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Cite this: Soft Matter, 2014, 10, 9220
Received 18th September 2014 Accepted 6th October 2014
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Studying the role of surface chemistry on polyelectrolyte adsorption using gold–thiol selfassembled monolayer with optical reflectivity† Plinio Maroni, Francisco Javier Montes Ruiz-Cabello and Alberto Tiraferri*
DOI: 10.1039/c4sm02093f www.rsc.org/softmatter
Self-assembled monolayers of thiols on gold are employed to study the role of surface chemistry on adsorption of polyelectrolytes to solid substrates. The suitability of these substrates is demonstrated in optical reflectivity, which combines high sensitivity to the possibility to precisely control the hydrodynamic conditions at the solid/water interface. Therefore, this system allows the determination of both the adsorbed amount and the kinetics of adsorption. The behavior of two representative strong polyelectrolytes of opposite charge is discussed as a function of pH and of concentration of a monovalent electrolyte in aqueous solutions. The application of equivalent substrates with varying surface chemistry sheds light on the role of different energetic contributions driving polyelectrolyte adsorption.
Water-borne adsorption of polyelectrolytes on solid substrates governs the properties of surfaces and of particle suspensions.1,2 The ability to control and to predict the amount of adsorbed polyelectrolyte is crucial to the understanding of surface properties and to the design of surface modication strategies. The development of adsorption layers on a planar substrate is usually probed with optical reectivity, ellipsometry, surface plasmon resonance, and the quartz crystal microbalance.1–5 Investigation of the adsorption kinetics requires well-dened hydrodynamic conditions at the solid/water interface. Among the common surface sensitive techniques, reectometry in the visible range is a versatile option in which the system geometry can be controlled precisely and in which substrates can be custom-made to have desired surface properties. The mass of the adsorbed lm and the kinetics of adsorption are inuenced signicantly by the molecule–substrate interactions. The role of surface chemistry is therefore crucial and, particularly, that of surface charge type and density. The important case of polyelectrolytes adsorbing on surfaces of Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland. E-mail: alberto.tiraferri@ unige.ch; Tel: +41 22 379 6421 † Electronic supplementary 10.1039/c4sm02093f
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opposite charge is well understood.1 In contrast, while the adsorption of polyelectrolytes on surfaces of like charge has been predicted in the presence of multivalent counterions6,7 and has been observed in several engineering systems,8–10 the mechanistic understanding of these phenomena is limited. As polyelectrolytes oen adsorb even in the absence of electrostatic attraction between chains and the adsorbing substrate, other energetic and entropic contributions may also play an important role.2,11,12 More efforts should be devoted to understand the mechanism of adsorption of polyelectrolytes on uncharged surfaces or on substrates bearing the same sign of charge as the polymer chain. A good way to probe the effect of surface chemistry is to use self-assembled monolayers (SAMs) with tailored terminal functionalities. SAMs have been extensively deployed to investigate the interactions of proteins with organic surfaces,13 a method that allowed identication of the functionalities that resist protein adsorption.14 This technique can be transferred directly to characterize the polyelectrolyte–surface interactions that inuence the development of adsorption layers.15 In this study, we demonstrate the suitability of gold–thiol SAMs in an optical reectivity instrument with well-dened hydrodynamics. SAMs of different hydrophobicity and charging properties were fabricated and characterized. These substrates were used to systematically study the role of surface chemistry on adsorption behavior of two representative strong polyelectrolytes, namely, anionic sodium poly(styrene sulfonate) (PSS) and cationic poly(diallyldimethyl ammonium chloride) (PDADMAC). We present the adsorbed amount and the adsorption kinetics of these charged polymers and we discuss these results in the light of the energetic interactions of the various polyelectrolyte–SAM combinations. The substrates used to fabricate the SAM lms consisted of an underlying layer of silicon, a middle adhesion layer of titanium, and an uppermost layer of sputter coated gold. The thickness of the gold layer was 8.5 0.5 nm, as determined by ellipsometry (Multiskop, Optrel, Berlin, Germany),5 while its root mean square surface roughness was approximately 0.5 nm.
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This value is signicantly lower than that of typical substrates used in the quartz crystal microbalance. The alkanethiol lms were adsorbed on the gold surfaces by immersing them overnight (>10 h) into a solution containing the corresponding thiol dissolved in ethanol (99.98%, VWR International, France).16 Solutions of 0.5 mM 11-mercapto-1-undecanol and 1-nonanethiol were used to form hydroxyl (OH) and methyl (CH3) terminated SAMs, respectively. Carboxyl (COOH) and amine (NH2) terminated SAMs were produced following an improved method described elsewhere.17 Briey, a solution of 0.5 mM 11mercaptoundecanoic acid with 2% (v/v) CF3COOH was used to form COOH SAMs, while solutions of 0.5 mM 11-amino-1undecanethiol hydrochloride with 3% (v/v) N(CH2CH3)3, were employed to fabricate NH2 SAMs. These lms were rinsed sequentially with ethanol, an ethanolic solution of 10% (v/v) NH4OH for carboxyl terminated lms or 10% (v/v) CH3COOH for amine terminated lms, and again ethanol. The quality of the SAM lms was assessed by measurement of the contact angle of water with the sessile drop method. The values of water contact angle evaluated for the four SAM types are reported in Table 1 and are consistent with reports in the literature.17–20 The wettability of SAM lms increased in the order CH3 < NH2 < COOH < OH. The surface potential of the SAM lms was estimated by means of the colloidal probe technique based on the atomic force microscopy. This technique consists on attaching a colloidal particle to a tipless cantilever and measuring the interaction forces between this particle and the investigated substrates. Sulfate latex beads with a diameter of 3 mm (Invitrogen) were used as probes. Measurements were conducted with a closed-loop atomic force microscope (MFP-3D, Asylum Research).21 Initially, the charging properties of the particle were calibrated by means of force measurements against a similar particle. The calibrated probe was then used to measure interactions with the planar SAM surface. The experimental forces as a function of probe–surface distance were tted using the Poisson–Boltzmann theory with the constant regulation approximation, to determine the electric potential of the monolayer lms in aqueous solution.21,22 Experiments were conducted in 1 mM NaCl at pH 4 and at pH 10. Under these conditions, the probe–substrate interaction was dominated at large distances by electrostatic forces. Therefore, we neglected other forces, such as Van der Waals or hydrophobic/hydrophilic interactions. The average of three measurements for the various SAMs is presented in Table 1. More details about this theory and the respective calculations can be found in the ESI† and in a previous publication.21 The potentials estimated for the surfaces functionalized with amine and carboxyl groups are consistent with the ionization behavior reported in the literature for similar thiol lms.23–28 All literature studies denote broadening of the ionization curves for the moieties of these monolayers compared to the respective free organic groups in aqueous solution. The retarded dissociation has been ascribed primarily to the electrostatic interaction and to hydrogen bonding of the neighboring ionized groups at the monolayer/solution interface.19,23–30 A negative surface potential was also observed for the two nominally uncharged
This journal is © The Royal Society of Chemistry 2014
Soft Matter Table 1 Contact angle of deionized water (pH 5.6) and surface potential of the SAM films (1 mM NaCl) used for adsorption experiments
Surface potential (mV) Surface
Water contact angle (deg)
pH 4
pH 10
Gold OH NH2 CH3 COOH
75 7 COOH > OH with sticking coefficients of 0.43, 0.36, 0.34, respectively; see also Fig. 1a and c. This trend is consistent with the variation in surface hydrophobicity. Accordingly, the development of the cationic PDADMAC layer was the slowest on the positive amine SAM; see Fig. 1b and d. The values of the sticking coefficient for the adsorption of PDADMAC in 10 mM NaCl were 0.35, 0.21, and 0.24 for CH3, OH, and NH2 SAMs, respectively. Similar conclusions can be drawn from Fig. 3c and d, which presents the adsorption rate coefficient as a function of prevailing salt concentration. In the presence of polyelectrolyte– substrate electrostatic attraction, deposition is favorable, and the measured adsorption rate was nearly uniform or decreased with increasing salt concentration. This reduction was caused by competition between ions originating from the background
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solution and the polyelectrolytes.40 On the contrary, when electrostatic repulsion dominated the interaction between polymeric chains and the substrates, higher ionic strength produced faster kinetics of adsorption. This result is rationalized with a decrease in the magnitude of the kinetic barrier for deposition with increased concentration of electrolyte in solution, due to compression of the double layer around the chains and at the surface/water interface. When this energy barrier was eliminated in high salt, van der Waals forces and other attractive interactions resulted in favorable deposition. Interestingly, while the mass of PSS adsorbed on COOH and OH lms increased signicantly in 100 mM, the related kinetics were reduced. Deposition of chains from solution onto the previously adsorbed layer might also occur at high salt, due to screening of the intermolecular electrostatic repulsion. This phenomenon may be responsible for the slow development of PSS layers on these two surfaces observed at the highest ionic strength. Self-assembled monolayers were applied to study the role of surface charge and hydrophobicity on adsorption of polyelectrolytes to solid substrates. Small changes of adsorbed mass, in the order of 103 mg m2 s1, were successfully probed, demonstrating the applicability of optical reectivity on these substrates. While gold surfaces are routinely used in surface plasmon resonance instruments, results presented here may potentially enlarge the pool of substrates and surface chemistries that can be investigated by optical reectometry, which represents a more exible and easily accessible technique. In particular, SAMs are remarkable substrates because their customized surface chemistry allows the systematic determination of the different energetic interactions governing polymer adsorption. In this study, charged polymers were shown to adsorb on all the different surfaces signicantly, thereby providing also some initial insight on the mechanism of adsorption of polyelectrolytes on surfaces of like charge. In particular, results suggest that adsorbed amounts were direct result of chain–chain interactions. These interactions were governed by the size of the electric double layer around the charged chains, which in turn was affected by the electric potential of the substrate and by the prevailing electrolyte concentration. Kinetics of adsorption were inuenced predominantly by the electrostatic interactions between polymeric chains and the adsorbing substrates. In the presence of electrostatic attraction, the adsorbed mass and the kinetics were the largest. If polyelectrolytes adsorbed on substrates with the same sign of potential, the kinetic barrier to deposition was reduced by higher ionic strengths, thus increasing the rate of adsorption signicantly. Surface hydrophobicity was observed to promote adsorption of polyelectrolytes, via interaction with the nonpolar segments of the polymeric chain. In this study, anionic PSS and cationic PDADMAC were chosen as examples of strong polyelectrolytes. The SAM-reectivity setup may be further applied to investigate the adsorption behavior of weak polyelectrolytes, uncharged polymers, or polymers characterized by different hydrophobicity. This research would further elucidate the various contributions to the driving force for adsorption of macromolecules on solid surfaces, as well as more clearly underline the role of nonelectrostatic interactions.
9224 | Soft Matter, 2014, 10, 9220–9225
Communication
Acknowledgements This work was supported by a Marie Curie Intra-European Fellowship to A.T. within the Seventh European Community Framework Programme (PIEF-GA-2012-327977), by the Swiss National Science Foundation, and by the University of Geneva.
References 1 I. Szilagyi, G. Trefalt, A. Tiraferri and M. Borkovec, So Matter, 2014, 10, 2479–2502. 2 G. J. Fleer, M. A. Cohen Stuart, J. M. H. M. Scheutjens, T. Cosgrove and B. Vincent, Polymers at Interfaces, Chapman and Hall, London, 1993. 3 N. G. Hoogeveen, M. A. Cohen Stuart and G. J. Fleer, J. Colloid Interface Sci., 1996, 182, 133–145. 4 L. E. Enarsson and L. Wagberg, Langmuir, 2008, 24, 7329– 7337. 5 M. Porus, P. Maroni and M. Borkovec, Langmuir, 2012, 28, 5642–5651. 6 M. Turesson, C. Labbez and A. Nonat, Langmuir, 2011, 27, 13572–13581. 7 L. Wang, H. J. Liang and J. Z. Wu, J. Chem. Phys., 2010, 133, 044906. 8 X. P. Jiang, C. Ortiz and P. T. Hammond, Langmuir, 2002, 18, 1131–1143. 9 A. Tiraferri, Y. Kang, E. P. Giannelis and M. Elimelech, Environ. Sci. Technol., 2012, 46, 11135–11144. 10 J. Y. Huang, K. S. Jackson and C. J. Murphy, Nano Lett., 2012, 12, 2982–2987. 11 S. L. Clark and P. T. Hammond, Langmuir, 2000, 16, 10206– 10214. 12 R. Maoz, J. Sagiv, D. Degenhardt, H. Mohwald and P. Quinn, Supramol. Sci., 1995, 2, 9–24. 13 K. L. Prime and G. M. Whitesides, Science, 1991, 252, 1164– 1167. 14 E. Ostuni, L. Yan and G. M. Whitesides, Colloids Surf., B, 1999, 15, 3–30. 15 C. Friedsam, A. D. Becares, U. Jonas, M. Seitz and H. E. Gaub, New J. Phys., 2004, 6, 9. 16 R. G. Nuzzo and D. L. Allara, J. Am. Chem. Soc., 1983, 105, 4481–4483. 17 H. Wang, S. F. Chen, L. Y. Li and S. Y. Jiang, Langmuir, 2005, 21, 2633–2636. 18 C. D. Bain and G. M. Whitesides, Angew. Chem., Int. Ed., 1989, 28, 506–512. 19 M. L. Wallwork, D. A. Smith, J. Zhang, J. Kirkham and C. Robinson, Langmuir, 2001, 17, 1126–1131. 20 G. M. Whitesides and P. E. Laibinis, Langmuir, 1990, 6, 87– 96. 21 F. J. Montes Ruiz-Cabello, G. Trefalt, P. Maroni and M. Borkovec, Phys. Rev. E: Stat., Nonlinear, So Matter Phys., 2014, 90, 012301. 22 W. B. Russel, D. A. Saville and W. R. Schowalter, Colloidal Dispersions, Cambridge University Press, Cambridge, 1989.
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23 W. A. Marmisolle, D. A. Capdevila, E. de la Llave, F. J. Williams and D. H. Murgida, Langmuir, 2013, 29, 5351–5359. 24 R. Schweiss, C. Werner and W. Knoll, J. Electroanal. Chem., 2003, 540, 145–151. 25 K. Hu and A. J. Bard, Langmuir, 1997, 13, 5114–5119. 26 E. Kokkoli and C. F. Zukoski, Langmuir, 2000, 16, 6029–6036. 27 R. Schweiss, P. B. Welzel, C. Werner and W. Knoll, Langmuir, 2001, 17, 4304–4311. 28 E. W. vanderVegte and G. Hadziioannou, J. Phys. Chem. B, 1997, 101, 9563–9569. 29 E. Kokkoli and C. F. Zukoski, J. Colloid Interface Sci., 2000, 230, 176–180. 30 C. T. Konek, M. J. Musorrati, H. A. Al-Abadleh, P. A. Bertin, S. T. Nguyen and F. M. Geiger, J. Am. Chem. Soc., 2004, 126, 11754–11755. 31 Y. H. M. Chan, R. Schweiss, C. Werner and M. Grunze, Langmuir, 2003, 19, 7380–7385. 32 H. J. Kreuzer, R. L. C. Wang and M. Grunze, J. Am. Chem. Soc., 2003, 125, 8384–8389. 33 M. Porus, P. Maroni and M. Borkovec, Sens. Actuators, B, 2010, 151, 250–255.
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34 L. Wagberg, Nord. Pulp Pap. Res. J., 2000, 15, 586–597. 35 I. Popa, B. P. Cahill, P. Maroni, G. Papastavrou and M. Borkovec, J. Colloid Interface Sci., 2007, 309, 28–35. 36 B. P. Cahill, G. Papastavrou, G. J. M. Koper and M. Borkovec, Langmuir, 2008, 24, 465–473. 37 E. Seyrek, J. Hierrezuelo, A. Sadeghpour, I. Szilagyi and M. Borkovec, Phys. Chem. Chem. Phys., 2011, 13, 12716– 12719. 38 D. Bauer, H. Buchhammer, A. Fuchs, W. Jaeger, E. Killmann, K. Lunkwitz, R. Rehmet and S. Schwarz, Colloids Surf., A, 1999, 156, 291–305. 39 R. Rehmet and E. Killmann, Colloids Surf., A, 1999, 149, 323– 328. 40 M. Jiang, I. Popa, P. Maroni and M. Borkovec, Colloids Surf., A, 2010, 360, 20–25. 41 A. Tiraferri, P. Maroni, D. C. Rodriguez and M. Borkovec, Langmuir, 2014, 30, 4980–4988. 42 N. Hansupalak and M. M. Santore, Langmuir, 2003, 19, 7423– 7426. 43 J. Blaakmeer, M. R. Bohmer, M. A. Cohen Stuart and G. J. Fleer, Macromolecules, 1990, 23, 2301–2309.
Soft Matter, 2014, 10, 9220–9225 | 9225
Published on 08 October 2014. Downloaded by Tulane University on 27/01/2015 20:14:42.
COMMUNICATION
Cite this: Soft Matter, 2014, 10, 9220
Received 18th September 2014 Accepted 6th October 2014
View Journal | View Issue
Studying the role of surface chemistry on polyelectrolyte adsorption using gold–thiol selfassembled monolayer with optical reflectivity† Plinio Maroni, Francisco Javier Montes Ruiz-Cabello and Alberto Tiraferri*
DOI: 10.1039/c4sm02093f www.rsc.org/softmatter
Self-assembled monolayers of thiols on gold are employed to study the role of surface chemistry on adsorption of polyelectrolytes to solid substrates. The suitability of these substrates is demonstrated in optical reflectivity, which combines high sensitivity to the possibility to precisely control the hydrodynamic conditions at the solid/water interface. Therefore, this system allows the determination of both the adsorbed amount and the kinetics of adsorption. The behavior of two representative strong polyelectrolytes of opposite charge is discussed as a function of pH and of concentration of a monovalent electrolyte in aqueous solutions. The application of equivalent substrates with varying surface chemistry sheds light on the role of different energetic contributions driving polyelectrolyte adsorption.
Water-borne adsorption of polyelectrolytes on solid substrates governs the properties of surfaces and of particle suspensions.1,2 The ability to control and to predict the amount of adsorbed polyelectrolyte is crucial to the understanding of surface properties and to the design of surface modication strategies. The development of adsorption layers on a planar substrate is usually probed with optical reectivity, ellipsometry, surface plasmon resonance, and the quartz crystal microbalance.1–5 Investigation of the adsorption kinetics requires well-dened hydrodynamic conditions at the solid/water interface. Among the common surface sensitive techniques, reectometry in the visible range is a versatile option in which the system geometry can be controlled precisely and in which substrates can be custom-made to have desired surface properties. The mass of the adsorbed lm and the kinetics of adsorption are inuenced signicantly by the molecule–substrate interactions. The role of surface chemistry is therefore crucial and, particularly, that of surface charge type and density. The important case of polyelectrolytes adsorbing on surfaces of Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland. E-mail: alberto.tiraferri@ unige.ch; Tel: +41 22 379 6421 † Electronic supplementary 10.1039/c4sm02093f
information
9220 | Soft Matter, 2014, 10, 9220–9225
(ESI)
available.
See
DOI:
opposite charge is well understood.1 In contrast, while the adsorption of polyelectrolytes on surfaces of like charge has been predicted in the presence of multivalent counterions6,7 and has been observed in several engineering systems,8–10 the mechanistic understanding of these phenomena is limited. As polyelectrolytes oen adsorb even in the absence of electrostatic attraction between chains and the adsorbing substrate, other energetic and entropic contributions may also play an important role.2,11,12 More efforts should be devoted to understand the mechanism of adsorption of polyelectrolytes on uncharged surfaces or on substrates bearing the same sign of charge as the polymer chain. A good way to probe the effect of surface chemistry is to use self-assembled monolayers (SAMs) with tailored terminal functionalities. SAMs have been extensively deployed to investigate the interactions of proteins with organic surfaces,13 a method that allowed identication of the functionalities that resist protein adsorption.14 This technique can be transferred directly to characterize the polyelectrolyte–surface interactions that inuence the development of adsorption layers.15 In this study, we demonstrate the suitability of gold–thiol SAMs in an optical reectivity instrument with well-dened hydrodynamics. SAMs of different hydrophobicity and charging properties were fabricated and characterized. These substrates were used to systematically study the role of surface chemistry on adsorption behavior of two representative strong polyelectrolytes, namely, anionic sodium poly(styrene sulfonate) (PSS) and cationic poly(diallyldimethyl ammonium chloride) (PDADMAC). We present the adsorbed amount and the adsorption kinetics of these charged polymers and we discuss these results in the light of the energetic interactions of the various polyelectrolyte–SAM combinations. The substrates used to fabricate the SAM lms consisted of an underlying layer of silicon, a middle adhesion layer of titanium, and an uppermost layer of sputter coated gold. The thickness of the gold layer was 8.5 0.5 nm, as determined by ellipsometry (Multiskop, Optrel, Berlin, Germany),5 while its root mean square surface roughness was approximately 0.5 nm.
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This value is signicantly lower than that of typical substrates used in the quartz crystal microbalance. The alkanethiol lms were adsorbed on the gold surfaces by immersing them overnight (>10 h) into a solution containing the corresponding thiol dissolved in ethanol (99.98%, VWR International, France).16 Solutions of 0.5 mM 11-mercapto-1-undecanol and 1-nonanethiol were used to form hydroxyl (OH) and methyl (CH3) terminated SAMs, respectively. Carboxyl (COOH) and amine (NH2) terminated SAMs were produced following an improved method described elsewhere.17 Briey, a solution of 0.5 mM 11mercaptoundecanoic acid with 2% (v/v) CF3COOH was used to form COOH SAMs, while solutions of 0.5 mM 11-amino-1undecanethiol hydrochloride with 3% (v/v) N(CH2CH3)3, were employed to fabricate NH2 SAMs. These lms were rinsed sequentially with ethanol, an ethanolic solution of 10% (v/v) NH4OH for carboxyl terminated lms or 10% (v/v) CH3COOH for amine terminated lms, and again ethanol. The quality of the SAM lms was assessed by measurement of the contact angle of water with the sessile drop method. The values of water contact angle evaluated for the four SAM types are reported in Table 1 and are consistent with reports in the literature.17–20 The wettability of SAM lms increased in the order CH3 < NH2 < COOH < OH. The surface potential of the SAM lms was estimated by means of the colloidal probe technique based on the atomic force microscopy. This technique consists on attaching a colloidal particle to a tipless cantilever and measuring the interaction forces between this particle and the investigated substrates. Sulfate latex beads with a diameter of 3 mm (Invitrogen) were used as probes. Measurements were conducted with a closed-loop atomic force microscope (MFP-3D, Asylum Research).21 Initially, the charging properties of the particle were calibrated by means of force measurements against a similar particle. The calibrated probe was then used to measure interactions with the planar SAM surface. The experimental forces as a function of probe–surface distance were tted using the Poisson–Boltzmann theory with the constant regulation approximation, to determine the electric potential of the monolayer lms in aqueous solution.21,22 Experiments were conducted in 1 mM NaCl at pH 4 and at pH 10. Under these conditions, the probe–substrate interaction was dominated at large distances by electrostatic forces. Therefore, we neglected other forces, such as Van der Waals or hydrophobic/hydrophilic interactions. The average of three measurements for the various SAMs is presented in Table 1. More details about this theory and the respective calculations can be found in the ESI† and in a previous publication.21 The potentials estimated for the surfaces functionalized with amine and carboxyl groups are consistent with the ionization behavior reported in the literature for similar thiol lms.23–28 All literature studies denote broadening of the ionization curves for the moieties of these monolayers compared to the respective free organic groups in aqueous solution. The retarded dissociation has been ascribed primarily to the electrostatic interaction and to hydrogen bonding of the neighboring ionized groups at the monolayer/solution interface.19,23–30 A negative surface potential was also observed for the two nominally uncharged
This journal is © The Royal Society of Chemistry 2014
Soft Matter Table 1 Contact angle of deionized water (pH 5.6) and surface potential of the SAM films (1 mM NaCl) used for adsorption experiments
Surface potential (mV) Surface
Water contact angle (deg)
pH 4
pH 10
Gold OH NH2 CH3 COOH
75 7 COOH > OH with sticking coefficients of 0.43, 0.36, 0.34, respectively; see also Fig. 1a and c. This trend is consistent with the variation in surface hydrophobicity. Accordingly, the development of the cationic PDADMAC layer was the slowest on the positive amine SAM; see Fig. 1b and d. The values of the sticking coefficient for the adsorption of PDADMAC in 10 mM NaCl were 0.35, 0.21, and 0.24 for CH3, OH, and NH2 SAMs, respectively. Similar conclusions can be drawn from Fig. 3c and d, which presents the adsorption rate coefficient as a function of prevailing salt concentration. In the presence of polyelectrolyte– substrate electrostatic attraction, deposition is favorable, and the measured adsorption rate was nearly uniform or decreased with increasing salt concentration. This reduction was caused by competition between ions originating from the background
Soft Matter, 2014, 10, 9220–9225 | 9223
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solution and the polyelectrolytes.40 On the contrary, when electrostatic repulsion dominated the interaction between polymeric chains and the substrates, higher ionic strength produced faster kinetics of adsorption. This result is rationalized with a decrease in the magnitude of the kinetic barrier for deposition with increased concentration of electrolyte in solution, due to compression of the double layer around the chains and at the surface/water interface. When this energy barrier was eliminated in high salt, van der Waals forces and other attractive interactions resulted in favorable deposition. Interestingly, while the mass of PSS adsorbed on COOH and OH lms increased signicantly in 100 mM, the related kinetics were reduced. Deposition of chains from solution onto the previously adsorbed layer might also occur at high salt, due to screening of the intermolecular electrostatic repulsion. This phenomenon may be responsible for the slow development of PSS layers on these two surfaces observed at the highest ionic strength. Self-assembled monolayers were applied to study the role of surface charge and hydrophobicity on adsorption of polyelectrolytes to solid substrates. Small changes of adsorbed mass, in the order of 103 mg m2 s1, were successfully probed, demonstrating the applicability of optical reectivity on these substrates. While gold surfaces are routinely used in surface plasmon resonance instruments, results presented here may potentially enlarge the pool of substrates and surface chemistries that can be investigated by optical reectometry, which represents a more exible and easily accessible technique. In particular, SAMs are remarkable substrates because their customized surface chemistry allows the systematic determination of the different energetic interactions governing polymer adsorption. In this study, charged polymers were shown to adsorb on all the different surfaces signicantly, thereby providing also some initial insight on the mechanism of adsorption of polyelectrolytes on surfaces of like charge. In particular, results suggest that adsorbed amounts were direct result of chain–chain interactions. These interactions were governed by the size of the electric double layer around the charged chains, which in turn was affected by the electric potential of the substrate and by the prevailing electrolyte concentration. Kinetics of adsorption were inuenced predominantly by the electrostatic interactions between polymeric chains and the adsorbing substrates. In the presence of electrostatic attraction, the adsorbed mass and the kinetics were the largest. If polyelectrolytes adsorbed on substrates with the same sign of potential, the kinetic barrier to deposition was reduced by higher ionic strengths, thus increasing the rate of adsorption signicantly. Surface hydrophobicity was observed to promote adsorption of polyelectrolytes, via interaction with the nonpolar segments of the polymeric chain. In this study, anionic PSS and cationic PDADMAC were chosen as examples of strong polyelectrolytes. The SAM-reectivity setup may be further applied to investigate the adsorption behavior of weak polyelectrolytes, uncharged polymers, or polymers characterized by different hydrophobicity. This research would further elucidate the various contributions to the driving force for adsorption of macromolecules on solid surfaces, as well as more clearly underline the role of nonelectrostatic interactions.
9224 | Soft Matter, 2014, 10, 9220–9225
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Acknowledgements This work was supported by a Marie Curie Intra-European Fellowship to A.T. within the Seventh European Community Framework Programme (PIEF-GA-2012-327977), by the Swiss National Science Foundation, and by the University of Geneva.
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