Anal. Chem. 2003, 75, 6011-6016
Technical Notes
Surfactant to Dye Binding Degree-Based Methodology for the Determination of Ionic Amphiphilic Compounds Rafael Fabios, Marı´a Dolores Sicilia, Soledad Rubio, and Dolores Pe´rez-Bendito*
Department of Analytical Chemistry, Faculty of Sciences, University of Co´ rdoba, Edificio Marie Curie, Campus de Rabanales, 14071 Co´ rdoba, Spain
A new analytical measurement parameter based on the effect of amphiphilic substances on the degree of binding of a surfactant to dye molecules, which induce the formation of surfactant premicellar aggregates, is presented. The theory for dye-surfactant intermolecular interactions in mixed surfactant systems, which assumes a mononuclear model for the formation of dye-induced premicellar aggregates, has been used to derive an expression that provides linear calibrations for the determination of amphiphilic compounds. The dye-surfactant interactions involved have been investigated, and the variables affecting the measurement analytical parameter have been discussed. The analytical applicability of the surfactant-dye binding degree method is demonstrated by quantifying major anionic surfactants at the nanogramsper-milliliter level and determining the total concentration of these amphiphilic substances in sewage samples (average recoveries ranged from 98 to 102%). Supramolecular assemblies have been used in many fields of analytical chemistry in order to improve existing methods and to develop new analytical procedures, micellar media being the most widely exploited for these purposes.1-8 Thus, the ability of micelles to alter the physicochemical properties of solutes has been extensively used in both equilibrium2,3 and kinetic4 determinations and different separation techniques.5-8 * To whom correspondence should be addressed. Phone/Fax: +34-957218644. E-mail:
[email protected]. (1) Love, L. J. C.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1132A1134A. (2) Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 169, 1-29. (3) Hinze, W. L. In Use of Surfactant and Micellar Systems in Analytical Chemistry; Mittal, K. L. Ed.; Plenum Press: New York, 1979; Vol. 1, pp 79-127. (4) Pe´rez-Bendito, D.; Rubio, S. Trends Anal. Chem. 1993, 12, 9-18. (5) Khaledi, M. G. Trends Anal. Chem. 1988, 7, 293-300. (6) Okada, T. J. Chromatogr., A 1997, 780, 343-360. (7) Corstjens, H.; Billiet, H. A. H.; Frank, J.; Luyben, K. C. A. M. J. Chromatogr., A 1995, 715, 1-11. (8) Carabias-Martı´nez, R.; Rodrı´guez-Gonzalo, E.; Moreno-Cordero, B.; Pe´rezPavo´n, J. L.; Garcı´a-Pinto, C.; Ferna´ndez Laespada, E. J. Chromatogr., A 2000, 902, 251-265. 10.1021/ac030105v CCC: $25.00 Published on Web 09/24/2003
© 2003 American Chemical Society
Introducing micelles in a chemical process can also alter their characteristic parameters, such as the critical micelle concentration (cmc), aggregation number, micellar structure, shape, etc., thus offering potential for the development of analytical methods based on no conventional measurement parameters. To date, the only analytical approach reported that has been based on the measurement of micellar parameters is the mixed aggregate method.9,10 It is based on measuring the critical micelle concentration value of mixtures of amphiphilic substances (i.e. surfactants, drugs, etc.), one of which is the analyte. Continuing with our investigations to explore the potential of using aggregation parameters of supramolecular assemblies for analytical purposes, we investigated here the possibility of using the effect of ionic amphiphilic substances on the degree of binding of a surfactant to dye molecules, which induces the formation of surfactant premicellar aggregates. Although neutral dyes can induce the formation of submicellar aggregates,11 they are commonly formed from interactions between dye and surfactants bearing charges of opposite sign.12-15 The dye can participate in the formation of these aggregates through its charged groups, which compensate the repulsion forces between ionic surfactant molecules bearing the same charge.12 Hydrophobic and specific interactions have also been reported to govern dye-induced premicellar aggregate formation.13 When two surfactants are present in aqueous dye solutions, interactions between them can result in a decrease in the degree of binding of surfactant to dye molecules,13 and this is the basis of the surfactant-dye binding degree (SDBD) method, presented here for the first time. In this work, the decrease of the degree of binding of the cationic surfactant didodecyldimethylammonium bromide (DDABr) to the anionic dye Coomassie Brilliant Blue G (CBBG) as a function of the total concentration of anionic surfactants (analytes) (9) Sicilia, D.; Rubio, S.; Pe´rez-Bendito, D. Anal. Chem. 1995, 67, 1872-1880. (10) Borrego, E.; Sicilia, D.; Rubio, S.; Pe´rez-Bendito, D. Trends Anal. Chem. 2001, 20, 241-254. (11) Ogino, K.; Kasuya, T.; Abe, M. Colloid Polym. Sci. 1988, 266, 539-546. (12) Rosendorfova´, J.; C ˇ erma´kova´, L. Talanta 1980, 27, 705-708. (13) Simoncˇicˇ, B.; Kert, M. Dyes Pigm. 2002, 54, 221-237. (14) Casero, I.; Sicilia, D.; Rubio, S.; Pe´rez-Bendito, D. Talanta 1997, 45, 167180. (15) Simoncˇicˇ, B.; Sˇ pan, J. Dyes Pigm. 1998, 36, 1-14.
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003 6011
Table 1. Analytical Figures of Merit of the Proposed Method for the Determination of Anionic Surfactants of the Type Alkylbenzenesulfonate [CnH2n+1(C6H4)SO3- Na+], Alkylsulfonate [CnH2n+1SO3- Na+], and Alkylsulfate [CnH2n+1SO4- Na+] anionic surfactant
n
slope ( SD, (M l/mg) × 105
SDBSo Petrelab P-550
12 10-14
1.88 ( 0.09 1.90 ( 0.09
SOSo SDeSo SDSo STSo SHSo
8 10 12 14 16
1.99 ( 0.06 1.9 ( 0.1 1.99 ( 0.06 1.99 ( 0.06 1.94 ( 0.09
SDeS SDS STS SHS
10 12 14 16
1.99 ( 0.09 1.92 ( 0.06 1.94 ( 0.06 1.99 ( 0.09
a
intercept ( s, (M) × 105
Alkylbenzenesulfonates -0.1 ( 0.1 0.1 ( 0.1 Alkylsulfonates -0.06 ( 0.09 0.04 ( 0.13 0.02 ( 0.13 0.04 ( 0.09 0.09 ( 0.09 Alkylsulfates
0.09 ( 0.09 0.04 ( 0.09 0.11 ( 0.06 0.09 ( 0.09
ra
Sy/xb × 106
βA ( SD
0.994 0.993
1.9 2.3
6.6 ( 0.3
0.996 0.991 0.996 0.996 0.998
1.2 1.8 1.3 1.3 1.3
4.3 ( 0.1 4.6 ( 0.2 5.4 ( 0.2 6.0 ( 0.2 6.4 ( 0.3
0.996 0.994 0.995 0.996
1.4 1.7 1.4 1.3
5.2 ( 0.2 5.5 ( 0.2 6.1 ( 0.2 6.9 ( 0.3
Correlation coefficient (nine different concentrations). b Standard deviation of residuals.
added to the aqueous DDABr-CBBG mixture was used for the practical demonstration of the new approach. The anionic surfactants selected were the most widely used in both industrial and domestic applications, namely, linear alkylbenzene sulfonates, alkylbenzene sulfonates, and alkylsulfates, with an alkyl chain length consisting of between 8 and 16 carbon atoms (Table 1). CBBG induces the formation of DDABr aggregates at concentrations far below its cmc, which can be monitored from changes in the spectral features of the dye.14 Therefore, the dye has two functions in the chemical system: it induces the formation of DDABr premicellar aggregates and acts as a photometric probe. The addition of anionic surfactants results in a strong interaction with DDABr and, hence, in decreased interactions CBBG-DDABr aggregates. This article deals with the principles that support the new methodology, the study of involved interactions, the effect of different variables on the new measurement parameter and the applicability of the developed method to the nonspecific determination of anionic surfactants in different sewage samples. THEORETICAL BACKGROUND The theoretical formulation provided by Simoncˇicˇ et al.13 assumes a mononuclear model for the formation of premicellar aggregates in diluted aqueous surfactant-dye mixtures. According to this treatment, the binding of monomeric surfactant to dye molecules bearing charge of opposite sign [i.e., a cationic surfactant (S+) which interacts with an anionic dye (D-)] can be presented as a set of multiple equilibria:
S+ + D- T DS S+ + DS T DS2+
The extent of the mononuclear premicellar aggregate formation can be expressed in terms of the degree of binding of cationic surfactant to dye anions, βD, which can be calculated from the relationship
βD )
mC - mC,M mD
(1)
where mC and mD are the total concentrations of cationic surfactant and dye in the aqueous surfactant-dye mixture, respectively, and mC,M is the concentration of the cationic surfactant in monomeric form. When a second surfactant of anionic nature (S-) is present in the aqueous surfactant-dye mixture, the interaction between surfactant molecules bearing charge of opposite sign results in a decrease in the degree of binding of the cationic surfactant to the anionic dye. In this case, βD can be calculated as follows,
m/C - mC,M - βAmA βD ) mD
(2)
where m*C is the total concentration of cationic surfactant required to reach a given degree of binding, βD, in the presence of a total concentration of anionic surfactant mA; and βA is the degree of binding of monomeric cationic surfactant to monomeric anionic surfactant. Under experimental conditions at which the βD value obtained in the presence of anionic surfactant is equal to that reached in its absence for a given concentration of dye and assuming mC,M does not depend on the concentration of anionic surfactant present in the aqueous solution, eqs 1 and 2 can be combined to give
S+ + DSn-1+(n-2) T DSn+(n-1)
m/C - mC ) βAmA
where the dye is the central group that interacts with monomers of cationic surfactant to form premicellar aggregates (DSn+(n-1)) with different stoichiometries (n).
on the basis of which linear calibration will be obtained by using a cationic surfactant as titrant and plotting the parameter (m/C mC) as a function of the concentration of anionic surfactant (mA).
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Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
(3)
Parameters m/C and mC can be easily obtained from titrations performed in the presence and absence of anionic surfactant in the titration vessel, respectively. The sensitivity for the determination of anionic surfactant will depend on its degree of binding to the cationic surfactant (βA). EXPERIMENTAL SECTION Apparatus. The equipment used consisted of a Mettler DL 40 Memotitrator equipped with a 10-mL autoburet, a fan stirrer, a titration vessel, and a Mettler GA 14 recorder. The detection unit was a Metrohm 662 spectrophotometer equipped with an immersion probe (1-cm path length). Dye-surfactant interactions were studied by using a Hitachi U-2000 spectrophotometer. Chemicals. Commercially available highest-grade chemicals were used throughout, without further purification. A 1.4 × 10-4 M aqueous solution of Coomassie Brilliant Blue G (CBBG, Sigma) was prepared by dissolving 0.125 g of the reagent in 1 L of distilled water with sonication for 15 min. This solution was prepared at least 24 h prior to use and was stable for 1 month. The buffer solution used consisted of 0.25 M triethanolamine (TEA, pH ) 7.0). A 1.0 × 10-3 M aqueous solution of didodecyldimethylammonium bromide (DDABr, Fluka) was also prepared. Solutions of the anionic surfactants, sodium dodecylbenzenesulfonate (SDBSo, Aldrich), sodium octanesulfonate (SOSo, Fluka), sodium decanesulfonate (SDeSo, Fluka), sodium dodecanesulfonate (SDSo, Fluka), sodium tetradecanesulfonate (STSo, Fluka), sodium hexadecanesulfonate (SHSo, Fluka), sodium decylsulfate (SDeS, Merck), sodium dodecylsulfate (SDS, Aldrich), sodium tetradecylsulfate (STS, Aldrich), and sodium hexadecylsulfate (SHS, Merck) were prepared at a concentration of 100 mg/L each in distilled water. These stock solutions remained stable for at least 1 month. Petrelab P-550, a commercial mixture of alkylbenzenesulfonates with the proportional composition of the different homologues 9.6% C10, 38.1% C11, 31.3% C12, 19.1%, C13 and 1% C14, was kindly supplied by Masso´ and Carol (Barcelona, Spain). Methanol was obtained from Panreac. Sample Collection and Preservation. Water samples were taken from two different sewage treatment plants (STP) in the region of Andalusia (Spain) located at Pozoblanco and Linares. The Pozoblanco STP received mainly domestic effluent, while the Linares STP received both domestic and industrial wastewater. Influent and effluent water samples from both STPs were collected in September and May 2002, respectively. They were preserved by addition of formaldehyde (1%, v/v) at the time of collection and stored at 4 °C in brown glass bottles until analysis. Procedures. Sample Pretreatment. A volume of sample containing 2.5-75 µg of total anionic surfactants (10-25 mL of influent and 100-250 mL of effluent) was filtered on glass fiber filters to remove solids. Cleanup and preconcentration of filtered samples were performed by solid-phase extraction (SPE) using reversedphase (RP) C18 columns (Varian 04302, 500 mg), which were attached to an SPE vacuum manifold (Varian VAC ELUT SPS 24). RP C18 columns were preconditioned by passing sequentially 10 mL of methanol and 10 mL of distilled water at a rate of 10 mL/min. Water samples were passed through the extraction column at a rate of
