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Ion exchange between divalent counterions in anionic micellar solution
Journal of Luminescence 82 (1999) 315}319
Ion exchange between divalent counterions in anionic micellar solution Joa o Baptista Sargi Bonilha, Antonio Claudio Tedesco*, Daniela M.A.F. Navarro, PatrmH cia Maria Nassar Departamento de Qun& mica da Faculdade de Filosoxa, CieL ncias e Letras de RibeiraJ o Preto, Universidade de SaJ o Paulo, 14040-901, RibeiraJ o Preto, Brazil Received 7 January 1998; received in revised form 4 May 1999; accepted 4 May 1999
Abstract The ion exchange between divalent cations in anionic micellar systems were investigated by a #uorescence quenching technique using the anionic ruthenium complex RuL\ (L"4, 4 -dicarboxy-2, 2 -bipyridine) as an extramicellar probe and the N, N -dimethyl-4, 4 -bipyridinium ion (MV>) as the quencher. Competitive ion exchange at the micellar surface between the divalent quencher counterion (MV>) and the divalent hexamethylethylenediammonium (HED>) counterion can be adequately described with a simple pseudophase ion-exchange formalism. The counterion exchange selectivity coe$cients are K+4 &#""1.3 and 1.4 for HED-decyl sulfate [HED(DeS)] and HED-dodecylsulfate [HED(DS)] micelles, respectively. These results show that the MV> cation only slightly more strongly bound to the micelle aggregates than the HED> cation. 1999 Elsevier Science B.V. All rights reserved. Keywords: Ion exchange; Fluorescence; Hexamethylethylenediammonium dodecyl sulfate; N, N -dimethyl-4, 4 -bipyridinium dichloride
1. Introduction The properties of micelles formed by surfactants like sodium dodecyl sulfate (SDS) have been studied for many years and the in#uence of organic and inorganic additives has been reported [1}3]. The presence of organic and inorganic cations can a!ect the composition of the micellar surface, due to the competitive ion exchange that occurs between these ions and the micellar counterions [4].
* Corresponding author. Fax: #55-16-6338151. E-mail address: [email protected] (A. Claudio Tedesco)
Several groups have studied the competitive binding of counterions to micellar interfaces in the presence of two (or more) counterions [5}12]. All of these works consider the micelle to be a distinct phase of the solution and describe the ion exchange process by an equilibrium of the type:
Z V W Z W X #> )8 W X #> . Z Z V V
(1)
K is the counterion exchange selectivity coe$cV W ient, de"ned as
8W 8V [> ] h V , K " V W [X ] h W
0022-2313/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 0 5 0 - 2
(2)
316
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
where the subscript `fa refers to the analytical concentration of the counterions free in the intermicellar aqueous medium and h and h are the fraction V W of the micellar surface charge compensated by the bound counterions. In previous works the selectivity coe$cients for monovalent}monovalent and monovalent} divalent counterion exchange were analyzed employing photophysical techniques [13}16]. Counterion exchange coe$cients for sodium dodecyl sulfate micelles were obtained from steady-state measurements, using cationic quenchers like Cu>, N, N -dimethyl-4, 4 -bipyridinium (MV>), and N-methyl-4-cyanopyridinium (MCP>). Fluorescence quenching data for the extramicellar anionic ruthenium complex, RuL\ (L"4, 4 -dicarboxy-2, 2 -bipyridine) were treated employing the pseudophase ion-exchange formalism. The values obtained for the selectivity coe$cients point to an in#uence of the hydrophobic character of the quencher ions present in anionic micellar solutions. This is the case, for example, for the selectivity coe$cients determined for MV>/Na> (K "7.0$1.0) and for Cu>/Na> V , (K "1.2$0.1) counterion exchange at the surV , face of SDS micelles [15]. In the present paper, we describe an extension of these studies to anionic surfactants (decyl- and dodecyl- sulfates) that have a divalent hydrophobic counterion, HED> (N, N, N, N , N , N -hexamethylethylenediammonium), rather than Na>. We have used RuL\ and MV> as the probe and quencher, respectively, to investigate the divalent} divalent MV>/HED> ion-exchange process at the surface of HED-decylsulfate [HED(DeS) ] and HED-dodecylsulfate [HED(DS) ] micelles. 2. Experimental section 2.1. Materials Sodium dodecylsulfate, SDS (Aldrich), was puri"ed by recrystallization from ethanol and purity checked by surface tensiometric determination of the critical micelle concentration (cmc). N, N Dimethyl-4, 4 -bipyridinium dichloride (methyl viologen, MVCl ) was purchased from Sigma, and
the sodium salt of tris-(4, 4 -dicarboxy-2, 2 -bipyridine) ruthenium (II), RuL\, was synthesized as previously described [14]. All solutions were prepared in ultrapure water from a Millipore system and all other solvents were analytical grade. 2.1.1. Preparation of hexamethylethylenediammonium iodide (HEDI2) A mixture of 18 ml (120 mmol) of N, N, N , N tetramethylethylenediamine (BDH) and 15 ml of methyl iodide dissolved in 70 ml of dry ethanol was stirred for 16 h at room temperature. Recrystallization of the resultant white solid from ethanol (active charcoal) provide 42.6 g (80% yield) of hexamethylethylenediammonium iodide. The purity was veri"ed by C, H, N microanalysis (C H N I . 1/2H O; found: C, 23.42; H, 5.64; N, 6.52; cal.: C, 23.37; H, 5.66; N, 6.70%), by iodide ion titration [17], which provide an apparent molecular weight of 409 g/mol, and by H NMR (80 MHz, D O): d 4.10 (s, 4 H), d 3.35 (s, 18 H). 2.1.2. Preparation of hexamethylethylenediamonium alkyl sulfates [HED(DS)2 and HED(DeS)2] Decyl and dodecylsulfate salts of HED> were precipitated by metathesis from the corresponding solutions of sodium alkyl sulfate and hexamethylethylenediammonium iodide. In each case, approximately 72 g of sodium alkyl sulfate were dissolved in 500 ml of 95% ethyl alcohol. Then 50 g of hexamethylethylenediammonium iodide was added and the resulting mixture stirred for 2 h at room temperature. The surfactants were recrystallized twice from ethanol, subjected to Soxhlet extraction with anhydrous acetone for 12 h, washed with ca. 1 l of chilled distilled water to remove all traces of iodide ion and dried in vacuum over P O . The absence of residual iodide ion in the detergent was veri"ed by the lack of precipitation with aqueous silver nitrate solution and by negative Lassaigne test [18]. Elemental analyses were satisfactory for both detergents: C H N S O . H O found: C, 55.72; H, 10.50; N, 4.09; calc. C, 55.99; H, 10.50; N, 4.08%; and C H N S O found: C, 54.02; H, 10.35; N, 4.47, calc. C, 54.19; H, 10.32; N, 4.52%.
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
2.2. Methods The cmc values for HED(DeS) and HED(DS) were obtained by surface tension and speci"c conductivity techniques, employing a Thomas model 70535 Du NouK y Surface Tensiometer, equipped with a Pt ring, and an Analion C-701 conductimeter, respectively. Successive aliquots of a concentrated stock solution of HED(DeS) or HED(DS) prepared in water with HEDI (0}0.035 M) were added to a thermostatted cell containing water or an aqueous solution of HEDI . Critical micelle concentrations (cmc) were determined from plots of apparent surface tension versus log [surfactant], or from the speci"c conductance as a function of the surfactant concentration, at 453C, 533C and 633C. The #uorescence quenching experiments with RuL\ as probe were performed in the absence and in the presence of micelles, using MV> as quencher. The solutions, contained in thermostatted #uorescence cuvettes, were prepared with RuL\ (3;10\ M), HEDI (0.0}0.035 M), and HED(DeS) or HED(DS) (0.01}0.035 M). Aliquots of a concentrated stock solution of the quencher (0.4 M) were added directly to the sample cuvette via a calibrated Hamilton microsyringe, and the emission intensities were registered after each addition. Fluorescence measurements were performed at 453C, 533C and 633C on a PerkinElmer LS-5B spectro#uorometer, equipped with a thermostatted sample holder.
3. Results and discussion Critical micelle concentrations (cmc) of HED(DeS) and HED(DS) obtained by surface tension and speci"c conductivity are listed in Table 1. In surfactant systems with multivalent ions the Kraft point is usually increased and the surfactants often precipitate at room temperature. An increase in the number of the carbon atoms in the alkyl chain also leads to a reduction in the solubility of these compounds. Consequently, the temperatures used to determine cmc values were: 453C, 533C and 633C for HED(DeS) , and 533C and 633C for HED(DS) . The cmc values found for these
317
Table 1 Values of cmc and a determined by surface tension and conductivity for HED alkyl sulfate detergents. Surfactants
[HED>] mM
¹ (3C)
cmc (mM)
HED(DeS)
0.0
25 25 25
45 45 53 45 53 63
6.48 7.20 6.60 3.00 3.30 3.40
(c) (C) (C) (C) (C) (C)
0.0
53
25 25
53 63
0.28 0.23 0.10 0.11
(c) (C) (C) (C)
HED(DS)
a
0.11 0.11
0.10 0.10
(c) " measured by surface tension; (C) " conductivity.
surfactants are lower than those of the same surfactants with Na> as counterion. For both surfactants, the dependence of the cmc on temperature is relatively small. Increasing the chain length, from decyl to dodecyl, decreases the cmc, as does the addition of HEDI . The values of a may be estimated from the ratio of slopes of the linear regions of plots of speci"c conductance as a function of surfactant concentration, above and below the critical micelle concentration [19], as well as from Corrin}Harkins plots [20]. The a values found by these two methods are similar (Table 1) and lower than that of 0.25 found for SDS [21], due to the in#uence of the divalent organic counterion on the properties of the micelles. As shown in the previous studies [14], the quenching of the luminescent probe RuL\ by MV> obeys the Stern}Volmer relationship in homogeneous solution: I3/I"1#K [MV>],
(3)
where I3 and I are the #uorescence intensities in the absence and presence of the quencher, respectively, and K is the Stern}Volmer quenching 14 constant. The quenching reaction is an electrontransfer process from the excited ruthenium complex to the quencher, forming the viologen cation radical [22].
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The variation of the Stern}Volmer quenching constant with salt concentration follows the extended Debye}HuK ckel relationship [23]:
where h and h are the fractional coverages &#" +4 of the aggregate surface by HED> and MV> given by
log K "log K3 !16 A (k) /(1#5.8k ), (4) 14 14 where k is the ionic strength, A is a constant (A"0.525 at 453C; 0.531 at 533C and 0.538 at 633C) and K3 is the Stern}Volmer constant at zero 14 ionic strength. The experimental variation of K with ionic strength in aqueous solution was 14 employed to determine the K3 values: 45 700 M\ 14 at 453C, 51 600 M\ at 533C and 65 600 M\ at 633C. The basic assumption of the method employed [13}16] is that the tetraanionic probe RuL\ is completely excluded from the negatively charged micelle, while the viologen cation partitions between the micellar and aqueous phases. Thus upon addition of anionic micelles, the concentration of the quencher in the aqueous phase decreases due to binding to the micellar surface, attenuating the quenching e$ciency. Considering that the quenching e$ciency is dependent on the concentration of free viologen cation in the aqueous phase, [MV>] , and that the quenching of the lumines cence of RuL\H by MV> follows normal Stern}Volmer kinetics, the equation describing the quenching is then
h "[HED>] /C , (8) &#" " h "[MV>] /C , (9) +4 " h3 "h #h "(1!a). (10) &#" &#" +4 The micellar fractional coverages and the bound and free micelle counterion concentrations are related by the pseudophase ion exchange mass balance relationships:
I3/I"1#K [MV>] . (5) 14 The total concentration of added quencher, [MV>] , is related to the [MV>] and the con centration of micelle-bound cation, [MV>] , by the mass-balance relationship:
The K values obtained from the experi+4 &#" mental data are 1.3 for HED(DeS) (10}35 mM detergent; 45}633C) and 1.4 for HED(DS) (5}35 mM detergent; 53}633C). In Fig. 1, representative Stern}Volmer plots for the quenching of RuL\ by MV> are presented; the curves corres pond to the predictions of the model (Eqs. (4)}(12)) for MV>/HED> ion exchange in HED(DS) micelles. Changes in the alkyl chain length have no signi"cant e!ect on K , consistent with pre+4 &#" vious "ndings that the ion exchange process at the micellar interface is not in#uenced by the micelle size [12]. The fact that the experimental values of K are only slightly longer than unity points +4 &#" to a similar a$nity of the alkylsulfate micelles surface for these two divalent organic cations, despite the large di!erence in chemical structure and charge distribution in the MV> and HED> ions.
[MV>] "[MV>] #[MV>] . (6) The appropriate value of K in the presence of 14 anionic micelles is calculated from the ionic strength dependence of K (Eq. (4)), using the 14 K3 values determined in homogeneous solution 14 and the e!ective intermicellar ionic strength (k , see below). This value of K is used to calculate 14 [MV>] iteratively from the experimental I3/I data. The corresponding counterion exchange selectivity coe$cient (K ) is then calculated from +4 &#" the relationship: K "(h [HED>] )/([MV>] h ), +4 &#" +4 &#"
(7)
(11) [HED>] "(1!a)C !h C , " +4 " [HED>] "aC #cmc#[HEDI ] #h C . " +4 " (12) In these equations, [HEDI ] is the added com mon counterion salt and C is the concentration of " aggregated HED(DeS) or HED(DS) , given by: C "C !cmc. " 2 As discussed previously [6,7,12}16], the e!ective ionic strength in the aqueous phase is determined by the ionic concentrations at the midpoint between micelles, which, in the present case, leads to k "2[HED] #2[MV>] #cmc#[HEDI ] . (13)
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
Fig. 1. Stern}Volmer plots of the quenching of RuL\ emission by MV> in HED(DS) micellar solutions in the presence of HEDI (25 mM) at 633C. Surfactant concentrations: (䊏) 5 mM, (䉱) 10 mM, (䢇) 25 mM and (䉬) 35 mM.
Acknowledgements This work was supported by grants from the Fundac7 a o de Amparo a` Pesquisa do Estado de Sa o Paulo, FAPESP (projects 91/0480-1 and 94/3505-3) and PADCT-FINEP (Project no. 65-92-0063-00). D.M.A.F. Navarro and P.M. Nassar acknowledge fellowship support from FAPESP. References [1] B. Lindman, H. Wennerstrom, Top. Curr. Chem. 87 (1980) 32. [2] C. Tanford, The Hydrophobic E!ect: Formation of Micelles and Biological Membranes, 2nd Edition, Wiley, New York, 1980.
319
[3] S. Tascioglu, Tetrahedron 52 (1996) 11 113. [4] C. Bunton, F. Nome, F.H. Quina, L.S. Romsted, Acc. Chem. Res. 24 (1991) 357. [5] F.H. Quina, H. Chaimovich, J. Phys. Chem. 83 (1979) 1844. [6] H. Chaimovich, J.B.S. Bonilha, M.J. Politi, F.H. Quina, J. Phys. Chem. 83 (1979) 1851. [7] F.H. Quina, M.J. Politi, I.M. Cuccovia, E. Baumgarten, S.M. Martins-Franchetti, H. Chaimovich, J. Phys. Chem. 84 (1980) 361. [8] D. Bartet, C. Gamboa, L. Sepulveda, J. Phys. Chem. 84 (1980) 272. [9] C. Gamboa, L. Sepulveda, R. Soto, J. Phys. Chem. 85 (1981) 1429. [10] E. Abuin, E.A. Lissi, J. Colloid Interf. Sci. 93 (1983) 562. [11] E. Abuin, E.A. Lissi, N. Bianchi, L. Miola, F.H. Quina, J. Phys. Chem. 87 (1983) 5166. [12] J.B.S. Bonilha, G. Chiericato, S.M. Martins-Franchetti, E.J. Ribaldo, F.H. Quina, J. Phys. Chem. 86 (1982) 4941. [13] J.B.S. Bonilha, T.K. Foreman, D.G. Whitten, J. Am. Chem. Soc. 104 (1982) 4215. [14] J.B.S. Bonilha, R.M.Z. Georgetto, A.C. Tedesco, L. Miola, D.G. Whitten, J. Phys. Chem. 93 (1989) 367. [15] P.M. Nassar, R.M.Z.G. Naal, S.H. Pauli, J.B.S. Bonilha, L.T. Okano, F.H. Quina, J. Colloid Interf. Sci. 190 (1997) 461. [16] J.B.S. Bonilha, R.M.Z. Georgetto, E. Abuin, E.A. Lissi, F.H. Quina, J. Colloid Interf. Sci. 135 (1990) 238. [17] O. Schales, S.S. Schales, J. Biol. Chem. 140 (1941) 879. [18] A.I. Vogel, Textbook of Practical Organic Chemistry, 5th Edition, Longman Scienti"c and Technical, New York, 1989. [19] J.E. Brady, D.F. Evans, G.G. Warr, F. Grieser, B.W. Ninham, J. Phys. Chem. 90 (1986) 1853. [20] M.L. Corrin, W.D. Harkins, J. Am. Chem. Soc. 69 (1947) 684. [21] R. Zana, J. Colloid Interf. Sci. 78 (1980) 330. [22] T. Miyashita, T. Murakata, Y. Yamaguchi, M. Matsuda, J. Phys. Chem. 89 (1985) 497. [23] J.N. Demas, J.W. Addington, J. Am. Chem. Soc. 98 (1976) 5800.
Ion exchange between divalent counterions in anionic micellar solution Joa o Baptista Sargi Bonilha, Antonio Claudio Tedesco*, Daniela M.A.F. Navarro, PatrmH cia Maria Nassar Departamento de Qun& mica da Faculdade de Filosoxa, CieL ncias e Letras de RibeiraJ o Preto, Universidade de SaJ o Paulo, 14040-901, RibeiraJ o Preto, Brazil Received 7 January 1998; received in revised form 4 May 1999; accepted 4 May 1999
Abstract The ion exchange between divalent cations in anionic micellar systems were investigated by a #uorescence quenching technique using the anionic ruthenium complex RuL\ (L"4, 4 -dicarboxy-2, 2 -bipyridine) as an extramicellar probe and the N, N -dimethyl-4, 4 -bipyridinium ion (MV>) as the quencher. Competitive ion exchange at the micellar surface between the divalent quencher counterion (MV>) and the divalent hexamethylethylenediammonium (HED>) counterion can be adequately described with a simple pseudophase ion-exchange formalism. The counterion exchange selectivity coe$cients are K+4 &#""1.3 and 1.4 for HED-decyl sulfate [HED(DeS)] and HED-dodecylsulfate [HED(DS)] micelles, respectively. These results show that the MV> cation only slightly more strongly bound to the micelle aggregates than the HED> cation. 1999 Elsevier Science B.V. All rights reserved. Keywords: Ion exchange; Fluorescence; Hexamethylethylenediammonium dodecyl sulfate; N, N -dimethyl-4, 4 -bipyridinium dichloride
1. Introduction The properties of micelles formed by surfactants like sodium dodecyl sulfate (SDS) have been studied for many years and the in#uence of organic and inorganic additives has been reported [1}3]. The presence of organic and inorganic cations can a!ect the composition of the micellar surface, due to the competitive ion exchange that occurs between these ions and the micellar counterions [4].
* Corresponding author. Fax: #55-16-6338151. E-mail address: [email protected] (A. Claudio Tedesco)
Several groups have studied the competitive binding of counterions to micellar interfaces in the presence of two (or more) counterions [5}12]. All of these works consider the micelle to be a distinct phase of the solution and describe the ion exchange process by an equilibrium of the type:
Z V W Z W X #> )8 W X #> . Z Z V V
(1)
K is the counterion exchange selectivity coe$cV W ient, de"ned as
8W 8V [> ] h V , K " V W [X ] h W
0022-2313/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 0 5 0 - 2
(2)
316
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
where the subscript `fa refers to the analytical concentration of the counterions free in the intermicellar aqueous medium and h and h are the fraction V W of the micellar surface charge compensated by the bound counterions. In previous works the selectivity coe$cients for monovalent}monovalent and monovalent} divalent counterion exchange were analyzed employing photophysical techniques [13}16]. Counterion exchange coe$cients for sodium dodecyl sulfate micelles were obtained from steady-state measurements, using cationic quenchers like Cu>, N, N -dimethyl-4, 4 -bipyridinium (MV>), and N-methyl-4-cyanopyridinium (MCP>). Fluorescence quenching data for the extramicellar anionic ruthenium complex, RuL\ (L"4, 4 -dicarboxy-2, 2 -bipyridine) were treated employing the pseudophase ion-exchange formalism. The values obtained for the selectivity coe$cients point to an in#uence of the hydrophobic character of the quencher ions present in anionic micellar solutions. This is the case, for example, for the selectivity coe$cients determined for MV>/Na> (K "7.0$1.0) and for Cu>/Na> V , (K "1.2$0.1) counterion exchange at the surV , face of SDS micelles [15]. In the present paper, we describe an extension of these studies to anionic surfactants (decyl- and dodecyl- sulfates) that have a divalent hydrophobic counterion, HED> (N, N, N, N , N , N -hexamethylethylenediammonium), rather than Na>. We have used RuL\ and MV> as the probe and quencher, respectively, to investigate the divalent} divalent MV>/HED> ion-exchange process at the surface of HED-decylsulfate [HED(DeS) ] and HED-dodecylsulfate [HED(DS) ] micelles. 2. Experimental section 2.1. Materials Sodium dodecylsulfate, SDS (Aldrich), was puri"ed by recrystallization from ethanol and purity checked by surface tensiometric determination of the critical micelle concentration (cmc). N, N Dimethyl-4, 4 -bipyridinium dichloride (methyl viologen, MVCl ) was purchased from Sigma, and
the sodium salt of tris-(4, 4 -dicarboxy-2, 2 -bipyridine) ruthenium (II), RuL\, was synthesized as previously described [14]. All solutions were prepared in ultrapure water from a Millipore system and all other solvents were analytical grade. 2.1.1. Preparation of hexamethylethylenediammonium iodide (HEDI2) A mixture of 18 ml (120 mmol) of N, N, N , N tetramethylethylenediamine (BDH) and 15 ml of methyl iodide dissolved in 70 ml of dry ethanol was stirred for 16 h at room temperature. Recrystallization of the resultant white solid from ethanol (active charcoal) provide 42.6 g (80% yield) of hexamethylethylenediammonium iodide. The purity was veri"ed by C, H, N microanalysis (C H N I . 1/2H O; found: C, 23.42; H, 5.64; N, 6.52; cal.: C, 23.37; H, 5.66; N, 6.70%), by iodide ion titration [17], which provide an apparent molecular weight of 409 g/mol, and by H NMR (80 MHz, D O): d 4.10 (s, 4 H), d 3.35 (s, 18 H). 2.1.2. Preparation of hexamethylethylenediamonium alkyl sulfates [HED(DS)2 and HED(DeS)2] Decyl and dodecylsulfate salts of HED> were precipitated by metathesis from the corresponding solutions of sodium alkyl sulfate and hexamethylethylenediammonium iodide. In each case, approximately 72 g of sodium alkyl sulfate were dissolved in 500 ml of 95% ethyl alcohol. Then 50 g of hexamethylethylenediammonium iodide was added and the resulting mixture stirred for 2 h at room temperature. The surfactants were recrystallized twice from ethanol, subjected to Soxhlet extraction with anhydrous acetone for 12 h, washed with ca. 1 l of chilled distilled water to remove all traces of iodide ion and dried in vacuum over P O . The absence of residual iodide ion in the detergent was veri"ed by the lack of precipitation with aqueous silver nitrate solution and by negative Lassaigne test [18]. Elemental analyses were satisfactory for both detergents: C H N S O . H O found: C, 55.72; H, 10.50; N, 4.09; calc. C, 55.99; H, 10.50; N, 4.08%; and C H N S O found: C, 54.02; H, 10.35; N, 4.47, calc. C, 54.19; H, 10.32; N, 4.52%.
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
2.2. Methods The cmc values for HED(DeS) and HED(DS) were obtained by surface tension and speci"c conductivity techniques, employing a Thomas model 70535 Du NouK y Surface Tensiometer, equipped with a Pt ring, and an Analion C-701 conductimeter, respectively. Successive aliquots of a concentrated stock solution of HED(DeS) or HED(DS) prepared in water with HEDI (0}0.035 M) were added to a thermostatted cell containing water or an aqueous solution of HEDI . Critical micelle concentrations (cmc) were determined from plots of apparent surface tension versus log [surfactant], or from the speci"c conductance as a function of the surfactant concentration, at 453C, 533C and 633C. The #uorescence quenching experiments with RuL\ as probe were performed in the absence and in the presence of micelles, using MV> as quencher. The solutions, contained in thermostatted #uorescence cuvettes, were prepared with RuL\ (3;10\ M), HEDI (0.0}0.035 M), and HED(DeS) or HED(DS) (0.01}0.035 M). Aliquots of a concentrated stock solution of the quencher (0.4 M) were added directly to the sample cuvette via a calibrated Hamilton microsyringe, and the emission intensities were registered after each addition. Fluorescence measurements were performed at 453C, 533C and 633C on a PerkinElmer LS-5B spectro#uorometer, equipped with a thermostatted sample holder.
3. Results and discussion Critical micelle concentrations (cmc) of HED(DeS) and HED(DS) obtained by surface tension and speci"c conductivity are listed in Table 1. In surfactant systems with multivalent ions the Kraft point is usually increased and the surfactants often precipitate at room temperature. An increase in the number of the carbon atoms in the alkyl chain also leads to a reduction in the solubility of these compounds. Consequently, the temperatures used to determine cmc values were: 453C, 533C and 633C for HED(DeS) , and 533C and 633C for HED(DS) . The cmc values found for these
317
Table 1 Values of cmc and a determined by surface tension and conductivity for HED alkyl sulfate detergents. Surfactants
[HED>] mM
¹ (3C)
cmc (mM)
HED(DeS)
0.0
25 25 25
45 45 53 45 53 63
6.48 7.20 6.60 3.00 3.30 3.40
(c) (C) (C) (C) (C) (C)
0.0
53
25 25
53 63
0.28 0.23 0.10 0.11
(c) (C) (C) (C)
HED(DS)
a
0.11 0.11
0.10 0.10
(c) " measured by surface tension; (C) " conductivity.
surfactants are lower than those of the same surfactants with Na> as counterion. For both surfactants, the dependence of the cmc on temperature is relatively small. Increasing the chain length, from decyl to dodecyl, decreases the cmc, as does the addition of HEDI . The values of a may be estimated from the ratio of slopes of the linear regions of plots of speci"c conductance as a function of surfactant concentration, above and below the critical micelle concentration [19], as well as from Corrin}Harkins plots [20]. The a values found by these two methods are similar (Table 1) and lower than that of 0.25 found for SDS [21], due to the in#uence of the divalent organic counterion on the properties of the micelles. As shown in the previous studies [14], the quenching of the luminescent probe RuL\ by MV> obeys the Stern}Volmer relationship in homogeneous solution: I3/I"1#K [MV>],
(3)
where I3 and I are the #uorescence intensities in the absence and presence of the quencher, respectively, and K is the Stern}Volmer quenching 14 constant. The quenching reaction is an electrontransfer process from the excited ruthenium complex to the quencher, forming the viologen cation radical [22].
318
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
The variation of the Stern}Volmer quenching constant with salt concentration follows the extended Debye}HuK ckel relationship [23]:
where h and h are the fractional coverages &#" +4 of the aggregate surface by HED> and MV> given by
log K "log K3 !16 A (k) /(1#5.8k ), (4) 14 14 where k is the ionic strength, A is a constant (A"0.525 at 453C; 0.531 at 533C and 0.538 at 633C) and K3 is the Stern}Volmer constant at zero 14 ionic strength. The experimental variation of K with ionic strength in aqueous solution was 14 employed to determine the K3 values: 45 700 M\ 14 at 453C, 51 600 M\ at 533C and 65 600 M\ at 633C. The basic assumption of the method employed [13}16] is that the tetraanionic probe RuL\ is completely excluded from the negatively charged micelle, while the viologen cation partitions between the micellar and aqueous phases. Thus upon addition of anionic micelles, the concentration of the quencher in the aqueous phase decreases due to binding to the micellar surface, attenuating the quenching e$ciency. Considering that the quenching e$ciency is dependent on the concentration of free viologen cation in the aqueous phase, [MV>] , and that the quenching of the lumines cence of RuL\H by MV> follows normal Stern}Volmer kinetics, the equation describing the quenching is then
h "[HED>] /C , (8) &#" " h "[MV>] /C , (9) +4 " h3 "h #h "(1!a). (10) &#" &#" +4 The micellar fractional coverages and the bound and free micelle counterion concentrations are related by the pseudophase ion exchange mass balance relationships:
I3/I"1#K [MV>] . (5) 14 The total concentration of added quencher, [MV>] , is related to the [MV>] and the con centration of micelle-bound cation, [MV>] , by the mass-balance relationship:
The K values obtained from the experi+4 &#" mental data are 1.3 for HED(DeS) (10}35 mM detergent; 45}633C) and 1.4 for HED(DS) (5}35 mM detergent; 53}633C). In Fig. 1, representative Stern}Volmer plots for the quenching of RuL\ by MV> are presented; the curves corres pond to the predictions of the model (Eqs. (4)}(12)) for MV>/HED> ion exchange in HED(DS) micelles. Changes in the alkyl chain length have no signi"cant e!ect on K , consistent with pre+4 &#" vious "ndings that the ion exchange process at the micellar interface is not in#uenced by the micelle size [12]. The fact that the experimental values of K are only slightly longer than unity points +4 &#" to a similar a$nity of the alkylsulfate micelles surface for these two divalent organic cations, despite the large di!erence in chemical structure and charge distribution in the MV> and HED> ions.
[MV>] "[MV>] #[MV>] . (6) The appropriate value of K in the presence of 14 anionic micelles is calculated from the ionic strength dependence of K (Eq. (4)), using the 14 K3 values determined in homogeneous solution 14 and the e!ective intermicellar ionic strength (k , see below). This value of K is used to calculate 14 [MV>] iteratively from the experimental I3/I data. The corresponding counterion exchange selectivity coe$cient (K ) is then calculated from +4 &#" the relationship: K "(h [HED>] )/([MV>] h ), +4 &#" +4 &#"
(7)
(11) [HED>] "(1!a)C !h C , " +4 " [HED>] "aC #cmc#[HEDI ] #h C . " +4 " (12) In these equations, [HEDI ] is the added com mon counterion salt and C is the concentration of " aggregated HED(DeS) or HED(DS) , given by: C "C !cmc. " 2 As discussed previously [6,7,12}16], the e!ective ionic strength in the aqueous phase is determined by the ionic concentrations at the midpoint between micelles, which, in the present case, leads to k "2[HED] #2[MV>] #cmc#[HEDI ] . (13)
J.B.S. Bonilha et al. / Journal of Luminescence 82 (1999) 315}319
Fig. 1. Stern}Volmer plots of the quenching of RuL\ emission by MV> in HED(DS) micellar solutions in the presence of HEDI (25 mM) at 633C. Surfactant concentrations: (䊏) 5 mM, (䉱) 10 mM, (䢇) 25 mM and (䉬) 35 mM.
Acknowledgements This work was supported by grants from the Fundac7 a o de Amparo a` Pesquisa do Estado de Sa o Paulo, FAPESP (projects 91/0480-1 and 94/3505-3) and PADCT-FINEP (Project no. 65-92-0063-00). D.M.A.F. Navarro and P.M. Nassar acknowledge fellowship support from FAPESP. References [1] B. Lindman, H. Wennerstrom, Top. Curr. Chem. 87 (1980) 32. [2] C. Tanford, The Hydrophobic E!ect: Formation of Micelles and Biological Membranes, 2nd Edition, Wiley, New York, 1980.
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