Langmuir 1989,5, 876-879
876
0
, 0
I
I
5
I
I
I
IO
Treatment
I
15
time
(rnin)
Figure 5. Advancing (0) and receding ( 0 )angles of CHJ2as a function of treatment time.
appear very similar to that of Figure 3e, but contact angle measurement exclude the occurrence of a composite surface. The reason is that in these cases both etching and oxidation are caused by the treatment, so that the surface is covered by a highly energetic, highly wettable layer, and water penetrates the crevices. On the other hand, Westerdahl et al.14*15 observed variations of the water advancing contact angle for helium plasma treated FEP samples, where flourine depletion and surface cross-linkingis known (28) Abu-Isa, I. A. Polym.-Plast. Technol. Eng. 1973, 2, 29.
to occur, while for oxygen plasma treated samples they obtained values close to untreated samples, indicating low alteration of surface chemistry. A prolonged oxygen plasma treatment of PTFE leads to deeply etched surfaces without modification of surface chemistry; thus, the only relevant variable is physical (roughness), and the wetting behavior is described by the theory of Wenzel (hydrophobicity increases with roughness), Cassie and Baxter (composite surfaces), and Johnson and Dettre (decrease of energy barriers between metastable states). The measurement of CH212contact angles offer a possibility to check the correctness of our hypothesis. Since the Young contact angle of CH212on PTFE is lower than 90°, according to theory no transition to a composite surface is possible. Thus a monotonic increme of hysteresis with roughness must be expected. The CH212contact angles reported in Figure 5 are in full agreement with theoretical expectations. In conclusion, the wetting behavior of oxygen plasma treated PTFE reflects, at short treatment time, the modification of surface chemistry due to the introduction of some oxygenated group. At longer treatment times, the surface is deeply etched without chemical modifications so that wettability is controlled by roughness. This work proved the sensitivity of contact angle hysteresis to the status of surfaces. Acknowledgment. We thank L. Pozzi for XPS spectra and G. Morelli for SEM micrographs. Registry No. PTFE, 9002-84-0; 02, 7782-44-7.
Ion-Expulsion Ultrafiltration-A New Method for Purifying Aqueous Streams Sherril D. Christian,*?+Edwin E. Tucker,? John F. Scamehorn,t Byung-Hwan Lee,? and K. James Sasakif Institute for Applied Surfactant Research, T h e University of Oklahoma, Norman, Oklahoma 73019, Department of Chemistry, T h e University of Oklahoma, Norman, Oklahoma 73019, and School of Chemical Engineering and Materials Science, The University of Oklahoma, Norman, Oklahoma 73019 Received November 3, 1988. I n Final Form: January 25, 1989 Ion-expulsion ultrafiltration (IEUF) is proposed as a new membrane separation method for removing ions from aqueous streams; ordinary ultrafiltration membranes, which block the passage of charged colloidal species on the basis of size exclusion alone, are used in the process. IEUF exploits the fact that the ion product of the counterion of a colloidal polyion and a co-ion, present at much lower concentration, will under favorable conditions be nearly the same in the retentate solution as in the permeate solution in ultrafiltration. This effect leads to a considerable enhancement in concentrationof the co-ion in the permeate solution. Semiequilibrium dialysis experiments show the large equilibrium extent of separation that can be achieved by using either micellar or polyelectrolyte solutions in membrane separations. Initial ultrafiltration resulta show that separation efficiencies can be quite large, although not as great as in equilibrium experiments. Research from our laboratories has indicated the effectiveness of colloid-enhanced ultrafiltration methods in removing solutes from aqueous streams.'-'l In these methods, small ions or molecules attach to macromolecular species (either surfactant micelles or polymer macroions) Department of Chemistry.
* School of Chemical Engineering and Materials Science. 0743-7463/89/2405-0876$01.50/0
which have been added to the aqueous stream. The solution is then processed by ultrafiltration, using a mem(1) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 257.
(2) Scamehorn, J. F.; Ellington, R. T.; Christian, S. D.; Penney, W.; Dunn, R. 0.;Bhat, S. N. AICHE Symp. Ser, 1986, 82, 48. (3) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1987, 22, 763.
0 1989 American Chemical Society
Letters brane having pore sizes small compared with the size of the dispersed species in solution; under favorable conditions, the resulting permeate solution will be practically pure water. In micellar-enhanced ultrafiltration (MEUF), micelles bind oppositely charged ions (counterions) and solubilize organic solute species in the hydrophobic micellar interior and in polar ionic regions near the micellar surface. In polyelectrolyte-enhanced ultrafiltration (PEUF), a soluble charged polymer binds oppositely charged multivalent ions. Membranes with at least a 5000-dalton molecular weight cutoff reject either micelles or the polyelectrolyte. The separation methods mentioned in the preceding paragraph rely on the tendency of micelles or polyions to bind solutes, which are then prevented from passing through the pores of an ultrafiltration membrane by size exclusion. We now report a new ultrafiltration method, in which the presence of a charged colloid causes the expulsion of ions having the same charge as the colloid. It is well-known that ionic micelles and polyelectrolytes in aqueous solution bind a certain fraction of the dissolved counterions (i.e., ions having a charge opposite that of the colloid), allowing the remainder of the counterions to migrate more or less freely.12-17 To a fair approximation, it has been shown that the thermodynamic activity of the counterions is equal to the concentration of the free or unbound ions. Moreover, the concentration of ions not bound to the polyions or micelles is frequently nearly a constant fraction of the total charge of the macroion, throughout relatively large ranges of concentration of this species. For example, spherical surfactant micelles, such as those of hexadecylpyridinium chloride (or cetylpyridinium chloride, abbreviated as CPC) and sodium dodecyl sulfate, frequently have degrees of counterion binding in the range 60-80%,12J3 at surfactant concentrations varying from just above the critical micelle concentration to several tenths molar. Similarly, poly(s0dium 4-styrenesulfonate) is 65-80% counterion bound at polymer sulfonate concentrations varying from very small values to at least 0.5 M.18 Underlying the rationale for ion-expulsion ultrafiltration is the fact that the concentration of free counterion (e.g., C1- in CPC solutions) is a significant and nearly constant fraction of the total concentration of the colloid. Thus, in an aqueous solution of 0.1 M CPC, the free chloride concentration is approximately 0.02 M. If a small con(4)Gibb, L.L.; Scamehorn, J. F.; Christian, s.D. J.Mernbr. Sci. 1987, 30. 67. (5) Smith, G. A.; Christian,S. D.; Tucker, E. E.; Scamehorn, J. F. In ACS Symp. Ser. 1987,342,184. (6)Bhat, S.N.; Smith, G. A.; Tucker, E. E.; Christian,S. D.; Scamehorn, J. F.; Smith, W. Ind. Eng. Chern. Res. 1987,26,1217. (71 Christian. S. D.: Bhat. S. N.: Tucker.' E. E.: Scamehorn. J. F.: El-Sayed, D. A..AIChE J. 1988,34,189. (8) Scamehorn, J. F.; Christian, S. D.; Ellington, R. T. In Surfactant-Bmed Separation Processes; Scamehorn, J. F.; Harwell, J. H., Eds., Marcel Dekker: New York, Chapter 2, in press. (9)Christian, S. D.;Scamehorn,J. F. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds., Marcel Dekker: New York, Chapter 1, in press. (10)Dunn, R.O.,Jr.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. in press. (11) Sasaki, K. J.; Burnett, S. L.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langrnuir, in press. (12)Rathman, J. F.; Scamehorn, J. F. J.Phys. Chern. 1984,88,5807. Rathman, J. F.; Scamehorn, J. F. Langrnuir 1987,3,372. (13)Sasaki, T.; Hattori, M.; Sasaki, J.; Nukina, K. Bull. Chern. SOC. Jpn. 1975,48,1397. (14)Beunen, J. A.; Ruckenstein, E. J.Colloid Interface Sci. 1983,96, 469. (15)Stigter, D. J. Phys. Chern. 1984,68,3603. (16)Oosawa, F.Polyelectrolytes; Marcel Dekker: New York, 1971. (17)Oman, S.;Dolar, D. Z. Phys. Chern. Neue Folge 1967,34, 1. (18)Schwarz, A,; Boyd, G. E. J. Phys. Chern. 1965,69,
Langmuir, Vol. 5, No. 3, 1989 877 retentate solution concn of CPC initial copper concn initial chloride concn ion product constant ultrafiltration membrane by electrical neutrality ion product constant permeate copper concn
[CPC] = 0.100 M [Cu2+]= 1.00 X lo4 M [Cl-] = 0.2 x 0.10 = 0.02 M [CU~+][C= ~ -1.0 ] ~ x 104(0.02)2= 4.0 X lo4 M3
------------------. 2[Cu2+]= [CI-] =-4[Cu2+I3 = [CU~+][C~ ]~ 4.0 X 10" M3 [Cu2+]= 2.15 X M permeate solution
Figure 1. An aqueous solution containing 0.100 M hexadecylpyridinium chloride (CPC) and 1.0 x lo4 Cu2+is placed in the retentate compartment of an ultrafiltration cell; the assumed concentration of free chloride ions is 20% of the CPC concentration, or 0.020 M. By assuming constancy of the ion product in the retentate and permeate solutions, and applying electrical neutrality, it is calculated that the concentration of Cu2+ in the permeate is 0.00215 M, 21.5 times greater than in the retentate.
centration of a cation not provided by the surfactant is also present in this solution (e.g., Cu2+),the activity of the neutral chloride (CuC12) will be acUcl2= [ C U ~ + ] [ C ~ - ] ~ , neglecting activity coefficients. If [Cl-] >> [Cu2+],then the mean ionic activity of the CuC1, will be considerably greater than [Cu2+]. When such a solution is allowed to pass through an ultrafiltration membrane which excludes the micelles of CPC, there will be a tendency for the permeate solution to have a greatly increased concentration of copper. One can estimate the maximum extent to which the permeate solution will be enriched in Cu2+by assuming that CuC12passes through the membrane at its equilibrium thermodynamic activity, as would be required in a Donnan eq~i1ibrium.l~Of course, in practical separations, the activity of CuC1, in the permeate stream may be expected to be somewhat smaller than the average activity of the salt in the retentate compartment, owing to diffusional resistances to ion transport. A related separation method, utilizing the fact that concentrations of co-ions are depleted in the vicinity of an ionic polymer, is ion exclusion or Donnan exclusion.2b22 This technique has been used to separate ionic and nonionic materials. It has also been reported that the addition of anionic polyelectrolytes to aqueous solutions of organic anions increases the rate of transfer of these species through membranes.23 Donnan effects have been considered in relation to reverse osmosis separation^,^^ but these effects pertain to membranes that can retard the passage of small anions or cations. Donnan dialysis and separations involving ion-exchange membranes have been used to concentrate ions of a given charge into a receiver electrolyte solution.%,% However, to our knowledge, these techniques have not been used in ultrafiltration separations. To illustrate the potential effectiveness of ion-expulsion ultrafiltration in increasing the concentration of a co-ion, we may consider the transfer of a solution containing initially 0.100 M CPC and 0.000100 M Cu2+ through a (19)Donnan, F. G. Z.Elektrochern. 1911,17,572. (20)Wheaton, R.M.; Bauman, W. C. Ind. Eng. Chern. 1953,45,228. (21)Wheaton, R.M.; Bauman, W. C. Ann. N. Y. Acad. Sci. 1953,57, 159. (22)Gluekauf, E.; Watts., R. E. Proc. R.SOC.London 1962,268A,339. (23)Higuchi, T.; Kuramoto, R.; Kennon, L.; Flanagan, T. L.; Polk, A. J. Am. Pharm. Assoc. 1954,43,646. (24) Lonsdale, H. K.; Pusch, W.; Walch, A. J. Chern. SOC.,Faraday Trans 1 , 1975,71,501. (25)Blaedel, W. J.; Haupert, T. J. Anal. Chern. 1966,38, 1305. (26)Cox, J. A.; Tanaka, N. Anal. Chern. 1985,57,2370.
878 Langmuir, Vol. 5, No. 3, 1989
membrane, assuming that the activity of CuC12 in the permeate solution equals that in the retentate (see Figure 1). The ion product for CuC12 for the solution initially in the retentate is approximately 1.00 X lo4 X 0.022 M3 = 4.0 X lo4 M3; neglecting activity coefficient effects, the same value of the ion product should obtain in the permeate solution. By applying the condition of electrical neutrality to the solution passing through the membrane (and assuming constancy of the ion product constant), we estimate that [Cu2+]in the permeate will be 2.15 X 10” M,more than 21 times the concentration of copper in the retentate. Simple arithmetic of this type also leads to the conclusion that the expulsion of Cu(1I) by the CPC solution becomes even greater as the concentration of Cu2+ decreases, at constant CPC concentration. We have previously used a technique called semiequilibrium dialysis (SED)**7r27*28 to study the equilibrium binding of ionic and molecular species by surfactant micelles. In semiequilibrium dialysis, a permeate solution is allowed to equilibrate with a retentate solution on opposite sides of a dialysis membrane, whereas in ultrdiltration processes, the aqueous stream is forced through the membrane. In numerous experiments, we have found that MEUF separations of molecules and ions occur practically at equilibrium; that is, the extent of separation that can be achieved with MEUF can be predicted almost quantitatively from the SED results. To test the concept of ion expulsion in membrane separations, we have utilized semiequilibrium dialysis experiments in which an aqueous solution containing 0.100 M CPC and either 0.001 or O.OOO1 M Cu2+is equilibrated for 1 day with a solution containing initially pure water, across a membrane having a molecular weight cutoff of approximately 6000 daltons. The table below lists results for duplicate determinations:
retentate compartment permeate compartment
ICu2+1after 1 day, M experiment 1 experiment 2 7.7 x 10” 2.2 x lo4 2.4 X lo4 7.9 X 10” 1.19 X 1.41 X lo-’ 1.37 x 104 1.19 x 10-3
Letters
permeate
retentate
‘ m
25
”.
0
75
50
25
100
125
(27) Christian S. D.;Smith, G. A.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1986,1, 564. (28) Chrietian S. D.; Smith, G. A.; Tucker, E.E.;Scamehorn, J. F. J. Solution Chem. 1986,15, 519.
175
200
Total volume of permeate, mL Figure 2. Ultrafiltration results for initial retentate solution consisting of 300 mL of 0.100 M hexadecyIpyridinium chloride and 1.0 X l0-r M CuCh at 30 OC. Data were obtained with 400-mL Nuclepore batch stirred cells, with 76-mm-diameter,5000-dalton molecular weight cutoff anisotropic cellulose acetate membranes, at an applied pressure of approximately 0.5 atm.
150 -
*
175
* permeate
125100
-
I
m m
75-
retentate
El
Q
PI
25
These data indicate that the average ratio of the copper concentrations (permeateretentate) is 15.3 in experiment 1and 60 in experiment 2, somewhat less than predicted, but well within the expected range, if activity coefficientk of ionic species and some transfer of CPC from the retentate into the permeate compartment are taken into account. (The fact that the final concentration of Cu2+ in the permeate solution exceeds that originally in the retentate solution results from the transfer of water from the permeate compartment into the retentate during the equilibration period. A material balance indicates that the volume of the retentate solution has increased by 25-35% in the two experiments because of the osmotic pressure developed.) Similar dialysis results have been obtained for chromate ion (CrOd2-), benzoate, and 1-naphthoate, expelled by poly(sodium 4-styrenesulfonate) and by Gantrez, a copolymer of maleic anhydride and vinylmethyl ether, obtained from GAF Corp. Comparable degrees of separation are obtained, although in each case ionic activity coefficient effects somewhat diminish the ion enhancement ratios from the numbers predicted by the simple ion product
150
0
Q
25
50
75
100
125
150
175
Xll
Total volume of permeate, mL Figure 3. Ultrafiltration results for initial retentate solution consisting of 300 mL of 0.050M Gantmz 5-95 (monomer molarity) at pH 5.9 and 1.0 X lo-‘ M sodium 1-naphthoate at 30 O C . Data were obtained with 400-mL Spectrum batch stirred cells, with 76-mm-diameter,5000-dalton molecular weight cutoff anisotropic cellulose acetate membranes, at an applied pressure of approximately 0.5 atm.
calculations. It is significant that monovalent QS well as divalent ions can be concentrated by the ion-expulsion technique, although the expulsion of monovalent coions is somewhat less effective. In contrast, our previous studies of colloid-enhanced ultrdiltratioli of counterions have shown that monovalent ions are much less effectively removed by the polyions than are multivalent ions. Stirred-cell ultrafiltration experiments have also been performed on solutions containing CPC and Cu2+and on solutions containing the polyelectrolytes sodium polystyrenesulfonate and Gantrez with naphthoate anion. At large fluxes, the extent of removal of copper (or the naphthoate ion) from the retentate compartment is considerably smaller than that predicted by the equilibrium assumption or observed in corresponding SED experi-
Langmuir 1989,5, 879-881 ments. Figures 2 and 3 indicate the separations achieved in initial ultrafitration experiments; concentrations of the target ion in the permeate and retentate solutions are plotted against the total volume of permeate collected (from an initial retentate volume of 300 mL). The theoretical or equilibrium ratio of permeate to retentate concentrations of the expelled ions is a factor of 5-10 greater than can be achieved in the ultrafitration experiments for which data are given in Figures 2 and 3. We are now experimenting with other types of ultrafiltration apparatus which provide larger membrane area to solution volume ratios. Such studies will be necessary to determine optimum conditions for achieving separations that approach the equilibrium separation ratios. Many potential applications of ion-expulsion ultrafiltration and related dialysis methods come to mind, including the removal of valuable or dangerous ionic species from aqueous reservoirs and streams. In analytical chemistry, the possibility of concentrating an ion for quantitative determination, and of expelling this species into a phase that is practically pure water, is eepecially attractive. In planning the purification of industrial waste streams, it is noteworthy that it will be possible to keep the reagent
879
(for example an ionic surfactant or polyelectrolyte) in the retentate solution for repeated use, avoiding the need to develop processes to regenerate the reagent, as is required in conventional ultrafiltration. A highly concentrated stream containing the expelled ion can be obtained, and as we noted above, the separation becomes better and better as the target ion is decreased in total concentration in the retentate. We emphasize that the ion-expulsion ultrafiltration method does not require the membrane to act specifically to prevent passage of particular anions or cations; IEUF is based solely on the ability of the membrane to prevent the transfer of a charged colloid (either surfactant micelle or polyelectrolyte) from the retentate to the permeate stream.
Acknowledgment. Financial support for this work was provided by the Office of Basic Energy Sciences of the Department of Energy, Grant no. DE-FG01-87FE61146, and the National Science Foundation, Grant CHE 8701887. R@tv NO.CPC, 123-03-8;CU,7440-50-8; CrO,*, 13907-454; Gantrez, 52660-25-0;poly(s0dium kstyrenesulfonate),25704-18-1; benzoate, 766-76-7; 1-naphthoate,3198-24-1.
Phase Transition of NzMolecules Filled in Micropores of Micrographitic Carbons Katsumi Kaneko,* Takaomi Suzuki, and Kazunori Kakei Department of Chemistry, Faculty of Science, Chiba University 1-33 Yayoi-cho, Chiba-shi 260, Japan Received October 18, 1988. I n Final Form: January 30, 1989 Adsorption isotherms of N2on activated carbon fibers (ACFs) and molecular sieving carbons (MSCs) have two steps at 0.004-0.005 and 0.008-0.009 of the relative pressure. X-ray diffraction shows ACF and MSC samples to consist of micrographites. The presence of the steps indicates that the monolayer of adsorbed N2molecules on the micropore walls shows a similar phase transition to the near-monolayerphase transition from fluid to disordered solid on the flat graphite surfaces.
Introduction Molecular processes on graphite and graphitized carbon black have been extensively studied, because the (001) surface of graphites is an ideal two-dimensional system. Several authors reported stepwise adsorption isotherms of N2 on graphite and graphitized carbon black. The studies are divided into two groups. One is concerned with the step near a relative pressure of 0.2-0.4 in the N2 isotherm,ll2 which was ascribed to condensation of second layer. Another step was found in the lower relative pressure range corresponding to monolayer formation (PIP, = 0.01-0.001). The latter is attributed to a twodimensional fluid to solid phase transition, which was shown by neutron scattering and thermodynamic studies.w Furthermore, the phase diagrams for N2 adsorbed (1) de Boer, J. H.; Limen, B. G.; van der Plas, Th.; Zondervan, G. J. J. Catal. 1965,4,649. (2)Brown, C. E.;Hall, P. G. Tram. Faraday SOC.1971, 67, 3558. (3) Kjem, J. K.; Passell, I,.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. Reu. 1976,13B,1446. (4)Rouquerol, J.; Partyka, S.; Rouquerol, F. J. Chem. Soc., Faraday Trans 1 1977, 73,306.
on graphitized carbon black and graphite have been revealed.'-" Physical adsorption of vapors on micropores of pore width less than 2 nm is called micropore filling;12 the micropore filling is assumed to be significantly different from the layer-by-layer adsorption on flat surfaces and capillary condensation in mesopores. The mechanism of the micropore filling is not fully established yet. Activated carbon fibers (ACFs) have uniform micropores of about 1-nm slit width.13 ACF consists of graphite-like micro( 5 ) Larher, Y. J. Chem. Phys. 1978,68,2257. (6)Grillet, Y.;Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1979, 70,239. (7)Butler, D.H.;Huff,G. B.; Toth, R. W.; Stewart, G. A. Phys. Rev. Lett. 1975,35,1718. (8) Piper, J.; Morrison, J. A.; Peters, C.; Ozaki, Y. J. Chem. SOC., Faraday Trans. 1 1983, 79,2863. (9)Chan, M. H.W.; Migone, A. D.; Miner, K. D. Phys. Reu. 1984,30B, 2681. (IO) You,H.;Fain, S. C. Faraday Discuss. Chem. SOC.1985,80,159. (11)Steele, W. A.; Vernov, A. V.; Tildesley, D. J. Carbon 1987,25,7. (12! Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982;Chapter 4.
0743-7463/89/2405-0879$01.50/00 1989 American Chemical Society
