Electrostatically Tunable Coadsorption of Sodium Dodecyl Sulfate and

Jan 30, 2001 - Finally, scaling of our coadsorption data with the bulk binding transitions (onset of cooperative binding and saturation of the polymer...
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Langmuir 2001, 17, 883-890

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Electrostatically Tunable Coadsorption of Sodium Dodecyl Sulfate and Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) Triblock Copolymer to Silica Alan D. Braem, Dennis C. Prieve, and Robert D. Tilton* Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received September 11, 2000. In Final Form: November 28, 2000 Interfacial properties can be tuned by exploiting polymer/surfactant interactions. We find that coadsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the amphiphilic triblock copolymer poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), Pluronic F108, to silica is extremely sensitive to SDS concentration and ionic strength. First, using a pyrene solubilization assay we identify the surfactant concentration regimes where different F108/SDS aggregates form in bulk solution at several ionic strengths. We then measure the total surface excess concentration of coadsorbing F108 and SDS using optical reflectometry. Above the critical aggregation concentration where F108/SDS aggregates form, the coadsorbed amount decreases with increasing surfactant concentration until an SDS concentration is reached at which adsorption is prevented entirely. Furthermore, although adsorbed layers containing only F108 are irreversibly adsorbed, F108/SDS layers are reversibly adsorbed. These results suggest that F108 is “shuttling” the normally nonadsorbing SDS to the silica surface. At high ionic strength, we find that sequential coadsorption followed by removal of SDS from the adsorbed layer results in an enhanced adsorbed amount of F108 (compared to direct adsorption of F108 in the absence of SDS). Thus, surfactant-free F108 layers can be “sculpted” into a different conformation by sequential processing with SDS. Finally, scaling of our coadsorption data with the bulk binding transitions (onset of cooperative binding and saturation of the polymer) indicates that changes in adsorbed amount occur at SDS concentrations both below where aggregates form and above the point where the polymer is saturated in the bulk.

Introduction When formulating commercial complex fluids such as coatings, inks, detergents, herbicides, cosmetics, dentifrices, or pharmaceutical suspensions, the introduction of surface active polymers and surfactants provides a powerful lever to manipulate macroscopic processing characteristics via adsorption and self-assembly. At the same time, this introduces strong multicomponent interactions that make it difficult to rationally design complex fluid formulations. Thus, there is a significant practical incentive to discern the mechanisms by which polymer-surfactant interactions exert their control over fundamental interfacial phenomena. The effects that a polymer or surfactant may have on the other’s adsorption depend on whether the polymer and surfactant are mutually repulsive or mutually attractive and also on the selectivity of the surface. A selective surface is one that adsorbs one of the individual components but not the other from a single component solution. Thus, a very simple categorization scheme can be used to describe the polymer/surfactant/surface interaction: (I) surfactant binds to polymer, and surface is selective; (II) surfactant binds to polymer, and surface is nonselective; (III) surfactant does not bind to polymer, and surface is selective; (IV) surfactant does not bind to polymer, and surface is nonselective. Of course, each of these categories may be further divided as one sees fit, for example, whether a selective surface favors the polymer or the surfactant, whether one or more components is ionic, and so forth. We are aware of investigations of type I,1-12 type II,5,6,13-33 type III,3,7 and type IV.3,9,18,22,29,34-41 The majority * Corresponding author. E-mail: [email protected]. (1) Anthony, O.; Marques, C. M.; Richetti, P. Langmuir 1998, 14, 6086.

of the type I systems studied in the literature are polyelectrolyte/ionic surfactant systems,1,2,4,5,7-10 and only (2) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (3) Bury, R.; Desmazieres, B.; Treiner, C. Colloids Surf., A 1997, 127, 113. (4) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (5) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550. (6) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2000, 16, 4478. (7) Asnacios, A.; Langevin, D.; Argillier, J.-F. Macromolecules 1996, 29, 7412. (8) Dedinaite, A.; Claesson, P. M.; Bergstro¨m, M. Langmuir 2000, 16, 5257. (9) Somasundaran, P.; Cleverdon, J. Colloids Surf. 1985, 13, 73. (10) Shubin, V. Langmuir 1994, 10, 1093. (11) Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1992, 153, 298. (12) Cosgrove, T.; Mears, S. J.; Thompson, L.; Howell, I. ACS Symp. Ser. 1995, 615, 196. (13) Esumi, K.; Takaku, Y.; Otsuka, H. Langmuir 1994, 10, 3250. (14) Rajagopalan, V.; Olsson, U.; Iliopoulos, I. Langmuir 1996, 12, 4378. (15) Purcell, I. P.; Liu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (16) Moudgil, B. M.; Somasundaran, P. Colloids Surf. 1985, 13, 87. (17) Regismond, S. T. A.; Gracie, K. D.; Winnik, F. M.; Goddard, E. D. Langmuir 1997, 13, 5558. (18) Stubenrauch, C.; Albouy, P.; Klitzing, R.; Langevin, D. Langmuir 2000, 16, 3206. (19) Wang, J.; Han, B.; Yan, H.; Li, Z.; Thomas, R. K. Langmuir 1999, 15, 8207. (20) Otsubo, Y. Langmuir 1994, 10, 1018. (21) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45. (22) Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 1993, 9, 284. (23) Duffy, D. C.; Davies, P. B. Langmuir 1995, 11, 2931. (24) Penfold, J.; Staples, E.; Tucker, I.; Creeth, A.; Hines, J.; Thompson, L.; Cummins, P.; Thomas, R. K.; Warren, N. Colloids Surf., A 1997, 128, 107. (25) Fleming, B. D.; Wanless, E. J.; Biggs, S. Langmuir 1999, 15, 8719. (26) Esumi, K.; Fujimoto, N.; Torigoe, K. Langmuir 1999, 15, 4613.

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three papers discuss an adsorbing neutral polymer/ionic surfactant system.6,11,12 The current investigation concerns an adsorbing neutral polyether and an anionic surfactant that is electrostatically repelled from the negatively charged surface. This type I system consists of a negatively charged silica surface, the anionic surfactant sodium dodecyl sulfate (SDS), and the amphiphilic triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) available commercially as Pluronic F108. All three components of this experimental system have widespread industrial applications. Some common themes emerge from the existing literature on type I systems. First, the adsorbing component may cause the normally nonadsorbing component to have a positive surface excess concentration at the interface, a process we call “shuttling.” In these cases, binding between the components prevails over the repulsion of the nonadsorbing component from the selective surface. Second, coadsorbed amounts can be either increased or decreased relative to single-component adsorption, and in some cases adsorption can be prevented altogether. Last, because the relative amounts of the components in a bound complex can be altered by changing the composition of the bulk solution, interfacial properties can be “tuned” through changes in bulk composition. For example, Anthony et al.,1 Lauten et al., 6 and Shubin10 observed changes in surface forces, layer thickness, and surface excess concentration, respectively, as a function of surfactant concentration. Neutral polymer/anionic surfactant binding in solution has been extensively studied and is reviewed elsewhere.42 Typically, there is a surfactant concentration below which no cooperative binding occurs, known as the critical aggregation concentration, or cac. The cac is generally insensitive to the polymer concentration. As the surfactant concentration is increased at a fixed polymer concentration, the amount of surfactant bound to polymer increases until a saturation concentration, or Csat, is reached. Any micelles that form beyond Csat are free micelles. There is also evidence from NMR,43 neutron scattering,44 and fluorescence spectroscopy45 that the polyether/anionic surfactant complex consists of distinct micellelike sur(27) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441. (28) Chari, K.; Antalek, B.; Kowalczyk, J.; Eachus, R.; Chen, T. J. Phys. Chem. B 1999, 103, 9867. (29) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (30) Babak, V. G.; Vikhoreva, G. A.; Lukina, I. G.; Kuznetsova, L. V. Colloid J. (Russian) 1997, 59, 131. (31) Izmailova, V. N.; Derkach, S. R.; Levachev, S. M.; Tarasevich, B. N.; Zotova, K. V.; Poddubnaya, O. Colloid J. (Russian) 1997, 59, 155. (32) Yamanaka, Y.; Esumi, K. Colloids Surf., A 1997, 122, 121. (33) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4977. (34) Esumi, K.; Oyama, M. Langmuir 1993, 9, 2020. (35) Harrison, I. M.; Meadows, J.; Robb, I. D.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3919. (36) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333. (37) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566. (38) Ghodbane, J.; Denoyel, R. Colloids Surf., A 1997, 127, 97. (39) Sastry, N. V.; Sequaris, J.-M.; Schwuger, M. J. J. Colloid Interface Sci. 1995, 171, 224. (40) Velegol, S. B.; Tilton, R. D. Langmuir 2001, 17, 219. (41) Esumi, K.; Iitaka, M.; Koide, Y. J. Colloid Interface Sci. 1998, 208, 178. (42) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins, 1st ed.; CRC Press: Boca Raton, FL, 1993. (43) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (44) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (45) Kim, J.-H.; Domach, M. M.; Tilton, R. D. J. Phys. Chem. B 1999, 103, 10582.

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factant clusters along the polymer backbone, that is, the string of pearls model. Recent simulations by Groot,46 however, suggest that complexation may produce either a string of pearls or a “bottlebrush” structure (a polymer chain surrounded by many surfactant “sidearms”), depending on whether the surfactant headgroup or tail dominates the interaction with the polymer. Whereas evidence indicates that the poly(ethylene oxide) (PEO) interaction with SDS yields a string of pearls structure,43-45 it is plausible that the hydrophobic tail interactions with poly(propylene oxide) (PPO) may promote the bottlebrush structure for a PPO/SDS complex. Thus, the structure of the F108/SDS complex is not known with certainty. In the current system, the F108 polymer has a high affinity for both the surface and the surfactant. Thus, the central feature of the coadsorption mechanism is the competition between polymer/surfactant binding and the surfactant’s electrostatic repulsion from the surface. This may be viewed as a competition between two mutually repellant species, the SDS and the silica surface, for binding to the polymer. We observe this competition by changing the relative concentrations of polymer and surfactant in the bulk solution. In so doing, we change the overall charge of the polymer/surfactant aggregates as well as the number of polymer segments that are bound to surfactant and are thereby unavailable to the surface. The ionic strength is an important variable, inasmuch as it tunes both short-range polymer/surfactant complexation and long-range electrostatic repulsions within mixed adsorbed layers. Before measuring coadsorption isotherms, we first examined the F108/SDS binding behavior in bulk solution. In addition to the expected cac and Csat transitions, we found additional transitions that likely reflect the different affinities for binding to the PEO and PPO blocks. For coadsorption, we measured total surface excess concentrations using optical reflectometry. We observed that the extent and reversibility of adsorption are extremely sensitive to the presence of SDS. Depending on the SDS concentration and the ionic strength, SDS complexation could either increase or decrease the adsorbed amount of polymer compared to adsorption from SDS-free polymer solutions. Experimental Section Materials. We obtained Pluronic F108 (average composition based on manufacturer’s literature PEO133-PPO50-PEO133, total molecular weight 14 600) in prill form as a gift from BASF Corp. We purchased SDS from Fluka (Micro-Select grade, >99% pure). By comparing to SDS purified by acetone washing,45 we found that any trace impurities in the as-received SDS sample did not affect coadsorption results in this system, so we did not further purify the surfactant. We purchased ACS grade NaCl, Chromerge, and ACS grade hydrochloric acid from Fisher, sodium hydroxide pellets from EM Science, and optical grade pyrene (>99% pure) from Aldrich Chemical Co. and used all reagents as received. We purified all water by reverse osmosis followed by treatment with the Milli-Q Plus system of ion exchange and organic adsorption cartridges from Millipore Corp. The pH of all solutions was unmodified from the air-equilibrated water pH of 5.5-6.0. The F108 concentration for all experiments was fixed at 1000 ppm. We conducted all experiments at 25 °C. All adsorption experiments were performed on oxidized silicon wafers. We obtained polished (60/40 scratch/dig) optical grade silicon wafers from Valley Design Corp. and oxidized them for 15-25 min in air at 1000 °C to generate 30-50 nm thick oxide layers. (We measured the thickness of the oxide layer on each individual wafer by in situ reflectometry prior to the start of each adsorption experiment.) After oxidation, we cleaned the (46) Groot, R. D. Langmuir 2000, 16, 7493.

Electrostatically Tunable Coadsorption wafers in Chromerge for 30 min, followed by a 30 min soak in 6 N hydrochloric acid. Finally, we soaked the wafers in 10 mM sodium hydroxide solution for 30 min, yielding a completely waterwettable, negatively charged silicon oxide surface. We rinsed the wafers profusely with water after each soak, stored them in water, and further rinsed with water prior to installing the wafer in the reflectometer flowcell. The wafers were never allowed to dry between the cleaning and the execution of adsorption experiments. Polymer/Surfactant Complexation in Solution. We used a pyrene solubilization assay based on ultraviolet-visible spectrophotometry to identify transitions in the SDS/F108 complexation. We detect transitions by virtue of the different pyrene solubilization powers provided by different types of aggregates.45 For these measurements, we added an excess of powdered pyrene to 5 mL of F108 or F108/SDS solutions in centrifuge tubes. The concentration of F108 was 1000 ppm in all cases. The samples were then sonicated by immersing the tubes into the bath of a Branson ultrasonic cleaner (model 1200) for at least 8 h. Following sonication, the samples were equilibrated to 25 °C and then centrifuged at 3000 rpm for 1 h to sediment all unsolubilized pyrene. Samples (3 mL) of the supernatant were placed into quartz cuvettes to measure their absorbance at 336 nm using a Varian Cary 300 Bio UV-visible spectrophotometer. We used the previously measured45 molar absorptivity for pyrene solubilized in SDS micelles, 2.06 × 10-5 M/cm, to calculate the solubilized pyrene concentration. Neither F108 nor SDS absorbs significantly at 336 nm. In cases where the solubilized pyrene concentration was large enough to yield absorbances greater than 1.0, we diluted the sample into a standard solution that contained 50 mM SDS and 1000 ppm F108 to prevent pyrene precipitation. Adsorption Measurements. Principles and applications of optical reflectometry are described elsewhere,36,37,47-53 and a detailed description of the particular reflectometry technique that we use is available.47 In this study, we use the scanning angle method to determine surface excess concentrations. Briefly, optical reflectometry is based on the change in the interfacial refractive index profile caused by adsorption. When the interface is illuminated by a parallel (p-) polarized laser beam at an angle of incidence close to the Brewster angle θb, changes in the interfacial refractive index profile produce a significant change in the intensity reflection coefficient, Rp. Changes in reflectivity can then be interpreted using a homogeneous two-layer optical model where the semi-infinite solution and silicon are separated by an oxide layer of known refractive index and thickness and an adsorbed layer. The optical model is numerically evaluated using the Abele`s matrix method54 and regressed against the data by χ2-minimization to obtain an optical average thickness, d, and an optical average refractive index, n, of the adsorbed layer. These parameters are model-dependent and cannot be reliably decoupled in the analysis, so they are not of value in expressing the true thickness or refractive index of the adsorbed layer. Nevertheless, the effective optical thickness of the adsorbed layer, d(n - n0) where n0 is the refractive index of the bulk solution, is a meaningful, model-independent property of the adsorbed layer. For a mixed adsorbed layer, each adsorbed species contributes to the effective optical thickness according to its refractive index increment, dn/dci, and its surface excess concentration, Γi, such that (47) Tilton, R. D. Scanning Angle Reflectometry and Its Application to Polymer Adsorption and Coadsorption with Surfactants. In ColloidPolymer Interactions: From Fundamentals to Practice; Farinato, R. S., Dubin, P. L., Eds.; John Wiley & Sons: New York, 1999; p 331. (48) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141. (49) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79. (50) Charron, J. Block Copolymer Adsorption to Phospholipid Monolayers at the Air-Water Interface. Ph.D. Dissertation, Carnegie Mellon University, Pittsburgh, PA, 1996. (51) Fu, Z.; Santore, M. M. Colloids Surf., A 1998, 135, 63. (52) Hayes, R. A.; Bo¨hmer, M. R.; Fokkink, L. G. J. Langmuir 1999, 15, 2865. (53) Heinrich, L.; Mann, E. K.; Voegel, J. C.; Schaaf, P. Langmuir 1997, 13, 3177. (54) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: New York, 1977.

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∑Γ dn/dc ) d(n - n ) i

i

0

(1)

i

Because F108 and SDS have similar refractive index increments of 0.13 ( 0.01 cm3/gm, they are optically equivalent, and we report the total surface excess concentration, that is, ΓSDS + ΓF108. Reflectometry does not directly provide any information on the relative amounts of each component in the adsorbed layer. The instrumentation we used is the same as described in Furst et al.37 with a rectangular slit flow cell except that we used a glass syringe to introduce solutions into the flow cell, rather than a peristaltic pump. All materials that contact the solution are either glass, stainless steel, Teflon, or fluorinated elastomers. For each change in solution composition, we flushed at least 30 cell volumes of new solution through the cell. The temperature was controlled at 25 °C by circulating constant temperature water from a bath through the flow cell housing.

Results and Discussion Polymer/Surfactant Complexation in Solution. By measuring the amount of pyrene solubilized by solutions containing F108 and SDS, we determined the surfactant concentrations that correspond to transitions in binding behavior. Figure 1 shows the total pyrene uptake in solutions containing 0.1 mM NaCl and 1000 ppm F108 as a function of SDS concentration. At very low SDS concentrations (regime I), the supernatant pyrene concentration is constant and has an average value of 0.66 µM. This, the measured solubility of pyrene in 1000 ppm F108 solution, matches the aqueous solubility of pyrene as measured by Kim et al.45 This indicates that F108 at 1000 ppm concentration does not solubilize pyrene and is therefore below its critical micelle concentration (cmc). The F108 cmc values reported in the literature are highly variable, ranging from 321 to 7140 ppm.55-58 At higher SDS concentrations, supernatant pyrene concentrations are significantly greater than 0.66 µM, indicating the presence of aggregates. Furthermore, the slope of each linear regime in Figure 1 may be interpreted as the solubilizing power of the micellar entity being formed in each concentration regime. Thus, the cac is indicated by the onset of pyrene solubilization, and Csat values are indicated by a change in solubilizing power from some value to the value corresponding to free SDS micelles. All cac and Csat values are summarized in Table 1, and solubilizing power results are summarized in Table 2. The solubilizing power of free SDS micelles has been measured previously by Kim et al.45 and ranges from 6.8 ( 0.1 µM pyrene per mM SDS in the absence of salt to 7.9 ( 0.1 µM pyrene per mM SDS in 100 mM NaCl. Kim et al.45 also measured the solubilizing powers of SDS complexes with PEO homopolymers. These range from 12.8 ( 0.5 µM pyrene per mM SDS in the absence of added salt to 17.0 ( 0.4 µM pyrene per mM SDS in 100 mM NaCl. We use these measurements to help discern what types of solubilizing aggregates are present in various concentration regimes in our system. Five distinct linear regimes are present in the data shown in Figure 1. Regime I is below the cac. It should be noted that although no cooperative aggregation occurs in regime I, the assay is insensitive to any noncooperative binding that might occur but would create no hydrophobic domain into which the pyrene might partition. Between 0.26 mM and 1.0 mM SDS (regime II), aggregates form (55) Batrakova, E. V.; Lee, S.; Li, S.; Venne, A.; Alakhov, V. Y.; Kabanov, A. V. Pharm. Res. 1999, 16, 1375. (56) Alexandridis, P.; Holzwart, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (57) Lopes, J. R.; Loh, W. Langmuir 1998, 14, 750. (58) Meilleur, L.; Hardy, A.; Quirion, F. Langmuir 1996, 12, 4697.

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Figure 1. Pyrene uptake by SDS solutions containing 1000 ppm F108. Each linear regime is labeled with a roman numeral. The lines are linear regression fits to the data in a given regime. The inset shows the low SDS concentration data. Table 1. Critical Micelle Concentrations and Major Polymer/Surfactant Binding Transitions, the Latter in 1000 ppm F108 Solutions NaCl concn (mM) cmc w/o polymer (mM) cac (mM) Csat (mM) 0.1 10.0 150.0

8.1 5.3 1.1

0.26 0.22 0.052

15.0 13.6 17.1

with a solubilizing power of 4.9 ( 0.32 µM pyrene per mM SDS, followed by aggregates with a solubilizing power of 2.6 ( 0.13 µM pyrene per mM SDS between 1.0 and 5.0 mM SDS (regime III). In regime IV, between 5.0 and 15.0 mM SDS, the aggregates have a solubilizing power of 13.2 ( 0.36 µM pyrene per mM SDS, approximately corresponding to the solubilizing power of PEO/SDS aggregates measured by Kim et al.45 Thus, the transition between regimes III and IV may represent a transition from PPO/ SDS interactions to PEO/SDS interactions, but this remains to be confirmed by other techniques. The slope of regime V is 8.6 ( 0.12 µM pyrene per mM SDS, indicating the presence of free micelles. Thus, we take the transition between regimes IV and V to be the Csat. Using the measured values of Csat and cac, we calculate that 4.25 g of SDS are bound per gram of F108 at saturation. To compare, Kim et al.45 found that 2.1 and 3.1 g of SDS were bound per gram of 10 000 MW PEO at saturation in 0 and 100 mM NaCl solutions, respectively. In Figure 2a,b, we present pyrene solubilization data for all three ionic strengths that we used in the coadsorption study. As expected, the cac decreases with increasing ionic strength. Of particular interest in Figure 2a is a qualitative change in behavior in the 150 mM NaCl data set, where an additional transition (“regime Ia”) is observed (six regimes as opposed to five for the other NaCl concentrations). The aggregates that form just above the cac in 150 mM NaCl have an unusually high solubilizing power (30.3 µM pyrene per mM SDS). This is higher than the values observed for any other aggregates formed at any ionic strength or SDS concentration, suggesting the formation of a unique structure. The next transition after the cac in 150 mM NaCl produces aggregates whose solubilizing power is similar to that of the aggregates formed at the cac for the lower NaCl concentrations. One possibility is that the high ionic strength switches the mode of PPO/SDS interaction from headgroup-dominated to tail-dominated, resulting in the bottlebrush structure mentioned above. The appearance of this as yet unde-

termined structure at 150 mM NaCl correlates with trends in the coadsorption data to follow. Kim et al.45 measured the PEO/SDS cac and found it to be 4.4 mM SDS in the absence of salt. Table 1 shows that the F108/SDS cac is an order of magnitude lower than this, providing evidence that the surfactant first binds to the hydrophobic PPO block of the F108 followed by the PEO block. Whereas the cac with PEO homopolymer consistently is approximately half of the cmc measured in the absence of polymer, the cac with F108 is approximately 3-5% of the cmc at all salt concentrations. The cac scales with salt concentration in the same manner that the cmc does. This constant scaling of the cac with the cmc is consistent with the recent theoretical work of Diamant et al.59 We note that there is some variability in the Csat measured at each ionic strength, but no monotonic trend is indicated. Single-Component Adsorption. We have measured F108 adsorption in the absence of SDS from unbuffered 1000 ppm solutions to silica at three different concentrations of added NaCl: 0.1, 10, and 150 mM. The F108 surface excess concentration on silica is 0.73 ( 0.05 mg/ m2, independent of NaCl concentration. This is similar to the extent of adsorption for similarly sized PEO homopolymers on silica, as measured by others.60 Although in many cases amphiphilic block copolymers such as F108 can adsorb in extended conformations, that is, “brushes” or “mushrooms,” the similarity of F108 and PEO homopolymer surface excess concentrations indicate that F108 adsorbs to silica in a homopolymer-like conformation, with both PEO and PPO segments bound to the surface, rather than a highly extended brush conformation. For comparison, optical reflectometry measurements show that F108 adsorbs to 3.0 mg/m2 at the hydrophobic air/ water interface,50 indicating the extent of adsorption that can be achieved when F108 does indeed adopt an extended conformation with PEO blocks stretched into solution. After each F108 adsorption experiment, we rinsed the adsorbed layer and allowed it to soak in F108-free solution for 20 h. This produced no detectable desorption at any NaCl concentration. Thus, as is often true of polymers, F108 adsorption to silica is irreversible over practical laboratory time scales. We have also attempted to measure SDS adsorption to the silica surfaces for the same range of SDS and NaCl concentrations used in our coadsorption experiments. In all cases, there was no detectable adsorption. It is possible that adsorption occurred below our detection limit for SDS (0.05 mg/m2 or 1000 Å2/molecule in this case). Coadsorption. Figure 3a shows the results of three sequential coadsorption experiments. In all experiments, the concentration of F108 was fixed at 1000 ppm. We used the same silica surface for all three of these experiments, without removal from the flow cell or drying between each experiment. Recall that wafers were kept in water continuously after cleaning, and they were mounted in the reflectometer while wet. After mounting, we allowed them to soak in water for an additional 10 min to check for surface active impurities before we introduced F108/ SDS solutions. We began with the highest SDS concentration. After attaining steady state for any SDS concentration, we continued the experiment by successive rinses with solutions that contained progressively lower SDS concentrations (but always a constant 1000 ppm F108 concentration). At every ionic strength, we found a (59) Diamant, H.; Andelman, D. Europhys. Lett. 1999, 48, 170. (60) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1994, 27, 3219.

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Table 2. Solubilizing Powers of SDS/F108 Aggregates solubilizing power (µM pyrene/mM SDS) NaCl concn (mM)

regime Ia

regime II

regime III

regime IV

regime V

0.1 10 150

N/A N/A 30.32 ( 0.44

4.88 ( 0.32 7.87 ( 0.45 11.03 ( 1.19

2.60 ( 0.13 3.21 ( 0.16 20.83 ( 0.66

13.15 ( 0.36 15.95 ( 0.90 15.37 ( 0.48

8.56 ( 0.12 8.25 ( 0.31 9.71 ( 0.86

Figure 2. Pyrene uptake by SDS solutions containing 1000 ppm F108 for three NaCl concentrations at low SDS concentrations (a) and for all SDS concentrations studied (b). Each symbol corresponds to solutions having a different NaCl concentration: (O) 0.1 mM NaCl, (3) 10 mM NaCl, and (9) 150 mM NaCl. The lines are linear regression fits to the data in a given regime. In (b), arrows indicate the intersection of regime IV and regime V, interpreted as Csat.

surfactant concentration at which no adsorption occurred. As we decreased the surfactant concentration from that point, the surface excess concentration increased, the only exception being the low SDS concentration regime at 150 mM NaCl, as we will discuss later. This trend in coadsorption behavior implies that the adsorbing species is an F108/SDS complex in all cases. The degree to which a complex adsorbs depends on a competition between attractive and repulsive interactions. Both the surfactant molecules and the surface have attractive interactions with the polymer segments. Repulsive interactions include the electrostatic repulsions between polymer-bound surfactants and the surface as well as lateral electrostatic repulsions between bound surfactants on neighboring chains. Complexes containing higher amounts of surfactant have both greater repulsive interactions as well as fewer unoccupied polymer segments that are free to bind to the surface, thus explaining the

Figure 3. Coadsorption from solutions containing 1000 ppm F108 and varying concentrations of SDS. (a) shows data at all SDS concentrations studied, and (b) focuses on low SDS concentrations. The symbols represent different NaCl concentrations: (O) 0.1 mM NaCl, (3) 10 mM NaCl, and (9) 150 mM NaCl. Bars on the ordinates indicate the surface concentration attained by adsorption from 1000 ppm F108 solutions in the absence of SDS (results are independent of NaCl concentration). Note that the coadsorption data sets are offset from the ordinate for visual clarity; the leftmost points in each data set correspond to 0 mM SDS. The lines in (b) indicate the upper and lower limits of regime Ia in the 150 mM NaCl pyrene uptake data.

observed decrease in total adsorption as the bulk surfactant concentration is increased. In addition to its effect on the total adsorbed amount, the presence of SDS dramatically changes the dynamics of the adsorbed layer. Whereas F108 layers adsorbed in the absence of SDS are irreversibly adsorbed, coadsorbed layers are reversibly adsorbed. We conducted several experiments wherein we alternated between high and low bulk surfactant concentrations rather than using the monotonic dilution employed in the standard sequential adsorption experiments. We also conducted experiments in which we adsorbed in a “single shot” at a particular SDS concentration. Experiments at all NaCl concentrations showed that the total adsorbed amount was path-

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independent;that is, the layers respond reversibly to changes in bulk composition, except near zero SDS concentration (see below). Furthermore, F108 layers initially adsorbed in the absence of SDS can be completely desorbed by the addition of F108/SDS solutions at the high surfactant concentrations where Figure 3a shows that no coadsorption occurs. Polymers adsorb irreversibly because of the large number of individually weak segment/surface interactions that sum to a very large adsorption energy. Because desorption requires simultaneous breaking of all segment/ surface contacts, there is a large energy barrier to desorption. The observed change in reversibility can be explained by a decrease in the number of segment/surface contacts, that is, a decreasing proportion of the F108 segments that reside in trains, as caused by the repulsive electrostatic interactions and the occupation of F108 segments by surfactant aggregates. Surfactant does not have this effect on reversibility in all polymer/surfactant coadsorption systems. The ability of the surfactant to overcome the irreversibility of polymer adsorption likely requires polymer/surfactant complexation. In a type IV system consisting of mutually repellant hexadecyltrimethylammonium bromide and poly-L-lysine coadsorbing to silica (a nonselective surface in that case), the polymer adsorption remained irreversible.36,40 Coadsorption Phenomena at Low Surfactant Concentrations. In Figure 3b, we focus on the low surfactant concentration regime of the same sequential adsorption experiments shown in Figure 3a. Several interesting features come to light. First, the adsorbed amount of F108 at zero surfactant concentration is pathdependent. After each coadsorption experiment, we extract SDS from the layer by rinsing with SDS-free F108 solutions. The resulting adsorbed amount (represented by the symbol at [SDS] ) 0 mM) differs from the adsorbed amount obtained by adsorbing F108 directly in the absence of SDS (represented by the bar on the ordinate). This is true at both 0.1 and 150 mM NaCl concentrations. Coadsorption in the presence of 150 mM NaCl produced a significant increase in the adsorbed amount of F108 at zero SDS concentration, whereas F108 adsorption was suppressed at 0.1 mM NaCl. In each case, we allowed the F108 layers that were originally formed through the sequential coadsorption process to equilibrate with SDSfree F108 solutions to determine whether the layers would eventually relax to the expected ∼0.73 mg/m2, yet there was no measurable change in surface excess concentration over at least a 1 h period of soaking. The surface concentration is closely related to the adsorbed polymer conformation. The observed difference in surface concentration indicates that sequential coadsorption followed by surfactant extraction can trap polymer layers into nonequilibrium structures that differ significantly from the structure formed by direct adsorption in the absence of SDS. In other words, we are able to alter the structure of surfactant-free polymer layers by processing with polymer/surfactant interactions. The resulting “surfactant processed” adsorbed layers are preserved after surfactant removal by virtue of kinetic trapping in persistent nonequilibrium states. In addition to the enhanced F108 adsorption in 150 mM NaCl solutions, we also observed a maximum in the total adsorbed amount at a relatively low SDS concentration. We view the maximum as further evidence for the presence of SDS in the adsorbed layer because the additional mass of surfactant may account for the difference between the maximum adsorbed amount and the zero-surfactant adsorbed amount.

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We offer two possible explanations for the enhancement of F108 adsorption that occurs in 150 mM but not 0.1 or 10 mM NaCl solutions. In the absence of energy barriers, adsorbed polymer chains would tend to relax to some equilibrium conformation at the surface. The presence of barriers to relaxation results in kinetically trapped nonequilibrium structures. When F108 adsorbs directly from SDS-free solutions, its surface concentration suggests that it adopts a homopolymer-like conformation, with both hydrophobic PPO and hydrophilic PEO blocks in contact with the surface. The resulting adsorbed layer may be trapped in a nonequilibrium conformation because of the energy barrier against PEO segment desorption and extension into solution. When SDS is present, it partially solubilizes and mobilizes polymer segments near the interface. Perhaps SDS-binding promotes F108 extension and thereby allows adsorbed chains to pack more densely on the surface. The pyrene solubilization results suggest that surfactants are bound only to the PPO block of the F108 at the lowest SDS concentrations. The repulsion of the SDS from the surface (normal interaction) stretches the chain away from the surface, while it remains anchored by the uncomplexed PEO blocks. The tendency of this electrostatic chain extension to increase the surface excess concentration would be tempered by the repulsive interaction between adjacent bound SDS aggregates (lateral interaction), because that would tend to increase the interchain spacing. As the bulk SDS concentration is decreased in the sequential coadsorption process, both the lateral and the normal electrostatic interactions are weakened because of the removal of charged SDS molecules from the layer. The resulting conformation will be determined by a competition between two dynamic processes: diffusion and adsorption of additional polymers or polymer/surfactant complexes to the surface versus a relaxation of the preadsorbed polymers to a flatter conformation. This competition is depicted schematically in Figure 4. If the normal repulsions dominate the lateral repulsions, preadsorbed chains will remain stretched while new chains are allowed to adsorb in the newly available area made accessible by the weakening of the lateral repulsions. This would provide a more extended layer with a higher surface excess concentration. On the other hand, if the lateral SDS/SDS repulsions dominate the normal SDS/surface repulsion the chains will relax to minimize the average lateral density of SDS aggregates. The polymer chains will flatten somewhat and prevent new chains from adsorbing as a result. Given the complex sensitivity of SDS/F108 binding to NaCl concentration, it is reasonable to postulate that the normal electrostatic SDS/surface repulsions do not necessarily scale with ionic strength in the same way as the lateral SDS/SDS repulsions. It is possible that the balance of normal and lateral electrostatic interactions becomes favorable to extension and enhanced adsorption only at the higher NaCl concentration, whereas the balance at the lowest NaCl concentration resists extension. The second hypothesis is suggested by the strong correlation between the SDS concentrations at which adsorption was enhanced and those at which the pyrene solubilization experiments indicated the formation of a unique aggregate structure in 150 mM NaCl solutions. The polymer/surfactant aggregate in question forms between 0.05 and 0.3 mM SDS (see Figure 2a). This concentration range is quite similar to the range over which the total surface excess concentration exceeded the ∼0.73 mg/m2 expected for F108 adsorption: 0-0.25 mM, with a maximum at 0.05 mM. Aggregation can increase

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Figure 4. Pictorial representation of one proposed mechanism for extension of an F108 layer. From left to right, the SDS concentration is being reduced. The top pictures represent a scenario in which normal repulsions dominate the adsorption behavior, and the bottom pictures represent a scenario in which lateral repulsions dominate. The arrows between aggregates and between the bound micelle and the surface indicate the strength of the interaction; larger, solid arrows represent relatively strong repulsion, and smaller, dashed arrows represent relatively weak repulsion.

adsorption, for example, if a bound SDS aggregate can serve as a cross-linking point for multiple F108 molecules. Surfactant cross-linking is commonly observed with hydrophobically modified polymers in solution at low SDS concentrations.61-63 For the F108 triblock structure, one may envision an F108 “micelle” with the PPO blocks enveloped in an SDS micelle at the core and uncomplexed PEO blocks in the corona. If these aggregates were to adsorb as SDS is being extracted from the adsorbed layer, the newly adsorbing polymer chains would be stretched and preassembled in close proximity. Contradicting the second hypothesis is the observation by Hecht et al.,64 that SDS inhibits Pluronic F127 selfassembly. Thus, we favor the first hypothesis. Furthermore, the correspondence between the unique aggregate formation in the bulk and the enhanced adsorption may still be explained in the framework of the first hypothesis. That is, it may be the case that the aggregate structure formed at low SDS concentrations in 150 mM NaCl is responsible for a shift in the balance of normal and lateral forces in the layer. Comparison of Bulk and Interfacial Aggregation. It is evident in Figure 3a that coadsorption is a function of ionic strength. In particular, the 10 mM NaCl coadsorption curve appears significantly rightward shifted in comparison to the other two ionic strengths. To test the degree to which ionic strength effects can be explained by the salt dependence of polymer/surfactant complexation as opposed to long-range electrostatic repulsions, we have scaled the SDS concentrations by the bulk binding transitions measured by pyrene solubilization. We define a degree of complexation, R, as

R)

CSDS - cac Csat - cac

(2)

Between the cac and Csat, R ranges from 0 to unity. Negative R values occur below the onset of cooperative aggregation at the cac, where we cannot rule out the possibility that noncooperative binding might occur. This scaling reflects the extent to which PPO and PEO segments are bound to SDS aggregates, but it does not capture any (61) Panmai, S.; Prud’homme, R. K.; Peiffer, D. G. Colloids Surf., A 1999, 147, 3. (62) Nilsson, S. Macromolecules 1995, 28, 7837. (63) Dualeh, A. J.; Steiner, C. A. Macromolecules 1991, 24, 112. (64) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866.

long-range electrostatic interactions. If the coadsorption curves are simply dictated by the degree of complexation alone, the curves for the three different ionic strengths should collapse onto one another. Figure 5a shows that this concentration scaling does not collapse the coadsorption curves. Thus, the salt effect cannot be completely attributed to changes in complexation. Long-range electrostatic intralayer repulsions are important. A consequence of intralayer electrostatic interactions is that the adsorbed amount continues to change for R > 1, where all F108 molecules in solution are saturated. This indicates that the complexation at the interface is different from complexation in the bulk, with more SDS being required to fully saturate and desorb chains at the interface. This reflects the depletion of the anionic surfactants from the negatively charged surface. It also reflects the ability of the silica surface to compete with SDS for binding to F108 segments. Both of these factors would tend to cause an increase in the apparent Csat for polymers at the surface. Figure 5a also indicates that the scaled concentration of SDS required to prevent adsorption entirely increases with increasing ionic strength. At 0.1 mM NaCl, approximately 22 mM SDS (R ) 1.5) is required to prevent adsorption, at 10 mM NaCl approximately 26 mM SDS (R ) 1.9) is required, and at 150 mM NaCl somewhere between 33 and 49 mM SDS (R ) 1.9-2.9) is required. As long-range electrostatic repulsions are screened by increased salt, more SDS is required to completely prevent adsorption. Figure 5b focuses on the low surfactant concentration range. For R < 0, that is, before the onset of cooperative binding in solution, the surface excess concentration remains sensitive to changes in the SDS concentration, at both 0.1 and 150 mM SDS. At 0.1 mM NaCl, the total adsorbed amount decreases with increasing SDS for R < 0, whereas the opposite is true at 150 mM NaCl. This suggests that there is some degree of noncooperative binding at the lowest SDS concentrations. Because the pyrene solubilization assay is sensitive only to the formation of micellelike aggregates, this would not be inconsistent with our measurements of the cac. The opposite qualitative trends observed for 0.1 and 150 mM NaCl concentrations highlight the competing effects of lateral and normal electrostatic repulsions within the layer. At 10 mM NaCl, these electrostatic interactions are balanced such that the adsorbed amount remains nearly constant near the cac. Finally, we note that the

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Figure 5. Coadsorption data scaled by SDS/F108 binding transitions. The scaled SDS concentration R is defined in the text by eq 2. (a) shows data at all SDS concentrations studied, and (b) focuses on low SDS concentrations. The symbols represent different NaCl concentrations: (O) 0.1 mM NaCl, (3) 10 mM NaCl, and (9) 150 mM NaCl. Bars on the ordinates indicate the adsorbed amount of 1000 ppm F108 in the absence of SDS. Note that the coadsorption data sets are offset from the ordinate for visual clarity; the leftmost points in each data set correspond to 0 mM SDS. The line at R ) 1 in (a) indicates where the SDS concentration is equal to Csat. The line at R ) 0 in (b) indicates where the SDS concentration is equal to cac.

maximum in the 150 mM NaCl data occurs right at the cac. This supports the notion that the difference between the zero surfactant adsorbed amount and the maximum is due to bound SDS in the layer and that only cooperative binding hinders adsorption of the polymer itself at this ionic strength.

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tions that SDS has in polymer-free solutions. Pyrene solubilization indicates several transitions in the binding behavior, corresponding to the formation of several different aggregate structures. At 150 mM NaCl, there is evidence that a unique aggregate structure forms at low SDS concentrations that is not formed at the lower ionic strengths. Adsorption from 1000 ppm F108 solution in the absence of SDS yields a surface excess concentration of 0.73 ( 0.05 mg/m2 on the silica surface and is irreversible, independent of NaCl concentration. The presence of SDS in solution controls both the extent and reversibility of F108 adsorption. For coadsorption at 1000 ppm F108 with varying SDS concentration and varying NaCl concentration, a surfactant concentration above which no adsorption occurs can always be found. As the surfactant concentration is decreased from this level, the adsorbed amount always increases until the cac is reached. Below the cac, the total adsorbed amount either increases, at low ionic strengths, or decreases, at high ionic strengths, as the SDS concentration is decreased. All coadsorbed layers are reversibly adsorbed. Scaling the coadsorption data with the solution binding transitions reveals that the extent of adsorption remains sensitive to SDS concentration both below the cac and above the Csat. This indicates that there are subtleties at the interface that are not captured by bulk binding behavior. For example, complexation behavior at the surface may differ from bulk complexation behavior because of competition between silica and SDS for polymer segments and the presence of an electric field that depletes SDS from the interfacial region. Additionally, noncooperative binding may be altering adsorption below the cac. The failure of the concentration scaling to capture the differences between adsorption at the various salt concentrations highlights the importance of long-range electrostatic interactions in the coadsorption mechanism. Finally, we note that sequential processing with coadsorbing surfactants provides a means to tune the structure of adsorbed polymer layers. After SDS is extracted from a coadsorbed layer at either 0.1 or 150 mM NaCl concentrations, the surface concentration that remains in the presence of SDS-free solutions of F108 is significantly different from the surface concentration attained by direct adsorption of F108 in the absence of SDS. In the case of 150 mM NaCl solutions, sequential processing with SDS enhances F108 adsorption in a manner that is consistent with a greater extension of the adsorbed polymer conformation. The degree of extension is currently under investigation and will be addressed in a future publication. We believe that other polymer/surfactant systems may exhibit this “sculpting” behavior. This would provide an additional degree of control over surface forces in colloidal systems.

Conclusions Pluronic F108 triblock copolymers associate strongly with SDS in solution. For NaCl concentrations between 0.1 and 150 mM, the critical aggregation concentrations are approximately 3-5% of the critical micelle concentra-

Acknowledgment. This material is based on work supported by the National Science Foundation under Grant Nos. CTS-9623849 and CTS-9711214. LA0013042