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Characterization of polyelectrolyte multilayers on mica and oxidized titanium by streaming potential and wetting angle measurements
Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 455–460
Characterization of polyelectrolyte multilayers on mica and oxidized titanium by streaming potential and wetting angle measurements Z. Adamczyk, M. Zembala ∗ , M. Kolasi´nska, P. Warszy´nski Institute of Catalysis and Surface Chemistry, Polish Academy of Science, Niezapominajek 8, 30-239 Cracow, Poland Received 27 February 2007; received in revised form 8 March 2007; accepted 8 March 2007 Available online 14 March 2007
Abstract Multilayer adsorption of polyelectrolytes on mica and electrochemically oxidized titanium was studied by the streaming potential method using a parallel-plate channel arrangement. Streaming potential data were complemented by wetting angle measurements. Two types of polyelectrolytes (PE) were used: polyallylamine hydrochloride (PAH), of a cationic type, and polysodium 4-styrenesulfonate (PSS) of an anionic type (both having molecular weight of ∼70,000). Layer by layer adsorption of polyelectrolytes from NaCl and TRIS solutions of various concentrations was studied in situ. It was demonstrated that after completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values appeared for multilayers terminated by PAH and PSS, respectively. This was in accordance with zeta potential of the polymers in the bulk measured by electrophoresis. It also was observed that streaming potential data correlated well with the wetting angle characteristics of dried polyelectrolyte films. Variations in the contact angle between 70◦ for PAH and 40◦ for PSS were exactly in phase with the variation in the zeta potential. The stability of polyelectrolyte films against prolong washing (reaching 24 h) was determined using the streaming potential method. It was revealed that both for mica and titanium, the PSS layer was considerably more resistant to washing, compared to the PAH layer. The experimental data obtained were consistent with a model postulating particle-like adsorption of polyelectrolytes. © 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption of polyelectrolytes on mica and titanium; PAH and PSS adsorption on mica and titanium; Polyelectrolyte adsorption; Multilayer adsorption of polyelectrolytes; Streaming potential measurements of polyelectrolyte adsorption
1. Introduction Consecutive deposition of anionic and cationic polyelectrolyte layers at solid/electrolyte interfaces proved an efficient method of preparing films of a desired architecture and functionality [1–6]. In contrast to the simplicity of film preparation by the layer-by-layer technique, studying their properties, is more complicated because of the film thickness being of the order of nanometers and its density being much lower than for the bulk polymeric material. As a result, polyelectrolyte films have been mostly studied under the dried state by using the optical methods like ellipsometry [7,8], X-ray reflectivity [2,9], quartz-crystal microbalance (QCM) [4], atomic force microscopy (AFM) [4,7], scanning probe microscopy [10], etc.
∗
Corresponding author. Tel.: +48 12 6395 134; fax: +48 12 4251 923. E-mail address: [email protected] (M. Zembala).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.03.013
More tedious experimental studies of wet films were performed by using total internal reflection fluorescence (TIRF) [11], ellipsometry [12], scanning angle reflectometry (SAR) [13], neutron reflectivity measurements [9,14], QCM [4] or tapping mode AFM [15,16]. Electrokinetic measurements represent another alternative for measuring in situ formation of polyelectrolyte multilayers on solid substrates. One is usually determining electrophoretic mobility of colloid particles covered by multilayers [17–19] or streaming potential and streaming current [8,13]. As substrates in the latter case one used fused quartz capillaries [13] or silicon wafers in the parallel-plate geometry [8]. A similar parallel-plate arrangement was applied for measuring multilayers of PAH/PSS on mica as reported in Ref. [20]. A considerable advantage of the electrokinetic methods is that they also can be exploited for determining the multilayer stability for various washing bath composition, pH, etc. [20]. The impact of the wet film studies by electrokinetic methods can be significantly increased when correlated with film characteristics of the collapsed monolayers under the dry state.
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One of the most interesting parameter of the latter is the interfacial energy that can be revealed by the contact angle measurements. Our studies will be concentrated on mica, used as the reference substrate and oxidized titanium known as an exceptionally suitable material for implant construction [21]. Surface modification of titanium by polyelectrolyte adsorption and consecutive attachment of proteins seems a promising technique for producing biocompatible films of targeted architecture. Despite significance of such procedures few systematic studies has been reported in the literature on this subject. Accordingly, the goal of this paper was to compare the build up and stability of polyelectrolyte multilayers on mica and modified titanium by using the complementary streaming potential and wetting angle methods. 2. Experimental 2.1. Materials and methods Natural ruby mica sheets supplied by Dean Transted Ltd. England were used as the substrate surfaces for polyelectrolyte adsorption. Thin sheets were freshly cleaved and used in each experiment without any pre-treatment. Metalic titanium (99.6% pure Ti) was supplied in the form of thin foils (dimensions 50 mm × 50 mm, thickness 30 m) by Goodfellow. The foil was cut into strips of the dimensions of 10 mm × 50 mm. The titanium substrate sheets were passivated electrochemically according to the method described in Ref. [22] by an anodic oxidation carried out in a phosphate buffer solution of pH 7.4 and the over-voltage of 80 V. This procedure, lasting 100 s, resulted in formation of a rather amorphous titania layer as confirmed by X-ray studies, having an averaged thickness of 150 nm and the micro roughness of 17 nm [22]. Preparing the thick titania layer of higher resistivity was advantageous for the streaming potential measurements that require non- or poorly-conductive substrates. The polyelectrolytes used were (i) the anionic poly(sodium 4-styrenosulfonate (PSS) and (ii) the cationic poly(allylamine) hydrochloride (PAH). Both polymers having the averaged molecular weight of 70,000 were purchased from Aldrich. Sodium chloride and tris(hydrooxyaminomethane) (TRIS), in mixtures with HCl serving as buffer for fixing pH, were commercial products of Fluka and Sigma, respectively. Ultrapure water (Elix&Simplicity 185 system, Millipore SA Molsheim, France) was used for the preparation of all solutions. 2.2. Experimental methods Streaming potential was determined using a home-made apparatus described in detail previously [23,24]. The cell consisted of two polished perfluoeroethylene (PTFE) blocks having inlet and outlet compartments. Two thin substrate surfaces (mica sheets or titanium foils) were placed on the blocks and separated by a PTFE gasket (serving as a spacer). The parallel plate channel of the dimensions: 2b × 2c × L = 0.025 cm × 0.33 cm × 3.6 cm was then formed by clamping together the blocks with two mica
sheets (or titanium foils) and the spacer, using a press under constant torque conditions. The streaming potentials Es measured when an electrolyte solution flows through the cell under regulated hydrostatic pressure difference P, were used for determination of the apparent zeta potential of the channel walls using the formula [20,23,24] 16α 16α ζ2 πη Es 1 L 4πη Es = ζc = ζ1 1 − 3 + 3 = λeff π π ζ1 ε P Rc bc ε P (1) where α = 2b/2c is the ratio of the thickness of the channel 2b to its width 2c, ζ 1 the zeta potential of mica and ζ 2 is the zeta potential of PTFE (side wall of the channel). Because for our channel geometry α = 0.076, the coefficient 16α/π3 was equal 0.04, which means that the correction term (−16α/π3 ) (1 − (ζ 2 /ζ 1 )) in Eq. (1) was much smaller than unity (if the substrate and PTFE zeta potentials are comparable), η is the solution viscosity, and L/Rc bc = λeff is the effective conductance of electrolyte in the channel incorporating the surface conductivity effect. The experimental procedure of determining the zeta potential of polyelectrolyte covered substrates described in detail previously [20] consisted of measuring first the zeta potential of bare substrate (mica or oxidized titanium), covering the substrate in situ by a desired number of polyelectrolyte layers and finally, without dismounting the channel, measuring the dependence of the streaming potential on the hydrostatic pressure difference Es = f(P) of polyelectrolyte covered surface. Contact angle measurements have been carried out according to a two-stage procedure: first the substrate was covered by a desired number of polyelectrolyte layers by dipping the substrate into the appropriate solution of a polyelectrolyte (alternatively PAH and PSS) of the concentration of 0.5 mg/ml (500 ppm), adsorbing the polymer under diffusion controlled transport (30 min) and washing with water for ca. 1 min. Then, the multilayer covered substrate was dried for 24 h in open air. Finally, wetting angle of sessile drops of water placed at random over the substrate surface has been determined using a home made thermostated apparatus with a chamber saturated with water vapour. The contact angle was determined by a sessile drop profile analysis [25,26]. The Young–Laplace equation was fitted to the drop profile and the contact angle was determined from a first derivative of the function describing the drop contour at the three-phase perimeter. The measurement time was ca. 30 s, so there have been no changes in the drop shape during this time. An average value of wetting angle has been calculated from at least 10 drops placed at various areas. Zeta potential of polyions in bulk solutions were determined by using the Zetasizer Nano ZS of Malvern. 3. Results and discussion 3.1. Electrokinetic characteristics of the system The electrokinetic characteristics of mica and modified titanium have been performed according to the above described streaming potential method. The influence of ionic strength,
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[20]. It also is interesting to observe that for such low values of this parameter, electrophoretic mobility remained practically independent of the shape of the polyelectrolyte [27]. It was found [20] that zeta potential of PSS was −95 mV at I = 10−3 M and −55 mV for 0.15 M NaCl solution. For PAH the zeta potential was found positive with smaller absolute values, i.e., 75 and 40 mV for I = 10−3 and 0.15 M, respectively. As can be observed, the absolute value of the zeta potential decreases monotonically with the increase in the ionic strength [20]. By adopting the cylindrical capacitor model it was estimated in Ref. [20] that the number of effective (electrokinetically accessible) charges per polyion q/e was 115 for PAH and q/e = 82 for PSS (at I = 10−3 M).
Fig. 1. Zeta potential of mica as a function of pH regulated by composition of TRIS + HCl mixture at ionic strength I = 10−3 M.
changed within the range 10−5 to 10−2 M (by addition of NaCl addition) on the zeta potential of mica was presented in [20] indicating a monotonic increase of zeta potential value from −87 mV for I = 10−3 M to −38 mV for I = 10−2 M. The pH dependence of mica zeta potential is presented in Fig. 1 at constant ionic strength equal to 10−3 M indicating that in the range of pH values between 3 and 9 mica surfaces is negatively charged. Zeta potential of passivated titanium was strongly pH dependent as can be observed in Fig. 2. The isoelectric point was found to be about 4.5 pH unit. This agrees with the data reported in Ref. [22]. Thus, for our experimental conditions (pH 7.4 and I = 10−3 M) the modified titanium surface was negatively charged exhibiting zeta potential of −52 mV. For I = 10−2 M and pH 7.4, we have measured ζ = −20 mV. The zeta potential of polyelectrolytes in the bulk was determined from electrophoretic mobility carried out by microelectrophoresis. The mobility has been converted to zeta potential by the H¨uckel formula. This is justified by the fact that the largest value of the κd/2 parameter (characterising the ratio of the polymer cross-section radius to the double-layer thickness κ−1 ) was 0.53 for PAH and 0.74 for PSS (at I = 0.15 M)
Fig. 2. Zeta potential of passivated titanium as a function of pH for ionic strength of: () 10−4 M, () 10−3 M and () 10−2 M.
3.2. Electrokinetic characteristics of poyelectrolyte multilayers Considerations presented in Ref. [20] suggest that polyelectrolyte molecules in the bulk can be viewed as elongated species rather than random coil (fuzzy) structures Thus, mechanisms of PE adsorption on charged surfaces can be treated as a process mainly governed by electrostatic interactions chain/interface with the dispersion interactions and entropic term in adsorption energy playing a less important role. In such a case polyelectrolyte chains will exhibit a strong tendency to adsorb side-on, minimizing the number of tails and loops, which was confirmed by Monte–Carlo simulations of Stoll [28]. Results of Stoll simulations [28] indicate that polyelectrolyte coverage versus ionic strength dependence should pass a maximum for ionic strength between 10−2 and 0.1 M. A non-monotonic dependence of zeta potentials on ionic strength with an extremum located at 10−2 M was obtained for densely packed PE layers at mica surface [20]. Here, basing on the above discussion one can expect that PE adsorption on charged support is proceeding analogously to irreversible adsorption mechanisms pertinent to colloids (that have been extensively studied theoretically and experimentally [29–33]) and consequently the zeta potential values determined for surfaces covered by adsorption layers should be related to adsorbed amount. These predictions seem consistent with our data shown in Figs. 3 and 4 where the dependence of the zeta potential of a consecutive polyelectrolyte layer is plotted against layer number. One can see in Fig. 3 that adsorption of a single PAH layer from a 500 ppm solution in 10−3 M electrolyte leads to the inversion of the sign of the apparent zeta potential of the mica substrate (equal −106 mV under these conditions) that attained +60 mV. Adsorption of a consecutive PSS layer leads again to zeta potential inversion that attained −77 mV. Then, adsorption of the third PAH layer leads again to zeta potential inversion that assumed this time a lower value of 32 mV. After completing of this layer the consecutive adsorption of PAH and PSS became quite periodic. As a consequence, a saw-like graph was obtained (see Fig. 3), fist reported by Caruso et al. [17] and Burke and Barrett [19] for polyelectrolyte adsorption on colloid particles. Similar graph also has been presented by Ladam et al. [13] for PAH and PSS adsorption on fused silica. However, the amplitude of these zeta potential oscillations was smaller than that observed by us.
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Fig. 3. The dependence of the apparent zeta potential ζ (a) and the contact angle (b) on the number of layers on mica. Polymers adsorbed from 500 ppm solutions in 10−3 M TRIS buffer pH 7.4.
Fig. 4. Same as in Fig. 3 but for polymer adsorption from 500 ppm solution in 0.15 M NaCl, streaming potential measured using 10−3 M TRIS solution of pH 7.4.
It is interesting to compare the zeta potential variations with contact angle changes measured for consecutive polyelectrolyte layers, see Fig. 3(b). It can be observed that adsorption of the first PAH layer (carried out from a 10−3 M TRIS buffer) increased the contact angle from a practically zero value for bare mica to 35◦ . However, deposition of next PSS and PAH layers did not change significantly contact angle values, so the dependence of the contact angle on the layer number showed, therefore, little periodicity. This behaviour would support the hypothesis of a sparse, side-on adsorption of consecutive polyelectrolyte layers, whose thickness was comparable with the polymer chain dimensions, i.e., about 1 nm. This is much lower than the range of the dispersion interactions, so the influence of the substrate on the dispersion interaction energy (governing the contact angle behaviour) remained significant even for multilayers built up of several PE layers. A quite different behaviour was observed for polyelectrolyte multilayers adsorbed from solutions characterised by high ionic strength (0.15 M), see Fig. 4. One can observe that in this case the periodicity in both zeta potential and contact angle was well pronounced and strictly correlated with each other. Accordingly, the zeta potential of the consecutive PAH layers was constant and equal 73 mV. For PSS this was −88 mV, respectively. It is interesting to note that these data are similar to the bulk values of
zeta potential of these polyelectrolytes measured by microelectrophoresis (equal 75 and −95 mV as mentioned previously). This agreement suggests that the streaming potential method used in our work can be used as an effective tool for determining zeta potentials of polyelectrolytes. The increase in the amplitude of the zeta potential variations of the layers adsorbed from higher concentration of salt (higher ionic strength) can be probably interpreted as due to increased thickness of the PAH and PSS layers due to screening the lateral electrostatic interactions among similarly charged polyelectrolyte chains. As a result, the mutual penetration of polyelectrolytes forming consecutive layers was expected quite insignificant. This hypothesis is supported by the contact angle variations shown in Fig. 4(b). One can see that the contact angle of consecutive PAH layers were almost constant and equal about 70◦ . On the other hand, the contact angle of the PSS layers was about 40◦ suggesting their significantly more hydrophilic character. Note also that there was practically no difference in contact angle between the first and consecutive layers of these polyelectrolytes. The more hydrophilic character of PSS may be caused by a higher degree of ionisation of the polyelectrolyte chain as discussed in Ref. [20]. Analogous results obtained for the oxidized titanium substrate are shown in Fig. 5. Polyelectrolyte multilayer deposition
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Fig. 6. The dependence of the reduced zeta potential of the multilayer ζ/ζ 0 on the rinsing time t: (), third PAH; (), second PSS; empty points, polymer adsorbed from a 10−3 M bath; full points, polymer adsorbed from a 0.15 M bath.
Fig. 5. The dependence of the apparent zeta potential ζ (a) and the contact angle (b) on the number of layers on mica (dotted line) and modified titanium (dashed line), polymers deposited from 500 ppm solutions in 0.15 M NaCl.
was carried out from 0.15 M NaCl solution whereas zeta potential measurements were done using the 10−3 M TRIS buffer pH 7.4. One can observe that after completion of the second layer the variations in zeta potential between consecutive layers are quite periodic. However, the amplitude of these variations for titanium was slightly smaller than for mica, since the maximum and minimum zeta potential value was 59 and −69 mV, respectively. Analogously, the variations of the contact angle became periodic for second and higher layers (see Fig. 5b). Thus, these results suggest that the influence of the solid substrate does not influence the contact angle after forming one polyelectrolyte bilayer. The results shown in Figs. 3–5 suggest that the streaming potential method used in this work enabled one to characterise electrokinetic properties (zeta potential) of polyelectrolyte mulitilayers. Additionally, the method can be exploited for determining the stability of monolayers against rinsing by electrolytes that cannot be realised by other method. The procedure of these experiments was such that upon depositing the desired number of polyelectrolyte layers, the channel was flushed with electrolyte (10−3 M TRIS buffer, pH 7.4) [20]. The wall shear rate of the flow was 2750 s−1 . For every half an hour interval the streaming potential of the channel was measured and the maximum washing time was 26 h. The results of stability of polyelectrolyte
layers at mica obtained in Ref. [20] are presented now in Fig. 6 rescaled in such a way to illustrate the effect of the ionic strength of the polymer adsorption bath. One can observe that changes in zeta potential of polyelectrolyte multilayers terminated with PSS were negligible. On the other hand, zeta potential of PAH terminated layers decreased significantly with time. This effect was especially pronounced if the multilayer deposition was carried out under the low ionic strength conditions (10−3 M TRIS buffer). In this case zeta potential was even converted to negative values for prolonged washing. Comparison the results obtained for rinsing effect on polyelectrolyte layers adsorbed from solutions in 0.15 M NaCl at mica and modified titanium is presented in Fig. 7. It is interesting to observe, that the change in zeta potential caused by washing is higher for passivated titanium than for the mica substrate indicating lower stability of PAH terminated multilayers formed at this substrate. It is not possible, without additional measurements, to unequivocally attribute differences in layer stability against
Fig. 7. The dependence of the reduced zeta potential ζ/ζ 0 of the multilayer on modified titanium (empty points) and mica (full points) on the rinsing time t: () PAH third layer; () PSS fourth layer, polymer deposition from a 0.15 M bath.
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washing shown in Figs. 6 and 7 to PAH desorption process. However, this hypothesis seems plausible because, as previously estimated the charge density on PAH chains [20] is smaller than for PSS chains. Moreover, the flexibility of PAH molecules seems higher due to their smaller thickness (0.83 nm) in comparison with PSS (1.17 nm) and more than two times larger length. This suggests that PAH adsorption on PSS may be associated with formation of a larger number of loops and tails, especially under low ionic strength conditions. As a result, the adsorption energy of PAH molecules may be smaller leading to partial reversibility of its adsorption or conformational changes in the polymer loops. Other experiments performed indicated that zeta potential of adsorbed layers of cationic polyelectrolytes was very sensitive to the presence of vestigial amount of silicate anions leached from glassware. Taking into account that washing experiments were done at pH 7.4 this explanation should also be taken into account. 4. Conclusions It was demonstrated using the electrokinetic measurements that deposition of consecutive polyelectrolyte layers on mica and modified titanium substrates is strictly correlated with periodic oscillations of zeta potential. The amplitude of these oscillations increased with the ionic strength of the polymer adsorption bath that was interpreted as the results of increased coverage and thickness of adsorbed layer. Zeta potential values of PAH and PSS terminated multilayers attained under high ionic strength conditions were very close to the zeta potential of these polymers in the bulk, measured by microelectrophoresis. It also was demonstrated that the variations in the zeta potential of multilayers were strictly correlated with periodic changes in the contact angle governing wetting properties of the polymeric film. Experimental data collected in this work allow one to conclude that the streaming potential and wettability measurements can be used as a sensitive tool for in situ monitoring of properties of polyelectrolyte layers on various substrates. Additionally, the streaming potential method can be exploited for determining the stability of each monolayer against rinsing that cannot be realised by other techniques. Acknowledgments This work was supported by the KBN Grant 4T09A 076 25. The authors would like to thank Dr B. Jachimska for performing zeta potential measurements of polyelectrolytes.
References [1] G. Decher, J.D. Honig, J. Schmitt, Thin Solid Films 210/211 (1992) 831. [2] Y.M. Lvov, G. Decher, H. M¨ohwald, Langmuir 9 (1993) 481. [3] G. Decher, Science 277 (1997) 1232. [4] Y.M. Lvov, K. Ariga, M. Onda, I. Ichinose, T. Kunitake, Colloid Surf. A: Physicochem. Eng. Aspects 146 (1999) 337. [5] P.T. Hammond, Curr. Opin. Colloid Interf. Sci. 4 (2000) 430. [6] Y.M. Lvov, Polyion-protein nanocomposite films, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 3, Marcel Dekker, New York, 2002, p. 4162. [7] J.D. Mendelsohn, C.J. Barett, V.V. Chan, A.J. Pal, A.M. Mayes, M.F. Rubner, Langmuir 16 (2000) 5017. [8] S. Schwarz, K.-J. Eichhorn, E. Wischerhoff, A. Laschewsky, Colloid Surf. A: Physicochem. Eng. Aspects 159 (1999) 491. [9] R. Steitz, V. Leiner, R. Siebrecht, R.V. Klitzing, Colloid Surf. A: Physicochem. Eng. Aspects 163 (2000) 63. [10] S.S. Shiratori, M.F. Rubner, Macromolecules 33 (2000) 4213. [11] R.V. Klitzing, H. M¨ohwald, Thin Solid Films 284/285 (1996) 352. [12] T.J. Halthur, U.M. Elofsson, Langmuir 20 (2004) 1739. [13] G. Ladam, P. Schaad, J.C. Voegel, P. Schaaf, G. Decher, F. Cuisinier, Langmuir 16 (2000) 1249. [14] D. Carri`ere, R. Krastev, M. Sch¨onhoff, Langmuir 20 (2004) 11465– 11472. [15] O.J. Rojas, Adsorption of polyelectrolytes on mica, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 1, Marcel Dekker, New York, 2002, p. 517. [16] V. Percec, C.-H. Ahn, G. Ungar, D.I. Yeardley, M. Moeller, S.S. Sheiko, Nature 391 (1998) 161. [17] F. Caruso, E. Donath, H. M¨ohwald, J. Phys. Chem. B 102 (1998) 2011. [18] E. Donath, D. Walther, V.N. Shilov, E. Knippel, A. Budde, K. Lowack, C.A. Helm, H. M¨ohwald, Langmuir 13 (1997) 5294. [19] S.E. Burke, C.J. Barrett, Langmuir 19 (2003) 3297. [20] Z. Adamczyk, M. Zembala, P. Warszy´nski, B. Jachimska, Langmuir 20 (2004) 10517. [21] M. Long, H.J. Rack, Biomaterials 19 (1998) 1621. [22] S. Roessler, R. Zimmermann, D. Scharnweber, C. Werner, H. Worch, Colloid Surf. B: Biointerf. 26 (2002) 387. [23] M. Zembala, Z. Adamczyk, Langmuir 16 (2000) 1593. [24] M. Zembala, Z. Adamczyk, P. Warszy´nski, Colloid Surf. A: Physicochem. Eng. Aspects 222 (2003) 329. [25] B. Janocha, G. Para, P. Warszy´nski, J. Matheis, Z. Adamczyk, Bull. Polish Acad. Sci. Chem. 48 (2000) 243. [26] M. Kolasi´nska, P. Warszy´nski, Bioelectrochemistry 66 (2005) 65. [27] B.J. Yoon, S. Kim, J. Colloid Interf. Sci. 128 (1989) 275. [28] S. Stoll, DNA-like polyelectroyte adsorption onto polymer colloids, in: A. Elaissari (Ed.), Colloidal Biomolecules, Biomaterials, and Biomedical Applications, Marcel Dekker, New York, 2004, p. 211. [29] P. Viot, G. Tarjus, S.M. Ricci, J. Talbot, J. Chem. Phys. 97 (1992) 5212. [30] J.W. Evans, Rev. Mod. Phys. 65 (1993) 1281. [31] Z. Adamczyk, J. Colloid Interf. Sci. 229 (2000) 477. [32] Z. Adamczyk, Adsorption of particles: theory, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 1, Marcel Dekker, New York, 2002, p. 499. [33] Z. Adamczyk, Adv. Colloid Inerf. Sci. 100–102 (2003) 267.
Characterization of polyelectrolyte multilayers on mica and oxidized titanium by streaming potential and wetting angle measurements Z. Adamczyk, M. Zembala ∗ , M. Kolasi´nska, P. Warszy´nski Institute of Catalysis and Surface Chemistry, Polish Academy of Science, Niezapominajek 8, 30-239 Cracow, Poland Received 27 February 2007; received in revised form 8 March 2007; accepted 8 March 2007 Available online 14 March 2007
Abstract Multilayer adsorption of polyelectrolytes on mica and electrochemically oxidized titanium was studied by the streaming potential method using a parallel-plate channel arrangement. Streaming potential data were complemented by wetting angle measurements. Two types of polyelectrolytes (PE) were used: polyallylamine hydrochloride (PAH), of a cationic type, and polysodium 4-styrenesulfonate (PSS) of an anionic type (both having molecular weight of ∼70,000). Layer by layer adsorption of polyelectrolytes from NaCl and TRIS solutions of various concentrations was studied in situ. It was demonstrated that after completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values appeared for multilayers terminated by PAH and PSS, respectively. This was in accordance with zeta potential of the polymers in the bulk measured by electrophoresis. It also was observed that streaming potential data correlated well with the wetting angle characteristics of dried polyelectrolyte films. Variations in the contact angle between 70◦ for PAH and 40◦ for PSS were exactly in phase with the variation in the zeta potential. The stability of polyelectrolyte films against prolong washing (reaching 24 h) was determined using the streaming potential method. It was revealed that both for mica and titanium, the PSS layer was considerably more resistant to washing, compared to the PAH layer. The experimental data obtained were consistent with a model postulating particle-like adsorption of polyelectrolytes. © 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption of polyelectrolytes on mica and titanium; PAH and PSS adsorption on mica and titanium; Polyelectrolyte adsorption; Multilayer adsorption of polyelectrolytes; Streaming potential measurements of polyelectrolyte adsorption
1. Introduction Consecutive deposition of anionic and cationic polyelectrolyte layers at solid/electrolyte interfaces proved an efficient method of preparing films of a desired architecture and functionality [1–6]. In contrast to the simplicity of film preparation by the layer-by-layer technique, studying their properties, is more complicated because of the film thickness being of the order of nanometers and its density being much lower than for the bulk polymeric material. As a result, polyelectrolyte films have been mostly studied under the dried state by using the optical methods like ellipsometry [7,8], X-ray reflectivity [2,9], quartz-crystal microbalance (QCM) [4], atomic force microscopy (AFM) [4,7], scanning probe microscopy [10], etc.
∗
Corresponding author. Tel.: +48 12 6395 134; fax: +48 12 4251 923. E-mail address: [email protected] (M. Zembala).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.03.013
More tedious experimental studies of wet films were performed by using total internal reflection fluorescence (TIRF) [11], ellipsometry [12], scanning angle reflectometry (SAR) [13], neutron reflectivity measurements [9,14], QCM [4] or tapping mode AFM [15,16]. Electrokinetic measurements represent another alternative for measuring in situ formation of polyelectrolyte multilayers on solid substrates. One is usually determining electrophoretic mobility of colloid particles covered by multilayers [17–19] or streaming potential and streaming current [8,13]. As substrates in the latter case one used fused quartz capillaries [13] or silicon wafers in the parallel-plate geometry [8]. A similar parallel-plate arrangement was applied for measuring multilayers of PAH/PSS on mica as reported in Ref. [20]. A considerable advantage of the electrokinetic methods is that they also can be exploited for determining the multilayer stability for various washing bath composition, pH, etc. [20]. The impact of the wet film studies by electrokinetic methods can be significantly increased when correlated with film characteristics of the collapsed monolayers under the dry state.
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One of the most interesting parameter of the latter is the interfacial energy that can be revealed by the contact angle measurements. Our studies will be concentrated on mica, used as the reference substrate and oxidized titanium known as an exceptionally suitable material for implant construction [21]. Surface modification of titanium by polyelectrolyte adsorption and consecutive attachment of proteins seems a promising technique for producing biocompatible films of targeted architecture. Despite significance of such procedures few systematic studies has been reported in the literature on this subject. Accordingly, the goal of this paper was to compare the build up and stability of polyelectrolyte multilayers on mica and modified titanium by using the complementary streaming potential and wetting angle methods. 2. Experimental 2.1. Materials and methods Natural ruby mica sheets supplied by Dean Transted Ltd. England were used as the substrate surfaces for polyelectrolyte adsorption. Thin sheets were freshly cleaved and used in each experiment without any pre-treatment. Metalic titanium (99.6% pure Ti) was supplied in the form of thin foils (dimensions 50 mm × 50 mm, thickness 30 m) by Goodfellow. The foil was cut into strips of the dimensions of 10 mm × 50 mm. The titanium substrate sheets were passivated electrochemically according to the method described in Ref. [22] by an anodic oxidation carried out in a phosphate buffer solution of pH 7.4 and the over-voltage of 80 V. This procedure, lasting 100 s, resulted in formation of a rather amorphous titania layer as confirmed by X-ray studies, having an averaged thickness of 150 nm and the micro roughness of 17 nm [22]. Preparing the thick titania layer of higher resistivity was advantageous for the streaming potential measurements that require non- or poorly-conductive substrates. The polyelectrolytes used were (i) the anionic poly(sodium 4-styrenosulfonate (PSS) and (ii) the cationic poly(allylamine) hydrochloride (PAH). Both polymers having the averaged molecular weight of 70,000 were purchased from Aldrich. Sodium chloride and tris(hydrooxyaminomethane) (TRIS), in mixtures with HCl serving as buffer for fixing pH, were commercial products of Fluka and Sigma, respectively. Ultrapure water (Elix&Simplicity 185 system, Millipore SA Molsheim, France) was used for the preparation of all solutions. 2.2. Experimental methods Streaming potential was determined using a home-made apparatus described in detail previously [23,24]. The cell consisted of two polished perfluoeroethylene (PTFE) blocks having inlet and outlet compartments. Two thin substrate surfaces (mica sheets or titanium foils) were placed on the blocks and separated by a PTFE gasket (serving as a spacer). The parallel plate channel of the dimensions: 2b × 2c × L = 0.025 cm × 0.33 cm × 3.6 cm was then formed by clamping together the blocks with two mica
sheets (or titanium foils) and the spacer, using a press under constant torque conditions. The streaming potentials Es measured when an electrolyte solution flows through the cell under regulated hydrostatic pressure difference P, were used for determination of the apparent zeta potential of the channel walls using the formula [20,23,24] 16α 16α ζ2 πη Es 1 L 4πη Es = ζc = ζ1 1 − 3 + 3 = λeff π π ζ1 ε P Rc bc ε P (1) where α = 2b/2c is the ratio of the thickness of the channel 2b to its width 2c, ζ 1 the zeta potential of mica and ζ 2 is the zeta potential of PTFE (side wall of the channel). Because for our channel geometry α = 0.076, the coefficient 16α/π3 was equal 0.04, which means that the correction term (−16α/π3 ) (1 − (ζ 2 /ζ 1 )) in Eq. (1) was much smaller than unity (if the substrate and PTFE zeta potentials are comparable), η is the solution viscosity, and L/Rc bc = λeff is the effective conductance of electrolyte in the channel incorporating the surface conductivity effect. The experimental procedure of determining the zeta potential of polyelectrolyte covered substrates described in detail previously [20] consisted of measuring first the zeta potential of bare substrate (mica or oxidized titanium), covering the substrate in situ by a desired number of polyelectrolyte layers and finally, without dismounting the channel, measuring the dependence of the streaming potential on the hydrostatic pressure difference Es = f(P) of polyelectrolyte covered surface. Contact angle measurements have been carried out according to a two-stage procedure: first the substrate was covered by a desired number of polyelectrolyte layers by dipping the substrate into the appropriate solution of a polyelectrolyte (alternatively PAH and PSS) of the concentration of 0.5 mg/ml (500 ppm), adsorbing the polymer under diffusion controlled transport (30 min) and washing with water for ca. 1 min. Then, the multilayer covered substrate was dried for 24 h in open air. Finally, wetting angle of sessile drops of water placed at random over the substrate surface has been determined using a home made thermostated apparatus with a chamber saturated with water vapour. The contact angle was determined by a sessile drop profile analysis [25,26]. The Young–Laplace equation was fitted to the drop profile and the contact angle was determined from a first derivative of the function describing the drop contour at the three-phase perimeter. The measurement time was ca. 30 s, so there have been no changes in the drop shape during this time. An average value of wetting angle has been calculated from at least 10 drops placed at various areas. Zeta potential of polyions in bulk solutions were determined by using the Zetasizer Nano ZS of Malvern. 3. Results and discussion 3.1. Electrokinetic characteristics of the system The electrokinetic characteristics of mica and modified titanium have been performed according to the above described streaming potential method. The influence of ionic strength,
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[20]. It also is interesting to observe that for such low values of this parameter, electrophoretic mobility remained practically independent of the shape of the polyelectrolyte [27]. It was found [20] that zeta potential of PSS was −95 mV at I = 10−3 M and −55 mV for 0.15 M NaCl solution. For PAH the zeta potential was found positive with smaller absolute values, i.e., 75 and 40 mV for I = 10−3 and 0.15 M, respectively. As can be observed, the absolute value of the zeta potential decreases monotonically with the increase in the ionic strength [20]. By adopting the cylindrical capacitor model it was estimated in Ref. [20] that the number of effective (electrokinetically accessible) charges per polyion q/e was 115 for PAH and q/e = 82 for PSS (at I = 10−3 M).
Fig. 1. Zeta potential of mica as a function of pH regulated by composition of TRIS + HCl mixture at ionic strength I = 10−3 M.
changed within the range 10−5 to 10−2 M (by addition of NaCl addition) on the zeta potential of mica was presented in [20] indicating a monotonic increase of zeta potential value from −87 mV for I = 10−3 M to −38 mV for I = 10−2 M. The pH dependence of mica zeta potential is presented in Fig. 1 at constant ionic strength equal to 10−3 M indicating that in the range of pH values between 3 and 9 mica surfaces is negatively charged. Zeta potential of passivated titanium was strongly pH dependent as can be observed in Fig. 2. The isoelectric point was found to be about 4.5 pH unit. This agrees with the data reported in Ref. [22]. Thus, for our experimental conditions (pH 7.4 and I = 10−3 M) the modified titanium surface was negatively charged exhibiting zeta potential of −52 mV. For I = 10−2 M and pH 7.4, we have measured ζ = −20 mV. The zeta potential of polyelectrolytes in the bulk was determined from electrophoretic mobility carried out by microelectrophoresis. The mobility has been converted to zeta potential by the H¨uckel formula. This is justified by the fact that the largest value of the κd/2 parameter (characterising the ratio of the polymer cross-section radius to the double-layer thickness κ−1 ) was 0.53 for PAH and 0.74 for PSS (at I = 0.15 M)
Fig. 2. Zeta potential of passivated titanium as a function of pH for ionic strength of: () 10−4 M, () 10−3 M and () 10−2 M.
3.2. Electrokinetic characteristics of poyelectrolyte multilayers Considerations presented in Ref. [20] suggest that polyelectrolyte molecules in the bulk can be viewed as elongated species rather than random coil (fuzzy) structures Thus, mechanisms of PE adsorption on charged surfaces can be treated as a process mainly governed by electrostatic interactions chain/interface with the dispersion interactions and entropic term in adsorption energy playing a less important role. In such a case polyelectrolyte chains will exhibit a strong tendency to adsorb side-on, minimizing the number of tails and loops, which was confirmed by Monte–Carlo simulations of Stoll [28]. Results of Stoll simulations [28] indicate that polyelectrolyte coverage versus ionic strength dependence should pass a maximum for ionic strength between 10−2 and 0.1 M. A non-monotonic dependence of zeta potentials on ionic strength with an extremum located at 10−2 M was obtained for densely packed PE layers at mica surface [20]. Here, basing on the above discussion one can expect that PE adsorption on charged support is proceeding analogously to irreversible adsorption mechanisms pertinent to colloids (that have been extensively studied theoretically and experimentally [29–33]) and consequently the zeta potential values determined for surfaces covered by adsorption layers should be related to adsorbed amount. These predictions seem consistent with our data shown in Figs. 3 and 4 where the dependence of the zeta potential of a consecutive polyelectrolyte layer is plotted against layer number. One can see in Fig. 3 that adsorption of a single PAH layer from a 500 ppm solution in 10−3 M electrolyte leads to the inversion of the sign of the apparent zeta potential of the mica substrate (equal −106 mV under these conditions) that attained +60 mV. Adsorption of a consecutive PSS layer leads again to zeta potential inversion that attained −77 mV. Then, adsorption of the third PAH layer leads again to zeta potential inversion that assumed this time a lower value of 32 mV. After completing of this layer the consecutive adsorption of PAH and PSS became quite periodic. As a consequence, a saw-like graph was obtained (see Fig. 3), fist reported by Caruso et al. [17] and Burke and Barrett [19] for polyelectrolyte adsorption on colloid particles. Similar graph also has been presented by Ladam et al. [13] for PAH and PSS adsorption on fused silica. However, the amplitude of these zeta potential oscillations was smaller than that observed by us.
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Fig. 3. The dependence of the apparent zeta potential ζ (a) and the contact angle (b) on the number of layers on mica. Polymers adsorbed from 500 ppm solutions in 10−3 M TRIS buffer pH 7.4.
Fig. 4. Same as in Fig. 3 but for polymer adsorption from 500 ppm solution in 0.15 M NaCl, streaming potential measured using 10−3 M TRIS solution of pH 7.4.
It is interesting to compare the zeta potential variations with contact angle changes measured for consecutive polyelectrolyte layers, see Fig. 3(b). It can be observed that adsorption of the first PAH layer (carried out from a 10−3 M TRIS buffer) increased the contact angle from a practically zero value for bare mica to 35◦ . However, deposition of next PSS and PAH layers did not change significantly contact angle values, so the dependence of the contact angle on the layer number showed, therefore, little periodicity. This behaviour would support the hypothesis of a sparse, side-on adsorption of consecutive polyelectrolyte layers, whose thickness was comparable with the polymer chain dimensions, i.e., about 1 nm. This is much lower than the range of the dispersion interactions, so the influence of the substrate on the dispersion interaction energy (governing the contact angle behaviour) remained significant even for multilayers built up of several PE layers. A quite different behaviour was observed for polyelectrolyte multilayers adsorbed from solutions characterised by high ionic strength (0.15 M), see Fig. 4. One can observe that in this case the periodicity in both zeta potential and contact angle was well pronounced and strictly correlated with each other. Accordingly, the zeta potential of the consecutive PAH layers was constant and equal 73 mV. For PSS this was −88 mV, respectively. It is interesting to note that these data are similar to the bulk values of
zeta potential of these polyelectrolytes measured by microelectrophoresis (equal 75 and −95 mV as mentioned previously). This agreement suggests that the streaming potential method used in our work can be used as an effective tool for determining zeta potentials of polyelectrolytes. The increase in the amplitude of the zeta potential variations of the layers adsorbed from higher concentration of salt (higher ionic strength) can be probably interpreted as due to increased thickness of the PAH and PSS layers due to screening the lateral electrostatic interactions among similarly charged polyelectrolyte chains. As a result, the mutual penetration of polyelectrolytes forming consecutive layers was expected quite insignificant. This hypothesis is supported by the contact angle variations shown in Fig. 4(b). One can see that the contact angle of consecutive PAH layers were almost constant and equal about 70◦ . On the other hand, the contact angle of the PSS layers was about 40◦ suggesting their significantly more hydrophilic character. Note also that there was practically no difference in contact angle between the first and consecutive layers of these polyelectrolytes. The more hydrophilic character of PSS may be caused by a higher degree of ionisation of the polyelectrolyte chain as discussed in Ref. [20]. Analogous results obtained for the oxidized titanium substrate are shown in Fig. 5. Polyelectrolyte multilayer deposition
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Fig. 6. The dependence of the reduced zeta potential of the multilayer ζ/ζ 0 on the rinsing time t: (), third PAH; (), second PSS; empty points, polymer adsorbed from a 10−3 M bath; full points, polymer adsorbed from a 0.15 M bath.
Fig. 5. The dependence of the apparent zeta potential ζ (a) and the contact angle (b) on the number of layers on mica (dotted line) and modified titanium (dashed line), polymers deposited from 500 ppm solutions in 0.15 M NaCl.
was carried out from 0.15 M NaCl solution whereas zeta potential measurements were done using the 10−3 M TRIS buffer pH 7.4. One can observe that after completion of the second layer the variations in zeta potential between consecutive layers are quite periodic. However, the amplitude of these variations for titanium was slightly smaller than for mica, since the maximum and minimum zeta potential value was 59 and −69 mV, respectively. Analogously, the variations of the contact angle became periodic for second and higher layers (see Fig. 5b). Thus, these results suggest that the influence of the solid substrate does not influence the contact angle after forming one polyelectrolyte bilayer. The results shown in Figs. 3–5 suggest that the streaming potential method used in this work enabled one to characterise electrokinetic properties (zeta potential) of polyelectrolyte mulitilayers. Additionally, the method can be exploited for determining the stability of monolayers against rinsing by electrolytes that cannot be realised by other method. The procedure of these experiments was such that upon depositing the desired number of polyelectrolyte layers, the channel was flushed with electrolyte (10−3 M TRIS buffer, pH 7.4) [20]. The wall shear rate of the flow was 2750 s−1 . For every half an hour interval the streaming potential of the channel was measured and the maximum washing time was 26 h. The results of stability of polyelectrolyte
layers at mica obtained in Ref. [20] are presented now in Fig. 6 rescaled in such a way to illustrate the effect of the ionic strength of the polymer adsorption bath. One can observe that changes in zeta potential of polyelectrolyte multilayers terminated with PSS were negligible. On the other hand, zeta potential of PAH terminated layers decreased significantly with time. This effect was especially pronounced if the multilayer deposition was carried out under the low ionic strength conditions (10−3 M TRIS buffer). In this case zeta potential was even converted to negative values for prolonged washing. Comparison the results obtained for rinsing effect on polyelectrolyte layers adsorbed from solutions in 0.15 M NaCl at mica and modified titanium is presented in Fig. 7. It is interesting to observe, that the change in zeta potential caused by washing is higher for passivated titanium than for the mica substrate indicating lower stability of PAH terminated multilayers formed at this substrate. It is not possible, without additional measurements, to unequivocally attribute differences in layer stability against
Fig. 7. The dependence of the reduced zeta potential ζ/ζ 0 of the multilayer on modified titanium (empty points) and mica (full points) on the rinsing time t: () PAH third layer; () PSS fourth layer, polymer deposition from a 0.15 M bath.
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washing shown in Figs. 6 and 7 to PAH desorption process. However, this hypothesis seems plausible because, as previously estimated the charge density on PAH chains [20] is smaller than for PSS chains. Moreover, the flexibility of PAH molecules seems higher due to their smaller thickness (0.83 nm) in comparison with PSS (1.17 nm) and more than two times larger length. This suggests that PAH adsorption on PSS may be associated with formation of a larger number of loops and tails, especially under low ionic strength conditions. As a result, the adsorption energy of PAH molecules may be smaller leading to partial reversibility of its adsorption or conformational changes in the polymer loops. Other experiments performed indicated that zeta potential of adsorbed layers of cationic polyelectrolytes was very sensitive to the presence of vestigial amount of silicate anions leached from glassware. Taking into account that washing experiments were done at pH 7.4 this explanation should also be taken into account. 4. Conclusions It was demonstrated using the electrokinetic measurements that deposition of consecutive polyelectrolyte layers on mica and modified titanium substrates is strictly correlated with periodic oscillations of zeta potential. The amplitude of these oscillations increased with the ionic strength of the polymer adsorption bath that was interpreted as the results of increased coverage and thickness of adsorbed layer. Zeta potential values of PAH and PSS terminated multilayers attained under high ionic strength conditions were very close to the zeta potential of these polymers in the bulk, measured by microelectrophoresis. It also was demonstrated that the variations in the zeta potential of multilayers were strictly correlated with periodic changes in the contact angle governing wetting properties of the polymeric film. Experimental data collected in this work allow one to conclude that the streaming potential and wettability measurements can be used as a sensitive tool for in situ monitoring of properties of polyelectrolyte layers on various substrates. Additionally, the streaming potential method can be exploited for determining the stability of each monolayer against rinsing that cannot be realised by other techniques. Acknowledgments This work was supported by the KBN Grant 4T09A 076 25. The authors would like to thank Dr B. Jachimska for performing zeta potential measurements of polyelectrolytes.
References [1] G. Decher, J.D. Honig, J. Schmitt, Thin Solid Films 210/211 (1992) 831. [2] Y.M. Lvov, G. Decher, H. M¨ohwald, Langmuir 9 (1993) 481. [3] G. Decher, Science 277 (1997) 1232. [4] Y.M. Lvov, K. Ariga, M. Onda, I. Ichinose, T. Kunitake, Colloid Surf. A: Physicochem. Eng. Aspects 146 (1999) 337. [5] P.T. Hammond, Curr. Opin. Colloid Interf. Sci. 4 (2000) 430. [6] Y.M. Lvov, Polyion-protein nanocomposite films, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 3, Marcel Dekker, New York, 2002, p. 4162. [7] J.D. Mendelsohn, C.J. Barett, V.V. Chan, A.J. Pal, A.M. Mayes, M.F. Rubner, Langmuir 16 (2000) 5017. [8] S. Schwarz, K.-J. Eichhorn, E. Wischerhoff, A. Laschewsky, Colloid Surf. A: Physicochem. Eng. Aspects 159 (1999) 491. [9] R. Steitz, V. Leiner, R. Siebrecht, R.V. Klitzing, Colloid Surf. A: Physicochem. Eng. Aspects 163 (2000) 63. [10] S.S. Shiratori, M.F. Rubner, Macromolecules 33 (2000) 4213. [11] R.V. Klitzing, H. M¨ohwald, Thin Solid Films 284/285 (1996) 352. [12] T.J. Halthur, U.M. Elofsson, Langmuir 20 (2004) 1739. [13] G. Ladam, P. Schaad, J.C. Voegel, P. Schaaf, G. Decher, F. Cuisinier, Langmuir 16 (2000) 1249. [14] D. Carri`ere, R. Krastev, M. Sch¨onhoff, Langmuir 20 (2004) 11465– 11472. [15] O.J. Rojas, Adsorption of polyelectrolytes on mica, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 1, Marcel Dekker, New York, 2002, p. 517. [16] V. Percec, C.-H. Ahn, G. Ungar, D.I. Yeardley, M. Moeller, S.S. Sheiko, Nature 391 (1998) 161. [17] F. Caruso, E. Donath, H. M¨ohwald, J. Phys. Chem. B 102 (1998) 2011. [18] E. Donath, D. Walther, V.N. Shilov, E. Knippel, A. Budde, K. Lowack, C.A. Helm, H. M¨ohwald, Langmuir 13 (1997) 5294. [19] S.E. Burke, C.J. Barrett, Langmuir 19 (2003) 3297. [20] Z. Adamczyk, M. Zembala, P. Warszy´nski, B. Jachimska, Langmuir 20 (2004) 10517. [21] M. Long, H.J. Rack, Biomaterials 19 (1998) 1621. [22] S. Roessler, R. Zimmermann, D. Scharnweber, C. Werner, H. Worch, Colloid Surf. B: Biointerf. 26 (2002) 387. [23] M. Zembala, Z. Adamczyk, Langmuir 16 (2000) 1593. [24] M. Zembala, Z. Adamczyk, P. Warszy´nski, Colloid Surf. A: Physicochem. Eng. Aspects 222 (2003) 329. [25] B. Janocha, G. Para, P. Warszy´nski, J. Matheis, Z. Adamczyk, Bull. Polish Acad. Sci. Chem. 48 (2000) 243. [26] M. Kolasi´nska, P. Warszy´nski, Bioelectrochemistry 66 (2005) 65. [27] B.J. Yoon, S. Kim, J. Colloid Interf. Sci. 128 (1989) 275. [28] S. Stoll, DNA-like polyelectroyte adsorption onto polymer colloids, in: A. Elaissari (Ed.), Colloidal Biomolecules, Biomaterials, and Biomedical Applications, Marcel Dekker, New York, 2004, p. 211. [29] P. Viot, G. Tarjus, S.M. Ricci, J. Talbot, J. Chem. Phys. 97 (1992) 5212. [30] J.W. Evans, Rev. Mod. Phys. 65 (1993) 1281. [31] Z. Adamczyk, J. Colloid Interf. Sci. 229 (2000) 477. [32] Z. Adamczyk, Adsorption of particles: theory, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, vol. 1, Marcel Dekker, New York, 2002, p. 499. [33] Z. Adamczyk, Adv. Colloid Inerf. Sci. 100–102 (2003) 267.