J. Phys. Chem. 1987, 91, 4549-4553
4549
Permselectivities of Polyelectrolyte Electrode Coatings As Inferred from Measurements with Incorporated Redox Probes or Concentration Cells Jody Redepenning and Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering,t California Institute of Technology, Pasadena, California 91 I25 (Received: January 26, 1987)
Permselectivitiesof coatings and membranes prepared from polyanionic Nafion and from a new type of polycationic copolymer were evaluated by two methods. (i) Electroactive couples incorporated in coatings on graphite electrode surfaces served as redox probes whose apparent formal potentials changed as the concentration of supporting electrolyte was varied. The magnitude of those changes allowed the permselectivities of the coating to be surmised. (ii) Thicker layers of the polyelectrolytes were used as membranes to separate supporting electrolyte solutions of differing concentrations. The potential difference between reference electrodes placed on either side of the membrane allowed the permselectivities of the membranes to be judged. Both of the methods indicated nearly ideal permselectivity of Nafion coatings and membranes at supporting electrolyte concentrations below ca. 2 M. Ideal permselectivity continued to be registered for a Nafion membrane by the redox probe method even after the membrane had been punctured so that the bulk cation transference number was far from unity. For the polycationic copolymer, both methods indicated increasing deviations from ideal anion permselectivity as the concentration of the supporting electrolyte was increased. Interpretations and some consequences of the observations are presented.
Recently, a quantitative analysis of the influences of supporting electrolyte concentration on the apparent formal potentials of redox reactants incorporated as probes in polyelectrolyte coatings was presented.’ Experimental measurements in reasonable agreement with the predictions of the analysis were obtained with polyanionic coatings of Nafion. It seemed likely that the ionic permselectivities2 of coatings inferred from measurements with incoporated redox probes might not provide a reliable indication of the properties of the portions of the coatings where no redox probe was present or where the redox probe present was not electroactive. In electrocatalytic applications of coatings the transport of substrate and supporting electrolyte throughout all parts of the coating can be essential so that it is important to establish what may and may not be inferred from measurements with redox probes about the transport properties and permselectivities of whole coatings. The present experiments were carried out for this purpose. Polyelectrolytes were employed in two formats: (i) as thin electrode coatings in which redox probes were incorporated; (ii) as thicker membranes that were used as separators in conventional concentration cells. In one case it proved possible to cast a membrane on a porous gold minigrid electrode so that both redox probe and concentration cell experiments could be conducted with the same membrane coating. The results indicate that care must be exercised in inferring permselectivities from redox probe measurements.
Experimental Section Materials. Os(bpy)3(PF6)zwas prepared from Os(bpy)JZ3 by metathesis with NH4PF6. K4Mo(CN), was prepared as described by Furman and Miller.4 Nafion (EW = 1100) was obtained as a 5 wt % solution from Aldrich. Gold minigrid (1000 lines/in.) was obtained from Buckbee/Mears, Inc. A block copolymer of styrene and (diethylaminomethy1)styrene was available from other studies in this l a b ~ r a t o r y . ~Its structure and composition are shown in Figure 1. Apparatus and Procedures. Electrochemical measurements were conducted in conventional H-type cells using a commercial instrument (BAS- 100 electrochemical analyzer, Bioanalytical Systems). Membrane potentials were measured by using concentration cells of the type shown in Figure 2. The membrane separator was a polymer-coated gold minigrid. The gold minigrid was sealed with 5-min epoxy between two 3 cm X 3 cm glass slides in which 1/4-in.holes were drilled. Electrical contact was made to the gold minigrid at one edge by attaching a copper wire with silver epoxy (Epoxy Technology, Inc.) and sealing the connection with an additional layer of 5-min epoxy. Polymer membranes Contribution No. 7521.
0022-3654/87/2091-4549$01.50/0
were cast on the gold minigrid by allowing individual drops of a solution of the dissolved polymer to evaporate on the minigrid while it was lying across the opening of one of the compartments of the concentration cell. This process was repeated until all of the pores in the grid were filled. For the block polymer, 1-2 wt % solutions in THF were employed. The 5 wt % solution of Nafion was used as received. “Solution-processed”6 Nafion membranes were prepared as follows: The 5 wt % solution of Nafion was diluted ca. fivefold with 1:l ethano1:water. NaOH, 2 M, was added to convert the Nafion into its sodium ion form and an equal volume of D M F was added to the resulting solution which was then heated at 110 OC to remove most of the alcohol and water. A 1 X 1 cm square of gold minigrid (40 lines/in.) was positioned in a shallow well of the same dimensions that was formed on a microscope slide by applying several layers of Teflon tape. A 100-pL aliquot of the solution of Nafion in D M F was applied to the minigrid and the solvent evaporated at 150 OC under atmospheric pressure. This step was repeated twice. Finally, the membrane was heated at 150 OC for 15 min under vacuum. After cooling, the minigridsupported membrane was wetted with water, carefully removed from the glass slide, and mounted in the concentration cell. Coatings of polymers on glassy carbon electrodes were cast by spin-coating lo-& aliquots of coating solutions onto 0.2-cm2 electrodes at 2000 rpm. The thickness of these dry films was found to be 0.05-0.20 pm based on profilometer measurements (Dektak 3030). Os(bpy)y was incorporated into Nafion coatings by immersing the films for 1-2 min in aqueous 0.1 mM solutions of Os(bpy),(PF,), which were also 0.1 M in sodium chloride. Formal potentials for incorporated redox probes were equated with the average of the anodic and cathodic peak potentials of cyclic voltammograms recorded at a scan rate low enough to assure that “thin-layer” behavior was obtained (10 mV/s). All potentials are reported with respect to a saturated sodium chloride calomel reference electrode (SSCE). Measurements were carried out at the ambient laboratory temperature, 22 f 2 O C .
Results and Discussion Assessment of Permselectivity of Nafion Coatings. In a recent report, the permselectivities of Nafion coatings were assessed by (1) Naegeli, R.; Redepenning, J.; Anson, F. C. J. Phys. Chem. 1986, 90, 6227. (2) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; p 339. (3) Creutz, C.; Netzel, T. L.; Okumura, M.; Sutin, N. J . Am. Chem. SOC. 1980, 102, 1309. (4) Furman, N. H.; Miller, C. 0. Inorg. Chem. 1950, 3, 160. ( 5 ) Sumi, K.; Anson, F. C. J . Phys. Chem. 1986, 90, 3845. (6) Moore, R. B.; Martin, C. R. Anal. Chem. 1986, 58, 2570.
0 1987 American Chemical Society
4550
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
Redepenning and Anson 120t
I-
x IOOi
> E
E
d
Block Copolymer, 8- 30 Figure 1. Formula of the polycationic block polymer, B-30, used in this study.
// *\\
I
o00 d 0 1 2
0I4
0I 6
0I8
1I 0
12 I
14 I
I6 I
L -J
Figure 3. Membrane potentials for Nafion membranes separating pairs of NaCl solutions. The activity of NaCl in the reference cell compart-
JG
H Figure 2. Schematic diagram of the cell employed in measurements with Nafion membrane-gold minigrid electrodes: (A) to protentiostat; (B) voltmeter; ( C ) saturated sodium chloride calomel reference electrodes; (D) platinum wire auxiliary electrode; (E) Nafion membrane-gold minigrid electrode; (F) O-ring seals; (G) Lucite cell (5 X 5 X 5 cm); (H) glass shroud. The two cell compartments are clamped together during measurements.
incorporating cationic redox couples in the coatings and determining their apparent formal potentials as a function of the composition and concentration of the pure supporting electrolyte solutions in which the measurements were made.l With ideally permselective coatings the apparent formal potential is expected to obey 1, where E,' is the real formal potential of the
(E,?,,,
= E,'
T [c~l + RIn ZF
ment was maintained constant at 41 mM. Solid line: calculated response Na+ ) form of the Nafion for an ideal cation permselective membrane; (I membrane as cast; ( 0 ) cast Nafion membrane after incorporation of Os(bpy),**; (A) "solution-processed" Nafion membrane (Nat form).
sites in the polyelectrolyte coating and the redox counterions, although hydrophobic interactions are also important with materials such as N a f i ~ n . ~Thus, the retained redox couples are necessarily confined to, or strongly associated with, the "Donnan domains" within the coatingsi0 which are the source of permselectivity. If some portions of the coatings are not ideally permselective because of structural or other differences, the measured values of will not necessarily be affected because the counterion transference number at the sites where the redox couples are retained could remain near unity. To examine this possibility, membrane potentials were measured in a two-compartment cell in which membranes of solution-processed Nafion cast on a porous gold minigrid were used to separate the two compartments. The difference in potential between two saturated calomel reference electrodes placed on either side of the membrane in sodium chloride solutions of differing concentrations was measured as a function of the ratio of the activities of the sodium chloride solutions. Some results are shown in Figure 3. For ideally cationic permselective membranes the measured potentials are expected to obey eq 2, where a, and a2 are the
[Cpl
incorporated redox couple within the coating and [C,] and [C,] are the concentrations of the countercation of charge Z in the solution and coating, respectively. Under conditions where the concentration of electroinactive counterions within the coatings remains contant as the supporting electrolyte concentration is varied, linear plots of (E,')appvs. In [C,]are expected. If the permselectivity of the coating is imperfect, so that coanions are also able to enter the coating and contribute to ionic transport within it, a more complex and nonlinear dependence of (Ei)app on In [C,] is anticipated that depends upon the relative concentrations and mobilities of the counter- and co-ions within the coatings.' With Nafion coatings and unipositive counterions, linear plots of (Epf),, vs. log [C,] were obtained with slopes near 59 mV per decade at room temperature.' This behavior was interpreted as a sign of the good permselectivity exhibited by Nafion.8 However, it is important to bear in mind that experiments in pure supporting electrolytes with redox couples ion-exchanged into polyelectrolyte coatings are only possible because the redox couples are retained for long periods by the coatings. This retention usually involves strong electrostatic interaction between the fixed charged
activities of sodium ion in the two solutions. Thus, linear plots of E,,, vs. log (a1/u2)are expected with slopes of 59 mV per decade at 25 OC. Os(bpy)j2+ was usually incorporated in the membrane before the membrane potentials were measured in order to provide conditions similar to those involved in experiments with thin Nafion coatings on graphite. If the Os(bpy)j2+ was not incorporated, the membranes exhibited considerably lower permselectivity (Figure 3). However, membranes with reasonable permselectivity in the absence of incorporated Os(bpy):+ resulted when solution-processed membranes (see Experimental Section) were employed. Apparently the internal cross-linking resulting from the incorporation of multiply charged cations has beneficial effects on the membranes similar to those resulting from processing at high temperatures as described recently by Moore and Martin.6 For activity ratios up to ca. 10 (0.4:0.04 M NaCl), the experimental values of potentials measured yith membranes in which Os(bpy) j2+ had been incorporated or with solution-processed membranes fall reasonably close to the calculated line that corresponds to ideal permselectivity. With higher concentrations of NaCl the experimental points fall increasingly below the theo-
(7) Reference 2, p 374 ff. (8) Eisenberg, A,; Yeager, H. L. Perfluorinated Ionomer Membranes; ACS Symp. Ser. No. 180; American Chemical Society: Washington, DC, 1982.
(9) Szentimary, M. N., Martin, C. R. Anal. Chem. 1984, 56, 1898. Moore, 111, R. B.; Wilkerson, J. E.; Martin, C. R. Anal. Chem. 1984.56, 2572. (10) Anson, F. C.; Saveant, J.-M.;Shigehara, K. J . Am. Chem. SOC.1983, 105. 1096.
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4551
Permselectivities of Polyelectrolyte Coatings
,
N 0.3 cm2) before a series of membrane potentials was measured in a concentration cell. As shown in Figure 4B, the presence of the hole reduces the cation permselectivity of the membrane. The response obtained presumably arises from both the intact membrane and the small hole in proportions that are a complex function of the ionic mobilities in both regions. The expected response calculated for a pure NaCl/NaCl liquid junction is given by the dashed line in Figure 4B. For pairs of NaCl solutions the junction potential, E,, is of the opposite sign from the membrane potential and the calculated slope of E, vs. log [ a l / a 2 ]is -12 mV per decade. The relatively constant membrane potentials obtained with the punctured membrane for log [ a l / a 2 ]values greater than unity may be the result of nonlinear combination of decreasing values of E, and increasing values of E,,,. The same punctured membrane was also used to measure values of ( E 3,,, for the incorporated O ~ ( b p y ) ~couple ~ + / ~in the same set of supporting electrolyte solutions. The resulting values of A(E;),,, are also plotted in Figure 4B. They all lie close to the calculated line corresponding to ideal permselectivity. It is clear that the same (punctured) membrane that had lost much of its permselectivity when examined as a separator in a concentration cell showed no sign of imperfect permselectivity when tested by means of the redox probe. This experiment supports the suggestion outlined above that redox probes may be subject to the effects of local Donnan potentials even though the overall coating or membrane in which they are incorporated is not ideally permselective. Behavior similar to that shown in Figure 4B might be anticipated for other types of electrode coatings that do not exclude co-ions. Examples include the “porous plugs” described some time ago by Wyllie and co-workersII,12and the membrane described recently by Majda and MoranI3 in which agarose and Nafion were combined to produce a coating that readily incorporated cationic reactants but, unlike pure Nafion coatings, did not prevent multiply charged anionic species such as Fe(CN)63- from permeating through the coating to be reduced at the underlying e1e~trode.l~ Assessment of Permselectivity of Polycationic Coatings. In an attempt to explore the behavior of intact coatings that were not ideally permselective, we examined the behavior of a recently described ternary copolymer, B-30, that contains tertiary amine groups that are protonated in aqueous media to produce polycationic materials with good anion exchange capacities5 (Figure 1). Coatings prepared from these copolymers swell more than Nafion coatings5y8and it was of interest to assess their permselectivities. Unfortunately, this class of copolymers required such thick coatings to obtain stable membranes on gold minigrids that cyclic voltammograms for redox probes incorporated in the thick membranes were too distorted to allow reliable values of (E,?,,, to be evaluated. However, simple concentration cell experiments of the type that were carried out with Nafion membranes were possible with supported membranes of B-30. In addition, the apparent formal potentials of anionic redox probes incorporated in thin coatings on graphite electrodes were measured for the M o ( C N ) ~ ~ - /and ~ - Fe(CN)63-/4- couples as a function of the concentration of the supporting electrolyte. The formal potentials of these redox couples in homogeneous solution as measured a t uncoated electrodes exhibit a strong dependence on the identity and concentration of the supporting electrolyte present.I4 The formal potentials increase with concentration in NaCl or MgC12 electrolytes, but they change much less in solutions of tetraethylammonium chloride. The behavior of the M o ( C N ) ~ ~ -couple / ~ - is shown in Figure 5 . For the Fe(CN)63-/4-couple, which behaves similarly, these changes have been interpreted as resulting from stronger ion-pair formation between the more highly charged, reduced form of the redox
-
-4OL
Figure 4. Responses of intact and punctured Nafion membranes to changes in the concentrations of NaCl bathing solutions: (A) intact membrane; (E)repeat of A after puncturing the membrane. Solid lines: calculated response of an ideally cation permselective membrane. Dashed line: calculated response for a NaCI/NaCI liquid junction with cation and anion transference numbers of 0.4 and 0.6, respectively; (m) mem-
brane potentials obtained from measurements with the concentration cell; ( 0 )apparent formal potehtials obtained from measurements with Os( b ~ y ) ~ ~ as + /a~redox + probe. The activity of NaCl in the reference compartment of the cell was maintained constant at 9 mM. retical line as expected when the concentration of one of the solutions becomes comparable to the codcentration of fmed charge sites inside the coating so that the Donnan exclusion of anions is less complete. Gold minigrids were used as supports for the Nafion membranes used as separators in the experiments with the concentration cells. In those cases where O ~ ( b p y ) , ~was + incorporated into the membranes, they could also be examined as electrode coatings by employing the gold minigrid support as a working electrode and adding an auxiliary electrode to the cell. In a series of experiments of this type, the values of for the Os(bpy)?+/2+ redox couple incorporated in the membrane separator were measured in the concentration cell, first with the auxiliary and reference electrodes in the more dilute solution on one side of the membrane, and then after moving them to the more concentrated solution on the other side of the membrane. The differences between the apparent formal potentials measured on either side of the membrane, A@,’),,,, are plotted in Figure 4A along with the corresponding membrane potentials and the calculated line corresponding to ideal permselectivities. Both sets of potentials fall fairly close to the line although the values of are all slightly greater than the membrane potentials. However, the agreement is reasonable considering the possible sources of experimental uncertainties; e.g., the cyclic voltammograms for the Os(b~y),+/~+ couple incorporated in the membrane showed greater separations in peak potentials than were obtained for the same couple in thinner coatings of Nafion on graphite, and membrane potentials measured with identically prepared membranes varied by as much as 5 mV. Thus, for Nafion coatings over a reasonably wide concentration range, it may be concluded that the high permselectivity indicated by the slopes of plots of (E,’),, for incorporated redox couples vs. log [C,] is representative of the permselectivity of the coating as a whole. However, for each coating material of interest it is necessary to establish independently that permselectivities indicated by shifts in apparent redox formal potentials with concentration also apply to the coating as a whole. A simple demonstration of the case in which a membrane coating that is not ideally permselective as a whole nevertheless appears permselective when examined by means of an incorporated redox probe is provided in Figure 4B. A membrane supported on a gold minigrid electrode was prepared as described above and O ~ ( b p y ) , ~was + incorporated in it. A small hole (-0.005 cm2) was then introduced into the center of the membrane (total area
(1 1) Sauer, M. C.; Southwick, P. F.; Spiegler, K. S.;Wyllie, M. R. J. Ind. Eng. Chem. 1955, 47, 2187. (12) Spiegler, K. S.; Yoest, R. L.; Wyllie, M. R. J. Discuss. Faraday SOC.
1956, 21, 174. (13) Moran, K. D.; Majda, M. J . Electroanal. Chem. 1986, 207, 13. (14) Hanania, G.; Imine, D.; Eaton, W.; George, P. J Phys. Chem. 1967, 71, 2022.
4552
Redepenning and Anson
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
,, ,
1201
./
/
A ' /
0
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I
/
/
'A
z
,
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I
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1
,
,
,
/
I
20
Figure 6. Membrane potentials for protonated B-30 membranes separating pairs of NaCl solutions. The concentration of NaCl in the reference cell compartment was as follows: (A) 10 mM; (B) 0.5 mM. The
460. 440 -
dashed line corresponds to ideal anion permselectivity.
420L
4001
1
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I
I
1
-3 0
I
I
I
I
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1
1
I
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I
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I
00
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Figure 5. Apparent formal potentials of the MO(CN)~~-/& couple as a function of the concentration of the supporting electrolyte. Solid lines: MO(CN)~'-/@ in protonated B-30. Dashed lines: MO(CN)~~-/@ in so-
lution. Supporting electrolyte: ammonium chloride.
( 0 ) NaC1, (A) MgCl,, ).(
tetraethyl-
couple and the alkali or alkaline earth cations of the supporting ele~trolyte.'~The tetraethylammonium cations are presumed to form much weaker ion pairs with both forms of the redox couple. Incorporation of the M O ( C N ) ~ >couple / ~ in coatings prepared from the polycationic block copolymer B-30results in a significant decrease in the apparent formal potential of the couple and an entirely different dependence on the supporting electrolyte concentration (Figure 5 ) . The negative ,shift in apparent formal potential produced by the incorporation shows that the more highly charged MO(CN)~& ion is less strongly bound by the polycationic coating than is Mo(CN),~-. This behavior contrasts with the behavior in homogeneous solution where the positive shift in formal potential in changing from tetraethylammonium to sodium chloride supporting electrolytes indicates that the more highly charged, reduced form of the redox couple forms the stronger ion pairs with the supporting electrolyte cation. This somewhat surprising result matches the behavior of the Fe(CN)63-/4-couple in coatings of It has been both B-30and protonated poly(~inylpyridine).~~,~~ attributed to a greatly diminished tendency toward ion pairing by the more highly charged half of the redox couples because of large differences in solvation energies inside and outside of the polycationic coatings.16 The decrease of the apparent formal potentials of the redox couple with a decrease in the concentration of the supporting electrolyte is the result expected when account is taken of the transfer of anions between the coating and the supporting electrolyte solution during the electrode process.' For an ideally anion-permselective coating the relevant half-reaction is (Mo(CN),~-), + X; + e- = (Mo(CN),~-), + X; (3) where the subscripts refer to the polycationic coating (p) or supporting electrolyte solution (s) and X- is the counteranion of the supporting electrolyte. The apparent formal potential of the MO(CN),~-/~couple within the coating, (E,9,,,, is therefore expected to obey eq 4, where E,' is the real formal potential of (E,?,,,
= E,'
RT Lx;1 +In -
F
[XS-I
(4)
(15) Oyama, N.; Shimomura, T.; Shigehara, K., Anson, F. C. J. Electroanal. Chem. 1980, 1 1 2, 27 1. (16) Braun, H.; Storck, W.; Doblholfer, K. J . Electrochem. Soc. 1983,130, 807.
the couple in the coating. Except for possible changes in swelling, the value of [X;] within coatings that contain much less of the redox couple than of the electroinactive counterions will not change with the concentration of supporting electrolyte, [X;]. Thus, vs. log [X;] are expected to according to eq 4,plots of be linear with slopes of -59 mV per decade at 25 "C. As shown in Figure 5 , apparently linear plots are obtained with the three different supporting electrolytes. However, the slopes of the lines are near -45 mV instead of the -59 mV per decade predicted by eq 4. &sentially the same behavior resulted with the Fe(CN):-/& couple. The deviant slopes indicate that coatings of B-30are not ideally anion permselective. Deviations from eq 4 are expected to become less pronounced as the electrolyte concentration is decreased because Donnan equilibrium causes the concentration of sorbed co-ions within the coating to decrease with the square of their concentration in s~lution.~'In addition, contributions to ion transport across the coating along pathways not subject to Donnan equilibria (e.g., via pinholes in the membrane) will also decrease with the concentration of supporting electrolyte.12 It was not possible to extend formal potential measurements of the type shown in Figure 5 to lower concentrations of supporting electrolyte because of the uncertainties introduced by the increased uncompensatable resistances that resulted. Fortunately, concentration cell experiments with membranes of the B-30copolymer as the separator could be extended to lower concentrations because measurements of values of E,, are made in the absence of current flow. Membrane potentials measured for B-30over a wide range of electrolyte concentration ratios are plotted in Figure 6. It is clear that the data points lie on curves rather than lines, as expected,' and that the membrane approaches the ideal permselective behavior predicted by eq 4 as the concentrations of NaCl are decreased. The concentration dependence of the formal potentials of the Mo(CN):-I4- couple in solution (Figure 5, dashed lines) indicates the presence of extensive ion pairing with sodium cations and much less with tetraethylammonium cations. At concentrations of NaCl where slopes less negative than -59 mV are obtained in Figures 5 and 6 sodium co-ions are presumably present within coatings and membranes of B-30. Nevertheless, the essentially identical slopes of the solid lines in Figure 5 signal the absence of significant ion pairing with sodium co-ions when the M o ( C N ) ~ ~ -anions /~are confined within B-30. The interaction of the M o ( C N ) ~ ~ - ~ anions with the cationic sites within B-30,which are present at much higher concentrations than any co-cations present within the polyelectrolyte, dominates the ion-pairing equilibria so that co-ions present within the coating are unable to form ion pairs despite their strong affinities for M O ( C N ) ~ in ~ -homogeneous solution.
(17) Reference 2, pp 136, 143.
J. Phys. Chem. 1987, 91, 4553-4555 Concluding Remarks The measurements reprted here have indicated the of conducting experiments with both concentration cells and incorporated redox probes in examining the permselective behavior of polyelectrolyte coatings and membranes. It is particularly noteworthy that coating permselectivities surmised from the concentration deDendence of incormrated redox probes’ may not reflect the permselectivity that would be obsdrved in typical concentration cell measurements with the same coating material
4553
employed as the membrane separator. However. the abilitv of redLx probes incorporated in t i e Donnan domains‘of imperfectly permselective membranes to respond as if the membrane were perfectly permselective could have advantages in sensor applications. For example, the electroanalytical selectivity of a coating might be preserved even when its bulk permselectivity had been compromised.
Acknowledgment. This work’was supported by the National Science Foundation and the U S . Army Research Office.
Localization of Methanol, Ethanol, and 2-Propanol at Micelles in Water: an NMR T,-Relaxation Study N. R. Jagatmathan,+K. Venkateswaran, F. G. Herring, G. N. Patey, and David C. Walker* Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1 Y6 (Received: February 2, 1987; In Final Form: April I , 1987)
Small aliphatic alcohols have been found to localize at dilute micelles rather than remain dispersed in water when surfactants are added, even at low concentrations. Evidence for this was obtained from the effects of added micelles on proton NMR spin-lattice relaxations induced by the microenvironment with and without paramagnetic ions. Use was made of the double pulse inversion-recoverytechnique ( r t - 7 r / 2 sequence). These alcohols are among the most water-soluble organic compounds known, so their localization indicates how strongly all nonionic molcules are forced from water’s structure. By comparison with benzene it is suggested that the alcohols locate at the micelle surface. It was not possible to evaluate the actual degree of localization but it must be very “substantial”. The most striking effect was that paramagnetic Mn2+-inducedrelaxations were greatly enhanced by negatively charged micelles and weakly reduced by positive micelles. This is consistent with Mnz+ being attracted into or repelled from the micelle’s double layer and is due to magnetic dipolar coupling of the localized proton with the electronic spin of the paramagnetic ion.
Introduction There has been a tendency in the past to assess the likelihood of a solute being found in the aqueous phase, rather than the hydrocarbon core, of a micelle system, by probability calculations based upon solubility data combined with volume ratios.’ When this is done for the dilute alcohol/micelle systems used here, one obtains a negligible probability of finding an alcohol molecule associated with the lipid (hydrophobic) phase. This can be seen from (i) the known distribution coefficients of 0.0016,0.008, and 0.03 from methanol, ethanol, and propanol in C6 to C12hydrocarbons compared to water,2 and (ii) the fact that the hydrophobic phase constituted only about 6% of the total volume of these solutions. On the basis of their relative solubility in water or hydrocarbon, the probability of finding the alcohol molecule in the hydrocarbon phase of these systems is merely 0.000 01 for methanol, 0.00005 for ethanol, and 0.0002 for propanol. However, recent muon spin rotation experiments (pSR) have produced a 104-fold enhancement in the muonium atom reaction cross section toward 2-propanol in water on addition of lo4 M concentrations of micelle^.^ Such enhancements can only be explained if 2-propanol is preferentially localized by the dilute micelle. Muonium localization has already been d e t e r m i ~ ~ eso d,~ the question now is, does 2-propanol localize significantly? (It might be noted that if only a fraction of 2-propanol is localized for a given solute/micelle concentration, then the calculated rate constant enhancement would be even larger, by the inverse of that fraction.) Localization is a transitory phenomenon. On the molecular level there is continual dynamic exchange of individual surfactants between the micellar and bulk phases, and an encased solute has a residence time of only lo-“ s or so.’ But, this is long compared
-
-
‘Present address: Solid State and Structural Chemistry, Indian Institute
of Science, Bangalore, 560012, India
0022-3654/87/2091-4553$01.50/0
to a muon’s lifetime (2 X lod s). Therefore each pSR measurement contributing to a muonium rate constant4 occurs on a solute distribution which is essentially “frozen-in-time”, because muonium’s existence is short relative to the diffusion time. N M R spin-lattice ( T1)-relaxation studiesS are also sensitive to this “equilibrium distribution” of solutes, and in the present study both intrinsic and paramagnetic-induced relaxation times were measured. In this way the microenvironments of the protons of the alcohols were probed as a function of added surfactant. (Chemical shifts caused by changes in microenvironment cannot be used in these studies since they are too small, even at 400 MHz, for aliphatic alcohols, though they can be seen due to ring-currentinduced effects with aromatic alcohols.6)
Experimental Section D 2 0 was used as the aqueous medium to avoid H20proton resonances. (This meant that the -OH groups of the alcohols (1) (a) Tanford, C. The Hydrcphobic Effect; Wiley: New York, 1973; pp 4-23. (b) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975; pp 44-47. (c) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982; pp 27-35. (2) Koreman, I. M.; Chernorukova, Z . G. Zh. Prikl. Khim. 1974, 47, 2523-2526. (3) (a) Venkateswaran, K.; Walker, D. C. Hyperfine Interact. 1986, 32, 559-564. (b) Venkateswaran, K.; Barnabas, M. V.; Stadlbauer, J. M.;
Walker, D. C., to be published. (4) (a) Hughes, V. W. Annu. Reu. Nucl. Sci. 1966, 16, 445-470. (b) Brewer, J. H.; Crowe, K. M.Annu. Rev.Nucl. Part. Sci. 1978,28, 239-326. (c) Walker, D. C. J . Phys. Chem. 1981, 85, 3960-3971. (5) Martin, M. L.; Delpeuch, J. J.; Martin, G. J. Practical N M R Spectroscopy; Heyden: London, 1980. (6) (a) Fendler, E. J.; Day, C. L.; Fendler, J. H. J. Phys. Chem. 1972, 76, 1460-1467. (b) Miyagishi, S.;Nishida, M. J . Colloid Interface Sci. 1980, 78(1), 195-199. (c) Ganesh, K. N.; Balasubramanian, D. J . Phys. Chem. 1982, 86, 4291-4293.
0 1987 American Chemical Society
