Competitive Adsorption of Neutral Comb Polymers and Sodium

Filter By. Menu Switch Switch View Sections. Previous Results. Next Results. ACS2GO © 2019. ← → → ←. Hide Menu Back ...
0 downloads 0 Views 177KB Size
Download PDF
7410

J. Phys. Chem. B 2008, 112, 7410–7419

Competitive Adsorption of Neutral Comb Polymers and Sodium Dodecyl Sulfate at the Air/Water Interface Nicolas Pe´ron,† Richard A. Campbell,‡ Tommy Nylander,‡ Ausvydas Vareikis,§ Ricardas Makuska,§ Tibor Gila´nyi,† and Ro´bert Me´sza´ros*,† Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eo¨tVo¨s Lora´nd UniVersity, H-1117, Budapest, Pa´zma´ny Pe´ter s. 1/A, Hungary, Department of Physical Chemistry 1, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden, and Department of Polymer Chemistry, Vilnius UniVersity, Naugarduko 24, LT-03225, Vilnius, Lithuania ReceiVed: October 19, 2007; ReVised Manuscript ReceiVed: April 3, 2008

The interfacial behavior of aqueous solutions of four different neutral polymers in the presence of sodium dodecyl sulfate (SDS) has been investigated by surface tension measurements and ellipsometry. The polymers comprised linear poly(ethylene oxide) with low and high molecular masses (103 and 106 Dalton (Da), respectively), and two high molecular mass methacrylate-based comb polymers containing poly(ethylene oxide) side chains. The adsorption isotherms of SDS, determined by Gibbs analysis of surface tension data, are nearly the same in the presence of the high molecular mass linear polymer and the comb polymers. Analysis of the ellipsometric data reveals that while a single surface layer model is appropriate for films of polymer alone, a more sophisticated interfacial layer model is necessary for films of SDS alone. For the polymer/ surfactant mixtures, a novel semiempirical approach is proposed to determine the surface excess of polymer, and hence quantify the interfacial composition, through analysis of data from the two techniques. The replacement of the polymer due to surfactant adsorption is much less pronounced for the high molecular mass linear polymer and for the comb polymers than for the low molecular mass linear polymer. This finding is rationalized by the significantly higher adsorption driving force of the larger polymer molecules as well as by their more amphiphilic structure in the case of the comb polymers. 1. Introduction Mixed solutions of polymers and surfactants are used in numerous products in the paint, pharmaceutical, and personal care industries. Therefore, considerable effort has been made to obtain a deeper understanding of the interactions between polymers and surfactants as well as their impact on their different physicochemical properties.1,2 Over the past few years, comb polymers have received increased attention as biodegradable components with added functionality in next generation products. These features arise from their interesting associative behavior in solution3–5 as well as their unique characteristics at interfaces.6–14 The surface properties of polymer/surfactant systems are controlled by the amount of each component adsorbed at the interface.1,15,16 The bulk solution properties are much better understood than the surface behavior, especially at the air/water interface, mainly because the information on the surface composition of multicomponent systems is not easily accessible. In the case of multicomponent adsorption at fluid interfaces, neutron reflection is the most powerful technique in terms of its ability to distinguish different chemical species.17–20 Important basic advances were gained in the field of mixed adsorption of polymers and surfactants, thanks primarily to the work of Thomas and co-workers.21,22 However, this approach requires isotopically labeled compounds and measurement time at large * Corresponding author. Email: [email protected]. Phone: +36 1 372 2514. Fax: +36 1 372 2592. † Eo ¨ tvo¨s Lora´nd University. ‡ Lund University. § Vilnius University.

scale facilities. Therefore, there is a need to develop simple and more accessible methods to probe mixed adsorption in a local laboratory. Among the various optical methods, ellipsometry is a particularly sensitive technique to characterize the optical properties of interfacial films. A common implementation of the technique is null ellipsometry, where two parameters (the ellipsometric angles Ψ and ∆) are measured simultaneously. In the thin film limit at the interface between two dielectric media, only one single independent parameter can be determined from the two ellipsometric angles, namely the coefficient of ellipticity.23,24 In this case, only a total adsorbed amount of the components can be determined, rather than the adsorbed amounts of any individual component,25–32 unless the surface concentration of the polymer is fixed or known.33–35 However, in most cases, the surface excess of the polymer is unknown. Therefore, the composition of the mixed surface layer of macromolecules and surfactants cannot be fully determined by ellipsometry without complementary information about the adsorbed amount of one of the components. Fourier transform infrared (FTIR) reflection techniques provide a spectral signal sensitive to the chemical species present at the air/water interfaces.36 In a recent study by Campbell et al.,37 ellipsometry was combined with external reflection Fourier transform infrared spectroscopy (ER-FTIRS) in order to determine the composition of the mixed surface layers of sodium dodecyl sulfate (SDS) and the oppositely charged poly(dimethyldiallylammonium chloride) at the air/water interface. The surface concentration of SDS was determined directly from ERFTIRS as a result of the very weak polymer resonances. Ellipsometric analysis provided the total adsorbed amount,

10.1021/jp710170d CCC: $40.75  2008 American Chemical Society Published on Web 06/03/2008

Competitive Adsorption of Neutral Comb Polymers SCHEME 1: Structural Formula of the Comb Polymer C100 (m/(n + m) ) 1) and the Copolymer C30 (m/(n + m) ) 0.30)

which enabled the determination of the surface excess of both components. Additionally, ER-FTIRS alone has been used to determine the composition of mixed adsorbed layers at the air/ water interface through the use of chemometric data analysis.38 An alternative way to determine the adsorbed amounts of polymer and surfactant is through analysis of the surface tension data with the Gibbs equation. However, this analysis is not straightforward for multicomponent systems if there is an interaction between the components.22,39 In recent work,39 it was shown that the Gibbs equation can be practically used only at surfactant concentrations below the critical aggregation concentration (cac), since above the cac, the polymer activity is not experimentally accessible. An additional difficulty is connected to the accuracy of the equilibrium surface tension measurements for these systems. There have been several studies that implemented the combination of surface tension measurements with ellipsometry to study mixed adsorption of polyelectrolytes and oppositely charged surfactants, assuming electro-neutral surface complexes of the components.40–42 The majority of these studies focused on the mixed adsorption of linear polymers and ionic surfactants at the air/water interface; the features of mixed surface layers of ionic surfactants with neutral comb polymers are largely unexplored. In the present paper, we combine surface tension measurements and ellipsometry to determine the composition of the mixed surface layers of an anionic surfactant with four different neutral macromolecules at the free aqueous surface. Our focus is on the surface properties of aqueous solutions of SDS in the presence of two linear polymers and two novel comb polymers. The two linear polymers are poly(ethylene oxide) with very different mass averaged molecular masses43: MP ) 103 Dalton (Da) (called “PEO 1 kDa”) and MP ) 106 Da (called “PEO 1000 kDa”). The two comb polymers are shown in Scheme 1. They contain poly(ethylene oxide) methyl ether methacrylate (PEOMEMA) and 2-hydroxyethyl methacrylate (HEMA) segments: a polymer called “C100”, with 100% PEOMEMA segments and a mass averaged molecular mass of 334 kDa, and a copolymer called “C30”, with 30% PEOMEMA and 70% HEMA segments and a mass averaged molecular mass of 1040 kDa. The composition of the mixed adsorbed layers of the low molecular mass PEO 1 kDa with SDS can be readily determined from the Gibbs analysis of surface tension data alone. However, this approach is not applicable to the determination of the surface excess of the high molecular mass polymers.39,44 An empirical approach, combining results from ellipsometry and surface tension measurements, is therefore proposed for the quantitative evaluation of the surface layers of long chain neutral polymers in the presence of SDS. Finally, we discuss the effect of surfactant adsorption on the composition of the mixed surface layers and its implications.

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7411 2. Experimental Section Materials. Sodium dodecyl sulfate (Aldrich) was twice recrystallized from a 1:1 benzene/ethanol mixture and dried for 70 h at 40 °C in vacuum. The critical micelle formation concentration (cmc) of the recrystallized SDS was 8.2 mmol/L without added salt and 1.6 mmol/L in 0.1 M NaBr, as determined by surface tension measurements at 25 °C. The NaBr (Reanal) was analytical grade. The water used was double distilled water for the preparation of samples for tensiometry and Milli-Q ultrapure water for ellipsometry. The poly(ethylene oxide) samples with molecular masses of MP ) 1 kDa and 1000 kDa (Aldrich) were used without further purification. Comb Polymer Synthesis. The comb polymer C100 with high density of PEO side chains was synthesized by free-radical polymerization of poly(ethylene oxide) methyl ether methacrylate macromonomer (molecular mass 2.08 kDa, ca. 45 units long poly(ethylene oxide) side chains) (PEOMEMA). The comb copolymer C30 with a low density of poly(ethylene oxide) side chains was synthesized by free-radical copolymerization of PEOMEMA and 2-hydroxyethyl methacrylate (HEMA) at the monomer ratio in the feed of PEOMEMA:HEMA ) 25:75 mol % (Scheme 1). PEOMEMA (50 wt % aqueous solution; Aldrich) and HEMA (Fluka) were used as received. Free-radical polymerization of PEOMEMA and its copolymerization with HEMA were carried out in polymerization tubes under nitrogen atmosphere in a 50:50 (wt/wt) mixture of 2-propanol and water in the presence of 0.3% azoisobutyronitrile (AIBN). The overall monomer concentration in the solution was 30%, and the polymerization time was 20 h. Specifically, 0.08 g of AIBN (0.487 mmol), 13.47 g of 50% aqueous solution of PEOMEMA (3.238 mmol), 1.2644 g of HEMA (9.716 mmol), 9.3333 g of 2-propanol, and 2.5977 g of water were placed in a polymerization tube for the synthesis of the copolymer C30. The tube was purged with nitrogen gas for 15 min and inserted in an oil bath kept at 60 °C. After the reaction, the content of the tube was quantitatively transferred into Visking dialysis tubing 27/ 32 (Serva, pore size 24 Å, exclusion limits 8-15 kDa) using a minimal amount of distilled water. The copolymer was dialyzed against distilled water for 5 days, changing the solvent two times per day. The dialyzed solution was concentrated with a rotating evaporator, and the polymer was extracted with chloroform and dried in a Petri dish at room temperature for 2-3 days. Finally, the product was dried in a vacuum oven (residual pressure 0.3 mbar) at 60 °C for 6 h to give 7.387 g of the copolymer (yield 92.3%). 1H NMR spectra of the samples dissolved in CDCl were 3 recorded on a UNITY INOVA VARIAN 300 MHz spectrometer at 29 °C. The molecular mass distribution of the polymers was determined by size exclusion chromatography (SEC) using Viscotek TDA 302 triple detection system with light scattering, viscosity, and RI detection. Columns used were 2x ViscoGel GMPWxl (Mixed Bed), eluent was aqueous 0.2 M NaNO3, pH = 7, eluent flow rate was 0.6 mL/min, concentration was approximately 1 mg/mL, and temperature was 35 °C. The composition of the copolymer C30 (HEMA ) 70.2 and PEOMEMA ) 29.8 mol %) was calculated according to the content of hydroxyl groups (OH- (mass %), Table 1), determined by acetylation with acetic anhydride in pyridine solution adapted for semimicroanalysis45 (the details of these calculations are given in the Supporting Information). The characteristic data of the comb methacrylate-based polymers containing PEO side chains are summarized in Table 1. 1H NMR spectra of the dialyzed polymers (see Supporting Information) showed no signals in the region of δ ) 5-6.5

7412 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Pe´ron et al.

TABLE 1: The Characteristic Data of the Investigated Poly(PEOMEMA) Polymer (C100) and Poly(PEOMEMA-HEMA) Copolymer (C30) polymer C100 C30

yield -OHa PEOMEMAb MP IVd (%) (mass %) (mol %) (kDa) MP/MNc (dL/g) 90.1 92.3

0 1.68

100 29.8

334 1040

5.1 7.9

0.23 0.56

a The content of hydroxyl groups in mass percentage. b The mol percentage of PEOMEMA units in the comb polymers. c The polydispersity index of the polymers. d The intrinsic viscosity of the samples.

ppm, indicating the absence of residual monomers and adequate sample purity. The yield of the polymers was high and reached ca. 90%. The average composition of the copolymer C30, estimated according to the content of hydroxyl groups, deviated slightly from the ratio of the monomer feed, showing an increased amount of PEOMEMA units in the copolymer: approximately 30 mol%, from which the name C30 is derived. It should be noted, however, that the determination of copolymer composition from 1H NMR spectra was not precise due to partial overlapping of the signal of protons that belonged to oxymethyl groups of PEOMEMA (3.38 ppm) with the signal of satellites of oxymethylene groups (3.64 ppm). The intrinsic viscosity (IV) of poly(PEOMEMA) (C100) is rather low, suggesting a compact molecular structure. The value IV ) 0.56 for copolymer C30 is much higher than the corresponding value IV ) 0.23 for the comb polymer C100, which is in accordance with the high molecular mass of this polymer and lower density of PEO attachments. The anionic polymerization mechanism of the PEO synthesis suggests a narrow (Poisson) distribution of the PEO molecular mass.46 This prediction was also proved in ref 47, where commercial PEO samples (which included Aldrich products as well) were investigated by size exclusion chromatography, and the polydispersity index was found to be MP/MN > 1.04. Thus the molecular mass of the PEO component of the comb polymers is well-defined and equal to 2 kDa, and MP/MN > 1.1. In contrast, as shown in Table 1, the polymers C100 and C30 have a large molecular mass distribution with polydispersity indices MP/MN ≈ 5-8. High polydispersity of the polymers is predetermined by bimodal molecular mass distribution with peaks at ca. 100 and 500 kDa (see Supporting Information). The more intense peak at 500 kDa and larger MP are characteristic for polymer C30, which contains a larger content of HEMA units and consequently more hydroxyl groups. We suggest that the peak at higher MP is formed during particular chain transfer to the polymer at high conversions, which should be more intense for the polymer containing accessible HEMA units. Chain transfer to the polymer should result in partly branched macromolecules, with the degree of branching higher for sample C30 than sample C100. Chemical analysis and SEC data were insufficient to provide adequate information about the compositional heterogeneity of copolymer C30. One should note, however, that the reactivity ratios of PEO-based macromonomers and of low molecular mass monomers of similar structure (e.g., butyl methacrylate and acrylamide)48–50 do not differ very much. Therefore, the composition of C30 macromolecules formed at various stages of copolymerization (at various conversion of the monomers) can differ; the macromolecules formed at the beginning of copolymerization are expected to contain more units of PEOMEMA than those formed at the end of the process. However, the effect of chemical heterogeneity on the properties of C30 should

Figure 1. Surface tension as a function of cSDS for solutions of SDS alone (O), (a) with 1 g/L of PEO 1 kDa (2), PEO 1000 kDa (0), and (b) C100 (]) and C30 ([). The solutions were prepared with 0.1 mol/L NaBr.

be minor, since variation in the chemical composition of C30 is rather small compared to the large difference in the number of PEOMEMA units between C30 and C100. Formation of block copolymers is not expected in this system. Surface Tension Measurements. Surface tension measurements were carried out by the pendant drop method.51 The setup and the procedure were described previously.39 The drops were formed at the tip of a stainless-steel needle with an external diameter of 2.425 ( 0.005 mm. All measurements were done at 25 ( 0.1 °C. The determination of the equilibrium surface tension, σ, of the mixed polymer/surfactant solutions is practically very difficult. This issue has been discussed in details in refs 39 and 44. In order to obtain a well-defined value of σ, we extrapolate numerically the values to a short surface lifetime at t ) 1 s. According to the results and reasoning of Mysels, the surface tension at t ) 1 s is firmly established here to represent the equilibrium surface tension for SDS concentrations larger than 0.05 mmol/L44. Previously, this procedure was proved to be feasible for the determination of the equilibrium surface tension of PEO/SDS mixtures.39 It should be noted that in the case of the polydisperse comb polymers the extrapolated σ values are characteristic of the local equilibrium of the surface layer at short time rather than of the true equilibrium surface tension of the solutions.52 Ellipsometry. Ellipsometry measures the change in polarization of an elliptically polarized light beam upon reflection at a planar interface. Polarized light can be regarded as the sum of two wave components, where one is parallel and the other normal to the plane of incidence. Reflection causes a phase shift ∆ between these two components, as well as a relative change

Competitive Adsorption of Neutral Comb Polymers

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7413

in amplitude by a factor tan Ψ. The ellipsometric angles Ψ and ∆ are connected to the overall Fresnel reflectivity coefficients of the parallel and perpendicular components, rp and rs, respectively:53

tan Ψ exp i∆ )

rp rs

(1)

The polymer/surfactant solutions were poured into a Petri dish (diameter 8 cm), and then the liquid surface was aspirated using a clean pipet connected to a suction pump. This process was done in order to remove contaminants as well as to start the measurements with well-defined initial concentrations of the polymer and surfactant at the interface. It took about 10 s before the interface was mechanically stable enough so that measurements could be taken. The ellipsometric angles Ψ and ∆ for the polymer/surfactant solutions in 0.1 M NaBr were measured at 25 °C using an Optrel null ellipsometer fitted with a Nd YAG laser (λ ) 532 nm). Ten values of ∆ were then recorded and averaged over a surface age of ∼10-40 s. The measurements were carried out at two angles of incidence φ ) 48° and 58°, i.e., on both sides of the Brewster angle φB ) Arctan(nsol/nair) ) 53.2°. Under the employed conditions, the refractive index of the solutions is nsol ) 1.336, calculated from the refractive indices of NaBr solutions.54 This value is obtained after a corrections using the Lorentz-Lorenz law and the density55 of salt solutions to account for the ambient temperature 25 °C. The contribution to nsol of the surfactant and of the polymer in solution are negligible. The refractive index of air, nair, was assumed to be 1.000. 3. Results and Discussion Surface Tension Data. Figure 1 shows measurements of σ of solutions of SDS alone and in the presence of the four polymers, each at a constant polymer concentration, cp, of 1 g/L. In the case of PEO 1 kDa, there is only one critical concentration, which is nearly identical to the cmc of pure SDS in 0.1 M NaBr (1.6 mmol/L). This finding is in line with recent observations that the interaction between SDS and PEO is negligible for polymer molecular masses of up to about MP ) 1 kDa.56 For the high molecular mass polymers (PEO 1000 kDa and the comb polymers C100 and C30), σ decreases as a function of log10(cSDS) with an increasing slope until a defined concentration, where the slope starts to decrease (∼1.1 mmol/L). This concentration, the cac, is indicative of surfactant binding to the polymer and is similar for all the high molecular mass polymers studied. The surface tension continues to decrease with increasing SDS concentration, due to the slight increase of the free surfactant concentration. At a second critical concentration (∼12 mmol/L, not in the range of Figure 1), σ becomes almost constant, since at this point free surfactant micelles start to form in the solution. We also note that the surface tension values are slightly lower for PEO 1000 kDa and for the comb polymers than for PEO 1 kDa at cSDS < cac. In recent work, it was shown that below the cac the σ[log10(cSDS)] curve tends to a lower universal limit with increasing PEO molecular mass (reached at MP ≈ 8 kDa).39 The comb polymers in the present study (Figure 1b) have the same effect on the surface tension as the high molecular mass linear polymer PEO above the universal limit (Figure 1a). As rationalized previously,39 in the presence of a large excess of NaBr, the surface excess of dodecyl sulfate ions ΓDS- for cSDS < cac can be determined from the surface tension isotherms measured at constant polymer concentration cP:

Figure 2. Surface excess of DS- calculated from eq 2 as a function of cSDS for solutions of SDS alone (O) and with 1 g/L of PEO 1 kDa (2), PEO 1000 kDa (0), and C100 (]). Solutions were prepared with 0.1 mol/L NaBr.

ΓDS- ) -

(

∂σ 1 RT ∂ lncSDS

)

(2) T,p,cP

Figure 2 shows the adsorption isotherms of SDS calculated using eq 2 for SDS alone and in the presence of three polymers. There were insufficient surface tension data to perform the calculation for polymer C30, but it is reasonable to consider the calculations for C100 as representative of both the comb polymers because the values of the σ[log10(cSDS)] curves of C100 and C30 overlap. As expected, ΓDS- is larger for pure SDS solutions than for solutions of SDS mixed with polymer, due to the absence of competitive adsorption from the macromolecules in the former case. The adsorption isotherms of the surfactant in the presence of the different polymers are similar. The values of ΓDS- are the same to within experimental error for the high molecular mass samples but are slightly elevated for PEO 1 kDa. The surface excess of polymer ΓP can be determined from surface tension isotherms measured as a function of cP at fixed concentration of surfactant provided that there is no polymer/ surfactant interaction in the bulk:39

ΓP ) -

( )

1 ∂σ RT ∂lncP

(3) T,p,cSDS

Table 2 shows calculated values of ΓP (in part reported previously39) for the PEO 1 kDa/SDS system. The polymer adsorption decreases significantly with increasing SDS concentration due to the increasing adsorption of the surfactant. At cSDS ) 0.8 mmol/L, significantly below the cmc, ΓP is more than 1 order of magnitude lower than the equivalent value recorded without the surfactant. The uncertainty in ΓP significantly increases with increasing the polymer molecular mass because the response in σ(cP) decreases to the level of the measurement error.44 Therefore, ΓP can be determined from the Gibbs analysis only for polymers of low polydispersity with a molecular mass of less than a few kDa, i.e., only for one of the four polymers that we studied here (PEO 1 kDa). In the following sections, we combine surface tension measurements and ellipsometry in order to determine the adsorbed amount of the high molecular mass polymers. Ellipsometry Data. In the thin film limit for monomolecular layers,57 i.e., where the film thickness d , λ, the ellipsometric angle Ψ is insensitive to the build up of a film between two dielectric and transparent media. The ellipsometric parameter ∆ is strongly affected by the properties of the film (i.e., surface excess, thickness, roughness, and anisotropy), while Ψ is most

7414 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Figure 3. The ellipsometric angle difference δ∆ (∆ for polymer/ surfactant solutions - ∆ for pure water) as a function of cSDS for solutions of SDS alone (O) and with 1 g/L of PEO 1 kDa (2), PEO 1000 kDa (0), C100 (]), and C30 ([) at an incident angle of φ ) 48° and SDS alone (b) and with 1 g/L of PEO 1 kDa (4) and PEO 1000 kDa (9) at an incident angle of φ ) 58°. The δ∆ values measured for pure polymer solutions (without SDS) are indicated by horizontal bars through the left axis. Solutions were prepared with 0.1 mol/L NaBr.

Figure 4. Schematic representations of an adsorbed surfactant film at the air/solution interface with (a) a single layer and (b) a dual layer model.

sensitive to changes in the properties of the two adjoining (liquid and gas) phases or to errors in the settings of the optical components or in the incident angle. In the systems studied in this work, the response of Ψ to the presence of surface active species was 2 orders of magnitude lower than the response of ∆. However, values of Ψ were recorded for each solution to confirm that there were no changes in the alignment of the ellipsometer; systematic variations in Ψ with changing surface coverage were observed to be commensurable with the standard error of the measurements. Figure 3 shows the ellipsometric angle difference δ∆ ) ∆ - ∆0, where ∆ is the value for the polymer/surfactant solution and ∆0 is the value for a clean water surface, for solutions of SDS alone and in the presence of the four polymers. The δ∆ values measured at φ ) 48° and at φ ) 58° have opposite signs because they are on opposite sides of the Brewster angle φΒ ) 53.2°. We note that the age of the interface in the ellipsometric measurements (between 10 and 40 s) is larger than the surface age (1 s) used for the determination of the “equilibrium” surface tension. However, ellipsometry is expected to be less sensitive than the surface tension to surface active impurities like dodecanol present even in samples of purified and recrystallized sodium dodecyl sulfate.44 For the pure SDS solutions, |δ∆| monotonically increases with increasing cSDS until a plateau is reached at the cmc. The constancy of |δ∆| beyond the cmc is expected as the surface excess of DS- is nearly independent of cSDS at this concentration range. For SDS solutions in the presence of polymer, |δ∆| differs significantly from the values obtained for SDS alone, an effect

Pe´ron et al.

Figure 5. The ellipsometric angle difference δ∆ as a function of the layer thickness d at the air/solution interface. Dashed line: δ∆ calculated from eq 4 using eq 7 for η. Solid line: δ∆ derived from the exact calculation starting from eq 4.63 of ref 60. The following parameters were used in both calculations: φ ) 48°, nsol ) 1.336, nair ) 1.000, and nlay ) 1.350.

that becomes more pronounced at low cSDS. The surfactant-free limits (marked with horizontal lines) show that the adsorbed amount of the pure polymers increases in the order: PEO 1 kDa < PEO 1000 kDa < C30 = C100. At high cSDS, |δ∆| converges toward a common limit for all the polymer-containing solutions, which is only slightly higher than the corresponding values measured for SDS alone. These trends suggest that the solution surface is rich in polymer at low cSDS and rich in surfactant at high cSDS. However, we stress that it is not valid to draw firm conclusions about the composition of the interface from ellipsometry alone. It is the combination of ellipsometry and surface tension measurements that enables us later to quantify the composition of the mixed polymer/surfactant films. In the following data analysis, we neglect the contribution of scattering by thermal capillary waves to δ∆, which is valid because of the minor influence of surface roughness on ∆ from changes in the surface tension.58 The quantity δ∆ in the thin film limit is first order with respect to the ratio l/λ (l is a length characteristic of the film thickness) by:59

δ∆ ) g(φ)

η λ

(4)

where g(φ) is a function that depends only on bulk properties and on the angle of incidence

g(φ) )

4πnairnsol2 cosφ sin2φ (nsol2 - nair2)[(nair2 + nsol2)cos 2φ - nair2]

(5)

and η is the ellipsometric thickness given by:

η)

∫-∞+∞

(n(z)2 - nair2) (n(z)2 - nsol2) n(z)2

dz

(6)

where z is the vertical coordinate with respect to the interface and n(z) is the refractive index at depth z. We stress here that the ellipsometric thickness η is a length typically in the subnanometer regime and does not correspond to the film thickness d. The validity of this first-order approximation (eqs 4–6) can be assessed through a comparison with an exact calculation for the model shown in Figure 4a, where a homogeneous and isotropic adsorbed layer delimited by sharp interfaces is assumed. With the definition n(z) ) nlay in the layer of thickness d, eq 6 gives:

Competitive Adsorption of Neutral Comb Polymers

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7415 TABLE 2: The Surface Excess of PEO 1 kDa Calculated from the Gibbs Analysis of Surface Tension Isotherms cSDS (mmol/L)

ΓP (µmol/m2)a

0 0.02 0.1 0.2 0.4 0.6 0.8

0.301 ( 0.035 0.289 ( 0.035 0.243 ( 0.035 0.168 ( 0.035 0.104 ( 0.027 0.074 ( 0.017 0.028 ( 0.017

a Equation 3 was applied to solutions in 0.1 mol/L NaBr at constant cSDS and varying bulk polymer concentration, cP, in the same polymer concentration range as reported previously.39

Figure 6. The ellipsometric thickness η (which was calculated from eq 4 using the δ∆ values at φ ) 48° and 58° in Figure 3) as a function of cSDS (the symbols correspond to those used in Figure 3). The η values for pure polymer solutions (without SDS) are indicated by horizontal bars through the left axis.

ηSDS )

(nlay2 - nair2)(nlay2 + 2) 3 nlay2

[(nsol2 + 2)ASDS (nsol2 - 1) VSDS] ΓDS- (10)

(nlay - nair ) (nlay - nsol ) d 2

η)

2

2

nlay2

2

(7)

Figure 5 shows calculations of δ∆ as a function of d using eqs 4–7 and from an exact calculation using eq 4.63 given in ref 60 (nlay ) 1.35 in both cases). The difference between the two approaches is not more than a few percent as long as δ∆ does not exceed 0.024 rad, which is the maximum value of our data for φ ) 48° (see Figure 3). We verified that this conclusion is general whatever the value of nlay is in the range 1.35 < nlay < 1.50. Therefore, the usage of eqs 4–7 is justified for the analysis of δ∆ in the present work. Figure 6shows calculations of the ellipsometric thickness η ) λ(δ∆)/g(φ) for solutions of SDS alone and in the presence of the four polymers, derived from the δ∆ values in Figure 3. Except for the comb polymers C100 and C30, the ellipsometric data were recorded at two angles of incidence to verify the experimental data. Note that the angle functions g(48°) ) 42.77 and g(58°) ) -9.97 have opposite signs. The calculated values of η from the data measured at φ ) 48° and 58° coincide, which confirms that ∆ was not influenced strongly by the incident angle and that the ellipsometer was properly aligned. Now we turn our attention to details of the mixed surface layers. Assuming a thin film with sharp interfaces, as sketched in Figure 4a, the Lorentz-Lorenz equation for the refractive index of mixtures gives an equation relating nlay and d:61

nlay2 - 1 nlay2 + 2

)

nsol2 - 1 nsol2 + 2

(1 - VSDS

˜ ΓDSΓP ΓDS- νP ) + ASDS + d d d ΓP AP ˜ (8) MP d

where VSDS is the partial molar volume of SDS, νP is the partial molar volume of the polymer, Γ˜ P is the polymer surface excess in mass per unit area, ASDS and AP are the molar refractivities of SDS and of the polymer, respectively, and MP is the molecular mass of the polymer. The variable d can be eliminated by combining eqs 7 and 8, and η can then be expressed as a sum of separate contributions from SDS and the polymer, which are linear with respect to ΓDS- or to Γ˜ P:

η ) ηSDS + ηP with

(9)

and

ηP )

(nlay2 - nair2) (nlay2 + 2) 3 nlay

2

[(nsol2 + 2)

AP MP

(nsol2 - 1) νP] ˜ ΓP (11) In order to test the validity of this approach, we compare the experimental value of ηP for PEO 1 kDa from Figure 6 with a calculation using eq 11. For the calculation, we use the values of νP and AP/MP in Appendix A and make the assumption that nlay lies between 1.36 and 1.42 (corresponding to a polymer volume fraction between 0.15 and 0.51). We choose PEO 1 kDa for the test because we have an independent measurement of the adsorbed amount of the pure polymer, Γ˜ P ) 0.301 ( 0.035 mg/m2, from surface tension analysis (Table 2). The calculation based on eq 11 gives the result ηP ) 0.054 ( 0.010 nm, which is in good agreement with the experimental value of ηP ) 0.062 ( 0.006 nm. This test on PEO 1 kDa has demonstrated the validity of using eq 11 in calculating values of ηP for the high molecular mass polymers, for which we do not have independent measurements of Γ˜ P. A good agreement between the adsorbed amounts calculated from the surface tension data and the ellipsometry data is not obtained, however, for pure SDS solutions. From our surface tension data, ΓDS- ) 4.3 µmol/m2 at bulk SDS concentrations around the cmc (Figure 2). This value is in good agreement with reported measurements from neutron reflectivity.63 The value ηSDS ) 0.193 ( 0.006 nm is derived by the insertion of ΓDS- ) 4.3 µmol/m2 into eq 10 and using the values of VSDS and ASDS/MSDS from Appendix A and the assumption that nlay is between 1.36 and 1.42. In contrast, the ellipsometric measurements give ηSDS ) 0.112 ( 0.005 nm around and above the cmc (Figure 6). This discrepancy demonstrates that the single surface layer model (given by eq 10) does not adequately represent the optical properties of the air/SDS solution interfacial region. In Appendix B, the measured ellipsometric thickness ηSDS ) 0.112 nm is rationalized by accounting for the complex stratified structure of the SDS surface layer sketched in Figure 4b (a dual surface layer model) to distinguish the headgroup and hydrocarbon chain regions of the monolayer. Other relevant factors include the roughness related to the thermal fluctuations, the ions in the diffuse double layer, and a slight decrease in the density of the hydrocarbon chain layer compared with the bulk condensed material.

7416 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Pe´ron et al.

Figure 7. Calculations of η as a function of cSDS for solutions of SDS with 1 g/L of PEO 1 kDa. The data are repeated from Figure 6 for incident angles of φ ) 48° (2) and 58° (4), and calculated from eq 11 using the data in Table 2( × ). Dotted line: fitting of eq 12. The value of ηP(0) is fixed to the value 0.0539 nm calculated from eq 11 with ΓP ) 0.301 mg/m2 from Table 2. The best fit value of c* is 0.378 mmol/ L.

Description of the Mixed Layers of Polymers and SDS. In light of the different optical models required to treat surface layers containing SDS or PEO, a semiempirical analysis of the experimental η values of the polymer/surfactant mixtures is proposed. The measured η values are divided into separate contributions from the polymer and the surfactant: η ) ηSDS + ηP. Furthermore, it is assumed that the theoretical ηp(ΓP) relationship from eq 11 holds in the presence of the surfactant. Since the Gibbs analysis of the surface tension data (eq 2) provides the surface excess of the surfactant regardless of the size of the macromolecule, this approach could provide reasonable estimates for ηP as well as for the adsorbed amount of the polymer, provided that the assumptions mentioned above are justified. As shown previously, for mixtures of SDS with the low molecular mass polymer PEO 1 kDa, the surface excesses of both the surfactant and the polymer can be determined from surface tension measurements. Therefore, the SDS + PEO 1 kDa mixture can be used as a benchmark calibration procedure to determine the surface excess of the larger polymers. Figure 7 shows the experimental η(cSDS) curve for the SDS + PEO 1 kDa mixture (triangles) replotted from Figure 6. Also shown in this figure is the estimated ηP(cSDS) curve for the polymer contribution (diagonal crosses), calculated using eq 11 and the data from Table 2. Empirically, these ηP(cSDS) data are found to be well described by the exponential plotted with a dotted line:

cSDS ηP(cSDS) ) ηP(0) exp() c*

(12)

where the parameter values are given in the caption of Figure 7. Let us suppose that the presence of surfactant molecules and their counterions in the surface layer does not affect the validity of the ηp(ΓP) linear relationship of eq 11. In the case of the SDS + PEO 1 kDa mixture, this assumption gives an effective ellipsometric thickness of SDS η’SDS, which is a function of ΓDS-:

η′SDS)η-ηP

(13)

where ηP is calculated using eq 12. Figure 8 shows the values of η′SDS calculated using eq 13 with respect to ΓDS- (triangles). As it is indicated by Figure 8, these data points are in good agreement with the ηSDS data

Figure 8. Contribution of SDS to η as a function of the surface excess of dodecyl sulfate ions ΓDS- determined from the Gibbs analysis for solutions of SDS alone (O) and in the presence of 1 g/L PEO 1 kDa calculated from eq 13 (4). The dashed line is the calibration line ηSDS ) k ΓDS-.

directly measured for SDS alone at large values of ΓDS- (circles). The η′SDS(ΓDS-) function is extrapolated from these points to the origin by a linear equation η′SDS(ΓDS-) ) kΓDS-, with k ) 0.25 × 10-3 m3 mol-1. The linearity of η(Γ) for surfactants at the air/water interface is a matter of ongoing discussion.64–66 The ellipticity coefficient Fj is widely studied in the literature instead of η, but the two quantities are proportional. For several nonionic and cationic surfactants, the η(Γ) relationship obtained from a combination of ellipsometry and neutron reflection measurements is linear.67–69 Moreover, previous measurements by ER-FTIRS combined with ellipsometry for SDS in the presence of a polymer37 appear to confirm well the linearity of the relationship between η′SDS and ΓDS-, which is also assumed in the present work. It should be noted, however, that the line in Figure 8 only demonstrates our calibration procedure and the data points do not provide evidence for the shape of the η(Γ) function of surfactants in general. The adsorption isotherms of SDS in the presence of the different polymers are very similar (Figure 2), so it is reasonable to conclude that the same effective η′SDS(ΓDS-) is valid for all four of the polymers studied in this work and not just for PEO 1 kDa. In this case, the adsorbed amount of the polymer can be determined from the following empirical formula:

˜ ΓP )

η - η′SDS(ΓDS-) (nlay - nair )(nlay + 2) 2

2

3nlay2

2

[(nsol2 + 2)

AP - (nsol2 - 1) νP] MP (14)

This equation allows the computation of Γ˜ P for all of the investigated polymers in the range cSDS < cac from the measured ellipsometric thickness η and the values of ΓDS- obtained independently from the Gibbs analysis of the surface tension data. In the case of the comb polymers, we also assume that the values of νP and AP/MP for PEO can be used for calculating the surface excess with eq 14. One should bear in mind that eq 14 is a semiempirical expression which is based on several crucial assumptions. Therefore, it is helpful to compare directly the calculated adsorbed amount of the polymers with the results of independent measurements. There are no available data in the literature for the adsorbed amounts of PEO 1000 kDa, C100, and C30 at the air/water interface in the presence of SDS. However, Cooke et al. demonstrated in a series of neutron reflection studies that above MP ) 25 kDa universal behavior may be expected, i.e.,

Competitive Adsorption of Neutral Comb Polymers

Figure 9. Surface excess of PEO 1000 kDa, Γ˜ P, as a function of the reduced concentration cSDS/cmc, calculated from eq 14 using the calibration curve for η′SDS (dashed line in Figure 8) (0). The error bars account for the assumption that 1.35 < nlay < 1.45 and for the standard deviation of the ellipsometric measurements. Reproduced data of PEO 25 kDa in the presence of SDS without added salt from neutron reflection measurements by Cooke et al. (b).71

Figure 10. Calculations of Γ˜ P as a function of cSDS in the presence of 1 g/L of PEO 1 kDa (2), PEO 1000 kDa (0), C100 (]), and C30 ([). Values are derived from eq 14 using the measured η values of the polymer/surfactant mixtures and the calibration curve for η′SDS(ΓDS-) (dashed line in Figure 8) for the high molecular mass polymers and are taken from Table 2 for PEO 1 kDa.

the size of the PEO molecules does not significantly influence the adsorbed amount of the polymer in the presence of dodecyl sulfate salts with different counterions.70,71 Figure 9 shows values of Γ˜ P for PEO 1000 kDa calculated using eq 14 and for PEO 25 kDa from neutron reflection measurements,71 each in the presence of SDS. It should be noted that in this latter study the experiments were carried out without added salt. The higher ionic strength (0.1 M NaBr) in our experiments means an increased adsorption driving force of the SDS at the same surfactant concentration. Therefore, the concentration of SDS is plotted on a relative cSDS/cmc scale. The results from the two experimental methods are in excellent agreement, which demonstrates the validity of the present semiempirical approach to determine the adsorbed amount of polymer from the ellipsometric data. Figure 10 shows values of Γ˜ P calculated using eq 14 for the three high molecular mass polymers and derived from surface tension analysis only for the low molecular mass PEO 1 kDa. The Γ˜ P value for the pure PEO 1000 kDa, taken as a measure of saturation adsorption, is significantly larger than for PEO 1 kDa. This finding is in line with earlier observations, where the saturation adsorbed amount of PEO was found to increase with the molecular mass, and above MP ≈ 20 kDa it became independent of the polymer size with a universal limit of ∼0.6-0.8 mg/m2.44,72,73 The values for comb polymers C100 and C30 are higher still (the difference between the saturation

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7417 adsorption of C100 and C30 is within the experimental error). The specific molecular architecture of the comb polymers can explain the large surface coverage. In particular the pronounced hydrophobic nature of the methacrylate backbone makes the molecule quite amphiphilic. Thus the hydrophobic part of the polymer is likely to be positioned at the air/water interface with the hydrophilic PEO side chains extending into the solution in a brush-like conformation.6 The main effect of increasing cSDS is that the surfactant replaces all four neutral polymers at the interface. In the case of PEO 1 kDa, the polymer adsorption is negligible at cSDS ≈ 1 mmol/L. For the high molecular mass samples, the adsorbed amount of polymer decreases gradually with increasing surfactant concentration for cSDS < cac. The adsorbed amount of polymer is roughly one-third at cSDS ≈ 1 mmol/L compared with the case without surfactant, and therefore polymer remains adsorbed at the air/solution interface even above the cac. It is interesting to note that despite the large differences in the segmental surface coverage of the polymers the surface tension isotherms of the mixtures of SDS with PEO 1000 kDa and with the comb polymers are the same within the experimental error (see Figure 1). We interpret the pronounced decrease in adsorption of PEO 1 kDa molecules as resulting from the significantly lower adsorption driving force of this polymer compared with the longer chain macromolecules. The gradual replacement of the high molecular mass polymers from the surface by SDS adsorption is not easy to explain, since the standard free energy change of adsorption for a long chain PEO as a whole is comparable with the energy of a chemical bond.44 We hypothesize that competitive adsorption of SDS is related to the changes in the average conformation (and therefore the effective adsorption driving force) of the adsorbed macromolecules with increasing SDS concentration. This phenomenon was observed for mixed adsorbed layer of PEO and SDS at various solid/ aqueous interfaces.74,75 However, this hypothesis requires further modeling and experimental investigations. Finally, let us comment on the surfactant concentration region cSDS > cac. In this case the proposed ellipsometric analysis cannot be performed, since the surface excess of SDS is not accessible from surface tension measurements. However, one might argue that the polymer/surfactant complexation significantly decreases the activity of the free polymer molecules between the cac and cmc, which may result in a diminishing adsorbed amount of the macromolecules. Furthermore, as the experimental data in Figure 3 revealed, the δ∆ values of all the investigated polymer/surfactant mixtures converge toward a common limit with increasing SDS concentration, which is only slightly above the values measured for pure SDS solutions at and above the cmc. This finding indicates qualitatively that the adsorbed layer consists mainly of SDS with a minor amount of residual polymer. Conclusion We have used a combination of surface tension measurements and ellipsometry to determine the surface composition of mixed layers of SDS with neutral linear and comb polymers. The analysis of the surface tension data below the critical aggregation concentration of SDS provided the surface excess of the surfactant in the presence of the different polymers. A single surface layer model is adequate for the analysis of the ellipsometric data for the polymer-containing films, but a more sophisticated stratified layer model is necessary for the treatment of pure SDS monolayers.

7418 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Pe´ron et al.

The surface tension isotherms for solutions of neutral polymers with SDS reveal common features for the high molecular mass linear PEO 1000 kDa and comb polymers C100 and C30. The largest surface segment densities were found for the comb polymers as a consequence of their more pronounced amphiphilic nature. We interpret this finding as evidence for a brush-like conformation at the interface where the hydrophobic methacrylate-based backbone is positioned at the air/water interface and the more hydrophilic PEO-based side chains are extended into the solution. PEO 1 kDa was almost completely replaced by SDS even at low SDS concentrations. In contrast, the high molecular mass polymers were still not entirely replaced by SDS at significantly higher surfactant concentrations, i.e., polymer remained adsorbed at the air/solution interface above the cac. We attribute this observation to the significantly higher adsorption driving force of the high molecular mass macromolecules as well as their more amphiphilic structure in the case of the comb polymers. Acknowledgment. This research was supported by a Marie Curie Research Training Networks, MRTN-CT-2004-512331 within the sixth European Community RTD Framework Programme. The work was also sponsored by the Hungarian Scientific Research Fund (OTKA-NKTH K-68027). R. Me´sza´ros is a Bolyai Ja´nos fellow of the Hungarian Academy of Sciences, which is gratefully acknowledged. We thank Ama´lia Mezei for assistance in the surfactant purification, Imre Varga for support with the pendant-drop instrument, and Zsolt Varga for designing a very helpful software utility. We also thank Colin Bain for critical reading of the manuscript and suggestions. Appendix A: The Ratio of A/M for SDS or PEO For a solution of SDS in water, we can apply the Lorentz-Lorenz relation: 2 ASDS n2 - 1 nw - 1 ) (1 - φSDS) + C 2 2 MSDS SDS n + 2 nw + 2

Appendix B The monolayer of SDS alone is assumed to be stratified, as sketched in Figure 4b, into two sublayers: the chain layer comprises the R12 dodecyl chains and the headgroup layer comprises water and the SO4Na groups. The total ellipsometric thickness with respect to pure water consists of four terms:

ηSDS)ηCL+ηHL+∆ηR+ηD

(B1)

where ηCL is the contribution of the alkyl-chain layer, ηHL is that of the head group layer, ∆ηR accounts for the roughness due to the capillary waves related to thermal agitation, and ηD is the contribution from the diffuse layer of counter- and coions. The chain layer is assumed to consist of dodecane where the relative density with respect to bulk liquid dodecane is denoted f (the relative density f is presumed to be between 0 and 1). The refractive index of this layer nCL can be obtained from solving the Lorentz-Lorenz equation:

nCL2 - 1 nCL2 + 2

) AR12

Fdodecane f Mdodecane

(B2)

Here, AR12 is the molar refractivity of the dodecyl chains, for which we used the interpolated value of 57.03 cm3/mol at λ ) 532 nm from the data by Vogel,79 Fdodecane) 0.7487 g/cm3 is the density of liquid dodecane55 and Mdodecane) 170.3 g/mol. The ellipsometric thickness contribution ηCL can be calculated as a function of f by eq 7 setting d ) (MdodecaneΓDS-)/(fFdodecane) and nlay ) nCL. For f ) 1, the calculated contribution is ηCL ) 0.107 nm. The value of ηCL decreases as f decreases, i.e., as the medium is diluted with void. The optical index of the headgroup layer nHL can be calculated by solving the equation:

nHL2 - 1 nHL2 + 2

)

nsol2 - 1 nsol2 + 2

(1 -

VSO4Na ΓDSdHL

) + ASO4Na

(A1)

ΓDSdHL (B3)

76

(A4)

Here VSO4Na is 36.45 cm3/mol by Vass et al. and ASO4Na is the molar refractivity of sulfate groups, which is assumed to be 11.34 cm3/mol.80 The variable dHL is the thickness of the headgroup layer, which is assumed to be 0.5 nm (the final result of the model depends little on the arbitrary value of dHL within this order of magnitude). The value found for nHL is 1.395. The resulting ellipsometric thickness contribution ηHL is 0.039 nm. A maximum ellipsometric thickness contribution due to the diffuse double layer can be estimated by using eq 7 where d is set to the Debye length (0rRT/2cF2)1/2 = 1.4 nm where 0 ) 8.85 10-12 J-1 C2 m-1, r = 80, c ) 102 mol/m3, and F ) 9.6 104 C mol-1 and where nlay is set to the optical index of water alone, i.e., without any ion: nlay ) 1.3346, while nsol ) 1.336. The resulting contribution to the ellipsometric thickness ηD is equal to -0.005 nm. The contribution of roughness due to the thermal capillary waves is the difference ∆ηR between ηR calculated62 for the surfactant solution and for pure water. The calculated contribution is ∆ηR ) -0.007 nm. Summing up the four terms, eq B1 gives ηSDS ) 0.135 nm for a value of f ) 1. Full agreement with the measured value 0.112 nm is found for f = 0.96, i.e., for a slightly lower density of the dodecyl chains with respect to bulk dodecane.

Here, CP denotes the polymer mass concentration. Setting νP ) 0.84 cm3/g and dn/dCP ) 0.134 cm3/g from Venohr et al.,78 eq A4 gives AP/MP ) 0.249 cm3/g.

Supporting Information Available: Additional experimental information is discussed in the text. This information is available free of charge via the Internet at http://pubs.acs.org.

where n and nw are the refractive indices of the solution and pure water, respectively, CSDS is the mass concentration of SDS (mass per unit volume), φSDS is the volume fraction occupied by SDS:

φSDS )

VSDS C MSDS SDS

(A2)

The known value of the refractive index increment dn/dCSDS needs to be converted into the ratio ASDS/MSDS. From eq A1, one can derive the following expression:

nw2 - 1 VSDS 6 nw ASDS dn ) 2 + C )0 (A3) MSDS nw + 2 MSDS (nw2 + 2)2 dCSDS SDS The calculated value is ASDS/MSDS ) 0.237 cm3/g using VSDS ) 237.7 cm3/mol from Vass et al.,76 dn/dCSDS ) 0.1195 cm3/g from Mysels and Princen77 and nw ) 1.3346 for water at 25 °C. Equivalently, for PEO:

nw2 - 1 6 nw AP dn νP + ) 2 2 2 dC CP)0 MP nw + 2 (nw + 2) P

Competitive Adsorption of Neutral Comb Polymers References and Notes (1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Nylander, T.; Samoshina, Y.; Lindman, B. AdV. Colloid Interface Sci. 2006, 123, 105. (3) Basak, P.; Nisha, C. K.; Manorama, S. V.; Souvik, Maiti.; Jayachandran, K. N. J. Colloid Interface Sci. 2003, 262, 560. (4) Duschner, S.; Gro¨hn, F.; Maskos, M. Polymer 2006, 47, 7391. (5) Middleton, H.; English, R. J.; Williams, P. A.; Broze, G. Langmuir 2005, 21, 5174. (6) Sartori, A.; Johner, A.; Viovy, J.-L.; Joanny, J.-F. Macromolecules 2005, 38, 3432. (7) Tian, P.; Uhrig, D.; Mays, J. W.; Watanabe, H.; Kilbey, S. M. Macromolecules 2005, 38, 2524. (8) Whitby, C. P.; Scales, P. J.; Grieser, F.; Healy, T. W.; Kirby, G.; Lewis, J. A.; Zukoski, C. F. J. Colloid Interface Sci. 2003, 262, 274. (9) Zhao, H.; Kang, X.; Liu, L. Macromolecules 2005, 38, 10619. (10) Zhou, Y.; Liedberg, B.; Gorochovceva, N.; Makuska, R.; Dedinaite, A.; Claesson, P. M. J. Colloid Interface Sci. 2007, 305, 62. (11) Nilsson, M.; So¨derman, O.; Johansson, I. Colloid Polym. Sci. 2006, 284, 1229. (12) Striolo, A.; Jayaraman, A.; Genzer, J.; Hall, C. K. J. Chem. Phys. 2005, 123, 64710. (13) van der Linden, C. C.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1996, 29, 1000. (14) Zhang, Z.; Ma, H.; Hausner, D. B.; Chilkoti, A.; Beebe, T. P. Jr. Biomacromolecules 2005, 6, 3388. (15) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (16) Holmberg, K. ; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; Wiley: New York, 2002. (17) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (18) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061. (19) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Bell, C. Langmuir 2006, 22, 8840. (20) Staples, E.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Lu, J. R. Langmuir 1993, 9, 1651. (21) Brumm, T.; Naumann, C.; Sackmann, E.; Rennie, A. R.; Thomas, R. K.; Kanellas, D.; Penfold, J.; Bayerl, T. M. Eur. Biophys. J. 1994, 23, 289. (22) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2007, 132, 69. (23) Monteux, C.; Llauro, M.-F.; Baigl, D.; Williams, C. E.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 5358. (24) Monteux, C.; Williams, C. E.; Bergeron, V. Langmuir 2004, 20, 5367. (25) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441. (26) Shubin, V. Langmuir 1994, 10, 1093. (27) Joabsson, F.; Lindman, B. Prog. Colloid Polym. Sci. 2000, 116, 74. (28) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499. (29) Abraham, T.; Giasson, S. Colloids Surf., A 2001, 180, 103. (30) Samoshina, Y.; Diaz, A.; Becker, Y.; Nylander, T.; Lindman, B. Colloids Surf., A 2003, 231, 195. (31) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753. (32) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 6692. (33) Konradi, R.; Ru¨he, J. Macromolecules 2005, 38, 6140. (34) Mann, E. K.; Lee, L. T.; He´non, S.; Langevin, D.; Meunier, J. Macromolecules 1993, 26, 7037. (35) Langevin, D. ReV. Inst. Fr. Pet. 1997, 52, 109. (36) Grandbois, M.; Desbat, B.; Salesse, C. Biophys. Chem. 2000, 88, 127. (37) Campbell, R. A.; Ash, P. A.; Bain, C. D. Langmuir 2007, 23, 3242. (38) Day, J. P. R.; Campbell, R. A.; Russell, O. P.; Bain, C. D. J. Phys. Chem. C 2007, 111, 8757. (39) Pe´ron, N.; Me´sza´ros, R.; Varga, I.; Gila´nyi, T. J. Colloid Interface Sci. 2007, 313, 389. (40) Asnacios, A.; Langevin, D.; Argillier, J.-F. Macromolecules 1996, 29, 7412. (41) Asnacios, A.; Klitzing, R.; Langevin, D. Colloids Surf., A 2000, 167, 189. (42) McLoughlin, D.; Langevin, D. Colloids Surf., A 2004, 250, 79.

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7419 (43) The mass averaged molecular mass is defined as MP ) (ΣiNiMi2)/ (ΣiNiMi) where Ni is the number of molecules of molecular mass Mi. The number averaged molecular mass is defined as MN ) (ΣiNiMi)/(ΣiNi). (44) (a) Mysels, K. J. Langmuir 1986, 2, 423. (b) Gila´nyi, T.; Varga, I.; Gila´nyi, M.; Me´sza´ros, R. J. Colloid Interface Sci. 2006, 301, 428. (45) Ogg, C. L.; Porter, W. L.; Willits, C. O. Ind. Eng. Chem. Anal. Ed. 1945, 17, 394. (46) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 337. (47) Barentin, C.; Muller, P.; Joanny, J. F. Macromolecules 1998, 31, 2198. (48) Xiao, H.; Pelton, R.; Hamielec, A. Polymer 1996, 37, 1201. (49) Horgan, A.; Saunders, B.; Vincent, B.; Heenan, R. K. J. Colloid Interface Sci. 2003, 262, 548. (50) Kawaguchi, S.; Winnik, M. A.; Ito, K. Macromolecules 1996, 29, 4465. (51) Hartland, S.; Hartley, R. W. Axisymmetric Fluid-Liquid Interfaces; Elsevier: Amsterdam, 1976. (52) An, S. W.; Thomas, R. K.; Forder, C.; Billingham, N. C.; Armes, S. P.; Penfold, J. Langmuir 2002, 18, 5064. (53) Rippner Blomqvist, R,.; Benjamins, J.-W.; Nylander, T.; Arnebrant, T. Langmuir 2005, 21, 5061. (54) Foss, J. G.; Schellman, J. A. J. Chem. Eng. Data 1964, 9, 551. (55) Lide, D. Handbook of Chemistry and Physics, 78th edition; C.R.C. Press: Boca Raton, FL, 1997-1998. (56) Me´sza´ros, R.; Varga, I.; Gila´nyi, T. J. Phys. Chem. B 2005, 109, 13538. (57) He´non, S.; Meunier, J. Brewster Angle Microscopy and Ellipsometry. In Modern characterisation methods of surfactant systems; Binks, B. P., Ed.; Surfactant Science Series, Marcel Dekker: New York, 1999. (58) Meunier, J. Light scattering by liquid surfaces and complementary techniques; Marcel Dekker: New York, 1992; p 333. (59) Motschmann, H.; Teppner, R. Ellipsometry in Interface Science. In NoVel Methods to Study Interfacial Layers; Miller, R., Moebius, D., Eds.; Elsevier: NY, 2001. (60) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and polarized light; North-Holland: Amsterdam, 1977; p 288. (61) Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W.Th.; Hemker, H. C. J. Biol. Chem. 1983, 258, 2426. (62) Casson, B. D.; Braun, R.; Bain, C. D. Faraday Discuss. 1996, 104, 209. (63) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf., A 1995, 94, 125. (64) Bain, C. D. Curr. Opin. Colloid Interface Sci. 1998, 3, 287. (65) Teppner, R.; Bae, S.; Haage, K.; Motschmann, H. Langmuir 1999, 15, 7002. (66) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15, 1400. (67) Battal, T.; Shearman, G. C.; Valkovska, D.; Bain, C. D.; Darton, R. C.; Eastoe, J. Langmuir 2003, 19, 1244. (68) Valkovska, D.; Wilkinson, K. M.; Campbell, R. A.; Bain, C. D.; Wat, R.; Eastoe, J. Langmuir 2003, 19, 5960. (69) Manning-Benson, S.; Parker, S. R. W.; Bain, C. D. Langmuir 1998, 14, 990. (70) Cooke, D. J.; Blondel, J.; A, K.; Lu, J.; Thomas, R. K.; Wang, Y.; Han, B.; Yan, H.; Penfold, J. Langmuir 1998, 14, 1990. (71) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (72) Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J.; Richards, R. W. Polymer 1996, 37, 109. (73) Henderson, J. A.; Richards, R. W.; Penfold, J.; Thomas, R. K.; Lu, J. R. Macromolecules 1993, 26, 4591. (74) Mears, S. J.; Cosgrove, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 997. (75) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4997. (76) Vass, S.; To¨ro¨k, T.; Ja´kli, G.; Berecz, E. J. Phys. Chem. 1989, 93, 6553. (77) Mysels, K. J.; Princen, L. H. J. Phys. Chem. 1959, 63, 1696. (78) Venohr, H.; Fraaije, V.; Strunk, H.; Borchard, W. Eur. Polym. J. 1998, 34, 723. (79) Vogel, A. I. J. Chem. Soc. 1946, 133. (80) The molar refractivity of the SO4Na group at λ ) 532 nm can be calculated by ASO4Na ) ASDS - AR12, where ASDS ) 68.4 cm3/mol from Appendix A and AR12 ) 57.06 cm3/mol from Vogel. 79

JP710170D