Langmuir 2003, 19, 9977-9980
Adsorption Properties of Polyethyleneimine on Silica Surfaces in the Presence of Sodium Dodecyl Sulfate Ro´bert Me´sza´ros,*,†,‡ Laurie Thompson,† Imre Varga,‡ and Tibor Gila´nyi‡ Unilever Research Port Sunlight, Quarry Road East, Bebington CH63 3JW, United Kingdom, and Department of Colloid Chemistry, Lora´ nd Eo¨ tvo¨ s University, P.O. Box 32, Budapest 112, H-1518, Hungary Received July 7, 2003. In Final Form: September 1, 2003
Introduction The surface characteristics of polymer/surfactant mixtures are crucial to understand, since these interfacial properties are directly connected to the benefit delivering capability of different products. The available literature is rather limited due to the experimental difficulties in the separate determination of polymer and surfactant adsorption properties in their mixed surface layer and the very complex theoretical treatment of these layers.1-11 In the case of oppositely charged polyelectrolytes and surfactants, the interfacial behavior is even more complicated.12 Shubin13,14 et al. investigated the surface properties of the aqueous solutions of SDS and a cationic, hydrophobically modified hydroxyethyl cellulose (quatrisoft), on mica and silica. The authors found irreversible characteristics and decreasing adsorption of the polymer with increasing surfactant concentration. Claesson et al. demonstrated with surface force methods and additional powerful techniques a good correlation between the bulk and interfacial properties and structure of oppositely charged amphiphiles and macromolecules.15-18 These works revealed that for the understanding of the mixed polyelectrolyte/ionic surfactant layers it is important to characterize the features of the bulk polyelectrolyte/ surfactant interaction as well as the individual adsorption properties of the polyelectrolyte onto the surface concerned. * Corresponding author. † Unilever Research Port Sunlight. ‡ Lora ´ nd Eo¨tvo¨s University. (1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 4. (2) Zana, R. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998; Chapters 1 and 10. (3) Argillier, J. F.; Ramachandran, R.; Harris, W. C.; Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242. (4) Harrison, I. M.; Meadows, J.; Robb, I. D.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3919. (5) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloids Surf., A 1997, 123, 283. (6) Moudgil, B. M.; Prakash, T. S. Colloids Surf., A 1998, 133, 93. (7) Fleming, B. D.; Wanless, E. J.; Biggs, S. Langmuir 1999, 15, 8719. (8) Mears, S. J.; Cosgrove, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 997. (9) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4997. (10) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999 149, 329. (11) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499. (12) Claesson, P. M.; Dedinaite, A.; Poptoshev, E. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Surfactant Science Series Vol. 99; Marcel Dekker: New York, 2001; pp 447-507. (13) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polym. Sci. 1994, 272, 1590. (14) Shubin, V. Langmuir 1994, 10, 1093.
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The different studies on the interaction of polyethyleneimine (PEI) with sodium dodecyl sulfate in aqueous solution have shown peculiar changes in the conductivity and pH,19-23 as well as revealed interesting aggregation behavior.24,25 As was demonstrated recently, the pH increase of the PEI solutions with increasing added surfactant concentration is the consequence of the charge density regulation of PEI due the noncooperative surfactant binding.25 It was also found that this system shows a unique conformational transition which correlates well with the observed phase properties of the PEI/SDS solutions. At low surfactant concentrations the PEI/SDS solution forms a thermodynamically stable, optically transparent system. With increasing surfactant concentration the PEI/SDS complexes shrink, and from a critical surfactant concentration the system can be considered as an unstable colloid dispersion of the collapsed polymer/ surfactant particles which coagulate in the precipitation concentration domain. Further increase in the SDS concentration can initiate the adsorption of the dodecyl sulfate (DS-) ions on the surface of the hydrophobic PEI/ DS- particles stabilizing electrostatically the dispersion of the polymer/surfactant particles in the postprecipitation concentration domain.25 Considering the individual adsorption properties of PEI, a comprehensive reflectometric study of the ionic strength and pH dependent adsorption as well as electrokinetic properties of aqueous PEI solutions on silica26 was undertaken recently. The results were interpreted according to a strong nonelectrostatic affinity of the PEI segments toward the silica surface. A significant overcompensation of the silica surface charge due to the adsorption of PEI was also observed.26,27 In this work we were interested in the adsorbed layer structure of PEI and its dependence on the presence of an oppositely charged surfactant. Therefore, the adsorption features of aqueous PEI/SDS solutions on silica surfaces were investigated by ellipsometry. Furthermore, these results were compared with the interfacial behavior of the SDS solutions on preadsorbed PEI layers. It will be shown that the properties of the mixed surface layers can be well interpreted via the individual adsorption properties of PEI as well as the characteristics of its interaction with SDS in the bulk solution. (15) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951. (16) Dedinaite, A.; Claesson, P. M.; Bergstrom, M. Langmuir 2000, 16, 5257. (17) Plunkett, M. A.; Claesson, P. M.; Rutland, M. W. Langmuir 2002, 18, 1274. (18) Claesson, P. M.; Fielden, M. L.; Dedinaite, A. J. Phys. Chem. B 1998, 102, 1270. (19) Van den Berg, J. W. A.; Staverman, A. Recl. Trav. Chim. (RECUEIL) 1972, 91, 1151. (20) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 1998, 14, 6101. (21) Bystryak, S. M.; Winnik, M. A.; Siddiqui, J. Langmuir 1999, 15, 3748. (22) Winnik, M. A.; Bystryak, S. M.; Chassenieux, V.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495. (23) Li, Y.; Ghoreishi, S. M.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 3093. (24) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5657. (25) Me´sza´ros, R.; Thompson, L.; Bos, M.; Varga, I.; Gila´nyi, T. Langmuir 2003, 19, 609. (26) Me´sza´ros, R.; Thompson, L.; Bos, M.; de Groot, P. Langmuir 2002, 18, 6164. (27) Poptoshev, E.; Rutland, M. W.; Claesson, P. M. Langmuir 2002, 18, 2590.
10.1021/la0352218 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003
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Experimental Section Materials. The PEI with a mean molecular weight of 7.5 × 105 g/mol was purchased from BASF in the form of a 33 wt % aqueous solution. The PEIs are hyperbranched polymers, containing the primary, secondary, and tertiary amine groups in a 1:2:1 ratio, with a wide molecular weight distribution, although no specific numbers are quoted for polydispersity.28 The sodium dodecyl sulfate was purchased from Sigma-Aldrich and was recrystallized from a 1:1 benzene/ethanol mixture. The cmc of the purified SDS was determined by surface tension measurements, and it was found to be 8.2 and 5.6 mM in water and 0.01 M NaCl, respectively, in good agreement with ref 29. The ACS reagents of HCl, NaOH (the carbonate free form), and the supporting electrolyte (NaCl) were also provided by SigmaAldrich. During the experiments ultraclean Millipore water was used for making solutions. As a surface substrate, strips of silicon wafers were used bearing thin layers of SiO2. The silica layers were made by thermal oxidation of the silicon wafer at 1000 °C for 30 min, resulting in a film thickness of 40 nm. The silica wafers were cleaned in concentrated persulfuric acid for 60 min. The purified samples were rinsed and then stored under Millipore water prior to the measurements. Ellipsometry. In this study an automated ELX-04 Minsearch ellipsometer (Riss EllipsometerBau GmbH, Ratzeburg, Germany) was used. This equipment monitors the ellipsometric angles, ψ and ∆, with an accuracy of 0.001° and 0.006°, respectively. All the measurements were performed at 22 ( 0.1 °C in a temperature controlled clean room at θ ) 70° angle of incidence and at the wavelength λ ) 632.8 nm. Assuming homogeneous, isotropic surface layers, the refractive index and thickness of the adsorbed layer (nads and dads, respectively) can be determined. The detailed description of this procedure and the ellipsometric technique can be found elsewhere.30-32 To determine the adsorbed layer properties, the optical constants of the solid substrate were characterized first in the ambient medium (salt or SDS solutions) without the adsorbing species. The solid phase was assumed to be formed by bulk silicon with a complex refractive index of N0 ) n0 - jk0 (where n0 and k0 are the real and imaginary parts of the complex refractive index of the silicon, respectively) and a transparent silica layer on it with a thickness of d1 and with a refractive index n1 (the imaginary refractive index of the silica layer k1 ≈ 0). The refractive index of the bulk solution (nb) was calculated according to the refractive indices of sodium chloride solutions, tabulated in ref 33, and by making use of the refractive index increment of SDS11 (dn/dc ) 0.12 cm3/g). n0, k0, n1, and d1 were determined by means of the supplier software assuming three isotropic, homogeneous planar layers (silicon, silica layer, ambient medium). (The fitted values were n0 ) 3.892 ( 0.006, k0 ) -0.04, n1 ) 1.474 ( 0.003, and d1 ) 42 ( 1 nm.) After the characterization of the substrate in the appropriate medium, the medium was quickly removed and then the polymer or the polymer/surfactant solutions (at the same pH, salt concentration, and SDS concentration as those in the medium) were immediately added into the cuvette with continuous stirring and the changes in the ellipsometric parameters were followed until a quasi-equilibrium (no further change in ψ and ∆) was reached. Practically, 60 min was ensured to meet this criterion. The experiments were performed in two different ways. In the first case the polyelectrolyte and the surfactant were added simultaneously into the cuvette at surfactant concentrations above and below the precipitation zone where the polymer/ surfactant solutions are transparent. In the other series of experiments the PEI was preadsorbed from aqueous solution for 30 min, and then, after careful rinsing with the medium, the surfactant solution was introduced into the cuvette. The indi(28) BASF literature on LUPASOL. (29) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36: Washington, DC, 1971; p 52. (30) Bain, C. D. Curr. Opin. Colloid Interface Sci. 1998, 3, 287. (31) Keddie, J. L. Curr. Opin. Colloid Interface Sci. 2001, 6, 102. (32) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927. (33) Huglin, M. B. Light Scattering from Polymer Solutions; Academic Press: London and New York, 1972; p 181.
Figure 1. Adsorbed layer thickness of PEI on silica as a function of pH in 0.01 and 0.1 M NaCl, respectively. The solid and dotted lines connect the experimental points in order to guide the eyes. cPEI ) 50 mg/dm3. vidual adsorption of SDS on silica was also tested, but no changes in the ellipsometric angles were detected within the experimental error in the investigated pH and ionic strength range. The refractive index and thickness of the adsorbed layer were calculated with the supplier software on the basis of a four layer model assuming optically isotropic and planar layers. In addition to the substrate silicon and the silica layer, the adsorbed layer and the bulk solution were introduced into the model. Having fixed the complex refractive index of the silicon and the refractive index and thickness of the silica layer (at the previously fitted values of n0, k0, n1, and d1), nads and dads can be numerically estimated via fitting the measured ψ and ∆ values to the calculated ones. On the basis of repeated measurements, a 10% error was found in the values of dads, probably due to the remaining inaccuracy of the silica substrate optical parameters. However, the reproducibility of the total optical adsorption, (nads - nb)dads, was much better, with an estimated 1% standard deviation. If there is more than one adsorbing component in the surface layer (in addition to the medium), then only the total optical adsorption, (nads - nb)dads, is well defined and the separation of this quantity into the individual adsorbed amounts could only be done by independent measurements of the adsorbed amount of one of the components or by making further assumptions.34 One should also keep in mind that the measured refractive index profile represents the inner, dense part of the adsorbed polymer layer.35 This means that the layer thickness predicted by ellipsometry is generally smaller than the corresponding hydrodynamic value obtained by light scattering or surface force measurements. Because of this, we will focus on the trends in the changes of dads and handle with care the absolute values of this quantity.
Results and Discussion In Figure 1 the adsorbed layer thickness of PEI is plotted against the pH at two ionic strengths. The surface layer thickness versus pH curves show a shallow minimum in both cases. As it is well-known, the charge density of the PEI and that of the silica surface vary conversely with the pH.26 At the high pH range, the silica surface is negatively charged whereas the PEI segments are slightly positively charged, which results in a relatively extended adsorbed layer. By lowering the pH, the protonation degree of the ethyleneimine groups increases; therefore, a significant flattening in the adsorbed layer takes place, especially at the lower ionic strength. A further decrease in pH, however, significantly lowers the surface charge density of silica, which results in a slight increase in the adsorbed (34) De Feijter, J. A.; Benjamin, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (35) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces, 1st ed.; Chapman and Hall: London, 1993; Chapter 7.
Notes
Figure 2. Total adsorption (9) and adsorbed layer thickness (O) of PEI/SDS solutions on silica as a function of the total SDS concentration without added supporting electrolyte. cPEI ) 50 mg/dm3, and pHin ) 9.7 At this pH the protonation degree of PEI Θ0 = 0.11. The pH was increased more than one pH unit at the highest surfactant concentration (to pH ) 10.8).25
layer thickness in the vicinity of the point of zero charge (pzc) of the surface (for silica the pzc is at pH = 226,36). These observations correlate well with the surface and polymer charge density dependent structural changes in the adsorbed layers of linear polyelectrolytes on oppositely charged surfaces,35 which is somewhat unexpected considering the hyperbranched structure of the PEIs. On the basis of their AFM pictures, Pfaff et al. suggested a bloblike PEI adsorbed layer on mica and polystyrene latexes where the adsorbed PEI molecules roughly keep their bulk lateral diameter but they significantly contract in their vertical direction to the surface.37 This interpretation fits with our findings and suggests a compact adsorbed layer structure, especially at low ionic strength and at highly charged states of the PEI and silica surface. Although the PEI sample is polydisperse, its mean hydrodynamic diameter can be estimated to be around 60 nm on the basis of dynamic light scattering measurements in 0.1 M NaCl. The observed, significantly smaller values of the adsorbed layer thickness can be in connection with the polydispersity of the PEI and/or with the pronounced contraction of the adsorbed PEI molecules even in the high pH ranges, due to the significant non-Coulomb affinity of this polyelectrolyte toward the silica. In Figure 2 the total adsorption and the adsorbed layer thickness are shown against the SDS concentration in those cases when the PEI and SDS added together to the silica at pHin ) 9.7. (pHin is the pH of the solution with the same PEI concentration but without SDS, with a protonation degree Θ0 = 0.11. Due to the addition of the surfactant, the pH increases to 10.8 at the largest surfactant concentration of Figure 2.25) As can be seen, a significant increase in the total adsorption takes place at lower SDS concentrations whereas above the precipitation concentration range the accumulation of the polymer and surfactant molecules in the adsorbed layer is small and gradually decreases with increasing surfactant concentration to nearly zero. The adsorbed layer thickness seems to be roughly constant within the experimental error in both concentration ranges, but it shows an approximate 2-fold decrease in the postprecipitation concentration domain compared to the low surfactant concentrations. (At intermediate surfactant concentrations precipitation occurs, which makes the optical measurements unfeasible.) (36) Behrens, S. H.; Grier, D. G. J. Chem. Phys. 2001, 115, 6716. (37) Pfau, A.; Schrepp, W.; Horn, D. Langmuir 1999, 15, 3219.
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These results can be interpreted if we assume that the PEI/SDS complexes, formed in the bulk solution, adsorb onto the silica surface. In the preprecipitation concentration range, the initial positive charges of the PEI are gradually compensated with increasing SDS concentration and the polymer/surfactant complex shrinks due to the binding of DS- ions.25 Therefore, the total adsorbed amount may increase due to the decreasing extent of the segment/ segment repulsion, the increasing hydrophobicity of the polymer/surfactant complex, and its decreasing size. On the other hand, in the postprecipitation domain the collapsed PEI/SDS particles possess a considerable negative surface charge density, which increases with increasing SDS concentration.25 Taking into account that the silica surface is pronouncedly negatively charged at high pH, there is a significant energy barrier hindering the adsorption of PEI/SDS complexes at this pH range. The addition of SDS also increases the bulk solution pH and, therefore, the silica surface charge density as well as the ionic strength, slightly influencing the repulsive interactions between the silica surface and the PEI/SDS particles. The adsorbed layer shows an approximate 2-fold shrinking in this concentration domain compared to the preprecipitation concentration range. This finding is in good agreement with the extent of the bulk solution collapse of the PEI/SDS complexes, which was detected by dynamic light scattering in the postprecipitation domain.25 In this concentration range no further change in the bulk hydrodynamic diameter of the collapsed PEI/ SDS particles was found with increasing surfactant concentrations. This observation explains why these compact adsorbed layers form and why their thickness is insensitive toward the surfactant concentration. In the studies of the interfacial behavior of polymer/ surfactant systems adsorbed on solid surfaces,11-18 significant irreversibility effects have been observed. The adsorbed layer structure and the adsorbed amount were found to significantly depend on the order of addition of the different components of the solutions. To test the effect of the order of addition on the adsorption features of the mixed PEI/SDS surface layers, measurements were also performed in those systems where surfactant adsorption took place onto the surface of PEI coated silica. In these series of experiments the PEI molecules were adsorbing first from PEI solutions on silica, which was followed by rinsing off the polymer solution with the medium of the same ionic strength and pH. No desorption was detected from the preadsorbed polymer layers in the time scale of the experiments (2 h). After that, the surfactant solution was added into the cell. In Figures 3 and 4 the total adsorption and the adsorbed layer thickness of the preadsorbed PEI layers are shown, respectively, as a function of the added surfactant concentration (at pH ) 5.8 and pH ) 9.9, in 0.01 M NaCl). This concentration of the added salt leveled out the ionic strength changes due to the increasing surfactant concentration in the experiments, but it was small enough to ensure the pronounced role of the electrostatic interactions. As can be seen in Figure 3, the total adsorption increases with increasing surfactant concentration according to a saturation-type adsorption isotherm which correlates with the monomer binding mechanism of the DS- ions to the PEI molecules.25 At pH ) 5.8 the adsorbed amount of the surfactant is considerably less than that at high pH. This observation can be explained by the lower adsorbed amount of PEI at this pH compared to the high pH conditions but also can be connected to the very compact structure of the PEI at this lower pH (Figure 1). This means that, because of the vertical contraction of the
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Figure 3. Optical, total adsorbed amount from SDS solutions on preadsorbed PEI layers as a function of SDS concentration. The PEI was adsorbing onto the silica wafer for 30 min, which was followed by a rinsing with the medium (0.01 M NaCl, O, pH ) 9.9; 9, pH ) 5.8). After that, the SDS solution was introduced at the same salt concentration and pH. cPEI ) 50 mg/dm3.
Figure 4. Adsorbed layer thickness of PEI/SDS layers on silica against the surfactant concentration under the same conditions as those in Figure 3. The PEI was adsorbing for 30 min, which was followed by a rinsing with the medium (0.01 M NaCl, O, pH ) 9.9; 9, pH ) 5.8). This was followed by the introduction of the SDS solution at the same salt concentration and pH. cPEI ) 50 mg/dm3.
adsorbed PEI molecules, the number of amine groups which are accessible to interact with the surfactant may significantly be reduced. These findings are in contrast with other observations where the total adsorbed amount was found to decrease
Notes
after the addition of SDS to the preadsorbed oppositely charged polyelectrolyte layers, indicating that desorption of the polymer took place.13,14 In our case the interaction with the SDS seems to be not efficient enough to unbind enough PEI segment contact from the silica surface and desorb the polymer into the solution phase (in the practical time scale of the experiments). The strong binding of the PEI to the silica surface is also demonstrated in Figure 4. The adsorbed layer thickness seems to be rather insensitive to the SDS concentration, and it is only slightly different from the individual layer thickness of PEI on silica at the same ionic strength and pH. As a summary it should be noted that these observations are in contrast with the recently reported surface behavior of polymer/surfactant mixtures where a significant reswelling of the preadsorbed polymer layers was found at higher surfactant concentrations due to the cooperative binding of the surfactant to the polymer.8-10,17 However, the binding of the dodecyl sulfate ions to the PEI was found to be basically noncooperative in the bulk solution,25 similarly to the adsorption mechanism of SDS on the PEI coated silica surface presented in the present paper. The preadsorbed PEI layers on silica were found to be very compact, and the ethyleneimine groups are irreversibly bound to the silica surface, which can prevent their further collapse or significant binding of the DS- ions within the adsorbed polymer. The thickness of the preadsorbed PEI layer was observed to be rather insensitive toward the presence of SDS. This finding suggests that the DS- ions might preferably bind to the outer surface shell of the bound PEI molecules, which did not cause detectable changes in the ellipsometric layer thickness due to the specific (possibly bloblike) structure of the mixed polymer/ surfactant layer. As shown above, the situation is different if the surfactant was adsorbing simultaneously with the PEI to the silica layer. In these cases the adsorbed layer shows a roughly 2-fold shrinking in the postprecipitation concentration range compared to the low surfactant concentration regime. This correlates with the extent of the collapse of the PEI/SDS complex in bulk solution which was recently detected by dynamic light scattering.25 Acknowledgment. This work was supported by the Hungarian Scientific Research Fund (No. T043621, No. F034838) and by the Ministry of Education (FKFP 0051/2001). LA0352218
