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Forces between Glass Surfaces in Aqueous Polyethylenimine Solutions Evgeni Poptoshev and Per M. Claesson* Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨ g 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 solutions of a branched cationic polyelectrolyte, polyethylenimine (PEI) MW ≈ 70 000 g/mol. Short incubation in 1 ppm PEI solution leads to neutralization of the glass negative surface charge. At this point the interaction forces are dominated by a bridging attraction detectable at separations below 10 nm. Prolonged incubation in the same solution results in charge reversal. Upon increasing the bulk polymer concentration, an additional adsorption takes place, and the magnitude of the observed charge reversal increases. In 50 ppm PEI an additional electrosteric force is present at short distances, and the force measured on approach remains repulsive at all separations. The pull-off forces measured on separation were shown to be dependent on the time the two surfaces spent in contact and increased with increasing contact time. In addition, the concentration dependence of the pull-off force in PEI solutions was compared to two linear, highly charged polyelectrolytes polyvinylamine (PVAm) and poly([2-(propionyloxy)ethyl]trimethylamonium chloride) (PCMA). It was found that the two linear polyelectrolytes induce lower adhesion between glass surfaces than PEI. Further, for the linear polyelectrolytes the adhesion force was not strongly influenced by the polyelectrolyte concentration for concentrations higher than 2 ppm. In contrast, PEI generates higher adhesive forces at low bulk concentrations, which then sharply decrease upon increasing the PEI concentration. These differences are discussed in terms of both the PEI highly branched structure and the fact that the PEI charge is concentration dependent due to changes in solution pH.
Introduction Polyethylenimine is a name commonly used for a rather large group of water-soluble, polyamines with varying molecular weight and structure. Ring-opening polymerization of ethylenimine yields a highly branched PEI containing primary, secondary, and tertiary amine groups in an approximate ratio 1:2:1.1,2 At a pH of 5.5-6, about 50% of these groups are protonated;3 i.e., PEI behaves as a moderately charged cationic polyelectrolyte. PEI is a weak polybase, and in more concentrated aqueous solutions the ionization is self-suppressed due to increase of solution pH.1 As expected from its cationic character, PEI readily adsorbs onto negatively charged surfaces, causing change neutralization and subsequent charge reversal.4 Under these conditions dispersed solids flocculate or redisperse depending on the polymer dosage. As with other cationic polyelectrolytes, these properties have been utilized in numerous industrial and scientific applications demanding control over stability of colloidal systems, papermaking,5 and biomedical sciences6 to mention just two. Generally, the mode of action of a particular polyelectrolyte depends on the type of interactions it generates upon adsorption to the surface of the dispersed phase. Charged * 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.
particles in water are mainly stabilized by electrostatic forces. Suppression of these forces (partial or full charge neutralization) may lead to flocculation. In some cases additional attraction is generated due to polymer bridging. Conversely, stabilization is realized via introducing repulsive forces, mainly of steric or electrostatic origin. Direct surface force measurements between polyelectrolyte-coated surfaces can therefore provide useful insight into the mode of action of a particular polyelectrolyte and its potential use as a flocculant or dispersant. While there have been quite a few studies on surface forces in the presence of linear polyelectrolytes,7-15 branched ones (and in particular PEI) have not been studied extensively. Claesson et al.4 investigated the interactions between PEI-coated mica surfaces using an interferometric surface force technique. Their main finding was that PEI adsorbs to the negatively charged mica surface, causing charge neutralization and charge reversal at higher polymer concentrations. The authors also reported a pH dependence of the interfacial properties of PEI due to change in polyelectrolyte charge density. In the present study, the forces between flame-polished glass (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.; Sjo¨stro¨m, L.; A° kesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (9) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface 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.
10.1021/la0112918 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/01/2002
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surfaces were measured in the presence of PEI in the concentration range 1-50 ppm. In some cases a comparison is made between PEI and other polymers previously studied, and the results are discussed in terms of the specific chemical structure and chain architecture. Materials and Methods Chemicals. Polyethylenimine with mean molecular weight of 70 000 g/mol was obtained from Polyscience Inc. and used without further purification. The polymer was supplied in a form of 30% aqueous solution, from which a 1000 ppm stock solution was prepared. Sodium chloride suprapure was purchased from Merck and used as received. Water was purified by Milli-RO 10 and RiOs 8 reverse osmosis units from Millipore and filtered through a 200 nm Millipore filter. The total organic carbon content of the water was monitored with an in-line TOC monitor (Millipore A10). In all cases the total organic carbon of the outgoing water did not exceed 10 ppb. Special care was taken in order to avoid contamination. All glassware was treated with hot chromosulfuric acid for at least 1 h and rinsed thoroughly with Milli-Q water prior to use. All procedures regarding solution preparation and instrument assembly were carried out inside a laminar flow cabinet. Surface Force Measurements. A noninterferometric surface force apparatus, better known as MASIF (measurements and analysis of surface interactions and forces), was used throughout this study. A detailed description of the instrument appears in a number of articles,16-18 and only a brief outline will be given here. The MASIF uses indirect surface separation detection, thus allowing the forces between any kind of hard surfaces to be determined as long as surface roughness and geometry requirements are met. Both interacting surfaces are attached to piezoelectric materials. They are enclosed in a stainless steel measuring chamber (volume ca. 10 mL) fitted with syringe ports for liquid exchange. The upper surface is mounted on a piezoelectric tube. During force measurement, the surface separation is varied continuously by applying a triangular voltage wave to the piezo tube. The lower surface is attached to a bimorph force sensor. It bends under the action of surface forces, which produces a charge in proportion to the bending. This charge is amplified and automatically recorded. After surface contact is reached the linear motion of the upper surface is directly transmitted to the bimorph, and the charge data can be converted into deflection since the piezo displacement is known. The force is then obtained by multiplying the deflection data by the spring constant of the bimorph, which is measured separately after the experiment. The results are presented as force scaled by the radius of interaction R, which is related to the free energy per unit area between flat plates GF(D), according to the Derjaguin approximation.19
F(D)/R ) 2πGF(D) 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. The macroscopic radius of curvature was measured with a micrometer after the experiment. Flame-polished glass surfaces were prepared by melting the end of a borosilicate glass rod in a butane-oxygen flame until a droplet with a radius of ca. 2 mm was formed. After allowing the surfaces to cool for a few seconds, they were immediately mounted in the instrument, and the measuring chamber was sealed in order to avoid airborne contamination. In the later part of the manuscript we will make some comparison between our results obtained with the MASIF technique with earlier results obtained by using the surface force apparatus (SFA). We note that both techniques use macroscopic surfaces 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 (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.
Figure 1. Force scaled by radius as a function of separation between glass surfaces in polyelectrolyte-free 0.1 mM NaCl solution. The solid line represents a force curve calculated using DLVO theory with constant surface charge boundary conditions and the following parameters: apparent surface potential at large separation Ψ0 ) -65 mV, Debye length κ-1 ) 30 nm, and Hamaker constant A ) 0.5 × 10-20 J.35 in any curvature effects on the adsorbed layer structure. Another difference between the MASIF and SFA is that the MASIF measurements are carried out considerably more rapidly. In case the relaxation rate of the adsorbed polymer layer is comparable to the measuring rate, one expects to observe a force that varies with the measuring speed. No such variations were observed over the range of measuring speeds used, except that the adhesion force was found to increase with contact time.
Results and Discussion Forces Measured on Approach. The force curve measured in polymer-free 0.1 mM NaCl solution is presented in Figure 1 together with a DLVO20,21 fit (solid line) using constant surface charge boundary conditions and a surface potential of -65 mV. It can be seen that the interaction follows the DLVO predictions closely down to separations of 3-4 nm. At shorter separations, an additional, non-DLVO repulsion is present. Instead of the expected jump into adhesive contact under the action of van der Waals forces, the interaction remains purely repulsive until surface contact is reached. No adhesion is observed on separation. The presence of a short-range non-DLVO repulsion between various types of glass and silica surfaces in aqueous solutions is well-documented22-24 but lacks definitive explanation at present. Two hypotheses have been proposed. The hydration force hypothesis23 attributes the additional repulsion to the energy needed to remove water molecules from the hydration layer of the surface bound cations and silanol groups. Other authors24 have proposed formation of a gellike layer at the silica-water interface and assumed the repulsion to be of steric origin. A detailed discussion of the problem is beyond the scope of this work. It can only be noted that, regardless of the mechanism, the extra repulsion vanishes upon polyelectrolyte adsorption. Introduction of a 1 ppm PEI solution into the measuring chamber changes the interaction completely. The results are shown in Figure 2. The double-layer repulsion is completely removed after 30 min of incubation, which indicates that PEI adsorption has led to a complete (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.
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Figure 2. Force scaled by radius as a function of separation between glass surfaces in 1 ppm PEI solution after 30 min of incubation. The solid line is the calculated van der Waals interaction.
neutralization of the glass negative surface charge. Instead, the surfaces start to attract each other at a separation of about 10 nm, and this attraction makes the surfaces jump inward from about 5 nm to hard wall contact between the adsorbed polymer layers. The jump occurs when the gradient of the force exceeds the spring constant of the bimorph. It can be seen that the attraction is longer ranged than the expected van der Waals force (solid line). The most likely cause of this is polymer bridging. Bridging is realized when segments of a polymer chain adsorbed to one of the surfaces become attracted (and consecutively attached) to the other surface or alternatively when polymer molecules in the gap between the surfaces are simultaneously attracted to both surfaces.8 It is worth noticing that linear chained polyamines with similar molecular mass13 generate considerably longer bridging force under similar conditions. For instance, in the case of polyvinylamine13 the attraction became detectable already at 25 nm. The difference can be explained by the difference in chain structure between PEI and PVAm. Linear polymers have a larger contour length than highly branched ones at a given molecular weight. Hence, the linear polymer chain can stretch further away from the surface than the branched one, which increases the range of the bridging force. Incubating the surfaces in the 1 ppm PEI solution for prolonged times results in another profound change to the interaction profile. The lowest curve in Figure 3 (open squares) shows the force measured on approach after 24 h of incubation. At large separations, a repulsive doublelayer force is present. Fitting DLVO theory (solid line) to the data resulted in a value of 60 mV for the apparent surface potential at large separations. Evidently, an additional adsorption has taken place during the incubation, and the surfaces acquire a positive charge. In contrast, no charge reversal is observed on glass surfaces incubated for 24 h in a 1 ppm polyvinylamine, PVAm, solution of low ionic strength.13 In fact, even at higher PVAm concentrations the magnitude of the potential remains limited (+28 mV at 10 ppm13), indicating a limited degree of charge overcompensation. A large degree of overcompensation of the surface charge of mica by PEI was also observed by Claesson et al.4 employing surface force and ESCA techniques. The enhanced charge reversal caused by PEI adsorption is attributed to the branched structure of PEI. This can be explained by the fact that branching by necessity leads to that a high number of charges are brought onto a small surface area by each adsorbing polyelectrolyte molecule. Charge overcompensation in general suggests that some nonelectrostatic interactions
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Figure 3. Force scaled by radius as a function of separation between glass surfaces in PEI solutions after 24 h of incubation: open squares, 1 ppm PEI; open circles, 10 ppm PEI. Solid lines are fits by DLVO theory with constant surface charge boundary conditions and the following parameters: Ψ0 ) 60 mV (1 ppm curve) and Ψ0 ) 90 mV (10 ppm curve). The rest of the parameters used are the same as in Figure 1.
between the polymer and the substrate surface is present.25 However, this can hardly explain the large difference between PEI and PVAm, keeping in mind their similar chemical structure. Hence, we suggest that the branching is the main effect behind the large degree of charge reversal observed in this study. At separations below about 10 nm the measured interaction is still less repulsive than the calculated DLVO force, although the jump-in distance is roughly consistent with that expected from the van der Waals attraction (around 2 nm). As in the case of 30 min incubation, it can be argued that an additional attraction is present due to bridging. Apparently, despite the charge overcompensation, there are still free adsorption sites on the glass surface capable of attracting PEI segments and causing bridging. This is consistent with the patchy adsorbed layer of PEI observed by AFM imaging26 and the patchy adsorbed layer structure suggested by for example Gregory.27 The reason that branched polymers are more prone to form patchy layers than linear ones is the large number of charges that are brought down on a small surface area by each polyelectrolyte. We note that charge regulation may also explain the shape of the force curve obtained in 1 ppm PEI, displayed in Figure 3. However, we regard this mechanism as unlikely since the forces at higher PEI concentrations (10 ppm curve, Figure 3) are well described by the constant charge model. We therefore propose that bridging is the cause for the deviation between measured and calculated force profiles. Increasing the bulk polymer concentration to 10 ppm results in a further increase in the magnitude of the doublelayer repulsion. Results are shown in Figure 3, open circles. An apparent surface potential at large separations of 90 mV was extracted from the DLVO fit to the curve. This indicates that an additional amount of PEI is adsorbed to the glass surface. More importantly, the interaction now follows the DLVO theory closely at all separations; i.e., the bridging force was not observed when bringing the surfaces together. This is an expected result considering the 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.
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Figure 4. Force scaled by radius as a function of separation between glass surfaces in 50 ppm PEI solution (open squares). For comparison, the curve recorded in 10 ppm PEI is also shown (filled diamonds).
that the surfaces are now approaching a state where they are fully coated by PEI. In absence of free adsorption sites the polymer segments from one of the layers will not be attracted to the opposing surface, and no bridging force will be present. The interaction measured after introducing a 50 ppm PEI solution into the measuring chamber is shown in Figure 4. For comparison, the result for 10 ppm is also plotted. It can be seen that at large separation the two curves are almost identical. The charge decreases somewhat at the higher concentration as indicated by the slightly lower magnitude of the double-layer repulsion. Further, the curves deviate significantly at separations below 3-4 nm. In the case of 50 ppm PEI no attraction is observed, and the total interaction remains repulsive at all distances. An attraction is however still present on separation (see below). To understand this, we have to consider that PEI is a weak polybase. The measured pH of the 50 ppm PEI solution drained from the MASIF measuring chamber was found to be 9.0 (compared to 6.3 at 10 ppm). At this pH less than 10% of the monomers are charged, compared to 50% at pH 5.5-6.3 Hence, we propose that the purely repulsive force observed on approach across the 50 ppm PEI solution is due to the decrease in PEI charge density, leading to higher adsorbed amounts and thicker adsorbed layers. Support for this hypothesis comes from an earlier study, using mica substrates, which shows that as the pH is increased, the adsorbed amount and layer thickness indeed increase and steric forces become more important.4 The adsorbed amount on glass is expected to be lower than on mica, due to lower surface charge density, whereas an increase in adsorbed amount and layer thickness with pH is expected to occur for both systems. An increase in layer thickness with decreasing charge density has also been theoretically predicted28 as well as experimentally observed for a range of copolymers with variable charge density.29,30 Further, thicker adsorbed layers generate an electrosteric type of repulsion when confined between the approaching surfaces. In our case the range of this repulsion is only 3-4 nm counted from the hard wall contact, indicating that no excessive swelling of the polyelectrolyte layer occurs. In similar experiments, using PEI and mica as a substrate in the interferometric SFA, an increase of the layer thickness by a factor of 1.6(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.
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Figure 5. Force measured on separation scaled by radius as a function of separation between glass surfaces in PEI solutions. The arrows point to the position reached after the outward jumps. Note the saturation of the bimorph signal at 1 ppm (lowest curve). In this case the pull-off force is determined by multiplying the spring constant with the outward jump distance. All measurements were carried out at a constant contact time of 20 s.
Figure 6. Pull-off force scaled by radius vs contact time in 1 ppm PEI solution. The data were collected after 24 h of incubation.
1.7 was found to occur when the polymer concentration was increased from 10 to 50 ppm.4 It should be noted that the charge density of the glass surface itself increases with increasing pH, which promotes further adsorption and formation of a thicker layer. Forces Measured on Separation. It was already mentioned that no adhesion is observed in polyelectrolytefree, salt solutions. However, introducing PEI into the system changes the forces measured on separation dramatically. There is now a negative (pull-off) force required to separate the surfaces from contact (see Figure 5). The magnitude of the pull-off force measured after prolonged incubation was found to dependent on several factors as will be discussed below. The dependence of the pull-off force on the time the surfaces spent in contact is shown in Figure 6. The MASIF instrument allows the surfaces to “rest” in contact for a selected period of time before they are separated. The presented data were collected at a constant driving speed (ca. 20 nm/s) of the piezo in order to minimize any ratedependent effects as discussed by Ruths et al.31 It can be seen that an increase in the contact time leads to a significant increase in the magnitude of the pull-off force. For the range of contact times studied here the pull-off force increased from 35.1 mN/m at 2.8 s to 48.5 mN/m (31) Ruths, M.; Granick, S. Langmuir 1998, 14, 1804.
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Figure 7. Pull-off force scaled by radius as a function of bulk polymer concentration: open circles, PEI MW ≈ 70 000 g/mol; open squares, PVAm MW ≈ 90 000 g/mol; open triangles, PCMA MW ≈ 1 × 106 g/mol. All measurements were carried out at a constant contact time of 20 s. The solid lines are for eye guidance only.
when the surfaces were allowed to remain in contact for 280 s. Longer contact times were impossible to obtain since electrical and thermal drifts develop on the bimorph during the rest period, which affects the reliability of the data. The results in Figure 6 suggest that the adhesion between PEI-coated surfaces is related to polymer chain relaxation phenomena. Apparently, once in contact, direct surface-polymer-surface bridges are formed. The adsorbed chains then relax, following a path that allows maximizing the entropy and the interaction with the surfaces; i.e., more bridges are formed, and the pull-off force increases as a result. Polymer chain interpenetration and entanglement effects, as suggested for similar results using grafted uncharged polymers,32 are likely to play a minor role in this case since (especially at low ionic strength) the polyelectrolyte chains experience strong electrostatic repulsion. The magnitude of the pull-off force was also found to be concentration dependent at constant contact time. Figures 5 and 7 illustrate this. For PEI (open circles in Figure 7) the magnitude of the pull-off force is comparatively high at low polymer concentrations. Above 5 ppm the pull-off force decreases sharply from 40 mN/m to about 15 mN/m at 50 ppm bulk concentration. For comparison, results for two linear polyelectrolytes are also present in Figure 7. Both polyvinylamine (PVAm) and poly[[2-(propionyloxy)ethyl]trimethylammonium chloride] (PCMA) behave similarly. The pull-off force increases at low concentrations and reaches a plateau value above 2 ppm. Comparing these results, two major differences are revealed. First, PEI generates much higher pull-off forces at low concentration. This is attributed to the branched polymer structure that aids the formation of patchy adsorbed layers26,27 and facilitates bridging in contact. This is seen as a major reason for the enhanced adhesion. Second, the pull-off force vs concentration profile for PEI is completely different compared with the other two polymers as shown (32) Plunkett, M. A.; Rutland, M. W. J. Adhes. Sci. Technol., in press.
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in Figure 7. This is due to the reduction of the PEI charge density as a result of the increased pH of the solution. This leads to weaker electrostatic interactions between the polymer and the surfaces, thicker adsorbed layers, and lower pull-off forces. Similar effects, i.e., thicker layers and lower adhesion forces with decreasing charge density of the adsorbing polyelectrolyte, are observed for polyelectrolytes with permanent charges (e.g., of the AMMAPTAC series).33,34 We note that the adhesion between polyelectrolyte-coated surfaces can also be reduced by adding an inorganic electrolyte.8 In the latter case, the increased ionic strength leads to screening of the electrostatic polymer-surface interactions and swelling of the adsorbed layers. These effects can be rationalized if considering that bridging forces control the adhesion between polyelectrolyte-coated surfaces. Hence, any effect that counteracts formation of polyelectrolyte bridges, e.g., lower charge density of the polyelectrolyte, lower surface charge density, higher ionic strength, and thicker adsorbed layer, will reduce the measured adhesion force. Conclusions In the present study we investigated the effect of branched cationic polyethylenimine on the interaction forces between negatively charged glass surfaces. The specific chemical nature and architecture of the PEI have a strong effect on the surfaces forces as revealed by comparison between the results reported here and those obtained with linear polyelectrolytes previously studied. The main differences are the following: (i) The range of the bridging attraction between PEIcoated glass surfaces is of shorter range as compared to the one generated by linear polyelectrolytes with similar molecular mass adsorbed to the same type of surface. This is due to the more compact, branched structure of the PEI molecule. (ii) PEI causes a pronounced charge reversal (+60 mV) already at a bulk concentration of 1 ppm. At higher concentrations the potential increases further to +90 mV. Such a large degree of overcompensation is not observed for linear polyelectrolytes under similar conditions. Thus, we attribute the major charge reversal observed for PEI to its branched structure. (iii) The pull-off force between PEI-coated glass surfaces is dependent on the contact time, indicating slow rearrangements of the polymer layer. No similar effects are observed for the linear polyelectrolytes. (iv) The pull-off force decreases with increasing PEI concentration due to an increase in bulk solution pH that reduces the charge density of the polyelectrolyte. Acknowledgment. E.P. acknowledges financial support from Bo Rydin’s foundation for scientific research. Discussions with Mark Rutland were highly appreciated. LA0112918 (33) Dahlgren, M. A. G.; Claesson, P. M. Nordic Pulp Pap. Res. 1993, 8, 62. (34) Claesson, P. M.; Dedinaite, A.; Poptoshev, E. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Marcel Dekker: New York, 2001; Surfactant Science Series Vol. 99, p 447. (35) Bergstro¨m, L. Adv. Colloid Interface Sci. 1997, 70, 125.
