Nuclear magnetic resonance study of the interaction between cation

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Nuclear Magnetic Resonance Study of the Interaction between Cation Exchange Resins and Alcohol and Water-Alcohol Mixtures William J. Casey' and Donald J. Pietrzyk2 The University of lowa, Department of Chemistry, Iowa City, Iowa 52242

Nuclear magnetic resonance data are reported for hydrogen-form, strongly acidic, cation exchange resins in water, methanol, ethanol, n-propanol, n-butanol, and water-alcohol mixtures. Gel and macroreticular (porous) type resins were investigated. The effect of crosslinking was considered in the gel type resin. A downfield shift in the peak for internal solvent was observed as a function of type of solvent. This behavior is discussed in terms of solvation of the sulfonic acid exchange sites on the resin. An observed shift for the interior solvent N M R peak in mixed solvent is discussed as a function of solvent composition. Comparison is made to p-toluenesulfonic acid in mixed solvent. Supporting data collected in mixed solvents were solvent uptake and solvent preference. Effects due to particle size, type of resin, and broadening are discussed.

Nuclear magnetic resonance (NMR) has been found very useful in the investigation of ion exchange resins suspended in water. Slow exchange between water within the resin and water external to the resin produces a separate signal from protons in both environments. Thus, the technique can be used for the characterization of cation and anion resins. Properties such as crosslinking, homogeneity, internal concentration of counterions, rate of exchange of water, hydration of counterions in the resin phase, and similarities between concentrated homogeneous solutions of electrolytes and the heterogeneous water-resin system have been evaluated (1-16). In some of these investigations, mixed solvents have been considered (9-11). Macro water levels in ion exchange resIPresent address, American Machine and Foundry Company, 689 Hope Street, Stamford, Conn. 06907. Author to whom reprint requests should be sent. (1) J . E . Gordon, Chem. Ind. /London), 1962, 267. (2) J . E. Gordon, J. Phys. Chem.. 6 6 , 1150 (1962) (3) R. H. Dinius, M . T. Emerson, and G . R. Choppin, J. Phys. Chem., 67, 1178 (1963). (4) D . Reichenberg and I , J . Lawrenson, Trans. Faraday Soc.. 59, 141 (1962). (5) R. H . Dinius and G . R. Choppin, J. Phys. Chem., 6 8 , 425 (1964) (6) J . P. deVilliers and J . R . Parrish, J . Poiymer Sci., Part A , 2 , 1331 (1964). (7) R . W. Creekmoreand C . N . Reilley, Ana/. Chem.. 42, 570 (1970). (8) R . W. Creekmore and C . N. Reilley, Ana/. Chem., 42, 725 (1970). (9) D. G . Howery, L . Shore, and B. H . Kohn. J. Phys. Chem.. 76, 578 (1972). (10) W . J . Blaedel, L . E. Brower, T. L. James, and J. H . Noggle, Ana/. Chem., 44, 982 (1972) (11) D . G . Howery and M . J . Kittay, J. Macromol. Sci.. Part A , 4 , 1003 (1970) (12) T. E. Gough, H . D. Sharma. and N . Subramanian, Can. J. Chem.. 48, 917 (1970). (13) A . Darickova, S. Doskocilova, S. Sevcik, and J . Stamberg. Poiym. Lett.. 8, 259 (1970) (14) L. S. Frankei, Can. J , Chem.. 48, 2432 (1970). (15) H . D . Sharma and N. Subramanian, Can. J. Chem., 49, 457 (1971). (16) H . D. Sharma and N. Subramanian, Ana/. Chem., 41, 2063 (1969). 1404

ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973

ins have been successfully determined by NMR (6, 16, 17). More recently, NMR has been used to study macroreticular (porous) ion exchange resins (18-20). These investigations have dealt primarily with ion exchange kinetics, measurement of hydrated porosity, and NMR spectral characteristics; these observations were also compared to those found for microreticular (gel) resins. Several investigations in this laboratory have illustrated the differences between the macro- and microreticular cation resins (SOBHtype) (21, 22). Since porous resins are particularly useful in nonaqueous and mixed solvents, it is important to establish the resin-solvent interaction because this information can serve as a guide for selecting a suitable resin and eluting conditions in a separation procedure and in selecting a resin to act as a catalyst in synthetic applications. The primary objective of this report is to utilize NMR techniques to investigate resin solvent interactions for the two types of resins in water, alcohol, and water-alcohol solvent systems.

EXPERIMENTAL Reagents. All ion exchange resins investigated were strong acid (S03H) type resins. Dowex 50W (D-50) resins, containing 2, 4, 8, and 12% crosslinking were obtained from J. T. Baker Chemical Co. (100-200 mesh, H form). Amberlyst 15 (A-15) and XE-284, a n experimental resin, were obtained from Rohm and Haas Co. as 25 (average) mesh beads. The resins were pretreated in a Soxhlet extractor alternating between methanol and benzene until the solvents remained colorless. The resins were subsequently treated in a column with eluting solutions in the order methanol. water, N a O H solution, sodium citrate solution, HC1 solution, and water. The resins were dried in a vacuum oven a t 110 "C and 8 m m for a t least 24 hr and then stored in closed containers in a desiccator (PzO5) (22). Other particle ranges for the A-15 resin were prepared by grinding in a blender and sieving of the dried powder (U.S. Standard) prior to the pretreatment procedure. Methods for purification of the alcohols were described previously (22). In general, the methods were chosen in order to minimize water contents. Procedure. Exchange capacities were determined by a column procedure ( 2 2 ) .A solution of 1F KNOB was passed through a column of H form resin of known, dried weight and the exchanged "03 titrated with XaOH. Capacities were found to be 5.36, 5.19, 5.12, 5.03, 4.72, and 2.87 mequiv of acid per gram of dry H form for D-50-X2, -X4, -X8, -X12, A-15, and XE-284, respectively. Mixed solvents were prepared by first weighing a n aliquot of water into a dry, tared flask. An aliquot of alcohol was added and the system weighed again. The mixtures were made a t 25 f 1 "C and maintained a t this temperature. From the weights and densities, the mole fraction of water in the mixture was calculated. The centrifugation procedure for the determination of solvent uptake has been previously described (23). In these studies, the (17) (18) (19) (20) (21) (22) (23)

H . D. Sharma and N . Subrarnanian, Ana/. Chem., 42, 1287 (1970). L . S. Frankei. Anal. Chem., 42, 1639 (1970). L . S . Frankel, J . Phys. Chem.. 75, 1211 (1971). L. S. Frankel,Ana/. Chem., 43, 1506 (1971). T. C. Gilmer and D. J . Pietrzyk, Ana/. Chem., 43, 1585 (1971). A . WiIks and D. J . Pietrzyk, Anal. Chem., 44, 676 (1972) D. J . Pietrzyk, Taianta, 1 6 , 169 (1969).

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A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973

1405

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Table I . Solvent Uptake and Molality

-600-

-

Resina

Solvent

D-50 X 2

Hz0 CH30H CzHsOH n-C3H70H n-CdHgOH

D-50 X 4

Hz0 CH30H CzH50H n-C3H70H n-CdHgOH Hz0 CH30H CZHSOH n-C3 H 7 0 H n-CdHgOH

r" -

-400

-300

-

d

2.0

4.0

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0.0

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Molality

D-50 X 8

Figure 2. Interior solvent peak relative to TMS vs. molality of solu-

tion inside the resin (D-50, 2, 4, 8, and 12% crosslinked) resin and solvent were equilibrated for at least 8 hr at 25 f 1 "C (for the more viscous mixtures, 48 hr were used) before centrifugation. In general, the solvent uptake d a t a were reproducible to within fl% or better. Water was determined with Karl Fischer reagent (Fisher Scientific) which had been diluted with methyl cellosolve. Methanol and sodium tartrate dihydrate were used as solvent and standard for the titration. In all cases, the resins were titrated within 1 hr (resin samples were stored in carefully sealed small glass containers) after being centrifuged with 10 min allowed for the titration. Titrations were done in a glove bag (N2) and end points were detected by the dead stop technique (Pt electrodes). YMR spectra were obtained with a Varian A-60 spectrometer. Dried resin was preswollen in a small dish with the appropriate solvent mixture and rapidly transferred to a n NMR tube which was filled with the same solvent. For pure organic solvent, the resin was added directly to the dry tube and the solvent added. A resin height of 5 cm was used, the tubes were capped and equilibrated at 25 f 1 "C for a t least 3 hr. Spectra obtained for samples equilibrated between 3 and 24 hr were identical. For the methanol-, ethanol-, and n-propanol-water mixtures, the internal NMR peak position was shown to be independent of resin to bulk solvent ratio (1 gram to 30 ml). For butanol-water, the aforementioned ratio had to be maintained for reproducible results. No change in peak position was observed for moderate temperature change. A TMS-CHC13 external reference was used for calibration. Depending on the experiment the peak positions were reproducible within the range of fl to 2 Hz.

RESULTS AND DISCUSSION A hydrogen form resin suspended in water will show well-defined resonances a t two well-separated positions in the NMR spectrum. One is due to the fluid or solvent in the interior of the resin while the second is due to the exterior solvent. The polymer itself does not contribute any peaks t o the spectrum. Furthermore, no spectral indication has been found up to this time which separates pore and gel type interior water which might be present in macroreticular type resin (19). NMR Spectra. Typical NMR spectra are shown in Figure 1. The peak furthest downfield for the cation resins corresponds to the interior solvent resonance while the next peak corresponds to the exterior solvent resonance. If DzO is introduced into the NMR tube, rapid equilibrium leads to a reduction of both the exterior and interior peaks. The interior resonance peaks, in general, are not so well defined in the alcoholic solvents as in water. The importance of the H-form exchange site is realized by comparison to resins without exchange sites. No interior solvent resonance is observed for XAD-2 resin in any of the solvents. (This resin is a neutral, macroreticular, spherical polystyrenedivinylbenzene copolymer. A similar result was obtained for a gel, 8% crosslinked polystyrenedivinylbenzene copolymer.) 1406

ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973

D-50 X 12

Hz0 CH30H C2H5OH n-C3H70H n-CdHgOH

A-1 5

Hz0 CH30H CzH50H n-C3H70H n-CdHgOH

XE-284

H20 CH30H CzH50H

Solvent uptakeb

Mole soivent, Mole S03H

Molality'

4.093 2.086 1.898 1.858 1.886 1.997 1.125 1.060 1.202 1.283 1.16 0.701 0.677 0.715 0.688 0.850 0.51 5 0.543 0.577 0.591 1.22 0.823 0.892 0.910 0.905 1.25 0.949 0.962

42.39 12.14 7.69 5.77 4.75 21.36 6.76 4.43 3.85 3.34 12 58 4.27 2.87 2.32 1.81 9.38 3.19 2.34 1.91 1.58 14.35 5.44 4.10 3.21 2.59 24.1 7 10.32 7.28

1.31 2.57 2.82 2.88 2.84 2.60 4.61 4.90 4.32 4.04 4.41 7.31 7.56 7.16 7.44 5.92 9.77 9.36 8.72 8.51 3.87 5.73 5.29 5.19 5.22 2.30 3.02 2.98

a Resins are in the H form D-50 resins are 100 to 200, A-15 IS 40-60, and XE-284 is 20-40 mesh Reported as g solvent/g dry resin. data for D-50 X 8 and A-15 are taken from ref 23. CThe capacities for the resins are reported in the Experimental section

Spectra in Pure Solvent. If an aqueous solution of a mineral acid is used, a linear paramagnetic shift for the acidic proton is observed as a function of acid concentration (24). A typical gel resin, such as the D-50 resin, produces chemical shifts analogous to this behavior. This similarity has been used to understand the gel resin-water interaction (2, 4, 7, 12). A shift for the internal solvent peak is also found when alcohols are used as solvents. In Figure 2, this shift (relative to TMS) is plotted us. molality for a series of different crosslinked D-50 resons. Table I lists the calculated molalities based on the experimentally determined solvent uptakes and resin capacities. Data for D-50 resin in water are included in Figure 2 for comparative purposes and agree with previously reported results (7, 22). It is apparent in Figure 2 that the shift in the alcohol solvents is similar to the shift observed for water. The slopes for the linear portions of the curves are nearly equivalent. These series of experiments, therefore, suggest that the alcohol molecules solvate the sulfonic acid group in a manner similar to the water molecules. The similarity between water and alcohols with respect to interactions with H-form resin have been suggested before. For example, difference in solvent uptake between Na and H-form resin was attributed to hydrogen bonding between alcohol and the latter resin (25, 26). More recently, heats of immersion observed for H-form resins in alcohols were also explained this way ( 2 2 ) . (24) G . C . Hood. 0. Redlich, and C . N. Reilley, J . Chem. P h y s , 22, 2067 (1962) (25) D. R. Reichenberg and W . F. Wall, d. Chem. Soc.. 1956, 3364. (26) F. Helfferich,"Ion Exchange," McGraw-Hill, New Y o r k , N . Y . , 1962.

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Solvation in the resin can be represented by the following equilibrium.

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External

Resin

(1)

The actual solvation number, 2.9, has been determined only for water ( 7 ) . From Figure 2, it can be seen that the downfield shift for the internal solvent follows the order n-butanol > npropanol > ethanol > methanol > water. However, because of exchange, the peak represents the contribution of each of the species, S03H, ROH, and ROHz+, as shown in Equation 1. Therefore, it follows that the probable order of shift is BuOHz+ > PrOHz+ > EtOHz+ > MeOH2+ > H 3 0 + . This is also consistent with the acidity that would be exhibited by the solvated proton; that is CH3CH2CHzCHOH2+ is the most acidic and H30+ is the least acidic. Spectra in Mixed Solvent. If water-alcohol mixtures are used, the internal and external resonance peaks in the presence of the resin are still observed. However, five species are now contributing to the system, S03H, ROH, ROHz+, H20, and H 3 0 + (see Equation 1).In order to understand this observation, a model compound of the resin, p-toluenesulfonic acid (PTS), was investigated in methanol-water mixtures. For this system, a single acidic, hydrogen peak, representing the average environment (PTS, HzO, H&+, MeOH, and MeOHZ+) is observed. Figure 3 illustrates the shift as a function of solvent composition. The concentration of PTS in all solutions was maintained a t 4.00M since this value approaches the concentration level in a typical resin such as D-50 X 8 (see Table I). As the composition of the system changes, the mole ratio of acid to solvent changes. Thus, the downfield shift is directly correlated to the mole fraction of PTS. Increased mole fraction of PTS results in a larger downfield shift. This downfield shift is also accomplished by an increase in the mole fraction of methanol. consequently, the net effect is a downfield shift because of the increased acid molality and because of the larger contribution from the more acidic alcohol solvated species to the average peak position (reaction 1). Figure 4 illustrates the shift of the interior solvent peak for several gel resins (D-50) containing different amounts of crosslinking and for two macroreticular resins (A-15 and XE-284) in water-ethanol mixtures. A similar set of data was obtained for methanol as solvent. Although npropanol and n-butanol were not examined as completely

D-50 X

Solvent 8

A-1 5

XE-284

Expressed as g

Hz0 H~O-C~HSOH C~HSOH Hz0 H~O-C~HSOH C~HSOH H2 0 HzO-C~H~OH CiHsOH

solvent/g

solution mole fraction H 2 0

Solvent uptakeU

1.000

1.16

0.246 0.000

0.91 8

1.000 0.246

0.000 1.000 0.246

0.000

0.677

1.22 0.980 0.892

1.25 1.007 0.962

d r y hydrogen form resin; see footnote a

Table I .

as methanol and ethanol, enough data were collected to state that the two former solvents generally affect the interior shift in the same manner as the latter two solvents. For all cases of mixed solvent, only one internal resonance peak was observed. This indicates that all solvent hydroxyl protons inside the resin are in rapid equilibrium with the acidic proton, just as in a homogeneous system (PTS in water-alcohol). Therefore, the observed position of the resonance is dependent on the weighed average of hydronium ions, protonated alcohol species, “free” water molecules, “free” alcohol molecules, and the sulfonic acid group. It can be seen from Figure 4 that the shift increases with increasing crosslinking for the series of gel resins (D50). This shift difference is due to a dependence on concentration. As the crosslinking increases, solvent uptake decreases or internal concentration increases (26). Thus, from Figure 4, it is apparent that a t any water-alcohol ratio, as the crosslinking decreases in the resin, the amount of solvent taken u p by the resin increases. Also, as the mole fraction of alcohol decreases, the total amount of solvent taken u p gradually increases. A difference in solvent uptake also accounts for the shift observed for the porous resins, A-15 and XE-284, in the alcohol-water system. Table I1 lists experimentally determined solvent uptake data for the XE-284, A-15, and D-50 X 8 resins which support the NMR observation. The fact that the porous resins take up ample quantities of solvent in comparison to the D-50 X 8 resin is not an indication of low crosslinking in the resin but rather an indication of low swelling (or contracting) because of a more permanent, rigid poA N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1 9 7 3

1407

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Table Ill. Resin Selectivity for Water over Alcohol Solvent

Resin

D-50 D-50 A-15

X 8 X 8

XE-284

D-50 X 1 2 D-50 X 1 2

External Internal solution mole solution mole fraction H20 fraction H20a

HzO-CH~OH HZO-CZH~OH H~O-CZHSOH HzO-CZH~OH HzO-CZH~OH HzO-CzHsOH

0.505

0.409 0.246 0.246 0.246 0.444 0.582

0.537

0.576 0.444 0.686 0.741

a Values reported are the average of three determinations.

01 100 CH30H

I

I

00

I

I

I

60

Per Cent

I

I

40

CH30H

1

I

20

by Volume

I

0 H20

Integration of the internal and external solvent hydroxy N M R peaks as a function of bulk solvent composition for Dowex 50 X 8, 100-200 mesh resin Figure 5.

rous-like property. In fact, the porous resins are highly crosslinked. On the basis of these experiments, it appears that the gel and porous resins are not directly comparahle on a simple relationship involving only the amount of crosslinking. A plot of NMR shift us. alcohol-water composition (Figure 4) is not linear in contrast to the observation for FTS (Figure 3) for the same solutions. However, the curves can be broken into four segments, each of which is consistent with the following explanation. In region IV, the shift is linear and covers from 40 to 60% of the total shift. This suggests a dependence on one, very acidic species. It is suggested that the protonated alcohol is the dominant species a t these conditions. The next section, region 111, is a transition zone prior to another linear segment and appears to be due to an increase in the influence of the hydronium ion species. Region I1 is linear and covers a small range in chemical shift. For example, a t 80% alcohol only 30% of the total shift is attained. This indicates that the dominant species is the hydronium ion. Furthermore, for this to occur the resin in this solvent region must show a large preference for water over alcohol. This preference by gel resins has been previously reported (25, 27-29). Table I11 lists additional data, which include results for porous resins, confirming the preference of water over alcohol even for the porous resins. Additional evidence for the solvent preference was obtained by integrating the internal and external solvent peaks (see Figure 5) as a function of solvent composition. The areas for the external and internal solvent increase with an increase in the per cent of water. Also, a t all compositions the area for the internal solvent is greater than for the external solvent. If the difference in integration is due only to the proton contributed from the dissociated sulfonic acid, the difference should appear as a constant (dashed line in Figure 5 ) . The deviation in the 50 to 100% alcohol region is evidence of the resin preference for water over alcohol, since for every molecule of water preferred over an alcohol molecule, there is a net gain of one proton. In region I, the lack of linearity is due to the system rapidly approaching complete dependence on the hydronium ion. Distribution coefficients, K D , for weak organic bases on these resins in alcohol-water mixtures (21) are consistent with the NMR observations in the same solvent mixtures. From 100% alcohol to 80% alcohol, the decrease in K D is (27) 0. 8.Bonner and J. C . Moorefieid, J. f h y s . Chem., 58, 555 (1954). (28) H. P. Gregor. D. Nobel, and M. H. Gottlieb, J. Phys. Chem., 59, 10

(1955). (29) R . W. Gabel and H. A. Strobel, J. f h y s . Chem., 60, 513 (1956).

1408

ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973

Table I V . Comparison of Distribution Coefficients for Weak Organic Bases with NMR Data in 100 and 80% Alcohol Distribution coefficient dataU Comp

o-Nitroaniline rn-Nitroaniline

NMR data

AKDb

11 51

5040 8770

p-Nitroaniline Caffeine

279 380 700 1230

Solvent

Resin

Methanol Ethanol n-Propanol Methanol Ethanol n-Propanol Methanol Ethanol n-Propanol Ethanol

D-50 X 8

A-1 5

XE-284

ANMRC

-76 -148 -198 - 38 -92 -150 -17 - 62

*

K D data taken from ref 27, 23; all are for A-15 H-form resin. l K D = KD(lOO% alcohol) - K ~ , ( 8 0 % )alcohol. l N M R = shift (100% alcohol) - shift(80% alcohol) for internal peak.

very rapid and approaches a minimum a t 80% alcohol. Similarly, the NMR chemical shift decreases rapidly. This is illustrated in Table IV where K D and NMR shift expressed as differences between 100 and 80% are compared. At 100% alcohol, the acidity in the resin interior decreases in the order n-propanol > ethanol > methanol. This is also the order for the change in K D and is consistent with suggestions that the retention of the weak organic base by the resin is an acid-base reaction (21). As water is added, the acidic character of the resin interior changes; similarly, K D decreases. A t 80% alcohol the resin preference for water is the strongest, and a t this point the greatest difference exists between internal and external solvent composition. Since the interior is richer in water, the exterior provides a more favorable environment for the weak base and a very small K D is observed. As the per cent alcohol is further decreased, the interior and exterior solvent composition become similar. Thus, the K D will increase since the acid-base reaction in the resin is still present. The minimum in K o , which results a t around 80% alcohol, is the lowest for n-propanol. This follows since the water preference for this system in comparison to the other alcohols is the largest. Table IV also lists the chemical shift difference between 100 and 80% alcohol. The greatest change occurs for npropanol. For given conditions the chemical shift difference is largest in the order D-50 X 8 > A-15 > XE-284. Consequently, it would be predicted that the largest change in KD (for bases) would occur for the D-50 X 8 resin and for the water-n-propanol system; the lowest for water-methanol and XE-284. Particle Size, Resin Comparison, and Band Broadening. Three major features characterize the NMR spectra of gel and porous H-form resins in water. Data for these spectra are summarized in Table V. It should be noted

Table V. Peak Width at Half Height and Separation between Internal and External Peaks as a Function of Particle Sizea Particle mesh size

1,Hz"

40-60 100-200 200-400 >400

-85 (-86) -82 -76 (-79) (-73)

Table VI. Peak Width at Half Height for Internal Solvent in Ethanol-Watera Mole fraction H20 external

Peak width at Peak width at half height for half height for internal solvent, external solvent Hz HZ

1.000 0.967 0.929 0.885 0.827 0.770 0.690 0.582 0.444 0.391 0.308 0.246 0.177 0.120 0.063 0.000

D-50 X 8 in H 2 0

3 (3) 5 6 (4) (6 5)

6 (7) 75 13 ( 8 ) (16)

A-15 in H20 40-60 60-80 100-1 20 120-200 >200

- 70

20-40 40-60 60-80 80-1 00 100-120 120-200 >200

-40

- 70 -67 - 68 - 67

7 8 10 13 18

7 8 8 8 13

XE-284 in H20

- 38 - 37 - 37 - 35

- 33 - 20c

8 5 12 15 16 22 C

45 7 7 75 11 25

C

C

a Data in parenthesis are taken from ref 2 A = Hz(externa1) Hz(interna1) Internal and external peaks overlap

D-50 X 2

D-50 X 4

D-50 X 8

D-50 X 12

3 3 3.5 4 4

3 3 3.5 3.5 4 4.5

5.5 5.5 6 6.5 7 7

7.5 8.5 8.5 8.5 9

5

-

4.5 5.5 6 6.5 7 8 8 9 10 -

13

5.5 6 6.5

7

8 9 10 11

10 11 11 11 12 14 15 17 18

10 11 12 13 16 17 21 25 30 36

A-15 13 15 16 18 21 21 23 27 32 36 39 43

74 90 98

XE-284

8 . 5 (226) 12 15 17 19 21 24 27 36

59c 68c 66c 8OC 1 ooc

a All resins 100-200 mesh except XE-284 which is 20-40 mesh. Value for 100-200 mesh. c' Broad, poorly-defined peak.

-

that in some cases, the particle size was obtained by sieving crushed resin and, therefore, the particles are not spherical beads. First, the separation between interior and exterior solvent peaks decreases as particle size decreases. At most, this change was observed to be 20 Hz. For very small sized beads ( -4-15 > XE-284. This order is consistent with the concentration dependency of the NMR signal from the internal protons. From Table I, the respective molalities are 4.41, 3.87, and 2.30. The third characteristic is band broadening. As particle size decreases (Table V), peak width a t half-height for the internal and external peaks increases. Peak width was also shown to be dependent on crosslinking and concentration of the alcohol. Data illustrating these effects on the internal solvent peak in mixed solvent are shown in Table VI. In general, broadening for the macroreticular resin is greater than with the gel resin. For example, the XE-284 resin has such broad peaks that in mixed solvent (Table VI) a 20-40 mesh size bead had to be used so that resolved spectra could be obtained a t all alcohol-water concentrations. Even with this size bead, the internal and external peaks a t high alcohol concentration were broad and overlapped. These observations are probably influenced by a lack of spherical beads but cannot be solely attributed to this factor since several of the mesh sizes were not obtained from crushed resin. In Table V, A-15 a t 40-60 mesh and XE-284 a t 20-40 and 40-60 mesh and in Table VI. XE-284 were not crushed fractions but sieved spherical resin beads. Also, all Dowex resins investigated were spherical. From early investigations with gel resins and water, it was suggested that interior line width would indicate

homogeniety of the interior structure of the resin (2) while others suggested that bead shape and the height of the resin sample relative to the sample coil in the instrument would influence the peak width ( 4 ) . Recently, Frankel has investigated the various parameters which may influence band broadening by comparing gel and porous resins of large controlled particle size in water (19). For these kinds of particles, he suggests that the principal influence on line broadening is due to the difference in susceptibility between the resin interior and exterior solvent. However, by using gel resins of very small particle size in water, the coalescence of the internal and external water peak and the band width as a function of particle size were suggested to be the result of exchange rate properties (13). Factors affecting peak broadening appear to be varied and complicated. However, several general conclusions regarding the gel and porous resin and the effect of alcohol on broadening can be stated. A decrease in chemical shift and an increase in band width as particle size decreases is observed for the porous resin as well as for the gel resin (see Table V) in water. As the crosslinking in the gel resin increases, the band width increases not only in water but also in pure alcohol and water-alcohol mixtures (see Table VI). Also, a greater change in internal peak width occurs for the porous resins in comparison to gel resins upon changing the solvent mixture from pure water to pure ethanol (see Table VI). One factor, which may influence band broadening and has only been briefly explored, is the degree of dissociation of the sulfonic acid groups in the resin. Recently, a fraction of the peak broadening in the NMR spectra (23Na magnetic resonance) of Na-form resins was attributed to gssociation between the sulfonic acid and the sodium ion ( 7 ) . It has also been reported that as the per cent of crosslinking increases, the acidity of the resin decreases (30). Received for review July 13, 1972. Accepted January 26, 1973. Taken from the Ph.D. Thesis submitted by W.J.C. to the University of Iowa, May 1971. The authors gratefully acknowledge the financial support of the National Institutes of Health (GM-15851). (30) 0. D. Bonner and A. L. Torres, J . Phys. Chem., 69, 4109 (1965)

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