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J. Phys. Chem. B 1998, 102, 1270-1278
Interactions between a 30% Charged Polyelectrolyte and an Anionic Surfactant in Bulk and at a Solid-Liquid Interface Per M. Claesson,* Matthew L. Fielden, and Andra Dedinaite Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden
Wyn Brown Department of Physical Chemistry, Uppsala UniVersity, P.O. Box 532, SE-751 32 Uppsala, Sweden
Johan Fundin Laboratoire de Physico-Chimie Macromole´ culaire, UniVersite´ Pierre et Marie Curie CNRS URA 278, ESPCI 10 rue Vauquelin, 75231 Paris Cedex 05, France ReceiVed: October 2, 1997; In Final Form: NoVember 24, 1997
The association between a 30% charged cationic polyelectrolyte and an anionic surfactant, sodium dodecyl sulfate (SDS), in 10 mM 1:1 electrolyte was investigated using surface force measurements and dynamic light scattering. The polyelectrolyte employed was a random copolymer of the neutral acrylamide and cationic [3-(2-methylpropionamide)propyl]trimethylammonium chloride (AM-MAPTAC-31). Light scattering measurements show that upon progressive addition of SDS to an AM-MAPTAC-31 solution the single coil size decreases until precipitation occurs at an SDS/MAPTAC ratio of just above 0.4. At SDS/MAPTAC ratios at or above 2, redispersion of the aggregates takes place. The interfacial behavior of AM-MAPTAC-31/SDS complexes was investigated in two ways. In one set of experiments a droplet containing a mixture of SDS and AM-MAPTAC-31 was placed between the surfaces and adsorption was allowed to occur from the aqueous mixture. It was found that the range of the steric force decreased when the SDS/MAPTAC ratio was increased from 0 to 0.4, indicating adsorption in a less extended conformation due to a decreased repulsion between the polyelectrolyte segments. At a ratio of 0.6 a compact interfacial complex was formed and the measured force was attractive over a small distance regime. A further increase in SDS/MAPTAC ratio resulted in precipitation of large aggregates at the surface, and reproducible force data could not be obtained. At an even higher SDS/AM-MAPTAC ratio of 4, individual aggregates were once again adsorbed at the surface. Hence, we find a good correspondence between association in bulk and at the solid surface. In another set of experiments the polyelectrolyte was first preadsorbed to mica surfaces and then SDS was added to the polyelectrolyte-free solution surrounding the surfaces. In this way precipitation of large SDS-polyelectrolyte aggregates onto the surfaces was avoided. Addition of SDS up to a concentration of 0.1 mM hardly affected the long-range interaction but gave an increased compressed layer thickness. A further increase in SDS concentrations to 1 mM results in a dramatic increase in the range of the force, suggesting formation of strongly negatively charged polyelectrolyte-surfactant complexes.
Introduction Polyelectrolytes and oppositely charged surfactants are present together in many applications such as washing, emulsification, particle deposition, rheology control and painting, just to mention a few. It is thus essential to understand how these types of molecules act together to give the desired properties of the product. The association of surfactant and polyelectrolyte changes the solubility of the polymer, which in many cases is essential for the synergistic effects obtained by mixing the two components. A complication is that polyelectrolytes and surfactants are mixed in bulk solution, whereas they really have to be effective at an interface. This is the case, for instance, in shampoos and during particle deposition and particle removal. Hence, a question that naturally arises is how the composition and structure of aggregates formed in bulk solution changes
when they come in contact with a fluid-fluid or fluid-solid interface. A second question of great practical importance is what happens with the concentrated polyelectrolyte-surfactant mixture when it is diluted with water. The association between polyelectrolytes and surfactants in bulk solution can, to a large extent, be understood by considering electrostatic and hydrophobic forces.1-6 For instance, by considering these two interactions, it is readily rationalized why the association process is highly cooperative and why the difference between the critical association concentration, cac, and the critical micellar concentration, cmc, decreases with increasing ionic strength. Consideration of hydrophobic and electrostatic forces is also sufficient to explain why a surfactant with a longer hydrophobic tail associates with a given polyelectrolyte at a lower surfactant concentration2,6,7 and why a given surfactant associates in a more cooperative way with a
S1089-5647(97)03216-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
Interactions at Solid-Liquid Interface more highly oppositely charged polymer.8 However, other aspects of the chemical structure of the polyelectrolyte and the surfactant are also important for the association behavior. For instance, a larger polyelectrolyte flexibility reduces the cac because it is easier for the polyelectrolyte to adopt a conformation that brings many charged segments close to the micellar surface.9 The chemical structure of the charged group of the surfactant is also important, and it has for instance been reported that alkylpyridinium halides interact somewhat more strongly with polyanions than alkyltrimethylammonium halides5 and that alkyl sulfates associate more easily with proteins than alkyl sulfonates and carboxylates.10 The reasons for this behavior are less well understood. The association between polyelectrolytes and surfactants at interfaces is more complicated, since one then also has to consider interactions between the interface and the surfactant as well as between the interface and the polyelectrolyte. The surface properties that ought to be of particular importance are the surface-charge density and the hydrophobicity. However, to our knowledge no systematic study has elaborated this point. A particular fruitful way to study association between polyelectrolytes and surfactants in bulk solution is to use dynamic light scattering that allows determination of aggregate sizes and size distributions and that also provides some information about interactions between polymer coils in bulk solution. This method has been successfully employed to study association between cationic polyelectrolytes and anionic surfactants.11 Association between polyelectrolytes and surfactants at interfaces can be studied in many ways. Some of the most rewarding methods are neutron reflectivity,12 ellipsometry,13 and surface force measurements.14 Previously, we have used the surface force technique to study the effect of surfactants on the forces between surfaces precoated with adsorbed polyelectrolytes.14-16 The primary information obtained from surface force measurements is the force acting between two macroscopic surfaces as a function of surface separation. From this, it is possible to draw conclusions about the types of forces that are important, as well as the extension and compressibility of the polyelectrolyte-surfactant aggregates adsorbed to the surface. These studies have shown that, at the solid-liquid interface, the association between polyelectrolytes and oppositely charged surfactants is a cooperative process, and that the cac at the surface is higher than in bulk solution, and also that the polyelectrolyte charge density is as important for the association process at the solid-liquid interface as it is in bulk solution. One important aim of the present investigation was to identify similarities and differences between polyelectrolyte-surfactant association in the bulk and at interfaces. To this end, we have addressed the association between a cationic polyelectrolyte and an anionic surfactant both in bulk and at a negatively charged solid surface. The bulk association was investigated by mainly dynamic light scattering, and the association between polyelectrolytes and surfactants at the solid-liquid interface was studied using the interferometric surface force technique. In one set of experiments we used surfaces precoated with polyelectrolytes and exposed them to surfactant solutions. In another set of experiments we allowed the bare surfaces to come into direct contact with a solution containing both polyelectrolyte and surfactant. From these experiments we see that many aspects of bulk and interfacial association are qualitatively similar, but we have also been able to identify some important differences. In particular, if one wishes to understand how bulk polyelectrolyte-surfactant mixtures behave at interfaces, it is essential to have the surface in contact with a solution containing both
J. Phys. Chem. B, Vol. 102, No. 7, 1998 1271 polyelectrolytes and surfactants, whereas preadsorbed polyelectrolyte layers exposed to a polyelectrolyte-free surfactant solution allow conclusions to be drawn about the cac at the surface. Experimental Section Materials. The polyelectrolyte used in the current study was a copolymer of the uncharged monomer acrylamide (AM) and the positively charged [3-(2-methylpropionamide)propyl]trimethylammonium chloride (MAPTAC) in the ratio 69:31. The polyelectrolyte (referred to in the remaining text as AMMAPTAC-31) was thus 31% charged and had a measured molecular weight of 780 000 g mol-1. The molecular weight of the MAPTAC monomer is 221 g mol-1; i.e., the average polyelectrolyte molecule contains just above 2000 charges. Sodium dodecyl sulfate (SDS, BDH, 99%) was used as received. Potassium bromide (KBr, Merck, pro analysi) was roasted at 500 °C overnight before use, whereas NaCl (Merck, pro analysi)) was used as received. Water was prepared by passage through a Millipore Milli-RO plus system followed by a Milli-Q 185 system. Immediately prior to the measurement, the water was degassed under vacuum using a water jet pump for at least 1 h. Dynamic Light Scattering. DLS measurements were made using the apparatus described briefly in ref 17. An ALV wideband, multi-τ digital autocorrelator has been used for data collection. The measured intensity autocorrelation function g(2)(t) is related to the field correlation function g(1)(t) by
g(2)(t) ) B(1 + β|g(1)(t)|2)
(1)
where β is a factor accounting for deviations from ideal correlation and B is a baseline term. For a continuous distribution of relaxation times, corresponding to an infinite range of particle sizes, the inverse Laplace transform (ILT) may be used:
g(1)(t) )
∫0∞A(τ) exp(-t/τ) dτ
(2)
ILT analysis was performed using a constrained regularization routine REPES.17,18 Although similar to CONTIN,19 the algorithm REPES differs in that it directly minimizes the sum of the squared differences between the experimental and calculated intensity-intensity autocorrelation functions g(2)(t) using nonlinear programming and allows the selection of the “smoothing parameter” P (probability to reject). Analysis of data encompassing 288 exponentially spaced grid points and a grid density of 12 per decade can be rapidly performed on an IBM-AT desk-top computer. Representation of the relaxation time distributions in the form of an τA(τ) versus log τ plots, with τA(τ) in arbitrary units, provides an equal area representation. Surface Force Measurements. Surface force measurements were conducted with a Mark II or Mark IV surface force apparatus using muscovite mica surfaces (New York Mica Corp., New York). Mica is an aluminosilicate mineral containing 2.1 × 1018 negative surface lattice sites per square meter due to isomophous substitution of aluminum for silicon. It was cleaved into 1-3-µm thick molecularly smooth squares with an area of about 1 cm2. These sheets were then coated on one side with a thin layer of silver using thermal deposition and glued silvered-side down to two semicylindrical silica lenses of radius of about 2 cm. The glue used was an epoxy resin (Shell Epikote 1004) that liquefies at increased temperature.
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The SFA is described in detail elsewhere.20,21 In brief, the two mica surfaces are brought together in gas or liquid, using a piezoelectric crystal and/or a synchronous motor. White light is introduced in the normal direction to the surfaces, which are mounted in a crossed cylinder geometry. The light undergoes multiple reflection between the silver layers, and an interference pattern, fringes of equal chromatic order, is created. The separation between the surfaces is calculated from the wavelengths of the standing waves. The lower surface is attached to a spring. The distance that the spring deflects is calculated as the difference between the distance moved by the piezo/ synchronous motor and the resulting change in separation between the surfaces. The force, F(D) is then calculated by multiplying the measured deflection by the spring constant of the double cantilever spring (measured at the end of each experiment). When the data are analyzed, the force is normalized by the local geometric mean radius R, which is approximately 2 cm. The local radius is determined from the shape of the standingwave pattern at each measuring position. According to the Derjaguin approximation,22 this quantity is related to the free energy of interaction per unit area between flat surfaces Gf(D):
F(D) ) 2πGf(D) R
(3)
with the condition that D , R. Since D is of the order of 10-6 m or less, this condition is satisfied. Another requirement is that the radius of the surfaces should be independent of D, but this is not always the case, since the surfaces deform under strong repulsive and attractive forces.23 When the gradient of the force ∂F/∂D exceeds the spring constant of the double cantilever spring the system becomes unstable and the surfaces jump together to the next stable region of the force curve. Experimental Procedures. Aqueous solutions containing both AM-MAPTAC-31 and SDS are turbid in a range of SDS/ MAPTAC ratios. For this reason the measuring chamber was not filled with the solution, but rather a small droplet of the liquid was placed between the mica surfaces. This has the advantage that the light used for interferometric measurements of surface separation does not pass through a large region of the turbid solution. One drawback is that the system becomes more sensitive to thermal drift and the precision in the measurements of weak forces decreases in comparison to the case when the whole measuring chamber is filled with solution. Forces between surfaces precoated with the polyelectrolyte were determined with the whole measuring chamber filled with the solution. After polyelectrolytes were introduced into the solution in the SFA, the system was left to equilibrate overnight before the force measurements were carried out. Before SDS was added, the polyelectrolyte solution was replaced by a polyelectrolyte-free 10 mM KBr solution. The removal of excess polymer from the liquid medium was ensured by rinsing the apparatus four times with 10 mM KBr. Forces were then measured after equilibration overnight. In the final step SDS was injected into the measuring chamber and thoroughly mixed as described above. After each surfactant addition, the mixture was left to stand overnight before measuring any force curves. Two SDS concentrations, 0.1 and 1 mM, were studied, and SDS stock solutions were filtered through a 0.2-µm filter before injection into the SFA. In 10 mM KBr, SDS was found to phase-separate upon increasing its bulk concentration to 10 mM, with large needlelike crystals precipitating out of solution.
Figure 1. Relaxation time distribution obtained by Laplace inversion of dynamic light scattering correlation function. The AM-MAPTAC31 concentration was 2000 ppm, and the SDS/MAPTAC ratios from top to bottom are 0, 0.1, 0.2, 0.3, 0.4, 2, and 3.3. At low SDS concentrations there are two peaks corresponding to a single chain (low log τ) and to a multichain (high log τ). At higher surfactant concentrations only a single peak corresponding to the surfactant-polyelectrolyte complex is observed.
Results Association of Polyelectrolytes and Surfactants in Bulk Solution. Relaxation time distributions obtained from Laplace inversion of normalized intensity correlation functions at a polymer concentration of 0.2% and at various surfactant concentrations are shown in Figure 1. In the absence of SDS, a bimodal distribution is observed. The fast mode corresponds to the translational diffusion of single polyelectrolyte coils and the slow mode to multichain aggregates. The apparent hydrodynamic radii Rh are 6.3 and 497 nm, respectively. The presence of the slow relaxation mode in polyelectrolyte solutions at low ionic strengths is often observed. It is plausible that it is due to the large expansion of the polyelectrolyte chains caused by long-range electrostatic repulsion, which gives rise to an ordering effect in the solution that prevents the chains from moving independently of each other through the solution volume.24 This mechanism is similar to the one that causes the formation of colloidal crystals, where a long-range doublelayer repulsion between charged monodisperse colloids drives the formation of ordered structures.25 Although such aggregates contribute greatly to the overall scattering in our experiments, it may be estimated that the concentration of the multichain aggregates (here denoted Cslow) is very low. Assuming that D is proportional to M-0.5, (Dslow/Dfast)2 ) Mfast/Mslow. For the present data, the ratio Mfast/Mslow ≈ 10-4. With approximately equal intensities for the two modes, one finds that Cslow ≈ 0.01% at Cp (∼Cfast) ) 0.2%. Thus, without surfactant present in solution the multichain aggregates are very few compared to the single chain coils. The position of the slow peak moves to shorter relaxation times as the surfactant concentration is increased, which corresponds to a transition to smaller aggregates. For example, the apparent Rh for these structures decreases from 325 to 87 nm as the SDS/MAPTAC ratio is increased from 0.1 to 0.2. The relaxation time distribution at SDS/MAPTAC ratios of 0.3, 0.4, 2.0, and 3.3 are unimodal, with a significantly greater degree of polydispersity for r ) 0.3 and 0.4 compared to when SDS is present in excess. The apparent Rh at ratios 0.3-0.4 are approximately 22 nm. The SDS/MAPTAC ratio 0.4 is close to the phase separation. Further increase in SDS concentration
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J. Phys. Chem. B, Vol. 102, No. 7, 1998 1273
Figure 2. Effective diffusion coefficient for the single chain as a function of surfactant concentration. The measurements were conducted at various AM-MAPTAC-31 concentrations, which from top to bottom, are 10000, 8000, 6000, 4000, and 2000 ppm.
results in a macroscopic phase separation showing that the polyelectrolyte-SDS complexes now attract each other. At high SDS/MAPTAC ratios, at or above r ) 2, the system is chargereversed and the particles are redispersed. The real Rh, calculated from the diffusion coefficient at infinite dilution, showed that the resolubilized complexes were larger than those formed prior to the phase-separation, owing to steric effects, since the former contains much more surfactant than the latter. Figure 2 depicts the diffusion coefficient for the single coil at different constant values of the polymer concentration as a function of SDS concentration. The concentration dependence of D in dilute binary polymer solutions can be expressed by a virial expansion:
Dz )
kT(1 - φ)2 (1 + 2A2Mwc + ...) f
(4)
where φ is the volume fraction of the complex, f is the friction coefficient, A2 is the second virial coefficient, Mw is the average molecular weight of the complex, and c is the concentration of the complex (g/mL). However, as we are studying D as a function of SDS concentration, one may insert the SDS concentration instead of the complex concentration. D decreases with increasing surfactant concentration, which is due to the reduced particle-solvent interaction potential as embodied in the second virial coefficient. The repulsive interactions between the coils decrease, which is due to the decreased electrostatic charge and increased hydrophobicity of the complex at higher SDS concentrations. This effect is the dominant factor in determining D. Rh of the complex decreases also as a function of the SDS/MAPTAC ratio. Forces between Polyelectrolyte-Coated Surfaces. The forces measured between bare mica surfaces immersed in 0.1 mM KBr (Figure 3) were dominated at large separations by an electrostatic double-layer repulsion whereas a van der Waals attraction was most important at distances below about 4 nm. This behavior is consistent with predictions of the DLVO theory. When forces calculated in the nonlinear Poisson-Boltzmann model were fitted to the measured interaction, it was found that the apparent surface potential was 52 mV and the Debye length 23 nm, corresponding to a 1:1 electrolyte concentration of 0.17 mM. The adhesion between the surfaces was 30 mN m-1. When the KBr concentration was increased to 10 mM, the range of
Figure 3. Force normalized by radius as a function surface separation between mica surfaces immersed in 0.1 mM KBr (filled squares) and in 10 mM KBr (triangles). Filled and unfilled symbols represent forces measured on approach and separation, respectively. Forces measured across a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 are represented by filled circles.
the double-layer force decreased with an apparent surface potential and Debye length of 32 mV and 3 nm, respectively. In this case no attractive force was observed at small separations because of the presence of a strong short-range repulsion. Such a force is commonly observed between mica surfaces in aqueous solutions with a sufficiently high salt concentration (above about 1-10 mM, depending on the type of salt present). It is partly due to the presence of a Stern layer that moves the plane of charge outward26,27 and partly due to a dehydration of adsorbed cations.28,29 No hysteresis was observed between forces measured on approach and forces measured on separation. AM-MAPTAC-31 was allowed to adsorb overnight onto the mica surfaces from a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 or a 10 mM KBr solution containing 1900 ppm of the polyelectrolyte. The purely repulsive forces measured across the 200 ppm solution are illustrated in Figure 3. They reached a measurable strength at a separation of about 100 nm. At a high compressive force the layer could be compressed to about 4 nm. The forces measured on separation were of slightly lower magnitude than those measured on approach. The repulsive forces measured across the 1900 ppm polyelectrolyte solutions were of even longer range and poorly reproducible. A likely reason for this is that when the surfaces are brought together, polyelectrolytes present in solution are trapped between them. When the polyelectrolyte-containing solution was replaced with a 10 mM KBr solution, the measured force decreased markedly in magnitude and became more reproducible. The repulsive force measured on approach reached a magnitude of 0.1 mN/m at a separation of between 90 and 130 nm and increased monotonically with decreasing separation (Figure 4). The minimum separation obtained under a high force was about 4 nm, i.e., the same as that reached in the 200 ppm AM-MAPTAC-31 solution. The forces measured on separation were less repulsive than those measured on approach. Unlike the situation with polymers present in solution, a weak adhesive minimum was found at a distance of about 130 nm (Figure 5). The large distance at which the minimum was observed suggests that the origin of the attraction is polyelectrolyte-bridging between the two surfaces. As a comparison, the forces measured in 0.1 mM KBr between AMMAPTAC-31 coated surfaces obtained by adsorption from a 20 ppm polyelectrolyte solution in 0.1 mM KBr are also shown in Figure 4. Under these circumstances, where the adsorption is close to its plateau value at this low ionic strength, a weak
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Figure 4. Force normalized by radius as a function surface separation between mica surfaces precoated with a AM-MAPTAC-31 layer. The triangles represent data where the polyelectrolyte layer was adsorbed from a 1900 ppm polyelectrolyte solution in 10 mM KBr, and then the forces were measured across a polyelectrolyte-free 10 mM KBr solution. Filled and unfilled symbols represent forces measured on approach and separation, respectively. The filled circles represent data obtained by adsorbing the polyelectrolyte from a 20 ppm 0.1 mM KBr solution and then measuring the forces across a polyelectrolyte-free 0.1 mM KBr solution.
Figure 5. Force normalized by radius as a function surface separation between mica surfaces precoated with an AM-MAPTAC-31 layer obtained by adsorption from a 10 mM KBr solution. The forces were measured across a polyelectrolyte-free 10 mM KBr solution containing no SDS (squares), 0.1 mM SDS (triangles), and 1 mM SDS (circles). Filled and unfilled symbols represent forces measured on approach and separation, respectively.
electrostatic double-layer force dominates the long-range interaction. At a separation of about 20 nm, a bridging attraction pulls the surfaces into contact at a separation of about 4 nm. The pull-off force measured on separation is about 1 mN/m. It is the strong electrostatic affinity between the polymer and the surface at the low ionic strength that results in a thin adsorbed layer and a pronounced bridging attraction. Forces between Surfaces Precoated with Polyelectrolytes in the Presence of Surfactants. The introduction of 0.1 mM SDS hardly affected the long-range force (Figure 6). Nevertheless, a clear difference was observed in the layer thickness obtained under a high compressive force. It increased from 4 nm before addition of SDS to around 6 nm in 0.1 mM SDS solution. This suggests that some surfactants are incorporated into the layer. Another clear difference was that when the compressive force was released in the presence of 0.1 mM SDS, the polyelectrolyte layer hardly expanded until a zero force was
Claesson et al.
Figure 6. Force normalized by radius as a function surface separation between mica surfaces precoated with an AM-MAPTAC-31 layer obtained by adsorption from a 10 mM KBr solution. The forces were measured on approach across a polyelectrolyte-free 10 mM KBr solution containing no SDS (squares), 0.1 mM SDS (triangles), and 1 mM SDS (circles).
Figure 7. Force normalized by radius as a function surface separation between mica surfaces precoated with an AM-MAPTAC-31 layer obtained by adsorption from a 10 mM KBr solution containing 1 mM SDS. Results for first approach (filled squares), first separation (unfilled squares), and second approach (filled circles) are shown.
reached (Figure 5). Hence, the presence of SDS in the compressed polyelectrolyte layer retarded its reexpansion because of hydrophobic interactions between surfactants associated with the polyelectrolytes. A further increase in SDS concentration to 1 mM dramatically increased the range of the repulsive force to about 180 nm (Figure 6). The force increased strongly with decreasing separation, displaying a distinct s shape when plotted on a logarithmic force scale. Even at very high loads, no clear hard wall was observed. Just as in the lower SDS concentration, the layer did not reswell when the compressive force was reduced. On a consecutive approach, the force was rather similar to the one observed during the first approach (Figure 7). Forces between Surfaces across Solutions Containing Both Polyelectrolytes and Surfactants. All measurements presented in this section were, unlike those discussed above, determined across a 200 ppm polyelectrolyte solution in 10 mM NaCl. The ratio of SDS to charged MAPTAC segments of the polyelectrolyte was varied. A new pair of mica surfaces were used for each experiment. The results obtained after at least a 12-h equilibration for SDS/MAPTAC ratios of 0, 0.2, and 0.4 are
Interactions at Solid-Liquid Interface
Figure 8. Force normalized by radius as a function surface separation between mica surfaces across a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 and various amounts of SDS. The SDS/ MAPTAC ratio was 0 (filled squares), 0.2 (unfilled squares), and 0.4 (filled triangles).
Figure 9. Force normalized by radius as a function surface separation between mica surfaces across a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 and SDS. The SDS/MAPTAC ratio was 0.4, and the forces were determined after about 1-2 h (unfilled squares) and after more than 12 h (filled triangles).
shown in Figure 8. In all three cases the measured force is purely repulsive and only slightly more repulsive on approach than on separation. For an SDS/MAPTAC ratio of 0 and 0.2, the force reaches a measurable strength at a separation of about 100-120 nm. Hence, addition of a small amount of SDS does not affect the interaction between the surfaces and the layer structure. When this ratio is increased further to 0.4, the range of the force decreases significantly to about 70 nm, showing that at this SDS concentration the incorporation of the surfactant in the layer renders it more compact. The layer thickness reached under a high compressive force was in all these cases about 4-6 nm. However, the force needed to reach this separation decreased with increasing surfactant concentration. At an SDS/MAPTAC ratio of 0.4, it was observed that the range of the force and the compressed layer thickness decreased with increasing equilibration time (Figure 9). This strongly indicates that it is polyelectrolyte-SDS aggregates that initially adsorb to the surfaces, and with time the structure of these aggregates changes in order to minimize the free energy of the system. It most likely involves the expulsion of SDS molecules from the polyelectrolyte-surfactant complex in order to increase the number of close contacts between negative surface sites and cationic polyelectrolyte segments. A similar change in the
J. Phys. Chem. B, Vol. 102, No. 7, 1998 1275
Figure 10. Force normalized by radius as a function surface separation between mica surfaces across a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 and SDS. The SDS/MAPTAC ratio was 0.6. Forces measured on approach and separation are represented by filled and unfilled squares, respectively. The arrow indicates an outward jump.
adsorbed layer with time was observed at higher, but not lower, surfactant concentrations. A further increase in the SDS/MAPTAC ratio to 0.6 results in a further decrease in the range and magnitude of the repulsive interaction (Figure 10). At separations below 10 nm a very steep repulsion is experienced. These results demonstrate that the adsorbed layer is considerably more compact compared with that at lower SDS concentrations. When the surfaces are separated, an attractive interaction is experienced that gives rise to a weak minimum in the force curve at a separation of about 9 nm. At an SDS/MAPTAC ratio of 1.0, the solution was very cloudy and reproducible forces could not be measured. Despite this, some qualitative features of interest were observed. The range of the force was very large, ∼2000 nm. Upon compression it was found that the adsorbed layers suddenly yielded and adopted a more compact form. Upon separation a rather strong attraction (∼10 mN/m) was experienced. These qualitative features suggest that the layers behave like a polyelectrolytesurfactant gel with a yield stress. One experiment was also conducted with an excess of SDS (SDS/MAPTAC ratio of 4). Here, the bulk polyelectrolytesurfactant aggregates have a net negative charge, but nevertheless, they still adsorb to the negatively charged surfaces. The range of the repulsive force is about 30 nm, and it increases rapidly with decreasing separation (Figure 11). On separation, a weak attractive minimum was often observed. Discussion Forces between Polyelectrolyte-Coated Surfaces. The forces acting between negatively charged mica surfaces coated with cationic polyelectrolytes have previously been investigated in great detail.30-36 One main finding of these studies is that in dilute electrolyte solutions, where electrostatic forces predominate, highly charged polyelectrolytes adsorb in a very thin layer on oppositely highly charged surfaces.34,36 Another finding is that the charges of the adsorbed polyelectrolyte slightly overcompensate the surface charge at the plateau value of the adsorption isotherm.34,36 Both these results are well understood from theoretical considerations37 and confirmed by the data obtained in 0.1 mM KBr presented in Figure 4. Dahlgren et al. found that the adsorbed layer of a 100% charged polyelectrolyte became considerably more extended when the adsorption occurred from solutions with a higher ionic
1276 J. Phys. Chem. B, Vol. 102, No. 7, 1998
Figure 11. Force normalized by radius as a function surface separation between mica surfaces across a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31 and SDS. The SDS/MAPTAC ratio was 4.0. Forces measured on approach and separation are represented by filled and unfilled squares, respectively.
strength.35 We have found that the same is true for AMMAPTAC-31. This point is clearly illustrated by the data shown in Figure 4. (The polyelectrolyte concentrations used are different at the two salt concentrations, but in 0.1 mM KBr the plateau value of the adsorption isotherm is reached already at an AM-MAPTAC-31 concentration of 20 ppm.) There are at least two reasons for this. First, the conformation of the polyelectrolyte in bulk solution becomes less extended when the salt concentration is increased. This effect can be illustrated by the effect of electrolyte on the electrostatic persistence length, which for a polyelectrolyte with 1 nm between the charges is about 170 and 1.7 nm in 0.1 and 10 mM 1:1 electrolyte, respectively.38 We expect that the electrostatic persistence length is somewhat less than this for AM-MAPTAC-31, since the charges are located on short side chains to the backbone. Second, the increased ionic strength screens the electrostatic attraction between the polyelectrolyte and the oppositely charged surface, making conformations with large loops and tails more favorable. The monotonically repulsive force measured between mica surfaces coated with AM-MAPTAC-31 in 10 mM KBr decays much too slowly to be an electrostatic double-layer force (Figure 4). Instead, this force is of steric origin caused by the confinement of extending polymer tails (and at shorter separations loops) in the gap between the surfaces. A distance of 4 nm is reached under a high compression, and at this stage most segments have to be present in loops and trains. The large steric force observed makes it impossible to determine the charge of the polyelectrolyte-coated surfaces from the force measurements. We note that, for preadsorbed AM-MAPTAC-31 layers in 10 mM KBr, no attractive force is present on approach whereas a weak bridging attraction is observed when two compressed polyelectrolyte layers are separated from each other. This is different compared to the situation when a thin polyelectrolyte layer is formed by adsorption from a dilute electrolyte solution (0.1 mM KBr). In this case a bridging attraction, of a rather short range (D < 20 nm), is observed also on approach. Forces between Surfaces across Solutions Containing Both Polyelectrolyte and Surfactant. It is well-known that charged surfactants associate with oppositely charged polyelectrolytes in bulk solution and at interfaces. However, comparatively little is known about interactions across solutions containing polyelectrolyte-surfactant aggregates and the structures formed when such aggregates adsorb to solid surfaces. In this study
Claesson et al. we find that the range of the forces decreases as the SDS/ MAPTAC ratio is increased from 0 to 0.6. Clearly, as more SDS is incorporated in the adsorbed layer, fewer and smaller tails are formed. There are at least two reasons for this. The net charge of the polyelectrolyte-surfactant complex decreases, and a hydrophobic attraction between surfactant tails develops when the surfactant concentration is increased. Both these effects favor a more compact layer structure. In bulk solution this corresponds to the decrease in size of the multichain aggregates as observed by dynamic light scattering. In bulk solution phase separation occurs for SDS/MAPTAC ratios between just above 0.4 and 2. This demonstrates that an attraction exists between the polyelectrolyte-surfactant aggregates. This attraction is also observed between surfaces coated with polyelectrolyte-surfactant aggregates. At an SDS/ MAPTAC ratio of 0.6 a weak repulsion still dominates the longrange interaction, but once the surfaces are brought into contact, an attraction develops due to intralayer association mediated by the hydrophobic tails of the surfactant. The weak long-range repulsion is sufficient to prevent precipitation of aggregates upon the surfaces. However, at an even higher SDS concentration (SDS/MAPTAC ) 1) the repulsion between the aggregates and the surface is too small to prevent precipitation. At this concentration we also observe a strong adhesion between the surfaces coated with thick polyelectrolyte-surfactant layers. When SDS is present in excess, the polyelectrolyte-surfactant aggregates are negatively charged and once again soluble in bulk solution as relatively small aggregates. This is also reflected by the surface interaction, which again is repulsive. Hence, again we find good qualitative agreement between interactions in bulk solution and at interfaces. However, there are also some differences. Let us take the situation at an SDS/ MAPTAC ratio of 0.4 as an example. Here, we find that the range of the forces is about 70 nm and the compressed layer thickness after a few hours is 8 nm, decreasing to 4 nm after equilibration overnight. In comparison the mean radius of gyration for the aggregates in bulk solution is 22 nm. Hence, if the aggregates adsorbed without any change in conformation, one would expect the range of the force to be at least 80 nm. The less long-ranged interaction observed shows that the aggregates flattens out on the surface. The reason for this is the strong affinity between the positively charged polyelectrolyte segments and the negatively charged surface. It seems very likely that the adsorption of the aggregates at the surface also results in some desorption of SDS from the aggregates. Further, a rather slow change in conformation of the adsorbed polyelectrolyte-surfactant complex, presumably involving further expulsion of negatively charged SDS molecules, takes place and results in the formation of a more compact layer (see Figure 9). Surfaces Precoated with Polyelectrolytes. Forces in the Presence of SDS. Addition of anionic SDS to a concentration of 0.1 mM had no significant effect on the long-range part of the force curve between surfaces precoated with AM-MAPTAC31 which is dominated by interactions between polyelectrolyte tails. On the other hand, the final-layer thickness was found to increase upon addition of SDS. The likely explanation for this finding is that it is more favorable for SDS to associate with the polyelectrolyte when the volume fraction of charged segments not bound to the surface increases. The reason for this is an increased possibility for SDS molecules in contact with charged segments to be sufficiently close to each other to experience a favorable hydrophobic interaction between the surfactant tails. Hence, since the volume fraction of charged
Interactions at Solid-Liquid Interface segments increases closer to the surface, and when two surfaces are pushed together, one would expect the association to occur more easily in the inner region of the polyelectrolyte layer (excluding the region in the direct neighborhood of the charged surface where also competition between SDS and negatively charged surface sites for the cationic segments has to be considered) than in the tail region. An increase in SDS concentration to 1 mM dramatically increased both the repulsive force and the gap between the surfaces at high compressive forces. This suggests a large increase in the number of SDS molecules associated with the polyelectrolyte layer and thus a significant recharging of the layer. As a result, the repulsion within the layer increases and the chains are forced to adopt a more extended conformation. At very large separations the observed decay length of the force is smaller than at intermediate separations, and at small separations the force becomes steeper again (Figure 6). Several theoretical approaches such as scaling theories39 and Monte Carlo simulations40 predict such a force profile for polyelectrolyte brushes. Miclavic and Marcelja, using an analytical mean-field approach, identified the most important force contributions in the different regions of such an s-shaped force curve.41 In the outermost part an electrostatic double-layer force predominates. In the intermediate region a change in conformation of the polyelectrolyte layer takes place that results in an inward shift of the plane of charge and a reduction in the slope of the logarithm of the force. Finally, in the innermost part of the force curve a steric force due to confinement of the polyelectrolyte chains dominates the interaction. It is plausible that the forces measured in the 1 mM SDS solution can be explained in a similar way. However, the situation with a layer consisting of SDS associated with AM-MAPTAC-31 is of course more complex than a system consisting of polyelectrolyte brushes. In particular, it is likely that the amount of surfactants in the layer changes when the surfaces are pushed close together. Hence, unlike in the theoretical models referred to above, it is likely that the layer charge in the present case is a function of the surface separation. The shape of the force profile on separation is very different in the presence and absence of SDS (Figure 5). The less extended repulsion in the presence of SDS suggests that the tails of the surfactants that are incorporated into the layer attract each other through a hydrophobic interaction. Thus, they mediate an attraction between the polyelectrolytes that slows the reexpansion of the layer when the compressive force is released. We are surprised that the same mechanism does not give rise to any significant attraction between the layers. The association between AM-MAPTAC-31 and SDS has also been studied at a lower salt concentration.16 In that study it was observed that, at an SDS concentration of about 0.08 mM, no significant swelling of the polyelectrolyte layer occurred, whereas at a concentration of 0.4 mM a cooperative association process between SDS and the polyelectrolyte resulted in a large swelling of the adsorbed layer. We do not have data at enough SDS concentrations to determine very precisely the critical association concentration for association between SDS and AMMAPTAC-31 at the mica surface (cacs). However, it is clear that in 0.1 mM KBr it is in the range 0.08 mM < cacs < 0.4 mM, and in 10 mM KBr it is in the range (0.1 mM < cacs < 1 mM). Comparison between Association between Polyelectrolytes and Surfactants in Bulk and at Interfaces Precoated with Polyelectrolytes. There are some clear differences between the association of surfactants and polyelectrolytes in bulk solution
J. Phys. Chem. B, Vol. 102, No. 7, 1998 1277 and at interfaces precoated with polyelectrolytes. In bulk solution the initial state of the association is favored by both electrostatic and hydrophobic forces. The latter acts primarily between the surfactant tails. When the cationic polymer associates with anionic surfactant molecules to form a nearly uncharged complex, the counterions of the polyelectrolyte will be released and a favorable increase in counterion entropy will occur. At higher surfactant concentration, further association between the cationic polyelectrolyte and the anionic surfactant leads to the development of a net negative charge of the complex. Clearly at this stage, the association is counteracted by electrostatic forces and will be solely driven by the hydrophobic interaction. The situation with an initially uncharged interface, here a negatively charged surface carrying a preadsorbed cationic polyelectrolyte layer that balances the surface charge, is similar to the recharging of initially uncharged polyelectrolytesurfactant aggregates in bulk solution. The association of an anionic surfactant with the preadsorbed polyelectrolyte layer leads to a recharging of the system already at the initial state. Hence, the association in this case is driven by the hydrophobic interaction and counteracted by electrostatic forces (i.e., confinement of the counterions to the electrical double layer outside the surface). Under the conditions studied here, SDS does not cause a net attraction between the surfaces precoated with polyelectrolytes. Instead, a steric force due to confinement of the polymer chains in the gap between the surfaces gives rise to a repulsive steric force when the surfaces approach each other. On separation, however, it is noted that the presence of SDS induces an attractive interaction between the polyelectrolyte chains that prevents their expansion. The situation is somewhat similar to the interactions between aggregates in bulk solution. Within the aggregates hydrophobic interactions between surfactants associated with the polyelectrolyte chains prevent the aggregates from dissolving into separate polymer chains, whereas a repulsion between separate aggregates prevents flocculation and a macroscopic phase separation. Conclusions The influence of SDS on a 2000 ppm solution of AMMAPTAC-31 in 10 mM 1:1 electrolyte has been assessed using dynamic light scattering. The light scattering data for the AMMAPTAC-31/SDS system in bulk solution show that addition of SDS, up to an SDS/MAPTAC ratio of 0.4, progressively reduced the hydrodynamic radius of the multichain aggregates as well as the repulsive interactions between single coils. Both these effects are due to an increased hydrophobicity and a reduced charge of the polyelectrolyte-surfactant complex. At higher SDS concentrations a precipitate was formed. It could be redispersed by increasing the SDS/MAPTAC ratio to about 2. The forces measured between surfaces across a 200 ppm AMMAPTAC-31-SDS solution were qualitatively consistent with the bulk behavior. When the SDS/MAPTAC ratio was increased from 0 to 0.6, the range of the repulsive force decreased and at the highest surfactant concentration, attraction was observed over a small distance regime. Just as for bulk solutions, this can be rationalized in terms of a decreased charge of the polyelectrolyte-surfactant layer and an increased hydrophobic interaction between surfactant tails. At an SDS/ MAPTAC ratio of 1, a thick layer was deposited onto the surfaces and the adhesion between these layers was strong. At a sufficiently high SDS/MAPTAC ratio, thin adsorbed layers
1278 J. Phys. Chem. B, Vol. 102, No. 7, 1998 were again formed because of the net negative charges of the polyelectrolyte-surfactant complex. The results also show that the aggregates flatten when they adsorb to the surface, and we suggest that SDS is expelled from the aggregates upon adsorption and during further slow conformational changes. Addition of SDS to a concentration of 0.1 mM to a solution in contact with surfaces carrying preadsorbed polyelectrolyte layers did not significantly affect the long-range force, whereas the compressed layer thickness was increased. This is consistent with an uptake of SDS within the adsorbed layer. Unlike the situation in the absence of SDS a decrease in compressive force did not result in any large expansion of the polyelectrolyte layer. This shows that SDS induces attractive interactions within the layers, which makes a compact conformation more favorable. A further increase in SDS concentration to 1 mM caused a significant increase in the repulsive force and compressed layer thickness due to a recharging of the adsorbed layer. Acknowledgment. This work was sponsored by the Human Capital and Mobility program (Contract No. CHRX-CT940655), the Swedish Natural Science Research Council (NFR), and the Swedish Research Council for Engineering Sciences (TFR). References and Notes (1) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263. (2) Hayakawa, K.; Santerre, J. P.; Kwak, C. T. Macromolecules 1983, 16, 1642. (3) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506. (4) Santerre, J. P.; Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1985, 13, 35. (5) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (6) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930. (7) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502. (8) Satake, I.; Takahashi, T.; Hayakawa, K.; Maeda, T.; Aoyagi, M. Bull. Chem. Soc. Jpn 1990, 63, 926. (9) Wallin, T.; Linse, P. Langmuir 1996, 12, 305. (10) Ananthapadmanabhan, K. P. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (11) Fundin, J.; Brown, W.; Vethamuthu, M. S. Macromolecules 1996, 29, 1195.
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