Polyelectrolyte Adsorption onto a Surface-Confined Surfactant

Julio Martinez, Raisa Talroze, Erik Watkins, Jaroslaw P. Majewski, and Pieter Stroeve. The Journal of Physical Chemistry C 2007 111 (26), 9211-9220...
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Langmuir 2003, 19, 2243-2248

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Polyelectrolyte Adsorption onto a Surface-Confined Surfactant Alexander B. Artyukhin, Kevin J. Burnham,† Andrei A. Levchenko,‡ Raisa V. Talroze, and Pieter Stroeve* Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 Received October 1, 2002. In Final Form: December 5, 2002 Using sodium dodecyl sulfate (SDS) as a surfactant and poly(diallyldimethylammonium chloride) (PDDA) as a polyelectrolyte, we formed a surfactant-polymer complex on a model surface of a positively charged 2-aminoethanethiol (cysteamine) self-assembled monolayer on gold. The complex formation is achieved by first SDS adsorption on cysteamine and subsequent adsorption of PDDA on the preformed SDS layer. Successful deposition of the PDDA layer on SDS without noticeable surfactant desorption shows that the complex formation is a fast process. The presence of SDS-PDDA complex on a cysteamine-modified gold substrate was observed by surface plasmon resonance (SPR) and Fourier transformed infrared spectroscopy. The thicknesses obtained from the SPR data were 1.5 nm for SDS and 1.1 nm for PDDA. Concentrations of SDS and PDDA in the parent solutions necessary to yield the complex upon adsorption were determined.

Introduction There is little understanding on the molecular level of interactions between surface-active species and polyelectrolytes at solid-liquid interfaces. Likewise, there is a lack of knowledge on the self-assembly properties of surfactants and polymers at interfaces. Knowledge about these self-assembled structures is important for applications in coatings, surface cleaning, wetting, cosmetics, controlled release, biosensors, and biomaterials.1-4 For example, there has been considerable interest in ultrathin molecular films made from the self-assembly of surfactants and ionic polymers at the air-water interface.5-7 The presence of surfactants gives surface tension lowering, and the use of ionic polymers can give rise to robust, ultrathin films. An area of research is the assembly of multilayers of alternating surfactants and ionic polymers to create semipermeable membranes for separating ions and low molecular weight species. By using different types of surfactants and ionic polymers in the multilayer stack, selective membranes can be designed. Another interest is in using ionic polymers with surfactants at the solidwater interface for selective barrier coatings for biosensors. It is also possible to use the self-assembly of surfactants on the solid-water interface to pattern the deposition of * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (530) 752-8778. Fax: (530) 7521031. † Present address: Department of Chemistry, University of California Davis, Davis, CA 95616. ‡ Present address: A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia. (1) Goddard, E. D. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabbau, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Uchida, M.; Kunitake, T.; Kajiyama, T. New Polym. Mater. 1994, 4, 199-211. (3) Tiberg, F.; Brinck, J.; Grant, L. Curr. Opin. Colloid Interface Sci. 1999, 4, 411-419. (4) Thunemann, A. F.; General, S. J. Controlled Release 2001, 75, 237-247. (5) Stroeve, P.; VanOs, M.; Kunz, R.; Rabolt, J. F. Thin Solid Films 1996, 285, 200-203. (6) Stroeve, P.; Hwa, M. J. Thin Solid Films 1996, 285, 561-563. (7) Bruinsma, P. J.; Stroeve, P.; Hoffmann, C. L.; Rabolt, J. F. Thin Solid Films 1996, 285, 713-717.

the polyelectrolyte on the surfactant. The deposition should lead to surface properties that are highly nonisotropic. Recent atomic force microscopy (AFM) studies3,8-18 of soluble ionic surfactants on different surfaces show they do not cover surfaces in uniform monolayers. AFM images of surfactants at the solid-liquid interface exhibit complex micelle formation at surfactant concentrations above and below the critical micelle concentration (cmc). Hemimicelles are often formed by the adsorption of charged surfactants on uncharged surfaces. Spherical hemimicelles may be encountered for concentrations below the cmc, while cylindrical hemimicelles are found at concentrations above the cmc. The formation of full micellar structures is often found in the adsorption of charged surfactants on surfaces of opposite charge. Concentrations at or above the cmc can favor the formation of cylindrical micelles.3 Taking into account the complex morphology of the surfaces covered by self-assembled surfactants, the latter may be considered as templates to control the surface assembly of polymer macromolecules. The key to putting to use the desirable properties of surface self-assembled surfactants lies in forming uniformly templated surfaces with a definite topology for adsorption and alignment of functional macromolecules of polyelectrolytes such as polypeptides, proteins, DNA, or synthetic polymer semiconductors. Creating the structure in the nanometer scale (8) Liu, J. F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 48954903. (9) Fleming, B. D.; Wanless, E. J. Microsc. Microanal. 2000, 6, 104112. (10) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 85588567. (11) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (12) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (13) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602-7607. (14) Kiraly, Z.; Findenegg, G. H. J. Phys. Chem. B 1998, 102, 12031211. (15) Jaschke, M.; Butt, H. J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384. (16) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920. (17) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (18) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413.

10.1021/la026640s CCC: $25.00 © 2003 American Chemical Society Published on Web 02/04/2003

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is important for micromanufacturing applications, such as micro- and nanosensors, and controlled adsorption of biopolymers. The well-known process of polyelectrolyte layer-bylayer adsorption on solid-water interfaces19 is different than the process of deposition of surfactant and polyelectrolytes.20-23 In the former process, the deposition process is started from a charged surface by sequential adsorption of alternatively charged polyelectrolytes, while in the latter process low molecular weight surface-active species are used instead of one of the polyelectrolytes. In this work, we focus on the use of sequential adsorption of a surfactant and a polyelectrolyte on the solid-water interface to create self-assembled composite materials. Patterning of solid surfaces via self-assembly using either polymers or surfactants is of recent interest,24-30 but patterning by sequential adsorption of a surfactant and a polymer on solid surfaces has not yet been addressed. This paper deals with the first observation of sequential adsorption first of surfactant and then polyelectrolyte on a self-assembled monolayer (SAM) of thiol on gold. The surfactant chosen in this study, sodium dodecyl sulfate (SDS), is a widely used surface-active agent, and the polymer is poly(diallyldimethylammonium chloride) (PDDA). This polymer serves as a model, which should prove the whole concept and show whether the surfactantpolyelectrolyte complex is stable at the surface and at what conditions. The model surface used in this work is a cysteamine (2-mercaptoethylamine) SAM on gold. Experimental Section Materials. SDS was purchased from Fluka and recrystallized three times from ethanol. Poly(diallyldimethylammonium chloride), 20% aqueous solution, was obtained from Polysciences, Inc. Cysteamine (2-aminoethanethiol) was received from Sigma. Water for experiments was purified by a NanoPure Diamond system (Barnstead) and had a resistivity of 18.2 MΩ cm. Ethanol used for thiol solution preparation, SDS crystallization, and glass slide cleaning was 200 proof (Gold Shield Chemical Co.). The refractive index matching fluid, sulfur in 1-iodonaphthalene, was obtained from Cargille Laboratories Inc. Preparation of Gold-Covered Substrates. Two methods were used to deposit 99.999% gold on high-refractive-index LaSFN9 glass slides (Schott, Germany) that have yielded considerably different results in subsequent surfactant adsorption experiments. Prior to gold deposition in either method, glass slides were cleaned by sonication at 55 °C first in water, then in 2% Helmanex solution, and finally in ethanol, for 15 min each with thorough water rinses between sonications. In method 1, (19) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (20) McQuigg, D. W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973-1978. (21) Li, Y. J.; Dubin, P. L.; Dautzenberg, H.; Luck, U.; Hartmann, J.; Tuzar, Z. Macromolecules 1995, 28, 6795-6798. (22) Kasaikin, V. A.; Litmanovich, E. A.; Zezin, A. B.; Kabanov, V. A. Dokl. Akad. Nauk 1999, 367, 359-362. (23) Litmanovich, E. A.; Kasaikin, V. A.; Zezin, A. B.; Kabanov, V. A. Dokl. Phys. Chem. 2000, 373, 121-124. (24) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52-59. (25) Jiang, X. P.; Zheng, H. P.; Gourdin, S.; Hammond, P. T. Langmuir 2002, 18, 2607-2615. (26) Lee, I.; Jiang, X. P.; Zheng, H. P.; Chen, K. M.; Rubner, M. F.; Kimerling, L. C.; Hammond, P. T. Abstr. Pap.sAm. Chem. Soc. 2001, 221, 164-COLL. (27) Lee, I.; Zheng, H. P.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 572-577. (28) Zheng, H. P.; Jiang, X. P.; Lee, I.; Chen, K. M.; Kimerling, L. C.; Rubner, M. F.; Hammond, P. T. Abstr. Pap.sAm. Chem. Soc. 2001, 221, 277-COLL. (29) Zheng, H. P.; Rubner, M. F.; Hammond, P. T. Langmuir 2002, 18, 4505-4510. (30) Zheng, H. P.; Lee, I.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 569-572.

Artyukhin et al. about 500 Å of gold was deposited on glass slides by heating the gold target in a crucible with an electron beam at a pressure below 5 × 10-6 Torr, with a gold deposition rate of 0.2 Å/s. In method 2, the slides were baked in a vacuum chamber (Edwards AUTO 306) at 250 °C for 2 h before deposition and then cooled to 30 °C. Gold was then deposited in the vacuum chamber by thermal evaporation of gold in a current-heated molybdenum boat. During gold evaporation, the vacuum was better than 3 × 10-6 Torr, the rate of deposition was 0.2-0.5 Å/s, and gold was deposited to a final thickness of 500 ( 20 Å which was measured by a calibrated built-in quartz crystal. The exact thickness of the gold layer prepared by both methods was determined by surface plasmon resonance (SPR) spectroscopy in every experiment. Goldcoated slides were kept in ethanol and used within 10 days after preparation. Preparation of Thiol Monolayer. Gold-coated LaSFN9 glass slides were covered with cysteamine monolayer by immersing them into a 5 mM cysteamine solution in ethanol for at least 14 h. Slides were then rinsed thoroughly with pure ethanol and dried in a stream of nitrogen. Measurement of SDS Surface Concentration. Since the change in index of refraction is linear with respect to changes in surfactant concentration, one can use the following relation to determine the surface concentration of SDS:31,32

cs )

d(nf - ns) dn/dc

(1)

where cs is the equilibrium excess surface concentration of SDS (units of µmol/m2), d is the film thickness (nm), dn/dc is the incremental change in the refractive index of solution with an increase in surfactant concentration (3.74 × 10-5 mM, measured with the Abbe refractometer), nf is the index of refraction of the SDS film (nf ) 1.46), and ns is the refractive index of pure solvent, in our case water (ns ) 1.330). Flow Cell. The flow cell was designed so that fully developed laminar flow was achieved prior to a solution entering the SPR measurement area. The cell was fabricated from Teflon with an O-ring (Apple Rubber Products Inc.) for a seal. The flow channel was 14 mm wide and 64 mm long. The gap between the glass slide and the opposite Teflon wall was 0.25 mm. The SPR measurement area was 40 mm from the inlet. The laser beam of the SPR has a cross-sectional area of about 2 mm2. Solutions were pumped using a 100 mL syringe (Popper & Sons Inc.) with a Cole-Palmer 74900 syringe pump or a peristaltic MasterFlex Digistaltic 77340-00 pump (Cole-Parmer). Results were independent of the choice of the pump system. Connections between the cell and feeding syringes and the output were made from Upchurch Scientific tubes and fittings. Surface Plasmon Resonance. Formation of the SDS layer and subsequent PDDA adsorption were monitored by SPR. The method is based on excitation of the surface plasmons by p-polarized light at the noble metal-dielectric interface.33 Details of the experimental procedure are described elsewhere.34,35 All experiments were carried out at room temperature of 24 ( 1 °C. Polymer Adsorption. Poly(diallyldimethylammonim chloride), solution in water or in 5 mM phosphate 7.0 buffer, was injected into the SPR flow cell with 1 or 3 mL disposable Monoject syringes (Sherwood Medical) after SDS adsorption had reached an equilibrium. A 0.1-0.5 mL air bubble was injected ahead of the PDDA solution in the same syringe to separate bulk SDS and PDDA solutions and prevent their mixing. Fourier Transform Infrared (FTIR) Spectroscopy of the Surface SDS-PDDA Complex. Fourier transform infrared spectra were collected on a Nicolet Prote´ge´ 460 setup equipped with a MCT detector in grazing angle reflection mode. Ten thousand scans were taken for each spectrum at 4 cm-1 (31) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (32) Defeijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759-1772. (33) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569-638. (34) Levchenko, A. A.; Argo, B. P.; Vidu, R.; Talroze, R. V.; Stroeve, P. Langmuir 2002, 18, 8464-8471. (35) Zhang, L. Q.; Longo, M. L.; Stroeve, P. Langmuir 2000, 16, 50935099.

Adsorption onto a Surface-Confined Surfactant

Figure 1. Isotherm of SDS adsorption on a cysteamine SAM on gold. Open symbols are for evaporated gold, and closed symbols are for e-beam-deposited gold: circles, recrystallized SDS in water; triangles, uncrystallized SDS in water; squares, recrystallized SDS in 5 mM phosphate 7.0. Error bars are shown for experiments repeated more than one time. Lines are drawn to aid the eye: solid, e-beam-deposited gold in water; dotted, evaporated gold in water; dashed, evaporated gold in 5 mM phosphate 7.0. resolution. Gold-covered LaSFN9 glass slides were used as substrates. The SDS layer was prepared by dipping the gold slide modified with cysteamine SAM into aqueous 8.1 mM SDSd25 solution for up to 1 h, and the slide was then pulled out of the solution and dried without rinsing. Cysteamine SAM on gold was used as a background. To obtain a surfactant/polyelectrolyte complex on the surface, a cysteamine-modified gold-coated slide was precoated with SDS-d25 as before and then dipped into a beaker with 0.2 M PDDA solution for 10 min. The PDDA solution was subsequently exchanged with pure water for seven volumes using a pump. During the rinse, the slide was always kept below the water-air interface. A small amount of perhydro SDS (1 mg/mL SDS in n-butanol as the spreading solution) was spread onto the water surface in the beaker using a microliter syringe, and the slide was then quickly removed from the solution and dried. This procedure was found to be necessary since no complex between SDS-d25 and the polymer was observed when the procedure was omitted. Transfer of the slide through the pure water-air interface with its high surface tension may desorb the SDS-PDDA complex on the SAM by causing the complex to spread on the water-air interface. For the background in this case, a cysteamine-modified gold substrate was immersed into 0.2 M PDDA solution and the subsequent procedure was the same as for the sample slide. The only difference is that the background slide was not in contact with SDS-d25 solution. This choice of the background would compensate for IR signals arising from probable surface contamination resulting from additional treatment. All drying was performed in ambient atmosphere.

Results and Discussion SDS Adsorption. We studied adsorption of anionic surfactant SDS on a monolayer of positively charged thiol cysteamine self-assembled on gold (Figure 1). Adsorption of SDS on cysteamine is governed by electrostatic attraction between dodecyl sulfate anions and positively charged ammonium headgroups of cysteamine chemisorbed on gold. Even though the exact pK of surface-confined cysteamine is not known, one may expect that it bears a positive charge in neutral aqueous solutions. Primary amines are well-known to be strong proton acceptors, and the pKa value of 2-hydroxyethylammonium in bulk water is 9.50 at 25 °C.36 Significant research has been done to (36) CRC Handbook of chemistry and physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992.

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determine the pKa values of different compounds on surfaces, and it is generally accepted that the pKa values are different from the bulk. To the best of our knowledge, only one work was devoted to studying basic properties of surface-adsorbed amines. It has been shown that the pKa of protonated forms of 4-mercaptoaniline and 4-mercaptopyridine SAMs on gold is higher than that in bulk37 but these compounds are considerably weaker bases than primary aliphatic amines. From this result, we can only conclude that pKa of protonated cysteamine on gold should be at least 9.5 or higher. To ensure consistency in degree of cysteamine protonation, we performed a series of adsorption experiments in 5 mM phosphate buffer at pH 7.0. Addition of the salt decreases the cmc of the surfactant,38 and at this ionic strength the cmc of SDS is 5.8 mM while in pure water it is 8.1 mM.39 At SDS concentrations higher than 0.5 cmc, substitution of buffer for water has no effect on the amount of SDS adsorbed (Figure 1). At lower SDS concentrations, less surfactant is adsorbed in buffer than in water. This difference is due to the screening of electrostatic interaction between SDS and cysteamine. At concentrations of SDS higher than 0.5 cmc (3-4 mM SDS), the ionic strength of the solution is primarily set by the surfactant itself and addition of 5 mM buffer increases it only 3 times. On the other hand, at 0.1 cmc (0.6-0.8 mM SDS) 5 mM buffer brings about a 10-fold increase in the ionic strength. Since the characteristic length of electrostatic interaction is inversely proportional to the square root of the ionic strength, the effect is further enhanced at lower concentrations. In a previous paper, we found that recrystallization of SDS was necessary and gave thinner surfactant layers on hydrophobic undecanethiol SAMs on gold.34 A probable reason for this phenomenon that has been observed by other researchers as well40 is contamination of commercially available SDS with dodecanol due to hydrolysis. Obviously, dodecanol is more hydrophobic than SDS and has a higher affinity to hydrophobic surfaces. Due to dodecanol adsorption at low SDS concentrations, the apparent SDS coverage is higher. However, above the cmc dodecanol is solubilized in SDS micelles, resulting in its desorption, and a maximum of the adsorption isotherm appears near the cmc. For SDS adsorption on positively charged cysteamine, we found that crystallization had no effect on the amount of SDS adsorbed (Figure 1), mainly because SDS has a higher affinity to cysteamine than dodecanol due to the electrostatic nature of the attraction. There is a difference between the two isotherms of SDS adsorption in water presented in Figure 1. Surface coverages in the two cases differ considerably. Our previous results (filled symbols) were obtained on gold deposited by heating a gold target by an electron beam (method 1, see Experimental Section for more details). Subsequent experiments carried out on gold prepared by thermal evaporation (method 2) reproducibly yielded a higher surface concentration of adsorbed SDS (Figure 1). We ascribe it to the difference in gold film preparation. The gold coating obtained by method 2 was more adsorptive, with higher cysteamine coverage and subsequently higher surface concentrations of SDS. PDDA Adsorption. A critical element in the formation of the structures under sequential adsorption of first surfactant and then polyelectrolyte is the competing (37) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385-387. (38) Newbery, J. E. Colloid Polym. Sci. 1979, 257, 773-775. (39) Clint, J. H. Surfactant aggregation; Blackie: Glasgow, 1992. (40) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017-1023.

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Figure 2. Kinetics of SDS adsorption at 1 cmc on a cysteamine SAM on gold and following PDDA adsorption from a 0.2 M solution on the SDS preformed layer, 5 mM phosphate buffer 7.0.

kinetics of surfactant desorption and polyion adsorption. To fabricate the structures, desorption of surfactant molecules from the SAM should be slower than adsorption of the polyion covering the surfactant on the surface. In the case of SDS adsorbed on cysteamine, desorption occurs within seconds upon rinsing with water or buffer.34 Another problem that might complicate interpretation of experimental results is formation of soluble or insoluble SDS-PDDA complex in bulk. To separate bulk SDS and PDDA solutions, an air bubble was injected ahead of the PDDA solution in the SPR cell. As a result, SDS and PDDA were never in contact in the bulk. To check that the injection of a 0.1-0.5 mL air bubble does not disturb the preformed SDS layer on the surface, the bubble was introduced into SDS solution flushing through the cell, that is, the solutions ahead of and behind the bubble were the same. SPR measurement showed that the reflectivity at a fixed angle of incidence (related to the amount of the material on the surface) was the same before and after purging the 0.5 mL bubble through the cell. Thus desorption of SDS did not occur. Time measurement revealed that introduction of the bubble caused a spike in reflectivity. The magnitude and direction of the spike were poorly reproducible. When an air bubble is flushed through, a thin film of liquid still remains on the substrate. Calculations show that in a narrow interval of the liquid film thicknesses (ca. from 200 to 700 nm), the magnitude and direction of the reflectivity spike dramatically depend on the thickness. Since we cannot control the thickness of the liquid film between the bubble and the solid with nanometer precision, the exact appearance of the spikes was always different. Kinetics of SDS adsorption from a 1 cmc solution in buffer, subsequent PDDA adsorption from a 0.2 M solution, and the following buffer rinse is shown in Figure 2. The equilibrium reflectivities measured for the SAM, SDS adsorbed on the SAM, and PDDA adsorbed on the SDS are given in Figure 3 as a function of the incident angle. The sequential adsorption of first SDS and then PDDA, both in phosphate buffer solution, results in a shift in the plasmon resonance peak (or angle) to higher values. This shift reflects the change in the refractive index and the thickness of the layers adsorbed on the surface. Using the Fresnel equations, the thickness or surface excess concentration can be calculated (see below). The instanta-

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Figure 3. SPR curves versus angle for the cysteamine SAM (solid line), SDS adsorbed on the SAM (dotted line), and PDDA adsorbed on the SDS (dashed line). Table 1. Thicknesses and Surface Concentrations of SDS on Cysteamine/Gold and PDDA on SDS/Cysteamine/Gold Obtained at Different SDS and PDDA Concentrations in 5 mM Phosphate 7.0 Buffer concentrations SDS (cmc) PDDA (M) 0.1 0.2 1 1.72 0.5 1 1.72

0.2 0.2 0.2 0.2 0.05 0.05 0.05

thicknesses (Å)

surface concentration (mol/m2)

SDS

PDDA

SDS

PDDA

3 5 17 20 19 20 16

5 7 11 16 3 5 3

1.03 1.71 5.82 6.84 6.50 6.84 5.48

1.14 1.6 2.51 3.65 0.69 1.14 0.69

neous increase in reflectivity upon PDDA injection anddecrease upon buffer rinse (Figure 2) are due to the significantly higher refractive index of 0.2 M PDDA solution (1.339) compared to water or diluted buffer (1.332). The net change in reflectivity after buffer rinse indicates the amount of the material remaining on the surface. Stability of the final layer is high since there is no indication of the surfactant or surfactant-polymer complex desorption upon rinsing with water or buffer. On the other hand, as was mentioned above, SDS desorbs fast when it is not fixed on the surface by a subsequent polymer layer. To obtain quantitative information about the SDSPDDA complex and find optimum conditions for its formation, we studied the effect of SDS and PDDA concentrations. Thicknesses of layers and amounts of SDS and PDDA obtained at different concentrations of complex components in buffer are compiled in Table 1. The thickness of the first SDS layer was measured by SPR after the equilibrium in SDS adsorption was achieved. It was followed by PDDA injection, preceded with an air bubble, and the system was allowed to equilibrate in the cell for a few minutes until the reflectivity reached a plateau. The thickness of the PDDA layer was obtained after subsequent buffer rinse to a constant reflectivity value. One can see from Table 1 that at low SDS concentrations, little PDDA is adsorbed due to the fact that the amount of SDS adsorbed in buffer considerably decreases at low SDS concentrations in the bulk (Figure 1). At the same time, adsorption of PDDA from a more diluted solution (0.05 M) yielded thin layers at any SDS coverage. The thickness of the PDDA layer formed at optimum conditions ([SDS] g 1 cmc, [PDDA] ) 0.2 M) is

Adsorption onto a Surface-Confined Surfactant

1.1 nm and is in the same range as values obtained for layer-by-layer polyelectrolyte structures41 and PDDA layers adsorbed on negatively charged thiol SAMs on gold.35 To gain further evidence of the formation of the surfactant-polyelectrolyte complex on the surface that prevents SDS desorption, we carried out an FTIR study of the surface complex. Spectra were collected in reflectance mode with the grazing angle of 80°. Since both SDS and PDDA contain CH2 groups having similar adsorption bands in IR, perdeutero SDS was used in this experiment to clearly distinguish it from the protonated polymer. It was also useful by virtue of the experimental procedure that was employed for surface complex preparation and isolation for FTIR study. We found that the transfer of a substrate with the formed surface complex through the water-air interface resulted in complex desorption, probably due to the high interfacial tension of pure water. To reduce the interfacial tension, SDS (perhydro) solution was spread with a microsyringe on the water surface just before pulling the slide from the beaker. A SDS monolayer formed on the water-air interface and lowered the surface tension to prevent desorption of the SDS-polymer complex from the SAM. The necessity of using additional reagents that might contaminate the complex on the surface and show up in IR spectra was another reason to use perdeuterated SDS (SDS-d25) for FTIR experiments. FTIR spectra of SDS-d25 and the SDS-d25-PDDA complex corresponding to vibrations of the SO3 group are shown in Figure 4a. We attribute the peak at 1045 cm-1 in SDSd25 to symmetric stretching and a broad one at about 1250 cm-1 to asymmetric stretching of the SO3 group.42-44 While the latter band is too broad to allow precise determination of its maximum, the former one definitely shifts from 1045 to 1037 cm-1 upon complex formation. It is known that SO3- modes are sensitive to the nature of the countercation.44 The IR spectral region corresponding to symmetric and asymmetric stretching of CD2 groups45 at 2097 and about 2200 cm-1, respectively, is shown in Figure 4b. The spectra (Figure 4) show that deuterated SDS is still present on the surface after immersion of the SDS-coated slide into 0.2 M PDDA solution and subsequent thorough rinsing with water. If a stable surfactant-polyelectrolyte complex was not formed on the surface, SDS-d25 would rapidly desorb from the cysteamine SAM and could not be seen in IR spectra. Successful formation of the complex means that the process of SDS desorption is slower than PDDA interaction with the surface-confined surfactant and adsorption on the surface. A possible schematic of the PDDA-SDS complex is shown in Figure 5. When SDS at or above the cmc is adsorbed on a surface of opposite charge, it will lead to cylindrical surface micelles aligned in a common direction.3,8-18,34 Adsorption of PDDA on top of the surface micelles could lead to the complex shown. Since PDDA is a random coiling polymer in solution, it is expected that PDDA will not have significant structure except for its positive charges interacting close with the negative charges of the SDS surface micelles. However, for stiff polymers, like polypeptide helices, Coulombic (41) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (42) Li, L. S.; Wang, R.; Fitzsimmons, M.; Li, D. Q. J. Phys. Chem. B 2000, 104, 11195-11201. (43) Ostrovskii, D.; Kjoniksen, A. L.; Nystrom, B.; Torell, L. M. Macromolecules 1999, 32, 1534-1540. (44) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623. (45) Stroeve, P.; Saperstein, D. D.; Rabolt, J. F. J. Chem. Phys. 1990, 92, 6958-6967.

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Figure 4. Grazing angle FTIR spectra of SDS-d25 adsorbed on a cysteamine SAM on gold and the SDS-d25-PDDA complex on a cysteamine SAM on gold: (a) 800-1400 cm-1 region, vibrations of SO3 group; (b) 2000-2300 cm-1 region, vibrations of CD2 group.

Figure 5. Schematic drawing of PDDA adsorbed on top of SDS hemimicelles. The drawing is not to scale.

interactions could lead to alignment of the polymer molecules on the surface micelles. Conclusions In this work, we showed the formation of a surface surfactant-polyelectrolyte complex on a solid-liquid

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interface by sequential adsorption of first a surfactant (SDS) and then a polyelectrolyte (PDDA). The complex is stable in aqueous environments and does not desorb or dissociate upon rinsing with water. Formation of the SDSPDDA complex was studied by surface plasmon resonance and FTIR. The influence of surfactant and polyelectrolyte concentrations on the formation of the surface complex was studied. The SDS and PDDA concentrations necessary

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for the surface complex formation were about 1 cmc and 0.2 M, respectively. Acknowledgment. The MRSEC program of the National Science Foundation under Award DMR-9808677 supported this work. LA026640S