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Langmuir 2005, 21, 9938-9945
Non-Surface Activity and Micellization of Ionic Amphiphilic Diblock Copolymers in Water. Hydrophobic Chain Length Dependence and Salt Effect on Surface Activity and the Critical Micelle Concentration† Ploysai Kaewsaiha, Kozo Matsumoto, and Hideki Matsuoka* Department of Polymer Chemistry, Kyoto University, Kyoto 615-8510, Japan Received June 14, 2005 We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior “normal” for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.
Introduction The defining characteristic of low molecular weight surfactants is their ability to decrease the surface tension of the air-water solution interface. In a solution, surfactants are adsorbed from the water phase onto the air/ water interface to form the Gibbs monolayer and the surfactants in bulk (unimer) and adsorbed molecules are in dynamic equilibrium. The adsorption induces the surface excess, which consequently results in the decrease of the surface tension. This is a common behavior for surface-active molecules, which can be described by the Gibbs adsorption isotherm.1 For nonionic amphiphilic diblock copolymers, a similar observation has been reported.2 Amphiphilic diblock copolymers with one polyelectrolyte block and one hydrophobic block have been viewed as ionic polymer surfactants,2 and their micelle formation behavior in solution has been investigated.3 Their solutions and interfacial characteristics have been investigated theo* To whom correspondence should be addressed. E-mail:
[email protected]. † Bob Rowell Festschrift. (1) Adamson, A. W. The Physical Chemistry of Surface, 4th ed.; Wiley: New York, 1982. (2) (a) Amphiphilic Block Copolymers-Self-Assembly and Application; Alexandridis, P., Lindman, B., Eds.; Elsevier: Amsterdam, 2000. (b) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (c) Hamley, I. W. Introduction to Soft Matter; Wiley: Chichester, U.K., 2000.
retically 4 and experimentally 5 in the past few years. The adsorption of hydrophobic blocks at the air/water interfaces gives rise to monolayers, in which the hydrophobic block adsorbed on the surface and water-soluble polyelectrolyte brushes dangling into the solution.6 Alternatively, the amphiphilic diblock copolymers can associate to form micelles with different morphologies in the solution.2 The self-assembly formation in aqueous solution at a concentration above the critical micelle concentration (cmc) results in the formation of micelles consisting of a hydrophobic core formed by hydrophobic blocks and the extended charged corona, which ensure solubility of micelles in water. 7-11 The effects of hydrophobic and hydrophilic chain lengths on cmc were duly investigated (3) Moffitt, M.;, Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (4) (a) Wittmer, J.; Joanny, J. F. Macromolecules 1993, 26, 2691. (b) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (c) Borisov, O. V.; Zhulina, E. B. Macromolecules 2002, 35, 4472. (5) (a) Fo¨ster, S.: Hermsdorf, N.; Leube, W.; Schnablegger, H.; Lindner, P.; Bo¨tcher, C. J. Phys. Chem. B 1999, 103, 6657. (b) Gabaston, L. L.; Furlong, S. A.; Jackson, R. A.; Armes, S. P. Polymer 1999, 40, 4505. (6) (a) Su, T. J.; Styrkas, D. A.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 6892. (b) Ahren, H.; Fo¨rster, H.; Helm, C. A. Macromolecules 1997, 30, 8447. (c) Mouri, E.; Matsumoto, K.; Matsuoka, H. J. Polym. Sci., Part B: 2004, 1921. (7) (a) Weber, S. E. J. Phys. Chem. B 1998, 102, 2618. (b) Guenoun, P.; Muller, F.; Delsanti, M.; Auvray, L.; Chen, Y. J.; Mays, J. W.; Tirrel, M. Phys. Rev. Lett. 1998, 81, 3872. (c) Fo¨rster, S.; Hemsdorf, N.; Bo¨ttcher, C.; Lindner, P. Macromolecules 2002, 35, 4096.
10.1021/la051584r CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005
Non-Surface Activity and Micellization
for polystyrene-b-poly(acrylic acid), and large and negligible effects were found, respectively.12 In addition, the structural change of their micelles and vesicles by solvent power tuning was also systematically studied.13,14 A unique self-assembly phenomenon, a surface micelle on water (sometimes called “jelly fish micelle”) was also found for amphiphilic ionic diblock copolymers.15,16 We have been investigating amphiphilic block copolymers from synthesis, structure and to self-assembling behavior for nonionic,11 weakly ionic,8 and strongly ionic9,10 block copolymers. Recently, we found a very unique selfassembling behavior of strongly ionized amphiphilic copolymers, that is, micellization without adsorption at the air/water interface.9,10 A similar observation was also found for amphiphilic diblock copolymers having a weak polyelectrolyte, which was fully neutralized, as a hydrophilic block.8 It is currently accepted that the common surfactants or amphiphiles are first adsorbed at the water surface to form the Gibbs monolayer and then form micelles in the solution above cmc. As a result, the surface tension for a surfactant solution has distinctive concentration dependence. With the increase in the concentration of surfactant, the surface tension of the solution decreases rapidly from the value for pure water (γ ) 73 mN/m) until a point of cmc at which it levels off and becomes almost independent of concentration. In contrast, the ionic block copolymers did not reduce the surface tension of the solution and only very small amounts of polymer adsorbed at air/water interface in the absence of salt, which was confirmed by X-ray reflectivity (XR). Nevertheless micelle formation was observed in the solution.9,10 This would be a totally new observation in the field of physical chemistry of surfaces and interfaces. The origin of non-surface activity, and hence the near lack of adsorption at the water surface was attributed to the image charge effect, which is a unique Coulomb effect only at interfaces, not act in bulk phase.9,10 On the other hand, a monolayer of amphiphilic diblock copolymer having strong acid groups, poly(hydrogenatedisoprene)-b-poly(styrene sulfonic acid) (PIp-h2-b-PSSNa) on the water surface has been investigated in detail by in situ X-ray reflectivity technique as a function of surface pressure, hydrophilic chain length, and salt concentration.17 In this work, we found a very interesting nanostructure of polyelectrolyte brush at the air/water interface, i.e., carpet/brush double layer structure. Moreover, it was also found that the monolayer is very stable against salt; the effect of added salt occurs only when the salt concentration in the bulk solution becomes comparable to or larger than the effective (in the sense of counterion condensation) ionic strength inside the brush layer. (8) Matsumoto, K.; Ishizuka, T.; Matsuoka, H.Langmuir 2004, 20, 7270-7282. (9) Matsuoka, H.; Matsutani, M.; Mouri, E.; Matsumoto, K. Macromolecules 2003, 36, 5321. (10) Matsuoka, H.; Maeda, S.; Kaewsaiha, P.; Matsumoto, K. Langmuir 2004, 20, 7412. (11) (a) Nakano, M.; Matsuoka, H.; Yamaoka, H.; Poppe, A.; Richter, D. Macromolecules 1999, 32, 697. (b) Nakano, M.; Matsumoto, K.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 4029. (12) Astafieva, I.; Zhong, X. F.; Eienberg, A. Macromolecules 1993, 26, 7339. (13) Luo, L.; Eisenberg, A. Langmuir 2001, 17, 6804. (14) Choucair, A.; Lavigueur, C.; Eisenberg, A. Langmuir 2004, 20, 3894. (15) Zhao, Z. L.; Quinn, J.; Rafailovich, M. H.; Sokolov, J.; Lennox, R. B.; Eisenberg, A.; We, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (16) Shin, K.; Rafailovich, M. H.; Sokolov, J.; Chang, D. M.; Cox, J. K.; Lennox, R. B.; Eisenberg, A.; Gibaud, A.; Huang, J.; Hsu, S. L.; Satija, S. K. Langmuir 2001, 17, 4955. (17) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Langmuir 2004, 20, 6754.
Langmuir, Vol. 21, No. 22, 2005 9939 Table 1. Characteristics of (Ip-h2)m-b-(SSNa)n Block Copolymers sample
ma:nb
Mn
Mw/Mnc
degree of sulfonationd
A B C D E F
6:50 25:40 38:50 62:40 88:50 131:54
6800 7500 9000 11 100 15 600 19 000
1.07 1.08 1.09 1.06 1.11 1.12
0.55 0.62 0.71 0.82 0.84 0.99
a Number-averaged degree of polymerization of the PIp-h 2 segment determined by 1H NMR for parent polymers in CDCl3. b Number-averaged degree of polymerization of the PSS segment determined by GPC for parent polymers (chloroform as an eluent with polystyrene standards). c Number-average molecular weight distribution of the block copolymer determined by GPC for parent polymers after hydrogenation (chloroform as an eluent with polystyrene standards). d Determined by elementary analysis.
Why can such an ionic block copolymer adsorb to the air/water interface and form a monolayer? The answer is the very long hydrophobic chains of the polymers used in the monolayer study in order to make them insoluble in water. As a result, they can easily adsorb to the air/water interface even if they have strong acid groups. This situation is expected to correspond to the stronger adsorption force than image charge repulsion. In this study, we used the same polymers, i.e., PIp-h2-b-PSSNa, but having shorter hydrophobic chain lengths to study the effect of hydrophobic chain length on adsorption behavior. Their fundamental properties, such as surface activity, self-assembling behavior in solution and at the air/water interface, and salt effect on these, have been fully studied. In addition, we have found the anomalous hydrophobic chain length dependence and salt concentration dependence of cmc. Here, log(cmc) did not decrease linearly with increasing hydrophobic chain length, which is a good contrast to the low molecular weight ionic surfactants. The cmc was not decreased but even increase with added salt concentration, which is exactly opposite to common low molecular weight ionic surfactants and has not been reported before to our best knowledge. Experimental Section Materials and Synthesis of Block Copolymers. The (PIph2-b-PSSNa) diblock copolymers were synthesized as reported previously.17 PIp-b-PS diblocks have been synthesized by living anionic polymerization. After hydrogenation of the PIp blocks (about 90% 1-4 addition with 9-10% 3-4 addition) and sulfonation of the polystyrene (PS) block, the resulting poly(styrenesulfonic acid) blocks were neutralized using sodium hydroxide. Then the organic solvent (chloroform) was evaporated. The PIp-h2-b-PSSNa solution was dialyzed against ultrapure water and the electric conductivity of dialyzed water was checked to determine the end point of dialysis. The degree of sulfonation was determined by elementary analysis. Six polymer samples with different hydrophobic block lengths but having almost the same hydrophilic chain length (the degree of polymerization, n, is about 50) were synthesized by changing the monomer composition in the polymerization process. Table 1 shows the characteristics of (PIp-h2-b-PSSNa). Water used for polymer sample preparation and for solution preparation was ultrapure water obtained using a Milli-Q system (Millipore, Pittsburgh, PA) whose resistivity was more than 18 MΩ cm. A typical low molecular weight ionic surfactant, sodium dodecyl sulfate (SDS, used for comparison), and other reagents, sodium hydroxide, and sodium chloride were purchased from Wako Pure Chemical Co. (Osaka, Japan) and used as received. o-Tolueneazo-bnaphthol (Oil Orange SS), a hydrophobic dye, was a product of Tokyo Kasei (Tokyo, Japan). Surface Tension Measurements. The surface tension of block copolymer aqueous solutions was measured with a FACE
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CBVP-Z surface tensitometer from Kyowa Interface Science Co., Ltd. (Tokyo, Japan) using a Pt plate in full automatic mode. The solution with the highest polymer concentration was prepared first, and then the other solutions were prepared by dilution with Milli-Q water before the addition of salt. The surface tension was measured about 2 h after the solution preparation in the glass cell without disturbance since there was almost no change of surface tension value for 2 h and even for 1 day after sample preparation. Solubilization Experiments. Hydrophobic dye solubilization experiments were carried out with a Hitachi UV/vis spectrophotometer U-3310 (Hitachi, Tokyo) with Oil Orange SS as a hydrophobic dye. The sample solutions with different polymer concentrations were prepared by the same procedure as those for the surface tension measurement. After addition of Oil Orange SS, the polymer solutions were left for one night and passed through 0.22 µm pore size filters before measurements. A 10 mm thick flat quartz cell (Hellma) was used, and pure water was used as a reference sample for the spectrometer. The specific absorption by Oil Orange SS at 495 nm, which is far from that by benzene ring in polymer unit (270 nm), was evaluated. Dynamic Light Scattering (DLS). Dynamic light scattering (DLS) experiments were carried out with a BI-30 goniometor with BI-3000AT correlator of Brookhaven Co. (Ronkonkoma, NY). Details of the DLS apparatus were as described previously.18 Part of the measurements was done with a DLS-7000 system (Otsuka Electronic, Osaka, Japan). The time-correlation functions were analyzed by the double exponential methods.19 The measurements were performed at five different scattering angles, and the diffusion coefficient was calculated from the slope of the straight line in the decay rate Γ vs q2 plot, with q as the scattering vector (Å-1). X-ray Reflectivity (XR). X-ray reflectivity (XR) was measured with a RINT-TTR-MA reflectometer (Rigaku Corp.) with Langmuir-Blodgett (LB) trough at the sample position. The details of the instrument, data collection, and data analysis have been fully described elsewhere.20 All measurements were performed with specular geometry.
Results Effect of Hydrophobic Chain Length on Air/Water Interfacial Properties. The polymer concentration dependence of surface tension γ of PIp-h2-b-PSSNa solutions in water and 1 M NaCl(aq) are shown in Figure 1. The hydrophobic dye solubilization data are also shown and the cmc thus estimated is indicated by vertical gray line in the figures. For samples A-C, the result shows that no substantial reduction in surface tension is noted in a range of polymer concentrations where micelle formation should occur for low molecular weight surfactants (0.1-1 g/mL). The absolute value of γ is almost equal to that for pure water (73 mN/m at 20°C). From these surface tension measurements, it can be said that only a very small amount of PIp-h2-b-PSSNa block was adsorbed at the air/water interface. This behavior of diblock copolymer is quite different from that generally observed for low molecular weight surfactants, where surface tension decreases with increasing concentration up to cmc and then saturates. It should be noted that the similar phenomenon, i.e., no reduction of surface tension, was also observed for fully neutralized weakly ionic,8 and strongly ionic9,10 amphiphilic diblock copolymer systems in our previous works. Hence, this curious phenomenon, that is, non-surface activity of water-soluble ionic amphiphilic diblock copolymer is a universal and common property. (18) Matsuoka, H.; Ogura, Y.; Yamaoka, H. J. Chem. Phys. 1998, 109, 6125. (19) Chu, B. Laser Light Scattering, Basic Principle and Practice, 2nd ed.; Academic: San Diego, CA, 1991. (20) (a) Yamaoka, H.; Matsuoka, H.; Kago, K.; Endo, H.; Eckelt, J.; Yoshitome, R. Chem. Phys. Lett. 1998, 295, 245. (b) Matsuoka, H.; Mouri, E.; Matsumoto, K. Rigaku J. 2001, 18, 54.
Kaewsaiha et al.
Figure 1. Surface tension of PIp-h2-b-PSSNa aqueous solutions and hydrophobic dye adsorption (495 nm) as a function of polymer concentration. Key: filled circle, surface tension of aqueous solution; opened circle, surface tension of 1 M NaCl aqueous solution; filled triangle, UV absorbance of aqueous solution; open triangle, UV absorbance of 1 M NaCl aqueous solution.
For samples D, E, and F, the surface tension slightly reduced at the high polymer concentration condition. This means that these polymers are practically adsorbed at the air/water interface to some extent. Note that these samples have long hydrophobic chains: the hydrophobic chain length is much longer than 3 times the hydrophilic chain length. It is interesting to note that the γ for samples E and F in a salt-free condition decreased with increasing polymer concentration above cmc which was determined by the dye method. Effect of Salt on Air/Water Interfacial Properties. The effect of added salt on surface tension was examined. The surface tension and dye solubilization data for polymer solution in 1 M NaCl condition are also shown in Figure 1. In 1 M NaCl aqueous solutions, some reduction in surface tension is noticed. It can be said that the added salt enhances the adsorption of the polymer onto the air/ water interface. In particular, for samples E and F, γ decreased with increasing polymer concentration and then became constant. In addition, this transition point is just on the cmc determined by dye solubilization. This behavior just looks like that for low weight molecular surfactants. To detect the polymer molecules adsorbed at the air/ water interface, we performed X-ray reflectivity measurements. Since our XR instrument is a θ-θ goniometer with a rotation plane in vertical geometry, the surface of liquids can be directly measured.20 Figure 2a shows XR profiles of 1 wt % SDS solution and parts b-e of Figure 2 show XR profiles of 1 wt % copolymer solutions. XR profiles in
Non-Surface Activity and Micellization
Figure 2. (a) XR profiles for PIp-h2-b-PSSNa aqueous solutions. Key: filled circle, water only; opened circle, polymer aqueous solution; filled triangle, polymer in 1 M NaCl aqueous solution. Polymer concentration ) 1%. (b) XR profiles in R*q4 vs q manner. The symbols are the same as in part a. The solid lines for sample B and D in R*q4 vs q plot is the fitting curve by two layer model (see text).
reflectivity (R) multiplied by q4 vs q manner were also shown to clarify a small undulation. The reflectivity profiles of samples B and C polymer solutions in pure water were similar to those of pure water but profiles of samples D and F and those for sample B polymer in salt solution showed an increase of reflectivity in the middle q range like that of SDS. This trend is more clearly observed in Rq4 vs q plot, which means the existence of a thin film at the air/water interface. Although the fringe in these profiles are weak and broad, the XR profiles were satisfactory reproduced by two layer model, consisting of upper hydrophobic layer and lower hydrophilic layer. The hydrophobic and hydrophilic layers of sample D in pure water were estimated to have a thickness of 9 and 12 Å,
Langmuir, Vol. 21, No. 22, 2005 9941
respectively with an interfacial roughness in the range of 2-6 Å, while those of sample B in 1 M NaCl solution were estimated to be 11 and 6 Å, respectively. These thickness values estimated are too small as a uniform polymer monolayer; probably a small amount of polymer was adsorbed on the water surface. In either case, direct confirmation of adsorption by XR measurement is in good agreement with reduction of the surface tension for samples D and F in Figure 1. Critical Micelle Concentration for PIp-h2-b-PSSNa. We performed solubilization experiments for (Ip-h2)m -b-(SSNa)n solutions at various polymer concentrations with addition of a water-insoluble hydrophobic dye. At low polymer concentrations, the solutions were clear and not colored. However, at certain polymer concentration, the solution was colored with dye and the color became deeper with increasing polymer concentration. As generally observed for low molecular weight surfactants, this phenomenon can be attributed to a solubilization of dye in the hydrophobic core part of polymer micelles. Hence, this apparent solubilization of water-insoluble hydrophobic dye in water means micelle formation in polymer solutions at a higher polymer concentration. The color change was quantitatively measured with a UV/vis spectrometer, and the absorbance was plotted against the polymer concentration also in Figure 1. As Figure 1 clearly shows, the absorbance started to increase rapidly at the polymer concentration of about 10-5 mol/L for all the samples. This turning point should be the critical micelle concentration (cmc) where unimers start to form micelles in solution. It should be noted that there was no serious reduction in the surface tension of sample A-D around the cmc, but a slight reduction was observed in surface tension of E and F. In particular, in the case of samples E and F, the surface tension of polymer solutions in 1 M NaCl(aq) showed the same behavior with low molecular weight surfactants, that is, the surface tension of the solution reduces with increasing polymer concentration up to the cmc and then saturates at cmc. Hydrodynamic Size of Polymer Micelles. DLS was used to confirm micellization of the block copolymer in aqueous solutions. Figure 3a shows the time correlation functions of scattered field g(1)(q,τ) for 1 wt % polymer aqueous solutions. The decay rate Γ values were evaluated and plotted against q2 in Figure 3b. From the slope of the straight line in Figure 3b, the translational diffusion coefficients were calculated and converted into hydrodynamic radius (Rh) by assuming the Stokes-Einstein equation (Table 2). The time correlation functions for sample A-F could be well reproduced by doubleexponential fitting. The smaller Rh values (fast mode) were 60-380 Å and should be those of micelles. The larger Rh, or the slow mode for the pure water solutions, which became smaller by the presence of salt, is thought to be due to interparticle electrostatic interaction and not to particle size.10 Discussion Mechanism of Micelle Formation with No Adsorption. The curious phenomenon, that is, polymers form micelles in aqueous solution without adsorption on the water surface, is observed for strongly ionized amphiphilic diblock copolymers having sulfonic groups.9,10,21 A similar observation was also found for weak acid systems (e.g., carboxylic acid) when they were fully neutralized.8 Guenoun et al.22 reported the same observation for poly(tert(21) Harada, T.; Matsumoto, K.; Matsuoka, H.Polym. Prepr. Jpn. 2004, 53, 1164.
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Kaewsaiha et al. Table 2. Hydrodynamic Radii (Rh) of (Ip-h2)m-b-(SSNa)n Micelles in Pure Water and in 1 M NaCl(aq) (nm) Evaluated by DLS in H2O
in 1M NaCl(aq)
sample
m:n
Rh
Rlarge
Rh
Rlarge
A B C D E F
6:50 25:40 38:50 62:40 88:50 131:54
15 6 17 12 38 21
55 23 66 90 120 95
18 6 19 8 33 15
50 14 50 31 120 60
butylstyrene)-b-poly(styrenesulfonate) at a very low concentration, and Rager et al.23 for poly(acrylic acid)-blockpoly(methyl methacrylate), although its origin was not discussed. Very recently, the similar phenomenon was also reported by Iddon et al.24 for poly(4-vinylbenzoate)b-poly(styrenesulfonate) at low pH condition where the weak polyacid chain behaves like a hydrophobic segment. As was confirmed by surface tension measurement and XR experiments, there are few polymer molecules at the air/water interface in the absence of salt, but an adsorption was induced by addition of salt.9,10 The foam formation ability is known to be enhanced by the addition of salt. On the other hand, the polymer micelles are formed in bulk solution phase although the electrostatic repulsion between ionic blocks in corona region of the micelle should be very strong. Hence, it is quite acceptable to assume that the origin of this peculiar phenomenon is due to some kind of electrostatic force only effective at the air/water interface. As was reported before, the contribution of image charge force at interface was suggested for the interpretation of this behavior.10,21 The interface consisted of water (dielectric constant w ) 78) and air (a ) 1), and many charges on the PSS chain should be localized in water phase very close to the interface to occur polymer adsorption. This situation results in the strong electrostatic repulsion from the interface for PSS chains. As was described previously,10 the polymers near the water surface were repelled from the interface by a strong Columbic repulsive force. The origin of the repulsive force is an image charge effect: apparently a charge with almost the same valency and the same sign is produced at the symmetric position with the interface. Hence, the polymers cannot localize near the interface, and enter the bulk water phase. Since the polymers have a long hydrophobic chain, they form micelles in bulk. The image charge effect at the air/water interface is not the first example, but has been known as an origin of negative adsorption of small simple ions. The surface tension of aqueous solutions of simple ions such as NaCl increases with increasing solute concentration due to the negative adsorption.25,26 In 1934, Onsager and Samaras proposed the existence of image charge effect as an origin of negative adsorption for simple ions.27,28 Contribution of the image charge effect was also pointed out for the adsorption of the polyelectrolyte homopolymer at air/water29 and monolayer/water30 interfaces. In our case, although the polymers have a long hydrophobic Figure 3. DLS results for solutions of samples A, B, C, and D. (a) Time correlation functions for the scattered field for PIph2-b-PSSNa solutions (1 wt %) in pure water (open circles) and in 1 M NaCl(aq) (open triangles) at 25 °C at the scattering angle of 90°. Solid lines are double-exponential fitted lines. (b) Decay rate Γ vs q2 plot for PIp-h2-b-PSSNa solutions in pure water (open circles) and in 1 M NaCl aq (filled circles). From the slope of the straight lines, the diffusion coefficient of the micelles was evaluated. Open and filled triangles are for no salt and 1 M NaCl conditions, respectively, for the slow mode.
(22) (a) Guenoun, P.; Davis, H. T.; Tirrell, M.; Mays, J. Macromolecules 1996, 29, 3965 (b) Fontaine, P.; Daillant, J.; Guenoum, P.; Alba, M.; Braslau, A.; Mays, J.; Petit, JM.; Rieutord, F. J. Phys. II 1997, 7, 401. (23) Rager, T; Wolfgang, H.; Meyer, W. H.; Wegner, G.; Mathauer, K.; Ma¨chtle, W.; Schrof, W.; Urban, D. Macromol. Chem. Phys. 1999, 200, 1681. (24) Iddon, P. D.; Robinson, K. L.; Armes, S. P. Polymer 2004, 45, 759. (25) Grinnell Jones, Wendell, A. Ray J. Am. Chem. Soc. 1937, 59, 187. (26) Jarvis, N. L.; Scheiman, M. A. J. Phys. Chem. 1968, 72, 74. (27) Onsager, L.; Samaras, N. N. J. Chem. Phys. 1934, 2, 528.
Non-Surface Activity and Micellization
Figure 4. Effect of salt on foam formation. (left) SDS 1% aqueous solution, (right) polymer B. Added NaCl concentration was 1 M.
segment, they also have a long ionic polymer chain as the hydrophilic segment, hence the image charge repulsion is superior to the hydrophobic adsorption force. However, it might be important to note that γ in our case did not increase with increasing polymer concentration. This means that there is no surface excess, nor remarkable surface depletion. One possibility is, as we proposed previously,10 the polymer concentrations in surface “phase” and bulk “phase” are almost the same. This idea is supported by the γ behavior in added salt systems. By addition of salt, the image charge repulsion is shielded and the polymers start to be adsorbed at the water surface. This situation can be confirmed by foam formation observation; i.e., the solution with added salt is more active for foam formation although it is still very less than conventional surfactant systems (Figure 4). Since the foam is stabilized by adsorption of molecules at the water surface of the thin water film, active foam formation means that more adsorption occurred. In our separate experiment on adsorption behavior of PSt-b-PSSNa at the air-water interface, we found that the increase in the degree of polymerization of PSSNa resulted in a drastic reduction of the amount of adsorption of block copolymers at interface.21 From these facts, the relationship between the charges on the PSS chain and the amount of adsorption of block copolymers is obvious and our opinion, i.e., the image charge prevents polymers from adsorbing at the air/water interface, should be supported. Effect of Hydrophobic Chain Length on Adsorption. In the case of samples A-D, since the hydrophobic chain length is comparatively short, the image force is superior to the hydrophobic forces. The polymer chains do not adsorb at the water surface due to the strong repulsion from the interface. Then, the polymers enter the water bulk phase, and micelles are formed in solution. This phenomenon is the same as that we described previously.9,10 However, samples E and F have a very long hydrophobic chain (more than 3 times longer the hydrophilic chain length), so in these cases, it is reasonable to assume that the hydrophobic force becomes superior to the image charge effect. Then, the polymers can easily adsorb onto the water surface, which results in a decrease of surface tension. Table 3 shows the relationship between hydrophobic chain length and adsorption. This is the first quantitative information for the balance between the hydrophobic effect and Coulomb effect at the interface. Simultaneous Adsorption and Micellization. Another interesting point for samples E and F in Figure 1 (28) Landau, L.D.; Lifshiz, E. M. Statistical Physics C, 3rd ed. (Course of Theoretical Physics, Volume 5); Oxford: Oxford, U.K., 1976; Chapter 15, section 15 (Japanese version; Iwanami: Tokyo, 1980). (29) Yim, H.; Kent, M. S.; Matheson, A.; Stevens, M. J.; Ivkov, R.; Satija, S.; Majewski, J.; Smith, G. S. Macromolecules 2002, 35, 9737. (30) Ahrens, H.; Baltes, H.; Schmitt, J.; Mo¨wald, H.; Helm, C. Macromolecules 2001, 34, 4504.
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is the relationship between the decrease in surface tension and the hydrophobic dye solubilization. For these samples in the absence of salt, the surface tension is continuously decreasing even at a higher polymer concentration above cmc. This means that both polymer adsorption at the interface and micelle formation in the bulk solution occur simultaneously. This interesting phenomenon is not common observation and this might be due to the almost balanced situation between an image charge repulsion and hydrophobic adsorption. In the presence of salt, the surface tension of these samples shows the common behavior, that is, first deceasing and then leveling off. Since the image charge force is shielded by the addition of salt, now the adsorption force is superior to the image charge repulsion, which results in the quite normal behavior for surfactants. In addition, the polymer concentration where the surface tension leveled off is almost the same as the cmc value determined by dye solubilization. Although the behavior of samples E and F in a 1 M NaCl condition is like that of a normal surfactant, the surface tension decrease is small (at least around 55 mN/m where normal surfactants sometimes show less than 30 mN/m). Extremely High Stability of Micelle Particles against Salt Addition. DLS experiments (Figure 3 and Table 2) show that the hydrodynamic radius (Rh) of the micelle particles is not largely affected by the addition of salt. In this study, we observed almost the same Rh values both in the absence of salt and in the presence of 1 M NaCl(aq), which is an extremely high salt concentration in the common sense of colloid science; e.g., seawater is ca. 0.6 M. Usually, electrostatically stabilized colloidal particles in dispersion, such as polystyrene latex, form aggregates by addition of salt at about 10-3-10-4 M. Micelle particles of low molecular weight ionic surfactants will change their shape and size or form large aggregates at high salt concentration on the order of 10-1 M. In this sense, this observation here is hard to believe from current colloid and surface science. However, similar phenomena were also observed for other strongly ionic amphiphilic diblock copolymers in our previous studies.9,10 A similar observation was also reperted for core-cross-linked polymer micelles with a polyelectrolyte corona.31 Hence, the extremely high stability of micelles is a universal property of this type of polymer. As has been described previously9,10 and also was experimentally confirmed by system analogous to polymer monolayer on water,17 this is due to a very high ion concentration inside the corona (brush) region of the micelles. When the ion concentration inside the corona is higher than that of the bulk solution, added salt ions cannot go into the corona region, which is the origin of the lack of change of micelle structure and high stability. Effect of Hydrophobic Chain Length on the Cmc. Figure 5 shows the cmc as a function of carbon number of hydrophobic chain (nc) for (a) no salt and (b) 1 M NaCl conditions. For the low molecular weight surfactants, the following relationship between cmc and nc has been widely accepted.32
log(cmc) ) a - bnc
(1)
A typical value of a for ionic alkylchain surfactants is in the range of 1-2 and is 0.3 for b, which means that cmc decreases in the order of magnitude by two additional (31) Matsumoto, K.; Hasegawa, H.; Matsuoka, H. Tetrahedron 2004, 60, 7197. (32) Tanford, C. The Hydrophobic Effect; John Wiley & Sons: New York, 1980; Chapter 7.
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Kaewsaiha et al.
Table 3. Adsorption Behavior of (Ip-h2)m-b-(SSNa)
n
Block Copolymers
decrease in surface tensiona
adsorption at water surfaceb
sample
m:n
lPIp-h2:lPSSNad
H2O
1M NaCl(aq)
H2O
1M NaCl(aq)
micelle formationc
A B C D E F
6:50 25:40 38:50 62:40 88:50 131:54
0.24 1.25 1.52 3.10 3.52 4.85
× × × × O O
× × ∆ O O
× × × ∆ ∆ ∆
O O O
O O O O O O
a Confirmed by surface tension measurement. b Confirmed by X-ray reflectometry. c Confirmed by dynamic light scattering. lPSSNa: ratio of hydrophobic and hydrophilic chain length.
d
lPIp-h2:
Figure 6. Cmc of PIp-h2-b-PSSNa aqueous solution as a function of ion concentration.
rapid decrease of up to the degree of polymerization (Pn) of PS of 30 and leveled off with further increase of Pn. They just pointed out that the free energy of micellization (∆G°) is not proportional to the length of polystyrene based on the following equation.
∆G° ) RT ln(cmc)
Figure 5. Effect of the hydrophobic chain length (carbon number nc) on cmc for (a) no salt and (b) 1 M NaCl conditions. Dashed lines are guides for the eyes.
methylene units in the hydrophobic chain. For sodium salt of sulfonated hydrocarbon surfactants, a ) 1.51 and b ) 0.30 was reported.33 In Figure 5, the log(cmc) was plotted against nc with nc as the number of carbon atoms in the hydrophobic chain. The log(cmc) value does not linearly decrease with increasing chain length, but shows a minimum at around nc ) 200. Obviously, the behavior below and above nc ) 200 is different. Below nc ) 200 where almost no adsorption occurs, the cmc decreases rapidly with increasing nc. This means that micelles are easily formed for longer hydrophobic chain molecules, which is quite reasonable. However, above nc ) 200 where the polymer molecules are more or less adsorbed at the water surface, the cmc is almost constant, even increasing with increasing hydrophobic chain length. This means that the micelles are hard to be formed for longer hydrophobic chain polymer, which is exactly opposite to eq 1. The details about this observation is not clear at this stage, but one possibility might be that adsorption of polymer molecules at water surface disturb micelle formation process in bulk solution. It might be interesting to note that Astafieva et al.12 also observed the similar behavior, i.e., a nonlinear relationship between log(cmc) and nc for polystyrene-bsodium polyacrylate, although they did not examine the surface tension change in their work. They observed a (33) Tartar, H. V.; LeLong, A. L. M. J. Phys. Chem. 1956, 59, 1185.
(2)
In our present case, also change of micellization mechanism might be predicted between below and above nc ) 200. In addition, it might be significant to note that Pn(PS) ) 30 means nc ) 240, which is very close to our finding of transition at nc ) 200. Anomalous Effect of Salts upon the Cmc. The effect of salts upon the critical micelle concentration for PIph2-b-PSSNa is characteristic. The addition of salt to low molecular weight ionic surfactant solutions, such as sodium dodecyl sulfate, decreases cmc. This has been known for a long time as the Corrin-Harkins experimental law34
log C0 ) - k log (C0 + Cs) - l
(3)
where C0 is the cmc of surfactant solution, Cs is the concentration of added salt, and k and l are the constant numbers. The most likely explanation is that addition of salt enhances screening of the electrostatic interactions between charged headgroups and favors the micellization process. Hence, the Corrin-Harkins law means the ionic micelles are more easily formed at a higher salt concentration. Figure 6 shows the cmc of PIp-h2-b-PSSNa as a function of salt concentration. Interestingly, the cmc did NOT decrease, it even increased, as the salt was added in contrast to low molecular weight surfactants, i.e., the Corrin-Harkins law. This situation means that micelles are harder to be formed at a higher salt concentration. The origin of this curious phenomenon is still a mystery at this stage, but we can make one speculation. Recall that the non-surface active ionic diblock copolymers do not adsorb on water surface but form micelles in solution (34) Corrin, M. L.; Harkins, W. D. J. Am. Chem. Soc. 1947, 69, 683.
Non-Surface Activity and Micellization
in the absence of salt, while those in the presence of salt are adsorbed at the interface. This situation means that micelle formation in a salt-added system is decelerated by adsorption compared to salt-free system in which all polymer molecules participate in micelle formation without adsorption. This might be one explanation for the suppressive effect of salt on micelle formation. One may notice that the increase of cmc for sample C is extremely large in Figure 6. We have no interpretation at present, but one possible explanation is that it is very close to the transition point as a function of hydrophobic chain length as is clear from Figure 5. This situation might make this polymer very sensitive to salt. Conclusions We investigated the adsorption behavior of an ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa at the air/water interface using surface tension measurement and X-ray reflectivity. Dye-solubilization and dynamic light scattering measurement were carried out to detect the micelle formation. We found that this polymer, with its short hydrophobic chain, formed micelles in the solution with only a small amount adsorbed at the air/ water interface. Alternatively, PIp-h2-b-PSSNa with its long hydrophobic chain adsorbed onto the water surface, which could be confirmed by X-ray reflectivity and the reduction of surface tension. The large hydrophobic force due to the long hydrophobic chains is superior to the image charge repulsion, which is thought to be the factor preventing adsorption. The results of these investigations clearly demonstrated the feasibility of controlling the adsorption of diblock copolymer by adjusting of the hydrophobic length. We also investigated the effect of salt on adsorption of the same copolymer on the water surface. The increase of
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adsorbed amount as a function of salt concentration was confirmed by X-ray reflectivity so we knew that the effect of salt enhances the adsorption of the polymer onto air/ water interface. The polymers with a long hydrophobic chain (more than three times longer than the hydrophilic chain) in 1 M NaCl solution behave like a normal low molecular weight surfactant. The critical micelle concentration of PIp-h2-b-PSSNa shows an anomalous dependence on hydrophobic chain length and added salt concentration. log(cmc) did not decrease linearly with the number of carbons in the hydrophobic chain but showed a minimum at about 200. The cmc did not decrease, it even increased, with increasing salt concentration. This is in good contrast with the Corrin-Harkins behavior for low molecular weight ionic surfactants. The ionic amphiphilic diblock copolymer is a kind of polymer, amphiphile, and also ion. However, its character is not a simple superposition of these characters. Its properties are quite different from those of low molecular weight ionic surfactants, or nonionic amphiphilic diblock copolymers, and further, from ionic polymers. Ionic amphiphilic diblock copolymer is a new class of molecules. Acknowledgment. We would like to express our sincere gratitude to Prof. Hiroshi Maeda, (Prof. Emeritus, Kyushu University, Japan) and Prof. Heinz Hoffmann (Bayreuth, Germany) for their valuable and fruitful discussions. This work was financially supported by a Grant-in-aid for Scientific Research (A15205017) from the Ministry of Education, Science, Culture, Sports, and Technology of Japan to whom our sincere gratitude is due. This work was also supported by the 21st century COE program, COE for a United Approach to New Materials Science. LA051584R
