1174
D. Nandan and A. R. Gupta
Solvent Sorption Isotherms, Swelling Pressures, and Free Energies of Swelling of Polystyrenesulfonic Acid Type Cation Exchangers in Water and Methanol Deoki Nandan and A. R. Gupta" Chemistry Division, Bhabha Atomic Research Center, Bombay 400085, India (Received July 13, 1976: Revised Manuscript Received January 3, 1977)
Swelling pressures (a)in H", Li", Na', and K" forms of Dowex 50W resins of 2,4, 8, and 12% DVB content in methanol and water have been determined from their solvent sorption isotherms, treating 1% DVB resins as reference "uncross-linked"exchangers. These swelling pressures have been discussed in terms of ionic solvation and ion pair formation in the resins. The linear relationships between a and the measured equivalent volumes of the various ionic forms have been discussed in terms of properties of soIvents particularly dielectric constant and ion-solvent interactions. The total electrostriction of the solvent at zero swelling pressure [ AVes(b)]and at maximum swelling [AVesl was found to be the same within experimental precision. The electrostriction per mole of solvent is much higher in MeOH than in water indicating stronger binding of methanol to cations in the resin phase. The integral free energies of swelling (AG,,) of these resins are negative. The -AG,, values follow the sequence H" > Li' > Na' > K" and 1%> 2% > 4 % > 8% > 1 2 % DVB. The contribution of the resin cross linking (swelling pressure) to the free energy of swelling of cross-linked resins has been computed with reference to 1% DVB resins. This contribution was found to be small and positive. It has been concluded that generation of swelling pressures is at the expense of secondary solvation of ions and osmotic effects.
Introduction have been reported earlier.8 From these solvent sorption isotherms, swelling pressures in the resinates have now Swelling behavior of polystyrenesulfonic acid (PSS)type been estimated using 1% DVB resinates as reference, cation exchangers in aqueous medium is well understood.'B2 "uncross-linked" resins. An attempt has also been made A number of studies on the water sorption isotherms of such resins and swelling pressures in them are a~ailable.~-~ to assess the separate contributions of ionic solvation and pressure-volume terms to the overall free energies of Such studies have shown that the swelling behavior of the swelling of the various resins. ion-exchange resins is governed by two processes: (i) hydration of the counterions and the ionogenic groups and Experimental Section (ii) osmotic effects. Swelling of the resinate stretches the polymer network and the resulting swelling pressure, then Methanol and water vapor sorption isotherms of the H', opposes the processes promoting the swelling of the resins. Li', Na", and K+ forms of 1-2% DVB Dowex 50W resins The same considerations would be applicable in a general have been obtained isopiestically covering the solvent way to the swelling of ion exchangers in any solvent. The activity range of 0.085-1.000 (MeOH) and 0.065-1.000 ion-solvent interactions (and indirectly ion-ion interac(water). For the lower methanolic activity range, LiC104 tions) would differ from solvent to solvent.8 solutions, and for the lower aqueous activity range, HzS04 Lately, there has been an increasing interest in ionsolutions were used. Solvent activities in these solutions exchange equilibria in mixed (aqueous-nonaqueous) and have been reported by Skabichevskii" (MeOH) and by nonaqueous solvents because of the enhanced selectivity Stokes and Robinson" (H20). Anhydrous LiC104 was prepared using lithium carbonate. All chemicals used were exhibited by ion exchangers in these The various factors governing the selective behavior of ion exchangers of AnalaR grade. The details of the isopiestic unit and in mixed and nonaqueous media have been identified."-13 other experimental details have been reported earlier.' Thermodynamically, the free energies of swelling of the Water and methanol sorption isotherms of 4,8,and 12% resinates, AGaW,in different solvents make significant DVB resins in the various ionic forms reported earlier have contributions to the exchanger selectivity. It has, generally, now been extended in the low solvent activity range from been observed that ion-exchan e resins swell to a lesser 0.175 to 0.085 (MeOH) and from 0.180 to 0.065 (HzO). extent in nonaqueous media.8,'0,B However, no systematic The time required for the attainment of isopiestic equilibrium for 1and 2% DVB resins was more than that investigation of the swelling behavior of ion-exchange for 4-12% cross linkings. At high solvent activities, a resins in nonaqueous media, which covers the solvent week's time was needed for equilibrium while at low acsorption isotherms and the swelling pressures, is available. tivities, upto 10 days time was required. The pressure-volume terms, which are insignificant from The methanol and water vapor sorption isotherms of 1% the point of view of ion-exchange selectivity, could be of DVB Dowex 50W resins in H", Li", Na', and K" forms significance in the individual free energies of swelling of are shown in Figure 1. The isotherms for 2% DVB resins the resinates. Besides, a comparative study of the swelling in different ionic forms were similar in shape. A typical behavior of PSS type resins in nonaqueous and aqueous set of methanol sorption isotherms of the variously media would help in a better understanding of the ioncross-linked H' resinate is shown in Figure 2. solvent and ion-ion interactions in the resin phase. The equivalent volumes of the vacuum dried 4,8, and In the present communication, the results of studies on 12% DVB resins in the H+, Li', Na', and K' forms were solvent sorption isotherms of various cross-linked (1%to estimated from resin densities measured using specific 12% DVB content) Dowex 50W resins in different ionic gravity bottles and toluene as the suspension liquid. The forms (H", Li", Na', or K') in methanol and water are equivalent volumes of these resins swollen in pure being reported. Similar data on 4,8, and 12% DVB resins The Journal of Physical Chemistry, Vol. 81, No. 12, 1977
1175
Solvent Sorption Isotherms of PSS in Water and Methanol 24
22
* m m * o c o * m m
20 18
16 14 12 10 8
a
l
m
m
o
6 4
o
l
0
r
m
i
2 0
J-9
t
-8
9
-7
C
-6 I -5
p
a t - 0 m r i c c * m m
4=
1
o
01
02
03
04
05
SOLVENT ACTIVITY
06
07
oa
0 09
IO
( aMeOH OR a,20)
5Z
c o o a m r i o o a m m r i
Figure 1. Methanol and water vapor sorption isotherms of Dowex 50wX1 resins at 298 K: ( 0 , O )H+ form; ( 0 , O )Li+ form; (@,ha) Na+ form; ( 0 , W ) K+ form.
methanol or water were also determined in a similar way using methanol or water in place of toluene. Surface dried resins were used for these determinations.
o r i a
m m
Results A. Swelling Pressures ( P ) . The swelling pressure in a resin swollen at a solvent vapor activity a, is given by
RT
n==ln-
a,
m m
l (11
Y
Y
m o a t -
vs
where V , is the partial molal volume of the solvent s (assumed to remain constant throughout the sorption process upto a, = 11,d , is the solvent activity inside the exchanger in equilibrium with outer vapor solvent activity a,. Generally a very low cross-linked resin (0.5-2% DVB) is treated as an “uncross-linked”reference exchanger.‘ At a, = 1,the solvent activity d, (inside the cross-linked resin under investigation) is given by the external phase activity at which the reference resin has the same molality as the fully swollen cross-linked exchanger. Swelling pressures have been determined from vapor sorption isotherms employing eq 1. In the present study, 1% DVB resins in the various ionic forms have been treated as the reference uncross-linked resins (Figure I). Swelling pressures computed for fully swollen resins (a, = 1) in methanol and water for 2, 4, 8, and 12% DVB resins in the H’, Li’, Na’, and K+ forms using eq 1 have been summarized in Table I. A comparison of P values in aqueous medium from the present study with those reported by Myres and Boyd‘ using 0.5% DVB reference resins shows a fair agreement except for 12% DVB resins (some differences are expected in resins from different batches). This indicates that the choice of 1%DVB resins is satisfactory. However, for making a comparison of swelling pressures in two different solvents, same resins should preferably be used as has been done in the present study. The data in Table I show that swelling pressures in MeOH medium are quite high for the Hfand Li+ forms
m m
m
m
m
* e l m
e
3:
4 E
G
c
o
N
m m r i m m m
U
The Journal of Physical Chemistry, Vol. 8 1 , No. 12, 1977
1176
D. Nandan and A. R. Gupta
TABLE 11: Free Energies of Swelling, AG,, (kJ/mol) of Dry H', Li+, Na+, and K+ Form Dowex 5 0 W Resinates of 1, 2, 4, 8, and 1 2 % Cross Linking6 in MeOH and Water a t 2 9 8 K
I 17
-AG,(MeOH)
%
crosslinking 1 2
HR
- AGsw (water)
LiR NaR KR IIR
LiR NaR KR
16.9 11.3 6.7 5.6 29.4 22.9 15.4 9.8 5.9 4.9 27.4 20.1 14.9 9.0 4.9 4.3 26.8 19.3 13.5 8.2 4.8 4 . 1 25.7 18.2 12.0 6.2 3.9 3.5 24.3 17.3
4
8 12
18.0 16.2 15.1 14.1 13.4
14.2 13.4 12.6 12.0 11.1
in equilibrium with the pure solvent).
V O
dl
02
04
03
05
" U
06
07
0.8
09
I
10
MeOH
Figure 2. Methanol vapor sorption isotherms of the Ht form of Dowex 50W resins of 1, 2, 4, 8, and 12% cross linkings at 298 K: 1% (0); 2 % (V);4 % (0); 8 % (Q; 12% DVB (A).
compared to their values for the Na+ and K+ forms. These differences are much more prominent for resins of higher cross-linking. A comparison of these 7r values with the corresponding values for water swollen resinates shows that H' and Li' forms show approximately as high values in MeOH as in water for all cross linking. On the other hand, Na" and K+ resins show considerably lower values in MeOH, in particular for the more highly cross-linked resins. The sequence for the swelling pressures in terms of ionic forms of resins in MeOH is thus TH+ > TL,+ > TN,+
> TK+.
The equivalent volumes (Ve)of the various resins at maximum swelling in both solvents have also been given in Table I. B. Swelling Free Energies. The swelling free energy, AG,,, of a dryion exchanger, iR, swollen upto activitya, is given by1i2 L
AG,,(iR) = -RTJas n, d In a, a,=O
+ RTn, In a,
-
V, = UT
or AG,,(iR) = - R T J a ~ ' l n ,d In a, a,=O
(at a, = 1)
(2)
where n, represents the number of moles of s absorbed at solvent vapor activity a,. Free energies of swelling of all resinates (H+-K', 1-12% DVB) have been computed from sorption isotherms using eq 2 (for a discussion of the extension of the n, vs. In a, curves to a, = 0, see ref 13) and summarized in Table 11. The extension of the reported sorption isotherms of 4,8, and 12% DVB resins to lower activities as well as a, = 1 has not resulted in any significant changes in AG,, values for these resins reported earlier.13 It should be emphasized here that AG,, values as reported in Table I1 represent the free energy change in the resin system when it goes from a certain reference state (dry ionic form) to its standard state (swollen resin The Journal of Physical Chemistry, Vol. 81, No. 12, 1977
Discussion Swelling Pressures. The earlier datal3 have indicated ion-pair formation in Na' and K+ forms of resins in methanol and appreciable solvation of the ions in H' and Li' forms of resins in the same solvent. In the aqueous medium, all the ionic forms show almost full solvation of the ions and no ion-pair formation." This possibility of ion-pair formation in some ionic forms of the resins in nonaqueous media has far reaching consequences for the swelling pressures in these resins. For all ionic forms in aqueous medium and the H' and Li+ forms in methanol, where ionic solvation is very prominent, swelling pressures are high and markedly influenced by resin cross linking (Table I). The effect of the resin cross linking is obviously due to the greater resistance of higher cross-linked resins to osmotic swelling. On the other hand, ion-pair formation in the Na' and K+ forms of resins in methanol results in very small swelling pressures and small variations in them with resin cross linking (Table I). It has been shown from the swelling pressure studies in aqueous medium that a low swelling pressure in a resin indicates greater preference of the resin for the ion.' The extremely low swelling pressures in the K' form of the presmt resins in methanol are in conformity with the high selectivity of these resins for K+ in methan01.'~ The increase in the swelling pressures in higher cross-linked resins should decrease the selectivity. The observed increase in selectivity of higher cross-linked resin is due to a relatively much greater increase in the swelling pressure of the Hf form, which is used as a reference resin for selectivity studies, with increasing cross linking. Equivalent Volumes of the Resins and Electrostriction of the Solvent. Boyd and Soldano3have observed a linear relationship between swelling pressures ( T ) and equivalent volumes (Ve)of the resin Dowex 50x8 in various ionic forms, i.e.
+b
(3 1
They observed an increase in the constant a, which refleds the elastic properties of the exchanger with the resin cross linking. If the constant a is truly a property of the resin structure and reflects the elasticity of the network, it should be independent of the solvent. The constant b has been interpreted by Boyd and Soldano3 as the volume occupied by the polymer network when the end-to-end chain distance is the same as in the monomer. Polystyrenesulfonic acid and its salts have highly extended structures in aqueous medium.'* The end-to-end chain distance in a polymer chain depends upon the interactions between adjacent ionogenic groups and the ionogenic group and the counterion. These interactions would depend upon the nature of the solvent. In a solvent of lower dielectric constant, "site binding" of the counterions would increase and this in turn would reduce the repulsion between adjacent ionogenic groups, leading to a decreased
1177
Solvent Sorption Isotherms of PSS in Water and Methanol
TABLE 111: Electrostriction Values a t R = 0 [ A v e , ( b ) ] and for Fully Swollen Dowex 50W Resins [ A DVB in H', Lit, Na' and K' Forms in Methanolic and Aqueous Systemsa
8%DVB
Ve ( dY '
a
H'
137
Li'
140
Na'
148
K'
152
7 (0.31) 11 (0.51) 13 (0.67) 14 (0.82)
10 (2.22) 9 (2.09) 8 (2.11) 9 (2.50)
21 (2.64) 12 (1.55) 11 (5.58) 11 (6.75)
25 (14.7) 8 (6.67) 8 (8.00) 8 (8.89)
Aqueous
resn
-
= Vr)
Aves
144 145 152 156
E,] of 4 and 8%
11 (0.89) 17 (1.50) 15 (1.50) 11 (1.26)
MeOH -
AFes(b)
15 (3.41) 12 (2.89) 12 (3.16) 7 (2.12)
AVe,
35 (6.36) 4 (1.01) 15 (8.38) 19 (11.80)'
Ayes(b)
32 (18.82) 5 (5.00) 12 (12.00) 12 (13.33)
Values in parentheses give electrostriction based on per mole of solvent absorbed. 260
220 -
240
200 180 160
-
- 140 8 I20 -
E 100 80 -
4--
40
I/
I/
eo -
7e[rn11
ieirnl~
Flgure 3. Variation of swelling pressure (9) with equivalent exchanger volume for Dowex 50W resins of 4, 8, and 12% DVB in MeOH and H,O: (0,O) 4 % DVB; (A,A) 8 % DVB; (E,l.) 12% DVB.
(v,)
extension of the resin network. This implies that constant b should depend upon the solvent. V, for the variously cross-linked resins in different ionic forms (Table I) have been plotted against P for methanol as well as aqueous systems (Figure 3). Though the points for methanolic systems look scattered, still a linear relationship between U,and P can be seen. The empirical relation, eq 3, is thus applicable to nonaqueous systems. The linear plots (different ionic forms but same cross linking) for different cross linkings in both the solvents converge to a common point a t zero swelling pressure. Again due to greater scatter of data in the methanolic systems, the convergence is not as good as in aqueous systems. Still a value of 180 f 5 mL for constant b can be derived from these plots for methanolic systems and 208 mL for aqueous systems. Boyd and Solda1-10~have also observed a comparable value of 192 mL for b for Dowex 50x8 resin in aqueous medium. The lower value of b in methanol is due to the lower dielectric constant of methanol which influences the endto-end chain distance in the unstrained form, as discussed above. The constant a is nearly the same for aqueous and methanolic systems for any specific cross linking. Its values (L atm-' mol-') for different cross linkings are as follows: 4% DVB, 0.14 f 0.02; 8% DVB, 0.9 f 0.1; and 12% DVB, 2.1 f 0.1. These data then confirm the following: (i) constant a is independent of the solvent but depends upon the cross linking and (ii) constant b is independent of cross linking but depends upon the solvent. From the above analysis, it is clear that resins swell to a volume b (VJ before swelling pressures start developing in the network, i.e., upto that point swelling pressures are zero. From the solvent sorption isotherms of the resins, the swelling pressures are zero upto the point where the
isotherms for the uncross-linked and cross-linked resins are identical, i.e., a t the solvent activity where the cross-linking effects appear in the sorption isotherms. It seems logical to identify Vbwith the zero swelling pressure point in the sorption isotherms. (This is also implied in the linear relationship between 7~ and the equivalent volumes of partially swollen resinates, observed by Boyd and sold an^.)^ The volume of the solvent imbibed by different resins in volume Vbcan then be computed. In this region of low solvent activity, the most important process taking place is the solvation of the ions. It is during this process that the solvent is subjected to the maximum amount of electrostriction. This electrostric_tion of the solvent in the resin swollen to volume Vbis [AV,,(b)] given by AVes(b)= IZ,'V,-
(Vb - Vr)
(4)
where n', is the number of moles of solvent absorbed by the cross-linked resin when its sorption isotherm separates out from the reference, uncross-linked resin, V, the molar volume of the solvent, and Vr the equivalent volume of the dry resin. The total electrostriction of the solvent when the resins are fully swollen (AV,,) can be calculated using the relation
(5) where n, is the number of moles of solvent in resin when swollen to the maximum. The computed values of AVes(b) and AVe, along with other details for Dowex 50WX4 and Dowex 50WX8 resins in water and methanol are given in Table 111. A comparison of AU,, and AVes(b)shows that they do not differ from one another by more than f3 mL, except in the case of the K+ form of Dowex 50WX8 in methanol. As these computations involve Ve,Vr, n's, n,, and V b , the values for ATe, and AVes(b)cannot be better than f3 mL. On the whole, one can conclude from the data in Table I11 that there is no further electrostriction of the solvent in the resin after it reaches a volume Vb. A similar behavior had been observed by the earlier workers als0.3319-22 They had noted that the partial molal volume of water in the resin becomes the same as the molar volume, after a certain stage of swelling has been reached. That stage in the swelling of the resins has now been shown to be the stage when the swelling pressures in the resin starts increasing from zero. From data given in ref 3, electrostriction of the solvent in the fully and partially swollen Dowex 50x8 (H' form) in water has been computed and was found to be the same at all stages of swelling beyond P = 0. This result when combined with the observed linearity between a and V, (partially swollen resins) suggests the constancy of the electrostriction of the solvent The Journal of Physical Chemistry, Vol. 81,
No. 12, 1977
1178
D. Nandan and A. R. Gupta
TABLE IV: A Comparison of AG, (kJ/mol) for Various Ionic Forms and Cross Linkings of Dowex 50W Resins for Methanolic and Aqueous Systems Computed on the Basis of Eq 6 and 7 2% DVB
Ionic form Ht Li' Na' K' H' Lit Na' K'
System MeOH MeOH MeOH MeOH H,O HZO HZO HZ 0
4% DVB
-
-
AG,,
Eq 7
Eq 6
Eq 7
Eq 6
Eq 7
Eq 6
Eq 7
0.54 0.48 0.06 0.03
0.46 0.38 0.02
0.71 0.71 0.06 0.03
0.76 0.72 0.02
1.08
1.32 0.92 0.03
1.31 1.20
1.08
1.12 1.12 0.70
1.14 0.80
0.60
0.57
1.12 1.12 0.75 0.62
1.32 1.20 0.04 0.01 1.21 1.58
0.01
1.02 0.63 0.46
1.08
+ RTJ?,=O a
= -RTJa~=' n , d In a, a,=O
n, d In a,
12% DVB
Eq 6
down to zero swelling pressure. In an earlier discussion of solvent sorption isotherms,' it has been suggested that the primary solvation of the ions is essentially complete in the solvent activity region where the solvent sorption isotherms of variously cross-linked resins are identical. The electrostriction data in Table I11 give further support to this interpretation. The electrostriction of the solvent generally increases with the electrolyte c ~ n c e n t r a t i o n .This ~ ~ effect can be readily seen if the ATes data in Table I11 are converted to AT,, per mole of solvent. The latter quantity generally increases with resin cross linking, and for the same cross linking it increases as the overall swelling of the resin decreases. The magnitude of the ion-solvent interactions is reflected in AV,,(b) per mole of solvent. For methanolic systems this is -11 mL compared to -2.5 mL in aqueous systems. The stronger binding of methanol to cations is clearly indicated. This is consistent with the free energies of transfer of the single cations from methanol to water.24 Free Energies of Swelling of the Resins. Swelling free energies, AG,,, would have the largest contribution from the free energies of solvation of the ions and thus would reflect the state of the ionic solvation in the resin phase. The data in Table I1 show that AG,, in the aqueous medium range from -29 (H' form, 1% DVB) to -11 kJ/mol (K' form, 12% DVB), thus showing appreciable solvation of the ions in the resin phase. In comparison AG,, for the Na' and K' forms of resins in methanol are -5 kJ/mol, indicating much lesser solvation of these ions in this medium. The free energies of swelling increase as the cross linking increases. The contribution of the cross linking, Le., of swelling pressure, to these free energies can be estimated by comparing the free energy of swelling of a cross-linked resinate (AG,,) with the free energy-of swelling of an uncross-linked reference resinate (AG,,) swollen to the same extent as the cross-linked one, Le., having the same molality in the resin phase. Using eq 2 for AG,, and AG,,, we obtain AG,,
8% DVB
- RTn,
In a, = AG,,
(6)
These differences between AG,, and AG,,, designated by AG,, (sp represents the swelling pressure), have been computed by graphical integration of n, vs. In a, plots between the appropriate limits. The computed values of AG,, for the various resinates in methanol and water have been tabulated in Table IV. These are positive quantities, emphasizing that the decrease in the free energy during the swelling of a cross-linked exchanger is less than in an uncross-linked one, swollen to the same extent. This then is the energy used in the extension of the resin network during the swelling process. Another approach to estimate AG,,, which uses swelling pressure data and directly gives The Journal of Physical Chemistry, Vol. 81, No. 12, 1977
1.00 0.06 0.03 1.25 1.36 0.97 0.74
0.01
-
0.01
0.08 0.03
1.33 1.51 0.78 0.71
1.31 1.59 1.14 0.97
1.01 0.82
the contribution of swelling pressures to swelling free energies, is as follows. Substituting for In a, in eq 2 from eq 1, we obtain
-
AG,,
=
-RT
1
+ Ing,
Jus" a,=O
n, d[
= -RT JEs
u,=O
2+
In as]
+n,RTr2
n, d lnZ, t n,RT I n a s
Transferring the first two terms on right-hand side, which are equal to AG,,, to left-hand side, we obtain AG,,
-
-
AG,,
= AG,,
(7) The values of AG,, from eq 7, computed by graphically integrating a vs. n, plots between the appropriate limits, are also given in Table IV. The good agreement between these two sets of AG,, values supports the above interpretation of swelling free energies of ion exchangers. As the primary solvation of the ions in the resin is essentially complete before the swelling pressures develop, the processes contributing to free energy changes in the solvent activity region where swelling pressures are positive are secondary solvation of the ions and the osmotic swelling. It is at the expense of these processes which involve comparativelysmall free energy changes that resin network is stretched. The small values of AG,, (- 1 kJ/ mol) are consistent with this analysis. The overall picture of the swelling of resins then emerges as follows. The solvation of the ions in the resin phase takes place at low solvent activities. This entails considerable electrostriction of the solvents. The swelling pressure remains zero while this is taking place though the volume of the resin increases. This volume increase, without swelling pressure, is due to the end-to-end chain distance in the polymer network approaching that of the unstrained monomer. The end-to-end polymer chain distance in turn depends upon the nature of the solvent and its interactions with the ions and ionogenic groups. As the polymer network in variously cross-linked resins or different ionic forms is identical, the unstrained fully stretched volume of different resins is also the same. In this region of solvent activity, the free energies of swelling are independent of resin cross-linking. In the region of higher solvent activity, resin network is stretched, swelling pressures develop, and the free energies of swelling are more positive, This loss of free energy, which is the energy spent in stretching the network, is at the expense of secondary solvation and osmotic swelling. There is no electrostriction of the solvent in this region. Acknowledgment. The authors wish to express their sincere thanks to Dr. M. D. Karkhanavala for his interest
Cu(I1)-Ethylenediamine Complexes on Zeolites
and encouragement during the course of this investigation.
References and Notes (1) F. Helfferich, "Ion Exchange", McGraw-Hill, New York, N.Y., 1962. (2) G. V. Samsonov and V. A. Pasechnik, Russ. Chem. Rev., 38, 547 (1969). (3) G. E. Boyd and B. A. Soldano, Z. Elektrochem., 57, 162 (1953). (4) H. P. Gregor, B. R. Sundheim, K. M. Held, and M. H. Waxman, J. Colloid Sci., 7, 511 (1952); J. Phys. Chem., 57, 974 (1953). (5) E. Glueckauf, Proc. R . SOC.(London), Ser. A, 214, 207 (1952). (6) G. E. Myers and G. E. Boyd, J . Phys. Chem., 80, 521 (1956). (7) 0. D. Bonner, V. F. Holland, and L. L. Smith, J . Phys. Chem., 80, 1102 (1956). (8) D. Nandan and A. R. Gupta, Ind. J. Chem., 12, 808 (1974). (9) G. J. Moody'md J. D. R. Thomas, Analyst, 93, 557 (1968). (10) Y. Marcus, Ion Exchange and Solvent Extractlon", Vol. 4, J. A. Marinsky and Y. Marcus, Ed., Marcel Dekker, New York, N.Y. 1973, pp 1-119.
1179
(11) A. R. Gupta, J. Phys. Chem., 75, 1152 (1971). (12) D.Nandan, A. R. Gupta, and J. Shankar, Ind. J . Chem., 10, 285 (1972); 11, 655 (1973). (13) D. Nandan and A. R. Gupta, J. Phys. Chem., 79, 180 (1975). (14) D. J. Pietrzyk, Talanta, 18, 169 (1969). (15) P. A. Skabichevskii, Russ. J. Phys. Chem., 44, 1162 (1970). (16) R. H. Stokes and R. A. Robinson, Id.Eng. Chem., 41, 2013 (1949). (17) G. E. Boyd, F. Vaslow, and S. Lindenbaum, J. Phys. Chem., 88, 590 (1964). (18) A. Caille and H. Daoust, J. Polym. Sci., Symp. Ser., No. 45, 153 (1974). (19) K. W. Pepper, D. Reichenberg, and D. K. Hale, J. Chem. Soc., 3129 (1952). (20) K. W. Pepper and D. Reichenberg, Z.Elekfrochem., 57, 183 (1953). (21) H. P. Gregor, F. Guttoff, and J. I. Bregman, J. ColloidSci., 8, 245 (195 1). (22) E. Hogfeldt, Acta Chem. Scand., 12, 162 (1956). (23) J. Padova, J . Chem. Phys., 40, 691 (1964). (24) J. Padova, J . Chem. Phys., 56, 1606 (1972).
Spectroscopic Characterization and Thermal Stability of Copper(II) Ethylenediamine Complexes on Solid Surfaces. 1. Synthetic Faujasites Types X and Y Paul PeigneurIt Jack H. Lunsford," Texas A & M University, Department of Chemistry, Coiiege Station, Texas 77843
Willy De Wilde, and Robert A. Schoonheydt Kathoiieke Universiteit Leuven, Centrum voor Opperviaktescheikundeen Coilddaie Scheikunde, De Croyiaan 42, 8-3030 Heverlee, Belgium (Received October 6, 1976) Publication costs assistedby the National Science Foundation (USA) and Diensten van het Wetenschapsbeieid (Belgium)
Bis(ethylenediamine)copper(II) complexes were ion exchanged from aqueous solution into zeolites X and Y. In Y-type zeolites only the bis complex was observed, but in X-type zeolites mono(ethylenediamine)copper(II) and aquo complexes were also detected in varying amounts, depending on the preparation. Exchange from solutions with an en:Cu ratio of one gives zeolites which contain bis, mono, and aquo complexes. Their relative amounts depend on the preparation conditions, the exchange level, and the type of zeolite. Tris(ethy1enediamine)copper(II)complexes were prepared by adsorption of ethylenediaminefrom the gas phase on dehydrated zeolites with small Cu(I1) loadings. The three complexes have apparent axial symmetry. The analysis of the EPR and electronic spectra of the bis complex on the surface shows a small but significant increase of the covalent character of the out-of-plane a orbitals with respect to the complexes in aqueous solution. The replacement of axially coordinated water molecules in solution by surface oxygens in zeolites is thought to be responsible for that effect.
1. Introduction The preparation, structure, and catalytic properties of copper(I1) zeolites have been the subject of numerous investigations involving various experimental technique^.'-^ Earlier studies dealt with the simple copper(I1) i ~ n . ~ More recently, attention has been given to complexes which may be synthesized within the zeolite f r a m e ~ o r k . ~ - l ~ These complexes are normally prepared by adsorbing the ligand from the gas phase into dehydrated copper(I1) zeolites. Ammonia, monodentate amines, or pyridine have been the most widely studied ligands. Cremers and co-~orkers'~,'~ have demonstrated that the ion-exchange properties may be drastically altered by complexing an ion before exchange. In an effort to understand this phenomenon we have attempted to characterize, by means of spectroscopic techniques, the copper-ethylenediamine complexes after they have been
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On Leave of absence from Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, De Croylaan 42, B-3030 Heverlee, Belgium.
exchanged into zeolites and certain clay minerals. In this paper data are presented on the mono(ethy1enediamine)copper(II) (Cu(en)2+),the bis(ethy1enediamine)copper(I1) (Cu(en)22+),and the tris(ethy1enediamine)copper(I1) (Cu(en)?+) complexes in the synthetic faujasites ~ X and Y. The ethylenediamine (en) molecule is one of the simplest bidentate ligands which is known to form stable complexes with a number of transition metal ions. In aqueous solution the logarithm of the stepwise stability constants for en with copper(I1) are 10.73 for the first and 9.31 for the second ligand.15 Since the log K 3 value is relatively small (log K3 N l),the tris complex can only be formed in concentrated ethylenediamine solutions. All three of these complexes have been studied extensively by EPR and optical spectroscopy as well as by x-ray diffraction, both in solution and in crystalline The mono, bis, and tris complexes have distinctly different EPR and optical spectra. The EPR and electronic spectra reflect variations in the anion that is associated with each The Journal of PhysicalChemistry, Vol. 81, No. 12, 1977
