Ind. Eng. Chem. Res. 1987, 26, 170-174
170
d = number of zones in the_reactor Da = Damkohler number, ( k / [ ( l+ ~ ) ~ ] ) ( L / v )-(tl)
De = effective diffusion coefficient
f ( u ) = dimensionless reaction rate function, [ u ( l
+
k = volume average reaction rate constant K = adsorption equilibrium constant L = reactor length L, = length of the ith zone n = integer characteristic of pellet shape, equals 0 for infinite slab, equals 1 for infinite cylinder, equals 2 for sphere R, = characteristic pellet dimension, half-thickness ( n = 0) or radius ( n = 1 and 2) s = x/R, S, = dimensionless location of the active catalyst within the pellet for the ith zone u =
cjcp
a[ = U ( S J a* = lower bifurcation point of the y vs. a curve from eq 3 E* = from eq 3, value of u where y ( u ) = -y(ii*) u = velocity of external fluid phase x = distance from pellet center X = reactor outlet conversion y = Z/I/ Y,= L l / L z = axial coordinate along the reactor Greek Symbols
P,
=
I$2/(n +
Y=
c,/c,o
6 = Dirac delta function e = bed void fraction u = dimensionless adsorption constant, KCP $* = Thiele modulus, k R P 2 / [ ( 1 u)*De] $ n ( ~ ) = {I- s for n = 0, In (l/s) for n = 1, and l / ( s - 1) for n = 2
+
Subscripts Oi = value at the inlet of the ith zone i = value at the outlet of the ith zone
Literature Cited Morbidelli, M.; Servida, A.; Varma, A. Ind. Eng. Chem. Fundam. 1982,21,27a. Morbidelli, M.; Servida, A,; Carra, S.; Varma, A. Ind. Eng. Chem. Fundam. 1985,24, 116. Morbidelli, M.; Servida, A.; Carra, S.; Varma, A. Ind. Eng. Chem. Fundam. 1986b,25, 313. Morbidelli, M.; Servida, A.; Varma, A. Ind. Eng. Chem. Fundam. 1986a, 25, 307.
* Author to whom correspondence
should be addressed.
Cassian K. Lee, Massimo Morbidelli, Arvind Varma* Department of Chemical Engineering University of Notre Dame Notre Dame, Indiana 46556 Received for review January 21, 1986 Accepted August 14, 1986
l)l$n(SJ
Permeation Characteristics of Amino Acids through a Perfluorosulfonated Polymeric Membrane Permeation of amino acids through perfluorosulfonated cation-exchange membranes, such as “Nafion”, has been found to depend strongly on pH and therefore on the relative dominance of the cationic, zwitterionic, or anionic species in solution. For the acid form of the membrane, permeation of amino acids was favored below their isoelectric p H values, at which pHs the solution contained dominant quantities of cationic species. In contrast, for the sodium-salt form of Nafion, the permeation was favored above the isoelectric pH, i.e., where anionic species prevailed. Transport data for six amino acids are reported for Nafion-117. Perfluorosulfonated cation-exchange membranes, such as Nafion (Nafion is a registered trademark of E. I. du Pont de Nemours & Co.), hold promise for various electrolytic applications (Govindan, 1982; Yeo, 1982; Kipling, 1982). The dominant commercial use of Nafion-type membranes is in the manufacture of caustic soda and chlorine by the electrolysis of brine. In this application the membrane is in the sodium-salt form, through which only sodium ion diffusion takes place from the anolyte to the catholyte. Electrical neutrality on both sides of the membrane in the cell is preserved by compensating electrode reactions. Though cation transport through Nafion has been extensively studied, its physicochemical mechanism is not well understood. Nafion ionomers are copolymers of tetrafluoroethylene and a sulfonated fluoro polymer. The generic chemical structure is ( CF 2 CF2 )”(C FCFp,,,)
I
(OCF2CF )lOCF2CH2X
c F3 where X = S03F,S03H, S03Na, etc., and n, m, and 1 are the numbers of corresponding repeating units. Analytical studies of Nafion membranes by small-angle X-ray scattering have shown the existence of a hydrophilic sulfonated 0888-5885/87/2626-0170$01.50/0
cluster network imbedded in the hydrophobic fluoropolymer matrix (Grierke and Hsu, 1982). Cations are believed to move through these clusters which are connected by narrow channels, the exact mechanism of such transport being still unknown. The hydrophilicity of Nafon, which is due to the presence of sulfonate-exchange sites, causes considerable swelling of the membrane in polar solvents such as water. Depending on the membrane composition and the experimental conditions, 2&30% by weight of water is common. Most transport studies using Nafion membranes have exploited the selective permeation of cations by using electrolytic reactions such as the formation of sodium hydroxide from sodium chloride. A notable exception is the work of Chum et al. (1983). They reported their chance discovery of the permeation of some carboxylic acids due to a concentration gradient alone. They postulated that these acids permeated as cationic species. However, no convincing experimental evidence was presented in this very preliminary work. Sikdar (1985a) reported permeation data for nine carboxylic acids through Nafion-11‘7 membranes in the acid form and after treatment with alkali. He showed that the diffusion rate of acetic acid through the acid membrane decreased with increasing ionization of the carboxylic acids, i.e., with increasing pH. The acetic acid diffused through Na+-Nafion-117, however, 8 1987 American Chemical Society
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 171 Table I. Amino Acids for Permeation Experiments
class polar R group
name L-glutamine
Mr 146
PK2
isoelectric PH
solubility in water at 25OC, g/100 g of water
9.13
5.65
3.6 (18 "C)=
2.34
9.6
5.97
25
2.34
9.69
6.01
16.72
1.83
9.13
5.48
2.97
2.18, ( ~ K = R 10.53)
8.95
9.47
freely soluble
2.09 ( ~ K = R 3.86)
9.82
2.98
0.5
formula
PK1 2.17
NHz
o>~CH~~~~COONH~+
nonpolar R group
glycine
75
H
I _ I NH~+
HCCOO
L-alanine
89
H
I
CHsCCOO
_
I
NH~+
L-phenylalanine
165
H
I
-
C~H~CH~CCOO
I
NH~+
positively charged R group
L-lysineb
146
H H~N+(CHZ),CCOO-
1
NH~+
negatively charged R group
L-aspartic acidb
133
-o
H
Dean, 1973. bGordon and Ford, 1972.
about 3 times more rapidly. This interesting property of the Na+-membrane was not thoroughly investigated. A kinetic study (Sikdar, 1985b) of glycine and alanine transport revealed that the fluxes obeyed MichaelisMenten-type saturation kinetics; Le., at a constant temperature, the flux through the membrane increased with concentration to reach a characteristic maximum flux for each solute, irrespective of the concentration difference. Sikdar's (198513) earlier publication dealt with the transport kinetics of two amino acids: glycine and L-alanine. This study extends the transport experiments to six amino acids selected from four classes of amino acids (Lehninger, 1982). According to this classification, on the basis of the polarity of the R group in the RCH(NH,+).COO- structural formula, R can be either polar or nonpolar and either positively or negatively charged. In particular, the effect of solution pH on the transport flux was examined. The pH determined the proportion of ionic components in solution, such as cations, anions, and zwitterions.
Experimental Section The apparatus of this study has been described elsewhere (Sikdar, 1985a,b). It consisted of a hollow glass cylinder, one end of which had a Nafion-117 membrane (equivalent weight 1100) fastened to it. The cylinder was immersed in a large reservoir of water. The membrane area exposed for transport was 15.5 cm2. The thickness of the dry membrane was 170 pm. In a typical permeation experiment, 250-300 mL of an aqueous solution of an amino acid was put in the inner vessel and, a t 23 f 1 "C,was allowed to diffuse vertically through the membrane to the receiver, which initially contained 1000 mL of deionized water. The pH of the water in the receiver was 5 in all permeation experiments. Both sides of the membrane were agitated, the source solution at the top by a laboratory stirrer and the receiver by a magnetic stirrer. Samples were taken from the source and the receiver and analyzed by a diode array spectrophotometer at a suitable wavelength chosen from the range 200-210 nm.
The H+-membranes for these experiments were prepared prior to a run by soaking the acid Nafion membrane, as obtained or as previously used, in deionized water at 60 "C for 2 h followed by two rinses with deionized water. The Na+-Nafion membrane was prepared by soaking a H+-Nafion membrane in 5% NaOH at 60 "C for 2 h, followed by two water rinses. No attempt was made to determine the extent of Na+ substitution in the membrane, which was assumed to be very nearly complete. A typical experiment was started with a liquid level in the source an inch higher than that in the receiver, primarily to observe leakage visually, if there was any. The unrecognizable change in the level of the inner source solution indicated negligible osmotic flux of water. The six amino acids of this study and their properties (Lehninger, 1982; Kirk-Othmer, 1978; Gordon and Ford, 1971) are listed in Table I. All samples (anhydrous Sigma grade, purity 99% or better) were obtained from Sigma Chemical Co., St. Louis, MO. For each experiment the solution was used immediately after preparation. The permeation results of this study are presented as percentages of the original amount of amino acid in the source vessel that diffused to the receiver as a function of time. The slope of this curve at any instant is proportional to the flux a t that time. For comparison, it is best to compute the initial flux. This was done by fitting the permeation data with a second-degree least-squares polynomial of the form
Y = bt
+ Ct2
(1)
where Y is the percent transported and t is the time (h). From eq 1dY/dt at t = 0 is b. Typically, such an empirical fitting yielded a high correlation coefficient (r2 = 0.99). Further comparison can be made by calculating permeability constants of the acids from the relationship initial flux = (P/L)ACi (2) where P = permeability constant (cm2/s),L = thickness of the membrane (cm), and ACi = initial concentration difference between the source and the receiver (mol/L). Since the thickness of the water-swelled membrane was
172 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 Table 11. Transport of Amino Acids initial source concn, source duration, initial flux, % PIL, acid mol/L pH h mol/(cm%) transported cm/s A. H+-Nafion a t 23 "C, Atmospheric Pressure 6.6 4.7 x 10-5 1.62 4.5 5 7.6 X glycine (25 "C) 5 7.8 x 10-8 6.2 4.7 x 10-5 1.64 4.5 glycine (25 "C) 24.0 2.6 x 10-4 4.5 4 3.4 x 10-8 L-alanine (25 "C) 0.13 11.5 7.7 x 10-5 4.5 4 5.12 X L-alanine (25 "C) 0.66 4.0 4 no transport L-phenylalanine 0.025 22.6 2.4 x 10-4 4.0 4 3.4 x 10" 0.14 L-glutamine 9.8 1.1 x 10-4 5 1.5 X 10" 0.14 7.0 L-glutamine 6.2 9.3 x 10-5 0.14 10.0 3 1.3 X L-glutamine 24.2 2.1 x 10-4 3.0 5 6.2 x 10-9 0.03 L-aspartic acid 0.03 1.0 1 1.1 x 10-8 9.2 3.7 x 10-4 L-aspartic acid 2.8 2.5 x 10-5 0.18 9.0 5 4.5 x 10-9 L-lysine L-alanine
0.13
4.5
comments membrane treated a t 60 "C for membrane boiled for 2 h membrane treated a t 60 "C for membrane treated at 60 "C for membrane treated a t 60 "C for membrane treated a t 60 "C for membrane treated at 60 "C for membrane treated a t 60 "C for membrane treated a t 60 "C for membrane boiled for 1 h membrane treated a t 60 "C for
2h 2' 2h 2h 2h 2h 2h 2h 2h
B. Na+-Nafion a t 25 "C, Atmospheric Pressure 1.9 3.9 X membrane treated with 5% Na a t 60 "C 3.5 4.3 x 10-9 for 2 h
acetic acid propionic acid
1.66 0.66
2.5 3.0
5 5
C. H+-Nafion a t 25 "C, Atmospheric Pressure 6.3 X 7.2 3.8 X 13.1 7.0 X 4.6 x lo-*
not measured, only rough comparisons can be made of P/L values, assuming that the swelled thickness did not vary significantly from solution to solution.
Results and Discussion H+-Nafion Membrane. It was reported before (Sikdar, 1985b) that the initial fluxes of glycine and alanine through the acid form of Ndion-117 increased with decreasing pH. In this study we confirmed that the initial flux through the acid form of Nafion-117 was more rapid below the isoelectric pH than above it for all six amino acids studied. A t pHs slightly below its isoelectric pH, an amino acid begins to form positively charged species. Contrariwise, a t pHs slightly above the isoelectric pH, anionic species begin to form. The proportions of cationic, zwitterionic, ' and anionic species present in a solution depend on the pH which in turn determines the equilibria R.CH(NH,+)COOH cationic
-H+
-H+
RCH(NH3')COOzwitterionic R.CH(NH,)COOanionic
For amino acids which have additional ionic groups, other species such as bipositive ions (e.g., for lysine) or binegative ions (e.g., for aspartic acid) can exist in the solution. In that case, additional equilibria have to be considered. The proportions of cationic, zwitterionic, and anionic species for an amino acid in aqueous solution can be calculated by the Henderson-Hasselbalch equation (Lehninger, 1982) pH = pKi + log ([baseJ/[acid]) (3) where pKi is either pK,, pK,, or pKR (for the ionizable R group) and [acid] and [base] are the concentrations of the acid and base species, respectively. In the following, the diffusive behavior of various amino acids as they undergo pH changes is presented. Nonpolar R Group. To illustrate the permeation characteristics of amino acids of this group, data for Lalanine at 25 "C and pH 4.5 for an initial concentration difference of 0.66 mol/L are reproduced in Figure 1 from our earlier work (Sikdar, 1985b). Eleven percent of the acid diffused in 4 h. The initial flux was 5.12 X mol/(cm*.s). This initial flux is the average of duplicate experiments having the corresponding fluxes of 5.13 x and 5.11 X mol/(cm2-s), respectively. The same membrane used in the first experiment was treated by
o Alanine, source pH= 4.5
0 bW -
membrane boiled for 1 h membrane boiled for 1 h
,2-
a
0 n
o Propiodc, source PH = 3.0 A Alanine.
source pH = 6.0
v)
2
< a
6-
i-
I2 W
0
En
4-
Initial Source Concentration = 0.66 mol
0
TIME, hr
Figure 1. Permeation of L-alanine through H+-Nafion-117 at 25 "C.
heating in water at 60 "C for 2 h before being used in the replicate experiment. The corresponding data for permeation a t the isoelectric pH of 6.0 are also plotted in Figure 1. The initial flux was significantly smaller (4 X lo4 mol/(cm2.s)) at this pH. The pH of the source solution in this experiment was adjusted by a dilute NaOH solution. The alanine diffusion was somewhat more rapid than that of propionic acid, the corresponding carboxylic acid, at the same concentration. The latter exhibited an initial flux of 4.6 X mol/(cm2.s) as shown. Glycine, though having the smaller molecular weight (M, 75), exhibited smaller P/L values than alanine. This is unexpected since glycine has very similar pK and isoelectric pH values. The comparison of experimental P/L data among compounds of this study is shown in Table 11. Glycine permeation data for two replicate experiments are plotted in Figure 2. For comparison, acetic acid and propionic acid data are also included in Table 11. P/L values, which reflected nonlinearity between flux and concentration, varied with pH and with concentration, as seen in Table 11. Perhaps, to compare transport data, it is best to look at the initial flux data. For example, alanine initial flux increased from 3.4 X to 5.12 X mol/ (cm2-s)when the concentrition was increased from 0.13 to 0.66 mol/L; the corresponding PIL values decreased by a factor of 3. It is not obvious from the data in Table I1 if, with a further increase in alanine concentration to 1.6
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 173
30 Table 111. Aspartic Acid Flux vs. pH time at percentage of species as which flux % / h computed, h pH cation zwitterion anion 9.0 0.5 1 92.5 7.5 0.01 7.0 1.5 3 9.8 79.3 10.9 5.0 2.5 4 0.5 41.8 57.7 2.9 3.5 6 9X 0.7 99.3 0.07 99.8 0.9 4.5 7 9 x 10-7
0 W t-
a
g v)
z a IL
i
W
0
a W
a
4
6
TIME, hr
Figure 2. Influence of amino acid structure and pH on permeation through H+-Nafion-117. Initial source concentrations: L-glutamine, 0.14 mol/L; L-aspartic acid, 0.03 mol/L; glycine, 1.62 mol/L (25 "C); and L-lysine, 0.18 mol/L.
mol/L, the alanine flux w ill be higher than the glycine flux. Our previously published work (Sikdar, 1985b) demonstrated that the fluxes of glycine and alanine saturate in the range 2.5-3.0 mol/L for H+-Nafion-117. Also, it was shown that at saturation concentrations, alanine transport was indeed faster by 50%. Phenylalanine, the other candidate from the group with a nonpolar R group, did not permeate to any measurable extent in the same time. Polar R Group. Glutamine, having an amide group and a larger molecular weight, permeated faster than glycine. This is seen by comparing the P / L values as well as the initial fluxes presented in Table 11. Permeation data for glutamine are shown in Table I1 and plotted in Figure 2. The strong pH dependence of the diffusion rate of an amino acid is illustrated by the glutamine data, shown in Figure 2. Three experiments were conducted a t pH 4,7, and 10, respectively, under otherwise identical conditions. For ACi = 0.14 mol/L, the initial flux dropped from 3.4 X mol/(cm2.s) at pH 4 to 1.5 X at pH 7 (higher than the isoelectric pH). For higher pHs, the initial flux dropped only slightly more, the pH 10 flux value being 1.3 x mol/(cm2.s). Since various ionic species were present in solution, it may be informative to correlate the initial fluxes with the cationic, zwitterionic, or anionic species. The solutions corresponding to pH 4 and pH 7 contained 98.5% and 99.3 7% zwitterionic glutamine species, respectively. The pH 4 experiment, which exhibited the higher flux, contained 1.5% cationic species, whereas the pH 7 experiment had 0.7% anionic species. But the pH 10 experiment had an enormous increase in the proportion of the anion to 88.1% (zwitterionic: 11.9%), yet the flux did not decrease appreciably. If the flux depended on the concentration of the zwitterion, a sharp drop in flux for the pH 10 experiment would have resulted according to the Fick's law of diffusion. Conversely, the presence of the negative ions was not a determining factor either. The importance of the presence of cationic species may be indicated here, although the proportion present in the pH 4 experiment may not be convincing. The effect of the cations on the transport fluxes was more pronounced for aspartic acid, as discussed next. Charged R Group. Apparently anomalous diffusion behavior was noted for the acids that have a charged R group. As seen in Figure 2, L-lysine exhibited lower per-
meation rates a t pH 9 than aspartic acid a t pH 3. These pHs were obtained just by dissolving these acids in slightly acidic (pH 5) deionized water. Both initial fluxes and the P / L values given in Table I1 for aspartic acid are higher than those for lysine. Moreover, the lysine molar concentration was 6 times greater. Poor flux data for lysine can be explained by the Donnan anion exclusion principle. The greater initial flux of aspartic acid may be attributed to the presence of cations in the solution. This point can be further elaborated by examining the ionic proportions of aspartic acid for the two experiments of Table 11. At pH 3, the proportions are 10.94% anion, 79.3% zwitterion, and 9.75% cation. A t pH 1, the proportions change to 0.01% anion, 7.5% zwitterion, and 92.36% cation. Clearly, the sharp increase in the initial flux and the P / L value, as shown in Table 11, can only be attributed to the cationic species. To demonstrate the effect of cations on transport more clearly, an experiment was conducted with aspartic acid in which the pH of the source solution was changed systematically from 1 to 7 over the duration of the experiment. By use of eq 1, the fluxes were then computed for each pH regime. Flux data in Table I11 clearly show that the flux was highest at pH 1 and then fell rapidly with increasing pH. As the pH of the solution increased, aspartic acid changed from its predominantly positively charged form (below its isoelectric pH of 2.98) to its predominantly negatively charged form. The percentages of the cationic, anionic, and zwitterionic species in the source solution at the pH values were calculated by eq 3 and are shown in Table 111. Two observations can be made from the numbers in Table 111. First, the rise of the flux values was approximately linear with the combined proportions of the cationic and zwitterionic species. The increase in flux as the pH decreased follows a monotonic rise in the proportion of cations and a corresponding decrease in the anions. Simultaneously, the zwitterions first increased and then decreased with pH. Second, the permeation of the anionic species, though small compared to the other species, is not zero. Aspartic acid can have a binegative ionic species as well. It has been omitted from Table I11 because its proportion was negligible. When eq 1is applied to obtain the fluxes at various pHs, a tacit assumption was made that the nonlinearity of the curve relating the permeate concentration to time was negligible, as may be seen generally in Figures 1 and 2. The aspartic acid data of Figure 2 show that this assumption is roughly valid. Na+-Nafion Membrane. Permeation characteristics changed dramatically when a membrane was employed in which the sulfonic acid groups were converted to the sodium-salt form. To illustrate this, permeation data at 25 "C for L-alanine, reproduced from an earlier publication (Sikdar, 1985b), are presented in Figure 3. Compared to the data for H+-membranes, the initial flux obtained at pH 4.5, for an initial source concentration of 0.13 mol/L, was an order of magnitude smaller. Only 3.5% was transported in 3.5 h, against 21% for the H+-membrane. The results of two other experiments, with a AC, of 0.67 mol/L at pH 6 and 10, respectively, showed that the flux
174 Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987
L-Alanine, Na+-Nafion, 25OC 0
w
a
01
Initial Source Concentration
0.13 mollL
Initial Source Concentration 0.67 mol/L
TIME, h r
Figure 3. L-Alanine transport through Na+-Nafion-117 and its pH dependence.
increased with increasing pH-an effect opposite to that for the H+-membrane. Data are presented in this communication to further corroborate the pH effect on the transport through the Na+-membrane. For example, when the pH was raised from 4.5 to 10, the flux, as shown in Figure 3, immediately increased 10-fold in the experiment with AC, = 0.13 mol/L. The different pH effects for the H+- and the Na+-membranes imply that, in contrast to the acid membrane which excluded amino acid anions, the Na+-membrane excluded cations. At present, there is no apparent explanation for the anion-exchanger-like behavior of the Na+-membrane. Future work needs to be directed to detect possible complexation with amino acid anions. On an absolute basis, however, permeation through the Na+-membrane was smaller. For example, for a AC, of 0.67 mol/L, the Na+-membrane exhibited an initial flux mol/(cm2-s) at pH 10 vs. 5.1 X mol/ of 3.8 X (cm2-s)for the H+-membrane at pH 4.5. Results of all transport experiments reported in this communication are summarized in Table 11. Transport data were reproducible as evidenced by a few available replicates, such as the L-alanine data of Table I1 for an initial source concentration of 0.66 mol/L. As mentioned before, the initial fluxes of two replicates were 5.13 X and 5.11 X loe8mol/(cm2.s), respectively. It was noted that the two different treatment procedures used did not yield different results. For instance, referring to Table I1 for glycine, the results obtained after pretreating the membrane at 60 "C for 2 h were virtually the same as those obtained after pretreatment by boiling.
Concluding Remarks The main purpose of this communication was to delineate the permeation behavior of amino acids through H+and Na+-Nafion-117 membranes at various pHs. We have seen two characteristics of amino acid transport through the acid form of a perfluorosulfonated membrane such as Ndion-117. First, the transport of the amino acids of this study through the H+-membrane was found to be favored when the cationic form predominated in solution compared to the zwitterionic form of the amino acid. As a result, the permeation rate increased with decreasing pH. The flux corresponding to the anionic species in solution was finite but small compared to those for the cationic and zwitterionic species. The greater flux of L-alanine than that of glycine, amino acids having similar isoelectric pH, pK,,
and pK, values, is apparently anomalous. Second, the permeation through the Na+-membrane was favored when the anionic species were predominant in the solution. Unlike the typical Donnan exclusion (Helfferich, 1962), according to which anions are excluded by a cation exchanger, cations were excluded by the Na+-membrane. However, permeation was greatly improved in alkaline pHs at which the amino acid was predominantly anionic. In the experiments presented here, unlike the sodium ion transport in the electrolysis of brine, the overall outcome was the transfer from the source to the receiver of zwitterionic molecules in which the charges are balanced. This was true even though the dominant ionic species in the source solutions were cationic, zwitterionic, or anionic, depending on the pH. It appears that any transport mechanism which explains the transfer of these molecules should be based on species-membrane interactions. In a previous study of the kinetics of glycine and alanine transport through the acid form of Nafion-117 (Sikdar, 1985b), such an interaction was also implied by the observation of flux saturation a t higher concentrations. A t this stage of the experimental investigation, these interactions, however, cannot be identified conclusively. Further investigation is needed, especially in the permeation through Nafion modified by substitution of the cations. Attempts to identify the transport species inside the membrane, as the permeation process occurs, should be very beneficial.
Acknowledgment I thank Laura Price Way for her technical support and John Persichetti for writing the least-squares program for data fitting. I am also appreciative of Du pont for supplying us with samples of Nafion-117. I am grateful to General Electric Company for permission to publish this work. This work was supported jointly by General Electric Company and National Bureau of Standards. Registry No. (L)-Glutamine, 56-85-9; glycine, 56-40-6; (L)alanine, 56-41-7; (L)-phenylalanine, 63-91-2; (L)-lysine, 56-87-1; (&aspartic acid, 56-84-8; nafion 117, 66796-30-3.
Literature Cited Chum, H. L.; Hauser, D. K.; Sopher, D. W. J . Electrochem. Soc. 1983, 2507. Dean, A. J., Ed.; Lange's Handbook of Chemistry, 11th ed.; McGraw-Hill: New York, 1973; pp 7-233. Gordon, A. J.; Ford, R. A. T h e Chemists Companion; Wiley: New York, 1972; p 44. Govindan, K. P. Salt Res. Ind. 1982, I 6 ( 2 ) , 22. Grierke, T. D.; Hsu, W. Y. In Advances in Chemistry Series; American Chemical Society: Washington, DC; 1982; Vol. 180, p 283. Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; p 134. Kipling, B. In Advances i n Chemistry Series; American Chemical Society: Washington, DC, 1982; Vol. 180, p 475. Kirk-Othmer, Encyclopedia of Chemical Technology; Wiley Interscience: New York, 1978; Vol. 2. Lehninger, A. L. Principles . of. Biochemistrv; - . Worth Publisher: New York; 1982. Sikdar. S.K. J. Memb. Sci. 1985a. 23(1). 83. Sikdar; S. K. J . Memb. Sci. 1985b;24(1); 59. Yeo, R. S. In Advances i n Chemistry Series; American Chemical Society: Washington, DC, 1982; Vol. 180, p 454.
* Present address: Center for Chemical Engineering, National Bureau of Standards, Boulder, CO 80303. Subhas K. Sikdar* General Electric Research & Development Center Schenectady, New York 12301 Received for review August 13, 1984 Revised manuscript received August 20, 1985 Accepted September 17, 1986
