant for understanding the behavior of the anion-exchange resins. In the following, the swelling properties of polystyrene methylene trimethyl ammonium type anion exchange resins, cross-linked by 4, 8, and 1G% divinylbenzene, and in the chloride and perchlorate forms will be reported for several water-miscible solvents. The parameters studied for each system are the uptake of each solvent as a function of the composition, so lhat the variation of the total swelling and the selectivity of the resin for the solvent and water can be calculated.
Experimental Section Materials. The resins used were comniercial samples of polystyrene methylene trimethylaniinoniuni type, Domex-1 X4, X8, and X16, ie., with nominal crosslinking of 4, 8, and 16% divinylbenzene, of 20 to 50 mesh size, In order to characterize the resin samples, (1) Previous paper in series: J. Penciner, I. Eliezer, and Y . Marcus, J. Phys. Chem., 69, 2955 (1965). (2) J. Korkisch in “Progress in Nuclear Energy, Series IX, Analytical Chemistry,” 9. 0. Stewart and H. A . Elion, Ed., Vol. 6, Pergamon Press, Oxford, 1966, pp 1-94. (3) K. A. Kraus and F. Kelson, Proc. Intern. Conf. Peaceful Uses A t ,
Energy, Geneva, 1955, 7, 113 (1956). (4) J. Korkisch, Sep. Sei., 1 , 159 (1966). (5) (a) 0 . D . Bonner and J. C. Moorefleld. J . Phys. Chem., 5 8 , 555 (1954); (b) H. P. Gregor, D. Noble, and M. H. Gottlieb, ibid., 59, 10 (1955): (c) 0. W. Davies and B. D. R. Owen, J . Chem. SOC., 1676 (1956): (d) D. Reichenberg and W. F. Wall, Ibid., 3364 (1956); (e) R. W. Gable and H. A. Strobel, J . Phys. Chem., 60, 513 (1956); (f) 0.W. Davies and A. Karebska, J. Chem. SOC., 4169 (1964); (g) R . Arnold and 9. 0 . Churns, ibid., 325 (1965); (h) H. Ohtaki, H. Kakihana. and K. Yamasaki, 2. Phys. Chem. (Frankfurt), 21, 224 (1959); (i) H. Ohtaki, ibid., 2 7 , 209 (1961). ( 6 ) (a) G . W. Bodamer and R. Kunin. Ind. Eng. Chem., 45, 2577 (1953): (b) H. Riickert and 0. Samuelson, A c t a Chem. Scand.. 1 1 , 703 (1957); (c) E. Sjostrom, L. NykYnen, and P. Laitinen, i b i d . , 16, 392 (1962). Volume 79, Number 9 March 1969
Y. MARCUSAND J. NAVEH
592
their density, water content in the air-dried state, and anion exchange capacity were determined. The density was determined at 22’ in 25 ml pycnometers, by displacement of n-dodecane, for samples dried over phosphoric anhydride in a vacuum desiccator. The density found for the dry resin in chloride form was 1.1281 g/ml for the X4 sample and 1.1273 g/ml for the X8 sample, reliable to 0.3 mg/ml. The water content of the air-dried free-flowing chloride-form resin samples was determined by drying in a vacuum desiccator over phosphoric anhydride to constant weight. Six samples of X4 resin and seven samples of X8 resin were tested. After 1 day of drying, 2.0 f 1.0% excess of the final weight remained, after 2 days 0.8 f 0.6%, and the value after 4 days represents the equilibrium dry weight of the resin (the deviations are one standard deviation from the mean). The water content of the air-dried resin is 98.6 f 0.4 for X4 resin, 69.1 f 0.4 for the X8 resin, and 33.3 f 0.6 for the X16 resin, in g of water/100 g of completely dried resin. The capacity of the phosphoric anhydride-dried chloride-form resin samples was determined by exchanging the chloride for perchlorate with an excess of the latter and titrating the chloride liberated. For six samples of X4 resin the value 4.04 f 0.01 was obtained, for eight samples of X8 resin 3.63 f 0.03, and for seven samples of X16 resin 2.32 f 0.02, all in mequiv/g of completely dried resin. Recalculated to the air-dried form, the numbers were 2.02,2.16, and 1.74 mequiv/g, respectively. The perchlorate form of the resin was prepared from the chloride form by the standard method of displacemeiit in a column, until no chloride could be detected in tlie effluent. The solvents used were all of analytical reagent grade, in the usual anhydrous or “absolute” form. The actual water content mas determined in each case by Karl Fischer titration and taken into account. Procedures. The resin undeiwent 1hc following treatnient before the determination of tlie swelling behavior. Through a column of thc resin werc passed in order 2 ill. aqueous solutions of sodium hydroxide, water, hydrochloric or perchloyic acid, water, sodium hydroxide, water, ethanol (to remove nonpolynierized inipuritics) , water, hydrochloric or perchloric acid, and water. The resin was then centrifuged at 3700 q m i for 15 miii in a standard procedure to obtain frcrflowing air-dried resin. Weighed portions of these resin samples werc then dried overnight in n vacuum desiccator over phosphoric anhydride. The weight of the dried perchlorate resin was nearly (1 0.0646) times the weight of the dried chloride resin, 6 being the capacity of the dry resin in chloride form, as expected from the increase in the equivalent weight. The deviations from this ratio were - 2.2 f 0.7% for six samples; i.e. , the drying of the chloride mas slightly
+
The JouTnal of Phyaical Chemist~y
less efficient than the drying of the perchlorate. Taking the latter to be absolute, the dried chloride resin retains 0.3 f 0.1 mol of water per equivalent of resin. To a weighed dry resin sample was added a ca. 40-fold excess by weight of a given solvent or aqueous solvent of known composition, and the mixture was equilibrated in a closed vessel at room temperature (ca. 22’) for several hours. The results obtained after 1 meek of equilibration were the same as those obtained after 2 hr, so that time allowed was in every case sufficient for reaching equilibrium. The resin was then quantitatively transferred into a tube with a sintered glass bottom, placed in a centrifuge tube, which mas then tightly closed, t o prevent evaporation of solvent. After centrifugation by the standard procedure, the resin was weighed and the water content determined by a Karl Fischer titration. Separate experiments showed that water in the resin can be determined with 99.5% accuracy compared with results obtained by complete drying, with an excess of Karl Fischer reagent being back-titrated. With the X4 resin the end point was sharp; with the X8 resin a small amount of reagent was strongly sorbed by the resin and had to be removed with a known excess of water to complete the titration. The overall precision (one standard deviation) of the total swelling, obtained by weighing of niultiplicate samples, was ea. f 3%, and so the precision of solvent in the swollen resin, obtained by difference, was ea. f 4%. The extreme difference between determinations was 5%. The equilibriuni composition of the solvent in which the resin was swollen was also obtained by difference. Since the liquid was at a ca. 40 times excess by weight, its composition changed only slightly by removal of solvent by the dry resin, and was obtained with good precision, ca. f 1%. Thc concentration of the solvent in the mixture was obtained from the amounts weighed in. The volume concciitratioii could be calculated by measuring the contraction on mixing.
Results Total Szuelling. The total wcight swelling of the resin samples of X4, X8, and X16 cross-linking has been measured in several solvcnts, with results shown in Table I, as moles of solvent taken up per equivalent of dry resin. The water content of the solvents was sufficiently small to avoid appreciable errors in the figures, within a precision of f 3%. The total specific swelling fl = f i s fiw has been measured as a function of composition for X4 and X8 resin for several water-solvent mixtures, and the results are shown in Figures 1 and 2, for chloride and perchlorate resins, respectively. In order to avoid a loss of accuracy by referring the results to inipreciscly known equivalents of resin (i.e,, ii/6), the primaw
+
593
ANIONEXCHANGE OF METALCOMPLEXEB Table I: Total Swelling of Dowex-1 Anion Exchangers in Several Solvents in Moles per Equivalent of Dry Resin Resin ypt-e
Solvent
X4 C1- X8 01- X16 Cl- X4 0 1 0 4 -
18.8 14.1 Water 6.8 8.5 Methanol Allyl alcohol 3.7 Ethanol 5.2 2.90 1-Propanol 3.5 2.35 2-Propanol 3.1 Formamide 11.0 8.2 Dimethylformamide Ethylene glycol Acetonitrile 2.03 2.45 Acetone 0.69 0.54 Dioxane 0.29 0.23
X8 Clod-
8.1 4.8
3.4 1.68
3.6 1.92
3.30 2.52
1.07 0.34
1.16 0.20
4.8 0.8 3.25 2.73 1.0 0.5
6.5 4.2
4.23.2
-0
data, moles of solvent per kg of dry resin, are given. Data have usually been obtained up t o ca. 98 mole % of the solvent in the equilibrium solution. In most cases 0.5-1.0 mol of water/kg of resin in perchlorate form and 1.0-2.0 mol of water/kg of resin in chloride I
I
I
I
L 40
-
0
co I
x 30
A L
-0
FA
q
20 2 MeOH
-
EtOH 10 PrOH
Y)
E
0
IC"
\ 20
40 60 80 x , m o l e % o f solvent
100
Figure 1. The total swelling 6 in mol of solvent/kg of dry resin in chloride form. Upper set of curves X8 resin, right-hand ordinate, lower set X4 resin, left-hand ordinate. Solvents: FA, formamide; MeOH, methanol; EtOH, ethanoI; and PrOH, I-propanol. Crosses, data for X7.5 resin from ref 60.
0
40 60 80 x,rnole % of solvent
20
100
Figure 2. The total swelling 7i in mol of solvent/kg of dry resin in perchlorate form. Upper set of curves, X8 resin, right-hand ordinate; lower set, X4 resin, left-hand ordinate. Symbols for solvents same as Figure 1, with addition of DMF,dimethylformamide.
form were retained on the resin and are included in the values shown. No attempt to remove the last traces of water was made. The only comparable data in the literature are those obtained for methanol and Dowex-1 X7.5 in chloride form,6cshown as crosses in Figure 1. Their agreement with our X8 data is quite good. The data available for ethano16b are for Dowex-2, which contains a 2-hydroxyethyl group instead of one methyl group in the quaternary ammonium exchange site. Because of the extra hydrogen bonding this permits, the results are not comparable with the present data. Solvent Selectivity. The data used to construct Figures 1 and 2 also permitted the evaluation of the solvent selectivity, Le., the mole fraction of the solvent in the resin, xs = ns/n as a function of its mole fraction in the equilibrium solution, XS. Some additional data have been obtained for a few solvents at given Volume 78, Number S March 1969
594
Y. MARCUSAND J. NAVEH I .o
I
0.8
0.8
-
-
x-4
I/
L.
0.8
0.6 -
0.6
I
X- 8
-
1t
/’
4
I
-
0.6
x-
I
1
xS
0.4
-
0.2
-
6
I/ A‘
/
012
014
016
018 xS
xS
Figure 4. The solvent selectivity, 28 as a function of X S , for perchlorate form resin; X8, upper part; X4, lower part. Symbols for the solvents same as in Figure 1.
Figure 3. The solvent selectivity, 2s as a function of 28, for chloride form resin: X8, upper part; X4, lower part. Symbols for the solvents same as Figure 1, with addition of 0, dimethylformamide; 0 ,acetonitrile; (3, urea; 0 ,ethylene glycol; 0 ,formaldehyde.
mole fractions, ie., iiot over the whole conipositioii range, and these have bern shown in Figure 3 along with the other data for chloride forin resin, while the data for the perchlorate forni resin are shown in Figure 4. Again, the data for niethanol aiid Dowex-1 X7.5 chloride in the literature6care shown as crosses in Figurc 3, in reasonable agreement with our data. For further correlation with published data, the selectivity results for the iiiterniediate 2s region, i.e., 0.2 < xs < 0.8, were treated according to the empirical equation suggested by Ruckert and Samuelson6b log ((1 - ZS)/ZS) = plog ( ( 1- z s ) / x s )
+ logk
(1)
and the values of the parameters p and log k shown iii Table I1 were obtained by a least-squares calculation. The values of p and log k are, of course, iiot iiidepei~dent,~” but the logarithmic data fit fairly well straight lines with the slopes p having the figures shown in the table, within the standard deviations shown. This procedure was used also by Ruckert and Samuelson, who found for several resins (Dowex-50 and Dowex-2) and couiiterions (Li+, S a + , K+, C1-, The Journal of Physical Chemistry
C104-, and S042-) with ethanol the value 11 = O.G, nearly the same as we found for Dowcx-1 X4 and X8 for chloride aiid perchlorate. The table shows that p is ?lot sensitive to the cross-linking and the anion also for the other solvents within the stated prrcision, but it is sensitive to the solvent. On the other hand, log I;, while not appreciably scnsitivc t o the cross-linking, is sensitive to both the anioii and the solvent. For all the systenir log li (Cl-) is larger than log I; (CIOL-), for ethanol tlic differeiicc being 0.28 =t 0,013. The corresponding differeiicc for D o ~ v e x - 2is ~ ~0.19, which is somewhat smaller, but nearly within the precision limits, and is certainly in the same direction.
Discussion The behavior of several aqueous-organic solvent mixtures has been presented, but the most complete data were obtained for the alcohols, methanol, ethanol, and 1-propanol, and these will therefore be discussed first. It is expedient to discuss three composition ranges of aqueous alcohols separately : the water-rich region, up to ca. xs = 0.2, the intermediate region, from xs = 0.2 to xs = 0.8, and the alcohol-rich region, above ca. zs = 0.8.
595
ANIONEXCHANGE OF METALCOMPLEXES ~~
~
Table 11:
Parameters
for the Empirioal Equation (1)
Solvent
Formamide Dimethylformamide
Methanol Ethanol 1-Propanol
Chloride----------log k
4
0.80 f0.04 0.80 f0.04
0.11 f0.02 0.09 f0.05
0.75 f 0 . 0 3 0.75 f0.03 0.55 f0.02 0.55 f0.02 0.25 f0.02 0.25 f0.02
0.16 f 0.04 0.21 f0.04 0.44 f0.04 0.46 f 0.03 0.76 f 0.05 0.74 f0.03
4 8 4 8
In the mater-rich region, the unexpected result that the alcohol is preferred in the resin over water is observed (Figures 3 and 4). The phenomenon is more pronounced for the perchlorate form of the resin than for the chloride form, but resins of X4 or X8 crosslinking yield approximately the same results. The preferred sorption decreases from propanol through ethanol to inethanol at the lowest alcohol concentrations, but reverses at the higher concentrations. Some results for 2-propanol and allyl alcohol with chlorideform resin show the same phenomenon of preferred sorption, even more than does 1-propanol. The explanation for these results should lie in the structure of water and the effect of low concentrations of alcohol on it, recently discussed in detail by Franks and Ives (ref 7, p 14). Water is much less structured in the resin than in the solution, and is less structured in the tight perchlorate form resins than in the swollen chloride form resins. There is then less water-water bond breaking, and more opportunities for alcoholwater bond formation, with overall decrease in enthalpy in the less structured situations. Furthermore, the larger the hydrocarbon portion of the alcohol, the more hydrophobic bonding it causes, and the more entropy is lost in the more highly structured phase. Therefore, again, an increase in entropy results if the alcohol is transferred into the resin phase. This behavior is similar to that exhibited by large ions with negligible specific affinity to the resin fixed ions, which are pushed from the structured external solution into the resin, according to the concepts of Diamond and Whitney.8 The x < 0.2 range is, however, not a simple one to understand, and although predominantly aqueous in character, “may be structurally very wide indeed.”’ The present work does not include results at such low alcohol concentrations or temperatures that anomalous (negative) deviations from Raoult’s and Henry’s laws and enhancement of the water structure9 are observed for the alcohols. It would be interesting to obtain data for the very lowest concentrations, where also the water-immiscible but partly soluble butanols could be used. Effects other than water structure could also be
\
log k
P
P
8 4 8 4 8
Perchlorate
r-
Resin -X
0.80 f 0 . 0 4 0.80 f0.04 0.76 f 0.04 0.75 f0.04 0.75 f0.03 0.75 f 0.03 0.55 f 0.02 0.55 f 0.02 0.25 f0.02 0.25 & 0.02
-0.03 -0.05 -0.10 -0.16
f 0.03 f0.01 f0.04 f0.01 0.09 f 0.04 0.06 f 0.04 0.20 f 0.02 0.14 f 0.04 0.36 f0.05 0.27 f 0.05
supposed to cause the observed behavior. However, hydration of the ions, small as it is for the large chloride, perchlorate, and trimethylmethylene ammonium ions, need not be more important than alcoholic solvation. The alcohols have similar dipole moments and should thus have similar solvation entha1pies,l0and a compensation between the effects on the cation, the anion, and the djspersion forces with the resin skeleton niay occur. Thus, in spite of the meager knowledge of the quantitative aspects of structure in the water-rich region, it is concluded that this, rather than other effects, is primarily responsible for the observed behavior in this region. In the intermediate composition region, where the normal structure of the water has been largely broken and replaced by small units of alcohol-water associates, the concept of solvent sorting by ions is useful,” superimposed on a direct interaction of the hydrocarbon parts of the solvents with the resin skeleton by dispersion forces. The resin phase can be taken to constitute the “vicinity of the ions,” mhile the external phase is, of course, free from ions. Using our symbols in Padova’sll eq 1, it becomes
I:(1 - zs)/zs]/[
(1 - 2s)/29] = 10“
(2)
This is the same as our eq 1, with CY = log k , except that our denominator has the power p < 1. Xo explanation can be offered for the values of p , although it is significant that they are independent of the resin and the ions, so that they probably are not due to selective solvation effects. Thus, the ratio of alcohol to water is lower in the resin over and beyond the ratio predicted from selective solvation. This may be due to steric effects, a point discussed further below. The CY values (Table 11), however, are qualitatively as expected: the span of selective solvation is larger for (7) F. Franks and D . J. G . Ives, Quart. Rev., 20, 1 (1966). “Ion Exchange,” Vol. 1, J. A. Marinsky, Ed., M . Dekker, New York, N . Y., p 297. (9) G.L. Bertrand, F . J. Millero, 0.Wu, and L. G . Hepler, J. Phys. Chem., 7 0 , 699 (1960). (10) B. Case and R. Parsons, Trans. Faraday Soc., 6 3 , 1224 (1967). (11) J. Padova, J. Phys. Chem., 7 2 , 796 (1968). ( 8 ) R . M . Diamond and D. 0.Whitney, in
Volume ‘79, Number 9 March 1969
Y. MARCUSAND J. NAVEH
596 the smaller anion chloride than the larger perchlorate, and water is the more preferred the larger the alcohol.11 In this respect it is important to note that the rejection of the alcohol is correlated with the lorn total smelling in this region. Thus, plots of f i e us. xs (not shown) for ethanol, 1-propanol, 2-propanol, and allyl alcohol show plateaus in the region 0.2 < xs < 0.8 or even slight maxima (for the perchlorate forms of the resins). In other words, as the mole fraction of water ~ apin the external solution decreases, f i decreases proximately linearly, while f i s remains constant. This behavior, as well as a maximum in the organic solvent content, was found also for the sorption of acetone on cation exchangers,jc and is shown only with solvents containing a substantial aliphatic portion, i e . , not with methanol, formamide, or diniethylformamide. It is conceivable that the solvents ethanol, propanol, and allyl alcohol saturate the resin by dispersion interactions with the organic skeleton, irrespective of solvent composition. This does not explain why there are 2-3 mol of solvent per benzene ring in the chloride form of the resin, and only ca. 1 mol per ring for the perchlorate form, since the dispersion interactions should be the same. Again, steric effects may be responsible for this. In the alcohol-rich region, the selectivity for water over alcohol becomes extreme. Other workers too have noted that certain solvents are not sorbed by ion exchangers unless some water is present. Thus no dioxane is sorbed on a cation exchanger unless it contains at least one mole of water per equivalent of resin.5f Thus, in the chloride form of the resin, where the residual water at the highest solvent concentration was still 0.3-0.6 mol/equiv, the lis vs. xs curves turned up again at the end of the plateau, whereas with the perchlorate form, where the residual water mas only 0.1-0.3 mol/equiv, the curves turned downward. In this region also, the differences between the various alcohols are very pronounced (Table I ) . Steric effects among the isomeric alcohols may explain the lower swelling by 2-propanol, in particular if association of the alcohols is taken into account.’ Interaction of the x electrons of the allyl alcohol with the aromatic rings may explain its relatively high swelling power. The two amides studied, formaniide and dimethylformamide, also show interesting behavior in that their total swelling curves with the perchlorate form of the resin show maxima (Figure 2), while they show only slight deviations from nonselective swelling in their mixtures with water (Figures 3 and 4 and Table 11). Finally, the higher swelling of the perchlorate form of the resin by dimethylformamide than by the with the Opposite behavior Of the chloride form of the resin, is Significant. It is known12
The Journal of Physical Chemistry
that aprotic dipolar solvents solvate anions the better the larger they are, so the stronger swelling of the perchlorate resin than of the chloride resin by dimethylformamide can be explained by this solvation effect, Formamide, although protic, forms weaker hydrogen bonds than do methanol and alcohols in general, and so shows the same effect. As for their aqueous solutions, they too break the structure of water and form strong associates.’3 With the chloride form of the resin, forinainide behaves regularly, in a manner very similar to methanol. It probably fits into the hydrogen-bonded structure of the water, while modifying it, and competes with the water for the solvation of the chloride ion through hydrogen bonding, of which it is capable. There is thus little selectivity for either water or formamide (and even methanol) over the whole composition range. For the perchlorate form of the resin, the situation is different. Both amides solvate the perchlorate ion strongly, even more so than does water, probably by virtue of their larger dipole moment (3.86 D for diniethylformamide, 3.68 D for formamide, compared with 1.85 for mater and 1.67 for methanol), As xs increases, the quantity f i s increases ~ causing the maxima more rapidly than f i decreases, in the fi curves in Figure 4. These are more pronounced in the altogether more highly swollen X4 resin, possibly because of the prevalence of heteromolecular associates in the intermediate composition range, which are more stable than the homomolecular a s s o c i a t e ~ , ’and ~ ~ ~constitute ~ the “free” solvent in the highly swollen resin. The tighter X8 resin contains only “solvating” solvent, with little “free” solvent, and in this role the molecules of the solvents act singly. There are, of course, some subtle features in the data for total swelling (Figures 1 and 2) and selective swelling (Figures 3 and 4) , which cannot be explained with the above crude factors. S o quantitative theory for predicting the smelling behavior can yet be proposed. However, the data presented are consistent with our present knowledge of mixed aqueous organic solvents, their structure, and solvation properties. Further insight should be gained from the volumetric behavior of the systems studied, and experiments along this line are nom in progress.
Acknowledgment. The authors wish to thank 5Ir. Mayo Nissini for technical help in carrying out the experiments. (12)J. Miller and A. J. Parker, J. Amer. Chem. Soc., 83, 117 (1961); A. J. Parker, Quart. Rev., 16, 163 (1962);A. J. Parker, J. Chem. sot., A , 220 (1966).
(13) B. E. Geller, Zh. ~ i z Khim., . 35, 1105 (1961): T.>f. Ivanova and B. E. Geller, ibid., 35, 1221 (1961); W. Kangro, Z. Physik. Chem., 32, 273 (1962);J. A. Rupley, J. Phys. Chem., 68, 2002 (1964) (14)E. N.Vasenko, Doki. Lvovsk. Politekh. Inst.. 1, 84 (1956).
