pubs.acs.org/Langmuir © 2010 American Chemical Society
Surface-Induced Rearrangement of Polyelectrolyte Complexes: Influence of Complex Composition on Adsorbed Layer Properties ‡ € Sedat Ondaral,† Caroline Ankerfors,‡,§ Lars Odberg, and Lars Wa˚gberg*,‡ †
Department of Pulp and Paper Technology, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey, ‡Department of Fibre and Polymer Technology, Royal Institute of Technology, 100 44 Stockholm, Sweden, and §Eka Chemicals AB, 445 80 Bohus, Sweden Received June 1, 2010. Revised Manuscript Received July 22, 2010 The adsorption characteristics of two different types of polyelectrolyte complexes (PECs), prepared by mixing poly(allylamine hydrochloride) and poly(acrylic acid) in a confined impinging jet (CIJ) mixer, have been investigated with the aid of stagnation point adsorption reflectometry (SPAR), a quartz crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM) using SiO2 surfaces. The two sets of PEC were prepared by combining high molecular mass PAH/ PAA (PEC-A) and low molecular mass PAH/PAA (PEC-B). The PEC-A showed a higher adsorption to the SiO2 surfaces than the PEC-B. The adsorption of the PEC-A also showed a larger change in the dissipation (ΔD), according to the QCM-D measurements, suggesting that the adsorbed layer of these complexes had a relatively lower viscosity and a lower shear modulus. Complementary investigations of the adsorbed layer using AFM imaging showed that the adsorbed layer of PEC-A was significantly different from that of PEC-B and that the changes in properties with adsorption time were very different for the two types of PECs. The PEC-A complexes showed a coalescence into larger block of complexes on the SiO2 surface, but this was not detected with the PEC-B. The size determinations of the complexes in solution showed that they were very stable over time, and it was therefore concluded that the coalescence of the complexes was induced by the interaction between the complexes and the surface. The results also indicated that polyelectrolytes can migrate between the different complexes adsorbed to the surface. The results also give indications that the preparation of PEC-B leads to the formation of two different types of polyelectrolyte complexes differing in the amount of polymer in the complexes; i.e., two populations of complexes were formed with similar sizes but with totally different adsorption structures at the solid-liquid interface.
Introduction By mixing oppositely charged polyelectrolytes in water, polyelectrolyte complexes can be formed by a process which is driven by the increase in entropy associated with the release of small ions from the ionic atmosphere surrounding the individual polyelectrolytes. More specific interactions, such as hydrogen bonding, also contribute to the interaction once the polyelectrolytes are in contact, but the main driving force for complex formation is the entropy gain following the release of counterions. The complexation most frequently results in three different types of complexes: soluble, colloidally stable, and coacervate complexes, depending on factors such as mixing ratio, polyelectrolyte concentration, polyelectrolyte charge density, molecular mass, pH, electrolyte concentration, the nature of ionic groups, the temperature, and the preparation conditions.1-6 The interest in and the use of polyelectrolyte complexes in large-scale industrial applications, such as coatings, binders, and flocculants, as well as in biotechnical and biomedical applications, *Corresponding author. (1) Kabanov, V. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003; p 47. (2) Tsuchida, E.; Abe, K. In Advances in Polymer Science; Cantow, H.-J., Dall'Asta, G., Dusek, K., et al., Eds.; Springer-Verlag: Berlin, 1982; Vol. 45, p 1. (3) Philipp, B.; Dautzenberg, H.; Linow, K.-J.; K€otz, J.; Dawydoff, W. Adv. Polym. Sci. 1989, 14, 91. (4) Biesheuvel, P. M.; Cohen Stuart, M. A. Langmuir 2004, 20, 2785–2791. (5) Dautzenberg, H.; Kriz, J. Langmuir 2003, 19, 5204–5211. (6) Kovacevic, D.; Borkovic, S.; Pozar, J. Colloids Surf., A 2007, 302, 107–112. (7) Hubbe, M. A.; Moore, S. M.; Lee, S. Y. Ind. Eng. Chem. Res. 2005, 44, 3068– 3074. € Yalc-ın, D.; Batıg€un, A.; Bayraktar, O. J. Therm. Anal. Calorim. (8) Malay, O.; 2008, 94, 749. (9) Fredheim, G. E. Biomacromolecules 2003, 4, 232–239. (10) Wan, A. C. A.; Tai, B. C. U.; Schumacher, K. M.; Schumacher, A.; Chin, S. Y.; Ying, J. Y. Langmuir 2008, 24, 2611–2617.
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has grown rapidly during recent decades.7-12 For these applications, it is very important to control the properties of the PEC in solution as well as the adsorption and conformational behavior of the PEC at the solid-liquid interface. Nevertheless, there are, to the knowledge of the authors, only a limited number of studies concerning the adsorption and conformational behavior of PECs at solid-liquid interfaces. A serie of studies, relevant to the present study, were conducted by Reihs and co-workers,13-15 who showed that the adsorption of the coacervate fraction of PECs, which was unprecipitated milky colored fraction obtained after centrifugation of a PEC solution of poly(diallyldimethylammonium chloride) (PDADMAC) and sodium poly(maleic acid-co-R-methylstyrene) (PMA-MS), onto a negatively charged surface modified with the polyelectrolyte multilayer technique, resulted in a higher surface coverage (≈26%) than with uncentrifuged PEC (≈10%). It was also found that the adsorbed coacervate particles had a homogeneous size distribution, whereas the uncentrifuged PEC particles aggregated with time. The coacervate particles had the same bulk properties, but their adsorption decreased with time13 as a result of aging of complex. Furthermore, the PEC particles prepared by mixing PDADMAC and poly(maleic acid-co-propylene) (PMA-P) adsorbed with a flattened, curved shape, whereas PEC particles prepared with PDADMAC and PMA-MS adsorbed as spherical caps.14 It was also found that the PEC particles of poly(L-lysine) (11) Silva, C. L.; Pereira, J. C.; Ramalho, A.; Pais, A. A. C. C.; Sousa, J. J. S. J. Membr. Sci. 2008, 320, 268–279. (12) Mazumder, M. A. J.; Shen, F.; Burke, N.A. D.; Potter, M. A.; St€over, H. D. H. Biomacromolecules 2008, 9, 2292–2300. (13) Reihs, T.; M€uller, M.; Lunkwitz, K. J. Colloid Interface Sci. 2004, 271, 69–79. (14) Reihs, T.; M€uller, M.; Lunkwitz, K. Colloids Surf., A 2003, 212, 79–95. (15) M€uller, M.; Reihs, T.; Ouyanf, W. Langmuir 2005, 21, 465–469.
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Table 1. Properties of PECs in 10 mM NaCl at pH 7.0 and at a Charge Ratio of 0.8a PEC name
Mw of polyelectrolytes size-std zeta potential-std (PAH/PAA) dev (nm) dev (mV)
PEC-A1 (BF) 70 000/50 000 78-0.4 þ54-2.5 PEC-A1 (AF) 70 000/50 000 82-1.4 þ50-3.4 PEC-A1 (BF-D) 70 000/50 000 88-0.2 þ62-0.9 PEC-A2 (BF) 70 000/50 000 58-0.7 þ46-3.8 PEC-B (BF) 15 000/5000 35-0.6 þ37-4.5 a AF: after filtration with the Millipore ultrafiltration cassettes with 100 and 10 kDa cutoff for PEC-A1 and PEC-B, respectively; BF: before filtration; and BF-D: before filtration-dialyzed.
and PMA-P produced by solution-casting had a needlelike shape whereas those of PLL/PMA-MS had a hemispherical shape,15 showing the effect of the molecular structure of the polyelectrolytes on the PEC conformation at the solid-liquid interface. In a different study, Kekkonen et al. found different types of adsorption kinetics of PECs produced from strong polyelectrolytes depending on the charge ratio and the pH of the adsorption medium.16 In the present study, the adsorption behavior of two different types of PECs, prepared by combining high molecular mass PAH/ PAA (PEC-A) and low molecular mass PAH/PAA (PEC-B) using a confined impinging jet (CIJ) mixer, has been investigated by means of stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D) using SiO2 surfaces. The conformation on the SiO2 surface was studied using atomic force microscopy (AFM). The aim was to establish a link between the solution structure of the complexes and the structure and stability of the adsorbed complexes.
Experimental Section Materials. Poly(allylamine hydrochloride) (PAH) polymers with different molecular weights (70 000 and 15 000 Da) were supplied by Sigma-Aldrich, and poly(acrylic acid) (PAA) polymers with 50 000 and 5000 Da molecular weight were purchased from Polysciences, Inc., and Sigma-Aldrich, respectively. The polymers were used in the PEC preparations without further purification. Colloidal silica (Bindzil15/500) was supplied by Eka Chemicals AB, Bohus, Sweden. Water used in all experiments was deionized water with Millipore quality. NaOH, HCl, and NaCl were all of analytical grade. The silicon wafers (boron-doped, p-type) used in the SPAR experiments were purchased from MEMC Electronic Materials SpA, Novara, Italy. QCM crystals coated with a silicon oxide coating (QSX 303/50 SiO2) were supplied by Q-Sense AB, Gothenburg, Sweden. PEC Preparation. The PECs were prepared in a CIJ mixer17 at 4 mequiv/L of a cationic charge concentration of PAH and 3.2 mequiv/L of negative charge concentration of PAA, resulting in a charge ratio (q-/qþ) of 0.8, in 10 mM NaCl at pH 7.0. The CIJ mixer consists of two opposing 1 mm jets (at an angle of 180° to each other) where the fluid is fed into a 5 mm diameter, cylindrical mixing chamber. The flows of PAH and PAA solution were adjusted to give a 2.1 ms mixing time, calculated according to a procedure outlined by Johnson and Prud’homme.17 The properties of the PECs used in the present study after preparation are given in Table 1. Stagnation Point Adsorption Reflectometry (SPAR). The adsorption of PECs at the solid-liquid interface was studied using a SPAR equipment from the Laboratory of Physical Chemistry and Colloidal Science, Wageningen University, The Netherlands. Silicon wafers were oxidized at 1000 °C after rinsing consecutively with water-ethanol-water and drying with nitrogen. The silica surfaces were hydroxylated by treatment with 10% NaOH for 30 s and rinsed (16) Kekkonen, J.; Lattu, H.; Stenius, P. J. Colloid Interface Sci. 2001, 234, 384. (17) Johnson, B. K.; Prud’homme, R. K. AIChE J. 2003, 49, 2264–2282.
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with water, dried with nitrogen, and then treated in a plasma cleaner for 30 s (PCD 002, Harrick Scientific Corp., Ossinging, NY). The thickness of the oxide layer on the silicon wafers was measured on each surface by means of a Rudolph ellipsometer (model 43702200E from Rudolph Research, Flanders, NJ) and was usually found to be ∼90 nm. In the calculation of the adsorbed amount, the exact layer thickness on each single wafer was used. Detailed information about the SPAR technique has been given by Dijt et al.18 Briefly, a beam of linearly polarized light is focused on the stagnation point of flow over the silica surface and reflected toward a detector in which parallel and perpendicular polarized components of light, Ip and Is, are separated by a beam splitter, and the intensity of each component is recorded as a voltage by two photodiodes. The Ip/Is ratio (S) is proportional to the reflectivity ratio of the surface and therefore changes with the adsorbed amount of solid polymer. The adsorbed amount (Γ) can be calculated as follows: Γ ¼ As- 1
ΔS S0
ð1Þ
where ΔS is the change in S, S0 is the reflectometer reading of the surface before adsorption, and As-1 is a sensitivity factor, proportional to the refractivity index increment dn/dc of PEC, calculated as 0.232 mL/g.19 The sensitivity factors of the PECs used in the experiments were determined by means of “Prof. Huygens” software (Dullware, The Netherlands).
Quartz Crystal Microbalance with Dissipation (QCM-D). The adsorbed amounts and viscoelastic properties of the adsorbed PECs layers on the bare SiO2 surface and the adsorption of colloidal silica on the SiO2 surface pretreated with PECs were determined using the QCM-D instruments (D300 and E4 models) from Q-Sense AB, Gothenburg, Sweden. With this technique, the resonant frequency, f, of the crystal decreases as the adsorbed mass, Δm, is increased. If the adsorbed mass is evenly distributed, rigidly attached, and small compared to the mass of the crystal, then f - f0 = Δf, where f0 is the resonant frequency of the bare crystal in the medium, is related to the adsorbed mass per unit surface, Δm, calculated via the Sauerbrey equation:20 Δm ¼
CjΔf j n
ð2Þ
where n is the overtone number and C is a constant that describes the sensitivity of the device to a change in mass. For the crystals used in the present work, C = 0.177 mg m-2 Hz-1 and n = 1, 3, 5, or 7. When the power to the crystal is switched off, there is a decay in the oscillation due to frictional losses in the crystal and in the adsorbed layer containing both polymer and bound solvent. The energy dissipation is characterized by a dissipation factor D, which is related to the decay time constant τ according to21 D ¼
1 πf τ
ð3Þ
where f is the resonance frequency. The QCM-D instrument measures the change in the dissipation factor ΔD = D - D0 during the adsorption process, where D0 is the dissipation factor of the pure quartz crystal immersed in the solvent and D is the dissipation factor when the polymer complexes are adsorbed. Generally a high dissipation means that the adorbed layers have a low viscosity and a low elastic modulus. Before using the QCM crystal, it was cleaned by rinsing consecutively with water-ethanol-water, dried with nitrogen gas, and (18) Dijt, J. C.; Stuart, M. A. C.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79–101. (19) Zintchenko, A.; Dautzenberg, H.; Tauer, K.; Khrenov, V. Langmuir 2002, 18, 1386–1393. (20) Sauerbery, G. Z. Phys. 1959, 155, 206–222. (21) H€oo€k, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. Langmuir 1998, 14, 729– 734.
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made hydrophilic by treatment in a plasma cleaner at 30 W for 2 min. After mounting the quartz crystal in the QCM cell, it was exposed to an aqueous buffer solution containing the same salt concentration and pH as used for preparation of the polymer solution in order to establish a stable baseline for the frequency and energy dissipation measurements. The adsorption experiments were then started by exchanging buffer with polymer solution. Residual polymer was removed from the cell by addition of the aqueous buffer solution. The QSoft software from Q-Sense was utilized to record the changes in the properties of adsorbed polymer layer at four different overtones during the adsorption process. Of these, the third overtone was used for evaluation, basically due to its stability. All QCM experiments were conducted at a constant temperature of 24.0 °C. In some experiments, the newest model of Q-Sense (E4 system) was used for the adsorption experiments. The main difference between this equipment and the earlier version of Q-Sense (D300 system) is that the frequency and dissipation are recorded while the solution is continuously fed to the surface. The flow speed was kept constant at 0.1 mL/min in all the experiments. Atomic Force Microscopy (AFM). The change in PEC conformation on SiO2 surface was studied by means of tapping mode atomic force microscopy using a Nanoscope III, Multimode SPM, Veeco Inc. All the experiments, in which standard rectangular noncontact silicon cantilevers (RTESP, Veeco Instruments Inc.) were used, were conducted under ambient conditions (23 °C and 50% relative humidity). The silicon wafers used for AFM images were treated in the same manner as described previously for SPAR. The silicon wafer was dipped into the 50 mg/L PEC solution (10 mM NaCl, pH 7.0) for different times (15 s, 5 min, and overnight). After PEC treatment, the surface was rinsed with 10 mM NaCl solution and dried with N2 gas. Size and Zeta Potential Properties of PECs. The average size and zeta potential of the complex particles were determined with dynamic light scattering (DLS) using a Zetasizer Nano ZS particle characterization system (Malvern Instruments Ltd., UK). The average size of the PEC particles is given as an intensity average calculated from the backscattered signal intensity. The electrophoretic mobility of the complexes was also evaluated with this equipment, and by performing the measurements at two different frequencies of the applied field, both small and large particles were included in the average mobility values presented. From the electrophoretic mobility, the zeta potential of the complexes was calculated using the Henry equation.22 Zeta potential data were achieved after 2 min equilibrating time. Ulrafiltration. To separate any free polyelectrolytes from the PEC formed, Pellicon XL Ultrafiltration Module Biomax cassettes with 100 and 10 kDa cutoff (Millipore Corp.) were used for PEC-A and PEC-B, respectively. PEC-A1 was also dialyzed with the aid of a porous cellulose acetate membrane (Spectrum, MWCO: 1000, Spectrum Laboratories Inc.) for 3 days against deionized water. The concentrations of PECs after filtration and dialysis were determined via a standard curve of count rate (kcps) vs concentration (R2 > 99.5) with the aid of the static light scattering mode of the Zetasizer Nano ZS equipment using toluene as a standard. Unfiltrated PECs were used to establish the standard curve by assuming that the time-averaged intensity of the scattered light is due mainly to the PEC particles compared to free polyelectrolyte in both 10 mM NaCl and deionized water.
The size distribution of PECs used for the adsorption experiments is given in Figure 1.
Figure 1. Size distribution of PECs as determined by the DLS intensity at a charge ratio (q-/qþ) of 0.8, pH = 7, and in 10 mM NaCl (AF: after filtration; BF: before filtration).
Results Size and Zeta Potential of the PECs Formed. The PEC properties in solution were determined from DLS as size and zeta potential for different PECs and differently stored PEC colloids, before and after filtration and also after dialysis. The size distributions for the PECs were found to be monomodal and significantly different depending on their preparation and treatment.
Figure 2. Size and zeta potential values during the storage of different PEC solutions: (a) average size with standard deviation; (b) average zeta potential with standard deviation as determined from DLS at pH = 7.0 and a salt concentration of 10 mM NaCl (BF: before filtration; AF: after filtration).
(22) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; pp 546-548.
Figure 2a,b shows the change in average size and zeta potential of PEC-A1 and PEC-B during storage at room temperature.
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Article Table 2. Δf, ΔD, and Δm Values Obtained by QCM-D (E-4 Model) Experimentsa Δf3/3(Hz)
ΔD (10-6)
Δm (mg/m2)
PEC-A1 (BF) 40.2 0.40 7.1 PEC-A1 (AF) 49.7 2.30 7.8 PEC-B (BF) 8.1 0.06 1.4 PEC-B (AF) 8.8 0.05 1.6 a PEC concentration: 50 mg/L; flow rate: 0.1 mL/min; NaCl concentration: 10 mM; pH: 7.0; and a charge ratio of 0.8 for the complexes.
Figure 3. Influence of ultrafiltration and dialysis on PEC adsorption at a charge ratio of 0.8, a pH of 7.0, and a salt concentration of 10 mM NaCl as a function of time. PEC concentration: 50 mg/L; pH: 7.0; BF: before filtration; AF: after filtration. Rinsing was performed with 10 mM NaCl solution at pH 7.0 for PEC-A1 (BF) and PEC-A1 (BF) and with deionized water at pH 7.0 for PEC-A1 (BF-dialysis). A second PEC addition was used to clarify if the surfaces could adsorb more PEC after rinsing.
Figure 4. SPAR data showing the effect of PEC concentration on the adsorbed amount for PECs formed at a charge ratio of 0.8, a pH of 7.0, and a salt concentration of 10 mM NaCl as a function of time. A second PEC addition was used to clarify if the surfaces could adsorb more PEC after rinsing.
The average size of the PECs remained constant during the storage, but the zeta potential changed for some of the PECs, depending on the conditions for their preparation. In particular, the ultrafiltrated and dialyzed complexes showed a decrease in zeta potential upon storage. Adsorption of PEC on SiO2 Surfaces. The adsorption of PECs onto SiO2 surfaces was investigated with both SPAR and QCM techniques. Figure 3 shows the adsorption of PEC-A1 on SiO2, determined via SPAR, after filtration and dialysis. As can be seen in the figure, when the PEC was filtered, the adsorbed amount increased. On the other hand, a significantly lower adsorbed amount was detected after dialyzing PEC-A1 against deionized water. The adsorbed amount was higher for PEC-A1 than for PEC-B, as shown in Figure 4. This figure also shows that the initial adsorption rate increased with increasing PEC concentration for both PEC-A1 and PEC-B. Langmuir 2010, 26(18), 14606–14614
The adsorption of both PEC-A1 and PEC-B on silicon oxide crystals was also investigated with the aid of the QCM-D (E-4 model), since this technique, apart from displaying the adsorbed amount including both solid complexes and immobilized liquid, also provides information about the viscoelastic properties of the adsorbed layers at a controlled flow rate, which was kept constant for all the experiments at 0.1 mL/min. Furthermore, by combining the SPAR and QCM measurements, it is possible to determine both the solid adsorbed amount and the immobilized liquid, as described earlier. As can be seen in Table 2, the shift in frequency (Δf) and adsorbed amount (Sauerbrey mass, Δm) following PEC-A1 adsorption onto silicon oxide surface was higher than after PEC-B adsorption. The dissipation shift (ΔD) was also found to be lower for PEC-B than for PEC-A1, indicating a more densely adsorbed layer in the case of the PEC-B. Removing free PAH and small PEC complexes from the solution by filtration enhanced the PEC adsorption on the SiO2 surface, as was also found with the SPAR equipment (Figure 3). As seen in Table 2, the ultrafiltered PEC-A1 complex showed a large increase in dissipation after filtration, indicating a lower density of the adsorbed layer following the filtration procedure. Generally, when polyelectrolytes are adsorbed onto oppositely charged surfaces, their conformations change depending mainly on their molecular weight, the charge density of the polyelectrolyte and of the surface, pH, and the electrolyte concentration. In the present study, the conformation of PECs having different sizes and different internal structures produced by changing the molecular weights of the PAH and PAA during PEC production has been studied, keeping the charge ratio of the PECs (q-/qþ = 0.8), the electrolyte concentration (10 mM NaCl), and the pH (7.0) constant in all the experiments. Images of PEC-A2 (which was produced under the same conditions as PEC-A1) and PEC-B particles adsorbed onto SiO2 surfaces obtained by tapping mode AFM are shown in Figures 5 and 6, respectively. For the AFM measurements, the negatively charged SiO2 wafer was kept in the 50 mg/L PEC-A2 and PEC-B solution at 10 mM NaCl and pH 7.0 for 15 s, 5 min, and overnight, and the SiO2 surfaces were thereafter rinsed with 10 mM NaCl solution and dried with N2 gas. As can be seen, the adsorption of PEC-A2 on the silicon oxide surface is definitely different from that of PEC-B. The PEC-A2 particles adsorbed very close together (Figure 5A) and tended to merge after longer adsorption times (Figure 5B,C). On the other hand, Figure 6 apparently shows two different fractions of PEC-B on the surface. For one fraction, encircled as seen in Figure 6A, a larger amount of adsorbed particles was observed at shorter contact times, i.e., less than 15 s. This fraction seemed to acquire a more flat conformation after 5 min (Figure 6B). After adsorption overnight, this fraction reconformed even more and could actually not be detected on the surface after this adsorption period (Figure 6C). A second fraction, initially adsorbed in a lower number concentration on the surface, basically kept the same conformation at the surface irrespective of the adsorption time. This indicates that PEC-B consisted of two different populations of complexes with DOI: 10.1021/la1022054
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Figure 5. AFM images (amplitude, height, and height profile from left to right) of PEC-A2 (BF) on silicon oxide surfaces after different adsorption periods (A: 15 s; B: 5 min; C: overnight; image size 5 5 μm, color scale 0-50 nm). The complexes had a charge ratio of 0.8, and they were prepared at pH = 7 in 10 mM NaCl. The location of the height profiles are shown in the height images.
basically the same size but with different amounts of polymer in the complexes. To clarify the phenomenon behind the different surface conformations of the PECs, the adsorption was monitored for longer times with the aid of Q-Sense D300 system, measuring both changes in frequency (Δf) and in dissipation (ΔD). The PEC was adsorbed to the SiO2 surface, and the flow was then stopped to ensure the same experimental conditions as used in the adsorption experiments preceding the collection of the AFM images. To evaluate the active positive charges of the PEC layers, colloidal silica particles were also allowed to adsorb onto the PEC-treated surfaces. Parts A and B of Figure 7 show the Δf and ΔD shifts, respectively, after both PEC and SiO2 adsorption. It can be seen in Figure 7A that the adsorption of PEC-A2 increased slightly with time in the long-term experiments, while the adsorption of PEC-B remained almost constant. More colloidal silica was adsorbed onto the PEC-treated surface after a long time of adsorption of the PECs than after adsorption for a short time compared with short adsorption time experiments. The amount of colloidal silica adsorbed onto the crystal pretreated with PEC-A2 was lower than that onto the crystal pretreated with PEC-B. In Figure 7B, ΔD decreased with time following the adsorption of PEC-A2, indicating a change in the PEC layer with time, 14610 DOI: 10.1021/la1022054
whereas it was stable for PEC-B. The silica adsorption on the PEC-treated crystal gave ΔD values of almost the same order of magnitude for both PEC-A2 and PEC-B.
Discussion Properties of PECs in Solution. From the size measurements shown in Figure 2a it is clear that the size of the PECs did not change significantly during storage; i.e., they were stable and did not aggregate. On the other hand, the zeta potential of the PECs, which had been filtered to remove excess free cationic polyelectrolyte (PAH) and small PEC particles, started to decrease after a short period. The same behavior was found for PEC dialyzed against deionized water. There may be several reasons for this behavior, but since the complexes tend to have the same initial potential, irrespective of molecular mass and filtration procedure, it may be suggested that the binding of excess cationic polyelectrolyte in the most external layer of the complexes is limited by the potential in the layer. Furthermore, the excess PAH layer will probably be in a nonequilibrium conformation, and there will be a restructuring of the layer with time, leading to a lower potential through a reconformation of the polyelectrolytes in the complexes. If excess polyelectrolyte is available, it will be possible to bind more molecules to the complexes, provided by anionic polyelectrolyte available in the Langmuir 2010, 26(18), 14606–14614
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Figure 6. AFM images (amplitude, height, and height profile from left to right) of PEC-B (BF) on silicon oxide surface after different adsorption periods (A: 15 s; B: 5 min; C: overnight; image size 5 5 μm, color scale 0-50 nm). The complexes had a charge ratio of 0.8, and they were prepared at pH = 7 in 10 mM NaCl. The location of the height profiles are shown in the height images.
complex. With no excess polymer, there will only be a decrease in the potential of the polyelectrolyte complex. This hypothesis can explain the difference between the complexes before and after filtration. It was also found that the PECs dialyzed against deionized water had higher initial zeta potential but also that the potential decreased with storage time. The increase in zeta potential is most probably caused by a decrease in salt concentration in the complex dispersion, changing the potential in the shear plane. The decrease in zeta potential indicates that the dialysis has removed some excess polyelectrolyte, which would be adsorbed to PEC particles providing more stable zeta potential readings. The initial increase in zeta potential after dialysis can be due either to a simple change in salt concentration or to a binding of excess polyelectrolyte. It has been also shown by Chollakup et al.23 that the interaction between polyelectrolytes increases in the absence of electrolyte during PEC formation, supporting the suggestion that binding of excess PAH to the complex surface leads to a higher initial zeta potential just after dialysis of the PEC. Adsorption of PEC to SiO2 Surfaces. The adsorption of PEC-A from high molecular weight PAH/PAA onto SiO2 surfaces (23) Chollakup, R.; Smitthipong, W.; Eisenbach, C. D.; Tirrell, M. Macromolecules 2010, 43, 2518–2528.
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was found to be higher than the adsorption of PEC-B from low molecular weight PAH/PAA. The same behavior was seen in both the QCM-D and the SPAR experiments. This difference in the adsorption of PECs with the same charge ratio (q-/qþ: 0.8) is due to the fact that the spherical complexes from PEC-A covered a larger area of the surface, as can be seen in the AFM images (Figures 5 and 6). Moreover, the higher ΔD value following PEC-A adsorption (Table 2 and Figure 7B) can be attributed to a lower viscosity and lower shear modulus of the adsorbed layer. The filtration of PEC solution to remove unreacted free polyelectrolyte and some unwanted removal of small PEC particles resulted in an increase in the adsorbed mass and a higher ΔD value for PEC-A1. The reason is simply that the removed material has a higher diffusion rate and will reach the surface before the larger complexes. Upon removal of these components, the larger complexes dominate the adsorption due to their larger dimensions, and therefore, the adsorption is higher and the adsorbed layer is probably also less compact; this explains the higher dissipation detected for the filtered sample. The lower adsorption after dialysis of PEC (Figure 3) is probably linked to the change in the interaction between PAH and PAA and between PEC particles and the SiO2 surface due to the removal of salt and some excess polyelectrolyte. The removal DOI: 10.1021/la1022054
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Figure 8. Initial rate of adsorption of PECs on the SiO2 surface at pH = 7.0, a salt concentration of 10 mM NaCl, and a charge ratio of 0.8 in the complexes.
Figure 7. QCM-D results for the adsorption of PEC-A2 and PECB onto SiO2 surfaces in both long-term and short-term experiments with the subsequent addition of SiO2 nanocolloids. Parts A and B show the Sauerbrey mass and the change in dissipation value (ΔD) as a function of time. Injection points of PEC, 10 mM NaCl (rinsing), and silica particles are shown as blue circle, blue square, and green square, respectively. PEC and silica concentration: 50 mg/L; NaCl concentration: 10 mM; pH: 7.0.
by dialysis of both added salt and salt released from the complexation reaction increases the electrostatic attraction between the PAH and PAA chains and the repulsion between the cationically charged complexes. There was a slight increase in the average size of PEC-A1 to 88 nm and a more significant increase in zeta potential to þ62 mV when the salt concentration was reduced. The increase in zeta potential can be attributed to the decrease in salt concentration and binding of PAH to PEC particles, as mentioned before. The charge of the SiO2 surface decreases as the salt concentration is decreased.24 Both the increase in zeta potential of PEC and the decrease in charge density of the SiO2 surface will lead to a lower adsorption as the salt is removed.25 The initial adsorption rate (dΓ/dt) for PEC-A1 and PEC-B was calculated from the slope of the initial adsorption stage as detected in SPAR measurements (given in Figure 3). Figure 8 shows that the two different PECs, which were used immediately after preparation without any further treatment, were adsorbed onto the surface at different rates depending upon the feed concentration. The rate of adsorption of PEC-A1 increased with increased feed concentration while the adsorption rate of PEC-B (24) Shubin, V. J. Colloid Interface Sci. 1997, 191, 372–377. (25) Van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538–2546.
14612 DOI: 10.1021/la1022054
first increased (between 5 and 50 mg/L) and then remained almost constant (between 50 and 200 mg/L). During the initial stage of adsorption, PEC-A1 particles can find free spaces on the surface and adsorption will be faster with increasing number concentration of particles.18 For PEC-B the constant rate of initial adsorption can be attributed to a limitation of the available surface. To understand and explain these differences, it is useful to study the AFM images of PEC-A2 and PEC-B taken at different adsorption times (15 s, 5 min, and overnight), as shown in Figures 5 and 6. In these images, it is obvious that the two PECs showed different behavior regarding both the adsorption mechanism and the conformation on the surface. In the case of PEC-A2 complexes, the small particles should theoretically reach the surface first due to their higher diffusion coefficient. Following this, the larger particles were adsorbed onto the surface. With increasing time of immersion of the SiO2 surfaces in the PEC solution, even larger PEC particles were adsorbed onto the surface. It is also clear that PEC particles started to assume a more flat conformation on the surface, and even more interesting, they start to coalesce into larger structures. This was most pronounced when the adsorption was allowed to proceed overnight. To the knowledge of the authors, this behavior has not been detected before. However, this coalescence of adsorbed complexes was not observed for the PEC-B particles in the AFM images. These images indicate the existence of two different types of PECs on the surface. Especially from the height and the phase images, it is clear that there is one fraction with a flat conformation and one fraction with a more extended conformation (height) on the surface. Despite the difference in height images, the particles have approximately the same size. This indicates the existence of two different fractions of PEC-B complexes with different densities, i.e., amounts of polyelectrolyte in the complexes. When the adsorption time was increased from 15 s to 5 min, the fraction with the lower density had flattened out even more and was difficult to distinguish unless both the AFM images were considered and compared. When the adsorption experiments were performed overnight, it was no longer possible to detect the fraction with the lower density, but considering that no more complexes are adsorbed onto the sites where these less dense complexes have been adsorbed, it is suggested that these particles have formed a flat adsorbed layer blocking any further adsorption of either of the two fractions of the PEC-B. There is no simple explanation of why there are different fractions formed for the Langmuir 2010, 26(18), 14606–14614
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It is important to link this new type of behavior of the PECs at the solid-liquid interface to how the modified surfaces will adhere to other surfaces or how they will bind other polyelectrolytes or colloids. In the present work, this was investigated by studying how SiO2 nanocolloids were adsorbed to a PEC-pretreated surface where the complexes were allowed to adsorb for different periods of time before they were exposed to the nanocolloids. The amounts of colloidal silica particles, including immobilized water onto the PEC-A2 and PEC-B layers, were calculated to be approximately 23 and 28 mg/m2, respectively, showing that the higher amount of active charge was on the PEC-B layer. The AFM images may seem contradictory but this might be explained by the crystal surface being fully covered with both extended globular PEC-B particles with higher density and undetectable PEC-B particles with lower density and free PAH, resulting in a greater binding of silica nanocolloids compared with the PEC-A2 layer.
Conclusions Figure 9. Illustration of PECs’ adsorption onto a SiO2 surface and the difference between the two systems where PEC-A2 shows a surface-induced coalescence whereas the PEC-B shows a site blocking by complexes with a lower density, i.e., due to the existence of two different types of complexes in the PEC-B dispersion.
different polyelectrolyte combinations, but both the adsorption experiments and the AFM experiments show this behavior. It can be suggested that the low molecular mass combination shows a phase behavior upon complexation similar to that suggested by Reihs et al.13 and K€otz and Beitz26 where two different phases are formed with two different concentrations of polyelectrolyte, and these two different types of polyelectrolyte complexes show a totally different reconformation behavior when adsorbed onto solid surfaces. In the case of PEC-A2 particles, this behavior is not seen, and complex particles are associated with each other when adsorbed to the solid surfaces, showing a behavior similar to that of polyelectrolyte multilayer formation at the solid/liquid interface. A similar discussion has also been presented by Sukhishvili et al.27 referring to the migration of polyelectrolytes from the polyelectrolyte multilayer contacted with polyelectrolyte solution, but to the knowledge of the authors, the present results are the first time that experimental results are presented showing this similarity of the PEC and the polyelectrolyte multilayer systems at the solid-liquid interface. There might naturally be alternative explanations to the adsorption behavior for PEC-B. It can for example be suggested that the rather broad size distribution of PEC-B could be enough to account for the disappearance of some of the complexes with adsorption time, i.e., small complex contained so little polymer that they will totally flatten with time. However, by studying the images in Figure 6, it is clear that some of the complexes that are disappearing with time have the same or even larger dimension than some of the complexes that remain identifiable on the surface after overnight adsorption. It must be stressed that the complexes in the PEC dispersion are stable over long storage times and that the agglomeration of the complexes is induced by the interaction with the surface. Figure 9 shows a schematic representation of the behavior of the two different types of complexes at the solid-liquid interface. (26) K€otz, J.; Beitz, T. Trends Polym. Sci. 1997, 5, 86–90. (27) Sukhishvili, A. S.; Kharlampieva, E.; Izumrudov, V. Macromolecules 2006, 39, 8873–8881.
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The solution and adsorption behaviors of two different PECs, prepared respectively by combining high molecular mass PAH/ PAA (PEC-A) and low molecular mass PAH/PAA (PEC-B), using a confined impinging jet (CIJ) mixer, were investigated in the present study. The conclusions areas follows: 1. The PECs formed were stable and did not aggregate during the storage at the charge ratio investigated. On the other hand, the zeta potential of filtered and dialyzed PEC particles was not stable and decreased after a short period. It is suggested that the excess cationic polyelectrolyte (PAH) in the solution adsorbed to the complexes, keeping the zeta potential constant with time. When excess PAH was removed by ultrafiltration or dialysis, the rearrangement of the polyelectrolytes in the complexes resulted in a decrease in the zeta potential, but the repulsion between the complexes in solution was obviously high enough to prevent complex aggregation. 2. The adsorption of PEC-A1 onto a negatively charged silicon oxide surface was higher and resulted in a higher dissipation value (ΔD) compared with PEC-B, indicating a lower viscosity and elastic modulus provided by larger PEC particles. The AFM images of the adsorbed PEC particles on the surfaces showed that the conformation and surface coverage of PEC-A2 and PEC-B were quite different. In the case of the PEC-A2 complexes, the larger particles were adsorbed to the surface at a slower rate than the small particles. When the adsorption time was increased, the SiO2 surface was fully covered by complex particles, and it was found that the PEC particles started to become more flat on the surface and to coalesce into larger structures. This was attributed to a surface-induced reconformation due to the migration of polyelectrolytes from one PEC complex to an adjacent PEC complex. In the case of the PEC-B complexes, two different fractions of complexes were observed on the SiO2 surface. One fraction of PEC-B complexes was, from the results, suggested to have a lower amount of polyelectrolyte in the complex and became flatter and undetectable on the surface, as shown by AFM images when the SiO2 surface was immersed overnight. The second fraction of PEC-B was suggested, from the results, to have a higher polyelectrolyte content and a more extended conformation on the surface, keeping its globular structure even after an extended adsorption time. 3. The active cationic charge on both the PEC-A2 and the PECB layers was investigated with aid of a subsequent colloidal silica adsorption onto these layers. A larger adsorbed amount of colloidal silica was detected on the PEC-B layer even though DOI: 10.1021/la1022054
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the surface coverage of PEC-B was indicated by the AFM images to be lower than that of the PEC-A2 layer. These results support that the first fraction of PEC-B particles with lower density and possibly also free PAH were still available on the surface, blocking the sites for the adsorption of the second fraction of PEC-B, while forming active cationic adsorption sites for the negatively charged colloidal silica particles.
14614 DOI: 10.1021/la1022054
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Acknowledgment. Dr. Sedat Ondaral and Professor Lars Wa˚gberg kindly acknowledge financial support from the Biomime Research Centre at KTH, and Caroline Ankerfors gratefully acknowledges financial support from Eka Chemicals AB, Bohus, Sweden, and the Knowledge Foundation through its graduate school YPK, at Chalmers Technical University, Gothenburg. Dr. Anthony Bristow is thanked for linguistic revision of the manuscript.
Langmuir 2010, 26(18), 14606–14614
