Surfactant Mixtures at Solid

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Adsorption and Desorption of Polymer/Surfactant Mixtures at Solid-Liquid Interfaces: Substitution Experiments D. Zimin,† V. S. J. Craig,‡ and W. Kunz*,† Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany, and Department of Applied Mathematics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 2000, Australia Received February 20, 2004. In Final Form: May 27, 2004

The adsorption of mixtures of aqueous solutions of cationic hydroxyethylcellulose polymer JR400 and anionic surfactant, sodium dodecyl sulfate, using atomic force microscopy (AFM) has been studied. Samples with various compositions from different regions of the ternary phase diagram presented in our previous work were imaged by atomic force microscopy on freshly cleaved mica, and hydrophobically modified mica and silica in soft-contact mode. A series of “washing” (subsequent injection of compositions with gradually decreasing polymer/surfactant ratio) and “scratching” (mechanical agitation of the surface material with an AFM tip) experiments were performed. It was revealed that the morphology of the adsorbed layer altered in a manner following the changes in morphology in the bulk solution. These changes were evidenced in cluster formation in the layer. The results suggest that the influence of the surface was limited to the formation of the adsorbed layer where the local concentrations of polymer and surfactant were higher than those in the bulk. All further modifications were driven by changes in the mixture composition in bulk. Force measurements upon retraction reveal the formation of network structures within the surface aggregates that will greatly slow structural reequilibration.

Adsorption of mixtures of polymers and surfactants at solid-liquid interfaces has been extensively studied in recent decades.1-3 Generally a fine interplay between hydrophobic, hydrophilic, and electrostatic interactions defines the final interaction and adsorption pattern.4,5 In the present article we study mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic polymer JR400 (Quatrisoft). The properties of both components and details of their interaction in bulk are described elsewhere.6,7 For this system where ionic polymer and surfactant bear charges of opposite sign, the electrostatic interactions are more significant than for any other type of polymer/surfactant system. Furthermore, the properties of the surface, especially its charge and wettability are important, as is the properties of the mixture to the final adsorption pattern. There are few ways to investigate the mutual influence between the mixture composition in the bulk and the surface properties during adsorption as an instantaneous change in the surface properties while the adsorption proceeds is difficult to undertake. No means currently exist to exclusively change the hydrophobicity of the surface during adsorption, and an attempt to change its charge (with the exception of metal surfaces8,9) is hardly possible without influencing the composition in bulk. On * To whom correspondence may be addressed. E-mail: werner. [email protected]. † University of Regensburg. ‡ The Australian National University. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Robb, I. D. Annu. Surf. Rev. 2000, 3, 97. (3) Nagarajan, R. Polym. Mater. Sci. Eng. 2001, 85 65. (4) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (5) Holmberg, K. Surfactants and polymers in aqueous solution, 2nd ed.; John Wiley & Sons: New York, 2003. (6) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73. (7) Zimin, D.; Craig, V. S. J.; Kunz, W. Langmuir 2004, 20, 2282.

the other hand, while exploring the same surface, an alteration in the composition of the mixture in the bulk is relatively simple. Investigations of both kinds have been performed in recent years.10,11 Some of them are of interest for the present article, as they deal with the SDS-JR400 system or with the atomic force microscopy technique on this or related systems. That is, SDS interacting with nonionic hydrophobically modified cellulose derivatives or with other hydrophobically modified cationic or nonionic polymers. Here we present a brief overview. Interesting studies have been made of the interactions between SDS and nonionic polymers: poly(vinylpyrrolidone) (PVP)12,13 and ethyl(hydroxyethyl)cellulose (EHEC). Joabsson et al. studied14 competitive adsorption of SDS and ethyl(hydroxyethyl)cellulose at silica and hydrophobized silica using ellipsometry and compared it to the association in bulk solution. On silica, addition of SDS caused an expansion of the preadsorbed polymer layer, while nonionic surfactant did not. On hydrophobized silica, SDS caused desorption of EHEC, completely replacing it at concentrations above the critical micelle concentration (cmc). This complete replacement could not be reached with hydrophobically modified EHEC (HMEHEC), thus showing the role of hydrophobic association between the adsorbed polymer and the surface. On a hydrophobic (8) Green, R. J.; Tasker, S.; Davies, J.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B. Langmuir 1997, 13, 6510. (9) Burgess, I.; Jeffrey, C. A.; Cai, X.; Szymanski, G.; Galus, Z.; Lipkowski, J. Langmuir 1999, 15, 2607. (10) Levchenko, A. A.; Argo, B. P.; Vidu, R.; Talroze, R. V.; Stroeve, P. Langmuir 2002, 18, 8464. (11) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366. (12) Fadnavis, N.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1984, 106, 2636. (13) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloids Surf., A 1997, 123-124, 283. (14) Joabsson, F.; Lindman, B. Prog. Colloid Polym. Sci. 2000, 116, 74.

10.1021/la0495581 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/12/2004

Adsorption of Polymer/Surfactant Mixtures

surface the competition between SDS and hydrophobic groups on the polymer determines the adsorption behavior. Fleming et al.15,16 studied the adsorption of the SDSPVP system on graphite (hydrophobic surface without charge) using atomic force microscopy (AFM) techniques. The AFM cantilever tip was used here to compress the adsorbed layer for evaluation of its mechanical properties. Interesting surfactant-polymer aggregates, templated by the highly ordered graphite surface, were observed. In contrast mica and silica are known to impose considerably less order on the surface organization. In our previous work on mica and silica substrates, we found that the size of polymer/surfactant aggregates was largely unchanged upon adsorption and their morphology was similar.7 Dedinate et al.17 compared the adsorption of mixtures of a highly charged cationic polyelectrolyte, poly{(propionyloxy)ethyl}trimethylammonium chloride (PCMA), and SDS using AFM and a surface force apparatus, using mica as a substrate. They also performed small-angle neutron scattering for investigations in bulk. Their results, as well as the results of others,11,18-20 reveal that the adsorption of established polymer/surfactant complexes from mixtures differs significantly from the adsorption of pure polymer, followed by addition of surfactant. The surface force techniques were used by Agrillier et al.21 and Shubin22 who showed that the polymers JR400 and LM200 (hydrophobically modified JR400) readily adsorbed at negatively charged surfaces and that this adsorption was affected by the concentration of the SDS present in the solution. SDS formed a complex with the adsorbed polymer layer, and at concentrations higher than the cmc of SDS, the surfactant caused desorption of the complex from the surface. Goddard,23 using fluorescence microscopy to study the adsorption of SDS and JR400 to hair, a negative surface, also found that SDS could cause a partial polymer desorption from the surface. An important general observation made during the direct investigations of adsorption of polymer/surfactant systems at solid-liquid interfaces is that the equilibrium establishment in these systems is extremely slow. The appearance and the properties of the adsorbed layer can change even after several days of equilibration. One main reason equilibrium is reached so slowly is that the polyelectrolyte is bound to the surface by a number of segments, each of which has a high affinity for the surface. In our system the mobility of the polymer will be dependent upon the number of segments bound to the surface. This can be interpreted in terms of the number of loops versus chains, a high proportion of chains corresponding to a large number of bound groups. In the presence of SDS the mobility is therefore determined by the relative affinity of SDS and the polymer for the hydrophobic surface. Hence at high polymer/surfactant ratios the mobility of the JR400 polymer chain on the surface will be low and consequently any desorption will be slow. (15) Fleming, B. D.; Wanless, E. J.; Biggs, S. Langmuir 1999, 15, 8719. (16) Fleming, B. D.; Wanless, E. J. Microsc. Microanal. 2000, 6, 104. (17) Dedinate, A.; Claesson, P. M.; Bergstroem, M. Langmuir 2000, 16, 5257. (18) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476. (19) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566. (20) Rojas, O. J.; Claesson, P. M.; Berglund, D.; Tilton, R. D. Langmuir 2004, 20, 3221. (21) Agrillier, J. F.; Ramachandran, R.; Harris, W. C.; Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242. (22) Shubin, V. Langmuir 1994, 10, 1093. (23) Regismond, S. T. A.; Heng, Y.-M.; Goddard, E. D.; Winnik, F. M. Langmuir 1999, 15, 3007.

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Figure 1. Ternary phase diagram of the system “SDS-JR400water”. The region of precipitation (turbidity) is shaded. Stars denote the composition of samples (with identifying labels) used for more detailed further investigations. The dotted line represents the theoretical composition of stoichiometrical equality between polymer charges and SDS, and the dashed line represents the composition where the precipitation maximum was observed experimentally. The large format numbers denote characteristic regions in the phase diagram: 1, preprecipitation region; 2, precipitation or turbidity region; 3, redissolution region; 4, region of high dilution.

It is important to mention that most studies, except the work of Dedinate et al.,17 were performed at relatively high surfactant concentrations, reaching up to the cmc or even 5 × cmc.11 There is scant information existing on the adsorption and desorption behavior of polymer/surfactant systems at high dilutions. In our previous work7 we studied mixtures of aqueous solutions of cationic hydroxyethylcellulose JR400 polymer and anionic sodium dodecyl sulfate using dynamic light scattering (DLS) and AFM. A ternary phase diagram was established showing three interesting realms of the polymer/surfactant-water mixture: a preprecipitation area of lowered viscosity compared to the pure polymer solution (polymer excess), a postprecipitation area (resolubilization at surfactant excess), and highly diluted compositions with stoichiometrical surfactant-polymer ratio close to that of maximum precipitation. The aggregate size in the bulk as determined by light scattering (R ) 10-300 nm) was found to correlate with the aggregate size at the surface determined by AFM over a wide composition range. The ternary phase diagram indicating the compositions chosen for these investigations is shown in Figure 1. The literature data15-17,24,25 and our own results suggest that when the components are mixed prior to adsorption, the composition of a polymer/surfactant mixture in bulk is of more importance to the adsorption pattern than the properties of the surface, on surfaces other than graphite. In the present study we extend our previous research to so-called washing experiments. In these experiments not only is the behavior of the surface layer investigated in contact with a bulk solution of defined composition but also the solution is gradually replaced by solutions with increasing surfactant-to-polymer ratio and the consequence of this solution replacement for the adsorbed layer is studied. In particular we seek to determine if the (24) Sakai, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 1203. (25) Braem, A. D.; Biggs, S.; Prieve, D. C.; Tilton, R. D. Langmuir 2003, 19, 2736.

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material is more easily removed from the surface. Furthermore, we recorded force-distance curves before and after “scratching” the surface. In this way, more detailed information on the stability and the elasticity of the layer could be obtained. Experimental Section Chemicals. All water used in the research was deionized water. For diluting chemicals and rinsing the fluid cell, MilliQ-Plus (Millipore) purified water was used. SDS from Merck and from ICN was used without further purification. The purity grade of SDS from both manufacturers is >99%. An aqueous stock solution of 1 wt % concentration was prepared at room temperature and stored before further dilution and use. JR400 polymer was obtained from Dow Chemicals. A stock solution of 1 wt % concentration was prepared at room temperature and then filtered to remove insoluble residues, before further dilution. Trimethylchlorosilane from Fluka (>90% purity) was used without further purification. Sample Preparation and Mixing. All concentrations are expressed in weight percent, for the sake of simplicity. According to Goddard6 and our previous work,7 the following approximations were used for all concentration calculations: the average molecule weight per charge of JR400 polymer was assumed to be 670, and the stoichiometric ratio 1:1 (the ratio at the point of electrostatic neutralization) was presumed to be achieved at the weight relation 1/2.3 between w/w solutions of surfactant and polymer, respectively. All mixtures were made from mixtures and dilutions of a 0.075 wt % solution of JR400 polymer and a 0.1 wt % solution of SDS corresponding to 0.42 of the cmc (8.1 mM) of the surfactant (later on called “working solutions”). These very high dilutions were needed to avoid the high viscosity that occurs at polymer prevalence at higher concentrations and, on the other hand, to have sufficiently large concentration ranges exhibiting clear solutions. To avoid precipitation during the mixing process, the mixtures were prepared in the following order: The quantitatively prevalent component (polymer or surfactant) was added, followed by water and then the minor component (surfactant or polymer). At compositions for which precipitation could not be excluded, the mixtures were stirred while adding the components. All samples were produced at room temperature. Generally, the samples were shaken for about 20 s after composition and left at least 10 h for equilibration. Preparation of Surfaces for AFM Imaging. Freshly cleaved mica (muscovite), hydrophobized mica, and hydrophobized flat silicon wafers were used as adsorption substrates. In some cases, the raw mica surface was pretreated in a plasma reactor in order to ensure a clean surface. A plasma reactor custom built in the Department of Applied Mathematics of the Research School of Physical Sciences and Engineering, Canberra, Australia, was used. The reactor uses a radio frequency generator, which permits the passive cleaning of adventitious carbon from inorganic surfaces, rendering them reproducibly clean with a high degree of surface hydroxylation. The device has a vacuum transference chamber for postcleaning reaction with reactive vapors, such as silanes. The raw mica surfaces were treated with a power of 10 W at 120 kHz for 20 s, at a total pressure of around 0.1 Torr, using a mixture of argon and water vapor. Some surfaces were made hydrophobic by treatment with trimethylchlorosilane (TMCS) according to the method of Hair.26,27 Silicon or mica wafers (10 × 10 mm2) were cleaned with carbon dioxide snow,28 followed by rinsing with redistilled ethyl alcohol, and dried under a stream of clean nitrogen gas. Additionally, the wafers were plasma treated and transferred and stored under vacuum. This chamber was subsequently connected to a glass vessel containing TMCS. When a valve was opened, the TMCS vapor entered the chamber and reacted with the prepared silica surface. The degree of surface modification was assessed using the contact angle of a sessile water droplet. It was 60-75° on silicon and ∼30° on mica surfaces. AFM Measurements. The AFM investigations were performed using a Digital Instruments Nanoscope III MultiMode (26) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 2372. (27) Hair, M. L.; Tripp, C. P. Colloids Surf., A 1995, 105, 95. (28) Sherman, R.; Hirt, D.; Vane, R. J. Vac. Sci. Technol., A 1994, 12, 1876.

Zimin et al. instrument (Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australia). The standard contact mode fluid cell was used. Cleaning of the cell was performed using Millipore water and redistilled ethyl alcohol followed by drying with a stream of nitrogen gas. The filling of the cell was performed by sample injection after mounting the cell and before cantilever approach. New cantilevers were used for every measurement. Before the cell was mounted, the cantilevers underwent plasma treatment identical to the substrate (described above), resulting in a hydrophilic, negatively charged cantilever tip, under the solution conditions employed. Imaging. The soft-imaging method of Manne et al. and Senden et al.16,29,30 was used. The advantage of this method is the fine control of the imaging force in the repulsive regime of the tipsample interaction, enabling the adsorbed layer to be imaged without damage. Adsorbed aggregates are generally only visible over a narrow range of applied force (10 nN) onto the surface, at the highest scan rate (60 Hz). Following this, the scanning was repeated over the larger area. This method is hereafter called “scratching”, as the cantilever may move the layer or its parts aside and expose the substrate surface. For strongly bound layers it may not remove the layer but may compress the layer partially, as seen by Fleming et al.16 This method permits the adhesion properties of the adsorbed layer to be probed. Recording and Evaluation of Force-Distance Curves. The force-distance curves were acquired on every sample, with an emphasis on distinguishing the difference between areas with different morphologies. When scratching had been applied, data were acquired inside and outside the scratched region. This provided information on the presence, or absence, of a polymer/ surfactant film, the rigidity of the layer, its elasticity, and hydration. The postmeasurement processing of images and force curves was performed using NanoScope 5.12r2 GUI software and Nanotec WSxM 1.2 software as well as Microcal Origin 7G and Microsoft Excel software.

Results The results of AFM investigations of adsorbed layers of the SDS-JR400 system of different compositions on different surfaces are shown below. First, the adsorption of mixtures in the high dilution region on different surfaces is shown and discussed in light of the force-distance curves and the images obtained after scratching. Then, two series of “washing” experiments are described. In these experiments mixtures with lower SDS:JR400 ratio were substituted by those with higher surfactant-polymer ratio in order to observe changes in structure and adhesion of the adsorbed layer. Comparison of the Same Mixture Adsorbed at Different Surfaces. Images of composition A (composition 27 in ref 7) adsorbed on native, freshly cleaved, mica, hydrophobized mica, and hydrophobized silica surfaces are presented in Figure 2. In each case a region of the surface film was subjected to scratching. In Figure 2A there is no evidence of adsorbed structures in the scratched region, yet no aggregates have accumulated on the borders of the scratched area. We interpret this as indicating that the aggregates have been removed from the surface by (29) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (30) Senden, T. J.; Drummond, C. J.; Ke´kicheff, P. Langmuir 1994, 10, 358.


Figure 2. Three-dimensional representations of 9 × 9 µm2 and 10 × 10 µm2 AFM deflection images of the layer adsorbed from a SDS/polymer/water mixture on hydrophobized silica (A), hydrophobized mica (B), and freshly cleaved (native) mica (C) after scratching, with a composition according to point A in Figure 1. The scratched regions are marked with white frames. The arrows and numbers in (B) denote the acquisition points of the force vs distance curves. The “waved” look of the underlying surface in (B) is an artifact associated with the “flattening” process whereby one scan line is situated in the z-axis relative to the prior one.

scratching. An alternative explanation is that the aggregates have been compressed into a different form, similar to the observation by Fleming et al.16 on graphite. However the change in height corresponding to the scratched region in our case is much bigger (∼5 nm) and this is too large to be attributed to mechanical compression. This indicates that the adsorption of polymer/surfactant aggregates is weak on this surface as sufficient SDS must be available to disengage the hydrophobic interactions between the polymer and the surface at this particular mixture composition. Force curves (discussed below) are particularly sensitive to small amounts of adsorbed polymer, and they reveal that the scratched surface is not completely free of polymer. A comparison of parts A-C of Figure 2 suggests that in this polymer/surfactant system the affinity for the surface is in the order hydrophobized silica < hydrophobized mica < native mica. Contrary to the observations for the silica surface, we see that although most of the aggregates (but not all) could be scratched aside from the hydrophobized mica surface, they did not desorb. On native mica, most of the aggregates remained on the surface but

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were altered in morphology, which indicates the strongest adsorption. It is apparent that the affinity between the SDS and the hydrophobic surface is aiding desorption. The adsorption can be further probed through the interaction forces between the tip and the substrate. In Figure 3 the interaction forces as a function of distance are presented for a single approach and retraction on the hydrophobized silica substrate, before (A) and after (B) scratching. The tip is negatively charged and hydrophilic, and therefore we expect that it bears adsorbed polymer/ surfactant aggregate in all cases. The repulsion on approach at distances beyond 2 nm has an exponential decay length of ∼10 nm, this is what is expected for a double layer force at this composition. Therefore we attribute this repulsion to an electrostatic repulsion at larger separations and a steric repulsion below 2 nm. The surface charge is likely to be positive due to charge overcompensation by the adsorbing polymer. Following scratching, the magnitude of the force on approach is greatly reduced and the form of the force is no longer exponential. Note that if an exponential decay is determined at large separations where the force is decaying most slowly, the decay length is only 4.5 nm, which is considerably shorter than the Debye length in these conditions (∼10 nm). The magnitude of the surface charge has clearly diminished and the interaction is now dominated by steric interactions. This indicates that the scratching process preferentially removes polymer chains from the surface and corresponds to the observation that the surface appears smoother after scratching for this system (see Figure 2A). The interaction forces on approach were highly reproducible. On retraction the forces varied for each measurement, as for each experiment the retraction measurement depends on the configuration of the few polymer chains that bridge between the two surfaces. The length and number of these chains will vary from one run to the next. Both systems show evidence of polymer on the surface. Adhesive force measurements in other systems are discussed in more detail below. As shown in Figure 2B, force curve measurements were performed on the locations indicated corresponding to a flat region produced by scratching (1), presented in Figure 4A, and a region of debris produced by scratching (2), Figure 4B. In Figure 4A a soft repulsion extending to approximately 15 nm is observed before the surfaces reach a harder contact. The decay length and form of the curve (∼3.0 nm at small separations and ∼5.5 nm at larger separations) is not consistent with an electrostatic interaction in this solution. Note that this contact is unlikely to be between the tip and the substrate. That is, polymer is present between the surfaces. The soft repulsion is attributed to electrostatic repulsion between polymer/ surfactant aggregates that extends beyond the film into solution. On retraction, evidence of polymer is seen in the adhesion that extends to greater than 100 nm. In the region of debris accumulation, Figure 4B, the force curve is substantially different and less reproducible. The soft repulsion in the curve is encountered at a larger separation of some 20 nm but is somewhat reduced in magnitude. It has an exponential decay length of ∼5 nm similar to the long-range component of the interaction shown in Figure 4A. A substantially firmer repulsion is encountered at ∼15 nm and continues until a force of ∼12 nN brings the surfaces into a hard contact or compliance region. No such compression behavior was observed in other regions of the surface. This indicates that the debris material is being compressed under the applied load more easily than the scratched polymer lying in the flat region (and also the unscratched polymer; results not shown). It is reasonable

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Figure 3. Force versus separation plots using a hydrophobized silica substrate before scratching (A) and after scratching (B) for sample A. The insert in part A shows the approach curve plotted on a log-normal scale to demonstrate the exponential decay of the force within a distance of ∼10 nm. This corresponds to the Debye length of the solution.

to assume that the density of the complex in this region is lower and represents the commencement of a mechanically induced resolubilization process. Washing Experiments. All washing experiments were performed using hydrophobized silica as the substrate, because the aggregates showed the smallest affinity for this substrate and were therefore more likely to reveal mechanical detergency. Two series of washing experiments were performed to investigate the possible desorption of adsorbed structures which can be caused by an increase of surfactant/polymer stoichiometric ratio. The compositions selected for this study are presented according to their position on the ternary phase diagram shown in Figure 1. While these experimental series were prepared, special attention was paid to the stoichiometric ratio of components. In addition to these samples, pure working solutions of JR400 polymer and SDS were used. To demonstrate the desorption of the polymer layer under the influence of relatively high concentrations of SDS, a different solution with an SDS concentration five times higher was used in the final part of the first series (0.5

wt %, corresponding to approximately two times the cmc of SDS). Desorption at high surfactant concentrations has been reported repeatedly by many groups.17,22,31 However, no information concerning this desorption phenomenon at high dilutions is available. Therefore we chose to investigate how to “wash off” (completely or partially) the adsorbed layer by increasing the surfactant-polymer ratio. Schemes of the experimental series are presented in Figure 5. The bars from left to right demonstrate the sequence by which the samples with polymer excess were substituted by surfactant-rich samples. Series I. The working solution containing 0.075% of JR400 polymer without any surfactant or dilution was imaged on the hydrophobized mica surface; see Figure 6A. The adsorbed layer formed on the surface was homogeneous, i.e., without any evident structure. Scratching did not cause any observable change. (31) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polymer Sci. 1994, 272, 1590.


Figure 4. Force measurements in different locations. Part A is a force curve obtained in the flat region (1) in Figure 2B after scratching. Part B is a force curve obtained in the region of debris accumulated from the scratching process (2). All measurements were made using a hydrophobized mica substrate in solution sample A.

From Composition B to Composition C. The next composition injected was B. No change could be observed or distinguished from the first composition, so the next composition, C, was injected. The image presented in Figure 6B was taken immediately after scratching. The scratched area in the right-hand bottom part of the image is distinguishable only after a remarkable increase of the image contrast, indicating that the adsorbed layer is stable against the mechanical treatment of the tip. From Composition C to Composition D. Composition C was displaced by composition D. This was the first composition change that resulted in a significant change in mechanical stability of the layer; see Figure 6C. The adsorbed layer has accumulated into a large amorphous structure during scratching. These large amorphous structures were unstable and were seen to disappear over a few minutes during imaging. It was observed that the material rearranged on the surface and in doing so “healed” the scratching scar, indicating that the mechanical perturbation had pushed the system away from equilibrium rather than toward it. From Composition D to Composition E. Composition E with stoichiometric ratio SDS:JR400 of 1:2 was injected next. This resulted in the formation of observable structures in the adsorbed layer for the first time. This can be seen in Figure 6D. The formation of such structures is indicative on an SDS induced swelling of the complex as observed in other systems previously.22 This process is preliminary to effective removal of the material. After the surface was scratched, see Figure 6E, a large amorphous structure was again seen. This was considerably more stable than the similar structure observed in the presence of composition D (Figure 6C).

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From Composition E to Composition F. Composition F equates to the theoretical stoichiometrical equality between polymer charges and SDS monomers and should therefore give a precipitation maximum (dotted line in the phase diagram in Figure 1). This composition could not be successfully imaged without the presence of artifacts as the solution injected into the fluid cell was turbid. Observations indicate that multiple clusters formed on the surface over many hours. Subsequent Replacement of Composition F by a 0.1% and Then a 0.5% SDS Solution. The injection of a pure SDS solution caused dramatic changes in the cell, especially after a long equilibration time. After 2 h of equilibration the removal of material from the surface by scratching could be achieved with ease. Further rinsing with a five times more concentrated working solution of SDS (0.5 wt %) resulted in a textured surface, shown in Figure 6F. The material in general is present as clusters, and scratching evidently is very effective in removing the majority of adsorbed material. We note that these clusters have now swollen to the extent that they have an appearance and size similar to that obtained by direct introduction of a surfactant-rich mixture.7 Series II. The second series of washing investigations was performed to obtain some additional information on the processes occurring at compositions very close to the precipitation region, in the preprecipitation region, and (more importantly) in the resolubilization region. The aim was to avoid turbidity-related artifacts and distortions that occurred in the first series of measurements. Two surfactant-rich mixtures were applied in the second series. Note that no pure polymer solution was injected at the commencement, as in the first series. The order of the sample substitution in this series is presented in Figure 5B. The images of compositions B, C, and D did not significantly differ from those obtained in the first series of substitution experiments. Therefore, only the results from the surfactant-rich mixtures are presented in this section. From Composition G to Composition H. Composition H when added into the fluid cell causeing changes in the adsorbed layer similar to those caused by SDS in the first series of washing measurements (compare Figures 7B and 6F). This influence, however, is moderate. Just after injection, as well as after scratching, the situations are similar to that with composition G: a relatively homogeneous layer is observed immediately after injection. Evidence of structure formation accumulates over time. Equilibration times and structure formation were somewhat slower in the second series of measurements, and the structures observed were not as clearly defined. This is attributed to the fact that no preadsorbed polymer layer was present that could act as a sort of “lubricant” between adsorbing polymer/surfactant complexes and the underlying surface, thus facilitating transformations. In this case, some SDS is always present, resulting in the formation of hydrophobic linkages between the polymer and the surface from the outset. Force Curves. In addition to producing images of the surface bearing the adsorbed polymer complex, AFM can be used to investigate the forces of interaction between the AFM tip and the surface. The adhesive force in particular can reveal the nature of the material adsorbed on the surface. On separation of the surfaces it is common that some of the adsorbed material forms a bridge between the surfaces and characteristic force-distance behavior can be observed as the material is forcibly extended. It is often the case that a single molecule can be probed using this very sensitive technique. However, this sensitivity

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Figure 5. Schematic representation of the order in which sample substitution took place (left to right) in “washing-off” experiments for two series denoted (A) and (B). The composition of the columns reflects the composition of the samples. Diamonds represent the reverse stoichiometric ratio, i.e., JR400 monomers:SDS molecules.

comes at the cost of reproducibility. Each experiment is unique and very large numbers of repeat measurements are required in order to build a quantitative ensemble picture of the conformation of the complex on the surface. Our results reveal that insufficient repeat experiments were conducted to demonstrate any clear quantitative changes between different compositions, due to the range of behavior observed. However, just a few experiments can reveal useful qualitative information, such as whether polymer complex is present and whether the complex is being extended into a good or poor solvent. Here we present just two examples of force curves. These were obtained from composition C (Figure 8) and after rinsing with 0.5% SDS (Figure 9) after the surface had undergone scratching. The examples demonstrate a rich variety of behavior even within a single measurement. This is uncommon in simple polymer systems and indicates the complexity introduced by addition of surfactant. For the composition C sample a single large secondary adhesion feature is fit using an entropic based (freely jointed chain, FJC) model.32 The Force, Fchain required to stretch a chain to a length D is given by

Fchain )

()

kBT D aL Lc

where kB is the Boltzmann constant, T is the absolute temperature, Lc is the contour length of the polymer chain being stretched, and L is the Langevin function. For this particular extension a rigid segment length of 0.85 nm and polymer contour length of 6 nm was used to obtain a fit. Note that this model does not incorporate charge interactions, yet good agreement between theory and experiment is obtained. At larger separations, two regions of constant force are observed at values of -0.2 and -0.1 nN, before the force returns to zero. This is attributed to pulling the polymer complex from a good solvent (adsorbed polymer complex) to a less good solvent (aqueous solution). The values are quantized based on the number of chains that are bridging between the surfaces, in this case it is initially two at separations of 25-30 nm, before dropping to one at separations of 33-66 nm. This form of the force-distance trace can result from the separation of a complex from an oppositely charged surface33,34 or indicate that the solvent is poor and therefore the extension of the complex into the (32) Flory, P. J. Statistical Mechanics of Chain Molecules; Hanser Publishers/Oxford University Press: New York, 1988. (33) Chaˆtellier, X.; Senden, T. J.; Joanny, J.-F.; Di Meglio, J. M. Europhys. Lett. 1998, 41, 303. (34) Chaˆtellier, X.; Joanny, J.-F. Phys. Rev. E 1998, 57, 6923.

solvent incurs an energy penalty. If the increase in surface area per unit extension is known, the surface energy of the complex can be calculated. As the exact conformation of the SDS molecules in the complex is not known, the surface area per unit extension can only be estimated. From the polymer dimensions, an estimated value of 3 nm2 m-1 is obtained; this yields a reasonable surface energy of 33 mJ m-2 for the complex. Figure 9 exhibits evidence that a networked structure is being extended from the surface as the surfaces are separated. Up to a separation of 125 nm a mildly increasing force is observed. At this point the force increases rapidly due to the elastic extension of a polymer chain. This force can be fit using the FJC model only if the zero of separation is shifted outward. This indicates that one end of the chain is not anchored on the surface but rather is anchored within a network structure which is extended from the surface. The polymer is not merely entangled, as disentanglement will easily occur on the time scale of the experiment, yielding a negligibly small force, but must have a degree of “cross-linking”. Pure JR400 does not normally form cross-links, therefore the adsorbed SDS structures must be responsible for the links between polymer chains. Evidence for this was seen in the force work performed by Shubin on a similar system.22 Such cross-linking may greatly slow polymer desorption and effectively prevent it from occurring even at high SDS: JR400 ratios. When the zero of separation is shifted by 74 nm the force can be fitted with the FJC model using a rigid segment length of 0.75 nm and polymer contour length of 35 nm. Note that a similar quality fit can be obtained using a zero shift of 71 nm and a rigid segment length of 1 nm and polymer contour length of 35 nm, so these values are clearly only semiquantitative. A similar event at larger separations has been fitted with the FJC model using a zero shift of 144 nm and a rigid segment length of 0.75 nm and polymer contour length of 45 nm. The highly structured region between the two FJC fitted regions is attributed to deformation of a complicated network structure. Note also that at large separations a plateau in the force curve is seen and the force has not yet returned to zero at a separation of 300 nm, indicating that polymer is still bridging the surfaces. Evaluation of the nanoscale structure of adsorbed complexes is complicated. The stoichiometry and structural arrangement of the adsorbed material is likely to be different from bulk due to the perturbing influence of the surface. The surface charge will favor adsorption of polymer over SDS and higher concentrations of polymer in the adsorbed layer compared to bulk may induce increased aggregation of SDS in micellar type structures.


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Figure 6. AFM deflection images obtained from the polymer/surfactant mixtures adsorbed on the hydrophobized silica surface in the sequence shown in Figure 5A. White squares, where present, denote the scratched areas. (A) 1 × 1 µm2 image of the working solution of JR400 polymer. (B) 5 × 5 µm2 image (enhanced contrast) of the working solutions of SDS (3.9%) and JR400 polymer (96.1%) (composition point C in Figure 1). (C) 5 × 5 µm2 image of the working solutions of SDS (7.55%) and JR400 polymer (92.45%), composition D, immediately after scratching. (D) 1 × 1 µm2 image of the working solutions of SDS (14%) and JR400 polymer (86%), composition E. The elongated form of structures is an artifact. (E) 5 × 5 µm2 image of the working solutions of SDS (14%) and JR400 polymer (86%), composition E. (F) 5 × 5 µm2 image of a polymer/surfactant layer after repeated rinsing with 0.5 wt % solution of SDS. The elongated form of structures in the bottom half of the image is an artifact due to the softness of the structures being perturbed by the imaging forcesonly the transverse size is relevant. The flat scratched area is clearly seen in the top part of the image. The SDS:JR400 ratio is increasing from image A to image F.

We suggest that analysis of a great many adhesion events similar to those described above may enable the structural arrangements of surface complexes to be unravelled in a quantitative manner, but such a study would best be performed in conjunction with appropriate molecular

dynamics studies to investigate the structural details of the complexes probed. It should be noted that the adhesion data indicate that even after scratching some polymer complex remains on the surface in all the samples studied, even at high levels

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Figure 7. AFM deflection images obtained from the polymer/surfactant mixtures adsorbed on the hydrophobized silica surface in the sequence shown in Figure 5B. The white squares denote the scratched areas. (A) 5 × 5 µm2 AFM deflection image of the working solutions of SDS (86.7%) and JR400 polymer (13.3%), composition G in Figure 1 (composition 85 in ref 7). The image was obtained after 45 min of equilibration and 5 min of scratching. Some structures are visible in the adsorbed layer. (B) 10 × 10 µm2 image of the working solutions of SDS (92.87%) and JR400 polymer (7.13%), composition H in Figure 1. The image was acquired immediately after scratching the surface for 5 min. The scratched areas are indicated by a white square.

Figure 8. Force measured on separation between the cantilever tip and a hydrophobized silica surface immersed in a fluid of the working solutions of SDS (3.9%) and JR400 polymer (96.1%), composition point C in series I captured after scratching. The large secondary adhesion is fitted using an entropic FJC model (see text). Horizontal lines indicate the force values associated with the two plateaus in the force versus distance curve.

of SDS. Unfortunately, the AFM technique is not able to quantify the adsorbed amount of polymer. A brief summary of washing investigations can be presented as follows. If the solution composition is changed, the adsorbed layer undergoes modifications similar to those that have occurred in bulk but the process occurs at a greatly reduced rate. These conformational and compositional alterations in the adsorbed layer have been investigated and reported in various papers, see, e.g., refs 1, 5, 14, 17, 22, and 35, at different conditions and at higher polymer concentrations or, more often, surfactant concentrations.11,18,19 Our results suggest that

similar polymer/surfactant arrangements take place at low concentrations too. Desorption of polymer at high SDS concentrations and its substitution by surfactant on the surface observed by Joabsson et al.14 using a nonionic polymer was not observed. This indicates that even at high concentrations the charge of the polymer is playing a role in adsorption. This is surprising as both the polymer/surfactant complex and the substrate are negatively charged at high SDS (35) Horn, D.; Klingler, J.; Schorf, W.; Graf, K. Prog. Colloid Polym. Sci. 1998, 111, 27.


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Figure 9. Force measured on separation between the cantilever tip and a hydrophobized silica surface immersed in a fluid after rinsing with 0.5% SDS in a location that had undergone scratching. The two large secondary adhesions are fitted using an entropic FJC model (see text) using an offset in the point of attachment, indicating a cross-linked network is present.

concentrations. The force curves presented indicate that extension of polymer from the surface into solution even at high SDS:JR400 ratios is energetically unfavorable and that SDS forms links between JR400 chains. This may explain the difficulty in removing the charged JR400 polymer from the surface relative to a nonionic polymer. Conclusions The surface has an influence on the properties of the adsorbed mixture: its adhesion to freshly cleaved mica is stronger than that to the hydrophobized mica, and the adsorption to the hydrophobized mica surface is in turn stronger than that to hydrophobized silica. A possible explanation could be the role of the electro-

static attraction and the thickness of the hydrophobizing layer. During the washing experiments, the properties of the adsorbed layer are prone to changes following those in the composition of the bulk solution. These changes are slow and occur even at low polymer and surfactant concentrations, where they can be detected with the help of scratching experiments. Complete removal of polymer from the surface was not observed even at high SDS:JR400 ratios with mechanical disturbance. Force curve analysis has indicated that SDS can act to form cross-links between JR400 chains, this will greatly slow the desorption process. LA0495581

Surfactant Mixtures at Solid

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