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Polyampholyte/Surfactant Complexes at the Water−Air Interface: A Surface Tension Study Mabya Fechner and Joachim Koetz* Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, Haus 25, 14476 Potsdam (Golm), Germany S Supporting Information *

ABSTRACT: The present paper is related to interactions between strongly alternating polyampholytes, i.e., copolymers of N,N′-diallyl-N,N′-dimethylammonium chloride and maleamic acid derivatives, varying in hydrophobicity and excess charges and the oppositely charged anionic surfactant sodium dodecyl sulfate (SDS). Surface tension measurements have revealed a complex behavior with the formation of polyampholyte−SDS complexes at water−air interfaces which depends on both the hydrophobic character of the polyampholyte and electrostatic attractive forces between the polyampholyte and the anionic surfactant in dependence on pH. Hereby, maleamic acid copolymers with additional carboxylic groups in the phenylic side chain show a significant lower surface tension at the critical association concentration (CAC) due to the formation of surface-active SDS complexes and multicomplexes. In the presence of only one carboxylic group in the pposition the CAC can be strongly shifted by varying the pH due to repulsive electrostatic interactions. designated as T1, T12, and T2. T1 refers to the CAC of the system, T12 represents the saturation point of the polymer from which all added surfactant molecules directly decrease the surface tension due to an adsorption of surfactant monomers onto the air/water interface, and T2 indicates the point where the interface becomes totally saturated with surfactant monomers (formation of free micelles in the volume phase) schematically shown in Scheme 1i. Further investigations of the poly(vinylpyrollidone) (PVP)/ SDS system led to a modification of the prior interaction model.17 In this system a decrease of surface tension can be observed at concentrations quite far below the pure surfactant solution from which one can conclude that the polymer/ surfactant complexes are surface-active themselves. Moreover, a maximum or a gradual decrease between T1 and T2 can sometimes be observed in addition to the plateau formation already mentioned (compare Scheme 1ii). Subsequent investigations in the presence of a polyelectrolyte, i.e., cationic-modified cellulose done by Goddard,18 could prove a pronounced effect of electrostatic interactions on the observed surface tension curve. Recent studies using various polyelectrolyte/surfactant combinations, i.e., poly(N,N′-diallylN,N′-dimethylammonium chloride) (PDADMAC)/SDS,19−21 poly(ethylenimine) (PEI)/SDS,22−24 poly(styrenesulfonate) (PSS)/CnTAB,25−27 poly(acrylic acid) (PAA)/SDS,28 acrylamidacrylamidomethylpropanesulfonate copolymer (PAMPS)/

1. INTRODUCTION The understanding of specific polymer−surfactant interactions in diluted aqueous systems is of high importance to many applications in chemical, pharmaceutical, cosmetic, and detergent products since the formation of those complexes influences solution as well as interfacial properties.1−4 While in aqueous mixtures of a nonionic polymer and an ionic surfactant the hydrophobicity and surface activity of the polymer are dominant, pH-dependent electrostatic interactions will be of prior importance in the case of oppositely charged polymer/ surfactant systems.5−7 The complexation of an oppositely charged low molecular weight surfactant onto the binding sites of a polyelectrolyte (PEL) in the volume phase starts after exceeding the so-called critical association concentration (CAC), which is several orders of magnitude lower than the critical micellization concentration (CMC) of the pure surfactant.6,7 Investigations on polymer/surfactant systems using surface tension measurements have shown that ionic surfactants are able to interact either with nonionic or with equally charged polymers.8,9 In case of a hydrophobic polymer those interactions are supposed to be a result of unspecific van der Waals interactions between the nonpolar segments of the polymer and the surfactant. Publications on those systems are quite rare, whereas the interaction mechanism between ionic surfactants and nonionic polymers has been investigated in more detail during the past decades.10−15 Early investigations on the interactions of poly(ethylene oxide) (PEO) and SDS, done by Jones et al.,16 revealed two to three characteristic changes in the surface tension curve, © XXXX American Chemical Society

Received: January 10, 2013 Revised: May 28, 2013

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Scheme 1. Schematized Course of Surface Tension (γ) as a Function of the Total Surfactant Concentration (c) in the Presence of Nonionic Polymers: (i) PEO/SDS System According to Jones16 and (ii) PVP/SDS System According to Thomas et al.17

Scheme 2. Schematized Course of Surface Tension (γ) as a Function of the Total Surfactant Concentration (c) in the Presence of Polyelectrolytes: (i) PSS/CnTAB System According to Taylor et al.27 and (ii) PDADMAC/SDS System According to Penfold et al.21

Scheme 3. Schematic Illustration of the Concentration-Dependent Formation of PA/SDS Complexes

C12TAB,29−31 DNA/C12TAB,31 and Xanthan/C12TAB31 have confirmed those results and have additionally revealed a more complex interaction mechanism compared to nonionic polymers. Structural investigations toward the composition of formed surface complexes of the PSS/CnTAB27 and PDADMAC/

SDS21 systems have pointed out a multilayer formation in the first system and depletion of the interface due to a favored formation of bulk phase complexes in the PDADMAC/SDS system. Therefore, characteristic changes in the surface tension curves can be observed, schematically shown in Scheme 2. B

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carboxylate)), PalPhCarb (poly(N,N′-diallyl-N,N′-dimethylammonium-alt-p-carboxyphenylmaleamic carboxylate)), and PalPhBisCarb (poly(N,N′-diallyl-N,N′-dimethylammonium-alt-3,5-bis(carboxyphenyl)maleamic carboxylate)) were synthesized by free radical polymerization according to the procedure already described previously.36 The molecular weights (Mη calculated in analogy to the PDADMAC homopolymer) for the different polyampholytes differ between 9000 and 21 000 g/mol (PalH = 21 000 g/mol; PalPh = 10 600 g/mol; PalPhCarb = 9100 g/mol; PalPhBisCarb = 15 000 g/ mol).36 Sodium dodecyl sulfate (SDS) (>99%) was purchased from Fluka and used without further purification. Hydrochloric acid (37%, Merck) and sodium hydroxide pellets (>99%, VWR) were used as received. Water was purified with the Modulab PureOne water purification system (Continental). 2.2. Surface Tension. For investigating PA/surfactant interactions a 0.05 wt % stock solution of the PA was prepared and equilibrated over a period of 24 h under shaking. The polymer solutions were then adjusted to pH 4 and pH 9 by adding an appropriate amount of HCl or NaOH. The 50 mM SDS solution was treated in a similar way. PA and surfactant solutions were filtered through a 450 nm syringe filter before use. Surface tension measurements were carried out by the Du-Noüy method on the digital tensiometer K10ST (Krüss) using a Haake thermostat (DC1). The surface tension of the pure SDS solution was detected by successive dilution of a 50 mM stock solution (pH 4, pH 9) to 25 mL pure water in volume increments of 200−400 μL. After the addition of each volume increment the sample was stirred for 2 min and equilibrated for another minute before data recording. The same procedure was used for measurements in presence of the polyelectrolyte PDADMAC as well as the polyampholytes PalH, PalPh, PalPhCarb, and PalPhBisCarb. The volume increments (0.5− 3000 μL) were adapted to the corresponding conditions to ensure an appropriate measurement precision.

Structural studies on the pH, charge density, and molar mass dependent assembly of formed surface complexes of the NaPAA/C12TAB,28 the PAMPS/C12TAB,31 and the PEI/ SDS24 systems show no direct correlation between the structural relation (monolayer, trilayer, multilayer, bulk phase complexes) at the air/water interface between T1 and T2 and the excess charge of the PA, schematically shown in Scheme S1 (Supporting Information). Thus, in the PAA system multilayers are only formed in the dissociated form at pH > pka° (polyanion), whereas primarily bulk phase complexes are built up if the PEL is nondissociated at pH < pka° (uncharged state). Polyampholytes (PA), which means polyelectrolytes with both anionic and cationic functional groups,32,33 offer special features by interacting with ionic surfactants.1,18 In general, electrostatic interactions in those systems can be directly changed from attractive to repulsive if the acidic and basic dissociation constants of the PA are known. According to that, a change of hydrophilicity of the PA has an impact on the ability of the formation of hydrophobic interactions. Hydrophobic interactions can be also influenced through the integration/addition of nonpolar groups (e.g., phenyl groups) or side chains (e.g., hydrophobic modified copolymers). In dependence on the structural and charge specific conditions different types of interactions with ionic surfactants can be considered. Nevertheless, investigations about PA/surfactant interactions under special accentuation of the surface activity of the resulting PA−surfactant complexes are rather scarce.34,35 Recently, we have reported that strongly alternating polyampholytes, which means maleamic acid copolymers with diallyldimethylammonium chloride, can be synthesized by free radical polymerization.36 The resulting amphiphilic copolymers PalH, PalPh, PalPhCarb, and PalPhBisCarb are well-defined model compounds with varying hydrophobicity and carboxyl functionality for studying the interactions with anionic surfactants, e.g., SDS. Recently, we have shown by means of potentiometry (with a SDS-selective electrode) and isothermal titration calorimetry (ITC) that in dependence on the type of polyampholyte used interactions with the surfactant can be controlled by varying the pH and the ionic strength.37 By incorporating the polyampholytes, i.e., PalH,38 PalPh,39 or PalPhBisCarb,39 into reverse microemulsion droplets a polyampholyte−SDS film tuning can be realized by varying the pH. Taking this knowledge into account, the aim of the present work was to investigate the surface activity of the resulting different PA/SDS complexes in dependence on structural features of the different polyampholytes used and the surrounding pH by using surface tension measurements. The cationic homopolymer, i.e., poly(diallyldimethylammonium chloride) (PDADMAC), was used as a reference polyelectrolyte of high charge density.

3. RESULTS AND DISCUSSION 3.1. PDADMAC System. The surface tension (γ) of the PDADMAC/SDS system as a function of the total surfactant concentration (cs,t) is shown in Figure 1.

2. EXPERIMENTAL SECTION

Figure 1. Surface tension (γ) as a function of the total surfactant concentration (cs,t) of the PDADMAC/SDS system.

2.1. Materials. Poly(diallyldimethylammonium chloride) was synthesized by free radical polymerization. The viscometrically determined molecular weight, Mη = 7800 g/mol, calculated according to the Kuhn−Mark−Houwink−Sakurada relationship (with K = 1.12 × 10−4 dL/g ; a = 0.82 in 1 M NaCl at 25 °C) was in good agreement with the molecular weights determined by size exclusion chromatography, i.e., Mw = 11 400 g/mol and Mn = 6400 g/mol. The strongly alternating, weak poylampholytes PalH (poly(N,N′diallyl-N,N′-dimethylammonium-alt-maleamic carboxylate)), PalPh (poly(N,N′-diallyl-N,N′-dimethylammonium-alt-N-phenylmaleamic

From the concentration-dependent surface tension curve of the pure SDS solution (after recrystallization of SDS from ethanol), the CMC of SDS (quite equal for pH 4 and 9) was calculated to 9.2 mM, in full agreement with relevant data (9.0−9.5 mM) already published by other authors.40 When water is replaced by a 0.05 wt % aqueous PDADMAC solution, a drastic reduction of the surface tension can be identified due to a so-called “synergetic” effect.19−21 Coulomb C

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Thus, three characteristic concentrations, namely c1, c2, and c3, with a decrease of surface tension can be identified for the PalH/SDS system at pH 4 and pH 9. A strengthened formation of PalH/SDS complexes takes place until the CAC of the system is reached (pH 4 ≈ 0.02 mM, pH 9 ≈ 0.03 mM). The subsequent decrease of surface tension could be either related to the formation of multilayers or a concurrence of surface and bulk phase complexes. A differentiation between the interaction mechanisms cannot be done without further measurements (e.g., neutron surface reflectivity), and thus the concentrations labeled with c* cannot be interpreted in more detail. The concentration c2 (pH 4 ≈ 5.4 mM, pH 9 ≈ 6.1 mM) can be related to the so-called saturation concentration at which all binding sites of the PalH in the bulk phase are occupied by surfactant molecules. Therefore, all added surfactant molecules are directly adsorbed onto the air/water interface to reduce the surface tension markedly. The characteristic concentration c3 (pH 4 ≈ 11.6 mM, pH 9 ≈ 12.2 mM) that corresponds to the CMC of the surfactant shows a shift toward higher concentrations for both pH values (pH 4, pH 9). It can be summarized that a change of the charge state from cationic (pH < IEP) to amphoteric (pH = IEP) shows no dramatic influence. However, a somewhat lower surface tension can be observed at pH 4. Therefore, one can conclude that formed PalH/SDS complexes are more surface active, if all carboxylic groups are protonated. That means a lower hydrophilicity of the copolymer (at low degree of dissociation) could be responsible for this behavior. 3.3. PalPh System. By incorporating a phenylic side chain into the polyampholyte, the hydrophobic character is drastically increased, reflected by a lower surface tension of the pure PA solution with 60.5 mN/m (at pH = 9) and 61.9 mN/m (at pH = 4) (compare Table 1). The principal shape of surface tension curve (Figure 3) is comparable to that of PalH with a moderate decrease in surface tension after exceeding the CAC (pH 4 ≈ 0.008 mM, pH 9 ≈ 0.009 mM) at pH 4 and the formation of a plateau at pH 9. Because of the higher hydrophobicity of the copolymer PalPh, an increased surface adsorption can be assumed. Therefore, the formation of multilayers can be expected. The saturation concentrations c2 (pH 4 ≈ 2.9 mM, pH 9 ≈ 4.2 mM) are lower compared to the PalH/SDS system. This in turn means that a comparatively lower amount of surfactant is needed for a saturation of PalPh. The CMC (pH 4 ≈ 11.6 mM, pH 9 ≈ 11.7 mM) is shifted to higher values independent of the pH value. However, lower surface tensions are achieved at pH 4 where all COOH groups are protonated and the copolymer thus bears more hydrophobic character. 3.4. PalPhCarb System. Because of the addition of a COOH group in the p-position to the amide function of the malamide comonomer unit (PalPh → PalPhCarb), the ability to form surface-active complexes at surfactant concentrations below the CAC (pH 4 ≈ 0.008 mM, pH 9 ≈ 0.19 mM) is strongly increased (Figure 4). Above the CAC the shape of the surface tension curve is comparable to that of the PDADMAC/ SDS system21 (schematically shown in Scheme 2) with the formation of a distinct maximum at c* (pH 4 ≈ 0.02 mM, pH 9 ≈ 0.5 mM) accompanied by a depletion of the interface as a consequence of the formation of bulk phase complexes. After exceeding the maximum the surface tension remains nearly unaffected at pH 4. However, when all COOH groups of PalPhCarb are deprotonated (pH > IEP), the copolymer behaves like a polyanion (compare ref 35), and the CAC is strongly shifted to higher values which is a direct result of

interactions between the copolymer and the oppositely charged head groups of the surfactant are responsible for this behavior. The initial decay of the surface tension, which starts quite far below compared to the titration of the pure SDS solution, can be related to an increased SDS adsorption, which is a result of the formation of surface active PDADMAC/SDS complexes. After exceeding a minimum in the surface tension curve, which can be attributed to the CAC (≈ 0.01 mM), a weak maximum is passed (c* ≈ 0.018 mM) which can be correlated to a partial depletion at the air/water interface due to a favored formation of bulk phase complexes. The subsequent decrease in surface tension is a consequence of the adsorption of SDS monomers at the air/water interface. The surface tension remains nearly constant after reaching another critical concentration (c* ≈ 0.042 mM). After exceeding a 1:1 stoichiometry the solution becomes turbid and the surface tension is decreased drastically at surfactant concentrations higher than c2 (≈ 0.9 mM), which is a result of the adsorption of surfactant monomers as a consequence of the saturation of the PEL in the volume phase. The shape of the surface tension curve of the PDADMAC/ SDS system is in good agreement with the observed trends already described in the literature.19 For that reason the formation of a monolayer of the surface-active PDADMAC/ SDS complexes due to the formation of attractive electrostatic interactions can be assumed concerning the mechanism of interface adsorption. Nevertheless, surface complexes at low surfactant concentrations are comparatively less surface active and depletion effects are less pronounced than described in the literature. Differences in the molecular weights of used PELs could be the reason therefore. 3.2. PalH System. The results of the surface tension measurements of the PalH/SDS system detected in a surfactant concentration range between 0 and 25 mM at pH 4 and pH 9 are illustrated in Figure 2. The addition of a maleamic

Figure 2. Surface tension (γ) as a function of the total surfactant concentration (cs,t) of the PalH/SDS system at pH 4 and pH 9.

comonomer (PDADMAC → PalH) leads to a decrease of the charge density of cationic groups and therefore to a change of surface tension. The shape of the surface tension curve now correlates with the pH-dependent curves for the systems NaPAA/C12TAB, PAMPS/C12TAB, and PEI/SDS (schematically shown Scheme S1 in the Supporting Information) where after an initial reduction of the surface tension due to the formation of surface-active PEL/surfactant complexes (monolayer) a plateau or slightly decreasing shift were determined. D

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Table 1. Surface Tension (γ0) of the 0.05 wt % Aqueous PEL Solutions, Critical Association Concentrations (CAC), Saturation Concentrations (c2), and Critical Micellization Concentrations (CMC) of the Different PEL/SDS Systems γ0 [mN/m] substance PDADMAC PalH PalPh PalPhCarb PalPhBisCarb

pH 4

pH 4

71.2 71.2 61.9 65.5 71.2

c2 [mM]

CAC (c1) [mM] pH 9

pH 9

pH 4

0.01 71.4 60.5 70.9 71.2

CMC (c3) [mM] pH 9

pH 4

0.021 0.008 0.008 0.006

0.032 0.009 0.190 0.006

Figure 3. Surface tension (γ) as a function of the total surfactant concentration (cs,t) of the PalPh/SDS system at pH 4 and pH 9.

5.4 2.9 0.05 0.01

pH 9 −

0.9 6.1 4.2 − −

11.6 11.6 11.4 12.1

12.2 11.7 6.70 11.8

Figure 5. Surface tension (γ) as a function of the total surfactant concentration (cs,t) of the PalPhBisCarb/SDS system at pH 4 and pH 9.

through the similarity of the surface tension curve in comparison to the PSS/C12TAB system described by Taylor et al., where a multilayer formation of strongly surface-active PEL/surfactant complexes could be verified experimentally in combination with neutron reflectivity.27

4. CONCLUSIONS Regarding the surface tensions of the pure PEL and PA solutions (compare Table 1), one can deduce that the existence of a phenylic side chain (PalPh), associated with an increase of the hydrophobic character of the copolymer, leads to an increase of surface activity which remains more or less unaffected even if the COOH groups are activated. The incorporation of another COOH group in the p-position to the amide function of the maleamide comonomer (PalPh → PalPhCarb) displays a lowering of the surface tension of the pure PA solution, whereas an activation (PalPhCarb, pH 9) or a change of position (PalPhCarb → PalPhBisCarb) of the acidic groups leads to a drastic lowering of the surface activity of the pure copolymer solution. Moreover, lower surface tensions can be observed at pH < IEP, which can be understood in terms of a higher hydrophobicity at pH 4. The drastic reduction of the surface tension which can be observed for all investigated systems below the CAC of SDS can be related to an adsorption of a monolayer of surface-active polyampholyte/SDS complexes (compare Scheme 3). In the region of medium surfactant concentrations different shapes of surface tension curves can be observed. While depletion effects can be detected for the PDADMAC/SDS and the PalPhCarb/ SDS systems, the formation of multilayers (schematically shown in Scheme 3) can be assumed for the systems PalPh/

Figure 4. Surface tension (γ) as a function of the total surfactant concentration (cs,t) of the PalPhCarb/SDS system at pH 4 and pH 9.

repulsive interactions between the equally charged PA and surfactant. 3.5. PalPhBisCarb System. The surface tension curves obtained at pH 4 and pH 9 (Figure 5) show a pronounced deviation to the PalPhCarb/SDS system with a drastic, nearly pH-independent decrease of surface tension below the CAC (pH 4 ≈ 0.006 mM, pH 9 ≈ 0.006 mM). At pH 4, a narrow plateau is passed subsequently (multilayer formation or bulk phase complexation) until at c2 (pH 4 ≈ 0.01 mM) the surface tension is further decreased due to the adsorption of surfactant molecules onto the air/water interface, remains constant, and thereafter (0.045 mM) merges with the surface tension curve at pH 9. Neither at pH 4 nor at pH 9 a saturation concentration can be determined, which is an indication of the formation of multilayers at the interface. This assumption is supported E

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(16) Jones, M. N. The Interaction of Sodium Dodecyl Sulfate with Polyethylene Oxide. J. Colloid Interface Sci. 1967, 23, 36−42. (17) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Adsorption of Sodium Dodecyl Sulfate at the Surface of Aqueous Solutions of Poly(vinylpyrrolidone) Studied by Neutron Reflection. Langmuir 1998, 14, 1637−1645. (18) Goddard, E. D., Ananthapandmanabhan, K., Eds. In Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (19) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Organization of Polymer−Surfactant Mixtures at the Air−Water Interface: Poly(dimethyldiallylammonium chloride), Sodium Dodecyl Sulfate, and Hexaethylene Glycol Monododecyl Ether. Langmuir 2002, 18, 5139−5146. (20) Mukherjee, S.; Dan, A.; Bhattacharya, S. C.; Panda, A. K.; Moulik, S. P. Physico-chemistry of Interaction between the Cationic Polymer Poly(diallyldimethylammonium chloride) and the Anionic Surfactants Sodium Dodecyl Sulfate, Sodium Dodecylbenzenesulfonate, and Sodium N-Dodecanoylsarcosinate in Water and Isopropyl Alcohol−Water Media. Langmuir 2011, 27, 5222−5233. (21) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, X. L.; Bell, C.; Breward, C.; Howell, P. The Interaction between Sodium Alkyl Sulfate Surfactants and the Oppositely Charged Polyelectrolyte, PolyDMDAAC, at the Air−Water Interface: The Role of Alkyl Chain Length and Electrolyte and Comparison with Theoretical Predictions. Langmuir 2007, 23, 3128−3136. (22) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Binding of Sodium Dodecyl Sulfate with Linear and Branched Polyethyleneimines in Aqueous Solution at Different pH Values. Langmuir 2006, 22, 1526−1533. (23) Halacheva, S. S.; Penfold, J.; Thomas, R. K.; Webster, J. R. P. Effect of Polymer Molecular Weight and Solution pH on the Surface Properties of Sodium Dodecyl Sulfate-Poly(Ethyleneimine) Mixtures. Langmuir 2012, 28, 14909−14916. (24) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Zhang, X. L. The Impact of Electrolyte on the Adsorption of Sodium Dodecyl Sulfate/Polyethyleneimine Complexes at the Air−Solution Interface. Langmuir 2007, 23, 3690−3698. (25) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. The Adsorption of Oppositely Charged Polyelectrolyte/Surfactant Mixtures: Neutron Reflection from Dodecyl Trimethylammonium Bromide and Sodium Poly(styrene sulfonate) at the Air/Water Interface. Langmuir 2002, 18, 4748−4757. (26) Taylor, D. J. F.; Thomas, R. K.; Hines, J. D.; Humphreys, K.; Penfold, J. The Adsorption of Oppositely Charged Polyelectrolyte/ Surfactant Mixtures at the Air/Water Interface: Neutron Reflection from Dodecyl Trimethylammonium Bromide/Sodium Poly(styrene sulfonate) and Sodium Dodecyl Sulfate/Poly(vinyl pyridinium chloride). Langmuir 2002, 18, 9783−9791. (27) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Adsorption of Oppositely Charged Polyelectrolyte/Surfactant Mixtures. Neutron Reflection from Alkyl Trimethylammonium Bromides and Sodium Poly(styrenesulfonate) at the Air/Water Interface: The Effect of Surfactant Chain Length. Langmuir 2003, 19, 3712−3719. (28) Zhang, J.; Thomas, R. K.; Penfold, J. Interaction of Oppositely Charged Polyelectrolyte−Ionic Surfactant Mixtures: Adsorption of Sodium Poly(acrylic acid)−Dodecyltrimethyl Ammonium Bromide Mixtures at the Air−Water Interface. Soft Matter 2005, 1, 310−318. (29) Jain, N. J.; Albouy, P.-A.; Langevin, D. Study of Adsorbed Monolayers of a Cationic Surfactant and an Anionic Polyelectrolyte at the Air−Water Interface. Role of the Polymer Charge Density. Langmuir 2003, 19, 8371−8379. (30) Jain, N. J.; Trabelsi, S.; Guillot, S.; McLouglilin, D.; Langevin, D.; Latelllier, P.; Turmine, M. Critical Aggregation Concentration in Mixed Solutions of Anionic Polyelectrolytes and Cationic Surfactants. Langmuir 2004, 20, 8496−8503. (31) Stubenrauch, C.; Albouy, P.-A.; v. Klitzing, R.; Langevin, D. Polymer/Surfactant Complexes at the Water/Air Interface: A Surface Tension and X-ray Reflectivity Study. Langmuir 2000, 16, 3206−3213.

SDS and PalPhBisCarb/SDS, whereas formed surface complexes of PalPhBisCarb bear the highest surface activity.

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ASSOCIATED CONTENT

* Supporting Information S

Scheme S1: surface tension as a function of the total surfactant concentration in the presence of polyelectrolytes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +49-331-9775220; Fax +49-331-9775054 (J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Björn Gamroth for the carefully performed repeating experiments.

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REFERENCES

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dx.doi.org/10.1021/la401576q | Langmuir XXXX, XXX, XXX−XXX