J. Phys. Chem. 1988, 92, 6461-6464
6461
Oxygen Diffusion via Cobalt-Porphyrin Complexes Fixed in a Polymer Matrix Eishun Tsuchida,* Hiroyuki Nishide, Manshi Ohyanagi, and Osamu Okada Department of Polymer Chemistry, Waseda University, Tokyo 160, Japan (Received: February 16, 1988; In Final Form: May 3, 1988)
Molecular oxygen diffusion through the [(~,(~’,a”,(~”’-~e~~-tetra~s(o-pi~alamidophenyl)porphinato]cobalt(II) (COP)complexed with various ligands and fixed in a transparent and rubbery poly(buty1 methacrylate) membrane was studied. Oxygen-binding kinetic and equilibrium constants of the COPcomplexes in the membrane are spectroscopically in situ determined. The diffusion coefficients of the dissolved oxygen according to Henry’s law and the adsorbed or coordinated oxygen to the complex according to the Langmuir isotherm are evaluated by analyzing oxygen permeation behavior through the membrane in terms of the combination of dual-mode transport theory and the spectroscopic data. The oxygen diffusion coefficient via the COPcomplex increases with the oxygen-dissociationkinetic constant from the COPcomplex.
Selective binding of molecular oxygen to a metalloporphyrin and a cobalt Schiff base complex fixed in a solid polymer has often been studied to provide a hemoglobin model or an adsorbent of oxygen.’” Although these complexes fixed in polymers sorbed oxygen reversibly, the oxygen adsorption and desorption occurred very slowly even in a finely powdered state with a large surface area and an oxygen-binding profile of the complexes fixed in the solid polymers has not been elucidated. Much research has been directed toward the liquid membrane systems containing mobile carriers, in view of their capability of highly selective transport of small guest molecule^.^*^ The liquid membrane containing hemoglobin as a mobile carrier of oxygen has been previously r e p ~ r t e d . ~ ,This ’ ~ was recently developed for an oxygen permselective liquid membrane by using cobalt Schiff base complexes as the mobile carrier of oxygen.” However, for the liquid membrane the membrane itself cannot be used under a differential gas pressure and the liquid medium containing the metal complex is vaporized in use. Thus, it is not feasible by using the liquid membrane to quantitatively study the oxygen permeation and to produce an oxygen-enriched membrane to separate oxygen from air. Attention has to be paid to a dry polymer membrane containing a metal complex as a fixed carrier that interacts specifically, reversibly, and rapidly with o ~ y g e n . ’ ~ , ’ ~ Recently, we succeeded in preparation of a dry polymer membrane containing a cobalt-porphyrin complex or a cobalt-Schiff base complex as the fixed carrier which sorbs and transports oxygen selectively in the Langmuir The permeability ratio of oxygen to nitrogen was greater than 10, and the permeation behavior was discussed with a dual-mode transport model. However, these studies were confined to the facilitation effect of the metal complex on oxygen transport. Discussions on the fa-
cilitation mechanism that clarify the correlation between the facility of oxygen diffusion and the oxygen-binding ability of metal complexes are effective in designing the facilitated transport membrane of oxygen. This paper described facilitated oxygen transport in the dry polymer membranes containing the cobaltporphyrin complexed with various axial, nitrogenous ligands and the effect of the oxygen-binding ability of the cobalt-porphyrin complexes fixed in the membrane on oxygen diffusion via the fixed complexes. The rubbery polymer membranes are prepared by homogeneously dispersing a [a,a’,a”,d”-meso-tetrakis(0-pivalamidophenyl)porphinato]cobalt(II) (COP) complex in poly(buty1 methacrylate) (PBMA). COP is complexed with various ligands, 1-methylimidazole (Im), 1,2-dimethylimidazole (MIm), pyridine (Py), 4-cyanopyridine (CPy), 4-aminopyridine (APy), and 4(dimethy1amino)pyridine (MAPy) to alter the oxygen-binding ability of the COPcomplex. The membranes are transparent and red wine colored, and the rapid and reversible oxygen binding to the fixed COPcomplexes can be observed in situ with the spectral change in the visible region. Oxygen exists and diffuses in the membrane as two populations: one dissolved in the polymer matrix according to Henry’s law and the other coordinated to the fixed complex according to the Langmuir isotherm. The diffusiveness of the oxygens is evaluated by analyzing the oxygen permeation behavior measured with a low-vacuum permeation method, in terms of the combination of the dual-mode transport model and the oxygen-binding kinetic and equilibrium constants of the fixed COP complexes. The advantages for this study on diffusion of a small molecule via a fixed carrier in a polymer matrix are that transport of the small molecule can be evaluated by both spectroscopic and permeation measurements and that the small molecule-binding ability of the fixed carrier can be varied without any influence on the physical property of the polymer matrix.
(1) Wang, J. H. J . Am. Chem. Soc. 1958, 80, 3168. (2) Leal, 0.;Anderson, D. L.; Bowman, R. G.; Basolo, F.; Burwell, R. L. J . Am. Chem. Soc. 1975, 97, 5152. (3) Tsuchida, E.; Nishide, H. Adu. Polym. Sci. 1977, 24, 1. (4) Tsuchida, E. J . Macromol. Sci., Chem. 1979, A13, 545. (5) Wohrle, D. Adu. Polym. Sci. 1983, 50, 45. (6) Gillis, J. N.; Sievers, R. E.; Pollock, G. E. Anal. Chem. 1985, 57, 1572. (7) Lamb, J. D.; Christensen, J. J.; Izatt, S. R.; Bedke, K.; Astin, M. S . ; Izatt, R. M. J. Am. Chem. SOC.1980, 102, 3399. (8) Newcomb, M.; Toner, J. L.; Helgeson, R. C.; Cram, D. J. J . Am. Chem. SOC.1979, 101, 4941. (9) Scholander, P. F. Science 1960, 131, 5 8 5 . (IO) Hemmingsen, E.; Scholander, P. F. Science 1960, 132, 1379. ( 1 1) Johnson, B. M.; Baker, R. W.; Matson, S.L.; Smith, K. L.; Roman, I. C.; Tuttle, M. E.; Lonsdale, H. K. J . Membr. Sci. 1987, 31, 31. (1 2) Nishide, H.; Ohyanagi, M.; Okada, 0.;Tsuchida, E. Macromolecules 1986, 19, 494. (13) Drago, R. S.;Balkus, K. J. Inorg. Chern. 1986, 25, 716. ( 1 4) Nishide, H.; Ohyanagi, M.; Okada, 0.;Tsuchida, E. Macromolecules 1987, 20, 417. ( 1 5 ) Nishide, H.; Ohyanagi, M.; Kawakami, H.; Tsuchida, E. Bull. Chem. Soc. Jpn. 1986, 59, 3213. (16) Tsuchida, E.; Nishide, H.; Ohyanagi, M.; Kawakami, H. Macromolecules 1987, 20, 1907.
Experimental Section Materials. [(Y,a’,(Y”,cr’”-meso-Tetrakis(o-pivalamidophenyl)porphinato]cobalt(II) (COP) was synthesized as in ref 17. Im, MIm, Py, CPy, APy, and MAPy were purified by distillation or recrystallization. COPwas complexed with the nitrogenous ligand in toluene under a nitrogen atmosphere. Toluene solutions of the COP complex and poly(buty1 methacrylate) (PBMA) (MW = 320000) are mixed, and the mixed toluene solution is carefully cast on a Teflon plate under an oxygen-free atmosphere, followed by drying in vacuo, to yield a transparent, red wine colored membrane with a thickness of 55-60 pm containing 2.5 wt 7% COP. Spectroscopic Measurements. Reversible oxygen binding to the COP complex fixed in the membrane is observed to be accompanied by a spectral change in the visible absorption (using a high-sensitivity spectrophotometer, Shimazu UV2000). The spectrophotometer is equipped with a cell having a gas inlet and
0022-3654/88/2092-6461$01.50/0
(17) Collman, J. P.; Brauman, J. I.; Coxsee, K. M.; Halbert, T. R.; Hayes, S . E.; Suslick, K. S . J . Am. Chem. Soc. 1978, 100, 2761.
0 1988 American Chemical Society
6462
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
--I
Tsuchida et al.
A
- 5.0
P
E 0 U
s
k
6
-8.0 Oxygen partial pressure
p(On)
mmktg
Figure 1. Oxygen-binding equilibrium curves of the CoP(L) complexes fixed in the PBMA membrane at 25 "C. L: Im ( O ) , MAPy (e),APy (e), M I m (a).Py (a),CPy (0).
outlet tube. A membrane sample is stuck on the cell wall. A gaseous mixture of oxygen and nitrogen is introduced into the cell through the gas inlet and outlet tube with a standing pressure such that the total pressure in the cell is 760 mmHg. Permeation Measurement. Oxygen permeation coefficients for various upstream gas pressures were measured with a low-vacuum permeation apparatus in the chamber with stable thermostating (Rika Seiki Inc. gas permeation apparatus K-315 N-03). The glass transition temperature is ca. 15 OC for the COPmembranes, and the membranes are in a rubber state at the temperatures for the permeability measurements. The pressure on the upstream side is maintained essentially constant. The pressures on the upstream and downstream sides are detected with a Baratron absolute pressure gauge (MKS Instruments Inc.). Nitrogen permeation coefficients are measured by the same procedure as for oxygen. The permeation coefficient was calculated from the slope of the steady-state straight line of the permeation curve. The time lag is measured from the crossing point of the steady-state straight line and the abscissa on the permeation curve.
Results and Discussion Oxygen Binding to the Complex in the Membrane. For example, the spectrum of the deoxy CoP(Py) complex (A,, = 528 nm) fixed in the membrane was changed to the spectrum with A,, = 545 nm assigned to the oxy CoP(Py) complex (02/Co = 1/ 1 adduct) immediately after exposure of the membrane to oxygen. The oxy-deoxy spectral change was reversible in response to a partial pressure of oxygen with isosbestic points a t 480, 538, and 667 nm. The visible absorption spectra in the COPcomplexes with the various ligands agreed with those for the corresponding complex in toluene solution. The oxygen-binding equilibrium constant ( K ) was determined from the oxygen-binding and -dissociation equilibrium measurement using Drago's equation.'* The oxygen-binding equilibrium curves of the COPcomplexes are given in Figure 1, which indicates that the equilibrium curves in accordance with a Langmuir isotherm. Figure 1 also shows that the oxygen-binding affinity of the COP complex can be controlled with the CoPcomplexed ligand species. K is plotted against the basicity (pK,) of the axial ligands (Figure 2). The linear relationship is the same as that previously summarized for the cobalt-protoporphyrin IX dimethyl ester complexed with various ligands in toluene s o l ~ t i o n . ' ~ The reversible oxy-deoxy spectral changes occur very rapidly; e.g., for 55-pm-thick membranes containing 2.5 wt % the oxygen-binding and -dissociation equilibria are established within a few minutes after exposure to oxygen or in vacuo at 25 OC (Figure 3). Apparent oxygen-binding and -dissociation kinetic constants (konand koR)to and from the COPcomplex fixed in the membrane were estimated by analyzing time courses of oxygen adsorption and desorption according to pseudo-first-order kinetics of the (18) Beugelsdijk, T.; Drago, R. S. J . Am. Chem. SOC.1975, 97, 6466. (19) Jones, R. D.; Surnrnerville, D. A,; Basolo, F. Chem. Reu. 1979, 79,
319.
1
I
2.0
0
I
4.0
I
I
8.0
6.0
I
10.0
I
PKa
Figure 2. Correlation of In (oxygen-binding equilibrium constant, K ) with pK, of the axial ligands (L) for the CoP(L) complexes fixed in the PBMA membrane at 25 O C . L: Im ( O ) , MAPy (e),APy (e),Py (a), CPY (0).
Time
s
Figure 3. Time course of the oxygen adsorption and desorption to and from the COPcomplexes fixed in the PBMA membrane at 25 OC. TABLE I: Apparent Oxygen-Binding and -Dissociation Rate Constants ( k , and kOm)and Oxygen-Binding Equilibrium Constant ( K ) O of the COPComplexes Fixed in the PBMA Membrane
ligand CPY PY MIm APY MAPy Im
105k,,, mmHg-' s-I 1.2
14
8.9 3.1 7.3 16
1O2kOrr,s-l 13 11 4.2 1.5 2.1 2.8
103K,mmHg-' 0.54 1.4 2.0 2.5
3.5 5.6
"Data at 25 O C . Thickness of membrane: 55 &m. Langmuir isotherm and are listed in Table I. The advantages of the membrane containing the COPcomplex are that the carrier maintains its rapid and reversible binding capability of the penetrant (oxygen) even after fixation in the polymer matrix and that kinetic and equilibrium constants of the penetrant binding to the carrier site can be evaluated in situ. Oxygen Permeation and Diffusion in the Fixed Carrier Membrane. The membrane containing the COPcomplex as a fixed carrier is assumed to sorb molecular oxygen by a dual mode: Henry's law sorption to the polymer domain and additional Langmuir sorption to the complex. Oxygen sorption and desorption to and from the COP complex in the membranes were rapid and in accordance with the Langmuir isotherm, as mentioned above. Thus, oxygen transport is expected to be accelerated by the additional Langmuir mode besides the Henry mode. Then, the oxygen permeation coefficient is represented as the sum of the Henry mode and the Langmuir mode transport (dual-mode transport theory):14 Here, P is the permeability coefficient, kD is the solubility coefficient for Henry's law, DD and Dc are the diffusion coefficients for the Henry and the Langmuir-type diffusion, C l , is the saturated amount of oxygen reversibly bound to the binding site or
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6463
Oxygen Diffusion via Cobalt-Porphyrin Complexes
-4.0
7 -I0 -3.0
-6 0 Y
c/
-7.0 -7 0
If
2
E m
I
-70 -
1
E
10.0
E u
3"
n~ E € u u
v ) " C vI N
E E
P0*(501 P%inert
Figure 5. Relationship between In K , In k,,, In kOrr,and the facilitated In k,, in transport efficiency of oxygen at 25 OC (In K in mmHg-' (O), mmHg-' s-' (O),In kerf in s-l (0)).
Figure 4. Effect of upstream gas pressure on permeation coefficient for the CoP(L)/PBMA membrane (oxygen, L: Im ( O ) , MAPy ( 0 ) ,APy (e),MIm (a),Py ( O ) , CPy (0); nitrogen, L: Py ( 0 ) )and for the inert Co*"P membrane (oxygen, L: (A)) at 25 OC.
fixed carrier, K is the oxygen-binding and -dissociationequilibrium constant, and p2 is the upstream gas pressure. Equation 1 is a function of p2, and P increases with a decrease in pz if the oxygen adsorbed to the COPcomplex is effectively mobile with adequate diffusiveness. Figure 4 shows the effect of upstream gas pressure (p2)on the permeability coefficients (Po,) in the membranes containing 2.5 wt % COP complexed with various ligands. Each Po, increases , is in accordance with eq 1. On with a decrease in p 2 ( 0 2 ) which the contrary, PN2is independent of p2(N2)because the fixed carrier does not interact with nitrogen. This is also supported by the fact that Po2 is independent of p 2 ( 0 2 )for a membrane containing the inert Co"'P(Py) complex (2.5 wt %), which does not interact with and does not bind to oxygen. Effects of the Axial Ligands of COP on Oxygen Permeation and Diffusion. Figure 4 also shows that the p 2 ( 0 2 )dependence of Po2is much affected by the COPcomplex species. For example, Po, is relatively large and drastically increases with a decrease in p 2 ( 0 2 )for th CoP(1m) membrane, while it is small and hardly influenced by p 2 ( 0 2 )for the CoP(CPy) membrane. Here, the facilitated transport efficiency of oxygen is presented as the ratio of Po,(at 50 mmHg)/Po2(in the membrane containing the inert and this facilitating ratio is Co"'P complex), (P02(SO)/P02,inerr), plotted against In K, In k,,, and In koflin Figure 5. The facilitating ratio is largest in the membrane containing the CoP(1m) complex with the largest K. The second largest ratio is observed for the CoP(Py) membrane with the largest koff. The koffvalueincreases in the order of APy, MAPy, MIm, and Py, which agrees with the order of the facilitating ratio. The efficiency of the facilitated oxygen transport by the Langmuir mode is found to be concurrently governed by the two factors K and kofp The effects of p 2 ( 0 2 )on Po, are analyzed by using eq 1; Le., Po, is plotted against 1/(1 + K p 2 ) (Figure 6). The plots show a linear relationship: The oxygen permeabilities in the membranes containing the COP complexed with various ligands as a fixed carrier can be explained in terms of the sum of the Henry mode attributed to the polymer matrix and the Langmuir mode at-
8.0
I
f 0
0.5
1.o
1 1 +KP2
Figure 6. Oxygen permeability in the CoP(L)/PBMA membrane plotted according to eq 1 at 25 'C. L: Im ( O ) , MAPy ( 0 ) ,APy (e), MIm ( O ) , PY (01, CPY (0).
tributed to the fixed carrier. In Figure 6 is also shown the extrapolation of linear Po, vs 1/(1 K p 2 ) plots to the intercept. The intercepts for the membranes containing various COPcomplexes coincide with each other, which strongly supports the validity of this study or that only oxygen-binding ability of the fixed carrier is variable without any change in physical property of the membrane. The oxygen permeability through only the polymer matrix can be calculated from this intercept, and it agrees with that directly measured for the PBMA membrane containing Co"'P (see Figure 4). This also supports the validity of this treatment. The slopes from the straight lines are very important to evaluate the oxygen diffusiveness via the COP complexes fixed in the membranes and will be used later. The time course of gaseous molecules permeation through membranes often shows an induction period (time lag) followed by a permeation with a constant slope (steady state). For a complex membrane the time lag in the permeation time course is to be enhanced because the complex interacts with the penetrant and retards its diffusivity in the membrane. The oxygen permeation time lag for the membrane containing the COP complex is also governed by both the Henry and the Langmuir modes. The time lag (0) for the oxygen permeation also depends on p 2 ( 0 2 ) ,as shown in Figure 7, in the same manner as the permeation coefficient. This behavior indicates that oxygen clearly interacts with the COP complex in the PBMA membrane. This is further supported by the results that B is independent of the upstream gas pressures for the nitrogen permeation in the CoP(Py) membrane and the oxygen permeation in the PBMA membrane containing the inert Co"'P(Py) complex, as reported previously for the CoP(1m) membrane.14 We also reported that Bo, and the p 2 ( 0 2 )dependency of Bo, for the CoP(1m) membrane decreased with temperature and explained that 80, and the p 2 ( 0 2 )dependency of Bo, were based on the oxygen binding to the complex
+
6464
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
'\ 8ot\
Tsuchida et al. TABLE 11: Diffusion Coefficient of Oxygen via the Fixed CoP(L) Complexes (Langmuir-Type Diffusion)"
I \
L
108Dc, em2 s-' WClDD)
Py
MIm
Im
MAPy
APy
3.9 0.06
2.0 0.03
1.4 0.02
0.65 0.01
0.35 0.00,
ODD = 7.0, X IO-' em2 s-', Ck = 0.20 em3 (STP) ~ m - and ~ , kD = 1.18 X em3 (STP) ern-) cmHg-'.
0" -19
- 20 In koti
Figure 8. Correlation of the oxygen-dissociation rate constant, koff,with the diffusion coefficient via the COP complex, Dc, for the CoP(L)/PBMA membrane at 25 OC. L: Im ( O ) , MAPy (e),APy (e), MIm ( c ) ) , Py
0 0 2 0 0 4 0 0 6 0 0 8 0 0 P2
(01,
"g
Figure 7. Effect of upstream gas pressure on permeation time lag for the CoP(L)/PBMA membrane (oxygen, L: Im (e),MAPy (e),APy (e), MIm ( c ) ) , Py ( O ) , CPy (0);nitrogen, L: Py ( 0 ) )and for the inert Co"'P membrane (oxygen, L (A))a t 25 OC
and enhanced at lower temperature because K of the complex increased with decrease in temperature. On the other hand, Figure 7 shows that Bo, and thep2(02)dependency of Os are independent of K of the complexes and increase with decrease in koff(see Table 1). Oxygen Diffusion and Mobility via the COP Complexes Fixed in the Membrane. The effect of p 2 ( 0 2 )on the time lag for the COP membrane was analyzed by using the theoretical equation (eq 2):14 the F R value calculated from the slope and the intercept of the linear relationship in Figure 6 was substituted in eq 2; the left term (Y) was plotted against the right term (X), to also give a linear relationship. 66'[1
+ FR/(1 + y)I3
(r) =
vbb) + FW,b) + (FR)Y*0,)112
-R+
1 + FRf,b) + ( F R ) ' f 4 b )
fob) + FRfib')
1
' W O , (2)
(FR)%b) where F = &IDD, R = CcK/kD,y = Kp2, and I is the thickness of the membrane. The functions of y , f O ( y ) - f 4 b ) ,were given in previous papers. DD and R ( C & K / k D )were calculated from the DD
f
slope and the intercept of the linear relationship for the CoP(1m) membrane in Figure 6, and F ( = Dc-DD) was given from F R (Table 11). The diffusiveness for the Langmuir mode in the CoP(1m) membrane is ca. 2% of the diffusiveness for the Henry mode. For the membranes containing the COP complexes with the other ligands, dual-mode transport parameters for the Cop( Im) membrane (see the footnote of Table I Q i 4are used to determine the other parameters to reduce errors in the calculations because there is few change in physical property in the membrane by the axial ligand species as mentioned above. F and DD are estimated by the calculated R and F R from the slopes of the straight lines and given in Table 11. The oxygen diffusion coefficient via the COP complexes fixed in the polymer matrix (Dc) is plotted against dissociation rate constant from the fixed COP complexes in Figure 8. The logarithmic plots of Dc vs koffshow a good linear relationship. Dc increases with koffand is independent of K. This result means that the penetrant dissociation kinetic constant from the fixed carrier is clearly reflected on the penetrant diffusion coefficient via the fixed carrier in the membrane.
Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of Macromolecular Complexes from the Ministry of Education, Science, and Culture, Japan.
