4302
Langmuir 1999, 15, 4302-4305
Adsorption of Sulfobetaine Polyampholyte on Silica Surfaces from Aqueous Salt Solutions† Tadaya Kato,* Masami Kawaguchi, and Akira Takahashi Department of Chemistry for Materials, Mie University, 1515 Kamihama-cho, Tsu, Mie 514-8507, Japan
Takayuki Onabe and Hisao Tanaka Toa Gosei Co. Ltd., 1-1 Funami-cho, Minato-ku, Nagoya 455-0027, Japan Received August 25, 1998. In Final Form: March 11, 1999 Adsorption of poly(N-3-sulfopropyl-N-methacrylooxyethyl-N,N-dimethyl ammonium betaine) (polysulfobetaine abbreviated as PSB), which is a neutral- and zwitterion-type polyampholyte, onto a silica surface from aqueous NaCl solutions was studied by adsorbance measurement and ellipsometry. Adsorbance A increases with decrease of added salt concentration Cs0, while the thickness of adsorbed layer L decreases with the decrease of Cs0. Comparison of L with the radius of gyration of PSB in NaCl solutions reveals the existence of collapsed (pancake)-, normal (fence)-, and elongated (pole)-like conformational regimes of PSB at the surfaces depending on the salt concentration. The salt distribution between the adsorbed layer and the bulk solution was estimated, and it was found that at low Cs0 positive ad(b)sorption of NaCl occurs while high Cs0 results in negative absorption of NaCl. The distribution of NaCl among adsorbed layers and bulk solution could be explained by Debye-Hu¨ckel theory, which takes into account the electrostatic attractive interaction among salt ions and zwitterion groups on PSB chain. It was also found that the polymeric osmotic and deformation pressure of adsorbed layer ∆π is related to the thickness of adsorbed layer L as ∆π ≈ L-9/4.
Introduction The remarkable feature of polyampholytes is the “antipolyelectrolyte” behavior, involving low solubility in water but enhanced solubility as well as greater chain expansion in the high salt concentration. This behavior originates from the attractive interaction among positive and negative charges on the chains. The addition of low molecular weight salt screens the attractive interaction depending on the salt concentration, and in addition, the excluded volume interaction among the polymer segments results in the chain expansion and solubility enhancement. Extensive theoretical and experimental studies of polyampholytes have been reported.1-10 Nevertheless, attention has not been focused on polymer adsorption of polyampholytes. Blaakmeer, Cohen Stuart, and Fleer11 studied the adsorption of polypeptide-type polyampholytes on charged lattices. The adsorption of terpolymer-type polyam* To whom correspondence should be addressed. † Presented at Polyelectrolytes ‘98, Inuyama, Japan, May 31June 3, 1998. (1) Candau, F.; Joanny, J.-F. Synthetic Polyampholytes in Aqueous Solutions. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, Fl, 1996. (2) Higgs, P.; Joanny, J.-F. J. Chem. Phys. 1991, 14, 1543. (3) Corpart, J. M.; Candau, F. Macromolecules 1993, 26, 1333. (4) McCormick, C. L.; Johnson, C. B. Macromolecules 1988, 21, 694. (5) Skouri, M.; Munch, J. P.; Candau, S. J.; Neyret, S.; Candau, F. Macromolecules 1994, 27, 69. (6) Ohlemacher, A.; Candau, F.; Munch, J. P.; Candau, S. J. J. Polym. Sci., Phys. Ed. 1996, 34, 2747. (7) Kantor, Y.; Kardar, M. Europhys. Lett. 1991, 14 ,421. (8) Kantor ,Y.; Kardar, M. Phys. Rev. E. 1995, 51, 1299. (9) Everaers, A.; Johner, A.; Joanny, j.-F. Macromolecules 1997, 30, 8478. (10) Kato, T.; Takahashi, A. Ber. Bunsenges. Phys. Chem. 1996, 100, 784. (11) Blaakmeer, J., Cohen Stuart, M .A., Fleer, G. J. J. Colloid Interface Sci. 1990, 140,314.
Figure 1. Poly(N-3-sulfopropyl-N-methacrylooxyethyl-N,Ndimethylammonium betaine) (PSB).
pholytes on negatively charged lattices and the colloid stability of complexes were reported by Neyret, Ouali, Candau, and Pefferkorn.12 Joanny13 and Dobrynin and Rubinstein14 treated theoretically the single-chain adsorption on charged surfaces. The existence of various conformational regimes, such as pole, fence, and pancake depending on the surface charges, has been theoretically deduced for the single-chain adsorption. We studied here the adsorption of polysulfobetaine (Figure 1) (abbreviated as PSB) onto a silica surface from aqueous NaCl solutions with various concentrations. The PSB sample is a neutral- and zwitterion-type polyampholyte, and both positive and negative charges locate on the same side chains as shown in Figure 1.10,15-18 In the analysis of the charge distribution of both polyion and salt ions between the adsorbed layer and the bulk (12) Neyret, S., Ouali, L., Candau, F., Pefferkorn, E. J. Colloid Interface Sci. 1995, 176, 86. (13) Joanny, J.-F. J. Phys. II (France) 1994, 4, 1281. (14) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. Macromolecules 1997, 30, 4332. (15) Monroy Soto, V. M.; Galin, J. C. Polymer 1984, 25, 254. (16) Nakaya, T.; Toyoda, H.; Imoto, M. Polymer J. 1986, 18, 881. (17) Schulz, D .N.; Pfeiffer, D. G.; Agarwal, P. K.; Larabee, J.; Kaladas, J. J.; Soni, L.; Handweker, B.; Garner, R. T. Polymer 1986, 27, 1734. (18) Wielema, T. A.; Engberts, J. B. F. N. Euro. Polym. J. 1990, 26, 639.
10.1021/la981102u CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999
Adsorption of Sulfobetaine Polyampholyte
Langmuir, Vol. 15, No. 12, 1999 4303
solution, which was determined from adsorbance and ellipsometry, we explicitly assumed the model of random polyampholytes. Of course, the charge distribution in the polyampholyte is important at the very low salt concentration. One of the features of zwitterion-type polyampholytes is that there is some possibility of intra-sidegroup association as well as intersegmental association of the same polymer chain. However, at sufficiently high salt concentration these associations are disrupted. We considered that the behavior of zwitterion-type polyampholytes may approach that of random polyampholytes.1-2,17 Experimental Section Materials. The polymer used in this study is poly(N-3sulfopropyl-N-methacrylooxyethyl-N,N-dimethylammonium betaine) (PSB). The polymer samples were prepared by radical polymerization using ammonium persulfate as an initiator. The polymer samples with various molecular weights were obtained by controlling the monomer to initiator mole ratio in the reaction vessel. The original samples were dissolved in NaOH aqueous solution and precipitated into concentrated gel by addition of methanol in order to remove impurities of the PSB sample. Then the samples were further purified by dialysis against pure water and freeze-dried. Some samples were fractionated according to the procedure as described previously.10 The nonporous Aerosil 130 silica (Degussa AG, Germany) was used as the adsorbent after being cleaned by the procedure described previously.19 The particle diameter was 20 nm, the specific surface area 130 ( 25 m2/g, and the surface content of silanol groups 3 SiOH/nm2. In the ellipsometry, a quartz crystal disk (φ ) 25.4mm, thickness 1.5 mm) (Koyo Co., Japan) was used as the adsorbent. NaCl was as received from Nakarai Tesq, Japan, and twicedistilled water was used. Molecular Weight and Radius of Gyration. Weight average molecular weight and radius of gyration were measured by the static light scattering method by using a LS601A automatic light scattering analyzer (Otsuka Electronics, Japan). Details of the measurement and the data of specific refractive index increment (∂n/∂c) of PSB solutions in various concentration of NaCl were reported previously.10 Adsorption Isotherms. Twenty milliliters of an aqueous NaCl solution of specified concentration, which also dissolved the known amount of PSB, was mixed with the silica (0.16 g) in a centrifuge tube with stopper, and the mixture was stirred gently by a magnetic chip for 48 h to attain equilibrium at 25 °C. The silica suspensions were centrifuged at 1000 rpm for 30 min to sediment the silica, and the supernatant was carefully removed. The PSB concentration in the supernatant, i.e., the equilibrium bulk concentration C, was determined by the refractive index increment measurement. Adsorbance A, expressed in g/cm2, was determined from the difference between the amount of the polymer initially added, C0, and that in the supernatant, C. Ellipsometry. A Shimadzu horizontal-type ellipsometer was used. The wavelength of the incident light was 546 nm, and the incident angle was 70°. The adsorbed layer thickness, L, and the refractive index, nf, of the adsorbed layer were calculated from the data of phase difference ∆ and relative amplitude reduction φ by using a computer. The refined computer program originally proposed by McCrackin20 was used. The measurement of ∆ and φ was continued for 2 or 3 days at 25 °C.
Results and Discussion Figure 2 displays the adsorption isotherms at various NaCl concentrations. It is clear that the adsorbance A in the plateau region increases with decreasing the bulk salt concentration Cs0, indicating an “anti-polyelectrolyte effect”,15-18 since in contrast the adsorbance of a typical (19) Kawaguchi, M.; Maeda, K.; Kato, T.; Takahashi, A. Macromolecules 1984, 17, 1666. (20) McCrackin, F. L. NBS Tech. Note (U.S.) 1966, 479.
Figure 2. Adsorption isotherms of PSB (Mw ) 7.0 × 105) onto silica. Cs0: 3, 0.06; O, 0.155; 0, 0.3; 4, 1.0; 9, 2 mol dm-3.
Figure 3. Adsorbed layer thickness L (PSB, Mw - 1.5 × 106) and radius of gyration plotted against Cs0. Bulk PSB concentration ) 0.002 g/mL.
polyelectrolyte from salt solution increases with the increase of Cs0. Figure 3 displays the salt concentration dependence of adsorbed layer thickness L at the plateau region of adsorption isotherm for the PSB Mw ) 1.5 × 106 sample. The value of L increases almost linearly with Cs0, while twice the radius of gyration 21/2 for an isolated PSB chain only increases slightly with Cs0. Thus, the salt concentration dependence of adsorbed layer thickness L and that of the radius of gyration are different. At low CS0, L > 21/2. Table 1 gives the adsorption data of PSB (Mw ) 1.5 × 106). Ellipsometry gives both the adsorbed layer thickness L and the refractive index nf of the adsorbed layer. A twocomponent system consists of polymer and solvent, and the adsorbance A can be calculated by
A ) (nf - n0)/(∂n/∂C)
(1)
where n0 is the refractive index of solvent and (∂n/∂C) is the refractive index increment of the polymer-solvent system. However, PSB-NaCl-H2O systems are three-
4304 Langmuir, Vol. 15, No. 12, 1999
Kato et al.
Table 1. Adsorption data of PSB (Mw ) 1.5 × 106) on SiO2 from aqueous NaCl solutionsa bulk NaCl concn Cs0 (mol dm-3)
adsorbance A × 107 (g cm-2)
thickness L (nm)
refractive index of adsorbed layer, nf
refractive index of bulk NaCl solution, n0
NaCl in layer Csf (mol dm-3)
0.06 0.155 0.3 0.7 1.0
1.75 1.2 1.14 0.9 0.77
8.4 25.3 43.6 129 202
1.4271 1.3454 1.3487 1.3398 1.3405
1.3372 1.3355 1.336 1.340 1.343
0.85 ( 0.15 0.5 ( 0.05 0.6 ( 0.05 0.7 ( 0.05 0.8 ( 0.05
a
Bulk polymer concentration ) (0.002) g/mL.
component systems, and eq 1 does not give the correct A. This can be easily understood by inspection of the values of nf and n0 at Cs0 ) 0.7 and 1 mol dm-3. At Cs0 ) 0.7 mol dm-3, nf Z n0, which means A should be zero according to eq 1 and at Cs0 ) 1mol dm-3, nf < n0 then A must be negative. However, the measured A is positive in both cases. This is the reason we determined A independently. Since we made use of different types of silica surfaces the adsorption on them may not be the same. Therefore, the following analysis is done on the presupposition that the adsorbance on both silica surfaces is the same. Thus, since A is known and nf is measured by ellipsometry, we can evaluate the NaCl concentration Csf in the adsorbed layer by applying the additivity rule of molecular refraction.21 The mean molar refractivity P h is given by
P h ) (nf - 1)M h /(nf2 + 2)F
(2)
where M h the mean molecular weight and F is the density of adsorbed layer. The terms of P h and M h are given by
∑Pifi M h ) ∑Mi fi P h)
(3) (4)
where fi denotes the mole fraction of component i. Density F is expressed by
F ) F0 + (M+ - F0V+0)C+/1000 + (M- - F0V-0)C-/1000 + (Mp - F0Vp0)Cp/1000 (5) where F0 is the density of water, V+0, V-0, and Vp0 are the apparent molar volumes, M+, M-, and Mp are the molecular weights, and C+, C-, and Cp are the molar concentrations of cation, anion, and polyampholyte ion, respectively. The Cp of PSB in moles per liter in the adsorbed layer is calculated by 1000A/M0L, where A is in g cm-2, L is in cm, and M0 is the molecular weight of the PSB monomer. The following values are used: PNaCl ) 9.23 cm3/mol, Pwater ) 3.73 cm3/mol, PPSB ) 69.35 cm3/mol, VNa+0 ) -1.55 cm3/mol,VCl-0 ) 18.3 cm3/mol, and VPSB0 ) 220 cm3/mol. The last column in Table 1, indicates the average NaCl concentration Csf in the adsorbed PSB layers thus determined. Although Csf contains considerable uncertainty, it should be noticed that Csf > Cs0 when Cs0 < 0.3 mol dm-3, Csf Z Cs0 at Cs0 ) 0.7 mol dm-3, and Csf < Cs0 at Cs0 ) 1 mol dm-3. This indicates that positive ad(b)sorption of NaCl occurs at low NaCl concentration, while at high ionic strength, the negative adsorption of NaCl occurs. Since the interionic or Debye radius r of zwitterion groups ranges from 0.34 to 0.45 nm depending on the internal rotation of -(CH2)3- groups and the r corresponds to salt concentration of 0.5-0.8 mol dm-3, we can understand that the accumulation or exclusion of NaCl (21) Stedman, M. Chem. Phys. Lett. 1968, 2, 457.
Figure 4. Plot of logarithm of salt distribution against electrostatic interaction term.
occurs by electrostatic interaction among salt ions and zwitterions. As far as moderate concentration of salt ions exist, solution and phase behavior15,17 of zwitterion polyampholyte do not differ from those of neutral polyampholyte that has random distribution of charges with a short distance between groups, which is comparable to the charge distance in the zwitterion groups. Then the salt distribution of the adsorbed layer a and the bulk solution b can be expressed as
ln(na/nb) ) 1/2(κa - κb)l
(6)
This equation was formulated by Higgs and Joanny.2 In eq 6, na and nb are the salt concentration in adsorbed layer and bulk, respectively, l is Bjerrum length l2 ) e2/ 4πkT, and κ2 ) 8πnl, but note that n contains both salt concentration and the zwitterion concentration. Making use of molar concentration, Figure 4 shows the plots of eq 6. Since in between and surroundings of both positive and negative charges on PSB chains there are hydrocarbon moities, we employed ) 50 as the effective dielectric constant. The experimental points fall on a diagonal line. Thus, the salt distribution between the adsorbed layer and the bulk solution is satisfactorily described by eq 6. The equality of osmotic pressures between the adsorbed layer and the bulk solution should also be held. The osmotic pressure of bulk solution is given by
π0/kT ) πp0/kT + 2n0 - nb(κb/3)l
(7)
where πp0 is the polymeric term, 2n0 is the salt concentration in the bulk, nb and κb contain the salt and zwitterion concentration. The adsorbed layer pressure includes the term due to the polymeric term πp and deformation (elastic or stretching) term pd,22-24 the ideal gas term for the salt (22) de Gennes, P. G. Macromolecules 1981, 14, 1837. (23) de Gennes, P. G. Macromolecules 1982, 15, 492.
Adsorption of Sulfobetaine Polyampholyte
Langmuir, Vol. 15, No. 12, 1999 4305
Figure 6. Plot of adsorbed layer thickness L against osmotic pressure ∆π/kT. The straight line is drawn with the slope -4/9. Figure 5. Plot of osmotic pressure ∆π/kT against average polymer volume fraction φ.
ions 2na, and the electrostatic correction term ntot(κa/3), which depends on ntot, i.e., zwitterion concentration +na in the adsorbed layer:
The data points are widely scattered and the slope appears to be around 2, not 2.25 in local good solvent conditions including both osmotic and deformation (stretching) contributions.22-24 But the observed thickness L is plotted against (∆π/kT) in Figure 6, and we obtain
π/kT ) (πp + pd)/kT + 2na - ntot(κa/3)l
L ) (∆π/kT)-4/9
(8)
From the calculation of the ideal gas term and the electrostatic correction, we can evaluate ∆π ) [(πp + pd) - πp0] by equating π ) π0. In Figure 5, ∆π/kT is plotted against the average polymer volume fraction φ calculated by adsorbance divided by thickness (A/L) in the adsorbed layer, neglecting the osmotic contribution of bulk solution. (24) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, N.Y., 1979.
(9)
Thus, [(πp + pd) - πp0] ∝ L-9/4. Comparison of Figures 5 and 6 shows that the adsorbed layer thickness may be determined by balancing the osmotic pressure and the elastic pressure of the adsorbed polymer. However, further research is necessary to separate the contribution of πP and Pd experimentally. For that purpose the more sophisticated experimental approach is highly desirable. LA981102U
