In Situ Morphology of Cationic Flocculants Adsorbed on Surfaces and

Tetsuya Arita,† Yoichi Kanda,† Hidenori Hamabe,‡ Tomoe Ueno,‡. Yasuhiko Watanabe,‡ and Ko Higashitani*,†. Department of Chemical Engineeri...
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In Situ Morphology of Cationic Flocculants Adsorbed on Surfaces and Their Interaction Forces Investigated by Atomic Force Microscopy Tetsuya Arita,† Yoichi Kanda,† Hidenori Hamabe,‡ Tomoe Ueno,‡ Yasuhiko Watanabe,‡ and Ko Higashitani*,† Department of Chemical Engineering, Kyoto University Yoshida, Sakyo-ku, Kyoto, 606-8501 Japan, and Advanced Technology Group, Research & Development Division, Kurita Water Industries Ltd., Tokyo, Japan Received January 28, 2003. In Final Form: April 21, 2003 Images of typical cationic flocculants of high molecular weight, poly[2-(acryloyloxy)ethyl(trimethyl)ammonium chloride] (PATC), adsorbed on mica surface in situ in water were observed by tapping-mode atomic force microscopy (AFM) under various conditions. The corresponding interaction forces between surfaces were measured using force-mode AFM, in order to know how the microstructure of polymers adsorbed on the surfaces affects the interaction and adhesive forces between surfaces. It was found that it takes a long time for cationic polymers of high molecular weight to relax completely in water, even though the dissolving process is enhanced by mixing. It seems that polymers relaxed in solutions are fundamentally spherical in shape but their structure becomes fragile as the size increases, so that highly swollen high polymers are easily extended by external disturbances to form various structures at their adsorption on surfaces. It was found that these characteristics of polymers adsorbed on surfaces are correlated well with the features of the interaction and adhesive forces between surfaces in the cases of both pure water and NaCl solutions.

1. Introduction Water-soluble cationic polymers of high molecular weight have been used as flocculants of colloidal particles in various fields, such as water and wastewater treatments, because they have the strong ability to bridge between negatively charged particles which are usually observed in nature. Hence large and strong flocs are formed by dosing the polymers into colloidal dispersions, which are easily separated by succeeding separation processes, such as sedimentation, flotation, and filtration. A large number of investigations on the characteristics of flocculants and flocculated particles have been carried out to develop polymer flocculants with high performance, where the effects of the concentration, molecular weight, and charge density of polymers on the flocculation rate were examined using macroscopic methods, such as optical observation, filtration, sedimentation, coagulation rate, and light scattering. As a result, the fundamental mechanisms of flocculation have been classified as follows:1-16 * To whom correspondence should be addressed. E-mail: higa@ cheme.kyoto-u.ac.jp. † Kyoto University. ‡ Kurita Water Industries Ltd. (1) Michaels, A. S. Ind. Eng. Chem. 1954, 46, 1485. (2) Black, A. P.; Birkner, F. B.; Morgan, J. J. J.sAm. Water Works Assoc. 1965, 57, 1547. (3) Marbire, F.; Audebert, R.; Quivoron, C. J. Colloid Interface Sci. 1984, 97, 120. (4) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448. (5) La Mer, V. K.; Healy, T. W. Rev. Pure Appl. Chem. 1963, 13, 112. (6) Watanabe, Y.; Kubo, K.; Sato, S. Langmuir 1999, 15, 4157. (7) Eriksson, L.; Alm, B. Water Sci. Technol. 1993, 28, 203. (8) Williams, H. R.; Fletcher, D. S.; Harris, E. E.; Lin, T. Y. Biochim. Biophys. Acta 1983, 757, 69. (9) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (10) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422. (11) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (12) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893.

(1) bridging flocculation, where tails or loops of a few polymers with high affinity to the particle surface make bridges between particle surfaces (2) flocculation by electrostatic-patch attraction, where the excessive charge of adsorbed polymers electrostatically attracts the bare areas of other particles with opposite charge (3) flocculation by charge neutralization, where the charge of the particle surface is neutralized by adsorbed polymers so that the particles attract each other by van der Waals force These mechanisms have been proposed, imagining the morphology of polymers adsorbed on surfaces without knowing their real molecular conformations on surfaces. Hence in situ and molecular-scale information on the morphology of adsorbed polymers is dearly needed to understand the detailed mechanism of flocculation of particles. Atomic force microscopy (AFM) has been employed not only to observe the molecular-scale roughness of surfaces, but also to measure the interactions between surfaces in situ in solutions. Interaction forces in polymer solutions have also been investigated extensively with AFM,17-23 and some mechanisms mentioned above were investigated.24-31 However, the correlation between the mor(13) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (14) Balastre, M.; Persello, J.; Foissy, A.; Argillier, J. F. J. Colloid Interface Sci. 1999, 219, 155. (15) Mabire, F.; Audebert, R.; Quivoron, C. J. Colloid Interface Sci. 1984, 97, 120. (16) Akesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91, 2461. (17) Akari, S.; Schrepp, W.; Horn, D. Langmuir 1996, 12, 857. (18) Pfau, A.; Schrepp, W.; Horn, D. Langmuir 1999, 15, 3219. (19) Regenbrecht, M.; Akari, S.; Fo¨rster, S.; Mo¨hwald, H. Surf. Interface Anal. 1999, 27, 418. (20) Zhu, M.; Schneider, M.; Papastavrou, G.; Akari, S.; Mo¨hwald, H. Langmuir 2001, 17, 6471.

10.1021/la034149a CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003

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phology of cationic polymers adsorbed on surfaces and the corresponding interaction forces between the surfaces has not been not clarified yet. In the present work, we carried out the observation of a cationic high polymer adsorbed on mica surface in situ in water, using tapping-mode AFM, to know the relaxation process of polymers in solution. The corresponding interaction and adhesive forces between surfaces are measured using force-mode AFM at the same time, and then the relations between the morphology of adsorbed polymers and the interaction and adhesive forces are investigated. 2. Materials and Experimental Methods 2.1. Materials. Typical cationic flocculants poly[2-(acryloyloxy)ethyl(trimethyl)ammonium chloride] (PATC) of molecular weight Mw ) 1.25 × 107 g/mol were used in most cases, but polymers of Mw ) 2.7 × 106 and 7.8 × 106 g/mol were employed when effects of molecular weight were examined. These polymers are highly charged because the functional group exists in each repeating unit. Pure water and NaCl solutions were used as the solvents. Pure water was prepared by purifying further distilled and ionexchanged water with a Milli-Q apparatus (Millipore). A stock solution of polymer concentration Cpoly ) 200 ppm was prepared by dissolving PATC in water and mixing for 30 min by a magnetic stirrer with a rotational speed of ca. 200 rpm in the beaker. The beaker was covered with aluminum foil to avoid damage of polymers from attack by ultraviolet rays.32,33 Then the solution was settled without further mixing in an incubator of 25.0 ( 0.1 °C, except when the effect of mixing was examined. This stock period, during which polymers disentangle and relax by their thermal motion, is defined as the settling time tset. At a given settling time, an aliquot of the solution was sampled and diluted in water to prepare a solution of Cpoly ) 1 ppm, except when the effect of polymer concentration was examined. After mixing for 1 min, the solution was used as the test solution. When effects of electrolyte were examined, the same procedure was repeated to prepare both the stock and test solutions of a given NaCl concentration. Muscovite mica plates, which were freshly cleaved prior to use, were used as the adsorbates to observe polymers adsorbed on them. Mica plates were carefully chosen by preliminary measurement with AFM, such that the surface roughness was less than 0.1 nm. When interaction forces between surfaces were measured, the mica plate was used as a flat surface and the surface of a silica particle of 20 µm in diameter was used as the other surface. 2.2. Measurements. Two kinds of measurements were carried out using a Digital Instruments multimode AFM connected to a Nanoscope III controller: (1) the observation of morphology of PATC molecules adsorbed on the mica surface in situ in water and (2) measurements of the corresponding interaction forces between silica and mica surfaces with adsorbed polymers. (21) Shu, L.; Schlu¨ter, A. D.; Ecker, C.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2001, 24, 4666. (22) Ebihara, K.; Koshihara, S.; Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Fujiki, M. Jpn. J. Appl. Phys. 1997, 36, L1211. (23) Stipp, S. L. S. Langmuir 1996, 12, 1884. (24) Biggs, S.; Proud, A. D. Langmuir 1977, 13, 7202. (25) Kelley, T. W.; Schorr, P. A.; Jounson, K. D.; Tirrell, M.; Frisbie, C. D. Macromolecules 1998, 31, 4297. (26) Kuhl, T.; Guo, Y.; Alderfer, J. L.; Berman, A. D.; Leckband, D.; Israelachvili, J. N.; Hui, S. W. Langmuir 1996, 12, 3003. (27) Israelachvili, J. N.; Tandon, R. K.; White, L. R. J. Colloid Interface Sci. 1980, 78, 431. (28) Dahlgren, M. A G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 66, 343. (29) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1984, 80, 865. (30) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1990, 86, 1363. (31) Courvoisier, A.; Isel, F.; Francois, J.; Maaloum, M. Langmuir 1998, 14, 3727. (32) Cichetti, O. Adv. Polym. Sci. 1970, 7, 70. (33) Tsuji, K. Adv. Polym. Sci. 1973, 12, 131.

Arita et al. For the in situ observation of adsorbed PATC molecules, the following procedure was carried out. The test solution mentioned above was poured into an AFM liquid cell with the adsorbate. After the solution was settled for 1 min to allow the polymer to adsorb on the mica surface, the bulk solution was replaced by the solvent, injecting it slowly into the cell several times, such that only polymers adsorbed on the adsorbate remained within the cell. Then the cantilever was mounted on the liquid cell and the cell was filled with the solvent again. This procedure prevents the adsorption of polymers on the cantilever tip. Two types of cantilevers made of Si were used: cantilever I is a popular cantilever, AC240 (Olympus), where the spring constant is 2 N/m, the resonance frequency is 70 kHz, and the nominal tip radius is presumably less than 10 nm; cantilever II is a highprecision cantilever, SI-DF20S (NANOSENSORS), where the spring constant is 11 N/m, the resonance frequency is 121 kHz, and the tip radius is guaranteed to be 2-5 nm with 80% confidence. Cantilever I was used in most cases, but cantilever II was employed only when clear images were needed. After the liquid cell was set up completely, the adsorbate surface was scanned using tapping-mode AFM to obtain the AFM image of adsorbed polymers. All experiments were conducted in an airconditioned room at 25 ( 3 °C. For the measurements of interaction forces, the following procedure was carried out. A colloidal probe was provided by gluing a silica particle on cantilever I with epoxy resin, as described elsewhere.28 The AFM liquid cell with the colloid probe and a mica plate with adsorbed polymers was prepared, following the same procedure described above, and then the interaction and adhesive forces were measured, using the popular method of force measurement given elsewhere.34

3. Results and Discussion 3.1. Relaxing Process of PATC Molecules in Water and Their Conformation. When a small amount of highly charged cationic polymers, such as PATC, are dissolved in water, we expect that they are soon swollen and disentangled by thermal motion and repulsive force due to the high charge of their chains. This relaxation process of PATC molecules of Mw ) 1.25 × 107 g/mol with settling time tset was investigated by observing the height image of PATC molecules adsorbed on mica by the method described above. Figure 1 shows a series of height images changing with time. Here we must pay attention to the fact that the scales of height and width are not necessarily the same among the images, and that images of adsorbed polymers do not represent their precise conformation in solution because they are the projections of polymers in solution into a two-dimensional surface. Nevertheless, they allow us to imagine their three-dimensional conformation in the solution. At tset ) 0 days, a few lump structures of hemispherical shape, whose width and height range from 300 to 450 nm and from 20 to 25 nm, respectively, were observed. This implies that coiled polymers are swollen almost spherically with water at the dosage, and then adsorb etectrostatically on the surface, deforming their structure into hemispherical shape. According to a simple random walk calculation,35 the size of a polymer in the solution is about 200 nm in diameter, so that we consider that each lump image represents an aggregate composed of entangled polymers. The lump size decreases gradually with tset until 7 days, and then fibril-flock structures appear at tset ) 10 days. The height of fibrils is found to be 0.4 nm, which coincides approximately with the height calculated from the polymer structure. The clearer images of extended polymers were obtained at tset ) 20 days, using cantilever II. The height (34) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (35) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: New York, 1979.

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Figure 2. Variation of average lump height of PATC molecules adsorbed on mica with time for stock solutions settling and mixing in the vessel, respectively (Mw ) 1.25 × 107 g/mol).

Figure 1. Variation of height images of PATC molecules on mica in situ in water with settling time (Mw ) 1.25 × 107 g/mol).

of the linearly extended polymer was also found to be 0.3-0.4 nm. This indicates that the fibril-like structure represents a single polymer chain. However, the width of the fibril chain is estimated to be ca. 4.0 nm, which is much greater than the predicted one, even though the contribution of the curvature of the cantilever tip is taken into account following the procedure proposed elsewhere.36 It is plausible to assume that various small loops of polymer chains are twisted and adsorb on the surface, either electrostatically or by the tapping force of the cantilever, to make crooks and clusters of the fibril.29,30,37 We consider that the width of the fibril chain is widened by these crooks. Apparently, however, more detailed analysis is required in order to know whether the above speculation is adequate or not. The above-mentioned relaxation process with time was shown in Figure 2, regarding the average lump height of adsorbed polymers as the measure. It is clear that the height decreases exponentially with tset, and it becomes less than 1 nm at tset g 10 days, where polymers are extended nearly as single polymers. It is important to note that it takes a surprisingly long time for high polymers to relax completely in water, even though they are highly charged. The effect of mixing of the stock solution was also examined for the sake of comparison, as shown in Figure 2. It is clear that the relaxation process is greatly enhanced by mixing, but it still takes about 2 days until polymers are relaxed sufficiently. Here the effect of molecular weight of PATC was examined. Parts a and b of Figure 3 show the height images of fully relaxed polymers of Mw ) 7.8 × 106 and 2.7 × 106 (36) Marczewski, A. W.; Higashitani, K. Comput. Chem. 1997, 21, 129. (37) Buja´n-Nu´_ez, n M. C.; Dickinson, E. J. Chem. Soc., Faraday Trans. 1996, 92, 2275.

Figure 3. Dependence of morphology of fully relaxed PATC molecules adsorbed on the surface on molecular weight.

g/mol, respectively. In the case of Mw ) 7.8 × 106 g/mol, highly extended molecules were observed at tset ) 10 days, as in the case of Mw ) 1.25 × 107 g/mol. On the other hand, most polymers were observed as nearly spherical in the case of Mw ) 2.7 × 106 g/mol even at tset ) 20 days. It is plausible to assume that a freely jointed polymer forms a spherical structure by the thermal motion of the chains, if it is in a stationary solution for a sufficiently long time, but this structure will become large but fragile as the charge density and molecular weight increase. Hence we consider that the reason spherical structures of polymers were observed for Mw ) 2.7 × 106 g/mol but not for Mw ) 1.25 × 107 and 7.8 × 106 g/mol is because the spherical structure for polymers of low molecular weight is strong enough against the surrounding disturbances, while those for higher polymers are too fragile to keep their spherical shape against disturbances. 3.2. Effect of Salt Concentration on Polymer Conformation. Here the effects of a typical salt, NaCl, on the morphology of adsorbed polymers were examined. Figure 4a shows their AFM images in a 1 × 10-4 M NaCl solution at tset ) 10 days. It is clear that the features of adsorbed polymers are not too different from those in pure water shown in Figure 1e. This indicates that this

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Figure 4. Typical height images of adsorbed PATC molecules in aqueous solutions of 1 × 10-4 and 0.1 M NaCl (Mw ) 1.25 × 107 g/mol). Figure 6. Comparison of features of force curves between tset ) 0 and 10 days. (Mw ) 1.25 × 107 g/mol).

Figure 5. Force curves between a mica plate and a silica sphere with polymers adsorbed in polymer solutions of 1 and 10 ppm (Mw ) 1.25 × 107 g/mol). The force curve in pure water is shown for the sake of comparison.

concentration of NaCl is too low to affect the structure of polymers in the solution. Parts b and c of Figure 4 indicate the images of adsorbed polymers in a 0.1 M NaCl solution at tset ) 0 and 10 days, respectively. It is interesting to note that hemispherical domains in Figure 4b are a little smaller than those in Figure 1a, where the width and height vary from 200 to 320 nm and from 15 to 20 nm, respectively. It is known that the charge of polymers in an electrolyte solution is more or less set by the counterions, so polymers tend to behave more freely and the size is reduced. This sealing effect is clearly confirmed when the morphology of coiled polymers in Figure 4c is compared with that of extended polymers in Figure 1e. It is also important to note that polymers in Figure 4c look spherical, of about 1 µm in diameter, but their height is ca. 0.8 nm. This implies that the polymers are nearly spherical in solution but extremely fragile. 3.3. Interaction Forces between Surfaces with Adsorbed PATC. It is very important to know how the morphology of adsorbed polymers illustrated in Figure 1 is related to the interaction and adhesive forces between surfaces, because the correlation is vital when polymers are used as flocculants. Figure 5 shows typical force curves between a mica plate and a silica particle in two polymer concentrations and that for pure water for the sake of comparison. At Cpoly ) 10 ppm, it appears that the surfaces are nearly neutralized by adsorbed polymers; the force curve is almost constant

and the adhesive force is small, although there exist a small repulsive force at separation h > 40 nm, a small attractive force at 10 < h < 20 nm, and a repulsive force again at h < 10 nm. The repulsive force at h > 40 nm coincides with that of pure water, so this force is the electrostatic repulsive force generated by the negative charge at the bare site of the surfaces. The attractive force at 10 < h < 20 nm will be caused by the electrostatic attraction between adsorbed polymers and the bare sites of the other surface. The repulsive force at h < 10 nm is considered to be attributable to the structural force due to the adsorbed polymers. For Cpoly ) 1 ppm, however, it is clear that there exists not only a big jump in the approaching force curve but also a strong adhesive force. These are fundamentally important features when polymers are used as flocculants. Hence the detailed characteristics of the interaction forces for Cpoly ) 1 ppm are discussed hereafter, comparing with conformations of adsorbed polymers shown in Figure 1. Figure 6a and Figure 6b show the typical force curves for tset ) 0 and 10 days, which correspond to the conformations of polymers observed in Figure 1a and Figure 1e, respectively. It is clear that the approaching force curve in Figure 6a jumps at h ∼ 40 nm and touches down at h ∼ 9 nm, and then a strong repulsive force acts between the surfaces. We consider that the jump at h ∼ 40 nm is caused by the electrostatic attraction between the excessive positive charge of polymer lumps and the bare site of the other surface, as easily estimated from Figure 1a. The repulsion at h < 9 nm is probably caused by the structural force due to adsorbed polymer lumps on both surfaces. It is important to know that there exists a hysteresis between approaching and separating force curves, which suggests that the strong bridging between surfaces by polymers is formed during their contact. In the case of tset ) 10 days, on the other hand, the approaching force curve is almost flat and touches down at h ∼ 0.5 nm after the jump at h ∼ 5 nm, as shown in the insert of Figure 6b. This value of h ∼ 0.5 nm coincides well with the height of adsorbed polymers in Figure 1e. It is especially important to note that the attraction is in

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Figure 8. Schematic drawing for the correlation among the morphology of polymers, their charges and interaction forces during the approaching and separating processes.

Figure 7. Distributions of the characteristic values of force curves defined in Figure 6: hj, ht, had, and Fad at tset ) 0 and 10 days (Mw ) 1.25 × 107 g/mol).

a small range in the approaching force curve, but the adhesion is very strong once two surfaces contact each other. As shown in Figure 1, polymers adsorb almost randomly on the surfaces at Cpoly ) 1 ppm, so that the strength of the interaction force depends on the contact point of two surfaces. Hence we repeated the same measurements at about 60 different points, using six pairs of test surfaces. Distributions of the jumping distance hj, the touch-down distance ht, the breakage distance of the surface contact had, and the adhesive force Fad, illustrated in Figure 6a, are shown in Figure 7. In the case of tset ) 0 days shown in Figure 7a, it is clear that the value of hj is widely distributed from 5 to 50 nm, as expected from the morphology of adsorbed polymers shown in Figure 1a. This large jumping distance cannot be explained by the direct bridging by polymers between two surfaces, because the lump height given in Figure 3 is smaller than the average jumping distance. This implies that the excessive positive charge of lumps attracts electrostatically the negatively charged bare site of the other surface, as illustrated in Figure 8a-1. This mechanism was known as the electrostatic-patch attraction. As for the magnitude of ht, the distribution peak appears at h ∼ 10 nm. This separation distance must be somehow correlated with the average lump height given in Figure 2. We consider that lumps are soft aggregates of swollen polymers, so lumps will be deformed and the value of ht becomes considerably smaller than the lump height when the colloid probe touches down strongly on the lumps, as illustrated in Figure 8a-2. Figure 7a-3 and Figure 7a-4 show the distributions of had and Fad, respectively. These

Figure 9. Rare force curve which does not have any jump in the approaching force curve but the attractive force even at a long separation distance in the separating force curve (tset ) 0 days, Mw ) 1.25 × 107 g/mol).

data indicate that the bridge between surfaces is elongated before the breakup and this adhesive force is much greater than that in pure water, ca. 2 mN/m given in Figure 5. Figure 7b shows the distribution of hj, ht, had, and Fad for tset ) 10 days. It is clear that the values of hj, ht, and had are much smaller than those for tset ) 0 days, as expected from the morphology of polymers shown in Figure 1. Despite these differences between tset ) 0 and 10 days, it is interesting to know that the magnitudes of Fad coincide with each other approximately. We consider this is because the charge density of adsorbed polymers per unit area is more or less the same whether they are adsorbed as lumps or as extended chains. Hence almost all cations of polymers are consumed to bridge between surfaces, when the polymer structure is crushed by a strong pushing force, as illustrated in Figure 8a-3 and Figure 8b-3; that is, nearly the same number of cations of polymers contribute to bridge between surfaces in both cases. Finally, we discuss the very rare case in which the jump is not found in the approaching force curve but the force is always repulsive until two surfaces contact, and the attractive force still exists even though two surfaces are separated enough on retraction, as shown in Figure 9. We consider that the repulsive force on approach is caused by the fact that lumps with excessive positive charge are facing each other at a complex configuration, which results in the complicated force curve with inflection points. The attractive force at a long separation will be attributable

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Figure 10. Comparison of features of force curves between tset ) 0 and 10 days in 0.1 M NaCl solution (Mw ) 1.25 × 107 g/mol).

to the entanglements of adsorbed polymers which were formed by the crush of two lumps on two surfaces at the contact. 3.4. Effect of Electrolyte on Interaction Forces. The force curves in a solution of 0.1 M NaCl, corresponding to the conformation of adsorbed polymers given in Figure 4b and Figure 4c, are obtained in Figure 10a and Figure 10b, respectively. Features of the force curve for tset ) 0 days are more or less similar to those in Figure 6a, although all the values of hj, ht, had, and Fad are much smaller, such as hj ∼ 15 nm as shown in the insert. As for the force curves for tset ) 10 days shown in Figure 10b, there exists no attractive force but the repulsive force appears at a larger separation than that in Figure 6b as shown in the insert. This corresponds well with the morphology given in Figure 4c. The adhesive force is extremely small in this case. The distributions of these values of hj, ht, had, and Fad are shown in Figure 11. Because finite values of hj, ht, had, and Fad exist for tset ) 0 days, we presume that the excessive charge within lumps is not neutralized completely within this settling time; that is, the outside of polymer lumps may be neutralized by adsorbed Cl-, but their core is not. Hence, when the lumps are crushed by the pushing force, the nonneutralized charge makes the bridge between surfaces and the bridging force appears on retraction. As for the distributions for tset ) 10 days, all the values of hj, ht, had, and Fad are very small. This is probably because polymers are swollen enough and neutralized by NaCl solution completely during this long settling time, as expected by Figure 4c. Hence there exists a big difference in the adhesive force between tset ) 0 and 10 days. This big difference is interestingly compared with the case that almost no difference exists in salt-free solutions as shown in Figure 7. 3.5. Characteristics of PATC as Flocculants. The above-mentioned features of PATC are extremely important when PATC is used as flocculants in water and wastewater treatments, because we often assume that the cationic polymers dissolved in a dilute solution adsorb

Figure 11. Distributions of the characteristic values of force curves defined in Figure 10: hj, ht, had, and Fad at tset ) 0 and 10 days in 0.1 M NaCl solution (Mw ) 1.25 × 107 g/mol).

immediately on surfaces with their loops and tails. However, the above results indicate that it takes an extremely long time for high polymers to relax completely even though the mixing procedure is introduced. Another important feature is that the surfaces with adsorbed polymers attract each other from a longer separation distance when polymers adsorb on the surface as big lumps, rather than when they adsorb as extended polymers. This indicates that aggregated polymers work as better flocculants than extended polymers; that is, the ability of polymers as flocculants decreases with the relaxation process of polymers. These features are especially important when flocculants are used in a concentrated electrolyte solution, because the adhesive force becomes extremely small for extended flocculants, as shown in Figure 11b-4. 4. Conclusion We observed images of cationic flocculants adsorbed on mica surface in situ in water by tapping-mode AFM under various conditions, and measured the corresponding interaction forces between surfaces using force-mode AFM, in order to know how the microstructure of polymers adsorbed on surfaces affects their interaction and adhesive forces between the surfaces. It is found that it takes a long time for cationic polymers of high molecular weight to relax completely in water, even though the dissolving process is enhanced by mixing. It seems that polymers relaxed in solutions are fundamentally spherical in shape but their structure becomes fragile as the size increases, so highly swollen polymers, such as cationic polymers of Mw ) 1.25 × 107 g/mol, are

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easily extended by external disturbances to form lumplike, flock-like, and then fibril-like structures at their adsorption on the surfaces. These characteristics of polymers adsorbed on the surfaces are found to have good correlation with the features of the interaction and adhesive forces between surfaces. When polymers in pure water are adsorbed as lumps, the long-range electrostatic attractive force acts between the polymer lump and the bare surface. When the polymers adsorb as extended polymers, the shortrange attraction acts between the surfaces, but the magnitude of adhesion does not depend on the morphology of adsorbed polymers, once the two surfaces contact sufficiently.

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In the case of electrolyte solutions, the characteristics of interactions are more or less similar to those for pure water, although the magnitude of interactions is reduced considerably, depending on the electrolyte concentration. However, it seems that the excessive charge within the lump core is not necessarily neutralized by the counterions of the electrolyte, but most charge is neutralized in the case of extended polymers. This results in the fact that the adhesive force is of finite magnitude in the case of lump polymers but is negligibly small in the case of extended polymers. LA034149A