2194
GILBERTE. JANAUER AND IRA 14. TURNER
The Selectivity of a Polystyrenebenzyltrimethylammonium-Type Anion-Exchange Resin for
Alkanesulfonatesla
by Gilbert E. Janauer and Ira M. Turnerlb Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 1Q001 (Received September 9, 1968)
The selectivity sequence for the series of n-alkanesulfonates (sodium salts) through n-heptanesulfonate was determined with analytical grade Dowex-1 type resin (chloride form) of 1, 4, 8, and 10% nominal DVB content. The variation of selectivity with resin loading and temperature was studied with the 8% cross-linked exchanger. Changes in selectivity behavior due to added organic solvents or urea were observed using 4 and 8% cross-linked resins. In aqueous media, selectivity was found to increase with increasing chain length of the counterions. Thermodynamicquantities were estimated from the selectivity data obtained at different temperatures. The exchange process 6- RSO3- e K O 3 - C1- was found to be entropy-directed for alkanesulfonates having a hydrocarbon tail consisting of more than four chain links. Hydrophobic interactions in the (external) aqueous solution phase are suggested as an important factor in controlling anionexchange resin selectivity for this family of aliphatic counterions.
+
Introduction The selectivity of strong and weak base anion-exchange resins for organic anions has been studied previously. It was found that with homologous series of anions such as the alkylcarboxylates or arylsulfonates resin selectivity generally increased with increasing size of the organic part of the anions. This was usually explained by assuming a superposition of the electrostatic interaction between exchange site and ionic “head group” of the anions, with some additional interactions ascribed to the organic “tails.” Because the latter should be responsible for any differentiation in a particular family of organic ions, it was of considerable interest to find out more about the nature of these interactions. Among the who reported work in this area severa14t5g7have suggested that van der Waals forces between the organic part of the anions and the resin matrix are, for the main part, responsible for the observed effect of increasing ion size upon selectivity. While not all investigators agreed in their conclusions, all but one research team6 considered the resin phase to be the only possible place where any selectivity could arise (which also had been a tacit assumption in earlier models of ion-exchange selectivity for all types of ionssng). It was pointed out, however, by Chu, Whitney, and Diamond6 that besides ion-site and ionmatrix interactions in the resin, ion-solvent, solventsolvent, and solvent-matrix interactions should be considered also. The selectivity model developed by Diamond and collaboratorsfl,’Ohas been very successful in predicting selectivity sequences for all kinds of anions. We were particularly interested in the postulated effect of local water structure in dilute aqueous solution upon resin selectivity for large organic ions, The Journal of Physical Chemistry
+
which is certainly one of the most striking features of this new selectivity model. Thus, the work reported in this paper was undertaken with the intent of testing the predictions of Diamond’s model with a series of organic anions which had not been previously studied. It was also hoped that more could be learned about the nature of the interactions in the solution phase. Previous results and models for interactions in aqueous solutionsl1-ls support the idea that local water structure may indeed be important for ion-exchange equilibria involving large hydrophobic anions. Gustafson and Lirio’4 reported recently the results of (1) (a) Presented in part before the Division of Physical Chemistry a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., Mar 31-April 5, 1968. (b) Participant in the SUNY/URP Program a t SUNY/Binghamton, 1967, and in the NSF/URP Program, summer 1968. (2) (a) S, Peterson and R. W. Jeffers, J . Amer. Chem. SOC.,74, 1605 (1952); (b) S. Peterson, Ann. N . Y . Acad. Sci., 57, 144 (1953). (3) H.P.Gregor, J. Belle, and R. A. Marcus, J. Amer. Chem. SOC., 76, 1984 (1954). (4) E.J. Shepherd and J. A. Kitchener, J. Chem. SOC.,86 (1957). (5) E. Tamamushi and K. Tamaki, Trans. Faraday Soc., 55, 1013 (1959). (6) B. Chu, D. C. Whitney, and R. M. Diamond, J . Inorg. Nucl. Chem., 24, 1405 (1962). (7) G. L. Starobinets and I. F. Gleim, Russ. J. Phys. Chem., 39, 1166 (1965). (8) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Co., Inc., New York, N. Y., 1962, Chapter 5. (9) D . Reichenberg, “Ion Exchange,” Vol. I, J. A. Marinsky, Ed., Marcel Dekker, Inc., New York, N. Y.,1966,Chapter 7. (10) R. M. Diamond and D. C. Whitney, ref 9, Chapter 8. (11) H . 8. Frank and M. W. Evans, J. Chem. Phys., 13, 607 (1945). (12) W. Kaunmann, Advan. Protein Chem., 14, 1 (1959). (13) G. Nembthy and H. A. Scheraga, J. Chem. Phys., 36, 3401 (1962). (14) R . L. Gustafson and J. A. Lirio, Ind. Eng. Chem. Prod. Res. Develop., 7, 116 (1968).
POLYSTYRENEBENZYLTRIMETHYLAMMONIUM-TYPE ANION-EXCHANGE RESIN research on the adsorption of organic anions by various anion-exchange resins, including a polystyrenebenzyltrimethylammonium-type resin of low cross-linkage similar to the resin used in this work. These workers determined selectivities for ethanesulfonate, benzenesulfonate, 2-naphthalenesulfonate, 2-anthraquinonesulfonate, t-butylcatecholsulfonate, dodecylbenzenesulfonate, and gallate ions (sodium salts, 25”, I = 0.10). The selectivities found increased with the number of aromatic rings and with the chain length of aliphatic substituents. The results were explained by a combination of electrostatic interaction and “hydrophobic bonding.” Gustafson and Lirio l4 also found evidence for stronger interaction of aromatic species with the resin matrix than for aliphatic ions containing an identical number of carbon atoms, which is not surprising if the possibility of x-x interactions between the aromatic systems in the resin phase is considered.16 It seemed advantageous to concentrate in our work on the aliphatic series of sulfonates to avoid enthalpic contributions to selectivity, arising from x-x interactions. Then, if the exchange enthalpies and entropies of exchange could be estimated from the temperature dependence of equilibrium selectivity, we could hope to gain some more insight into the nature of the effect of water structure upon strong base resin selectivity for aliphatic hydrophobic anions. Experimental Section Analytical grade, recycled polystyrenebenzyltrimethylammonium-type resins (AG-1, Bio-Rad Laboratories, Richmond, Calif ,) of nominal cross linkages of 1,4,8, and 10% divinylbenzene (DVB) were obtained in the chloride form. Sodium salts of alkanesulfonic acids were supplied and in several cases custom-synthesized by Distillation Products Industries, Rochester, N. Y. The purity of these alkanesulfonates (AS) was better than 99% on a dry weight basis. Approximately 0.2 M stock solutions were prepared by dissolving weighed quantities of these materials in distilled water. Stock solutions were then analyzed by passing aliquots through columns packed with AG-1x4 resin in the chloride form and by titrating the displaced chloride ions using the Volhard method. Other chemicals were all reagent grade, except for organic solvents, which were Fisher Certified ACS grade. Before use, all ion-exchange resins were washed with pure ethanol, rinsed with large quantities of distilled water, and air-dried for about 1 week. The water content of the air-dried chloride resins was determined by drying to constant weight in a vacuum oven. For determination of selectivity coefficients l-g batches of the air-dried resins were equilibrated with solutions containing the particular AS (and sodium chloride when appropriate) a t a constant ionic strength of I = 0.04. Equilibration was achieved in all cases by agitating for 24 hr in a ground-glass-stoppered erlen-
2 195
meyer flask on a water bath shaker whose temperature control allowed regulation to =k0.5”. After equilibration both the resin phases and aliquots of the equilibrium liquid phases were analyzed for chloride ion and material balance established. I n additional experiments it was also established that equilibrium could be attained from both sides. When equilibrium selectivity coefficients were determined a t 40, 50, 60, and 70°, the entire procedure for the separation of resin from the equilibrium solution was performed inside an oven which was kept at the appropriate temperature. Approximately 10 sec elapsed from the moment when an erlenmeyer flask was taken out of the thermostated water bath shaker until the moment when the bulk of the equilibrium mixture was transferred to the preheated filter assembly in the oven. Both flask and filtering apparatus then remained in the oven and the door was closed. Transfer of the remainder of the resin from the flask onto the preheated Buchner funnel was achieved by recycling the filtrate at intervals of 10 min during which the oven was completely closed. While this procedure was somewhat cumbersome and crude, the reproducibility of repetitive measurements was good when compared to that obtained a t 25 and 20”. The crude temperature control in these procedures was believed to be adequate in view of the small variation of selectivity coefficients with temperature (see Figure 5). Selectivity coefficients in mixed solvent systems were determined at 25” and at an ionic strength of I = 0.02. Solvent mixtures were prepared by adding measured volumes of the aqueous electrolyte solutions and the organic components directly into the equilibration flask. The mole percentage of organic component was then calculated using its density at 25” and neglecting any changes in solution composition due to selective swelling of the resins. I n the case of dioxane, where selective solvent uptake is quite marked” a maximum change of less than 1 mol % was calculated from literature data. This error becomes smaller as the water percentage of the mixtures increases and is considerably smaller with all other solvent systems used. Analyses for chloride in resin phases after equilibration, as well as capacity determinations, were all carried out by argentometric titration using a modification of a recently reported rapid procedure for capacity determinations for basic ion-exchange resins. I n the original procedure by Polyanskii, et a2.,l8 the chloride resin is washed with absolute ethanol and covered with an excess of 10% w/w solution of potassium sulfate. The (15) W. Slough, Trans. Faraday Soc., 55, 1036 (1959). (16) D. A. Skoog and D. M. West, “Fundamentals of Analytical Chemistry,” Holt, Rinehart, and Winston, New York, N. Y.,1963, Chapter 12. (17) C.W. Davies and B. D. R. Owen, J. Chem. SOC.,1676 (1956). (18) N. G.Polyanskii, M. A. Shaburov, and A. A. Efimov, Russ. J . Anal. Chem., 19, 1192 (1964). Volume 78, Number 7 July 1969
2196
GILBERTE. JANAUER AND IRA M. TURNER
displaced chloride is then titrated directly with silver nitrate using the Mohr method.l8 We found this procedure to give excellent agreement with capacities determined by a conventional elution procedurelg but less satisfactory for determining chloride in resin batches of low chloride content. However, addition of 50 ml of 0.1 M standard silver nitrate solution 15 min after slurrying the resinin 75 ml of 10% potassium sulfate solution (acidified with nitric acid) drove the reaction to completion, and the Volhard titration yielded good and reproducible results over the entire range of resin chloride contents. Consequently, the same convenient procedure was employed for both selectivity coefficient and capacity determinations. The latter were always carried out simultaneously with every run to avoid errors due to possible changes in moisture content of the air-dried resins. All selectivity coefficients reported in this paper were obtained by this procedure. The uncertainty of the concentration selectivity coefficients reported in this paper was *3%. At low resin loadings the error reached * 5 % according to our estimate from experimental errors entering into the mass law expression KCIRSoa = mequiv of RS03-/g of resin [Cl-] mequiv of Cl-/g of resin [RS03-]
(1)
Results
2.0
1.5
1.0
M 0 K W rO’
0.5 0 0 -!
0.0
-0.5
I
-1.0 IO
I
I I I I I I 0.40 0.60 0.80 RESIN MOLE FRACTION OF RSOg,F R S 0 3 I
0.20
I.
1.m
Figure 1. The variation with resin mole fraction ( 2 ~ ~ of 0 ~ ) equilibrium selectivity coefficients a t 25’ for the exchange of n-alkanesulfonates with chloride ion on AG-1x8 resin.
The thermodynamic equilibrium constant for a uniunivalent ion-exchange reaction B e A B can be defined as
estimated as being very close to unity at the low ionic strength (I = 0.04) of our solutions and could therefore be neglected considering the error limits of our experimental results.
and may be rewritten
Table I: Selectivity Constants for the Ion Exchange of n-Alkanesulfonate Ions with Chloride Ion on AG-1x8 Resin a t 25”
+
+
(3)
Selectivity constant,
where KIABis the “corrected selectivity coefficient”
KOIR801
1.00 0;35 iz 0.02 0.36 f 0.03 0.55 i 0.05 0.77 0.04 1.78 f 0.16 4.34i 0.29 15.2 f 2.9 1
(4)
andjB andj.4 the activity coefficients in the resin phase. With appropriate conventions for the reference statess19 the integral
can be evaluated in the usual way from the corrected selectivity coefficients determined over the entire mole fraction range. Table I is a list of the selectivity constants a t 25” for the 8% cross-linked resin AG-1x8 and the n-alkanesulfonates up to n-heptanesulfonate calculated from the experimental selectivity data at different resin loading shown in Figure 1. The correction factors (Y*:N~cI/ Y ~ R R S Ofor ~ ) the ~ experimental values of KClRso8were The Journal of Physical Chemistry
Figure 2 shows a plot of log KaRSoaa t 25” for half resin loading (XRB,,= 0.5) os. chain length of the AS for all resins studied. It has been showne that the corrected selectivity coefficient a t a resin mole fraction of X = 0.5 is generally a good approximation for the thermodynamic selectivity constant, i.e. K A ~
(19) Reference 8, Chapter 4.
[K’AB]~..O.rj
(6)
POLYSTYRENEBENZYLTRIMETHYLAMMONIUM-TYPE ANION-EXCHANGE RESIN
2197
1.00
m 0 Wr'
0.50
a0 Y
0.0 0
-0.50 T A K E N FROM R E F . ( $ )
t
-1.00 2
I
5
4
3
NUMBER OF C H A I N - L I N K S
7
6
,N
-
-
20 MOLE
30
PERCENT
40
IO MOLE
Figure 2. The effect of increasing hydrocarbon chain length on equilibrium selectivity coefficients a t 25' of 1, 4, 8, and 10% DVB cross-linked benzyltrimethylammonium resin for n-alkanesulfonates.
IO
0
50
60
DIOXANE
Figure 3. The effect of 1,4-dioxane on equilibrium selectivity coefficients at 25" of AG-1x4 resin for n-alkanesulfonates.
even when log K Ais~not a simple function of resin loading. This seems confirmed by our own results obtained with the 8% cross-linked resin as seen from a compari-
20 PERCENT
30 DIMETHYL
40
50
€0
SULFOXIDE
Figure 4. The effect of dimethyl sulfoxide on equilibrium selectivity coefficients at 25' of AG-1x4 resin for n-alkanesulfonates .
son of K C I values ~ ~ ~(Table ~ I) with corresponding values (Table IIC), which were obtained by interpolation on least-squares lines through points determined on both sides of 8 = 0.5. Figures 3 and 4 illustrate the effect of admixed dioxane and dimethyl sulfoxide, respectively, on the experimental selectivity coefficients of the Ag-1x4 resin for *AS a,s a function of composition of the solvent mixture a t 25". An ionic strength of I = 0.02 was maintained in all mixed solvent systems, and no attempt ~ to0 was made to keep the resin mole fraction 8 ~ close a value of 0.5 (XRSO~ actually varied between 0.2 and 0.7). The 4% cross-linked resin was used to ensure a reasonable equilibration rate even at the higher concentrations of the organic solvents. Table I11 gives a summary of selectivity data obtained with AG-1x4 and AG-1x8 resins in all mixed solvent systems studied a t a composition of 50 mol % of the organic solvent. Again, the ionic strength was 0.02 and 8 ~ 8 was 0 ~ variable. Column 5 of Table I11 shows selectivity coefficients of the AG-1x4 resin for AS a t resin loadings of 8 = 0.5, obtained a t I = 0.04 in the presence o f 6 M urea. 'The last column gives the ~ in aqueous solution (from corresponding K ' o . values Table 11). Figure 5 summarizes selectivity data obtained at different temperatures in the usual way as a Van't Hoff plot. Points a t all temperatures other than 25" represent K'0.5 values at the particular temperature. These are again assumed to give a fair estimate of the Volume 73, Number 7 July 1969
~
GILBERT E. JANAUER AND IRAM. TURNER
2198 ~
~
~~
~~
Table I1 : Equilibrium Selectivity Coefficients a t 25” for the Exchange of Alkanesulfonates with Chloride Ions on AG-1x1 Resin
Resin mole fraction, Exchange ion
fRSOs
Equilib seleotivity coeff,
Equilib seleotivity ooeff a t half resin loading,”
KCIRsos [K’01Rs0sl~-o.5
Equilib Resin mole fraotion, Exchange ion
CH+3HCHzSOa
ZR8Os
-
selectivity coeff,
Equilib selectivity coeff at half resin
loading,=
KCIRsos [K’C1R80’k-0,6
1.31 1.28 1.37
1.32
0.42 0.53 0.77 0.79
0.95 0.89 0.84 0.80
0.90
0.32 0.55 0.56
0.97 0.84 0.81 0.77 0.79
0.86
1.70 1.62 1.58 1.61
1.66
0.33 0.48 0.57 0.63 0.68
0.39 0.50 0.68 0.75 0.44 0.53 0.57
1.61 1.52 1.47
1.55
CHa(CHz)zSOa-
0.43 0.54 0.58
1.16 1.09 1.10
1.16
0.38 0.45 0.81
6.31 5.70 4.29
5.69
CHs(CHz)aSOa-
0.37 0.54 0.64 0.71 0.75
1.65 1.34 1.30 1.20 1.15
1.47
0.48 0.53 0.82
13.8 12.3 13.2
13.2
0.37 0.41 0.57 0.75
1.66 1.82 1.69 1.79
1.74
12.2 10.5 8.08 9.09
11.7
CHa(CHz)4SOa-
0.48 0.65 0.76 0.88
CHa(CHz)5SOa-
0.39 0.46 0.55
8.50 8.06 7.42
7.79
A. On AG-1x1 resin CHsSOs-
CHaCHzSOs-
CHs(CHz)aSOa-
0.35 0.49 0.59 0.77
38.3 30.7 28.2 10.6
C. On AG-1x8 resin CHsSOs-
0.45 0.49 0.52 0.59 0.63
0.34 0.35 0.35 0.34 0.35
0.35
CHaCHzSOs-
0.45 0.49 0.52 0.59 0.63
0.34 0.34 0.34 0.33 0.35
0.34
CHa(CHz)zSOa -
0.41 0.44 0.47 0.51 0.63
0.57 0.56 0.54 0.52 0.51
0.53
CHa(CHa)aSOs-
0.34 0.45 0.50 0.53 0.60
0.82
0.76
0.78 0.75 0.75 0.73
0.42 0.48 0.50 0.51 0.55
1.66 1.58 1.42 1.47 1.33
30.4
B. On AG-1x4 resin CHsSOa-
0.46 0.50 0.65
0.58 0.58 0.58
0.58
CHsCHzSOa -
0.30 0.43 0.58 0.62
0.54 0.52 0.51 0.51
0.52
CHs(CH2)Kh-
0.48 0.58 0.64
0.70 0.70 0.69
0.70
(CHa)zCHSOa-
0.44 0.46 0.53
0.59 0.61 0.59
0.60
CHa(CHz)aSOa-
0.35 0.51 0.61 0.72
1.01 0.96 0.94 0.99
0.98
Th,e Journal of Physical Chemietry
CH~(CHB)~SO~-
1.48
POLYSTYRENEBENZYLTRIMETHYLAMMONIUM-TYPE ANION-EXCHANGE RESIN
2199
Table I1 (Continued)
Resin mole fraction, Exchange ion
3RSOa
CHa(CHz)sSOa-
0.30 0.46 0.50 0.52 0.66 0.37 0.43 0.49 0.50 0.54
Equilib selectivity coeff,
Equilib selectivity coeff at half resin loading," Exchange ion
KCIRsos [K'ClRsosl?-0.6
4.76 3.85 3.79 3.71 3.17
3.84
19.6 17.3 14.2 12.0 12.7
Resin mole fraction, XRSOa
Equilib seleativity coeff,
KolRsOa [ K ' C l R ~ O ~ I Z ~ O . 5
CHs(CHz)zSOa-
0.44 0.51 0.60
0.54 0.52 0.48
0.52
CHs(CHz)aSOa-
0.45 0.62 0.76 0.80
1.11 0.98 0.99 1.01
1.08
1.88 1.70 1.60
1.85
4.42 4.39 4.16 4.11
4.43
13.7
D. On AG-1x10 resin 0.33 0.50 0.64
0.38 0.34 0.35
0.36
0.34 0.48 0.55 0.65
0.41 0.40 0.39 0.36
0.39
Equilib selectivity coeff at half resin loading,'
CHa(CHz)5SOa-
0.47 0.59 0.80 0.91
23.0 15.9 9.7
22.1
' Interpolated from plots of the individual K values given in column 3.
Table I11 : Equilibrium Selectivity Coefficients a t 25" for the Ion Exchange of n-Alkanesulfonates with Chloride on AG-1 Resins in the Presence of 50 Mol % ' of Various Organic Solvents (XRSOa Variable, I = 0.02) or in 6 M Urea (XRSO~ = 0.5, I = 0.04) Compared with K ' c ~ ~ " 'in Aqueous Medium (XRSoa = 0.50, I = 0.04) Reain
AG-1x4
AG-1x8
Exchange ion
----------
Solvent---
7
Methanol
Acetone
Dioxane
DMSO
Urea
Water
CHaSOaCHaCHzSOsCHa(CHz)zSOaCHa(CHz)aSOaCHa(CHz)rSOsCHs (CHz)5SOaCHa(CHz)eSOa-
0.61 0.52 0.62 0.58
0.46 0.45 0.48 0.45
...
0.63
0.31
0.10 0.21 0.12 0.11 0.06 0.04 0.03
0.40 0.32 0.28 0.42 0.95 2.20 5.85
0.58 0.52 0.70 0.98 1.65 5.66 13.1
CHaSOaCHsCHzSOaCHs(CHz)zSOaCHdCHz)aSOaCHa(CHz)SOaCHa(CHM0aCHa(CHz)eSOa-
0.39 0.40 0.42 0.44
0.33 0.30 0.29 0.19
...
0.42
0.11
0.35 0.34 0.53 0.76 1.47 3.82 13.6
...
...
...
...
...
...
...
corresponding selectivity constants. The slopes of the curves in Figure 5 are constant with the best straight lines shown in the graph. It is assumed that AH does not vary appreciably over the temperature range of this study which may be a fair approximation judging from the data (with the exception of methanesulfonate for which a slight curvature in the log K US. 1/T plot is apparent above 40'). Direct calorimetry would, in
...
0.19 0.18 0.12 0.10 0.08 0.08
...
... 0.59 0.51 0.50 0.72 1.25
...
... ... ... ... ... ...
... ... ... ...
...
...
principle, be a preferable technique for the determination of AH of exchange, but the lack of data for mean molal activity coefficients and apparent molal heat contents for the higher homologs of the n-AS series would still prevent a rigorous calculation of the thermodynamic quantities. The values for AGO at 25" given in Table IV represent the standard free energy of exchange for 1 mol of Volume 78, Number 7
July 1969
2200
GILBERTE. JANAUER AND 1.5(
I
I
I
I
I
I
IRA
n/r. TURNER
and are expected to give a reasonable estimate of the standard enthalpies of exchange. Entropy changes were computed using the fundamental equation
I
AG = AH - TAX
I.0C
(9)
and are expected to give a rough estimate for the actual values of ASo at 25" with the approximate magnitudes and the trends certainly significant. The accuracy of all our estimates depends on how well the particular values of K'0.6will approximate the actual selectivity constants.
m :+ 0.5C
L O Y 0
0
Discussion
-I
0.oc
4
-0.X
30
28
32
3 4
RECIPROCAL T E M P E R A T U R E , T
X IO-'
36
K-I
Figure 5. Selectivity of the AG-1x8 resin for n-alkanesulfonates RS a function of reciprocal temperature.
Table IV : Thermodynamic Quantities" for the Ion Exphange of n-Alkanesulfonate Ions with Chloride Ion on AG-1x8 a t 25" Exchange ion
AGO,
AH,
kcal/mol
kca1/ mo1
CHaSOa0.61 i: 0.03 CHaCHzSOa 0 . 6 0 f . 0.06 CHa(CHz)2SOa0.35 f.0.03 0.16 f 0.01 CHs(CHi)aSOaCHs(CHz)4SOs- -0.34 f 0.03 CHs(CH8)aSOa- -0.86 f.0.06 0.30 CHs(CH2)eSOa- -1.60
AS, eu
0.49 f 0.12 -0.40 f 0.20 1.26 f 0.09 2.1i: 0 . 4 1.65 f 0.19 4 . 4 k 0.9 1.85 f 0.27 5.7 f 1 . 2 1.46 f 0.19 6.1 f 1.3 1.54 f 0.46 8.0 f 3 . 0 1.38 f 0.30 10.0 f 4 . 1
5 The AH and A S values in columns 3 and 4 are not true standard heats and entropies of exchange but probably reasonable estimates (see text for explanation).
AS with 1mol of chloride (pure resinates of unit activity at equilibrium with 0.04 rn solutions of the corresponding electrolytes at activity 1). They were calculated using the relation AGO = -2.3RT
J-
ZRSOs = 1
log K'CIRSoadXFtSOa (7)
XRSOa=O
AH values at 25' were obtained by means of the equa-
tion d log. K
AH
ie., from the slopes of the least-squares lines in Figure 5 The Journal of Physical Chemistry
The role of the solvent in strong base anion-exchange resin selectivity for large hydrophobic anions has recently been discussed by Diamond and Whitney.lo According to these authors, the normal hydrogen-bonded water structure is to a great extent disrupted inside the resin phase, which will cause preference of the resin for ions which fit less readily into the ordered external water phase. This model can be tested by studying homologous series of organic anions in which for instance the size of a hydrocarbon chain is varied but the rest of the ion including the head group remains the same. We have chosen the family of aliphatic sulfonates as these anions derive from strong parent acids and exhibit (up to a chain length of eight carbon atoms) normal 1,l-electrolyte behavior.20 I n establishing the new selectivity sequence (see Tables I and I1 and Figures 1 and 2), we were able to show that the predictions of Diamond's model are valid for the series of n-AS with strong base resins up to 10% DVB content. We have also obtained some more information on the nature of the effect of external water structure in the exchange of these hydrophobic anions. The log KclRSoavalues plotted in Figure 1 increase slowly toward low resin mole fractions for the lower AS but curve up more rapidly in this same direction for the higher AS homologs. Such behavior is quite common and has caused for the AG-1x8 resin a relatively small ~ ~the~ discrepancy between the true values of K C I and corresponding interpolated K values determined for R = 0.5 (see Figure 2 and Table IIC). Table I is a list of the selectivity constants calculated from Figure 1 using eq 7. The selectivity of the 8% cross-linked resin in the chloride form increases slowly up to n-pentanesulfonate which is the first AS to be preferred over the chloride ion. Further increase in length of the hydrocarbon chain of the ion CHs(CHdnSO3- produces a much more rapid increase of resin selectivity. values given in Table I1 we have From the plotted all those for the n-alkanesulfonates in Figure 2. I n addition to these, Table IIB gives the values for 2(20) E. L. McBain, W. B. Dye, and S. A. Johnston, J. Amer. Chem. SOC.,61, 3210 (1939).
~
POLYSTYRENEBENZYLTRIMETHYLAMMONIUM-TYPE ANION-EXCHANGE RESIN
220 1
invalidate this qualitative argument. The effect of propanesulfonate, 3-methyl-l-butanesulfonate, p-toluaddition of dimethyl sulfoxide (see Figure 4) on resin enesulfonate, and 2-propene-1-sulfonate obtained with selectivity for the C6 and C7 homologs is quite similar the 401,DVB resin. It is seen that the straight-chain but less pronounced than the effect of dioxane. The isomers are preferred over the corresponding branchedselectivity for all other AS is considerably less affected chain isomers. This was also found for the series of by the addition of DMSO. It has been mentioned that alkylcaxboxylate ions6g10 and may be caused by a DMSO, which has at room temperature a dielectric constronger structure-making tendency of the straightstant of about 48,24 has itself some local structure26and chain species. 1 1 , 2 1 The comparatively high selectivity also interacts with water to form a stoichiometric of the resin for 2-propene-1-sulfonate is believed to readduct.26 Thus, it is probably much less effective for sult from a combination of the hydrophobic effect in the destroying the local structure of the external water external phase and the interaction of the free electrons phase than dioxane. at the carbon-carbon double bond with the aromatic Table I11 compares various organic solvents with a systems of the resin matrix. On the other hand, prespect to their ability for decreasing resin selectivities toluenesulfonate is taken up less by the resin than nfor various n-AS at a concentration of 50 mol % of the heptanesulfonate which has the same total number of organic component in the external solution. Appreciacarbon atoms. While we expect considerable interin resin selectivity for the lower AS were action in the resin phase for the aromatic c ~ u n t e r i o n , ~ ble ~ ~ changes ~~ only produced with dioxane, and the power of all its structure-making tendency in the external phase, if organic solvents to decrease resin selectivity for higher any, is probably much smaller than for the correspondAS falls off with increasing polarity of the solvent. ing aliphatic counterion.22 DMSO and methanol are less effective than acetone It is interesting to compare qualitatively our data which in turn is much less effective than dioxane. It for AS with those of Chu, Diamond, and Whitney6 obmay be reasonable to assume that this is the order of tained for the corresponding alkylcarboxylates (AC) increasing tendency to interfere with the hydrogenwith a resin of 10% DVB content. The effect of parent bonded water structure in aqueous mixtures of these acid strength could be expected to cause exactly what solvents. Although factors such as selective solvent is seen in Figure 2, namely, that the AS are preferred uptake by the resin and preferential solvation of ions over the corresponding AC, at least initially. However, were not explicitly considered, it seems possible to conthe two selectivity series start to approach each other, clude that the observed bulk effects of organic solvents after a chain length of five is reached, and we notice upon resin selectivity are evidence in favor of water simultaneously a substantial increase in the slope of the structure as a major driving force in the exchange of AS (and probably also AC) selectivity curve. These hydrophobic ions. It should be mentioned that Gustaftwo facts may be related to the progressively diminishson and Lirio14 also found a strong decrease in resin ing influence of the headgroups on the water structure alongside the hydrophobic chains. It has been preselectivity for naphthalenesulfonate when organic solviously shown by Clifford and Pethica2athat the influvents such as aliphatic alcohols or ketones were present in the external solution phase. ence of the headgroup in n-alkylsulfates extends over some five carbons atoms in the aliphatic chain. If their The influence of urea upon resin selectivity is illusconclusion is also valid for the n-alkanesulfonate series, trated in the last column of Table 111. The effect in 6 the effect of increasing chain length upon resin selectivM aqueous urea solution (at I = 0.04) is not as great as ity should become the dominant factor at some critical the effect caused by addition of some of the organic number of methylene groups per aliphatic counterion. solvents but is still quite noticeable. Several workers At this point we would expect the selectivities for the have shown that urea effectively decreases hydrophobic alkanesulfonates and alkylcarboxylates of corresponding interactions in s o l u t i ~ n . ~ Therefore, ~-~~ this result also chain lengths to converge. suggests that water structure is important for strong Figure 3 shows how the addition of l14-dioxane to base resin selectivity for hydrophobic anions. the external solution effectively removes selectivity for higher n-alkanesulfonates and even reverses the aqueous (21) J. A. V. Butler, Trans. Faraday Soc., 33, 229 (1937). selectivity sequence of the 4% cross-linked resin. (For (22) 0. D. Bonner, J . Phys. Chem., 72, 2512 (1968). the sake of clarity, we have left out some experimental (23) J. Clifford and B. A. Pethica, Trans. Faraday SOC.,60, 1483 points near cross-overs of some curves in Figure 3.) (1964). This behavior may be attributed to effective destruc(24) J. N. Butler, J . ElecfroanaE. Chem., 14, 89 (1967). tion of the water structure by the organic solvent, which (25) H. L. Schlaefer and W. Schaffernicht, Angew. Chem., 72, 618 (1967). eliminates a major part of the driving force for exchange A. J. Parker, Int. Sei. Technol., 28 (Aug 1965). (26) of the higher AS homologs. The fact that in water(27) M. J. Schick, J . Phys. Chem., 68, 3585 (1964). dioxane systems the resin phase will be relatively en(28) M. F. Emerson and A. Holtzer, ibid., 71, 3320 (1967). riched with water over almost the complete range of (29) J. M. Corkill, J. F. Goodman, S. P. Harrold, and J. R. Tate, solvent compositions studied can only enhance but not Trans. Faraday Soc., 63, 240 (1967). Volume 78,Number 7 July 1060
2202
GILBERTE. JANAUER AND IRA M. TURNER
As described above (see previous section) the results shown in Figure 5 were used to compute the thermodynamic quantities listed in Table IV. It is now seen RSOa $ C1 that the exchange reaction is enthalpy controlled up to n-butanesulfonate but becomes entropy controlled as a chain length of four carbon atoms is exceeded. It has been shown that hydrophobic solutes “tighten” the water structure around themselves and thereby decrease the entropy of the aqueous phase.ll-la This entropy may, on the other hand, be regained by removing the hydrophobic solutes from the solution, e.g., here by the ion-exchange process. It is interesting to note that Scheraga and coworkerP used an aqueous suspension of cross-linked polystyrene beads as a model system for studying the thermodynamics of “hydrophobic bonding” for carboxylic acids and aliphatic alcohols. The values for A S per methylene group (transferred from the aqueous phase to the polystyrene phase) reported by these authors30 are of similar magnitude as the average increments in A S of exchange for successive AS found in this work (see Table IV). To explain the seemingly irregular increase of AH (and associated increase of AS) for the first four members of the AS series, one might argue along the line of a conflict between the influence of the structurebreaking sulfonate groups1 and the increasing structuremaking tendency of the growing aliphatic chain. One would expect AH to become more positive as the hydrocarbon chain grows, as removing the hydrophobic tails (by the ion-exchange process) from the solution should reduce the number of hydrogen bonds in the dilute aqueous phase.la This is, of course, the trend we actually observed up to a chain length of four carbon atoms, but we have at this point no satisfactory explanation for the apparent decrease (or leveling off) of AH beyond a chain length of four. If, however, the interpolated values for the higher AS would be in error, then we would expect them to be on the low side, and the resulting errors in AH would go in this same direction also. The increase of A S of exchange with growing chain length continues through the entire series of n-AS studied and is in accord with the expected increase of hydrophobic interactions for these species in the dilute aqueous phase. The fact that A S per added chain link seems to become approximately constant beyond % chain of four carbon atoms may be related to the vanishing of the disturbing head-group effect at this distancezaas discussed earlier. A transition from over-all structure-breaking to overall structure-making behavior was recently shown to occur with the series of tetraalkylammonium ions (R4N+)as the alkyl chains increased in size, and the results of a complete thermodynamic study of the ionexchange selectivity of strong acid cation-exchange resins for this family of cations were recently published by Boyd and L a r s ~ n . While ~ ~ some of the conclusions reached for spherically symmetrical hydrophobic cations
a+
The Journal of Physical Chemistry
+ ma
may not apply to the case of linear aliphatic anions, it is significant that these authors found, by accurate calorimetric measurements, enthalpic and entropic trends very similar to the ones observed by us for the AS. They concluded from their results that hydrophobic interactions played an important role and that van der Waals forces were not important for resin selectivity in their system.
Conclusions (1) The selectivity sequence for alkanesulfonates was with all resins studied exactly the one predicted from Diamond’s model of strong base resin selectivity in dilute aqueous solutions. lo (2) The removal or decrease of resin selectivity for n-AS caused by the addition of organic solvents or urea to the aqueous phase constitutes new evidence for a close relation between local water structure and strong base resin selectivity for large hydrophobic ions. (3) The observed trend in AH of exchange for the 8% cross-linked resin with increasing chain length in the n-AS series does not support van der Waals attraction in the resin phase as the major selectivity determining factor for aliphatic anions. (4) The trend and approximate values determined for A S of exchange with growing chain Iength of the n-AS suggest that hydrophobic interactions of the structuremaking hydrocarbon chains of the aliphatic counterions in the dilute aqueous phase are a major driving force for the exchange of aliphatic anions on strong base resins. ( 5 ) From a comparison of selectivity data for alkanesulfonates and alkylcarboxylates6 it appears that head-group effects in aqueous solutions of straight-chain aliphatic anions fall off beyond a distance corresponding to four chain segments. Therefore, the selectivity constants for the anions CH,(CH2).X- may tentatively be expected to approach one another at higher chain lengths, irrespective of the nature of the head group X. Acknowledgments. One of the authors (G. E. J.) wishes to acknowledge financial support received for this project from the State University of New York Research Foundation in the form of two SUNY Faculty Fellowships and a Grant-in-Aid. Special thanks are due to Professor R. M. Diamond for encouragement, generous advice, and stimulating discussions. We also want to thank Dr. H. S. Sherry for helpful suggestions. For help with certain calculations and some enlightening discussions we would like to express our gratitude to Professors R. A. Robinson and D. D. Konowalow. (30) H. Schneider, G. C. Kresheck, and H. A. Scheraga, J . Phys. Chem., 69, 1310 (1965). (31) R. W. Gurney, “Ionic Processes in Solution,” MoGraw-Hill Book Co., Inc., New York, N. Y.,1953. (32) G. E. Boyd and Q. V. Larson, J . Amer. Chem. Soc., 89, 6038 (1967).
2203
PHOTOCHEMICAL DECOMPOSITION OF NITROSYL FLUORIDE Professors E. E. Schrier and B. McDuffie have contributed valuable suggestions throughout the project.
Miss D. House, Miss J. Bogusky, and Miss N. Mrassikoff have assisted in various experiments.
The Photochemical Decomposition of Nitrosyl Fluoridela by A. L. Floreslb and B. deB. Darwent The Maloney Chemistry Laboratory, The Catholic University of America, Washington, D . C . 40017 (Received September 17, 1968)
The photochemical decomposition of gaseous nitrosyl fluoride has been studied at pressures between 5.0 and 20 mm in the presence of ethylene and other hydrocarbons and inert gases. In the presence of C2H4, with or without inert gases, the products are NO and vinyl fluoride. The rates of formation of both NO and CzHsF are decreased by increasing pressure of CzH4 or inert gas; the ratio CZHsF/NO is constant a t 0.5 0.1 up to about 50 mm pressure. At higher pressures RNo becomes constant and finite, but R o ~ H ~ approaches F zero. The results are interpreted on the basis of the formation of electronically and, possibly, vibrationally excited ONF" which may decompose unimolecularly or either be deactivated or induced to decompose by collision with another molecule, The F atoms add to CzH4to form highly chemically activated C2HAF' which decomposes at low pressure and is collisionally deactivated a t high pressure.
*
I. Introduction Although the uv absorption spectrum of ONF has been reported,2 essentially nothing is known about the photochemical decomposition of that molecule. We have studied the photolysis of gaseous ONF in the presence of ethylene and other hydrocarbons a t room temperature.
11. Experimental Section A , Apparatus. Attempts were made to construct suitable reaction systems from Pyrex and Teflon. Even after vigorously drying the ONF and flaming the glass under vacuum, we found that a rapid reaction occurred between ONF and pyrex to produce NOz. Typically, the absorption spectrum of ONF vanished completely in about 20 min. A Teflon reaction system was constructed and shown to be free from conventional leaks at the various joints. However, the Teflon appeared to be somewhat permeable to air; it also slowly emitted a condensable gas and strongly adsorbed the reactants which were desorbed only very slowly under vacuum. The reaction system was finally fabricated from nickel, for the reaction vessel, and monel for the connecting tubing and loop traps. The valves were camoperated monel diaphragm type Hoke valves (CK 113) with Kel-F gaskets. Pressures were measured on a Bourdon-type spiral gauge made from Kel-F tubing (1 mm 0.d. and 0.1-mm wall). The gauge was found to be sensitive to 0.1 Torr. The monel and nickel connections were silver soldered. The window was a
fluorite disk (49.5-mm diameter and 6.5 mm thick) which was attached to the cylindrical nickel reactor by a thin coating of epoxy glue. The surfaces were rendered inert by repeated fluorination. Approximately 300 mm of ONF was admitted, allowed to stand for a day, and evacuated. That process was repeated until the dark reaction was negligible, e.g., less than 1 pmol of NO formed in 30 min from 10 mm of ONF. The volume of the reaction vessel was 1978 cm3. The light was obtained from a medium-pressure mercury arc. The unfiltered light was collimated by a quartz lens and covered the entire window. The experiments were done in random order to minimize the effect of a possible systematic change of intensity. B. Reagents. The ONF was obtained in cylinders from the Ozark Mahoning Co., Tulsa, Okla. Typically, it contained approximately 2% NO as an impurity. The absorption spectrum was measured between 2000 and 5500 A in a fluorinated nickel cell with fluorite windows on a Bausch and Lomb Model 505 spectrophotometer. No identifiable impurity was detected, except for a weak absorption a t 2200 A, which may have (1) (a) This work was supported by the Office of Naval Research, Department of the Navy, under Contract No. Nonr. 2249(10) with the Catholic University of America. (b) Abstracted in part from a dissertation submitted by A. L. Flores to the Graduate School of Arts and Sciences in partial fulfillment of the requirements for the Ph.D. degree from the Catholic University of America. (2) H. J. Bertin, Jr., Ph.D. Dissertation, Stanford University, 1967.
Volume 79, Number 7 Julg 1969
