ADSORPTION OF ORGANIC IONS BY ANION EXCHANGE RESINS R I C H A R D 1. G U S T A F S O N A N D
Research
ljiii5ion,
JOSEPH
A.
LlRlO
Kohm and Haa5 Co , I>hiiade/phin l'n
19l.li
The selectivities of poly( N,N,N-trimethylvinylbenzylommonium) chloride (I), the protonated form of poly( 3-N,N-dimethylaminopropylacrylamide) ( II) and poly( 3-N,NrN-trirnethylammoniurnpropylacrylamide) chloride ( Ill) anion exchange resins for ethanesulfonate ( ES), benzenesulfonate ( BS), 2-naphthalenesulfonate (NS), 2-anthraquinonesulfonate (AQS), fed-butylcatecholsulfonate (TBCS), dodecylbenzenesulfonate (ABS), a n d gallate (GAL), ions have been measured a t 25OC. and = 0.10. The following values of molal selectivity coefficients, K Z , , for the binding of organic ions by resin I were obtained: ES, 0.67; BS, 7.4; NS, 133; GAL, 150; TBCS, 400; AQS, 1290; ABS, 32,000. The selectivities of all the resins for organic species increase markedly as the number of aromatic rings in the organic ion increases. For species containing a single benzene ring, the selectivity increases as the chain length of the aliphatic substituent increases. The selectivities of the above resins for a given organic species decrease in the order I > II N= Ill. The high selectivities observed are produced by a combination of electrostatic interactions and hydrophobic bonding. The influence of the latter effect i s reduced considerably by the addition of nonaqueous solvents. The selectivity of resin Ill for naphthalenesulfonate decreases in various solvents in the order H2O > > > 5 0 % CHiOH > 50% CiH-,OH > 50% n-C,H-OH Y 50% (CH i)K O .
ALTHOUGH several
studies of the binding of organic cations by cation exchange resins have been reported, virtually no measurements of the selectivities of anion exchangers for organic ions have been undertaken. An understanding of the nature of such binding is of considerable importance since one of the chief problems associated with the use of anion exchange resins in water treatment is the virtually irreversible fouling of the resins by such naturally occurring materials as fulvic and humic acids, and knowledge concerning the binding of organic ions by anion exchangers will aid in the proper development and selection of resins for use in situations in which organic fouling is a potential problem. I n this study, the influence of the chemical structure of the resin upon the equilibria and kinetics of binding of model organic species has been evaluated.
7.9,
---,
Experimental
' (--L
Kesins. Samples of poly(iV,N,L'V-trimethylvinylbenzylammonium) chloride ( I ) , poly ( 3-N,AJ-dimethylaminopropylacrylamide) ( I I ) , and poly(3-N,hr,~V-trimethylammoniumpropylacrylamide) chloride ( I 11) were conditioned by alternate treatments with 1M NaOH and 1M HC1.
, _ , \ - ,
116
II
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Following the final exhaustive washings with HC1, the weak base resin (11) was rinsed with ethanol and the quaternary ammonium materials were rinsed with water. All samples were air-dried a t room temperature. Resin capacities were determined by elution of chloride with NaNO, and subsequent coulometric titration of chloride ion. Properties of the various resins are summarized in Table I. Keagents. Most of the organic reagents were the best grades obtainable from Eastman Organic Chemicals Department. Stock solutions ( 0 . 2 M ) of the mono-sodium salts o f ethanesulfonic acid (ES) , benzenesult'onic acid (HS),2-naphthalenesulfonic acid (SS). ?-anthraquinonesulfonic acid ( A Q S ) . tert-butylcatecholsulfonic acid (TBCS), gallic acid (GAL), and dodecylbenzenesulfonic acid (ABS) were prepared. The ABS. which was obtained from the Soap and Detergent Associa-
Table 1. Properties of Chloride Form Resins Used in Measurements of Selectivity for Organic Ions
Resin IA IB IC 11 111
Crosslinher
4LtD V B ~ 4'r DVB'
C. HLO Anion Exchange Capacity, Meq.;G. Meq. Hydrated Resin 3.60 3.63 4.20 5.71 4.53'
0.465 0.342 0.303 0.252 0.369
I Poly(N,N,iV-trimethylvinylbenzylammonium)chloride I1 Poly(iV,N-dimethylaminopropylacrylamide) I11 Poly(N,N,N-trimethylammoniumpropylacrylamide)chloride "Small amounts ( I to 2%) of D V B used but crosslinking through methylene bridges also contributes significantly. * Diuinylbenzene. Of the total anion exchange capacity of 4.53 rneq.,:g., 4.36 is quaternary ammonium capacity. Balance of capacity is provided by functional groups of structure I I .
tion, S e w York, and which contained 36%; NaSOd, was passed through a column of Amberlite XAD-2 (a high surface area styrene-divinylbenzene adsorbent) in order t o adsorb the ABS. T h e column was rinsed with water to remove all traces of inorganic salts, after which the ABS was eluted with acetone. The solvent was evaporated on a hot plate and the resultant sirupy ABS solution was diluted with water. This stock solution was standardized by passing an aliquot through a column of chloride form Amberlite IRA-401 and determining chloride ion in the effluent. The concentrations of the other reagents were determined by passage of aliquots through H * form Amberlite IR-120, followed by titration of the acid produced. Equilibrations. Samples of chloride form resins (3.000 + 0.002 meq.) were equilibrated with 100-ml. volumes of solution, so prepared that the total concentration of organic and chloride ions was 0.100N, except in the cases of the GAL and TBCS systems in which the total ionic concentration was 1.00M. The concentration of chloride in the aqueous phase was determined coulometrically, while the concentrations of the aromatic organic species were determined spectrophotometrically a t the following wavelengths: BS, 262 mfi; NS, 273.5 my, AQS, 255.5 mfi; TBCS, 290 mp; GAL, 250 mfi; ABS, 225 mp. The concentration of sodium ethanesulfonate was determined by subtracting the chloride concentration from the total anion concentration as determined by passage of an aliquot of solution through a column of chloride form Amberlite IRA-400, followed by titration of chloride ion. The concentrations of species in the resin phases were calculated by difference. Kinetic Experiments. Rates of adsorption of organic ions were measured in infinite bath experiments as described by Boyd et d.(1947). Dilute solutions (0.001M) of organic ions were passed through shallow beds of resin beads a t a sufficiently rapid rate so that the change in concentration upon contact with the resin was less than 2%. After contact was maintained for a specified period, the resin was flushed with water and the residual chloride in the resin was determined. The amount of organic taken up was assumed to be equal to the number of milliequivalents of chloride desorbed. T o prevent loss of HC1 because of hydrolysis in the case of the weakly basic resin 11, the stock solutions of organic salts were also 0.001M in HC1. Since the selectivities for the organic ions are high, it is felt that the effect of this small amount of chloride upon the rate of adsorption is negligible.
Results and Discussion
Equilibria. The values of selectivities of resin IA and the hydrochloride form of I1 for various organic ions are shown in Figures 1 and 2. The selectivity coefficients, Kpl were calculated by the relationship
K&
=
meq. 0- in resin meq. ~ 1 in- resin
.- [Cl-], [O-1,
where 0 - represents the molar concentration of organic ion. Values of K increase markedly as the aromaticity of the organic ion increases or the aqueous solubility of the organic ion decreases. Such high selectivities cannot be explained in terms of electrostatic interactions alone, since the charge densities of the organic ions are considerably less than those of the chloride ions which they displace. I t appears that a combination of electrostatic and hydrophobic bonding produces this unusual selectivity. Measurements of adsorption of aromatic sulfonates by high surface area styrene-divinylbenzene copolymers have shown that, although reasonably high affinities exist, the mutual repulsions of neighboring groups on the surface of the adsorbent seriously hinder adsorption. I n the present case, neutralization of these charges by the positive charges of the functional groups of the resin permits high degrees of interaction. Measurements of the binding of BS, NS, and AQS by a high surface area (335 sq. meters per gram) styrenedivinylbenzene adsorbent showed (Gustafson et al., 1968) that the order noted above is found in the case in which only physical adsorption via hydrophobic bonding is
3.c
QUINONESULFONATE
i
N A P HT HA L ENE s u L F o AT E
2 .c
0log K C , -
1.c "
n
BE N Z E N E S U LFON ATE
C
E T H A N E SU LFON ATE
I
(
02
I
1
03
04
I 05
1
06
I
07
C E
r X0-
Figure 1 . Plots of lot KF, vs. x b for binding of organic ions by resin IA V O L . 7 N O . 2 JUNE 1 9 6 8
117
involved. The following binding constants, K , were obtained in an aqueous medium a t 25" by the use of the Langmuir equation, c - -c 1
-
in which c is the molar concentration of sorbate in equilibrium with the adsorbent, q is the number of equivalents of sorbate adsorbed per gram of dry adsorbent, and b is a constant equal to the number of equivalents of sorbate bound per gram of resin a t the point a t which monolayer coverage is obtained: BS, 0.58; NS, 17; AQS, 170. Because hydrophobic bonding should play a less extensive role in the case of acrylate resins, relative to that for polystyrene-based materials, it is not surprising that the selectivities of the poly(N,N-dimethylaminopropylacrylamide) resin (11) are less than those of the polystyrene-based resin (IA). An interesting observation with respect to the binding of aromatic ions is that the selectivity coefficients, K f , , decrease with increasing organic loading in the case o f the styrene-DVB resin (except in the case of the IA-AQS system), whereas the reverse trend is observed in the acrylate resin system. The former trend is that normally observed and expected on the basis of theoretical considerations (Harris and Rice, 1954). The latter effect, in which the affinity for organics increases with increasing organic content in the resin, may possibly be produced by hydrophobic bonding of the entering organic ion to organic groups already adsorbed by the resin. The data obtained upon the binding of various organic ions by other resins in aqueous NaCl solution are shown in Table 11. The greater selectivity of IA, relative to that of IC, for naphthalenesulfonate is somewhat surprising since, for simple inorganic species, selectivity increases with increasing crosslinking of the resin. The fact that the opposite trend is observed may be associated with steric effects. Selectivities in Mixed Solvents. Studies on the adsorption of organic molecules and ions by high surface area styreneDVB copolymers have shown that the large free energies of adsorption from aqueous solution are produced mainly by large positive entropy changes (Kauzmann, 1959; Nem-
b ' K b
z
-
BENZENESULFONATE
A
E T H A N ESU LFO N ATE
I
I
1
03
04
05
I I
02
I 07
06
r
0-
:,
Figure 2. Plots of log vs. x b for binding ions by the hydrochloride form of resin I1
of
organic
Table II. Selectivity Coefficients for Adsorption of Organic Species by Ion Exchange Resins
IA
Organic Ion NS
IB
TBCS
Resin
IB
GAL
IB
ABS
IC
KS
I11
NS
0 - in Resin, Me9 0.752 1.496 2.204 0.48 0.94 1.78 2.50 0.50 1.17 2.18 0.071 0.141 0.71 1.41 2.12 2.83 3.48 3.57 0.940 1.457 1.902 0.482 0.987 1.606
Cl in Resin, Meq. 2.246 1.503 0.794 2.94 2.48 1.64 0.92 2.92 2.25 1.24 2.93 2.86 2.29 1.60 0.89 0.18 0.00 0.00 2.060 1.544 1.098 1.749 1.234 0.620
[O 1.78 x 7.50 x 2.58 x 3.37 x 8.89 x 2.81 x 5.92 x 1.20 x 4.44 x 9.48 x
I. 10 10 10 10 10 10 10 10 10 10
IC1
' ' '
'
'
6.0 x 10 2.8 x 10 6.9 x 10 ' 2.32 x 10 ' 7.6 x 10 ' 2.09 x 10 ' 6.63 x 10 ' 1.776 x 10 3.62 x 10 ' 4.71 x 10 7.56 x 10 ' 1.302 x 10 '
I-
0.1010 0.1005 0.0984 1.000 1.005 1.014 1.021 1.001 1.007 1.018 0.1006 0.1013 0.1012 0.1013 0.1011 0.1006
K;/ 190 133 106 486 429 391 467 142 118 189
3.2 x 10' 3.5 x lo4 6.8 x 10'
0
b
0.0974 0.0963 0.0943 0.0964 0.0937 0.0882
67.1 51.2 45.1 5.64 9.90 17.5
xo 0.251 0.499 0.735 0.141 0.275 0.520 0.730 0.146 0.342 0.637 0.024 0.047 0.237 0.470 0.707 0.943 1.16 1.19 0.313 0.486 0.634 0.216 0.444 0.721
Results cannot be estimated, since little or no ABS is present in solution. 'Presence of ABS interferes with coulometric chloride analysis. ~
118
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
ethy and Scherega, 1962). Enthalpy changes, as in cases of ion exchange reactions, are very small-i.e., 0 to 4 kcal. per mole. Frank and Evans (1945) have pointed out that when a nonpolar molecule dissolves in water, the solvent structure is modified in the direction of greater crystallinity-that is, so-called “icebergs” are formed. Upon adsorption of the organic species by an adsorbent or ion exchange resin, the hydrogen-bonded iceberg structure is broken up with an accompanying large entropy increase. This behavior is peculiar to water. Similar iceberg structures are not formed in nonaqueous solvents, even those as polar as low molecular weight alcohols. Since large favorable entropies of adsorption are not involved when reactions are carried out in nonaqueous solvents, the free energy changes are low and depend mainly on small enthalpy changes. This should be true, not only for physical adsorption, but for the case of ion exchange sorption of organic species as well. Hence, the selectivity of anion exchangers for organic ions should decrease markedly in nonaqueous or mixed solvents. The results of measurements of the binding of naphthalenesulfonate ions by resin IA in 25, 50, and 7570 methanol solutions are shown in Table 111. The selectivity coefficient drops from 133 at xlvS = 0.5 in water to 3 in 75% CHaOH. A K value as low as 3 indicates that a resin containing naphthalenesulfonate ions may easily be completely regenerated by a few bed volumes of methanolic brine. Of course, natural foulants, such as humic and fulvic acids, have selectivities much greater than the selectivity of naphthalenesulfonate. Nevertheless, it is reasonable to expect that brine solutions of lower alcohols or acetone will function well in the regeneration of organically fouled resins, particularly if an acrylate-based resin, which has a low selectivity for organic species, is used. I n general, the lower the solubility parameter (Burrell, 1955), 6, of a water-miscible solvent, the more efficiently it will function in regeneration of ion exchange resins. Several such solvents are listed below in the order of their expected decreasing regeneration efficiency (Gardon, 1966) : 6 , CulOj,
Solwnt 2-Butanone 2-Propanone 1-Butanol 1-Propanol Ethanol Methanol Water
Cm!‘ 9.3 10.0 11.4 11.9 12.7 14.5 23.2
Table 111. Selectivity Coefficients for Adsorption of Sodium Naphthalenesulfonate by Resin IA in Methanol-Water Mixtures
CH,IOH, Vol. 74 0 0 25 25 25 50 50 50 75 75 75
Fraction of resin sites occupied by NS ions.
of resin 11. The substitution of a methyl group for a proton has little effect on the affinity of the resin for an organic counterion. I n summary, it is of interest to compare the selectivities of resin IA for organic ions in order to show the relationship of structure to selectivity.
KO 50% CHiOH 50% C>H,OH 50% n-CIH-OH 50% (CHI),CO
~
K
Organic Ion Ethanesulfonate Benzenesulfonate Naphthalenesulfonate Gallate tert-Butylcatecholsulfonate Anthraquinonesulfonate D odecylbenzenesulfonate
gl
ut x = 0.5 in Salt Solution 0.67 7.4 133 150 400 1290 32,000
The selectivity coefficient increases as the degree of aromaticity increases. For species containing a single benzenering, K & increases as the hydrophobicity of the substituent increases. Similar results have been shown by Kressman and Kitchener (19491, who measured the binding of a number of quaternary ammonium ions by a sulfonated phenol-formaldehyde resin. In the latter case selectivity coefficients, K&, , increased in the order tetramethylammonium (3.7) < tetraethylammonium (5.0) < trimethyl-n-amylammonium (8.2) < phenyldimethylethylammonium (25.2) < phenylbenzyldimethylammon-
Resin IA
K $’ ut X
190 133 106 47.0 36.0 40.2 9.29 8.28 8.63 2.93 2.94 3.03
Table IV. Kinetics of Adsorption of Organic Ions by Various Anion Exchange Resins
Values o f K g s which were obtained for resin I11 a t x = 0.5 in various solvents are:
Soloent
Kgs
X1V.t
0.251 0.499 0.735 0.323 0.643 0.902 0.308 0.580 0.824 0.247 0.348 0.501
0
Organic Ion Benzenesulfonate
Naphthalenesulfonate
= S 0.5
11.3 1.86 0.82 -0.4 -0.4
The value of 11.3 obtained in the aqueous medium is close to the value of 12.3 which was interpolated from the data of Figure 2 obtained for the protonated form
Anthraquinonesulfonate
I1
Anthraquinonesulfonate
Contact Time,Min.
f”
1.o 2.5 4.0 8.0 2.0 4.0 8.0 4.0 8.0 20.0 2.1 4.0 9.8
0.16 0.42 0.63 0.75 0.32 0.42 0.65 0.10 0.13 0.77 0.23 0.26 0.65
a Meq. organic species in resin ut time tlmeq. resin used; upproximutely 0.40 meq. resin used.
VOL. 7 N O . 2 JUNE 1 9 6 8
119
ium (44.2). The value of the selectivity coefficient was roughly proportional to the number ( N ) of carbon atoms in contact with the adsorbent surface in the cases of the aliphatic substituents. However, upon the substitution of aromatic for aliphatic groups, a pronounced increase in the KEH / N ratio was observed. This suggests that the van der Waals interaction of T electrons of the aromatic ring of the sorbate molecule with those of the aromatic resin matrix is of considerably higher energy than that of aliphatic carbon atoms with the polymer. Kinetics of Adsorption. Soldano and Boyd (1953) have shown that as the selectivity of polystyrenesulfonate resins for cations increases in the order N a + < Zn2’ < Y” < Th4+,the rate of diffusion within the resin phase decreases markedly. Similarly, comparison of the equilibrium data of Figures 1 and 2 and the kinetic data of Table IV shows that as the selectivity increases in the order BS < NS < AQS, the rate of adsorption increases in the order AQS < NS < BS. In the case of the less selective resin 11, the rate of adsorption of AQS is considerably greater than the rate observed for IA.
Literature Cited
Boyd, G. E., Adamson, A. W., Myers, L. S., Jr., J . A m . Chem. SOC.69, 2836 (1947). Burrell, H., Znterchem. Reu. 14, 3, 31 (1955). Frank, H. S., Evans, M. W., J. Chem. Phys. 13, 507 (1945). Gardon, J. L., “Encyclopedia of Polymer Science and Technology,” H. F. Mark, Tu’. G. Gaylord, N. M. Bikales, eds., Interscience, New York, 1966. Gustafson, R. L., Albright, R. L., Heisler, J., Lirio, J. A., Reid, 0. T., Jr., IND. ENG. CHEM. PROD.RES. 7, 107 (1968). DEVELOP. Harris, G., Rice, S. A., J . Phys. Chem. 58, 725 (1954). Kauzmann, W., Aduan. Protein Chem. 14, 1-63 (1959). Kressman, T. R. E., Kitchener, J. A., J . Chem. SOC.1949, 1208. Nemethy, G., Scheraga, H. A., J . Phys. Chem. 66, 1773 (1962). Soldano, B. A., Boyd, G. E., J . A m . Chem. SOC.75, 6097 (1953). RECEIVED for review January 31, 1968 ACCEPTED March 20, 1968
ULTRAFINE ION EXCHANGE RESINS BARBARA
J .
SCHULTZ‘
A N D
E V A N
H .
C R O O K
Research Laboratories, Rohm and Haas Co., 5000 Richmond Street, Philadelphia, Pa.
19137
Amberlite Ultrafine ion exchange resins are prepared in strong acid (-SOZNa), weak acid (-COONa), strong base -N( CH3)3CI, weak base [-N-(CHS)Z)], and nonionic form. Their particle size (0.5 to 1.5 microns) is much smaller than conventional ion exchange resins (450 microns) or micropowders (30 microns). This results in much faster ( 5 to 15 times) ion exchange kinetics for the ultrafine resins vs. conventional ion exchange resins. The individual particles of ultrafine resin are microspheres or agglomerates of microspheres. Such resins may be readily incorporated into paper or other suitable substrates to give rapid kinetics of ion exchange and simultaneously overcome the high pressure drop inherent with their small size. An example showing fulvic acid removal (84%) is presented. Ultrafine resins show dispersant activity toward inorganic pigments such as CaCos and T i 0 2 and because of their own pigment-like dimensions may be readily incorporated into such pharmaceutical formulations as lotions, ointments, and aerosols.
RECENTLY, a new
useful modification of ion exchange resins and polymeric adsorbents has been developed: the ultrafine resin or adsorbent (particle diameter of 0.5 to 1.5 microns) which is differentiated from the conventional ion exchange resin and polymeric adsorbent (ca. 600 microns) by its extremely small particle size. Resins I Present address, Rohm and Haas Research Laboratories, Spring House, Pa. 19477
120
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
of such dimensions can be particularly useful in systems which cannot tolerate coarse, bulky beads, and their availability should consequently extend the use of ion exchange resins into areas previously excluded to standard resins. Examples in the drug industry include topical formulations where rapid release of a reactive moiety is desired or for use as a drug carrier for oral consumption. I n either case, the presence of a micropowder resin, such
