Electrophoretic Effects of the Adsorption of Anionic Surfactants to Poly

Aug 3, 2007 - Electrophoretic Effects of the Adsorption of Anionic Surfactants to Poly(dimethylsiloxane)-Coated Capillaries. Maria F. Mora, Carla E. G...
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Anal. Chem. 2007, 79, 6675-6681

Electrophoretic Effects of the Adsorption of Anionic Surfactants to Poly(dimethylsiloxane)-Coated Capillaries Maria F. Mora,† Carla E. Giacomelli,‡ and Carlos D. Garcia*,†

Department of Chemistry, The University of Texas at San Antonio, San Antonio, Texas 78249, and INFIQC, Departamento de Fisicoquimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cordoba, Ciudad Universitaria, Cordoba, Argentina

Poly(dimethylsiloxane) (PDMS) is one of the most convenient materials to construct capillary electrophoresis microchips. Even though PDMS has many advantages, its use is often limited by its hydrophobicity. Although it is well-known that the surface properties of PDMS can be modified by anionic surfactants, very little is known regarding the driving forces or the electrophoretic consequences of the adsorption of anionic surfactants. In this work, the adsorption of alkyl surfactants on PDMS was studied by performing electroosmotic flow (µEOF) measurements. In order to mimic the behavior of PDMS microchannels, fused-silica capillaries were coated with PDMS and used for the µEOF measurements. This approach allowed using standard CE instrumentation and provided significant advantages over similar experiments performed on microchips. The change in the µEOF in the presence of surfactants was correlated to the surfactant adsorbed amount which, plotted versus surfactant concentration, gives an adsorption isotherm. The adsorption isotherms were obtained using alkyl surfactants with different chain lengths and head groups. According to our results, the interaction of alkyl surfactants with the PDMS surface is determined by a combination of hydrophobic and electrostatic interactions, where the former is more significant than the latter. The affinity of each surfactant for the PDMS surface was calculated by fitting the adsorption profiles with a Langmuir equation and, in the case of single-charged surfactants, correlated to the corresponding cmc value. Capillary electrophoresis (CE) is a family of separation techniques driven by an electric potential difference applied across a small capillary (1 mmol L-1 are expected to dominate the ζ potential performance of hydrophobic polymer substrates through adsorption at the surface.1,8 Among other polymeric substrates, poly(dimethylsiloxane) (PDMS) is considered one of the most convenient materials to fabricate CE microchips.9,10 PDMS is a flexible polymer with repeated units consisting of -(O-Si-(CH3)2)-. PDMS is optically transparent, nontoxic, and allows easy and quick fabrication of microdevices with minimum instrumentation requirements. However, the hydrophobic nature of PDMS often limits its applications.11 The hydrophobicity of PDMS is responsible for unstable migration times, peak tailing, and low separation efficiency.12,13 Consequently, different coating procedures have been used to (1) Kirby, B. J.; Hasselbrink, E. F., Jr. Electrophoresis 2004, 25, 203-213. (2) Zhou, M. X.; Foley, J. P. Anal. Chem. 2006, 78, 1849-1858. (3) Yin, X.-B.; Wang, E. Anal. Chim. Acta 2005, 533, 113-120. (4) Wang, J. Electroanalysis 2005, 17, 1133-1140. (5) Dolnik, V. Electrophoresis 2004, 25, 3589-3601. (6) Trojanowicz, M.; Szewczynska, M.; Wcislo, M. Electroanalysis 2003, 15, 347-365. (7) Garcia, C. D.; Dressen, B. M.; Henderson, A.; Henry, C. S. Electrophoresis 2005, 26, 703-709. (8) Kirby, B. J.; Hasselbrink, E. F., Jr. Electrophoresis 2004, 25, 187-202. (9) Wheeler, A. R.; Trapp, G.; Trapp, O.; Zare, R. N. Electrophoresis 2004, 25, 1120-1124. (10) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J.; Whitesides, G. M. Electrophoresis 2000, 21, 27-40. (11) Seo, J.; Lee, L. P. Sens. Actuators, B 2006, 119, 192-198. (12) Linder, V.; Verpoorte, E.; Thormann, W.; de Rooij, N. F.; Sigrist, H. Anal. Chem. 2001, 73, 4181-4189. (13) Doherty, E. A.; Meagher, R. J.; Albarghouthi, M. N.; Barron, A. E. Electrophoresis 2003, 24, 34-54.

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reduce the hydrophobicity of PDMS.14-17 Among others, surfactants are one of the most convenient coatings for a number of reasons. First, surfactants adsorb spontaneously to PDMS, and therefore, no instrumentation, chemical modification, or specific training is required. Second, the surfactant concentration (solution and surface) can be controlled. Third, most surfactants can be removed from the surface by dilution (rinsing the surface with background electrolyte). Fourth, by simply increasing the concentration above the critical micelle concentration (cmc), the same molecules can also be used as a pseudostationary phase.18 Fifth, the structure of the surfactant can be chosen to meet specific needs.19 Sixth, surfactants can also increase the electrochemical response.20,21 It is well-known that the adsorption of surfactants changes the hydrophobic/hydrophilic character of the surface on which they are adsorbed. In the case of ionic surfactants, they also alter the surface charge.22 While the interactions of cationic surfactants with hard surfaces have been extensively studied by Lucy, Koopal, and others,23-36 the interactions with soft surfaces (like PDMS) have had only limited study.7,37-40 Although it is well accepted that sodium dodecyl sulfate (SD12S)34,37,41-44 as well as other anionic surfactants,7 can adsorb to PDMS and modify the electroosmotic (14) Janini, G. M.; Issaq, H. J.; Muschik, G. M. J. Chromatogr., A 1997, 792, 125-141. (15) Liu, Y.; Fanguy, J. C.; Bledsoe, J. M.; Henry, C. S. Anal. Chem. 2000, 72, 5939-5944. (16) Hu, S.; Ren, X.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Electrophoresis 2003, 24, 3679-3688. (17) Hu, S.; Ren, X.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. L. Langmuir 2004, 20, 5569-5574. (18) Terabe, S. Anal. Chem. 2004, 76, 240A-246A. (19) Mohanty, A.; Dey, J. J. Chromatogr., A 2005, 1070, 185-192. (20) Wallingford, R. A.; Ewing, A. G. Anal. Chem. 1988, 60, 258-263. (21) Garcia, C. D.; Henry, C. S. Anal. Chim. Acta 2004, 508, 1-9. (22) Jo´dar-Reyes, A. B.; Ortega-Vinuesa, J. L.; Martı´n-Rodrı´guez, A. J. Colloid Interface Sci. 2006, 297, 170-181. (23) Baryla, N. E.; Melanson, J. E.; McDermott, M. T.; Lucy, C. A. Anal. Chem. 2001, 73, 4558-4565. (24) Melanson, J. E.; Baryla, N. E.; Lucy, C. A. Anal. Chem. 2000, 72, 41104114. (25) Wang, C.; Lucy, C. Anal. Chem. 2005, 77, 2015-2021. (26) Yassine, M. M.; Lucy, C. A. Anal. Chem. 2005, 77, 620-625. (27) Cunliffe, J. M.; Baryla, N. E.; Lucy, C. A. Anal. Chem. 2002, 74, 776-783. (28) Atkin, R.; Craig, V. S.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219-304. (29) Chorro, M.; Chorro, C.; Dolladille, O.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 1999, 210, 134-143. (30) Paria, S.; Khilar, K. C. Adv. Colloid Interface Sci. 2004, 110, 75-95. (31) Sabah, E.; Turan, M.; Celik, M. S. Water Res. 2002, 36, 3957-3964. (32) Stodghill, S. P.; Smith, A. E.; O’Haver, J. H. Langmuir 2004, 20, 1138711392. (33) Ye, M.; Zou, H.; Liu, Z.; Ni, J.; Zhang, Y. J. Chromatogr., A 1999, 855, 137145. (34) Wang, C.; Lucy, C. A. Electrophoresis 2004, 25, 825-832. (35) Torn, L. H.; Koopal, L. K.; Keizer, A. d.; Lyklema, J. Langmuir 2005, 21, 7768 -7775. (36) Torn, L. H.; de Keizer, A.; Koopal, L. K.; Lyklema, J. J. Colloid Interface Sci. 2003, 260, 1-8. (37) Ocvirk, G.; Munroe, M.; Tang, T.; Oleschuk, R.; Westra, K.; Harrison, D. J. Electrophoresis 2000, 21, 107-115. (38) Badal, M. Y.; Wong, M.; Chiem, N.; Salimi-Moosavi, H.; Harrison, D. J. J. Chromatogr., A 2002, 947, 277-286. (39) Kang, J.; Yan, J.; Liu, J.; Qiu, H.; Yin, X.-B.; Yang, X.; Wang, E. Talanta 2005, 66, 1018-1024. (40) Wang, A.-J.; Xu, J.-J.; Chen, H.-Y. Anal. Chim. Acta 2006, 569, 188-194. (41) Huang, H.-Y.; Chuang, C.-L.; Chiu, C.-W.; Chung, M.-C. Electrophoresis 2005, 26, 867-877. (42) Garcia, C. D.; Henry, C. S. Electroanalysis 2005, 17, 223-229. (43) Bao, N.; Xu, J.-J.; Zhang, Q.; Hang, J.-L.; Chen, H.-Y. J. Chromatogr., A 2005, 1099, 203-206.

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flow (µEOF), very little is known regarding the driving forces or the electrophoretic consequences of the adsorption of anionic surfactants. In general, there are different models that can be used to describe the adsorption of a solute on a solid surface.26,30,34,35,45,46 Among them, the simplest is the one proposed by Langmuir;47 which assumes a monolayer coverage, a homogeneous surface, and an absence of lateral interactions. Under this model, the adsorption can be described using eq 2, where Γ is the adsorbed amount (mol/m2), Γmax is the maximum adsorbed amount (mol/ m2), C is the adsorbate concentration in solution (in equilibrium with the surface), and K is a constant related to the affinity of the adsorbate for the surface.48

Γ ) Γmax

KC 1 + KC

(2)

In this work, we investigated the adsorption of different alkyl surfactants (varying in chain length and head group) to PDMS by performing electroosmotic flow measurements. The observed increments in µEOF were attributed to the adsorption of surfactants to the PDMS surface. Consequently, the adsorption isotherms (∆µEOF vs surfactant concentration) were processed using the Langmuir equation to obtain the affinity constant (K), which was then correlated to the corresponding cmc. In order to evaluate the effects of these dynamic coatings, the separation of six phenolic compounds by CE was also studied using surfactantcoated PDMS capillaries. EXPERIMENTAL SECTION Reagents and Solutions. All chemicals were analytical reagent grade and used as received. Sodium tetraborate (Na2B4O7‚ 10H2O) and sodium hydroxide were purchased from Fisher Scientific (Fair Lawn, NJ). Poly(diallyldimethylammonium chloride) (PDDA; average MW ∼ 200 000-350 000, 20 wt %), 3-aminopropylethoxysilane (APTES), sodium chloride, dichloromethane, and methanol were purchased from Sigma-Aldrich (Saint Louis, MO). Sodium 2-ethylhexyl sulfate (SE8S), sodium decyl sulfate (SD10S), sodium dodecyl sulfate (SD12S), sodium tetradecyl sulfate (ST14S), and dodecanoic acid (D11COOH) were purchased from Sigma (Saint Louis, MO). Sodium mono-n-dodecyl phosphate (SD12P) was purchased from Alfa Aesar (Ward Hill, MA). Phenol (Ph), 2,4-dimethyl phenol (DMP), 4-hydroxyphenylacetic acid (HPA), trans-4-hydroxy-3-methoxycinnamic acid (FA), 4,6-dinitro-o-cresol (DNOC), and vanillic acid (VA) were obtained from Sigma. Stock solutions (5 mmol L-1) of phenols were prepared weekly in methanol and stored at 4 °C until use. Aqueous solutions of phenols and surfactants were prepared using 18 MΩ·cm water (NANOpure Diamond, Barnstead; Dubuque, IA) and were filtered using a hollow-fiber filter (0.2 µm, (44) Nagata, H.; Tabuchi, M.; Hirano, K.; Baba, Y. Electrophoresis 2005, 26, 2247-2253. (45) Atkin, R.; Craig, V. S.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236-244. (46) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219-304. (47) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1403. (48) Spildo, K. In Encyclopedia of Surface and Colloid Science.; Hubbard, A. T., Ed.; Marcel Dekker, Inc.: New York, 2002; Vol. 1, pp 465-481.

Barnstead). The pH of the solutions was adjusted using either 1 M NaOH or 1 M HCl (Fisher Scientific) and measured using a glass electrode and a digital pH meter (Orion 420A+, Thermo; Waltham, MA). The background electrolyte (BGE) used for all the experiments was prepared with 10 mmol L-1 tetraborate buffer and 20 mmol L-1 NaCl (pH 9.2) in order to minimize changes in the µEOF due to variations in the ionic strength and viscosity of the electrolyte. Apparatus and Procedures. Unless noted, a BeckmanCoulter P/ACE MDQ (Fullerton, CA) capillary electrophoresis system and fused-silica capillaries (50 µm i.d. × 375 µm o.d.. × 30 cm long; Polymicro Technologies; Phoenix, AZ) were used. Data acquisition was performed using Karat 32 software (Beckman-Coulter) on an IBM personal computer. Unless otherwise noted, samples were introduced into the capillary by a 5-s, 0.5 psi pressure injection (∼6.5-nL sample plug)49 and subsequently separated (E ) 333.3 V/cm) at 25 °C. Direct UV detection was performed using a wavelength of 280 nm, through the capillary at a window located 20 cm from the inlet. Measurement of the cmc Values. The corresponding cmc values were measured at 25 °C, according to a procedure described by Jacquier and Desbene.50 Briefly, the electrophoretic mobility of a neutral marker (naphthalene, Sigma-Aldrich) as a function of surfactant concentration (in the premicellar and micellar regions) was followed. For these experiments, solutions containing increasing concentrations of surfactants were prepared by dissolving the corresponding amount of solid in the selected buffer (10 mmol L-1 tetraborate buffer + 20 mmol L-1 NaCl) and then adjusting the pH to 9.2, as necessary. In all the cases, bare silica capillaries were used. The obtained results were in good agreement with previously reported values obtained under similar conditions.50-54 Coating Procedures. Several procedures have been reported for coating silica capillaries with PDMS.9,38 For these experiments, the silica capillaries were initially preconditioned by sequentially rinsing them with 0.1 M NaOH (4 min), water (10 min), and dichloromethane (5 min). A fresh, uncured 10:1 (w/w) mixture of Sylgard 184 silicone elastomer and curing agent (Dow Corning; Midland, MI) was dissolved in dichloromethane to form a 10% (w/v) PDMS solution. Then, the capillaries were rinsed with the 10% (w/v) PDMS solution in dichloromethane for 10 min, dried with N2 for 12 min, and finally cured in a convective oven for 2 h at 65 °C. In order to mimic the procedure used to fabricate PDMS microchips, the PDMS-coated capillaries were placed in an air plasma cleaner (Harrick PDC-32G Plasma Cleaner/Sterilizer; Ithaca, NY) for 1 min to oxidize the PDMS surface and used within the next 30 min. As previously reported, the PDMS-coated capillaries9,38 showed the same behavior (µEOF and adsorption behavior) as standard PDMS microchips,7,21,55 while providing very reproducible results, autosampling capabilities, low cost, and (49) McLaren, D. G.; Boulat, O.; Chen, D. D. Electrophoresis 2002, 23, 19121920. (50) Jacquier, J. C.; Desbene, P. L. J. Chromatogr., A 1995, 718, 167-175. (51) Sehgal, P.; Mogensen, J. E.; Otzen, D. E. Biochim. Biophys. Acta 2005, 1716, 59-68. (52) Gharibi, H.; Rafati, A. A. Langmuir 1998, 14, 2191-2196. (53) Kanicky, J. R.; Shah, D. O. Langmuir 2003, 19, 2034-2038. (54) Walde, P.; Wessicken, M.; Radler, U.; Berclaz, N.; Conde-Frieboes, K.; Luisi, P. L. J. Phys. Chem. B 1997, 101, 7390-7397. (55) Ding, Y.; Mora, M. F.; Garcia, C. D. Anal. Chim. Acta 2006, 561, 126132.

temperature control. Because the material used in these experiments is the same material used in soft lithography, the results could be directly correlated to PDMS microchips. Additional experiments comparing PDMS and dichloromethane-dissolved PDMS can be found in the Supporting Information. These experiments also support the possibility of using PDMS-coated capillaries as an alternative to PDMS microchips. For some of the experiments (see Results and Discussion), the PDMS capillaries were preconditioned and then coated with an additional layer of a cationic polymer, PDDA. In this case, the capillaries were initially rinsed with a 0.8% (w/v) solution of PDDA in water for 30 min, followed by rinsing with a 0.08% (w/v) PDDA solution for 10 min, and finally rinsed with acetate buffer (20 mmol L-1, pH 4.7) for 5 min, as described by Luong et al.56 When plain fused-silica or PDMS-coated capillaries were silanized, a procedure described by Zhang et al. was used (capillary rinsed with a 2% (w/v) APTES solution in methanol for 2 h, at room temperature).57 µEOF Measurements. Capillary coatings are usually characterized by µEOF measurements. Different methods have been used to measure the µEOF,15,58-61but probably the two most common are the current monitoring62 and the neutral marker method.59 It has been previously reported that both methods generate similar results.9 In this work, silica capillaries coated with a PDMS layer and a standard CE instrument were used to measure the µEOF by the neutral marker method. To calculate the µEOF, the migration time of a neutral marker (methanol) was used. The reported µEOF values correspond to the average and standard deviation calculated from at least five consecutive runs. In all the experiments, the µEOF for bare PDMS (no surfactant added) was first measured (µEOF-PDMS). Next, the capillary was thoroughly rinsed (5 min, 20 psi) with the surfactant-containing background electrolyte and the µEOF measured again (µEOF-SURF). Then, the change in µEOF (∆µEOF ) µEOF-SURF - µEOF-PDMS) was calculated and plotted as function of the surfactant concentration. RESULTS AND DISCUSSION In order to ensure that the observed increases in µEOF correspond only to changes in the PDMS surface (due to an adsorption process) and not to changes in the solution, the µEOF as a function of SD12S concentration was investigated using silica and PDMS-coated silica capillaries. The obtained results are shown in Figure 1. SD12S was selected for these experiments because it does not adsorb on silica surfaces63,64 and therefore does not change the ζ potential of the capillary wall. According to our results, when silica capillaries are used, the migration time of the neutral marker (and consequently, the µEOF) was not affected by (56) Luong, J. H.; Bouvrettea, P.; Liu, Y.; Yang, D. Q.; Sacher, E. J. Chromatogr., A 2005, 1074, 187-194. (57) Zhang, Z. L.; Crozatier, C.; Le Berre, M.; Chen, Y. Microelectron. Eng. 2005, 78-79, 556-562. (58) Pittman, J. L.; Henry, C. S.; Gilman, S. D. Anal. Chem. 2003, 75, 361-370. (59) Culbertson, C. T.; Ramsey, R. S.; Ramsey, J. M. Anal. Chem. 2000, 72, 2285-2291. (60) Martin, R. S.; Gawron, A. J.; Lunte, S. M. Anal. Chem. 2000, 72, 31963202. (61) Gilman, S. D.; Chapman, P. J. In Microchips Capillary Electrophoresis. Methods and Protocols; Henry, C. S., Ed.; Humana Press: Totowa, NJ, 2006; pp 187202. (62) Huang, X.; Gordon, M. J.; Zare, R. N. Anal. Chem. 1988, 60, 1837-1838. (63) Berthod, A.; Girard, I.; Gonnet, C. Anal. Chem. 1986, 58, 1356-1358. (64) Thibaut, A.; Misselyn-Bauduin, A. M.; Grandjean, J.; Broze, G.; Jerome, R. Langmuir 2000, 16, 9192-9198.

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Figure 1. Effect of the addition of SD12S to the background electrolyte when bare silica (0) or PDMS-coated capillaries (b) are used. Conditions: 10 mmol L-1 tetraborate, 20 mmol L-1 NaCl (pH 9.2), applied potential 10 kV.

the addition of SD12S to the background electrolyte (in the 0-0.5 mmol L-1 range). Because the same µEOF values were obtained, we can also conclude that the addition of SD12S (up to 0.5 mmol L-1) does not significantly modify the viscosity of the solution (see Supporting Information). On the other hand, when PDMS-coated silica capillaries were used, the addition of SD12S produced a significant increase in µEOF (µEOF with no SD12S ) (4.8 ( 0.2) × 10-4 cm2 V-1 s-1; µEOF with 0.5 mmol L-1 SD12S ) (7.3 ( 0.2) × 10-4 cm2 V-1 s-1). This increment in µEOF (in the 0-0.5 mmol L-1 range) can therefore be only attributed to a change in the ζ potential, which ultimately depends on the surface charge and can be affected by the adsorption of surfactants.5,7,33,36,40,43 As a result, the change in µEOF (∆µEOF ) µEOF-SURF - µEOF-PDMS) can be directly correlated to the adsorbed amount of surfactant, and the plot ∆µEOF versus surfactant concentration represents an adsorption isotherm. Furthermore, the maximum µEOF value obtained at high surfactant concentrations corresponds to surface saturation. The slight decrease in µEOF observed at SD12S concentrations higher than 0.5 mmol L-1 (in both silica and PDMS-coated capillaries) could be attributed to a combination of binding of counterions (Na+) to the capillary wall,2 and slight changes in the ionic strength and viscosity of the solution, probably produced by initial stages of the formation of micelles in solution. Additional experiments supporting this explanation are detailed in the Supporting Information. Similar decreases were also observed for the other surfactants (see Figure 3). Because this work is focused on studying the effects of the addition of surfactants to the capillary surface (and not in the solution), this decrease in µEOF was not used for the data analysis. As can be also observed in Figure 1, smaller error bars were obtained for silica capillaries (when compared to PDMS-coated silica). Further details regarding the interaction of surfactants with PDMS are discussed in the following paragraphs. Effect of Surface Charge. At any interface, there is always an unequal distribution of electrical charges between the two phases, which allows the development of a potential difference across the interface. This potential of the electrical double layer has been exploited to adsorb cationic surfactants on hard anionic surfaces, and it is responsible not only for their typical 6678 Analytical Chemistry, Vol. 79, No. 17, September 1, 2007

Figure 2. Variation on the µEOF for different concentrations of SD12S in PDMS (9), silanized PDMS (b), and PDDA-coated PDMS (2). Conditions: PDMS-coated capillary, 10 mmol L-1 tetraborate, 20 mmol L-1 NaCl (pH 9.2), applied potential 10 kV. Lines included to guide the eye.

head-to-surface orientation but also for the formation of surface aggregates.24,26,30,32,34,45,65-67 Although the PDMS surface is generally considered hydrophobic,12,13 it has randomly distributed negative charges at the surface that are responsible for the µEOF.1,8,43 These negative charges however, may drive surfactants to interact with the surface with a particular orientation. Consequently, the effect of the surface charge on the adsorption process of SD12S was studied using bare PDMS (negatively charged), silanized PDMS (neutral), and PDMS coated with a cationic polymer (PDDA). As can be observed in Figure 2, the highest change in the µEOF (∆µEOF) was observed when SD12S was adsorbed to the PDDA-coated PDMS (positively charged). This observation is not surprising because SD12S can now interact with PDMS through hydrophobic and (favorable) electrostatic interactions. In the case of silanized PDMS, the adsorption was also higher than bare PDMS. This behavior could be attributed to the absence of repulsive negative charges on the capillary wall, allowing the adsorption of higher amounts of SD12S to the PDMS surface. It is important to note that when the silica capillary was silanized with APTES,57 the addition of 0.5 mmol L-1 SD12S to the background electrolyte induced a significant increase in the µEOF (µEOF with no SD12S ) (0.33 ( 0.01) × 10-4 cm2 V-1 s-1; ∆µEOF with 5 mmol L-1 SD12S ) (2.4 ( 0.2) × 10-4 cm2 V-1 s-1). These findings indicate that the absence of the characteristic hydrophobic sites of PDMS precludes the adsorption of SD12S to silica and highlight the importance of the hydrophobic interactions in the adsorption of alkyl surfactants to PDMS. Effect of Tail Length. It is well-known that the hydrophobicity of the surfactants can significantly affect the adsorption behavior. For that reason, the change in the ∆µEOF as function of the concentration of the four alkyl sulfates (with alkyl chains ranging from 8 to 14 carbon atoms) was studied. As can be observed in Figure 3, all surfactants produced a significant increase in the µEOF of the PDMS-coated capillary. As previously reported,7,37-39,68 this increase in µEOF was attributed to an increase in the surface (65) Zhmud, B.; Tiberg, F. Adv. Colloid Interface Sci. 2005, 113, 21-42. (66) Rojas, O. J.; Claesson, P. M.; Berglund, K. D.; Tilton, R. D. Langmuir 2004, 20, 3221-3230. (67) Nagamine, N.; Nakamura, H. Anal. Sci. 1998, 14, 405-406.

Figure 3. Variation on the µEOF for different concentrations of SE8S (9), SD10S (b), SD12S (2), and ST14S ([). Conditions: PDMS-coated capillary, 10 mmol L-1 tetraborate, 20 mmol L-1 NaCl (pH 9.2), applied potential 10 kV. Lines correspond to the best fitting using eq 3.

Figure 4. Variation on the µEOF for different concentrations of SD12S (2), SD12P (9), and D11COOH (b). Conditions: PDMS-coated capillary, 10 mmol L-1 tetraborate, 20 mmol L-1 NaCl (pH 9.2), applied potential 10 kV. Lines correspond to the best fitting using eq 3.

charge, produced by the adsorption of the anionic surfactant to the capillary wall. Although it is not evident (because of the logarithmic scale in Figure 3), the slope of the adsorption isotherms systematically increases as the chain length increases, indicating more favorable hydrophobic surface-monomer interactions. Consequently, the highest slope was obtained for ST14S, for which the lowest concentration was required to produce significant increases in the µEOF. The highest affinity of ST14S for PDMS could be attributed to its highest hydrophobic character, when compared to the rest of the surfactants. In fact, the measured cmc for ST14S (0.7 mmol L-1) is 185 times smaller than the measured cmc for SE8S (139 mmol L-1). Importantly, similar ∆µEOF -4, SD S (1.96 ( 0.02) × 10-4, MAX values (SE8S (2.01 ( 0.03) × 10 10 -4 SD12S (2.35 ( 0.02) × 10 , and ST14S (2.34 ( 0.05) × 10-4 cm2 V-1 s-1) were obtained at 1 mmol L-1 for SE8S, SD10S, and SD12S, or 0.025 mmol L-1 for ST14S. These results suggest that, regardless of the structure, the PDMS surface is saturated at those concentrations. It is also important to point out that in order to avoid the formation of micelles (which could affect the tM of the neutral marker), the surfactant concentrations used in all the experiments were lower than the corresponding cmc values (vide infra, Table 1). The observation that a single plateau was obtained in all the adsorption isotherms suggests that the arrangement of surfactants on the surface is similar in all the studied cases and that no aggregates are present on the PDMS surface.28,30,45,46,69 Furthermore, the fact that the µEOF is significantly affected by the addition of surfactants at concentrations below the cmc indicates that these surfactants adsorb as monomers and not as micelles or hemimicelles. These are significant differences with respect to previously reported systems involving cationic surfactants.28,35,36,45 Effect of Head Group. The structure of the adsorbed layer depends on the packing of the molecules, which, at high surface concentrations, also depends on the surfactant-surfactant repul-

sions and steric constrains among adsorbate species.30,70 In order to investigate the effect of the type and charge of head group on the adsorption isotherms, a set of surfactants with different head groups (SD12S, SD12P, D11COOH) was selected. It is important to note that, at the selected conditions (pH 9.2), SD12S and D11COOH have only one formal negative charge, while SD12P has two negative formal charges.71 As shown in Figure 4, all the surfactants showed similar adsorption profiles, in which the µEOF (or ∆µEOF) increased with the surfactant concentration until a constant value was reached. The first remarkable difference is the lower affinity of D11COOH with respect to SD12S (see calculated values in Table 1). This difference could be attributed to the fact that D11COOH has one less aliphatic carbon than SD12S. The second difference is that although SD12P has a similar affinity for the surface (when compared to SD12S), lower ∆µEOF MAX values were obtained ((1.8 ( 0.1) × 10-4 cm2 V-1 s-1). These results could be attributed to a combination of electrostatic repulsions of adsorbed SD12P molecules (which limit the adsorption process) and differential binding of counterions (Na+, in this case) to the capillary wall.2 Data Treatment. In order to correlate the surfactant affinity with a readily available parameter (such as the cmc), the isotherms in Figure 3 and Figure 4 were processed to obtain the affinity constant (K). For that purpose, eq 2 was rewritten as eq 3, where

(68) Wang, A.-J.; Xu, J.-J.; Zhang, Q.; Chen, H.-Y. Talanta 2006, 69, 210-215. (69) Lucy, C. A.; Baryla, N. E.; Yeung, K. K. In Capillary Electrophoresis of Proteins and Peptides: Methods in Molecular Biology; Strege, M. A., Lagu, A. L., Eds.; Humana Press: Totowa, NJ, 2004; Vol. 276, pp 1-14.

KC ∆µEOF ) ∆µEOFMAX 1 + KC

(3)

Γ was replaced by the change in µEOF (∆µEOF) and the maximum adsorbed amount (Γmax) was replaced by ∆µEOF MAX. For the sake of convenience, PDMS was treated as a homogeneous surface, and neither lateral interactions between surfactant molecules nor the effect of supporting electrolyte was (70) Sivakumart, A.; Somasundamn, P. Langmuir 1994, 10, 131-134. (71) Lukas, D. S.; Peter, W.; Luisi, P. L.; Wilfred, F. v. G. Eur. Biophys. J. 2001, 30, 330-343.

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Table 1. cmc Valuesa and Best-Fitting Parametersb Obtained for the Selected Surfactants

SE8S SD10S SD12S ST14S D11COOH SD12P

cmc (mmol L-1)

∆µEOF MAX (10-4 cm2 V-1 s-1)

K (103 mol-1 L)

R2

139 15 2 0.7 7 4

2.5 ( 0.2 2.0 ( 0.1 2.4 ( 0.1 2.4 ( 0.1 2.2 ( 0.1 1.8 ( 0.1

3.1 ( 0.5 5.5 ( 0.4 100 ( 10 1100 ( 80 20 ( 1 160 ( 20

0.980 0.992 0.990 0.995 0.993 0.980

a Measured in 10 mmol L-1 tetraborate, 20 mmol L-1NaCl, pH 9.2, and applied potential 10 kV.82 b Calculated using eq 3.

Figure 5. Correlation between the affinity constant of single-charged alkyl surfactants (calculated using eq 3) for the PDMS surface and their corresponding 1/cmc value (measured in 10 mmol L-1 tetraborate, 20 mmol L-1NaCl, pH 9.2, and applied potential 10 kV). Line included to guide the eye.

considered.72 As shown in Figure 3 and Figure 4 (where points correspond to experimental data and lines correspond to the best fitting obtained using eq 3), the adsorption isotherms showed a good agreement with Langmuirian behavior (R2 > 0.98). Although the report of Langmuir-like isotherms is infrequent in the literature, our experimental results are in agreement with previous reports describing the adsorption of SD12S to other hydrophobic materials.72,73 Because only one plateau was observed, these fittings also indicate that the formation of hemimicelles (on the surface) is rather improbable under the selected experimental conditions.23,30,46,74,75 Table 1 summarizes the fitting parameters obtained for ∆µEOF MAX, K, and the correlation coefficient (R2) as well as the corresponding cmc (measured under the selected experimental conditions) for the selected alkyl surfactants. As can be observed, the affinity for the surface (K) is higher for surfactants with longer aliphatic chains, which are in fact more hydrophobic. For example, the calculated affinity of ST14S for PDMS is ∼355 times higher than the corresponding value obtained for SE8S (see Table 1). It has been reported that the stability of dynamic coatings could be correlated with the respective cmc of the adsorbate.26,28,34,45 (72) Gurkov, T. D.; Dimitrova, D. T.; Marinova, K. G.; Bilke-Crause, C.; Gerber, C.; Ivanov, I. B. Colloids Surf., A 2005, 261, 29-38. (73) Wolgemuth, J. L.; Workman, R. K.; Manne, S. Langmuir 2000, 16, 30773081. (74) Diamant, H.; Ariel, G.; Andelman, D. Colloids Surf., A 2001, 259, 183185. (75) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5418-5425.

6680 Analytical Chemistry, Vol. 79, No. 17, September 1, 2007

Because no such data were available, the affinity constant of each surfactant for PDMS (calculated using eq 3) was correlated to each corresponding 1/cmc value. By doing this, it would be possible to estimate the affinity constant of other anionic surfactants and then calculate the corresponding change in the µEOF of a PDMS channel upon the addition of other alkyl surfactants (at a known concentration). Because the simplest model was used, surfactants with different charge (SD12P) and different surfaces (vide supra) were omitted. As expected (see Figure 5), higher cmc values lead to lower affinities for the PDMS surface. The main reason for this behavior is that, while the number of carbons is just an indication of the hydrophobicity of the tail, the cmc accounts for both, the hydrophobic character and the electrostatic interactions of the head group. Although these results (obtained with single-charged alkyl surfactants) seem to follow a clear trend, they come in contrast to adsorption experiments performed with different surfactants such as SD12P (2-), PA (cmc ) 0.05 mmol L-1),76 and sodium deoxycholate (DOCh, cmc ) 5 mmol L-1).77 As reported elsewhere,7 the structure and molecular weight of PA and DOCh engender a significant effect on their interaction with PDMS. It is clear that more tests still need to be done in order to propose a rational modification scheme that fits most surfactants. It has been reported78 that silica exposes 4-5 silanol groups/ nm2. This charge allows the development of a µEOF of (5.76 ( 0.01) × 10-4 cm2 V-1 s-1 (under the experimental conditions selected for these studies, Figure 1). Therefore, if is assumed that only the exposed negative charges are responsible for the generation of µEOF in PDMS,1,43 and that the ζ potential is proportional to the surface charge,22,79 it can be estimated (from µEOF PDMS ) (4.3 ( 0.2) × 10-4 cm2 V-1 s-1, n ) 5, Figure 1) that bare PDMS exposes ∼3-4 negative groups/nm2. Because the ∆µEOF was attributed to a change in surface charge, the average maximum ∆µEOF value could be correlated to a maximum adsorbed amount of 2 surfactant molecules/nm2 of PDMS. In order to support this estimation, the cross sectional area of SD12S was calculated using a computational model (HyperChem v6.03 and PM3 optimization). According to the model (see Supporting Information), a molecule of SD12S can be described as a cylinder with a cross sectional area of 0.46 nm2, which would allow the adsorption of a maximum of 2 surfactant molecules/ nm2 of PDMS. These results indicate that the studied surfactants might form a single layer, with the hydrophobic tails orienting toward (or grafted in) the PDMS surface, and that head-to-head interactions are minimized (except in the case of SD12P), probably due to the high concentration of ions in the BGE (10 mmol L-1 tetraborate + 20 mmol L-1 NaCl). These results are also in good agreement with a model recently proposed by Baba’s group.44 Effect of Surfactants on the Separation of Phenolic Compounds. In order to analyze electrophoretic effects of surfactants with different chain lengths, the separation of six phenolic compounds was studied. In order to saturate the PDMS surface, the concentration of surfactant used was 1 mmol L-1 for SE8S, SD10S, and SD12S and 0.01 mmol L-1 for ST14S. The (76) Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544-6554. (77) Matsuoka, K.; Moroi, Y. Biochim. Biophys. Acta 2002, 1580, 189-199. (78) Horvath, J.; Dolnik, V. Electrophoresis 2001, 22, 644-655. (79) Sefcik, J.; Verduyn, M.; Storti, G.; Morbidelli, M. Langmuir 2003, 19, 47784783.

Table 2. Migration Times, Plate Numbers, and Skew for the Separation of Six Selected Phenolic Compounds Using PDMS-Coated Capillary with SE8S, SD10S, SD12S, and ST14Sa PDMS

DMP Ph DNOC HPA FA VA a

SE8S

SD10S

SD12S

ST14S

tM

N

S

tM

N

S

tM

N

S

tM

N

S

tM

N

S

2.54 2.87 4.35 4.75 6.04 7.25

31000 45000 51000 49000 47000 42000

0.86 0.79 0.91 1.02 0.96 1.22

2.08 2.33 3.01 3.29 3.75 4.18

17000 34000 55000 55000 51000 50000

0.83 0.82 0.92 1.00 1.05 1.02

1.88 2.03 2.60 2.75 3.08 3.35

17000 32000 47000 46000 46000 47000

0.89 0.87 1.01 0.98 1.05 1.02

1.83 1.99 2.52 2.68 2.99 3.24

15000 29000 51000 49000 55000 54000

0.88 0.86 0.95 0.96 0.98 1.05

1.79 1.92 2.44 2.57 2.87 3.10

16000 29000 50000 48000 49000 52000

0.87 0.84 0.93 0.98 1.00 0.97

Other conditions as described in Figure 6. Migration time (tM) expressed in minutes. N, number of theoretical plates/m. S, peak skew.

Figure 6. Electropherograms obtained in surfactant-coated capillaries corresponding to a mixture of 35.6 µg/mL DMP (1), 39 µg/mL phenol (2), 15.9 µg/mL DNOC (3), 31.8 µg/mL HPA (4), 14.5 µg/mL FA (5), and 15.3 µg/mL VA (6). (A) No added surfactant, (B) 1 mmol L-1 SE8S, (C) 1 mmol L-1 SD10S, (D) 1 mmol L-1 SD12S, and (E) 0.1 mmol L-1 ST14S. Conditions: 10 mmol L-1 tetraborate, 20 mmol L-1NaCl (pH 9.2), applied potential 10 kV.

corresponding electropherograms obtained are shown in Figure 6. As the result of the adsorption of surfactants to the PDMS surface, reductions in the migration times were observed for all the surfactants, when compared to bare PDMS. Improvements in peak skew (S) were also observed, particularly in the peaks corresponding to FA and VA. These improvements in peak shape can be attributed to a decrease in hydrophobic interactions between the PMDS surface and the analytes. The migration times, theoretical plates, and skew obtained for the six phenols are summarized in Table 2. Examples about the effect of surfactants on the electrophoretic separation of other analytes have been also previously reported.26,34,38,69,80,81 (80) Melanson, J. E.; Baryla, N. E.; Lucy, C. A. TrAC, Trends Anal. Chem. 2001, 20, 365-374. (81) Pranaityte, B.; Padarauskas, A. Electrophoresis 2006, 27, 1915-1921. (82) Lin, C.-E.; Wang, T.-Z.; Chiu, T.-C.; Hsueh, C.-C. J. High Resolut. Chromatogr., 1999, 22, 265-270.

CONCLUSIONS The adsorption of alkyl surfactants on PDMS was studied by performing electroosmotic flow (µEOF) measurements. According to the results, the adsorption of alkyl surfactants with the PDMS surface is driven by a combination of hydrophobic and electrostatic interactions, where the former was more significant than the latter. The adsorption isotherms, obtained with surfactants of different chain lengths and head groups, showed a Langmuirian behavior. In general, the affinity constant increases as the cmc decreases. Although the amount required to obtain the maximum µEOF value depends on the structure of the selected surfactant, all the selected surfactants produced an increase in µEOF, a decrease in the analysis time, and improvements in the peak shape, with respect to bare PDMS. According to our results, it is possible to use the corresponding cmc value to estimate the change in µEOF upon addition of single-charged surfactants at a certain concentration. Further experiments are still required to develop a theoretical model that fits most of the surfactants and allows a more rational selection of the coating conditions. ACKNOWLEDGMENT This project was financially supported by The University of Texas at San Antonio. Authors thank Grant Merrill (Department of Chemistry at UTSA) for the calculations involving HyperChem, Dave Johnson’s lab (Department of Chemistry at UTSA) for their help with the NMR experiments, and Dr. Denes Marton (Department of Radiology at UTHSCSA) for the XPS measurements. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review May 10, 2007. Accepted June 28, 2007. AC070953G

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