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Counterion Exchange Selectivity in Detergent–Polymer Aggregates
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
190, 461–465 (1997)
CS974879
Counterion Exchange Selectivity in Detergent–Polymer Aggregates Patricia M. Nassar,* Rose M. Z. Georgetto Naal,* Silvia H. de Pauli,* Joa˜o B. S. Bonilha,* ,1 Laura T. Okano,† and Frank H. Quina† ,1 *Departamento de QuıB mica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, 14040-901 Ribeira˜o Preto, Brazil; and †Instituto de QuıB mica, Universidade de Sa˜o Paulo, C.P. 26.077, 05599-970 Sa˜o Paulo, Brazil Received December 19, 1996; accepted March 24, 1997
In aqueous solution, the interaction between sodium dodecyl sulfate (SDS) and poly(ethylene glycol) (PEG) results in the formation of small aggregates or clusters of SDS attached to the PEG polymer chain. Selectivity coefficients for exchange of two monovalent (N-methyl-4-cyanopyridinium cation and Tl / ) and two divalent (methylviologen cation and Cu 2/ ) counterions at the surface of SDS–PEG clusters, determined employing photophysical techniques, are similar, but not identical, to those for exchange at the surface of SDS micelles in the absence of PEG. The principal factor affecting ion exchange selectivity in SDS–PEG clusters does not appear to be aggregate size or surface charge density but rather the presence of poly(oxyethylene) subunits at the aggregate surface. q 1997 Academic Press Key Words: detergent–polymer interaction; ion exchange; fluorescence; sodium dodecyl sulfate.
INTRODUCTION
One of the most extensively studied detergent – polymer interactions is that between the anionic detergent sodium dodecyl sulfate ( SDS ) and the nonionic polymer poly ( ethyleneglycol ) ( PEG ) in aqueous solution ( 1 – 5 ) . The SDS – PEG interaction is highly cooperative and occurs over a rather well-defined range of detergent concentration. The onset of the interaction, or critical aggregation concentration ( cac ) of the detergent, is only slightly dependent on PEG concentration. The detergent concentration at which the polymer saturates with SDS increases in approximately linear fashion with [ PEG ] . The interaction is independent of PEG molecular weight ( for MW ú 4000 ) , is somewhat enhanced by the addition of salt, and decreases with increasing temperature ( 2 – 5 ) . Specific counterion effects on the SDS – PEG interaction have also been investigated ( 6, 7 ) . Although several anionic surfactants interact with PEG, other types of detergents ( cationic, non-ionic, or zwitterionic ) appear to have little or no affinity for binding to PEG ( 3 – 5 ) . 1
To whom correspondence should be addressed.
The PEG–SDS interaction results in the formation of small aggregates or clusters of SDS attached to the PEG polymer chain. These SDS–PEG clusters are smaller than normal SDS micelles, are more highly dissociated, and appear to have a somewhat more ‘‘open’’ structure (2–5, 7, 8). The PEG polymer chains are reported to be located at the surface of the SDS clusters, but estimates of the fraction of the PEG segments bound to the micelles vary rather widely (4, 5). Although a number of theoretical models have been proposed to explain the onset of the interaction (cac), the saturation, and the aggregate size, none provides a completely satisfactory description of all of the aspects of the PEG–SDS interaction (3–5). Photophysical techniques (9, 10) have played a prominent role in studies of counterion binding to micelles. Thus, for example, selectivity coefficients for monovalent–monovalent, monovalent–divalent, and divalent–divalent counterion exchange have been determined from steady-state (11– 15) and time-resolved (16) measurements of the quenching of the fluorescence of micelle-solubilized aromatic hydrocarbons by counterions. In the present work, we have employed two photophysical probe techniques to determine counterion exchange selectivities at the surface of SDS–PEG clusters. The selectivities are similar, but not identical, to those at the surface of SDS micelles in the absence of PEG. EXPERIMENTAL
Materials. PEG of nominal molecular weight 10,000 (ATPEG-10000; Atlas-Ultra Divisa˜o QuıB mica) was employed throughout. Sodium dodecyl sulfate (SDS; Merck, for biochemical use) was purified by recrystallization from absolute ethanol. Purity was checked by surface tensiometric determination of the critical micelle concentration (cmc) in the usual manner. N-Methyl-4-cyanopyridinium (MCP / ) chloride (17) and the sodium salt of the luminescence probe (18) tris(4,4 *-dicarboxybipyridine)ruthenium(II) (RuL 40 3 ) were prepared and purified by literature procedures. Pyrene (Aldrich) was purified as described previously (11). Methyl viologen (MV 2/ ) chloride and analytical reagent grade so-
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dium chloride (Merck, p.a.), sodium sulfate (Merck, p.a.), thallium(I) sulfate (BDH AnalaR), and copper sulfate pentahydrate (Reagen S.A., p.a.) were used as received. Methods. All measurements were performed at 307C. All solutions were prepared in distilled, freshly deionized water and PEG stock solutions were stirred gently at 607C for at least 1 h to eliminate interpolymeric entanglement (19). Apparent surface tensions were determined with a du Nou¨y tensiometer equipped with a Pt ring. Steady-state emission measurements were performed with either a Perkin–Elmer LS-5B or a Hitachi F4500 fluorescence spectrometer. Pyrene fluorescence decay curves were collected by the single-photon counting technique using an Edinburgh Analytical Instruments Model FL-900 Lifetime Spectrometer (H2 flashlamp gas, 337 nm excitation, 390 nm emission). Experiments with RuL 40 as probe. Aliquots (2.5 mL) 3 05 M), SDS of aqueous solutions containing RuL 40 3 (3 1 10 (0.010–0.060 M), NaCl (0.020–0.100 M), and PEG (0, 0.20, 0.40, or 4.00% PEG wt/vol) were thermostated in Teflon-stoppered 1 cm path length fluorescence cuvettes (Hellma). Successive aliquots of a concentrated (0.30 M) stock solution of MCP / or MV 2/ were added to the sample cuvette via a calibrated Hamilton microsyringe, recording the emission intensity (465 nm excitation/625 nm emission) following each addition. Experiments with pyrene as probe. Pyrene (5 1 10 06 M) was added to air-equilibrated aqueous solutions of SDS (0.040 M, freshly prepared) and PEG (4 wt %) containing Na2SO4 (0.025, 0.050, 0.075, or 0.100 M). Aliquots of concentrated aqueous stock solutions of the quenchers (Tl / or Cu 2/ ) were added with the aid of a Hamilton microliter syringe, and the probe fluorescence decay was determined. Fluorescence decays in the absence of quencher were analyzed utilizing the standard single exponential decay routines of the FL-900 operating software. The corresponding decay curves in the presence of quencher were fit to the Infelta– Tachiya equation (20) F(t) Å A1 exp[ 0 A2t 0 A3 {1 0 exp( 0 A4t)}]
[1]
using the micelle quenching module of Edinburgh Analytical Instruments Level 2 analysis software (Version 1.60). RESULTS AND DISCUSSION
The binding of counterions to SDS aggregates can be treated in terms of competitive ion exchange (21) between the foreign counterion X (of charge z X ) and the detergent counterion Na / : K X / Na
Xaq / ÉzX ÉNamic B Xmic / ÉzX ÉNaaq .
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The corresponding counterion exchange selectivity coefficient, KX / Na , is defined as KX / Na Å ( uX /[Xaq ]) 1 /Éz X É ([Naaq ]/ uNa ),
[3]
where the subscript aq refers to the analytical concentration of counterions free in the intermicellar aqueous medium and uX and uNa are the fractional coverages of the aggregate surface by X and Na / , respectively. The micellar fractional coverages and the aqueous counterion concentrations are related to each other by the classical pseudophase ion-exchange mass balance relationships (12, 21, 22): uX Å ÉzX É([Xmic ]/[SDSag ]) Å ÉzX É([XT ] 0 [Xaq ])/[SDSag ]
uNa Å (1 0 a ) 0 uX
[Naaq ] Å a[SDSag ] / cac / [Naad ] / [SDSag ] uX .
[4] [5] [6]
In these equations, [SDST ], [XT ], and [Naad ] are the total concentrations of detergent and of added foreign and common counterion salt, cac is the critical aggregation concentration (cmc in the absence of PEG or cac in the presence of PEG), a is the degree of counterion dissociation of the aggregate, and [SDSag ] Å [SDST ] 0 cac is the concentration of aggregated SDS. Experimental determination of either [Xaq ] or uX at known total concentrations of detergent and added salt allows one to calculate the value of KX / Na from Eqs. [3] – [6]. In the present work, quenching of the luminescence of the tris(dicarboxybipyridine)ruthenium(II) anion (RuL 40 3 ) by the monovalent N-methyl-4-cyanopyridinium cation (MCP / ) and the divalent methylviologen cation (MV 2/ ) was employed to determine selectivity coefficients for exchange of these two organic counterions at the surface of SDS micelles and of SDS–PEG aggregates. The basic assumption of this method, which has been described in detail previously (18), is that the tetraanionic probe RuL 40 3 is completely excluded from the negatively charged SDS micelle or SDS–PEG aggregate. As a consequence, the efficiency of quenching of the RuL 40 3 luminescence by a positively charged counterion depends only on the concentration of the quencher counterion in the aqueous phase, [Xaq ]. Furthermore, since the intermicellar aqueous phase is a homogeneous, macroscopic phase, the quenching obeys the simple Stern–Volmer equation: I 7 /I Å 1 / KSV [Xaq ].
[7]
In order to estimate [Xaq ] from the ratio of RuL 40 3 luminescence intensities in the absence and presence of quencher (I 7 /I), the value of KSV, must be known or estimated independently. Since the probe is a tetraanion and the quencher
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COUNTERION EXCHANGE SELECTIVITY IN AGGREGATES
TABLE 1 Selectivity Coefficients for MCP//Na/ and MV2//Na/ Counterion Exchange at the Surface of SDS–PEG Aggregates Determined from Quenching of the Luminescence of RuL40 3
[NaCl] (mM)
[SDS] (mM)
20 50 80 100
10–20 10–30 10–30 10–30
KMCP//Na/
KMV2//Na/
% PEG
% PEG
0.2%
0.4%
6.5 { 1
8.8 { 0.5
5.5 { 1
9.0 { 0.5
4% 8.4 8.7 8.7 8.3
counterions are positively charged, the magnitude of the quenching rate constant, and hence KSV, is highly dependent on the ionic strength, m, of the intermicellar aqueous phase. Values of KSV determined as a function of added NaCl in the absence of SDS are adequately described by the conventional extended Debye–Hu¨ckel relationship, log KSV Å log K 7SV 0 4.4zX m1 / 2 /(1 / 2.9m1 / 2 ),
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0.6 0.9 1 1
0.4%
5.3 { 0.5
5.1 { 0.5
4.6 { 0.5
4.6 { 0.5
4% 3.5 3.5 3.6 3.6
{ { { {
0.1 0.1 0.2 0.1
that the quenching of the fluorescence of a micelle-bound probe such as pyrene is sensitive to the local concentration of quencher counterion at the micelle surface (11, 20). As we have recently shown (16), analysis of the fluorescence decay of pyrene in micellar SDS permits direct determination of uX from the coefficients of the Tachiya–Infelta equation (Eq. [1]) via the relationship
[8]
with Stern–Volmer constants at zero ionic strength, K 7SV , of 14,000 M 01 and 39,800 M 01 , respectively, for the Nmethyl-4-cyanopyridinium (MCP / ) and methylviologen (MV 2/ ) cations. In the presence of SDS micelles, the effective ionic strength in the intermicellar aqueous phase is given by the pseudophase ion exchange mass balance expression: m Å [Naaq ] / (z X ) 2[Xaq ]. The values of m and K 7SV are employed in Eq. [8] to calculate KSV, which is in turn used in Eq. [7] to calculate [Xaq ] from the experimental ratio of RuL 40 3 luminescence intensities in the absence and presence of quencher (I 7 /I). Application of this method to MCP / /Na / and MV 2/ / Na / exchange (18) at the surface of SDS micelles in the absence of PEG gave values of 7 { 1 for both ions. The corresponding selectivity coefficients for exchange of these ions at the surface of SDS–PEG aggregates are collected in Table 1. The values employed for a were 0.25 for SDS micelles (23) and 0.56 for SDS–PEG aggregates; reported a values for SDS–PEG aggregates are 0.54 at the cac (8) and 0.65 (24) and 0.56 (25) above the cac. Critical aggregation concentrations (cac) for the onset of the interaction of SDS with PEG are insensitive to the PEG concentration (0.2 and 0.4% PEG) and were estimated from the relationship (8): log cac Å 03.47 0 0.46 log(cac / [Naad ]). In all experiments with PEG, care was taken to maintain the concentration of SDS above the cac and below that at which saturation of the polymer by SDS occurs (19). The second method employed to determine counterion exchange selectivity coefficients takes advantage of the fact
{ { { {
0.2%
uX Å (ÉzX É/Nag )(A2 0 1/ t7 / A3 A4 ) 2 /[A3 (A4 ) 2 ],
[9]
where t7 is the pyrene fluorescence lifetime measured in the absence of the counterionic quencher and Nag is the aggregation number of the SDS micelles or clusters. Aggregation numbers of SDS–PEG clusters, determined by the technique of Turro and Yekta (26), have been reported elsewhere (8); the values for micellar SDS were calculated from the relationship (23) log Nag Å 2.23 / 0.23 log[Naaq ]. Selectivity coefficients determined in this manner for MV 2/ /Na / , Tl / /Na / , and Cu 2/ /Na / exchange at the surface of SDS– PEG clusters (0.040 M SDS and 4% PEG) and for Tl / / Na / and Cu 2/ /Na / exchange at the surface of micellar SDS are collected in Table 2. TABLE 2 Selectivity Coefficients for MV2//Na/, Tl//Na/, and Cu2//N/ Counterion Exchange at the Surface of SDS–PEG Aggregates (0.040 M SDS; 4% PEG) and SDS Micelles (0.040 M SDS) Determined from Analyses of the Fluorescence Decay of Pyrene in Solutions Containing Added Na2SO4 (0.025, 0.050, 0.075, or 0.10 M) KX/Na
SDS–PEG aggregates
KMV2//Na/ KTl//Na/ KCu2//Na/
5 {2 6.7 { 1 2.1 { 0.4
a
SDS micelles
5{1 1.3 { 0.3a (1.2 { 0.1)b
Reference 16. Determined from Cu2/ quenching of the luminenscence of intermicellar RuL40 3 . b
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The basic assumptions implicit in the use of Eq. [9] are that the micelles or aggregates are not highly polydisperse and that the statistical distribution of the quenchers among the available micelles obeys a Poisson distribution (20). In all cases in the present work, added common counterion salt was present and the number of bound quencher counterions was restricted to less than one per micelle or aggregate. Under these conditions, the statistical distribution of the bound quencher can be safely approximated by a Poisson distribution and any effects of the small amount of added quencher counterion on a, the cmc or cac, and the aggregation number of the micelles or clusters should be negligible. In SDS micelles, pyrene and MV 2/ have been shown to form a ground state charge-transfer complex (27) and analogous ground state complexation also occurs in SDS–PEG clusters. However, as shown by Gehlen and De Schryver (27), the dynamic part of the quenching of pyrene fluorescence by MV 2/ obeys the Tachiya–Infelta equation (Eq. [1]), implying that Eq. [9] is still valid for this system. A final assumption of the method in the case of SDS–PEG clusters is that the quenching is confined to the clusters, with no contribution from pyrene emission and/or quencher cations bound to PEG chain segments located outside of the clusters. The possibility of weak interactions between pyrene and PEG in aqueous solution is suggested by the observation of an intramolecular interaction of this type in pyrene endlabeled PEGs (28); however, upon addition of SDS, the pyrene-labeled end groups selectively solubilize in the SDS– PEG clusters (28). Similarly, in the present system, the existence of a weak pyrene–PEG interaction in the absence of SDS is indicated by the increase in the pyrene fluorescence lifetime from 134 ns in water to 142 ns in the presence of 0.4% PEG and to 170 ns in 4% PEG. In the SDS–PEG clusters, however, pyrene exhibited a single exponential lifetime of 203 ns in all cases, indicative of exclusive solubilization of pyrene in the clusters. Although binding of cations to PEG can be significant in solvents such as methanol (29), hydrophilic cations typically bind only weakly, if at all, to PEG in aqueous solution in the absence of hydrophobic anions (30). The possibility of binding of Cu 2/ to PEG in the absence of SDS was, nonetheless, examined experimentally by determining the second order rate constants (kq ) for quenching of pyrene by Cu 2/ from pyrene fluorescence lifetime data in the presence and absence of Cu 2/ . The lack of appreciable binding of Cu 2/ is indicated by the observation that kq decreased from 5.6 1 10 9 M 01 s 01 in water to 3.8 1 10 9 M 01 s 01 in aqueous 0.4% PEG to 2.4 1 10 9 M 01 s 01 in 4% PEG. As mentioned in the Introduction, SDS–PEG clusters are known to be smaller than SDS micelles formed in the absence of PEG and to have a much higher degree of counterion dissociation and a more ‘‘open’’ structure (2–5, 7). Differences have also been reported in the intracluster mobility and solubility of molecules in the SDS–PEG clusters
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(8). Despite these differences in structure and, presumably, surface charge density between SDS micelles and SDS– PEG aggregates, the exchange selectivities are found to be comparable for the two monovalent counterions (MCP / and Tl / ). Indeed, for a given ionic detergent headgroup type, monovalent counterion exchange selectivity is usually found to be rather insensitive to aggregate size, surface curvature (32, 33) or moderate changes in aggregate surface charge density (15). The divalent counterion MV 2/ exhibits a somewhat lower selectivity and Cu 2/ a distinctly higher selectivity at the surface of the SDS–PEG aggregates. The selectivity coefficients for MCP / /Na / and MV 2/ /Na / exchange exhibit modest dependences on the PEG concentration, though no consistent trend is apparent. Recent studies indicate that 1H NMR spectroscopic data is the method of choice for obtaining more detailed information on the factors contributing to the binding and exchange of such organic counterions at charged interfaces (31). One of the most attractive proposals for the origin of ion exchange selectivity between inorganic ions is the partial dehydration hypothesis of Morgan et al. (33). This model attributes differences in selectivity at different surfaces to differences in the extent of partial dehydration of the two counterions. It is not unreasonable to expect that the presence of poly(oxyethylene) subunits at the surface of the SDS–PEG clusters might influence the extent of counterion dehydration relative to that at the SDS micellar surface. Interestingly, Cu 2/ is one of the few ions for which partial dehydration cannot adequately account for the observed selectivity trends (33), suggesting that additional factors are responsible for the enhanced Cu 2/ / Na / selectivity at the surface of the SDS–PEG clusters. ACKNOWLEDGMENTS This work was supported by grants from the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP; Projects 91/0480-1 and 1994/ 3505-3) and PADCT-FINEP (Project 65-92-0063-00). P.M.N. acknowledges fellowship support from FAPESP; L.T.O. acknowledges fellowship support from CAPES (Coordenac¸a˜o de Aperfeicoamento de Pessoal de NıB vel Superior).
REFERENCES 1. Goddard, E. D., and Ananthapadmanabhan, K. P. (Eds.), ‘‘Interactions of Surfactants with Polymers and Proteins.’’ CRC Press, Boca Raton, FL, 1993. 2. Goddard, E. D., Colloids Surf. 19, 255 (1986). 3. Brackman, J. C., and Engberts, J. B. F. N., Chem. Soc. Rev. 22, 85 (1993). 4. Goddard, E. D., in ‘‘Interactions of Surfactants with Polymers and Proteins’’ (E. D. Goddard and K. P. Ananthapadmanabhan, Eds.), p. 123. CRC Press, Boca Raton, FL, 1993. 5. Lindman, B., and Thalberg, K., in ‘‘Interactions of Surfactants with Polymers and Proteins’’ (E. D. Goddard and K. P. Ananthapadmanabhan, Eds.), p. 203. CRC Press, Boca Raton, FL, 1993. 6. Maltesh, C., and Somasundaran, P., J. Colloid Interface Sci. 157, 14 (1993).
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COUNTERION EXCHANGE SELECTIVITY IN AGGREGATES 7. Abuin, E., Lissi, E., Quina, F., and Paredes, S., Bol. Soc. Chil. Quim. 40, 65 (1995). 8. Nassar, P. M., Nogueira, L. C., Bonilha, J. B. S., Leiva, O. W., and Quina, F. H., J. Braz. Chem. Soc. 6, 173 (1995). 9. Kalyanasundaram, K., ‘‘Photochemistry in Microheterogeneous Systems.’’ Academic Press, Orlando, FL, 1987. 10. Malliaris, A., Int. Rev. Phys. Chem. 7, 95–121 (1988). 11. Abuin, E., Lissi, E., Bianchi, N., Miola, L., and Quina, F. H., J. Phys. Chem. 87, 5166 (1983). 12. Lissi, E., Abuin, E., Sepu´lveda, L., and Quina, F. H., J. Phys. Chem. 88, 81 (1984). 13. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 112, 178 (1986). 14. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 143, 97 (1991). 15. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 151, 594 (1992). 16. Okano, L. T., Alonso, E. O., Waissbluth L., O. L., and Quina, F. H., Photochem. Photobiol. 63, 746 (1996). 17. Bonilha, J. B. S., Chiericato, Jr., G., Martins-Franchetti, S. M., Ribaldo, E. J., and Quina, F. H., J. Phys. Chem. 86, 4941 (1982). 18. Bonilha, J. B. S., Georgetto, R. M. Z., Abuin, E., Lissi, E., and Quina, F. H., J. Colloid Interface Sci. 135, 238 (1990). 19. Cabane, B., and Duplessix, R., J. Phys. (Paris) 43, 1529 (1982).
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20. Gehlen, M. H., and De Schryver, F. C., Chem. Rev. 93, 199 (1993). 21. Bunton, C. A., Nome, F. J., Quina, F. H., and Romsted, L. S., Acc. Chem. Res. 24, 357 (1991). 22. Quina, F. H., and Chaimovich, H., J. Phys. Chem. 83, 1844 (1979). 23. Alonso, E. O., and Quina, F. H., Langmuir 11, 2459 (1995). 24. Zana, R., Lang, J., and Lianos, P., in ‘‘Microdomains in Polymer Solutions’’ (P. Dubin, Ed.), p. 357. Plenum Press, New York, 1985. 25. Witte, F. M., and Engberts, J. B. F. N., J. Org. Chem. 52, 4767 (1987). 26. Turro, N. J., and Yekta, A., J. Am. Chem. Soc. 100, 5951 (1978). 27. Gehlen, M. H., and De Sshryver, F. C., J. Phys. Chem. 97, 11242 (1993). 28. Quina, F. H., Abuin, E., and Lissi, E., Macromolecules 23, 5173 (1990). 29. Quina, F., Sepu´lveda, L., Sartori, R., Abuin, E. B., Pinto, C. G., and Lissi, E. A., Macromolecules 19, 990 (1986). 30. Sartori, R., Sepu´lveda, L., Quina, F. H., Lissi, E., and Abuin, E., Macromolecules 23, 3878 (1990). 31. Bachofer, S. J., and Simonis, U., Langmuir 12, 1744 (1996). 32. Morgan, J. D., Napper, D. H., Warr, G. G., and Nicol, S. K., Langmuir 10, 797 (1994). 33. Morgan, J. D., Napper, D. H., and Warr, G. G., J. Phys. Chem. 99, 9458 (1995).
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CS974879
Counterion Exchange Selectivity in Detergent–Polymer Aggregates Patricia M. Nassar,* Rose M. Z. Georgetto Naal,* Silvia H. de Pauli,* Joa˜o B. S. Bonilha,* ,1 Laura T. Okano,† and Frank H. Quina† ,1 *Departamento de QuıB mica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, 14040-901 Ribeira˜o Preto, Brazil; and †Instituto de QuıB mica, Universidade de Sa˜o Paulo, C.P. 26.077, 05599-970 Sa˜o Paulo, Brazil Received December 19, 1996; accepted March 24, 1997
In aqueous solution, the interaction between sodium dodecyl sulfate (SDS) and poly(ethylene glycol) (PEG) results in the formation of small aggregates or clusters of SDS attached to the PEG polymer chain. Selectivity coefficients for exchange of two monovalent (N-methyl-4-cyanopyridinium cation and Tl / ) and two divalent (methylviologen cation and Cu 2/ ) counterions at the surface of SDS–PEG clusters, determined employing photophysical techniques, are similar, but not identical, to those for exchange at the surface of SDS micelles in the absence of PEG. The principal factor affecting ion exchange selectivity in SDS–PEG clusters does not appear to be aggregate size or surface charge density but rather the presence of poly(oxyethylene) subunits at the aggregate surface. q 1997 Academic Press Key Words: detergent–polymer interaction; ion exchange; fluorescence; sodium dodecyl sulfate.
INTRODUCTION
One of the most extensively studied detergent – polymer interactions is that between the anionic detergent sodium dodecyl sulfate ( SDS ) and the nonionic polymer poly ( ethyleneglycol ) ( PEG ) in aqueous solution ( 1 – 5 ) . The SDS – PEG interaction is highly cooperative and occurs over a rather well-defined range of detergent concentration. The onset of the interaction, or critical aggregation concentration ( cac ) of the detergent, is only slightly dependent on PEG concentration. The detergent concentration at which the polymer saturates with SDS increases in approximately linear fashion with [ PEG ] . The interaction is independent of PEG molecular weight ( for MW ú 4000 ) , is somewhat enhanced by the addition of salt, and decreases with increasing temperature ( 2 – 5 ) . Specific counterion effects on the SDS – PEG interaction have also been investigated ( 6, 7 ) . Although several anionic surfactants interact with PEG, other types of detergents ( cationic, non-ionic, or zwitterionic ) appear to have little or no affinity for binding to PEG ( 3 – 5 ) . 1
To whom correspondence should be addressed.
The PEG–SDS interaction results in the formation of small aggregates or clusters of SDS attached to the PEG polymer chain. These SDS–PEG clusters are smaller than normal SDS micelles, are more highly dissociated, and appear to have a somewhat more ‘‘open’’ structure (2–5, 7, 8). The PEG polymer chains are reported to be located at the surface of the SDS clusters, but estimates of the fraction of the PEG segments bound to the micelles vary rather widely (4, 5). Although a number of theoretical models have been proposed to explain the onset of the interaction (cac), the saturation, and the aggregate size, none provides a completely satisfactory description of all of the aspects of the PEG–SDS interaction (3–5). Photophysical techniques (9, 10) have played a prominent role in studies of counterion binding to micelles. Thus, for example, selectivity coefficients for monovalent–monovalent, monovalent–divalent, and divalent–divalent counterion exchange have been determined from steady-state (11– 15) and time-resolved (16) measurements of the quenching of the fluorescence of micelle-solubilized aromatic hydrocarbons by counterions. In the present work, we have employed two photophysical probe techniques to determine counterion exchange selectivities at the surface of SDS–PEG clusters. The selectivities are similar, but not identical, to those at the surface of SDS micelles in the absence of PEG. EXPERIMENTAL
Materials. PEG of nominal molecular weight 10,000 (ATPEG-10000; Atlas-Ultra Divisa˜o QuıB mica) was employed throughout. Sodium dodecyl sulfate (SDS; Merck, for biochemical use) was purified by recrystallization from absolute ethanol. Purity was checked by surface tensiometric determination of the critical micelle concentration (cmc) in the usual manner. N-Methyl-4-cyanopyridinium (MCP / ) chloride (17) and the sodium salt of the luminescence probe (18) tris(4,4 *-dicarboxybipyridine)ruthenium(II) (RuL 40 3 ) were prepared and purified by literature procedures. Pyrene (Aldrich) was purified as described previously (11). Methyl viologen (MV 2/ ) chloride and analytical reagent grade so-
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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dium chloride (Merck, p.a.), sodium sulfate (Merck, p.a.), thallium(I) sulfate (BDH AnalaR), and copper sulfate pentahydrate (Reagen S.A., p.a.) were used as received. Methods. All measurements were performed at 307C. All solutions were prepared in distilled, freshly deionized water and PEG stock solutions were stirred gently at 607C for at least 1 h to eliminate interpolymeric entanglement (19). Apparent surface tensions were determined with a du Nou¨y tensiometer equipped with a Pt ring. Steady-state emission measurements were performed with either a Perkin–Elmer LS-5B or a Hitachi F4500 fluorescence spectrometer. Pyrene fluorescence decay curves were collected by the single-photon counting technique using an Edinburgh Analytical Instruments Model FL-900 Lifetime Spectrometer (H2 flashlamp gas, 337 nm excitation, 390 nm emission). Experiments with RuL 40 as probe. Aliquots (2.5 mL) 3 05 M), SDS of aqueous solutions containing RuL 40 3 (3 1 10 (0.010–0.060 M), NaCl (0.020–0.100 M), and PEG (0, 0.20, 0.40, or 4.00% PEG wt/vol) were thermostated in Teflon-stoppered 1 cm path length fluorescence cuvettes (Hellma). Successive aliquots of a concentrated (0.30 M) stock solution of MCP / or MV 2/ were added to the sample cuvette via a calibrated Hamilton microsyringe, recording the emission intensity (465 nm excitation/625 nm emission) following each addition. Experiments with pyrene as probe. Pyrene (5 1 10 06 M) was added to air-equilibrated aqueous solutions of SDS (0.040 M, freshly prepared) and PEG (4 wt %) containing Na2SO4 (0.025, 0.050, 0.075, or 0.100 M). Aliquots of concentrated aqueous stock solutions of the quenchers (Tl / or Cu 2/ ) were added with the aid of a Hamilton microliter syringe, and the probe fluorescence decay was determined. Fluorescence decays in the absence of quencher were analyzed utilizing the standard single exponential decay routines of the FL-900 operating software. The corresponding decay curves in the presence of quencher were fit to the Infelta– Tachiya equation (20) F(t) Å A1 exp[ 0 A2t 0 A3 {1 0 exp( 0 A4t)}]
[1]
using the micelle quenching module of Edinburgh Analytical Instruments Level 2 analysis software (Version 1.60). RESULTS AND DISCUSSION
The binding of counterions to SDS aggregates can be treated in terms of competitive ion exchange (21) between the foreign counterion X (of charge z X ) and the detergent counterion Na / : K X / Na
Xaq / ÉzX ÉNamic B Xmic / ÉzX ÉNaaq .
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The corresponding counterion exchange selectivity coefficient, KX / Na , is defined as KX / Na Å ( uX /[Xaq ]) 1 /Éz X É ([Naaq ]/ uNa ),
[3]
where the subscript aq refers to the analytical concentration of counterions free in the intermicellar aqueous medium and uX and uNa are the fractional coverages of the aggregate surface by X and Na / , respectively. The micellar fractional coverages and the aqueous counterion concentrations are related to each other by the classical pseudophase ion-exchange mass balance relationships (12, 21, 22): uX Å ÉzX É([Xmic ]/[SDSag ]) Å ÉzX É([XT ] 0 [Xaq ])/[SDSag ]
uNa Å (1 0 a ) 0 uX
[Naaq ] Å a[SDSag ] / cac / [Naad ] / [SDSag ] uX .
[4] [5] [6]
In these equations, [SDST ], [XT ], and [Naad ] are the total concentrations of detergent and of added foreign and common counterion salt, cac is the critical aggregation concentration (cmc in the absence of PEG or cac in the presence of PEG), a is the degree of counterion dissociation of the aggregate, and [SDSag ] Å [SDST ] 0 cac is the concentration of aggregated SDS. Experimental determination of either [Xaq ] or uX at known total concentrations of detergent and added salt allows one to calculate the value of KX / Na from Eqs. [3] – [6]. In the present work, quenching of the luminescence of the tris(dicarboxybipyridine)ruthenium(II) anion (RuL 40 3 ) by the monovalent N-methyl-4-cyanopyridinium cation (MCP / ) and the divalent methylviologen cation (MV 2/ ) was employed to determine selectivity coefficients for exchange of these two organic counterions at the surface of SDS micelles and of SDS–PEG aggregates. The basic assumption of this method, which has been described in detail previously (18), is that the tetraanionic probe RuL 40 3 is completely excluded from the negatively charged SDS micelle or SDS–PEG aggregate. As a consequence, the efficiency of quenching of the RuL 40 3 luminescence by a positively charged counterion depends only on the concentration of the quencher counterion in the aqueous phase, [Xaq ]. Furthermore, since the intermicellar aqueous phase is a homogeneous, macroscopic phase, the quenching obeys the simple Stern–Volmer equation: I 7 /I Å 1 / KSV [Xaq ].
[7]
In order to estimate [Xaq ] from the ratio of RuL 40 3 luminescence intensities in the absence and presence of quencher (I 7 /I), the value of KSV, must be known or estimated independently. Since the probe is a tetraanion and the quencher
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TABLE 1 Selectivity Coefficients for MCP//Na/ and MV2//Na/ Counterion Exchange at the Surface of SDS–PEG Aggregates Determined from Quenching of the Luminescence of RuL40 3
[NaCl] (mM)
[SDS] (mM)
20 50 80 100
10–20 10–30 10–30 10–30
KMCP//Na/
KMV2//Na/
% PEG
% PEG
0.2%
0.4%
6.5 { 1
8.8 { 0.5
5.5 { 1
9.0 { 0.5
4% 8.4 8.7 8.7 8.3
counterions are positively charged, the magnitude of the quenching rate constant, and hence KSV, is highly dependent on the ionic strength, m, of the intermicellar aqueous phase. Values of KSV determined as a function of added NaCl in the absence of SDS are adequately described by the conventional extended Debye–Hu¨ckel relationship, log KSV Å log K 7SV 0 4.4zX m1 / 2 /(1 / 2.9m1 / 2 ),
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0.6 0.9 1 1
0.4%
5.3 { 0.5
5.1 { 0.5
4.6 { 0.5
4.6 { 0.5
4% 3.5 3.5 3.6 3.6
{ { { {
0.1 0.1 0.2 0.1
that the quenching of the fluorescence of a micelle-bound probe such as pyrene is sensitive to the local concentration of quencher counterion at the micelle surface (11, 20). As we have recently shown (16), analysis of the fluorescence decay of pyrene in micellar SDS permits direct determination of uX from the coefficients of the Tachiya–Infelta equation (Eq. [1]) via the relationship
[8]
with Stern–Volmer constants at zero ionic strength, K 7SV , of 14,000 M 01 and 39,800 M 01 , respectively, for the Nmethyl-4-cyanopyridinium (MCP / ) and methylviologen (MV 2/ ) cations. In the presence of SDS micelles, the effective ionic strength in the intermicellar aqueous phase is given by the pseudophase ion exchange mass balance expression: m Å [Naaq ] / (z X ) 2[Xaq ]. The values of m and K 7SV are employed in Eq. [8] to calculate KSV, which is in turn used in Eq. [7] to calculate [Xaq ] from the experimental ratio of RuL 40 3 luminescence intensities in the absence and presence of quencher (I 7 /I). Application of this method to MCP / /Na / and MV 2/ / Na / exchange (18) at the surface of SDS micelles in the absence of PEG gave values of 7 { 1 for both ions. The corresponding selectivity coefficients for exchange of these ions at the surface of SDS–PEG aggregates are collected in Table 1. The values employed for a were 0.25 for SDS micelles (23) and 0.56 for SDS–PEG aggregates; reported a values for SDS–PEG aggregates are 0.54 at the cac (8) and 0.65 (24) and 0.56 (25) above the cac. Critical aggregation concentrations (cac) for the onset of the interaction of SDS with PEG are insensitive to the PEG concentration (0.2 and 0.4% PEG) and were estimated from the relationship (8): log cac Å 03.47 0 0.46 log(cac / [Naad ]). In all experiments with PEG, care was taken to maintain the concentration of SDS above the cac and below that at which saturation of the polymer by SDS occurs (19). The second method employed to determine counterion exchange selectivity coefficients takes advantage of the fact
{ { { {
0.2%
uX Å (ÉzX É/Nag )(A2 0 1/ t7 / A3 A4 ) 2 /[A3 (A4 ) 2 ],
[9]
where t7 is the pyrene fluorescence lifetime measured in the absence of the counterionic quencher and Nag is the aggregation number of the SDS micelles or clusters. Aggregation numbers of SDS–PEG clusters, determined by the technique of Turro and Yekta (26), have been reported elsewhere (8); the values for micellar SDS were calculated from the relationship (23) log Nag Å 2.23 / 0.23 log[Naaq ]. Selectivity coefficients determined in this manner for MV 2/ /Na / , Tl / /Na / , and Cu 2/ /Na / exchange at the surface of SDS– PEG clusters (0.040 M SDS and 4% PEG) and for Tl / / Na / and Cu 2/ /Na / exchange at the surface of micellar SDS are collected in Table 2. TABLE 2 Selectivity Coefficients for MV2//Na/, Tl//Na/, and Cu2//N/ Counterion Exchange at the Surface of SDS–PEG Aggregates (0.040 M SDS; 4% PEG) and SDS Micelles (0.040 M SDS) Determined from Analyses of the Fluorescence Decay of Pyrene in Solutions Containing Added Na2SO4 (0.025, 0.050, 0.075, or 0.10 M) KX/Na
SDS–PEG aggregates
KMV2//Na/ KTl//Na/ KCu2//Na/
5 {2 6.7 { 1 2.1 { 0.4
a
SDS micelles
5{1 1.3 { 0.3a (1.2 { 0.1)b
Reference 16. Determined from Cu2/ quenching of the luminenscence of intermicellar RuL40 3 . b
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The basic assumptions implicit in the use of Eq. [9] are that the micelles or aggregates are not highly polydisperse and that the statistical distribution of the quenchers among the available micelles obeys a Poisson distribution (20). In all cases in the present work, added common counterion salt was present and the number of bound quencher counterions was restricted to less than one per micelle or aggregate. Under these conditions, the statistical distribution of the bound quencher can be safely approximated by a Poisson distribution and any effects of the small amount of added quencher counterion on a, the cmc or cac, and the aggregation number of the micelles or clusters should be negligible. In SDS micelles, pyrene and MV 2/ have been shown to form a ground state charge-transfer complex (27) and analogous ground state complexation also occurs in SDS–PEG clusters. However, as shown by Gehlen and De Schryver (27), the dynamic part of the quenching of pyrene fluorescence by MV 2/ obeys the Tachiya–Infelta equation (Eq. [1]), implying that Eq. [9] is still valid for this system. A final assumption of the method in the case of SDS–PEG clusters is that the quenching is confined to the clusters, with no contribution from pyrene emission and/or quencher cations bound to PEG chain segments located outside of the clusters. The possibility of weak interactions between pyrene and PEG in aqueous solution is suggested by the observation of an intramolecular interaction of this type in pyrene endlabeled PEGs (28); however, upon addition of SDS, the pyrene-labeled end groups selectively solubilize in the SDS– PEG clusters (28). Similarly, in the present system, the existence of a weak pyrene–PEG interaction in the absence of SDS is indicated by the increase in the pyrene fluorescence lifetime from 134 ns in water to 142 ns in the presence of 0.4% PEG and to 170 ns in 4% PEG. In the SDS–PEG clusters, however, pyrene exhibited a single exponential lifetime of 203 ns in all cases, indicative of exclusive solubilization of pyrene in the clusters. Although binding of cations to PEG can be significant in solvents such as methanol (29), hydrophilic cations typically bind only weakly, if at all, to PEG in aqueous solution in the absence of hydrophobic anions (30). The possibility of binding of Cu 2/ to PEG in the absence of SDS was, nonetheless, examined experimentally by determining the second order rate constants (kq ) for quenching of pyrene by Cu 2/ from pyrene fluorescence lifetime data in the presence and absence of Cu 2/ . The lack of appreciable binding of Cu 2/ is indicated by the observation that kq decreased from 5.6 1 10 9 M 01 s 01 in water to 3.8 1 10 9 M 01 s 01 in aqueous 0.4% PEG to 2.4 1 10 9 M 01 s 01 in 4% PEG. As mentioned in the Introduction, SDS–PEG clusters are known to be smaller than SDS micelles formed in the absence of PEG and to have a much higher degree of counterion dissociation and a more ‘‘open’’ structure (2–5, 7). Differences have also been reported in the intracluster mobility and solubility of molecules in the SDS–PEG clusters
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(8). Despite these differences in structure and, presumably, surface charge density between SDS micelles and SDS– PEG aggregates, the exchange selectivities are found to be comparable for the two monovalent counterions (MCP / and Tl / ). Indeed, for a given ionic detergent headgroup type, monovalent counterion exchange selectivity is usually found to be rather insensitive to aggregate size, surface curvature (32, 33) or moderate changes in aggregate surface charge density (15). The divalent counterion MV 2/ exhibits a somewhat lower selectivity and Cu 2/ a distinctly higher selectivity at the surface of the SDS–PEG aggregates. The selectivity coefficients for MCP / /Na / and MV 2/ /Na / exchange exhibit modest dependences on the PEG concentration, though no consistent trend is apparent. Recent studies indicate that 1H NMR spectroscopic data is the method of choice for obtaining more detailed information on the factors contributing to the binding and exchange of such organic counterions at charged interfaces (31). One of the most attractive proposals for the origin of ion exchange selectivity between inorganic ions is the partial dehydration hypothesis of Morgan et al. (33). This model attributes differences in selectivity at different surfaces to differences in the extent of partial dehydration of the two counterions. It is not unreasonable to expect that the presence of poly(oxyethylene) subunits at the surface of the SDS–PEG clusters might influence the extent of counterion dehydration relative to that at the SDS micellar surface. Interestingly, Cu 2/ is one of the few ions for which partial dehydration cannot adequately account for the observed selectivity trends (33), suggesting that additional factors are responsible for the enhanced Cu 2/ / Na / selectivity at the surface of the SDS–PEG clusters. ACKNOWLEDGMENTS This work was supported by grants from the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP; Projects 91/0480-1 and 1994/ 3505-3) and PADCT-FINEP (Project 65-92-0063-00). P.M.N. acknowledges fellowship support from FAPESP; L.T.O. acknowledges fellowship support from CAPES (Coordenac¸a˜o de Aperfeicoamento de Pessoal de NıB vel Superior).
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