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Concomitant Adsorption of Poly(ethylene oxide)-b-poly(E-caprolactone) Copolymers and Sodium Dodecyl Sulfate at the Silica-Water Interface P. Vangeyte,† B. Leyh,‡ C. De Clercq,‡ L. Auvray,§ A.-M. Misselyn-Bauduin,| and R. Je´roˆme*,† Center for Education and Research on Macromolecules, University of Lie` ge, Sart-Tilman B6a, B-4000 Lie` ge, Belgium, Laboratoire de Dynamique Moleculaire, University of Lie` ge, Sart-Tilman B6c, B-4000 Lie` ge, Belgium, Laboratoire Le´ on Brillouin (CEA-CNRS), Saclay, 91 191 Gif sur Yvette, France, Laboratoire des Mate´ riaux Polyme` res aux Interfaces, University of Evry, Baˆ t. des Sciences, 91 025 Evry Cedex, France, and Universite´ Catholique de Louvain, Place de l’Universite´ 1/B4, 1348 Louvain-la-Neuve, Belgium Received December 1, 2004. In Final Form: June 21, 2005 Upon addition of silica to aqueous solutions of poly(ethylene oxide)-b-poly(-caprolactone) copolymers (PEO-b-PCL) and sodium dodecyl sulfate (SDS), adsorption of the solutes occurs at the silica-water interface. The amount of the adsorbed constituents has been measured by the total concentration depletion method. Small-angle neutron scattering experiments (SANS) have been carried out to investigate the structure of the adsorbed layer. Although SDS is not spontaneously adsorbed onto hydrophilic silica, adsorption is observed in the presence of PEO-b-PCL diblocks, in relation to the relative concentration of the two compounds. Conversely, SDS has a depressive effect on the adsorption of the copolymer, whose structure at the interface is modified. Copolymer desorption is however never complete at high SDS content. These observations have been rationalized by the associative behavior of PEO-b-PCL and SDS in water.
Introduction Polymer-surfactant complexes and their behavior at solid/liquid interfaces play a key role in several industrial applications, such as enhanced oil recovery, detergency, cosmetics, etc.1 These systems are controlled by the complex interplay of concomitant phenomena, including micellization of the surfactant, association of the surfactant with the polymer, and adsorption of the species formed in solution at the solid surface. Two main situations have to be distinguished. First, the polymer and the surfactant do not interact mutually in water and they adsorb spontaneously at the interface. This situation of competitive adsorption prevails when the polymer and the surfactant bear electrical charges of the same sign and adsorb on an oppositely charged surface.2 Weakly interacting systems, such as nonionic polymer mixed with a cationic surfactant, e.g., poly(ethylene oxide), PEO, with dodecyltrimethylammonium bromide, DTAB, are also involved in competitive adsorption on silica,3 which modulates the structure of the solid* To whom correspondence should be addressed. E-mail:
[email protected]. † Center for Education and Research on Macromolecules, University of Lie`ge. ‡ Laboratoire de Dynamique Moleculaire, University of Lie ` ge. § Laboratoire Le ´ on Brillouin (CEA-CNRS) and Laboratoire des Mate´riaux Polyme`res aux Interfaces, University of Evry. | Universite ´ Catholique de Louvain. (1) (a) Goddard, E. D. Applications of Polymer-Surfactant Systems. In Interactions of Surfactants with polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds; CRC Press: Boca Raton, 1993; Chapter 10. (b) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley & Sons Ltd: Chichester, 1998. (c) Goddard, E. D.; Ananthapadmanabhan, K. P. Applications of Polymer-Surfactant Systems. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Chapter 2. (2) Moudgil, B. M.; Somasundaran, P. Colloids Surf. 1985, 13, 87. (3) Esumi, K.; Iitaka, M.; Koide, Y. J. Colloid Interface Sci. 1998, 208, 178.
liquid interface and thus the stability of the silica dispersion in water. In the second case, the polymer interacts with the surfactant in water, which perturbs the adsorption profile of each compound. For instance, Maltesh et al. observed that the adsorption of PEO on alumina was promoted by sodium dodecyl sulfate (SDS), and that the SDS adsorption on silica was favored by PEO,4 as a result of the polymer-surfactant complexation at the interface. In this respect, the concentration regime for the polymer and the surfactant as well as the mixing procedure (addition order of the constituents) plays an important role.2,4a,5 The behavior of polymer-surfactant pairs at solidliquid interfaces has been reviewed by Misselyn-Bauduin.6 Additional studies have been published recently about interaction of surfactants with polyelectrolytes7 and nonionic homopolymers8 in water, in the presence of solid surfaces. However, only few examples of amphiphilic polymers and surfactants have been examined under the same conditions. Adsorption of hydrophobically modified polyelectrolytes and cellulose has been studied at different solid-water interfaces (polystyrene latex, mica, or silica) in the presence of several surfactants (sodium dodecyl (4) (a) Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1992, 153, 298. (b) Maltesh, C.; Somasundaran, P.; Ramachandran, R. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1990, 45, 329. (5) (a) Dedinaite, A.; Claesson, P. M.; Bergstro¨m, M. Langmuir 2000, 16, 5257. (b) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951. (6) Misselyn-Bauduin, A. M. Polymer-Surfactant Interactions: Their Behavior at Solid/Liquid Interface. In Handbook of Detergents Properties; Zoller, U., Broze, G., Eds.; Marcel Dekker: New York, 1999; Vol. 2, p 157. (7) (a) Maurdev, G.; Gee, M. L.; Meagher, L. Langmuir 2002, 18, 9401. (b) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366. (c) Dedinaite, A.; Bastardo, L. Langmuir 2002, 18, 9383. (8) (a) Mears, S. J.; Cosgrove, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 997. (b) Otsuka, H.; Ring, T. A.; Li, J. T.; Caldwell, K. D.; Esumi, K. J. Phys. Chem. B 1999, 103, 7665. (c) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999, 149, 329. (d) Fleming, B. D.; Wanless, E. J.; Biggs, S. Langmuir 1999, 15, 8719.
10.1021/la047051k CCC: $30.25 © 2005 American Chemical Society Published on Web 07/23/2005
Concomitant Adsorption of PEO-b-PCL and SDS
sulfate (SDS), pentaethylene oxide mono n-dodecyl ether (C12E5), tetradecyltrimethylammonium bromide (TTAB), and hexadecyltrimethylammonium bromide (CTAB)).9 In another example, a PEO-b-PPO-b-PEO amphiphilic block copolymer (Pluronics) has been adsorbed at the silicawater interface in the presence of SDS.10 A major conclusion is that the adsorption is influenced by the polymer-surfactant aggregation in water. This work aims at investigating the behavior of amphiphilic poly(ethylene oxide)-b-poly(-caprolactone) (PEOb-PCL) diblock copolymers, upon addition of sodium dodecyl sulfate (SDS), at the silica-water interface. Previous investigations have shown that these diblocks form polydispersed micelles in water in equilibrium with free polymer chains.11 These micelles consists of a hydrophobic PCL core surrounded by a shell of extended PEO chains.11 Addition of a small amount of SDS to PEOb-PCL aqueous solutions results in mixed copolymer/ surfactant aggregation. Upon increasing the SDS concentration, the copolymer/surfactant aggregates are progressively and ultimately disassembled.12 In contrast to SDS, which does not adsorb on hydrophilic silica in water,4a a monolayer of copolymer micelles is adsorbed at the silica-water interface possibly with interstitial free chains. Compared to poly(ethylene oxide), the amount of the copolymer adsorbed is higher.13 In this work, emphasis has been placed on the adsorption of PEO-b-PCL/SDS mixtures in water at the surface of silica. Quantitative data have been collected by the total concentration depletion method. Small-angle neutron scattering (SANS) experiments have also been performed in order to investigate the structure of the adsorbed layer, and the data have been analyzed according to a formalism detailed elsewhere.13 Experimental Section Materials. The amphiphilic PEO-b-PCL copolymers were synthesized by converting the hydroxyl end-group of PEO into an Al alkoxide, thus a macroinitiator for the ring opening polymerization of -caprolactone. The synthesis was detailed elsewhere.14 SDS, high purity grade, and SDSd25 (98%D) were supplied by BDH and Aldrich, respectively, and used without further purification. Porous silica X015M was purchased from Biosepra and used as received. The particle size was in the 40 to 110 µm range, the specific area was 31 m2/g, and the average pore size was 1260 Å. The copolymer was dissolved in water (0.1 wt %) by heating at 82 °C for 12 min under vigorous stirring, and the solution was maintained at room temperature for 18 h.11 For a 0.1 wt % SDS solution, 20 mg of SDS was added by the appropriate amount of the copolymer solution (20 g), and the mixture was stirred for at least 24 h and left at rest for an additional 24 h. A mixture of H2O/D2O (40/60 wt/wt) was substituted for water in case of SANS experiments, and SDS was replaced by a mixture of protonated and deuterated SDS (SDSd/SDS ) 54/46 wt/wt), for contrast matching purposes (cf. infra). (9) (a) Shubin, V. Langmuir 1994, 10, 1093. (b) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polym. Sci. 1994, 272, 1590. (c) Dedinaite, A.; Claesson, P. M.; Nygren, J.; Iliopoulos, I. Prog. Colloid Polym. Sci. 2000, 116, 84. (d) Joabsson, F.; Thuresson, K.; Blomberg, E. Langmuir 2001, 17, 1506. (e) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499. (f) Lauten, R. A.; Kjøniksen, A. L.; Nystro¨m, B. Langmuir 2000, 16, 4478. (10) Braem, A. D.; Prieve, D. C.; Tilton, R. D. Langmuir 2001, 17, 883. (11) Vangeyte, P.; Leyh, B.; Heinrich, M.; Grandjean, J.; Bourgaux, C.; Je´roˆme, R. Langmuir 2004, 20, 8442. (12) Vangeyte, P.; Leyh, B.; Auvray, L.; Grandjean, J.; MisselynBauduin, A. M.; Je´roˆme, R. Langmuir 2004, 20, 9019. (13) Vangeyte, P.; Leyh, B.; Rojas, O. J.; Claesson, P. M.; Heinrich, M.; Auvray, L.; Willet, N.; Je´roˆme, R. Langmuir 2005, 21, 2930. (14) Vangeyte, P.; Je´roˆme, R. J. Polym. Sci, Part A, Polym. Chem. 2004, 42, 1132-1142.
Langmuir, Vol. 21, No. 17, 2005 7711 Adsorption. The amount of each component adsorbed onto silica was determined by the total concentration depletion method. Typically, 100 mg of silica was immersed in 2 mL of a polymer-surfactant mixture and gently shaken for 48 h. The silica particles were then separated by sedimentation, and both the PEO-b-PCL and SDS concentrations were determined by 1H NMR by using peaks at 3.65 ppm for the copolymer (ethylene oxide protons), at 0.87 ppm for SDS (terminal methyl protons), and at 5.3 ppm for methylene chloride (internal reference in a coaxial NMR tube (Aldrich) at constant 0.15 wt % concentration). A calibration curve was set up for each experiment. A delay of 5 s between each sequence of pulses was enough for the relaxation to be complete. Small-Angle Neutron Scattering (SANS). SANS was carried out with the PACE diffractometer at the Laboratoire Le´on Brillouin in Saclay (France), with a wavelength of 5 Å, a wavelength resolution of 10%, and a beam diameter of 10 mm. Sample-to-detector distances of 2.8 and 4.7 m allowed us to cover a range of scattering vectors, q, of 0.014-0.15 Å-1 and 0.0040.044 Å-1, respectively. The detector consists of 30 concentric rings of 1 cm width. Samples were prepared as for adsorption analysis. After separation, the silica particles were rinsed two times with a 40/60 wt/wt H2O/D2O mixture in order to eliminate nonadsorbed copolymer chains. They were finally added into a quartz cell (2 mm path length) as a suspension in H2O/D2O. The electronic and ambient background noise and the sample holder contribution were subtracted from the scattered intensity by standard procedures. The scattered intensities were converted into macroscopic scattering cross sections per unit volume (unit: cm-1) by using the incoherent scattering of water. The polymer contribution to the scattering cross section was determined by subtracting the cross section for wet bare silica weighed by its volume fraction. The intensity scattered by a quaternary system (silica/ copolymer/SDS/solvent), I(q), contains several partial inter- and intramolecular correlation terms.15 However, the contribution to I(q) of one constitutive component can be suppressed by tuning the scattering length density of the solvent with respect to that of this component, thus by adjusting the composition of H2O/ D2O mixtures. The contrast of silica was matched with a 40/60 (wt/wt) H2O/D2O mixture. Similarly, the silica/SDS contrast was matched using a 46/54 (wt/wt) SDS/deuterated SDS mixture.16 This allowed us to investigate selectively the structure of the adsorbed copolymer. SANS data can be analyzed on the basis of two models,13 i.e., adsorption of copolymer chains with formation of a swollen PEO layer around the silica particles, and adsorption of a monolayer of copolymer micelles at the silica/water interface. This latter model, detailed elsewhere,13 takes into account both the micellar form factor P(q) and the bidimensional organization within the micellar monolayer. The scattering cross section, dΣ/dΩ, is given by eq 1-3.
dΣ (q) ) hIpp(q) + I˜pp(q) dΩ
(1)
The profile term, hIpp, is equal to
( )[
S Γ hIpp(q) ) 2π V M h
2
]
2 M h1 M h2 P(q) (n - ns) + (n - ns) (2) d1 1 d2 2 q2
The correlation term, I˜pp, can be expressed by eq 3:
I˜pp(q) )
( )[
]
2 M h2 M h1 (n1 - ns) + (n2 - ns) P(q) d1 d2 Nmic σ π dθ sin θ 1 - 2π dF FJ0(qF sin θ) 0 S 0
πσ2 S Γ 2 V M h
∫
2
[
∫
]
(3)
P(q) is the form factor of the single micelles, normalized to 1 at q ) 0. J0(x) is the zero-order Bessel function. Nmic/S is the number (15) (a) Higgins, J. S.; Benoıˆt, H. C. Polymers and Neutron Scattering; Oxford University Press: Oxford, 1996. (b) Auvray, L. Scattering by Polymers at Interfaces. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, T., Eds.; North-Holland: Amsterdam, 1991.
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Vangeyte et al.
Table 1. Molecular Characteristics of the PEO-b-PCL Block Copolymers Used in This Study samples
Mn EO/CL EO/CL Mn PEOa PCLb NAc NBc molar % wt %
PEO114-b-PCL8 5000 PEO114-b-PCL19 5000
950 114 2200 114
8 19
93 86
83/17 69/31
polydispersity 1.10 1.14
a Determined by SEC with PEO standards. b Calculated from the copolymer composition (1H NMR) and Mn of PEO. c NA and NB are the numbers of monomer units for the PEO and PCL blocks, respectively.
of micelles adsorbed per unit surface. S/V is the specific interfacial area (total interfacial area divided by the sample volume). Γ is the adsorbed copolymer mass per unit surface. The indices 1 and 2 refer to the hydrophobic and hydrophilic blocks, respectively. h 2 are the molar masses of the copolymer, the M h, M h 1, and M hydrophobic block, and the hydrophilic block, respectively. di is the density and ni the scattering length density of the block i. Several pieces of information can be extracted from the fitting of the micellar model to the experimental macroscopic cross sections by a nonlinear least-squares procedure: (i) the structural parameters of the micelles (core radius and corona thickness), (ii) the average intermicellar distance (σ), and (iii) the adsorbed mass per unit surface (Γ). At small q values, P(q) may be described by the Guinier’s law, which provides the radius of gyration, Rg, of the micelles. Alternatively, if a core-shell model is used in the whole q range, both the core radius (R) and the corona thickness (L) can be inferred.17,18 In the absence of SDS, our previous experiments confirmed the validity of the micellar model.13
Results The reciprocal influence of SDS and poly(ethylene oxide)-b-poly(-caprolactone) copolymers on their respective adsorption profile at the silica/water interface has been studied by measurement of adsorption isotherms and by SANS experiments. Two diblock copolymers that differ on the length of the PCL block, i.e., PEO114-b-PCL19 and PEO114-b-PCL8, have been studied. Their molecular characteristics are listed in Table 1. Because the pH of the aqueous solutions of the copolymer-surfactant mixtures is systematically in the 5.5 to 6.2 range, the net electric charge on silica is negative (isoelectric point at pH ) 2.5).19 Maltesh et al. have studied the adsorption of PEO and SDS on silica. The total amount of adsorbed material depends on whether PEO and SDS are premixed or are adsorbed in a sequential manner,4a consistent with other observations on the addition sequence of the adsorbates.5a,9 In this study, however, the adsorption was systematically performed with preformed PEO-b-PCL/SDS solutions. Adsorption of PEO-b-PCL. The adsorption isotherms of PEO114-b-PCL19 on silica have been recorded in the presence of SDS. Figure 1 illustrates how these isotherms depend on the SDS concentration (0.1 and 1 wt %). SDS concentration has been chosen on the basis of previous results about the assembling of SDS and the diblock copolymer in water.13 At low SDS concentration, SDS interacts not only with the copolymer micelles but also with the copolymer unimers, whose concentration is however low in relation to a low cmc. At higher concentration, SDS triggers the progressive disruption of the (16) (a) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4997. (b) Marshall, J. C.; Cosgrove, T.; Jack, K. Langmuir 2002, 18, 9668. (17) (a) Lin, Y.; Alexandridis, P. J. Chem. Phys. B 2002, 106, 12124. (b) Yang, L.; Alexandridis, P.; Steyler, D. C.; Kositza, M. J.; Holzwarth, J. F. Langmuir 2000, 16, 8555. (c) Pedersen, J. S. Adv. Colloid Interface Sci. 1997, 70, 171. (18) Fati, D. Master Thesis, University of Lie`ge, 1999. (19) Hunter, R. J. Applications of Zeta Potential. In Zeta Potential in Colloid Science. Principles and Applications; Ottewill, R. H., Rowell, R. L., Eds.; Academic Press: London, 1981; Chapter 6, p 219.
Figure 1. Adsorption isotherms on silica for neat PEO114-bPCL19 (4), and for PEO114-b-PCL19 added with 0.1 wt % SDS ([) and 1 wt % SDS (*). Measurements by the total concentration depletion method.
copolymer micelles and, ultimately, copolymer unimer/ SDS hybrid dominates (Scheme 1). Figure 1 shows that, at a concentration lower than 0.05 wt %, the copolymer adsorption is slightly enhanced by 0.1 wt % SDS. At higher copolymer concentration, 0.1 wt % SDS has a depressive effect on Γmax of the neat diblock. A 10-fold increase in the SDS concentration dramatically restricts the PEO114-bPCL19 adsorption, over a very large concentration range. Only at copolymer concentration higher than 0.4 wt %, the adsorbed amount starts to increase, although it remains 4-5 times lower than Γmax in the absence of surfactant (1.6 mg/m2).13 In a second series of experiments, the SDS concentration has been changed at constant concentration of the diblock copolymer (Figure 2). Two copolymer concentrations (0.15 wt % and 0.5 wt %) have been selected on the basis of the adsorption isotherms for neat copolymer (Figure 1). Indeed, the maximum of copolymer adsorption (1.6 and 1.1 mg/m2 for PEO114-b-PCL19 and PEO114-b-PCL8, respectively) is observed when the initial concentration is 0.5 wt % (i.e., an equilibrium concentration of 0.2 wt %), whereas half-saturation is noted for an initial concentration of 0.15 wt % (i.e., an equilibrium concentration of 0.03 wt %). As a rule, the addition of low amounts of SDS results in a rapid and important decrease in the amount of adsorbed copolymer (Figure 2), which however levels off at a SDS concentration that depends on the copolymer composition and content. The copolymer desorption is never complete as is the case for PEO.20 The PEO114-bPCL19 exhibits an unusual behavior, i.e., an initial increase in copolymer adsorption up to 0.3 and 0.15 wt % SDS for 0.5 and 0.15 wt % copolymer concentration, respectively. Nevertheless, the maximum adsorbed amounts (1.3 and 0.65 mg/m2) are lower than Γmaxcopo in the absence of SDS. Adsorption of SDS. Similarly, the adsorption of SDS has been measured in the presence of PEO114-b-PCL19 at initial concentrations of 0.15 and 0.5 wt %, respectively. Although the adsorption of neat SDS at the silica/water interface is very low,4a the PEO114-b-PCL19 copolymer triggers the SDS adsorption. At low SDS concentration ( 0.3). A previous work showed that R ) 0.3 is the composition at which a transition between aggregated and disrupted PEO114-bPCL19 chains occurs. Figure 3 shows that a high amount of SDS is adsorbed only when the SDS content is 1 wt %. Another way to emphasize this situation consists of plotting SDS adsorption versus R (Figure 6). The high adsorption at large R values results from the destructuration of the copolymer aggregates by SDS with simultaneous adsorption at the silica-water interface. SDS has actually two opposite tendencies: it interacts with the copolymer micelles in solution which results in their disruption, and it is driven to the silica surface by the adsorbed copolymer. The micelle/SDS interaction is consistent with the well-known PEO/SDS interactions and the PCL/SDS interactions observed by NMR 2D-NOESY experiments.12 These combined interactions are responsible for the progressive disruption of the copolymer aggregates. The adsorption at the silica interface is more likely driven by hydrophobic interactions and by H-binding between Si-OH at the silica surface and PEO to which SDS is complexed. Depending on the relative concentration of each species, the system will favor one or the other situation in order to minimize the free energy of the system. It must be noted that, at R < 0.3, the amount of adsorbed SDS starts to decrease in agreement with the transition observed in the bulk.12 As far as the SANS experiments are concerned, the applicability of the micellar model has to be considered. Indeed, the intensity scattered by the SDS/PEO114-b-PCL19 mixtures is weak at R higher than 0.3, although a significant amount of copolymer is adsorbed (Figure 2). Therefore, the weak scattering does not indicate the absence of copolymer at the interface but rather the nonaggregation of the copolymer in interaction with SDS. Additional evidence for the close relation between the situation in bulk and at the interface may be found in the
Figure 6. Dependence of the amount of SDS adsorbed as a function of the SDS molar fraction (R) determined by the total depletion method, at constant SDS content of 1 wt %.
SDS/copolymer binding ratio calculated in solution and at the interface. The amount of copolymer and SDS adsorbed can be determined from Figures 2 and 3 at copolymer and SDS contents of 0.5 wt % and 1 wt %, respectively. It results that the binding ratio is 0.9, thus close to the 1.1 ratio found in solution.12 Finally, the adsorption of PEO114-b-PCL19 at very low SDS content is worth being noted. Indeed, a low amount of SDS slightly triggers the copolymer adsorption which is maximum at approximately 0.2-0.3 wt % SDS (Figure 2). This behavior contrasts with PEO/SDS/silica and PEOb-PPO-b-PEO/SDS/silica systems, for which the amount of polymer adsorbed decreases even at the lower SDS concentration.4a,10 Moreover, Esumi et al. reported the same type of behavior for the adsorption of poly(vinyl pyrrolidone)-lithium dodecyl sulfate-lithium perfluorooctanesulfonate mixtures onto alumina, graphite, and polystyrene latex.8,24 According to these authors, a mixed complex is preferentially formed at the interface at low surfactant content, prior to formation in the bulk. In this work, the interfacial situation is also closely related to the interactions occurring in the bulk. At low surfactant content, although the copolymer aggregation persists, the adsorption capacity of the copolymer aggregates changes as a result of interaction with small amounts of SDS. A higher affinity for the silica/water interface is noted, which accounts for a small increase in adsorption. This effect is more clearly observed for the PEO114-b-PCL19 diblock than for the PEO114-b-PCL8 copolymer. The PEO114-b-PCL19 diblock contains, indeed, a longer hydrophobic block at constant PEO length able to stabilize more efficiently the copolymer aggregates against SDS. In conclusion, the experimental observations reported in this work confirm that the adsorption of the diblock/ SDS mixtures at silica/water interface is dominated by the species formed in solution. Conclusions The adsorption of PEO-b-PCL/SDS mixtures at the silica-water interface has been investigated. Adsorption isotherms and SANS experiments have shown the crucial role of the copolymer/surfactant associations in the liquid phase on the adsorption at the interface. According to previous studies,12 the molar ratio (R) between SDS and the diblock is of prime importance for their mutual interaction in solution. Indeed, for PEO114-b-PCL19 at R < 0.3, SDS interacts with the copolymer aggregates and modifies them only to a small extent. In contrast, at R > (24) (a) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45. (b) Otsuka, H.; Esumi, K. J. Colloid Interface Sci. 1995, 170, 113.
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0.3, SDS is responsible for the disruption of the copolymer aggregates, and the released copolymer chains interact with the surfactant as it commonly occurs in case of homopolymer/surfactant pairs, such as the PEO/SDS and the PVP/SDS ones. The adsorption data are consistent with the aggregation/ deaggregation pattern observed in water. At R < 0.3, the adsorbed species are typically the copolymer aggregates although their adsorption capacity is slightly changed as a consequence of their interaction with SDS. As a rule, less copolymer is adsorbed compared to the absence of surfactant, and the adsorption of SDS is slightly higher than in the absence of copolymer. At R > 0.3, the amount of adsorbed copolymer further decreases but remains significant, whereas the adsorption of SDS strongly increases at higher SDS content. SANS experiments confirm that the copolymer is no longer adsorbed as aggregates in this concentration regime. The adsorbed layer at the silica/water interface thus consists of a relatively low amount of copolymer chains complexed by SDS similarly to what is observed for adsorbed homopolymer/SDS pairs, such as PEO/SDS and PVP/SDS.
Vangeyte et al.
This general behavior is illustrated in Scheme 1. However, two main differences must be noted, depending on whether SDS is added to a homopolymer (PEO, PVP) or a diblock (PEO-b-PCL): (i) ΓmaxSDS is higher in the presence of the diblock and (ii) desorption of the PEO-b-PCL copolymer is never complete in the presence of a huge excess of SDS although it is a common feature for adsorbed PEO and PVP, respectively. Acknowledgment. The authors are very much indebted to the “Fonds pour la Formation a` la Recherche dans l’Industrie et l’Agriculture” (FRIA) for a fellowship to P.V., to the “Belgian Science Policy” for financial support in the frame of the “Interuniversity Attraction Poles Programme (PAI V/03)sSupramolecular Chemistry and Supramolecular Catalysis”, and to the European CommunitysAccess to Research Infrastructures Action of the Improving Human Potential Program (HPRI) for financial support and access to facilities at the Laboratoire Le´on Brillouin (Saclay). LA047051K
