Selectivity coefficients for ion exchange in micelles of hexadecyltrimethylammonium bromide and chloride

NOTES Selectivity Coefficients for Ion Exchange in Micelles of Piexadecyltrimethylammonium Bromide and Chloride For a number of micelle-modifiedreactions and equilibria, surfactant profiles and the effects of added salts have been successfully analyzed using pseudophase ionexchange models (1). However, the application of such models requires a knowledge of selectivity coefficients (Kx/y) for all counterionic species present in the system (2). Reported methods for determining Kx/v values include absorption spectroscopy (3), fluorescence quenching of micelle-incorporated (4) or water-soluble probes (5), conductivity (6), ion selective electrode measurements (7), ultrafiltration (8), complex formation (8), and kinetic measurements (9-11). The purpose of the present note is to report values of Kx/y for the exchange of a series of anions in micellar hexadecyltrimethylammonium bromide (CTAB) determined using three different methods, including a relatively rapid and versatile fluorescence quenching technique. These values are compared to available Kx/v values from the literature and with Kx/v values obtained in micellar hexadecyltrimethylammonium chloride (CTAC1). The present data thus serve as a source of Kx/v values for pseudophase ion-exchange analysis of micellar effects on reactivity and equilibria in CTA solutions. The Kx/v values reported here (see Table I) were determined by two fluorescence quenching methods (Methods I and II), described elsewhere (4), and by an absorption spectroscopic method (3) based on the competitive displacement of the 2-furoate ion by added salt (Method III). In this last method analytical concentrations of bound (FUb) and free (Fur) furoate ion were determined from the sample absorbance at 255 nm in 0.020 M Tris-HBr buffer, pH 8, using the relationship Abs = cflFuf[ + EbIFub[ with ~f = 9410 + 320 M -1 cm -~ (independent of buffer or added salt up to at least 0.2 M). The value of~b (6180 ___ 200 -1 cm -1) Was obtained from the absorptivity o f CTAFu (CMC = 7.7 × 10-4 M) (12), prepared by neutralization of CTAOH (11) with furoic acid, and assuming a net degree of micellar dissociation (c0 equal to 0.20 (13). Selectivity coefficients K~u~ and KFu/Z were evaluated from the change in furoate anion absorption upon addition of CTAB or CTAC1 to solutions of sodium furoate or upon addition of common (NAY) or foreign (NaZ) salt to a solution containing a fixed amount of sodium furoate and detergent (CTAB or CTAC1). Data were fitted by multiple regression analysis on a TRS-80 microcomputer using Eqs. [4], [8], and [13] to [18] of

Ref. (14). (X = Fu-; Y = Br (CTAB) or CI-(CTAC1); and Z = anion of added foreign salt.) The fit of the data is exemplified for the Br-/Fu- exchange in Fig. 1A. The fluorescence quenching data could also be reproduced with a single value of the selectivity coefficient, as shown in Fig. 1B for Br-/NO~ exchange with pyrene as probe. The most extensive set of data has been obtained for B r / N O 3 and Br/C1- exchange for which all methods give excellent agreement. As in the case of previous methods, the present results are derived from an analysis in terms of simple pseudophase ion exchange, assuming that all ions are either "bound" or "free." This assumption, and the resulting mass balance equations, may not be valid if the capacity of the electrical double layer is significant (5). Moreover, different methods may discriminate differently between "bound" and "free" counteflon concentrations. Nevertheless, the data show that even the ultrafiltration technique (8), which measures intermicellar ions, gives Kx/v values that agree with those obtained with methods which sense local concentrations of bound ions (e.g., Method I). This implies that, at least for relatively strongly bound ions such as NO~, Br-, and CI-, the basic analysis of the pseudophase ion exchange model is adequate: Furthermore, since several of the Kx~c values were obtained employing a reference anion (2-furoate in Method III or iodoacetate in Method II) and using both CTAB and CTAC1, the agreement obtained implies that relative counterion selectivities can be transferred from one system to another. The agreement between KBr/Xvalues for several other counterion exchange processes is not as satisfactory. This is particularly noticeable for ions which bind inefficiently (i.e., OH- and F-). The discrepancy in the KBr/Xvalues for F - and OH- may be due to errors in mass balance calculations when one of the ions binds poorly, or to inherent defects of the simple pseudophase ion exchange model (17), Indeed, errors due to neglect of counterion accumulation in the diffuse double layer should be most important for weekly bound counterions (18) when methods which measure intermiceUar counteflon concentrations are employed to estimate the selectivity coefficient. Finally, the data of Table I point to the relative importance of several factors that may contribute to counterion selectivity. From the results for the series I-/Br-/C1-/F- and acetate/chloroacetate/bromoacetate/iodoacetate, it appears that the complexing capacity of the ions and/or polarizability are much more important factors than size. Indeed, neglect of contri-

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294

NOTES TABLE I

Selectivity Coefficients for Bromide/Anion Exchange (KBr/X)in Micelles of Hexadecyltrimethylammonium Bromide and Chloridea CTAB Anion

Furoate Acetate Chloroacetate Bromoacetate Iodoacetate NO3 C1FOH H2PO~ HCO~

Ib

IV

28

18 + 4 3 _+ 0.5 2 + 0.5 1 + 0.2 0.9 + 0.1 6 + 1 23 -22 + 5 22 + 5

0.8 4.2 ~42 ~42 --

CTACI III d

Ill

1.05 _+ 0.05 13

0.73 + 0.1 5.0

Reference

17

10.2 (8)

1.2 + 0.2 4.6 50 + 10 50 + 10

0.7 (7), 0.9 (8), 0.9 (15) 5.1 (8), 3 (15), 5.0 (16) 23 (8) 21 (8), 12.5 (9)

13

a CTAB and CTAC1 were purified as described previously (9, 10) and all inorganic salts were analytical grade or superior. 2-Furoie acid (Aldrich) was recrystallized from CC14 prior to use. b Method I: Bromide quenching of naphthalene or pyrene fluorescence: data analyzed using Eqs. [6] and [8][ 11] of Ref. (4). c Method II: Competitive displacement of the iodoacetate ion (IAC) with pyrene as fluorescence probe (4), assuming KBr/X = KIAC/x/KIAC/Br. d Method III: Furoate absorption method described in this work, assuming KFu/v/KFumr = KBr/Y.

butions from these factors potentially represents a defect in theoretical models which presume that ion size is the determining factor for counterion selectivity (19).

0,45

i

A

m 0,40 o

0,35

( tb

io

go

NoBr

(mM)

4'o

5'o

1,2

o

H "-

I~I

1,0

Y

I*0

2'0

ACKNOWLEDGMENTS This work was supported in part by grants from the Funda9~o de Amparo ~.Pesquisa do Estado de S~o Paulo (FAPESP), Conselho Nacional de Desenvolvimento Cientifico e Tecnolrgico (CNPq), Financiadora de Estudos e Projetos (FINEP B-76-81-295-00-00). N.B. and L.M. acknowledge partial leaves of absence from the Instituto de Bioci~ncias, Letras e Ci~ncias Exatas, UNESP, S~o Jos6 do Rio Preto. This collaboration was stimulated by travel funds from PNUD-UNESCO (RLA 024) and a collaborative Research Project from the Conselho Nacional de Investigaci6n Cientffica e Tecnol6gica (CONYCIT-Chile)-CNPq. REFERENCES

N o N O 3 (rnM)

FIG. 1. Representative data fits for (A) Method III displacement of the furoate ion (5 X 10-5 M) by added bromide ion in micellar CTAB (8.3 × 10-3 M); (B) Method II, enhancement of pyrene (2 × 10-6 M) :fluorescence upon displacement of bromide ion from micellar CTAB (1.0 x 10-2 M) by added nitrate ion. Journal of Colloid and Interface Science, Vol. 96, No. 1, November 1983

la. Chaimovich, H., Aleixo, R. M. V., Cuccovia, I. M., Zanette, D., and Quina, F. H., in "Solution Behaviour of Surfactants Theoretical and Applied Aspects" (K. L. Mittal, and E. J. Fendler, Eds.). Plenum, New York, 1982. lb. Fendler, J. H., "Membrane Mimetic and Chemistry." Wiley-Interscience, New York, 1982.

NOTES 2. Quina, F. H., Politi, M. J., Cuccovia, 1. M., Baumgarten, E., Martins-Franchetti, S. M., and Chaimovich, H., J. Phys. Chem. 84, 272 (1980). 3. Barter, D., Gamboa, C., and Sepfilveda, L., J. Phys. Chem. 84, 272 (1980). 4. Lissi, E., Abuin, E., Bianchi, N., Miola, L., and Quina, F. H. J. Phys. Chem., in press. 5. Gianni, M. F., Doctoral Thesis, Instituto de Quimica, Universidade de Sao Paulo, Brasil, 1982. 6. Bunton, C. A., Ohmenzetter, K., and Sepfllveda, L., J. Phys. Chem. 81, 2000 (1977). 7. Larsen, J. W., and Magid, L. J., J. Amer. Chem. Soc. 96, 5774 (1974). 8. Gamboa, C., Sep6lveda, L., and Soto, R., J. Phys. Chem. 85, 1429 (1981). 9. Chaimovich, H., Politi, M. J., Bonilha, J. B. S., and Quina, F. H., J. Phys. Chem. 83, 1951 (1979). 10. Bonilha, J. B. S., Chiericato, G., Jr., Martins-Franchetti, S., Ribaldo, E., and Quina, F. H., J. Phys. Chem~ 86, 4941 (1982). 11. Bunton, C. A., Romsted, L. S., and Thamavit, C., J. Amer. Chem. Soc. 102, 3900 (1980). 12. Cuccovia, I. M., Aleixo, R. M. V., Erismann, N. E., Zee, N. T. v.d., Schreier, S., and Chaimovich, H., J. Amer. Chem. Soc. 104, 4544 (1982). 13. Romsted, L. S., PhD Thesis, Indiana University Bloomington, Indiana, 1975. 14. Quina, F. H., and Chaimovich, H., J. Phys. Chem. 83, 1844 (1979). 15. Abuin, E. B., and Lissi, E., J. Colloid Interface Sci., in press. 16. Calculated from the data of Burrows, H. D., For-

295

mosinho, S. J., Fernanda, M., Paiva, J. R., and Rasburn, E. J., J. Chem. Soc. Faraday Trans. H 76, 685 (1980) using Method I (4). 17. Bunton, C. A., Romsted, L. J., and Savelli, G., J. Amer. Chem. Soc. 101; 1253 (1979); Nome, F., Rubira, A. F., Franco, C., and Ionescu, L. G., J. Phys. Chem. 86, 1881 (1982). 18. Quina, F. H., and Gianni, M. F., to be published. 19. Linse, P., Gunnarson, G., and J6nson, B., J. Phys. Chem- 86, 413 (1982). ELSA B. ABUIN EDUARDOLISSI Departamento de Qufmica Fac. Ciencia Universidad de Santiago de Chile Av. Ecuador 3469 Santiago, Chile PEDRO S. ARAUJO* REGINA M. V. ALEIXO* HERNAN CHAIMOVICH* NATALBIANCHI~ LAERTE MIOLA~ FRANK H. QUINA~ *Departamento de Bioqufmica and tDepartamento de Qufmica Fundamental Instituto de Qufmica Universidade de Sdo Paulo SSo Paulo Caixa Postal 20.780 SSo Paulo, S. P., Brazil Received February 14, 1983 ; accepted March 21, 1983

Journalof Colloidand InterfaceScience, Vol.96, No. 1, November1983