Interactions between a 30 Charged Polyelectrolyte and an Anionic Surfactant in Bulk and at a Solid−Liquid Interface

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  Interactions between a 30 Charged Polyelectrolyte and an Anionic Surfactant in Bulk and at a Solid−Liquid Interface
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  Interactions between a 30% Charged Polyelectrolyte and an Anionic Surfactant in Bulk andat 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 Uni V  ersity, P.O. Box 532, SE-751 32 Uppsala, Sweden Johan Fundin  Laboratoire de Physico-Chimie Macromole ´ culaire, Uni V  ersite ´  Pierre et Marie Curie CNRS URA 278, ESPCI 10 rue Vauquelin, 75231 Paris Cedex 05, France Recei V  ed: October 2, 1997; In Final Form: No V  ember 24, 1997  The association between a 30% charged cationic polyelectrolyte and an anionic surfactant, sodium dodecylsulfate (SDS), in 10 mM 1:1 electrolyte was investigated using surface force measurements and dynamiclight 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 measure-ments show that upon progressive addition of SDS to an AM-MAPTAC-31 solution the single coil sizedecreases until precipitation occurs at an SDS/MAPTAC ratio of just above 0.4. At SDS/MAPTAC ratios ator above 2, redispersion of the aggregates takes place. The interfacial behavior of AM-MAPTAC-31/SDScomplexes was investigated in two ways. In one set of experiments a droplet containing a mixture of SDSand AM-MAPTAC-31 was placed between the surfaces and adsorption was allowed to occur from the aqueousmixture. It was found that the range of the steric force decreased when the SDS/MAPTAC ratio was increasedfrom 0 to 0.4, indicating adsorption in a less extended conformation due to a decreased repulsion between thepolyelectrolyte segments. At a ratio of 0.6 a compact interfacial complex was formed and the measuredforce was attractive over a small distance regime. A further increase in SDS/MAPTAC ratio resulted inprecipitation of large aggregates at the surface, and reproducible force data could not be obtained. At aneven 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 setof experiments the polyelectrolyte was first preadsorbed to mica surfaces and then SDS was added to thepolyelectrolyte-free solution surrounding the surfaces. In this way precipitation of large SDS - polyelectrolyteaggregates onto the surfaces was avoided. Addition of SDS up to a concentration of 0.1 mM hardly affectedthe long-range interaction but gave an increased compressed layer thickness. A further increase in SDSconcentrations 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 presenttogether in many applications such as washing, emulsification,particle deposition, rheology control and painting, just tomention a few. It is thus essential to understand how thesetypes of molecules act together to give the desired propertiesof the product. The association of surfactant and polyelectrolytechanges the solubility of the polymer, which in many cases isessential for the synergistic effects obtained by mixing the twocomponents. A complication is that polyelectrolytes andsurfactants are mixed in bulk solution, whereas they really haveto be effective at an interface. This is the case, for instance, inshampoos and during particle deposition and particle removal.Hence, a question that naturally arises is how the compositionand structure of aggregates formed in bulk solution changeswhen they come in contact with a fluid - fluid or fluid - solidinterface. A second question of great practical importance iswhat happens with the concentrated polyelectrolyte - surfactantmixture when it is diluted with water.The association between polyelectrolytes and surfactants inbulk solution can, to a large extent, be understood by consideringelectrostatic and hydrophobic forces. 1 - 6 For instance, byconsidering these two interactions, it is readily rationalized whythe association process is highly cooperative and why thedifference between the critical association concentration, cac,and the critical micellar concentration, cmc, decreases withincreasing ionic strength. Consideration of hydrophobic andelectrostatic forces is also sufficient to explain why a surfactantwith a longer hydrophobic tail associates with a given poly-electrolyte at a lower surfactant concentration 2,6,7 and why agiven surfactant associates in a more cooperative way with a 1270  J. Phys. Chem. B  1998,  102,  1270 - 1278S1089-5647(97)03216-1 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 01/22/1998  more highly oppositely charged polymer. 8 However, otheraspects of the chemical structure of the polyelectrolyte and thesurfactant are also important for the association behavior. Forinstance, a larger polyelectrolyte flexibility reduces the cacbecause it is easier for the polyelectrolyte to adopt a conforma-tion that brings many charged segments close to the micellarsurface. 9 The chemical structure of the charged group of thesurfactant is also important, and it has for instance been reportedthat alkylpyridinium halides interact somewhat more stronglywith polyanions than alkyltrimethylammonium halides 5 and thatalkyl sulfates associate more easily with proteins than alkylsulfonates and carboxylates. 10 The reasons for this behaviorare less well understood.The association between polyelectrolytes and surfactants atinterfaces is more complicated, since one then also has toconsider interactions between the interface and the surfactantas well as between the interface and the polyelectrolyte. Thesurface properties that ought to be of particular importance arethe surface-charge density and the hydrophobicity. However,to our knowledge no systematic study has elaborated this point.A particular fruitful way to study association betweenpolyelectrolytes and surfactants in bulk solution is to usedynamic light scattering that allows determination of aggregatesizes and size distributions and that also provides someinformation about interactions between polymer coils in bulksolution. This method has been successfully employed to studyassociation between cationic polyelectrolytes and anionic sur-factants. 11 Association between polyelectrolytes and surfactantsat interfaces can be studied in many ways. Some of the mostrewarding methods are neutron reflectivity, 12 ellipsometry, 13 andsurface force measurements. 14 Previously, we have used thesurface force technique to study the effect of surfactants on theforces between surfaces precoated with adsorbed poly-electrolytes. 14 - 16 The primary information obtained fromsurface force measurements is the force acting between twomacroscopic surfaces as a function of surface separation. Fromthis, it is possible to draw conclusions about the types of forcesthat are important, as well as the extension and compressibilityof the polyelectrolyte - surfactant aggregates adsorbed to thesurface. These studies have shown that, at the solid - liquidinterface, the association between polyelectrolytes and oppositelycharged surfactants is a cooperative process, and that the cac atthe surface is higher than in bulk solution, and also that thepolyelectrolyte charge density is as important for the associationprocess at the solid - liquid interface as it is in bulk solution.One important aim of the present investigation was to identifysimilarities and differences between polyelectrolyte - surfactantassociation in the bulk and at interfaces. To this end, we haveaddressed the association between a cationic polyelectrolyte andan anionic surfactant both in bulk and at a negatively chargedsolid surface. The bulk association was investigated by mainlydynamic light scattering, and the association between polyelec-trolytes and surfactants at the solid - liquid interface was studiedusing the interferometric surface force technique. In one setof experiments we used surfaces precoated with polyelectrolytesand exposed them to surfactant solutions. In another set of experiments we allowed the bare surfaces to come into directcontact with a solution containing both polyelectrolyte andsurfactant. From these experiments we see that many aspectsof bulk and interfacial association are qualitatively similar, butwe have also been able to identify some important differences.In particular, if one wishes to understand how bulk polyelec-trolyte - surfactant mixtures behave at interfaces, it is essentialto have the surface in contact with a solution containing bothpolyelectrolytes and surfactants, whereas preadsorbed polyelec-trolyte layers exposed to a polyelectrolyte-free surfactantsolution allow conclusions to be drawn about the cac at thesurface. Experimental SectionMaterials .  The polyelectrolyte used in the current study wasa copolymer of the uncharged monomer acrylamide (AM) andthe positively charged [3-(2-methylpropionamide)propyl]tri-methylammonium chloride (MAPTAC) in the ratio 69:31. Thepolyelectrolyte (referred to in the remaining text as AM-MAPTAC-31) was thus 31% charged and had a measuredmolecular weight of 780 000 g mol - 1 . The molecular weightof the MAPTAC monomer is 221 g mol - 1 ; i.e., the averagepolyelectrolyte molecule contains just above 2000 charges.Sodium dodecyl sulfate (SDS, BDH, 99%) was used asreceived. Potassium bromide (KBr, Merck, pro analysi) wasroasted at 500  ° C overnight before use, whereas NaCl (Merck,pro analysi)) was used as received. Water was prepared bypassage through a Millipore Milli-RO plus system followed bya Milli-Q 185 system. Immediately prior to the measurement,the water was degassed under vacuum using a water jet pumpfor at least 1 h. Dynamic Light Scattering .  DLS measurements were madeusing the apparatus described briefly in ref 17. An ALV wide-band, multi- τ   digital autocorrelator has been used for datacollection. The measured intensity autocorrelation function g (2) ( t  ) is related to the field correlation function  g (1) ( t  ) bywhere    is a factor accounting for deviations from idealcorrelation and  B  is a baseline term. For a continuousdistribution of relaxation times, corresponding to an infiniterange of particle sizes, the inverse Laplace transform (ILT) maybe used:ILT analysis was performed using a constrained regularizationroutine REPES. 17,18 Although similar to CONTIN, 19 thealgorithm REPES differs in that it directly minimizes the sumof the squared differences between the experimental andcalculated 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 agrid density of 12 per decade can be rapidly performed on anIBM-AT desk-top computer. Representation of the relaxationtime distributions in the form of an  τ   A ( τ  ) versus log  τ   plots,with  τ   A ( τ  ) in arbitrary units, provides an equal area representa-tion. Surface Force Measurements .  Surface force measurementswere conducted with a Mark II or Mark IV surface forceapparatus using muscovite mica surfaces (New York MicaCorp., New York). Mica is an aluminosilicate mineral contain-ing 2.1  ×  10 18 negative surface lattice sites per square meterdue to isomophous substitution of aluminum for silicon. It wascleaved into 1 - 3-  µ m thick molecularly smooth squares withan area of about 1 cm 2 . These sheets were then coated on oneside with a thin layer of silver using thermal deposition andglued silvered-side down to two semicylindrical silica lensesof radius of about 2 cm. The glue used was an epoxy resin(Shell Epikote 1004) that liquefies at increased temperature. g (2) ( t  ) )  B (1 +   | g (1) ( t  ) | 2 ) (1) g (1) ( t  ) ) ∫ 0 ∞  A ( τ  ) exp( - t   /  τ  ) d τ   (2) Interactions at Solid - Liquid Interface  J. Phys. Chem. B, Vol. 102, No. 7, 1998   1271  The SFA is described in detail elsewhere. 20,21 In brief, thetwo mica surfaces are brought together in gas or liquid, usinga piezoelectric crystal and/or a synchronous motor. White lightis introduced in the normal direction to the surfaces, which aremounted in a crossed cylinder geometry. The light undergoesmultiple reflection between the silver layers, and an interferencepattern, fringes of equal chromatic order, is created. Theseparation between the surfaces is calculated from the wave-lengths of the standing waves. The lower surface is attachedto a spring. The distance that the spring deflects is calculatedas the difference between the distance moved by the piezo/ synchronous motor and the resulting change in separationbetween the surfaces. The force,  F  (  D ) is then calculated bymultiplying the measured deflection by the spring constant of the double cantilever spring (measured at the end of eachexperiment).When the data are analyzed, the force is normalized by thelocal geometric mean radius  R , which is approximately 2 cm.The local radius is determined from the shape of the standing-wave pattern at each measuring position. According to theDerjaguin approximation, 22 this quantity is related to the freeenergy of interaction per unit area between flat surfaces  G f  (  D ):with the condition that  D ,  R . Since  D  is of the order of 10 - 6 m or less, this condition is satisfied. Another requirement isthat the radius of the surfaces should be independent of   D , butthis is not always the case, since the surfaces deform understrong repulsive and attractive forces. 23 When the gradient of the force  ∂ F   /  ∂  D  exceeds the spring constant of the doublecantilever spring the system becomes unstable and the surfaces jump together to the next stable region of the force curve. Experimental Procedures . Aqueous solutions containingboth AM-MAPTAC-31 and SDS are turbid in a range of SDS/ MAPTAC ratios. For this reason the measuring chamber wasnot filled with the solution, but rather a small droplet of theliquid was placed between the mica surfaces. This has theadvantage that the light used for interferometric measurementsof surface separation does not pass through a large region of the turbid solution. One drawback is that the system becomesmore sensitive to thermal drift and the precision in themeasurements of weak forces decreases in comparison to thecase when the whole measuring chamber is filled with solution.Forces between surfaces precoated with the polyelectrolytewere determined with the whole measuring chamber filled withthe solution. After polyelectrolytes were introduced into thesolution in the SFA, the system was left to equilibrate overnightbefore the force measurements were carried out. Before SDSwas added, the polyelectrolyte solution was replaced by apolyelectrolyte-free 10 mM KBr solution. The removal of excess polymer from the liquid medium was ensured by rinsingthe apparatus four times with 10 mM KBr. Forces were thenmeasured after equilibration overnight. In the final step SDSwas injected into the measuring chamber and thoroughly mixedas described above. After each surfactant addition, the mixturewas left to stand overnight before measuring any force curves.Two SDS concentrations, 0.1 and 1 mM, were studied, and SDSstock solutions were filtered through a 0.2-  µ m filter beforeinjection into the SFA. In 10 mM KBr, SDS was found tophase-separate upon increasing its bulk concentration to 10 mM,with large needlelike crystals precipitating out of solution. ResultsAssociation of Polyelectrolytes and Surfactants in BulkSolution .  Relaxation time distributions obtained from Laplaceinversion of normalized intensity correlation functions at apolymer concentration of 0.2% and at various surfactantconcentrations are shown in Figure 1. In the absence of SDS,a bimodal distribution is observed. The fast mode correspondsto the translational diffusion of single polyelectrolyte coils andthe slow mode to multichain aggregates. The apparent hydro-dynamic radii  R h  are 6.3 and 497 nm, respectively. The presenceof the slow relaxation mode in polyelectrolyte solutions at lowionic strengths is often observed. It is plausible that it is dueto the large expansion of the polyelectrolyte chains caused bylong-range electrostatic repulsion, which gives rise to anordering effect in the solution that prevents the chains frommoving independently of each other through the solutionvolume. 24 This mechanism is similar to the one that causesthe formation of colloidal crystals, where a long-range double-layer repulsion between charged monodisperse colloids drivesthe formation of ordered structures. 25 Although such aggregatescontribute greatly to the overall scattering in our experiments,it may be estimated that the concentration of the multichainaggregates (here denoted  C  slow ) is very low. Assuming that  D is proportional to  M  - 0.5 , (  D slow  /   D fast ) 2 )  M  fast  /   M  slow.  For thepresent data, the ratio  M  fast  /   M  slow ≈ 10 - 4 . With approximatelyequal intensities for the two modes, one finds that  C  slow ≈ 0.01%at  C  p  ( ∼ C  fast )  )  0.2%. Thus, without surfactant present insolution the multichain aggregates are very few compared tothe single chain coils. The position of the slow peak moves toshorter relaxation times as the surfactant concentration isincreased, which corresponds to a transition to smaller ag-gregates. For example, the apparent  R h  for these structuresdecreases from 325 to 87 nm as the SDS/MAPTAC ratio isincreased 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 greaterdegree of polydispersity for  r  ) 0.3 and 0.4 compared to whenSDS is present in excess. The apparent  R h  at ratios 0.3 - 0.4are approximately 22 nm. The SDS/MAPTAC ratio 0.4 is closeto the phase separation. Further increase in SDS concentration Figure 1.  Relaxation time distribution obtained by Laplace inversionof dynamic light scattering correlation function. The AM-MAPTAC-31 concentration was 2000 ppm, and the SDS/MAPTAC ratios fromtop to bottom are 0, 0.1, 0.2, 0.3, 0.4, 2, and 3.3. At low SDSconcentrations there are two peaks corresponding to a single chain (lowlog  τ  ) and to a multichain (high log  τ  ). At higher surfactant concentra-tions only a single peak corresponding to the surfactant - polyelectrolytecomplex is observed. F  (  D )  R  ) 2 π  G f  (  D ) (3) 1272  J. Phys. Chem. B, Vol. 102, No. 7, 1998   Claesson et al.  results in a macroscopic phase separation showing that thepolyelectrolyte - SDS complexes now attract each other. At highSDS/MAPTAC ratios, at or above  r  ) 2, the system is charge-reversed and the particles are redispersed. The real  R h ,calculated from the diffusion coefficient at infinite dilution,showed that the resolubilized complexes were larger than thoseformed 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 coilat different constant values of the polymer concentration as afunction of SDS concentration. The concentration dependenceof   D  in dilute binary polymer solutions can be expressed by avirial expansion:where  φ  is the volume fraction of the complex,  f   is the frictioncoefficient,  A 2  is the second virial coefficient,  M  w  is the averagemolecular weight of the complex, and  c  is the concentration of the complex (g/mL). However, as we are studying  D  as afunction of SDS concentration, one may insert the SDSconcentration instead of the complex concentration.  D  decreases with increasing surfactant concentration, whichis due to the reduced particle - solvent interaction potential asembodied in the second virial coefficient. The repulsiveinteractions between the coils decrease, which is due to thedecreased electrostatic charge and increased hydrophobicity of the complex at higher SDS concentrations. This effect is thedominant factor in determining  D .  R h  of the complex decreasesalso as a function of the SDS/MAPTAC ratio. Forces between Polyelectrolyte-Coated Surfaces .  Theforces measured between bare mica surfaces immersed in 0.1mM KBr (Figure 3) were dominated at large separations by anelectrostatic double-layer repulsion whereas a van der Waalsattraction 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 - Boltzmannmodel were fitted to the measured interaction, it was found thatthe apparent surface potential was 52 mV and the Debye length23 nm, corresponding to a 1:1 electrolyte concentration of 0.17mM. The adhesion between the surfaces was 30 mN m - 1 . Whenthe KBr concentration was increased to 10 mM, the range of the double-layer force decreased with an apparent surfacepotential and Debye length of 32 mV and 3 nm, respectively.In this case no attractive force was observed at small separationsbecause of the presence of a strong short-range repulsion. Sucha force is commonly observed between mica surfaces in aqueoussolutions with a sufficiently high salt concentration (above about1 - 10 mM, depending on the type of salt present). It is partlydue to the presence of a Stern layer that moves the plane of charge outward 26,27 and partly due to a dehydration of adsorbedcations. 28,29 No hysteresis was observed between forces mea-sured on approach and forces measured on separation.AM-MAPTAC-31 was allowed to adsorb overnight onto themica surfaces from a 10 mM NaCl solution containing 200 ppmAM-MAPTAC-31 or a 10 mM KBr solution containing 1900ppm of the polyelectrolyte. The purely repulsive forcesmeasured across the 200 ppm solution are illustrated in Figure3. They reached a measurable strength at a separation of about100 nm. At a high compressive force the layer could becompressed to about 4 nm. The forces measured on separationwere of slightly lower magnitude than those measured onapproach. The repulsive forces measured across the 1900 ppmpolyelectrolyte solutions were of even longer range and poorlyreproducible. A likely reason for this is that when the surfacesare brought together, polyelectrolytes present in solution aretrapped between them. When the polyelectrolyte-containingsolution was replaced with a 10 mM KBr solution, the measuredforce decreased markedly in magnitude and became morereproducible. The repulsive force measured on approachreached a magnitude of 0.1 mN/m at a separation of between90 and 130 nm and increased monotonically with decreasingseparation (Figure 4). The minimum separation obtained undera high force was about 4 nm, i.e., the same as that reached inthe 200 ppm AM-MAPTAC-31 solution. The forces measuredon separation were less repulsive than those measured onapproach. Unlike the situation with polymers present insolution, a weak adhesive minimum was found at a distance of about 130 nm (Figure 5). The large distance at which theminimum was observed suggests that the srcin of the attractionis polyelectrolyte-bridging between the two surfaces. As acomparison, the forces measured in 0.1 mM KBr between AM-MAPTAC-31 coated surfaces obtained by adsorption from a20 ppm polyelectrolyte solution in 0.1 mM KBr are also shownin Figure 4. Under these circumstances, where the adsorptionis close to its plateau value at this low ionic strength, a weak Figure 2.  Effective diffusion coefficient for the single chain as afunction of surfactant concentration. The measurements were conductedat various AM-MAPTAC-31 concentrations, which from top to bottom,are 10000, 8000, 6000, 4000, and 2000 ppm. Figure 3.  Force normalized by radius as a function surface separationbetween mica surfaces immersed in 0.1 mM KBr (filled squares) andin 10 mM KBr (triangles). Filled and unfilled symbols represent forcesmeasured on approach and separation, respectively. Forces measuredacross a 10 mM NaCl solution containing 200 ppm AM-MAPTAC-31are represented by filled circles.  D  z ) kT  (1 - φ ) 2  f   (1 + 2  A 2  M  w c + ...) (4) Interactions at Solid - Liquid Interface  J. Phys. Chem. B, Vol. 102, No. 7, 1998   1273  electrostatic double-layer force dominates the long-range inter-action. At a separation of about 20 nm, a bridging attractionpulls the surfaces into contact at a separation of about 4 nm.The pull-off force measured on separation is about 1 mN/m. Itis the strong electrostatic affinity between the polymer and thesurface at the low ionic strength that results in a thin adsorbedlayer and a pronounced bridging attraction. Forces between Surfaces Precoated with Polyelectrolytesin the Presence of Surfactants .  The introduction of 0.1 mMSDS hardly affected the long-range force (Figure 6). Neverthe-less, a clear difference was observed in the layer thicknessobtained under a high compressive force. It increased from 4nm before addition of SDS to around 6 nm in 0.1 mM SDSsolution. This suggests that some surfactants are incorporatedinto the layer. Another clear difference was that when thecompressive force was released in the presence of 0.1 mM SDS,the polyelectrolyte layer hardly expanded until a zero force wasreached (Figure 5). Hence, the presence of SDS in thecompressed polyelectrolyte layer retarded its reexpansionbecause of hydrophobic interactions between surfactants as-sociated with the polyelectrolytes.A further increase in SDS concentration to 1 mM dramaticallyincreased the range of the repulsive force to about 180 nm(Figure 6). The force increased strongly with decreasingseparation, displaying a distinct s shape when plotted on alogarithmic force scale. Even at very high loads, no clear hardwall was observed. Just as in the lower SDS concentration,the layer did not reswell when the compressive force wasreduced. On a consecutive approach, the force was rathersimilar to the one observed during the first approach (Figure7). Forces between Surfaces across Solutions Containing BothPolyelectrolytes and Surfactants .  All measurements presentedin this section were, unlike those discussed above, determinedacross a 200 ppm polyelectrolyte solution in 10 mM NaCl. Theratio of SDS to charged MAPTAC segments of the polyelec-trolyte was varied. A new pair of mica surfaces were used foreach experiment. The results obtained after at least a 12-hequilibration for SDS/MAPTAC ratios of 0, 0.2, and 0.4 are Figure 4.  Force normalized by radius as a function surface separationbetween mica surfaces precoated with a AM-MAPTAC-31 layer. Thetriangles represent data where the polyelectrolyte layer was adsorbedfrom a 1900 ppm polyelectrolyte solution in 10 mM KBr, and then theforces were measured across a polyelectrolyte-free 10 mM KBr solution.Filled and unfilled symbols represent forces measured on approach andseparation, respectively. The filled circles represent data obtained byadsorbing the polyelectrolyte from a 20 ppm 0.1 mM KBr solutionand then measuring the forces across a polyelectrolyte-free 0.1 mMKBr solution. Figure 5.  Force normalized by radius as a function surface separationbetween mica surfaces precoated with an AM-MAPTAC-31 layerobtained by adsorption from a 10 mM KBr solution. The forces weremeasured across a polyelectrolyte-free 10 mM KBr solution containingno SDS (squares), 0.1 mM SDS (triangles), and 1 mM SDS (circles).Filled and unfilled symbols represent forces measured on approach andseparation, respectively. Figure 6.  Force normalized by radius as a function surface separationbetween mica surfaces precoated with an AM-MAPTAC-31 layerobtained by adsorption from a 10 mM KBr solution. The forces weremeasured on approach across a polyelectrolyte-free 10 mM KBr solutioncontaining no SDS (squares), 0.1 mM SDS (triangles), and 1 mM SDS(circles). Figure 7.  Force normalized by radius as a function surface separationbetween mica surfaces precoated with an AM-MAPTAC-31 layerobtained by adsorption from a 10 mM KBr solution containing 1 mMSDS. Results for first approach (filled squares), first separation (unfilledsquares), and second approach (filled circles) are shown. 1274  J. Phys. Chem. B, Vol. 102, No. 7, 1998   Claesson et al.
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