Forces between Glass Surfaces in AqueousPolyethylenimine Solutions

Evgeni Poptoshev and Per M. Claesson*

Department of Chemistry, Surface Chemistry, Royal Institute of Technology,Drottning Kristinas Vag 51, SE-100 44 Stockholm, Sweden, and Institute for Surface

Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden

Received August 14, 2001. In Final Form: January 3, 2002

Interaction forces between flame-polished glass surfaces were measured in a range of aqueous solutionsof a branched cationic polyelectrolyte, polyethylenimine (PEI) MW ≈ 70 000 g/mol. Short incubation in1 ppm PEI solution leads to neutralization of the glass negative surface charge. At this point the interactionforces are dominated by a bridging attraction detectable at separations below 10 nm. Prolonged incubationin the same solution results in charge reversal. Upon increasing the bulk polymer concentration, an additionaladsorption takes place, and the magnitude of the observed charge reversal increases. In 50 ppm PEI anadditional electrosteric force is present at short distances, and the force measured on approach remainsrepulsive at all separations. The pull-off forces measured on separation were shown to be dependent onthe time the two surfaces spent in contact and increased with increasing contact time. In addition, theconcentration dependence of the pull-off force in PEI solutions was compared to two linear, highly chargedpolyelectrolytes polyvinylamine (PVAm) and poly([2-(propionyloxy)ethyl]trimethylamonium chloride)(PCMA). It was found that the two linear polyelectrolytes induce lower adhesion between glass surfacesthan PEI. Further, for the linear polyelectrolytes the adhesion force was not strongly influenced by thepolyelectrolyte concentration for concentrations higher than 2 ppm. In contrast, PEI generates higheradhesive forces at low bulk concentrations, which then sharply decrease upon increasing the PEIconcentration. These differences are discussed in terms of both the PEI highly branched structure and thefact that the PEI charge is concentration dependent due to changes in solution pH.

Introduction

Polyethylenimine is a name commonly used for a ratherlarge group of water-soluble, polyamines with varyingmolecular weight and structure. Ring-opening polymer-ization of ethylenimine yields a highly branched PEIcontaining primary, secondary, and tertiary amine groupsin an approximate ratio 1:2:1.1,2 At a pH of 5.5-6, about50% of these groups are protonated;3 i.e., PEI behaves asa moderately charged cationic polyelectrolyte. PEI is aweak polybase, and in more concentrated aqueous solu-tions the ionization is self-suppressed due to increase ofsolution pH.1

As expected from its cationic character, PEI readilyadsorbs onto negatively charged surfaces, causing changeneutralization and subsequent charge reversal.4 Underthese conditions dispersed solids flocculate or redispersedepending on the polymer dosage. As with other cationicpolyelectrolytes, these properties have been utilized innumerous industrial and scientific applications demand-ing control over stability of colloidal systems, papermak-ing,5 and biomedical sciences6 to mention just two.Generally, the mode of action of a particular polyelectrolytedepends on the type of interactions it generates uponadsorption to the surface of the dispersed phase. Charged

particles in water are mainly stabilized by electrostaticforces. Suppression of these forces (partial or full chargeneutralization) may lead to flocculation. In some casesadditional attraction is generated due to polymer bridging.Conversely, stabilization is realized via introducingrepulsive forces, mainly of steric or electrostatic origin.Direct surface force measurements between polyelectro-lyte-coated surfaces can therefore provide useful insightinto the mode of action of a particular polyelectrolyte andits potential use as a flocculant or dispersant.

While there have been quite a few studies on surfaceforces in the presence of linear polyelectrolytes,7-15

branched ones (and in particular PEI) have not beenstudied extensively. Claesson et al.4 investigated theinteractions between PEI-coated mica surfaces using aninterferometric surface force technique.Theirmain findingwas that PEI adsorbs to the negatively charged micasurface, causing charge neutralization and charge reversalat higher polymer concentrations. The authors alsoreported a pH dependence of the interfacial properties ofPEI due to change in polyelectrolyte charge density. Inthe present study, the forces between flame-polished glass

* Corresponding author.(1) Molyneux, P. Water-Soluble Synthetic Polymers Properties and

Behavior; CRC Press: Boca Raton, FL, Vol. II, 1984.(2) Horn, D. Polymeric Amines and Ammonium Salts; Pergamon

Press: Oxford, 1980.(3) Lindquist, G. M.; Stratton, R. A. J. Colloid Interface Sci. 1976,

55, 45.(4) Claesson, P. M.; Paulson, O. E. H.; Blomberg, E.; Burns, N. L.

Colloids Surf. A 1997, 123-124, 341.(5) Retention of Fines and Fillers during Papermaking; Tappi Press:

Atlanta, GA, 1998.(6) Schurer, J. W.; Kalicharan, D.; Hoedemaeker, P. J.; Molenaar, I.

J. Hystochem. Cytochem. 1978, 26, 688.

(7) Claesson, P. M.; Dahlgren, M. A. G.; Eriksson, L. Colloids Surf.A 1994, 93, 293.

(8) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.;Sjostrom, L.; A° kesson, T.; Jonsson, B. J. Phys. Chem. 1993, 97, 11769.

(9) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. ColloidInterface Sci. 1994, 166, 343.

(10) Dahlgren, M. A. G.; Hollenberg, H. C. M.; Claesson, P. M.Langmuir 1995, 11, 4480.

(11) Holmlberg, M.; Wigren, R.; Erlandsson, R.; Claesson, P. M.Colloids Surf. A 1997, 129-130, 175.

(12) Kawanishi, N.; Christenson, H. K.; Ninham, B. W. J. Phys. Chem.1990, 94, 4611.

(13) Poptoshev, E.; Rutland, M. W.; Claesson, P. M. Langmuir 1999,15, 7789.

(14) Poptoshev, E.; Rutland, M. W.; Claesson, P. M. Langmuir 2000,16, 1987.

(15) Hartley, P. G.; Scales, P. J. Langmuir 1998, 14, 6948.

2590 Langmuir 2002, 18, 2590-2594

10.1021/la0112918 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 03/01/2002

surfaces were measured in the presence of PEI in theconcentration range 1-50 ppm. In some cases a com-parison is made between PEI and other polymers previ-ously studied, and the results are discussed in terms ofthe specific chemical structure and chain architecture.

Materials and MethodsChemicals. Polyethylenimine with mean molecular weight

of 70 000 g/mol was obtained from Polyscience Inc. and usedwithout further purification. The polymer was supplied in a formof 30% aqueous solution, from which a 1000 ppm stock solutionwas prepared. Sodium chloride suprapure was purchased fromMerck and used as received.

Water was purified by Milli-RO 10 and RiOs 8 reverse osmosisunits from Millipore and filtered through a 200 nm Milliporefilter. The total organic carbon content of the water was monitoredwith an in-line TOC monitor (Millipore A10). In all cases thetotal organic carbon of the outgoing water did not exceed 10 ppb.

Special care was taken in order to avoid contamination. Allglassware was treated with hot chromosulfuric acid for at least1 h and rinsed thoroughly with Milli-Q water prior to use. Allprocedures regarding solution preparation and instrumentassembly were carried out inside a laminar flow cabinet.

SurfaceForceMeasurements.A noninterferometric surfaceforce apparatus, better known as MASIF (measurements andanalysis of surface interactions and forces), was used throughoutthis study. A detailed description of the instrument appears ina number of articles,16-18 and only a brief outline will be givenhere. The MASIF uses indirect surface separation detection, thusallowing the forces between any kind of hard surfaces to bedetermined as long as surface roughness and geometry require-ments are met. Both interacting surfaces are attached topiezoelectric materials. They are enclosed in a stainless steelmeasuring chamber (volume ca. 10 mL) fitted with syringe portsfor liquid exchange. The upper surface is mounted on apiezoelectric tube. During force measurement, the surfaceseparation is varied continuously by applying a triangular voltagewave to the piezo tube. The lower surface is attached to a bimorphforce sensor. It bends under the action of surface forces, whichproduces a charge in proportion to the bending. This charge isamplified and automatically recorded. After surface contact isreached the linear motion of the upper surface is directlytransmitted to the bimorph, and the charge data can be convertedinto deflection since the piezo displacement is known. The forceis then obtained by multiplying the deflection data by the springconstant of the bimorph, which is measured separately after theexperiment. The results are presented as force scaled by theradius of interaction R, which is related to the free energy perunit area between flat plates GF(D), according to the Derjaguinapproximation.19

In the case of interaction between two spheres, R is given by R) (r1r2/r1+ r2), where r1 and r2 are the radii of the spheres. Themacroscopic radius of curvature was measured with a micrometerafter the experiment.

Flame-polished glass surfaces were prepared by melting theend of a borosilicate glass rod in a butane-oxygen flame untila droplet with a radius of ca. 2 mm was formed. After allowingthe surfaces to cool for a few seconds, they were immediatelymounted in the instrument, and the measuring chamber wassealed in order to avoid airborne contamination.

In the later part of the manuscript we will make somecomparison between our results obtained with the MASIFtechnique with earlier results obtained by using the surface forceapparatus (SFA). We note that both techniques use macroscopicsurfaces with a radius of about 2 mm (MASIF) and 20 mm (SFA).The radius of curvature is in both cases much too large to result

in any curvature effects on the adsorbed layer structure. Anotherdifference between the MASIF and SFA is that the MASIFmeasurements are carried out considerably more rapidly. In casethe relaxation rate of the adsorbed polymer layer is comparableto the measuring rate, one expects to observe a force that varieswith the measuring speed. No such variations were observedover the range of measuring speeds used, except that the adhesionforce was found to increase with contact time.

Results and DiscussionForces Measured on Approach. The force curve

measured in polymer-free 0.1 mM NaCl solution ispresented in Figure 1 together with a DLVO20,21 fit (solidline) using constant surface charge boundary conditionsand a surface potential of -65 mV. It can be seen that theinteraction follows the DLVO predictions closely down toseparations of 3-4 nm. At shorter separations, anadditional, non-DLVO repulsion is present. Instead of theexpected jump into adhesive contact under the action ofvan der Waals forces, the interaction remains purelyrepulsive until surface contact is reached. No adhesion isobserved on separation. The presence of a short-rangenon-DLVO repulsion between various types of glass andsilicasurfaces inaqueoussolutions iswell-documented22-24

but lacks definitive explanation at present. Two hypoth-eses have been proposed. The hydration force hypothesis23

attributes the additional repulsion to the energy neededto remove water molecules from the hydration layer ofthe surface bound cations and silanol groups. Otherauthors24 have proposed formation of a gellike layer atthe silica-water interface and assumed the repulsion tobe of steric origin. A detailed discussion of the problem isbeyond the scope of this work. It can only be noted that,regardless of the mechanism, the extra repulsion vanishesupon polyelectrolyte adsorption.

Introduction of a 1 ppm PEI solution into the measuringchamber changes the interaction completely. The resultsare shown in Figure 2. The double-layer repulsion iscompletely removed after 30 min of incubation, whichindicates that PEI adsorption has led to a complete

(16) Ederth, T. Novel Surfaces for Force Measurements. Ph.D. Thesis,Royal Institute of Technology, Stockholm, 1999.

(17) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205.(18) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv.

Colloid Interface Sci. 1996, 67, 119.(19) Derjaguin, B. Kolloid Z. 1934, 69, 155.

(20) Derjaguin, B.; Landau, L. Acta Physiochem. 1941, 14, 633.(21) Verwey, E. G. W.; Overbeek, J. T. G. The Theory of the Stability

of Liophobic Colloids; Elsevier: Amsterdam, 1948.(22) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface

Sci. 1994, 165, 367.(23) Chapel, J.-P. Langmuir 1994, 10, 4237.(24) Yaminsky, V. V.; Ninham, B. W.; Pashley, R. M. Langmuir 1998,

14, 3223.

F(D)/R ) 2πGF(D)

Figure 1. Force scaled by radius as a function of separationbetween glass surfaces in polyelectrolyte-free 0.1 mM NaClsolution. The solid line represents a force curve calculated usingDLVO theory with constant surface charge boundary conditionsand the following parameters: apparent surface potential atlarge separation Ψ0 ) -65 mV, Debye length κ-1 ) 30 nm, andHamaker constant A ) 0.5 × 10-20 J.35

Aqueous Polyethylenimine Solutions Langmuir, Vol. 18, No. 7, 2002 2591

neutralization of the glass negative surface charge.Instead, the surfaces start to attract each other at aseparation of about 10 nm, and this attraction makes thesurfaces jump inward from about 5 nm to hard wall contactbetween the adsorbed polymer layers. The jump occurswhen the gradient of the force exceeds the spring constantof the bimorph. It can be seen that the attraction is longerranged than the expected van der Waals force (solid line).The most likely cause of this is polymer bridging. Bridgingis realized when segments of a polymer chain adsorbedto one of the surfaces become attracted (and consecutivelyattached) to the other surface or alternatively whenpolymer molecules in the gap between the surfaces aresimultaneously attracted to both surfaces.8 It is worthnoticing that linear chained polyamines with similarmolecular mass13 generate considerably longer bridgingforce under similar conditions. For instance, in the caseof polyvinylamine13 the attraction became detectablealready at 25 nm. The difference can be explained by thedifference in chain structure between PEI and PVAm.Linear polymers have a larger contour length than highlybranched ones at a given molecular weight. Hence, thelinear polymer chain can stretch further away from thesurface than the branched one, which increases the rangeof the bridging force.

Incubating the surfaces in the 1 ppm PEI solution forprolonged times results in another profound change tothe interaction profile. The lowest curve in Figure 3 (opensquares) shows the force measured on approach after 24h of incubation. At large separations, a repulsive double-layer force is present. Fitting DLVO theory (solid line) tothe data resulted in a value of 60 mV for the apparentsurface potential at large separations. Evidently, anadditional adsorption has taken place during the incuba-tion,andthesurfacesacquireapositive charge. Incontrast,no charge reversal is observed on glass surfaces incubatedfor 24 h in a 1 ppm polyvinylamine, PVAm, solution of lowionic strength.13 In fact, even at higher PVAm concentra-tions the magnitude of the potential remains limited (+28mV at 10 ppm13), indicating a limited degree of chargeovercompensation. A large degree of overcompensation ofthe surface charge of mica by PEI was also observed byClaesson et al.4 employing surface force and ESCAtechniques. The enhanced charge reversal caused by PEIadsorption is attributed to the branched structure of PEI.This can be explained by the fact that branching bynecessity leads to that a high number of charges arebrought onto a small surface area by each adsorbingpolyelectrolyte molecule. Charge overcompensation ingeneral suggests that some nonelectrostatic interactions

betweenthepolymerandthesubstratesurface ispresent.25

However, this can hardly explain the large differencebetween PEI and PVAm, keeping in mind their similarchemical structure. Hence, we suggest that the branchingis the main effect behind the large degree of charge reversalobserved in this study.

At separations below about 10 nm the measuredinteraction is still less repulsive than the calculated DLVOforce, although the jump-in distance is roughly consistentwith that expected from the van der Waals attraction(around 2 nm). As in the case of 30 min incubation, it canbe argued that an additional attraction is present due tobridging. Apparently, despite the charge overcompensa-tion, there are still free adsorption sites on the glass surfacecapable of attracting PEI segments and causing bridging.This is consistent with the patchy adsorbed layer of PEIobserved by AFM imaging26 and the patchy adsorbed layerstructure suggested by for example Gregory.27 The reasonthat branched polymers are more prone to form patchylayers than linear ones is the large number of chargesthat are brought down on a small surface area by eachpolyelectrolyte.

We note that charge regulation may also explain theshape of the force curve obtained in 1 ppm PEI, displayedin Figure 3. However, we regard this mechanism asunlikely since the forces at higher PEI concentrations (10ppm curve, Figure 3) are well described by the constantcharge model. We therefore propose that bridging is thecause for the deviation between measured and calculatedforce profiles.

Increasing the bulk polymer concentration to 10 ppmresults in a further increase in the magnitude of the double-layer repulsion. Results are shown in Figure 3, open circles.An apparent surface potential at large separations of 90mV was extracted from the DLVO fit to the curve. Thisindicates that an additional amount of PEI is adsorbed tothe glass surface. More importantly, the interaction nowfollows the DLVO theory closely at all separations; i.e.,the bridging force was not observed when bringing thesurfaces together. This is an expected result consideringthe large degree of charge overcompensation, indicating

(25) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.;Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall:London, 1993.

(26) Akari, S.; Schrepp, W.; Horn, D. Ber. Bunsen-Ges. Phys. Chem.1996, 100, 1014.

(27) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448.

Figure 2. Force scaled by radius as a function of separationbetween glass surfaces in 1 ppm PEI solution after 30 min ofincubation. The solid line is the calculated van der Waalsinteraction.

Figure 3. Force scaled by radius as a function of separationbetween glass surfaces in PEI solutions after 24 h of incuba-tion: open squares, 1 ppm PEI; open circles, 10 ppm PEI. Solidlines are fits by DLVO theory with constant surface chargeboundary conditions and the following parameters: Ψ0 ) 60mV (1 ppm curve) and Ψ0 ) 90 mV (10 ppm curve). The restof the parameters used are the same as in Figure 1.

2592 Langmuir, Vol. 18, No. 7, 2002 Poptoshev and Claesson

that the surfaces are now approaching a state where theyare fully coated by PEI. In absence of free adsorption sitesthe polymer segments from one of the layers will not beattracted to the opposing surface, and no bridging forcewill be present.

The interaction measured after introducing a 50 ppmPEI solution into the measuring chamber is shown inFigure 4. For comparison, the result for 10 ppm is alsoplotted. It can be seen that at large separation the twocurves are almost identical. The charge decreases some-what at the higher concentration as indicated by theslightly lower magnitude of the double-layer repulsion.Further, the curves deviate significantly at separationsbelow 3-4 nm. In the case of 50 ppm PEI no attractionis observed, and the total interaction remains repulsiveat all distances. An attraction is however still present onseparation (see below). To understand this, we have toconsider that PEI is a weak polybase. The measured pHof the 50 ppm PEI solution drained from the MASIFmeasuring chamber was found to be 9.0 (compared to 6.3at 10 ppm). At this pH less than 10% of the monomers arecharged, compared to 50% at pH 5.5-6.3 Hence, we proposethat the purely repulsive force observed on approach acrossthe 50 ppm PEI solution is due to the decrease in PEIcharge density, leading to higher adsorbed amounts andthicker adsorbed layers. Support for this hypothesis comesfrom an earlier study, using mica substrates, which showsthat as the pH is increased, the adsorbed amount andlayer thickness indeed increase and steric forces becomemore important.4 The adsorbed amount on glass isexpected to be lower than on mica, due to lower surfacecharge density, whereas an increase in adsorbed amountand layer thickness with pH is expected to occur for bothsystems. An increase in layer thickness with decreasingcharge density has also been theoretically predicted28 aswell as experimentally observed for a range of copolymerswithvariable chargedensity.29,30 Further, thickeradsorbedlayers generate an electrosteric type of repulsion whenconfined between the approaching surfaces. In our casethe range of this repulsion is only 3-4 nm counted fromthe hard wall contact, indicating that no excessive swellingof the polyelectrolyte layer occurs. In similar experiments,using PEI and mica as a substrate in the interferometricSFA, an increase of the layer thickness by a factor of 1.6-

1.7 was found to occur when the polymer concentrationwas increased from 10 to 50 ppm.4 It should be noted thatthe charge density of the glass surface itself increaseswith increasing pH, which promotes further adsorptionand formation of a thicker layer.

Forces Measured on Separation. It was alreadymentioned that no adhesion is observed in polyelectrolyte-free, salt solutions. However, introducing PEI into thesystem changes the forces measured on separationdramatically. There is now a negative (pull-off) forcerequired to separate the surfaces from contact (see Figure5). The magnitude of the pull-off force measured afterprolonged incubation was found to dependent on severalfactors as will be discussed below.

The dependence of the pull-off force on the time thesurfaces spent in contact is shown in Figure 6. The MASIFinstrument allows the surfaces to “rest” in contact for aselected period of time before they are separated. Thepresented data were collected at a constant driving speed(ca. 20 nm/s) of the piezo in order to minimize any rate-dependent effects as discussed by Ruths et al.31 It can beseen that an increase in the contact time leads to asignificant increase in the magnitude of the pull-off force.For the range of contact times studied here the pull-offforce increased from 35.1 mN/m at 2.8 s to 48.5 mN/m

(28) Linse, P. Macromolecules 1996, 29, 326.(29) Kjellin, M. U. R.; Claesson, P. M.; Audebert, R.J. Colloid Interface

Sci. 1997, 190, 476.(30) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M.

Langmuir, in press. (31) Ruths, M.; Granick, S. Langmuir 1998, 14, 1804.

Figure 4. Force scaled by radius as a function of separationbetween glass surfaces in 50 ppm PEI solution (open squares).For comparison, the curve recorded in 10 ppm PEI is also shown(filled diamonds).

Figure 5. Force measured on separation scaled by radius asa function of separation between glass surfaces in PEI solutions.The arrows point to the position reached after the outwardjumps. Note the saturation of the bimorph signal at 1 ppm(lowest curve). In this case the pull-off force is determined bymultiplying the spring constant with the outward jump distance.All measurements were carried out at a constant contact timeof 20 s.

Figure 6. Pull-off force scaled by radius vs contact time in 1ppm PEI solution. The data were collected after 24 h ofincubation.

Aqueous Polyethylenimine Solutions Langmuir, Vol. 18, No. 7, 2002 2593

when the surfaces were allowed to remain in contact for280 s. Longer contact times were impossible to obtainsince electrical and thermal drifts develop on the bimorphduring the rest period, which affects the reliability of thedata. The results in Figure 6 suggest that the adhesionbetween PEI-coated surfaces is related to polymer chainrelaxation phenomena. Apparently, once in contact, directsurface-polymer-surface bridges are formed. The ad-sorbed chains then relax, following a path that allowsmaximizing the entropy and the interaction with thesurfaces; i.e., more bridges are formed, and the pull-offforce increases as a result. Polymer chain interpenetrationand entanglement effects, as suggested for similar resultsusing grafted uncharged polymers,32 are likely to play aminor role in this case since (especially at low ionicstrength) the polyelectrolyte chains experience strongelectrostatic repulsion.

The magnitude of the pull-off force was also found to beconcentration dependent at constant contact time. Figures5 and 7 illustrate this. For PEI (open circles in Figure 7)the magnitude of the pull-off force is comparatively highat low polymer concentrations. Above 5 ppm the pull-offforce decreases sharply from 40 mN/m to about 15 mN/mat 50 ppm bulk concentration. For comparison, results fortwo linear polyelectrolytes are also present in Figure 7.Both polyvinylamine (PVAm) and poly[[2-(propionyloxy)-ethyl]trimethylammonium chloride] (PCMA) behave simi-larly. The pull-off force increases at low concentrationsand reaches a plateau value above 2 ppm. Comparingthese results, two major differences are revealed. First,PEI generates much higher pull-off forces at low concen-tration. This is attributed to the branched polymerstructure that aids the formation of patchy adsorbedlayers26,27 and facilitates bridging in contact. This is seenas a major reason for the enhanced adhesion. Second, thepull-off force vs concentration profile for PEI is completelydifferent compared with the other two polymers as shown

in Figure 7. This is due to the reduction of the PEI chargedensity as a result of the increased pH of the solution.This leads to weaker electrostatic interactions betweenthe polymer and the surfaces, thicker adsorbed layers,and lower pull-off forces. Similar effects, i.e., thicker layersand lower adhesion forces with decreasing charge densityof the adsorbing polyelectrolyte, are observed for poly-electrolytes with permanent charges (e.g., of the AM-MAPTAC series).33,34 We note that the adhesion betweenpolyelectrolyte-coated surfaces can also be reduced byadding an inorganic electrolyte.8 In the latter case, theincreased ionic strength leads to screening of the elec-trostatic polymer-surface interactions and swelling of theadsorbed layers. These effects can be rationalized ifconsidering that bridging forces control the adhesionbetween polyelectrolyte-coated surfaces. Hence, any effectthat counteracts formation of polyelectrolyte bridges, e.g.,lower charge density of the polyelectrolyte, lower surfacecharge density, higher ionic strength, and thicker adsorbedlayer, will reduce the measured adhesion force.

ConclusionsIn the present study we investigated the effect of

branched cationic polyethylenimine on the interactionforces between negatively charged glass surfaces. Thespecific chemical nature and architecture of the PEI havea strong effect on the surfaces forces as revealed bycomparison between the results reported here and thoseobtained with linear polyelectrolytes previously studied.The main differences are the following:

(i) The range of the bridging attraction between PEI-coated glass surfaces is of shorter range as compared tothe one generated by linear polyelectrolytes with similarmolecular mass adsorbed to the same type of surface. Thisis due to the more compact, branched structure of the PEImolecule.

(ii) PEI causes a pronounced charge reversal (+60 mV)already at a bulk concentration of 1 ppm. At higherconcentrations the potential increases further to +90 mV.Such a large degree of overcompensation is not observedfor linear polyelectrolytes under similar conditions. Thus,we attribute the major charge reversal observed for PEIto its branched structure.

(iii) The pull-off force between PEI-coated glass surfacesis dependent on the contact time, indicating slow rear-rangements of the polymer layer. No similar effects areobserved for the linear polyelectrolytes.

(iv) The pull-off force decreases with increasing PEIconcentration due to an increase in bulk solution pH thatreduces the charge density of the polyelectrolyte.

Acknowledgment. E.P. acknowledges financial sup-port from Bo Rydin’s foundation for scientific research.Discussions with Mark Rutland were highly appreciated.

LA0112918

(32) Plunkett, M. A.; Rutland, M. W. J. Adhes. Sci. Technol., in press.

(33) Dahlgren, M. A. G.; Claesson, P. M. Nordic Pulp Pap. Res. 1993,8, 62.

(34) Claesson, P. M.; Dedinaite, A.; Poptoshev, E. In PhysicalChemistry of Polyelectrolytes; Radeva, T., Ed.; Marcel Dekker: NewYork, 2001; Surfactant Science Series Vol. 99, p 447.

(35) Bergstrom, L. Adv. Colloid Interface Sci. 1997, 70, 125.

Figure 7. Pull-off force scaled by radius as a function of bulkpolymer concentration: open circles, PEI MW ≈ 70 000 g/mol;open squares, PVAm MW≈90 000 g/mol; open triangles, PCMAMW ≈ 1 × 106 g/mol. All measurements were carried out at aconstant contact time of 20 s. The solid lines are for eye guidanceonly.

2594 Langmuir, Vol. 18, No. 7, 2002 Poptoshev and Claesson