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Polyelectrolyte-Surfactant Layers: Adsorption of Preformed Aggregates versus Adsorption of Surfactant to Preadsorbed Polyelectrolyte Andra Dedinaite, Per M. Claesson,* and Magnus Bergstro¨m Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P. O. Box 5607, SE-114 86 Stockholm, Sweden Received August 26, 1999. In Final Form: February 4, 2000 The character of adsorbed layers containing both polyelectrolyte and surfactant depends on the type of polyelectrolyte used and the surfactant concentrations, as demonstrated by several recent studies. However, the layer properties also depend on the experimental pathway. This shows that the adsorbed layers can be trapped in quasi-equilibrium states and that the true equilibrium is reached only after experimentally inaccessible time scales. This has to be kept in mind when comparing different results reported in the literature. The present article highlights these effects using a system consisting of a highly charged cationic polyelectrolyte, poly{(propionyloxy)ethyl}trimethylammonium chloride (PCMA), and an anionic surfactant, sodium dodecyl sulfate (SDS). The adsorbed layer properties were determined using mainly surface force measurements and atomic force microscope (AFM) imaging. We also present some small-angle neutron scattering data for bulk PCMA-SDS complexes formed between the polyelectrolyte and the surfactant. They demonstrate the presence of a characteristic correlation length of about 37-39 Å. A similar characteristic length scale is also observed in some of the adsorbed layers, both employing the AFM and the surface force apparatus. It may be interpreted as the distance between surfactant loaded polyelectrolyte chains.
Introduction Polyelectrolyte-surfactant aggregates adsorbed at solid-liquid interfaces are often trapped in nonequilibrium states. This is an important issue in many processes utilizing combinations of surfactant and polyelectrolyte. One example of a common application is the use of polyelectrolyte and surfactant mixtures in hair-care products, including shampoos.1 It is straightforward that, in such applications, the relevant time scale is in the range from seconds to minutes and during this short time no equilibrium state is reached. Several experimental studies have been devoted to polyelectrolyte-surfactant interactions at interfaces. For example, Shubin et al.2,3 investigated how the structure of cationic hydrophobically modified cellulose adsorbed on negatively charged surfaces, mica and silica, was affected by anionic, cationic, and nonionic surfactants. Later, Claesson et al.,4 Fielden et al.,5 and Kjellin et al.6 studied interactions between acrylamide based cationic polyelectrolytes of various charge densities (100, 30, and 10%, respectively) adsorbed on negatively charged mica surfaces and how the layer properties changed when an anionic surfactant, sodium dodecyl sulfate (SDS), was added to the solution. Anthony et al.7 explored the interactions between mica surfaces coated with cationic (1) Reich, C. In Hair Cleansers, 2nd ed.; Reich, C., Ed.; Dekker: New York, Basel, Hong Kong, 1997; Vol. 68. (2) Shubin, V. Langmuir 1994, 10, 1093. (3) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polym. Sci. 1994, 272, 1590. (4) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008. (5) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366. (6) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476. (7) Anthony, O.; Marques, C. M.; Richetti, P. Langmuir 1998, 14, 6086.
guar in water and SDS solutions. The systems in these investigations were at equilibrium, or at least aimed to be so. The research on adsorption from polyelectrolytesurfactant mixtures is a comparatively recent effort,8 and many aspects, particularly nonequilibrium phenomena, are not very well understood. Pagac et al. studied adsorption and coadsorption of cationic surfactant with cationic polyelectrolytes and found that the effect of the surfactant on the polyelectrolyte adsorption was very sensitive to the order in which the surfactant and the polyelectrolyte were exposed to the surface. Different pathways to the same final bulk solution composition produced significantly different adsorption results.9 Similar conclusions were reached by Neivandt et al.10 who studied coadsorption of cethyltrimethylammonium bromide and poly(styrenesulfonate) on silica. Recently, Dedinaite et al.11 studied how the interactions between negatively charged mica surfaces were affected by adsorption of polyelectrolyte-surfactant aggregates formed in bulk solution. They used a cationic polyelectrolyte and an anionic surfactant. The charge of the aggregates could be varied from being highly positive to highly negative by changing the surfactant concentration. Some nonequilibrium aspects in these systems were noted. For instance, the slow spreading of positively charged polyelectrolyte-surfactant aggregates on negatively charged surfaces was briefly discussed. It was also noticed that the order of adding polyelectrolytes and surfactants affects the properties of the adsorbed layers. In this article, we present a more detailed investigation of these nonequilibrium phenomena. We compare the properties of (8) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566. (9) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333. (10) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (11) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951.
10.1021/la9911537 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/10/2000
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Figure 1. Chemical structure of poly{2-(propionyloxy)ethyl}trimethylammonium chloride, PCMA.
adsorbed polyelectrolyte-surfactant aggregates at identical bulk solution compositions, but we vary the experimental paths leading to the given bulk concentration. The results obtained are strikingly different, which highlights the fact that equilibrium is reached extremely slowly in polyelectrolyte-surfactant systems. This has to be kept in mind when comparing scientific results reported in the literature as well as when aiming at improving technological processes. Experimental Section Materials. The polyelectrolyte used in this study was poly{(propionyloxy)ethyl}trimethylammonium chloride, PCMA, with a molecular weight of 1.6 × 106 g/mol. Each monomeric segment of the polyelectrolyte carries one positive charge as shown in Figure 1. The polyelectrolyte was synthesized and kindly provided for us by the Laboratoire de Physico-Chimie Macromoleculaire, Universite´ Pierre et Marie Curie, Paris. The surfactant, sodium dodecyl sulfate (SDS), especially pure grade for biochemical work (>99%), was purchased from BDH and used as received. Potassium bromide, KBr (Merck, pro analysi) was roasted at 500 °C overnight before use. The water was purified using a Millipore Milli-RO plus system followed by a Milli-Q 185 system. In addition, before being used in the surface force apparatus (SFA), it was deaerated for at least 1 h using a water jet pump. Freshly cleaved muscovite mica (Reliance, New York) was used as a substrate. Potassium ions located at the muscovite mica surface can easily be exchanged by other cations present in the solution. This feature of the mica surface is of importance when considering adsorption of cationic PCMA or PCMA-SDS aggregates. Surface Forces. The forces acting between muscovite mica surfaces coated with polyelectrolyte or polyelectrolyte-surfactant aggregates were studied with a surface force apparatus, SFA, using Mark II12 and Mark IV13 models. The technique is described in detail in ref 12. In brief, the forces are measured between two macroscopic surfaces positioned in crossed cylinder geometry. One of the surfaces is attached to a double cantilever spring. A force acting between the two surfaces causes the spring to deflect. From the deflection, knowing the spring constant, the acting force can be calculated. The surface separation is determined interferometrically. The interacting surfaces are transparent and silvered on their backsides. White light is shone through the surfaces and multiply reflected between the silver layers. As a consequence of a constructive interference for certain wavelengths, fringes of equal chromatic order (FECO) are created and their wavelength can be measured in a spectrometer. The distance is determined by comparing the wavelengths of the FECO when the two surfaces are in contact and apart. The results of the surface force measurements are plotted as force normalized by mean undeformed geometric radius, F/R, as a function of surface separation, D. The distance resolution is 2 Å. The F/R detection limit for our droplet experiments was about 5 × 10-5 N/m and for filled box experiments about 1 × 10-5 N/m. We note that the force normalized by radius for the crossed cylinder geometry used in the experiments is related to the free energy of interaction per unit area (Gf) between flat surfaces (F/R ) 2πGf). This relation is valid when the radius of the surfaces (≈2 cm) is much larger than the range of the measured forces and when the surfaces do not deform due to the action of the surface (12) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (13) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135.
Figure 2. Schematic representation of the two experimental procedures used. Procedure I: Step 1, polyelectrolyte is adsorbed on the mica surface from a 60 µL droplet containing 20 ppm PCMA and 1 × 10-4 M KBr. Step 2, the droplet is diluted approximately 6000 times with a pure 1 × 10-4 M KBr solution. Step 3, SDS is added to a concentration of 8 × 10-3 M ) 1 cmc SDS. Step 4, the added SDS is exchanged with a pure 1 × 10-4 M KBr solution again. Procedure II: Step 1, polyelectrolyte (20 ppm) and surfactant (1 cmc) are first mixed in 1 × 10-4 M KBr bulk solution, and then the mixture is allowed to adsorb on the mica surface from a 60 µL droplet. Step 2, the droplet is diluted approximately 6000 times with a 1 cmc SDS and 1 × 10-4 M KBr solution. Step 3, this solution is exchanged with pure 1 × 10-4 M KBr. forces. The pressure between flat surfaces can thus be calculated from the slope of the measured force curve. In this study we aimed at elucidating nonequilibrium aspects of polyelectrolyte-surfactant aggregate adsorption. With this in mind we used two experimental schemes (Figure 2), which led to the same bulk solution composition but through different paths. The surface interactions were measured after each single step. Procedure I was as follows: First, the polyelectrolyte was adsorbed on the mica surface from a 60 µL droplet containing 20 ppm PCMA. Next, to remove the PCMA from the bulk solution, the measuring chamber was filled with ≈340 mL of a 1 × 10-4 M KBr solution, which means that PCMA was diluted approximately 6000 times (PCMA bulk concentration ≈ 3 ppb). In the following step, SDS was injected to a bulk concentration of 1 critical micelle concentration (cmc, 8.3 × 10-3 M). And finally, the SDS solution was drained out from the measuring chamber, and instead the chamber was again filled with a 1 × 10-4 M KBr solution. To ascertain that SDS was removed from the bulk solution, the process of filling and draining pure 1 × 10-4 M KBr was repeated several times. Procedure II included the following steps: First, the PCMA-SDS aggregates formed in bulk solution were adsorbed on the mica surface from a 60 µL droplet containing a mixture of 20 ppm PCMA and 1 cmc SDS. Next, PCMA was diluted 6000 times by filling the measuring chamber with a 1 cmc SDS solution. And finally, the SDS solution was exchanged with a 1 × 10-4 M KBr solution. We note that 1 × 10-4 M KBr was present in all solutions used. Atomic Force Microscope Imaging. The principals of atomic force microscopy (AFM) has been described in detail by Binnig et al.14 In our study we used a Nanoscope III, Digital Instruments, Santa Barbara, CA. In brief, a very sharp tip,
Polyelectrolyte-Surfactant Layers attached on a weak cantilever spring, is scanned across a surface. When no force is acting between the tip and the surface, the cantilever remains undeflected. When the tip and the surface are in close proximity to each other, a force acts on the tip, thus forcing the cantilever to deflect. A laser beam is shone on the backside of the cantilever and reflected to a split photodiode that is employed to detect the bending of the spring. The AFM images of polyelectrolytes or polyelectrolyte-surfactant aggregates adsorbed on a mica surface were obtained in contact mode in the liquid using silicone ultralevers (Park Scientific, CA), which prior to use were exposed to ultraviolet light (about 9 mW cm-2 at 253.7 nm) for 1-2 h. We used cantilevers with spring constants of 0.12 ( 0.02 N/m. In the contact mode the tip is moved across the surface in a raster pattern. At the same time the sample height is regulated in response to the force acting between the sample and the tip. With perfectly selected gains, the deflection of the tip is kept constant and a true topographical height image is obtained (ignoring tip broadening effects). We used a dual screen mode and captured both height and deflection data simultaneously. A height image maps the sample vertical position, and a deflection image maps the tip deflection. Both images contain information about the surface features. The adsorbed layers were imaged using a very weak force. The force acting between the sample and the tip during imaging was due to an electrostatic double-layer repulsion. Hence, there was no direct contact between the tip and the sample, which minimizes the risk of damaging the adsorbed layers while dragging the tip over the surface. The same imaging method has been used before. See, for example, ref 15. The images were obtained at a scanning speed of 10 Hz. The fluid cell is made of glass and constructed so that it can be sealed with the help of a rubber ring positioned in a grove. Thus, a small liquid volume (0.1 mL) can be injected into the cell, and, if needed, the liquid can be easily exchanged. The adsorption of the PCMA or PCMA-SDS aggregates was accomplished simply by introducing an aqueous solution of desired composition into the fluid cell and allowing the tip and the freshly cleaved substrate, mica, to equilibrate with this solution for 30 min before imaging. AFM images were taken after each step of procedure I, Figure 2. Small-Angle Neutron Scattering. The SANS data were obtained with the D11 instrument at Institut Max von LauePaul Langevin, ILL, Grenoble, France. The scattering curves were obtained using three different settings, sample-to-detector distances 1.25, 5.5, and 12 m, and a neutron wavelength of 8 Å, covering the Q range 7 × 10-4 to 0.22 Å-1. The samples contained 1000 ppm PCMA and various amounts of deuterated SDS. The solvent used was either D2O, emphasizing the scattering from PCMA, or a mixture of 80% H2O and 20% D2O, in which the polyelectrolyte is contrast matched.
Results Surfactant Association with Preadsorbed Polyelectrolyte Layers. The forces acting between mica surfaces across a solution containing 20 ppm PCMA and 1 × 10-4 M KBr are shown in Figure 3. The dominating force at large separations is a weak, up to 0.18 mN/m, electrostatic double-layer repulsion that becomes detectable at separations below 1000 Å. The double-layer force is much weaker than before addition of PCMA, showing that most of the negative charges on the mica surface have been neutralized by the cationic polyelectrolyte. At a distance of 130 Å this repulsion is overcome by an attractive force pulling the surfaces into a separation of 10-14 Å, where a deep adhesive minimum of 110 mN/m is measured. Apparently, PCMA adopts very flat conformations on mica surfaces when the ionic strength is low. At the force minimum the polyelectrolyte layer formed on each surface is just 5-7 Å thick despite the molecular weight of the polymer being 1.6 × 106 g/mol. After the PCMA had been diluted by 1 × 10-4 M KBr to a (14) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (15) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmiur 1994, 10, 4409.
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Figure 3. Force normalized by radius as a function of surface separation. The forces were measured between two mica surfaces interacting across a solution containing 20 ppm of PCMA and 1 × 10-4 M KBr (O). The forces were measured between mica surfaces coated with PCMA after replacing the polyelectrolyte containing solution with a pure 1 × 10-4 M KBr solution (b). The arrows show inward jumps due to the presence of an attractive force.
Figure 4. Force normalized by radius as a function of surface separation. The forces were measured between mica surfaces precoated with PCMA across an essentially polyelectrolytefree (3 ppb) solution containing 1 cmc SDS and 1 × 10-4 M KBr.
concentration of 3 ppb (essentially polyelectrolyte-free solution), no force was acting between the mica surfaces until they were less than approximately 150 Å apart. Just like before the dilution, an attractive force pulled the surfaces into a separation of 10-14 Å. The measured pulloff force from this position was 120 ( 5 mN/m. We note that these results agree well with those reported previously using highly positively charged polyelectrolytes, PCMA and p o l y { 3 - ( 2 - m e t h y l p r o p i o n a m i d o ) p r o p y l} trimethylammonium chloride, MAPTAC,4,16 even though the magnitude of the pull-off force appears to vary substantially (65-250 mN/m). Following the experimental procedure I, upon injection of SDS to a concentration of 1 cmc, a dramatic swelling of the adsorbed PCMA layers takes place due to incorporation of SDS into the adsorbed polyelectrolyte layers. The corresponding surface interactions are displayed in Figure 4. The force curve displays pronounced oscillations with a periodicity of about 40 Å. The strengths of both the repulsive and the attractive force branches increase with decreasing surface separation. In a previous study we have shown that this type of oscillating force curve is observed between PCMA-coated mica surfaces when the SDS concentration is at or above 0.2 cmc.4 In this study we used very thin pieces of mica that made it easy to detect (16) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 166, 343.
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Figure 5. Force as a function of separation between an AFM silicon tip and a mica surface precoated with PCMA (adsorbed from a 20 ppm solution) measured across a polyelectrolyte-free 1 cmc SDS solution. The distance is measured relative to the “hard wall” defined by the constant compliance region reached under a high force. The thick line represents the force measured on approach and the thin one the force measured on separation. The arrow indicates the force used when imaging the polyelectrolyte-surfactant layer.
Figure 6. Force normalized by radius as a function of surface separation. Here we compare the forces measured across identical bulk solutions (1 × 10-4 M KBr), but with different surface history. The forces acting between mica surfaces precoated with PCMA before swelling the layer with SDS are shown (b) together with those measured after first swelling the layer with a 1 cmc SDS solution and when exchanging the SDS solution with a pure 1 × 10-4 M KBr solution (O).
even weak oscillations. This is most likely the reason we observe more oscillations in the present study compared to in the previous one (6 compared to 5). The forces measured between a PCMA-coated mica surface and a silicon AFM tip across a 1 cmc SDS solution are shown in Figure 5. The long-range force is dominated by an electrostatic double-layer force that was put to use when imaging the layer (see below). Two distinct oscillations are observed in the force curve (Figure 5). Similarly, as observed using the SFA (Figure 4), the periodicity of these oscillations is about 40 Å. Hence, both techniques monitor essentially the same internal structure of the adsorbed layer. Replacing the 1 cmc SDS solution by a 1 × 10-4 M KBr solution results in a total disappearance of the oscillating force profile (Figure 6). Instead, the measured interactions resemble those observed between the mica surfaces coated with PCMA across a 1 × 10-4 M KBr solution before SDS was added. Hence, the effect of swelling the polyelectrolyte layer with SDS is to some extent reversible. However, some significant differences are observed. The final separation into which the surfaces are brought by the bridging attraction is 40 Å, i.e., 3-4 times larger than before swelling the polyelectrolyte layer by SDS. Further, the pull-off force has decreased from 120 mN/m before
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Figure 7. Force normalized by radius as a function of surface separation. The forces measured between mica surfaces across a solution containing 20 ppm PCMA, 1 cmc SDS, and 1 × 10-4 M KBr. The line has a slope identical to that of an electricaldouble-layer force at the present ionic strength.
SDS addition to 14.5 ( 5 mN/m. It also appears that the long-range surface interaction (at distances g 200 Å) now is weakly repulsive. Adsorption of Polyelectrolyte-Surfactant Aggregates Formed in Bulk Solution. We now consider the case when polyelectrolyte and surfactant are mixed outside the SFA to obtain a solution containing 20 ppm PCMA (1.03 × 10-4 M charged segments) and 1 cmc SDS (8.3 × 10-3 M). Thus there is a large excess of surfactant over polyelectrolyte segments. Such a solution is turbid because of the formation of large and negatively charged polyelectrolyte-surfactant aggregates.11 This solution was placed between the mica surfaces (procedure II, step 1), and the surface force curve was determined (Figure 7). The slope of the force measured at large distances (70170 Å) is consistent with that of an electrical double-layer force at the known ionic strength and equal to about 33 Å. In the theoretical calculation of the Debye length we have ignored the contribution of the polyelectrolyte to the ionic strength. The reason is that the negatively charged polyelectrolyte-surfactant complexes will be depleted from the region between the surfaces and they do not contribute to the screening of the double-layer force. This result is consistent with previous observations of doublelayer forces in systems containing highly charged species.17,18 At distances smaller than 70 Å, the slope of the measured force is steeper than expected from double-layer theory and we conclude that a steric force contribution becomes predominant (Figure 7). The same forces were observed on approach and on separation. It is clear that some polyelectrolyte-surfactant aggregates are adsorbed on the surface. We have argued previously that adsorption of the negatively charged aggregates on the negatively charged surface is accompanied by expulsion of some SDS from the aggregates.11 Next, the dilution of PCMA to a bulk concentration of 3 ppb by filling the measuring chamber with a 1 cmc SDS solution increases the range of measured force (Figure 8, unfilled symbols, and Figure 9). Now, the decay length of the long-range force (about 65 Å) is no longer consistent with that of an electrical-double-layer force at the given ionic strength (33 Å). At shorter separations, 50 Å and less, the slope of the force becomes significantly steeper. Thus, it is not possible to desorb PCMA-SDS aggregates attached to the mica surface by merely diluting the bulk (17) Marra, J.; Hair, M. L. J. Colloid Interface Science 1989, 128, 511. (18) Claesson, P. M.; Ninham, B. Langmuir 1992, 8, 1406.
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Figure 8. Force normalized by radius as a function of surface separation. The forces measured between mica surfaces precoated with PCMA across an essentially polyelectrolyte-free (3 ppb) solution containing 1 cmc SDS and 1 × 10-4 M KBr are represented by (9). The forces obtained after PCMA-SDS aggregates have been adsorbed from a PCMA-SDS mixture containing 20 ppm PCMA and 1 cmc SDS and subsequently diluted by 1 cmc SDS solution is shown by (0).
Figure 9. Force normalized by radius as a function of surface separation. The forces were measured between mica surfaces coated with PCMA-SDS aggregates adsorbed from a mixture containing 20 ppm PCMA and 1 cmc SDS (O), after exchanging this mixture with a pure 1 cmc SDS solution (9), and, next, after exchanging the 1 cmc SDS solution with a pure 1 × 10-4 M KBr solution (0). The lines have slopes identical to those of electrical-double-layer forces at the present ionic strengths.
polyelectrolyte-surfactant mixture with a surfactant solution, at least not within the experimental time scale of 2 days. It is interesting to note that no oscillations in the force curve were observed, contrary to the case when a 1 cmc SDS solution was contacted with preadsorbed polyelectrolyte layers on the mica surfaces. Finally, the 1 cmc SDS solution was replaced by a 1 × 10-4 M KBr solution. This resulted in two important changes in the surface interaction (see Figure 9). First, the force becomes much more long-ranged. This is simply due to the decreased ionic strength that allows the double layer force to extend to larger separations. This force contribution has a decay-length of 300 Å in 1 × 10-4 M KBr, which is consistent with the decay length of the force measured at distances above 150 Å. Second, the range of the steric force decreases from above 300 Å in 1 cmc SDS solution to less than 150 Å in 1 × 10-4 M KBr. AFM Images. The adsorbed polyelectrolyte layers were imaged using AFM. The images of PCMA on mica obtained across a 20 ppm PCMA solution were the same as after dilution with 1 × 10-4 M KBr. In both cases the AFM images were nearly featureless, demonstrating that the adsorbed polyelectrolyte layer was smooth and homoge-
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neous (Figure 10). This is consistent with the very flat layer (Figure 3) observed with the surface force apparatus. The AFM images obtained after injecting a 1 cmc SDS solution in the measuring chamber (step 3 in experimental procedure I) show that the adsorbed polyelectrolyte layers are no longer flat and smooth. Instead, large-scale topographical features are observed with typical lateral dimensions of 200 nm (Figure 11). The height differences observed in the images are typically around 40 Å. We note that the deflection of the cantilever is not absolutely constant during imaging and the height difference may be underestimated by as much as 10 Å due to this effect. The image clearly demonstrates that SDS is incorporated in the preadsorbed polyelectrolyte layer, as also demonstrated by the surface force technique (Figure 4). After removing SDS from the bulk solution and flushing the measuring cell with 1 × 10-4 M KBr, the adsorbed polyelectrolyte-surfactant layers have collapsed back on the surface (Figure 12). However, even though the bulk solution composition is identical to that of Figure 10, the structure of the adsorbed layer is different. It is seen that some material on the surface is collected in small lumps. It seems likely that some SDS remains entrapped in the collapsed adsorbed layer. Small-Angle Neutron Scattering. The small-angle neutron scattering curve for a sample containing 1000 ppm PCMA and 0.5 wt % deuterated SDS is shown in Figure 13. It was obtained using two different solvents. In the first solvent, D2O, the scattering contribution from deuterated SDS is negligible. In the other solvent (80% H2O and 20% D2O), the polyelectrolyte is contrast matched as evidenced by a flat scattering curve for a sample containing only the polyelectrolyte in this solvent mixture. Nevertheless, the scattering curves look very similar in the two solvents. This shows that the surfactant and the polyelectrolyte have similar distributions in the aggregates. The difference at intermediate Q-values is most likely due to the presence of some free SDS micelles. We base this statement on the fact that no evidence for the presence of micellar-like aggregates was observed at SDS concentrations below 0.5 wt %, whereas an additional scattering peak typical of free SDS micelles was observed when the SDS concentration was increased to 2 wt %. The mean size of the aggregates is too large to be determined from these data. A peak due to correlation between scattering centers is observed at Q ) 0.16-0.17 Å-1, corresponding to a characteristic distance of 37-39 Å. We have previously suggested that SDS form discrete micellarlike aggregates along the PCMA backbone,4 i.e., a structure similar to that formed by SDS associated with poly(ethylene oxide).19,20 However, the SANS data presented in Figure 13 are inconsistent with this model. Rather it appears that mixed association structures are formed between the polyelectrolyte and the surfactant and that these structures are separated by 37-39 Å. One may suggest that the characteristic distance corresponds to the distance between surfactant loaded polyelectrolyte chains. Further results from the SANS study will be reported in a forthcoming article. Discussion Crucial Differences between the SFA and the AFM. There are several important differences between the SFA and the AFM. Here we will highlight a few of them that are relevant for the present study. In the AFM a sharp tip probes the local structure of the adsorbed layer. (19) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (20) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19.
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Figure 10. AFM deflection image of a PCMA layer adsorbed on a mica surface from a 20 ppm PCMA solution in 1 × 10-4 M KBr. The image was taken in contact mode in a liquid cell filled with a polyelectrolyte-free 1 × 10-4 M KBr solution.
On the other hand, in the SFA one uses two macroscopically curved surfaces, and the measured force is sampled over a rather large surface area (of the order of 1000 µm2, with the exact value depending on the local radius and the range of the force). When the adsorbed layer is homogeneous, the Derjaguin approximation21 is applicable, which allows us to relate the measured force to the free energy of interaction per unit area between flat surfaces (provided the forces are small enough to ignore surface deformation effects). However, when the surface is inhomogeneous, this is no longer the case since some parts of the surface may then contribute unproportionally more to the measured interaction. This is for instance the case when the SDS swelled layer has been collapsed by removal of the SDS from the bulk solution. The AFM image shows that some material is collected in aggregates on the surface (Figure 12). Even though these aggregates only cover a small fraction of the surface, the work needed to deform them will dominate the interaction at small separations. As a consequence the adsorbed layer thickness appears to have increased from 10 to 14 Å before addition of SDS to 40 Å after collapsing the SDS swelled layer (Figure 6). The AFM image (Figure 12) makes it plausible that the average thickness has increased considerably less. Another difference between the SFA and the AFM is that the SFA measures the absolute layer thickness (relative to contact between the two mica surfaces). The AFM image, on the other hand, shows how the topography of the surface varies at a given force load. For a homogeneous polyelectrolyte layer or a swelled polyelectrolyte-surfactant layer, the tip is never in direct contact with the mica surface. Hence, in Figures 10-12 the height referred to is the roughness of the layer observed under a given load. It should be emphasized that these images contain no information about the thickness of the adsorbed (21) Derjaguin, B. Kolloid-Z. 1934, 69, 155.
layer. However, from the force curve measured between the AFM tip and the sample, it appears that by applying a higher load, the tip can penetrate a further 100 Å down into the layer. We note that the absolute layer thickness as a function of load is directly obtained with the SFA. Adsorption/Desorption Processes. Before discussing the results obtained in the present investigation, we need to spend some time considering the properties of the mica surface. Mica is a layered aluminosilicate mineral. Each layer is strongly negatively charged due to isomorphous substitution where some silicon atoms are replaced by aluminum. These charges are compensated for by monovalent ions, mostly potassium, located between the layers. The number of such ions on the outer surface is 2.1 × 1014 cm-2. When the mica surface is immersed in water, the potassium ions are dissolved and partly exchanged by hydrogen ions.22,23 The exchange is not complete, but the mica surface is strongly negatively charged in dilute salt solutions. When a highly charged cationic polyelectrolyte, such as PCMA, is introduced in the low ionic strength solution, it is strongly adsorbed to the surface. The adsorption is accompanied by a release of solvated hydrogen ions, and the number of adsorbed segments are close to the number of mica lattice charges,16 i.e. 2.1 × 1014 cm-2. Due to the strong affinity between the polyelectrolyte and the oppositely charged surface a very thin adsorbed layer is obtained, as observed in this study and previous reports.4,16 The electrical double-layer repulsion that is observed when the PCMA is present in bulk solution is very weak, and it disappears when the polyelectrolyte is removed from the solution. This result suggests that the number of adsorbed cationic charges, mainly from the polyelectrolyte but also with some contribution from hydrogen ions,24 initially was slightly larger than the lattice charge. Further desorption of PCMA in 1 × 10-4 (22) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (23) Claesson, P. M.; Herder, P. C.; Stenius, P.; Eriksson, J. C.; Pashley, R. M. J. Colloid Interface Sci. 1986, 109, 31.
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Figure 11. AFM image of preadsorbed PCMA layers taken in contact mode in a liquid cell containing a 1 cmc SDS solution. The force used when imaging is illustrated by the arrow in Figure 5. Part a is a 3D representation of the height image, and part b shows the height variation along a line across the image.
M KBr solution is negligible, which is well-understood theoretically.25 It has also been shown experimentally using a similar highly charged polyelectrolyte, MAPTAC, by the method of X-ray photoelectron spectroscopy (XPS)26 that about 20% of the polymer was desorbed when the salt concentration (NaCl) was increased to 0.1 M. The situation is more complex when instead of a 1:1 inorganic electrolyte a charged surfactant is present in the solution in contact with the polyelectrolyte-coated surface. The anionic surfactant ion will, just like an inorganic anion, be attracted to positive charges on the polyelectrolyte molecule and repelled by negative charges on the mica surface. The electrostatic interactions are the same for the two types of anions. However, when the anion is a surfactant, there is a strong hydrophobic interaction that drives the surfactant to self-assemble. In a polyelectrolyte-free bulk solution this occurs at the cmc. With polyelectrolytes present in the bulk solution, the selfassembly occurs at a much lower concentration, the critical (24) Blomberg, E.; Claesson, P. M.; Fro¨berg, J. C. Biomaterials 1998, 19, 371. (25) Cohen Stuart, M. A.; J., F. G. Annu. Rev. Mater. Sci. 1996, 26, 463. (26) Rojas, O.; Ernstsson, M.; Neuman, R. D.; and Claesson, P. M., submitted.
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association concentration, cac. When the polyelectrolyte has a hydrophilic backbone, the main reason is that the polyelectrolyte is a very efficient counterion to the micelle, which is well-understood theoretically.27-30 In such a case the polyelectrolyte chain is located outside the micelle. On the other hand, mixed association structures containing polyelectrolyte segments and surfactants may form when the polyelectrolyte has a (slightly) hydrophobic backbone and/or hydrophobic side chains.31 In this case the polyelectrolyte is partly mixed in with the surfactant hydrocarbon region. The critical association concentration between polyelectrolytes adsorbed to a surface and surfactants, cacs, is expected to depend on the nature of the polyelectrolyte, the surfactant, and the surface. For the PCMA-mica-SDS system we have previously shown that cac < cacs < cmc, with the cacs ) 0.1-0.2 cmc. Hence, at this concentration the chemical potential of the surfactant has become sufficiently high to allow it to compete efficiently with negative surface sites for association with cationic polyelectrolyte segments. Despite the incorporation of SDS in the adsorbed layer, only a limited desorption of polyelectrolytes occurs. For instance, it has been shown that exposing a mica surface precoated with MAPTAC, which is similar in structure to PCMA, to a 1 cmc SDS solution for 24 h led to a reduction of the adsorbed amount of the polyelectrolyte by just about 20%.26 The molecular mass of the MAPTAC polymer used was 4.8 × 105 g/mol, which is less than one-third of the molecular weight of the PCMA used in this study. Thus, we expect that an even smaller fraction of the initial PCMA layer is desorbed by the surfactant in the present case. We note that both incorporation of SDS to the adsorbed layer and partial desorption of PCMA due to the presence of SDS will result in the buildup of a negative surface charge on the initially uncharged PCMA-coated mica surface. Finally, we note that even though an adsorbed polymer cannot easily be removed by dilution, it can as a rule easily be exchanged with another polymer with marginally higher adsorption affinity.32 From studies of protein adsorption, using labeled proteins, it is also well-known that a protein in solution can exchange with a similar adsorbed protein,33 and the same ought to be true for any other polymer. This may be the explanation why the range of the repulsive force increases when the solution containing 20 ppm PCMA and 1 cmc SDS is replaced by a polyelectrolyte-free 1 cmc SDS solution (i.e. going from step 1 to step 2, procedure II in Figure 2). The effect is illustrated in Figure 9. It cannot be due to the marginally decreased ionic strength. Instead it is evident that when the bulk polyelectrolyte is removed by dilution with SDS solution, the previously established equilibrium between the bulk and the surface is disturbed. As long as PCMA is present in solution, there will be an exchange between adsorbed and bulk polyelectrolytes. Naturally, the adsorbed chains that extend out into solution are more easily exchanged than those residing close to the surface with most of their segments. This mechanism favors formation of a compact layer. However, as soon as the polyelectrolytes are removed from solution, the exchange mechanism will (27) Wallin, T.; Linse, P. Langmuir 1996, 12, 305. (28) Wallin, T.; Linse, P. J. Phys. Chem. 1996, 100, 17873. (29) Wallin, T.; Linse, P. J. Phys. Chem. B 1997, 101, 5506. (30) Wallin, T.; Linse, P. Langmuir 1998, 14, 2940. (31) Linse, P.; Piculell, L.; Hansson, P. In Models of polymer-surfactant complexation; Linse, P., Piculell, L., Hansson, P., Eds.; Dekker: New York, 1998; Vol. 77. (32) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (33) Norde, W. Adv. Colloid Interface Sci. 1986, 25, 267.
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Figure 12. AFM deflection image of PCMA layers as they appear at the end of experimental procedure I, i.e., the layer was first swelled by exposure to a 1 cmc SDS solution that subsequently was replaced with pure 1 × 10-4 M KBr. Patches of adsorbed material are clearly seen, showing that the layer no longer is homogeneous. The image was taken in contact mode.
Figure 13. log-log plot of scattering intensity vs scattering vector for a sample containing 1000 ppm PCMA and 0.5 wt % deuterated SDS (1.00E+03, for example, means 1.00 × 103). The curves were recorded in D2O (() and in a mixture of 80% H2O and 20% D2O (O).
stop and the adsorbed polyelectrolyte chains may adopt more extended conformations without being removed from the surface. We propose that this is the explanation for the swelling observed when the polyelectrolyte is removed from solution. Origin of the Oscillating Force Profile Observed between Preadsorbed PCMA Layers Swelled by SDS. Preadsorbed PCMA layers are strongly swelled by association with SDS. This indicates that part of the polyelectrolyte chain is desorbed from the surface. However, the polyelectrolyte as a whole remains attached to the mica surface for a period of at least several days. When the swelled layers are pushed together, oscillating force curves are observed both with the SFA (Figure 4) and the AFM (Figure 5). The reason is that the internal structure of the adsorbed layer changes in order to minimize the free energy when the surfaces are brought together. We note that similar, but weaker, oscillations with larger periodicity have been observed due to packing of free SDS
micelles in the confined space between two surfaces.34 Such oscillating force curves have also been reported for micellar CTAB solutions.35 However, these forces were observed only well above the cmc (0.25 M ) 30 cmc in the case of SDS). We have previously argued4 that the oscillations are due to the presence of small SDS micellarlike structures stabilized by the polyelectrolyte. However, our recent SANS results (Figure 13) do not seem to be consistent with this picture. Instead, the peak observed at 0.16-0.17 Å-1, corresponding to a correlation distance of 37-39 Å, might be interpreted as the distance between two surfactant-loaded polyelectrolyte chains, i.e., the mesh size within the layer. This characteristic distance agrees very well with the periodicity of the oscillating forces observed with the SFA and the AFM. Oscillating forces across single foam films stabilized by polyelectrolytesurfactant mixtures have recently been reported.36 In one of the papers the periodicities of the oscillations were correlated with the mesh size in the bulk polyelectrolyte solution.37 Similar results were obtained by Milling38 using AFM. He studied forces acting between a flat surface and a colloidal probe across a concentrated polyelectrolyte solution and observed a force curve displaying weak oscillations. Our results are different from the ones mentioned above because we have no polyelectrolyte present in solution, but the characteristic length scale is a property of the adsorbed polyelectrolyte-surfactant layer (and the internal structure of dispersed bulk aggregates). It seems likely that the structure of our polyelectrolyte-surfactant aggregates is similar to those (34) Hartley, P. G. In Measurement of Colloidal Interactions Using the Atomic Force Microscope; Hartley, P. G., Ed.; Wiley: New York, 1999; p 253. (35) Richetti, P.; Ke´kicheff, P. Phys. Rev. Lett. 1992, 68, 1951. (36) Bergeron, V.; Langevin, D.; Ascacios, A. Langmuir 1996, 12, 1550. (37) Asnacios, A.; Espert, A.; Colin, A.; Langevin, D. Phys. Rev. Lett. 1997, 78, 4974. (38) Milling, A. J. J. Phys. Chem. 1996, 100, 8986.
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of the mesomorphous polyelectrolyte-surfactant phases described by Antonietti et al.39-42 There are several reasons why more oscillations are observed with the SFA than with the AFM. First, in the SFA experiment both surfaces are coated with a surfactant-swelled polyelectrolyte layer, whereas in the AFM this is the case only for the substrate surface but not for the tip. Second, the SFA measures the average force over a much larger surface area than that probed by the AFM tip, which means that the resolution in force divided by local radius (F/R) is better for the SFA than for the AFM. Hence, weak oscillations can easily be missed by the AFM. Third, SFA measurements are carried out at a much slower rate than the AFM measurements. The force curve displayed in Figure 4 took about 6 h to measure, whereas the AFM force curve in Figure 5 took fractions of a second to record. This means that oscillations due to slow rearrangements (seconds to minutes) within the polyelectrolyte-surfactant layer are easily detected with the SFA, but not with the AFM. Interactions between Preadsorbed Polyelectrolyte Layers across 1 × 10-4 M KBr Before and After Swelling the Layer with SDS. Even though the bulk composition at step 2 and step 4 (procedure I in Figure 2) is the same, the state of the adsorbed layer is clearly different, as illustrated by the force curves displayed in Figure 6 and the AFM images in Figures 10 and 12. This is true even after more than 48 h equilibration, demonstrating the slow approach toward the “true” equilibrium state. These changes indicate that not all of the surfactant is desorbed, but some remains entrapped in the polyelectrolyte layer. Evidently, the polyelectrolyte is not as homogeneously distributed on the surface in distinction to the case of adsorption from the surfactant-free solution. The layer thickness is larger and the adhesion is reduced at step 4 compared to step 2. We propose that the weaker adhesion is a consequence of the larger layer thickness that makes bridging less favorable.43 Further, the incorporation of SDS in the layer means that less polyelectrolyte segments are available for bridge formation. Effects due to the Order of Adding Polyelectrolyte and Surfactant. In a previous study Dahlgren et al. showed that the order in which polyelectrolytes and simple salt were added to the solution had a strong effect on the resulting adsorbed layer structure and the range of the steric force.44 This difference persisted for several days (the duration of the experiment), showing that the approach toward a true equilibrium state was very slow. In light of this it is only natural that the order in which polyelectrolytes and surfactants are added also affects the properties of the adsorbed layer. This is for instance the case for the adsorbed amount of polyelectrolyte. The amount of PCMA adsorbed onto mica from a mixture containing 20 ppm PCMA and 1 cmc SDS has been found to be 0.084 mg/m2.11 On the other hand, when PCMA is adsorbed from an SDS-free 20 ppm polyelectrolyte solution, the adsorbed amount is 0.7 mg/m2, i.e., 8 times larger.16 We argued above that less than 20% of the polyelectrolyte will desorb when exposed to a 1 cmc SDS (39) Antonietti, M.; Kaul, A.; Thu¨nemann, A. Langmuir 1995, 11, 2633. (40) Antonietti, M.; Burger, C.; Effing, J. Adv. Mater. 1995, 7, 751. (41) Antonietti, M.; Maskos, M. Macromolecules 1996, 29, 4199. (42) Antonietti, M.; Wenzel, A.; Thu¨nemann, A. Langmuir 1996, 12, 2111. (43) Dahlgren, M. A. G.; Waltermo, Å.; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; Åkesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (44) Dahlgren, M. A. G.; Hollenberg, H. C. M.; Claesson, P. M. Langmuir 1995, 11, 4480.
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solution. With the large difference in adsorbed amount of PCMA at step 3, procedure 1, and step 2, procedure 2 (Figure 2), it is obvious that the surface interaction must be different even though the composition of the bulk solution is the same. We note that oscillating forces are observed when the adsorbed amount of PCMA is relatively high, but not when it is small (Figure 8). This seems natural since the oscillations arise due to changes in the internal structure of the polyelectrolyte-surfactant layer under confinement. Another reason for the lack of oscillations in the force curve between surfaces coated with adsorbed aggregates may be that the structural units, giving rise to the characteristic length scale observed with SANS, have a random orientation relative to the surface. Replacing the 1 cmc SDS solution with a 1 × 10-4 M KBr solution does not result in a complete desorption of SDS but rather causes a deswelling of the PCMA-SDS aggregates (step 4, procedure I, and step 3, procedure II). The removal of SDS from the bulk solution reduces the net negative charge of the adsorbed layer, which explains the collapse. However, it is at first glance surprising that not all SDS is desorbed by dilution, but this can be rationalized as suggested Ilekti et al.,45 who investigated aqueous systems containing sodium polyacrylate and cethyltrimethylammonium bromide. The authors showed that the phase separation in these systems and the deswelling of the concentrated phase can be induced by addition of water. This phenomenon is accounted for by the fact that the gain in entropy for the small counterions that are released into the dilute phase is larger at low ionic strength. The forces measured after deswelling the polyelectrolyte-surfactant layer by removal of the SDS from the solution is different depending on which of the two procedures has been followed. This is not surprising since the adsorbed amount is different. The fact that a distinct double-layer force is observed when the adsorbed amount is low (Figure 9), but only a weak long-range repulsion when the adsorbed amount is large (Figure 6), is also as expected. More surprisingly, the range of the steric force is larger when the adsorbed amount is smaller. This might indicate formation of a more heterogeneous adsorbed layer containing larger aggregates when procedure II is followed; i.e., the adsorption of polyelectrolyte-surfactant aggregates leads to a more uneven distribution of material on the surface than first adsorbing the polyelectrolyte and then adding the surfactant. Attempts to test this suggestion by imaging the surfaces after adsorption of the PCMA-SDS aggregates failed. No reproducible images could be obtained, which may indicate that the aggregates are too weakly attached to the surface. Conclusions We have shown that adsorbed layers on mica consisting of a highly charged cationic polyelectrolyte, PCMA, and an anionic surfactant, SDS, are readily trapped in metastable states and that “true” equilibrium is not established over a period of several days. In fact, our study does not tell us the nature of the “true” equilibrium. One main reason equilibrium is reached so slowly is that the polyelectrolyte is bound to the surface by many segments, each of which has a high affinity for the surface. Hence, the mobility of the chain on the surface will be low and likewise the desorption will be slow. Another important reason is that stoichiometric PCMA-SDS aggregates are water-insoluble and very stable toward dilution with (45) Ilekti, P.; Piculell, L.; Tournilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344.
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water. Hence, once they are formed on the surface they are difficult to remove. We have also shown that when a preadsorbed polyelectrolyte layer is exposed to a 1 cmc SDS solution, an oscillating force curve with a periodicity of about 40 Å is obtained. Similar results are obtained both with the SFA and the AFM. Small-angle neutron scattering experiments show that about the same characteristic length scale is present within polyelectrolytesurfactant aggregates dispersed in aqueous solution. The SANS data are, however, not consistent with micellarlike aggregates of SDS in a bead-and-necklace structure along the polyelectrolyte and rather suggest that mixed aggregates containing both polyelectrolyte segments and
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surfactant are formed. Hence, it seems likely that the internal structure is reminiscent of those described by Antonietti et al. for other polyelectrolyte-surfactant systems. Acknowledgment. We acknowledge the assistance of Dr. Isabelle Grillo at ILL. M.B. acknowledges financial support from the Centre for Surfactants Based on Natural Products (SNAP). We thank Lachlan Grant for introducing us to the AFM imaging technique. Professor Srinivas Manne is thanked for constructive discussions. LA9911537
