Noninteracting versus Interacting Poly(N-isopropylacrylamide

Feb 23, 2005 - Kalyanasis Sahu , Kenneth B. Eisenthal , and V. Faye McNeill ... Bruno Jean , Lay-Theng Lee , Bernard Cabane and Vance Bergeron...
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J. Phys. Chem. B 2005, 109, 5162-5167

Noninteracting versus Interacting Poly(N-isopropylacrylamide)-Surfactant Mixtures at the Air-Water Interface Bruno Jean* and Lay-Theng Lee Laboratoire Le´ on Brillouin, C.E. Saclay, 91191 Gif-sur-YVette Cedex, France ReceiVed: October 7, 2004; In Final Form: January 4, 2005

Two polymer-surfactant mixtures have been studied at the air-water interface using neutron reflectivity and surface tension techniques. For the noninteracting system poly(N-isopropylacrylamide) (PNIPAM)/ octaethyleneglycol mono n-decyl ether (C10E8), the adsorption behavior is competitive and driven purely by surface pressure (Π). When Πpolymer > Πsurfactant, the surface layer consists of almost pure polymer, and for Πpolymer < Πsurfactant, the polymer is displaced from the surface by the increasing pressure of the surfactant. Beyond the CMC, the polymer is completely displaced from the surface. For the interacting system PNIPAM/ sodium dodecyl sulfate (SDS) where the two species interact strongly in the bulk beyond the critical aggregation concentration (CAC), the surface behavior is more original. Earlier neutron reflectivity studies investigated PNIPAM adsorption behavior where the SDS was contrast-matched to the solvent. In the present study, complementary measurements of SDS adsorption where PNIPAM is contrast-matched to the solvent give a complete view of the surface composition of the mixed system. At a constant polymer concentration, with increasing SDS, three main regimes are obtained. For CSDS < CAC, adsorption is governed by simple competition and PNIPAM is predominant at the interface. At intermediate SDS concentration (CAC < CSDS < x2, where x2 indicates the predominance of free SDS micelles), interfacial behavior is governed by bulk polymer-surfactant interaction. Adsorbed polymer is displaced from the interface to form PNIPAM-SDS complex in the bulk. SDS adsorption remains weak since most of the SDS molecules are used to form bulk polymer-surfactant aggregates. Further increase in SDS concentration results in continued displacement of PNIPAM and an abrupt increase in SDS adsorption. This is a result of saturation of bulk polymer chain with adsorbed micelles. Interestingly, beyond x2, PNIPAM is not completely displaced from the surface. A mixed PNIPAM-SDS adsorbed layer with enhanced packing of the SDS monolayer is formed.

Introduction Polymers are often associated with surfactants in dispersed systems such as emulsions or foams that are used in a wide variety of important industrial applications such as enhanced oil recovery, cosmetics, food, or healthcare. Mixtures of neutral polymers with anionic surfactants have been studied extensively, but attention has mainly been drawn to the bulk properties of those systems.1 However, interfacial properties often play a predominant role in the behavior of systems with high surfaceto-volume ratios. Adsorption at the air-solution is thus an important issue that needs to be addressed taking into account all the relevant parameters. For example, adsorption at the interface is often the result of a delicate balance between polymer-surfactant aggregates in the bulk and at the surface. Therefore, of particular importance are the type and strength of the polymer-surfactant interactions. Polymer-surfactant mixtures can be divided into three categories: noninteracting systems (e.g., neutral polymer/nonionic surfactants), weakly interacting systems (e.g., most of neutral polymer/anionic surfactants such as PEO/ SDS2,3), and strongly interacting systems (charged polymer/oppositely charged surfactant4,5). In the strongly interacting systems category can be added a special case of neutral polymer/anionic surfactant mixture, namely, poly(N-isopropylacrylamide) (PNIPAM) and sodium dodecyl sulfate (SDS). PNIPAM is a thermosensitive polymer that * Corresponding author. E-mail: [email protected].

exhibits lower critical solubility temperature (LCST) behavior: it is soluble in water at low temperatures but phase-separates when heated above a critical temperature, Tc, of around 33 °C.6 Above the critical aggregation concentration (CAC) (i.e., the surfactant concentration at which the polymer chains and the surfactant molecules start to interact), small angle neutron scattering studies have shown that PNIPAM and SDS form a necklace structure consisting of several micellar aggregates adsorbed on a polymer chain, similar to that found in PEOSDS system.7 The formation of a necklace structure by PNIPAM and SDS takes place both above and below the Tc. Other solution properties such as conductivity and rheology of the PNIPAMSDS system8,9 are also consistent with the formation of a solution of necklaces. The strong interaction of PNIPAM and SDS can be characterized by a low value of the critical aggregation concentration (CAC), around 0.8 mM, determined by various techniques6,8,10 (as compared to 3 mM for PVP/SDS11 and 4.5 mM for PEO/ SDS12) and also by a large value of the excess free energy of polymer surfactant interaction, -1356 cal/mol (as compared to -566 cal/mol for PVP/SDS and -326 cal/ mol for PEO/SDS).13 As shown in recent literature, neutron reflectivity is an effective technique that provides detailed information about the surface layer that complements surface tension measurements.2,3,14,17-19 In particular, contrast variation can be used to measure the adsorption of individual species in a mixed system.19 In a previous study,13 we reported on the effect of

10.1021/jp0454265 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005

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SDS on the adsorption of PNIPAM at the air-water interface using neutron reflectivity where the surfactant molecules were contrast-matched to the solvent (protonated PNIPAM and deuterated SDS in D2O). Richardson and co-workers also reported on the adsorption behavior of PNIPAM in the presence of SDS using the same technique but an inverse system (deuterated PNIPAM in nonreflecting H2O/D2O mixture).18 Their main conclusions are consistent with ours. In the present paper, we show a complementary study of the SDS adsorption behavior in the presence of PNIPAM, where the latter is contrast-matched to the solvent. These studies allow us to obtain a precise and complete picture of the adsorption of the PNIPAM/ SDS strongly interacting system at the air-water interface. For comparison, we report also the results of a noninteracting system, PNIPAM/octaethyleneglycol mono n-decyl ether (C10E8). Experimental Procedures Materials. Octaethyleneglycol mono n-decyl ether (C10E8) was purchased from Nikko Chemicals, Japan, and used as received. Protonated SDS was purchased from BDH, England and recrystallized in ethanol, and deuterated SDS was purchased from Isotec, France and was used as received. Protonated PNIPAM was prepared by polymerization using the methodology of Schild and Tirrell. N-Isopropylacrylamide monomer purchased from Eastman Kodak Co. was recrystallized in a mixture of hexane and benzene. After three cycles of degassing through freezing and thawing, polymerization was carried out in benzene at 50 °C by free radical polymerization using azobis(isobutyronitrile) from Alfa Chemicals as an initiator. The polymerized material was dissolved in acetone and precipitated in hexane. Deuterated PNIPAM was purchased from Polymer Science, where a similar polymerization procedure was used. The molecular weights of the polymers were determined using size exclusion chromatography coupled with light scattering. The protonated and deuterated PNIPAM samples had molecular weights Mw ) 190 000 and 250 000 and a polydispersity index of 2.7 and 3.6, respectively. Surface Tension. Surface tension measurements were performed with a Kruss K12 tensiometer using the Wilhelmy plate method. Protonated polymer and surfactant samples were used to prepare the aqueous solutions. A Plexiglas cover was placed over the solution during the measurements to avoid contamination and to reduce evaporation. Successive measurements were carried out at different time intervals, and the surface tension value was taken to be in equilibrium if it did not vary by more than 0.2 mN/m over a period of 10 min. The temperature of the solutions was controlled to (0.2 °C using a circulating temperature bath. Neutron Reflectivity Measurements. Specular neutron reflectivity experiments were carried out on the time-of-flight reflectometers EROS and DESIR at the Laboratoire Le´on Brillouin, CEA Saclay. For the noninteracting mixture C10E8/ PNIPAM, polymer adsorption was measured using deuterated PNIPAM and protonated C10E8 that was almost contrastmatched to the solvent (80% H2O and 20% D2O by volume). In the case of the interacting SDS/PNIPAM mixture, SDS adsorption was measured using deuterated SDS and protonated PNIPAM that is contrast-matched to the solvent (80% H2O and 20% D2O by volume). In this contrast scheme, the scattering length density of deuterated SDS is +6.73 10-6 Å-2 and that of PNIPAM is 0.90 10-6 Å-2, very close to the value of 0.83 10-6 Å-2 for the solvent. Complementary measurements of PNIPAM adsorption using SDS contrast-matched to the solvent have also been performed.13

Figure 1. Surface tension of C10E8 and PNIPAM: C10E8 alone (×) and C10E8 + PNIPAM (O), Cp ) 1 g/L, Mw ) 1.9 × 105, and T ) 23 °C.

The solution sample was contained in a Teflon cell (15 × 5 × 0.3 cm), which was completely enclosed in a thermostated aluminum cell with quartz windows to allow the neutron beam to enter and exit the cell. Additional enclosure in a second aluminum cell allowed us to reduce condensation of water vapor on the quartz windows and to control the temperature to (0.1 °C. The grazing incidence angle used was 1° with an angular resolution of about 3%. Results and Discussion Noninteracting System. The surface tension data of C10E8 at T ) 23 °C are shown in Figure 1. In the absence of PNIPAM, the CMC of C10E8 is around 1 mM, and the limiting surface tension value is 36.5 m Nm-1. In the presence of 1 g/L PNIPAM, over a large range of surfactant concentration, the surface tension remains constant at 42 m Nm-1; this value corresponds to that of the pure PNIPAM solution.15 As the surfactant concentration is increased to around 0.2 mM, the curve for the PNIPAM-C10E8 mixture is superimposed with that for surfactant alone. At this point, the surface tension of the pure C10E8 solution yields 42 m Nm-1. Thus, given a mixture of noninteracting polymer and surfactant molecules, at low surfactant concentrations where the surface pressure of the adsorbed surfactant is lower than that of the polymer, the pressure of the surface layer is that of the pure polymer. Beyond a critical surfactant concentration where the surfactant surface pressure exceeds that of the polymer, the reverse is true. This absence of a synergistic effect followed by a perfect superposition of the surface tension curves indicates a purely competitive behavior of the two species at the interface. For Πpolymer > Πsurfactant, the surface layer consists of almost pure polymer, and for Πpolymer < Πsurfactant, the polymer is displaced from the surface by the increasing pressure of the surfactant. The displacement of polymer from the surface can be seen more clearly in Figure 2. The adsorption density of PNIPAM measured by neutron reflectivity, for Cp ) 1 g/L, is shown as a function of C10E8 concentration. (Here, the protonated C10E8 is contrast-matched to the solvent (80% H2O and 20% D2O by volume), and the excess reflectivity signal is due to adsorbed deuterated PNIPAM. In fitting the reflectivity curves, a power law profile is used to describe the concentration profile of the adsorbed polymer layer. The adsorption density is obtained by integration of the concentration profile. Details are given elsewhere.15) The corresponding surface pressure isotherm is

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Figure 2. Adsorption density of PNIPAM measured by neutron reflectivity vs C10E8 concentration at T ) 22 °C (b, left y-scale). Corresponding surface pressure isotherm deduced from surface tension measurements (0, right y-scale). Figure 4. Normalized reflectivity of deuterated SDS at the surface of a solution containing protonated PNIPAM at T ) 20.2 °C for different SDS concentrations: 3 × 10-4 M (O); 2 × 10-3 M (]); 8 × 10-3 M (1); 1.35 × 10-2 M (4); and 3 × 10-2 M (×). Protonated PNIPAM (Cp ) 1 g/L, Mw ) 1.9 × 105) is contrast-matched to the solvent (80: 20 H2O/D2O mixture in volume). The solid lines through the data points are best-fit curves using a one-layer model.

Figure 3. Surface tension of SDS and PNIPAM: SDS alone (×) and SDS + PNIPAM (O), Cp ) 1 g/L, Mw ) 1.9 × 105, T ) 21 °C.

also shown. At low surfactant concentrations, polymer adsorption is insensitive to the presence of surfactant. Beyond the critical concentration of around 0.2 mM where the surface pressure of the surfactant starts to dominate, the polymer is displaced from the surface. Beyond the CMC, the polymer is completely displaced from the surface. Thus, PNIPAM-C10E8 is a good example of a noninteracting system where the adsorption behavior is competitive and governed purely by surface pressure. Interacting System. Figure 3 shows the surface tension data for the PNIPAM-SDS mixture. These data have been presented and discussed elsewhere,13 but we present them here for comparison purposes with the noninteracting case stated previously. At very low concentrations of SDS, the surface tension of the PNIPAM-SDS mixture remains constant, as in the case of PNIPAM-C10E8. At around 0.1 mM, the surface tension decreases gently but continuously with SDS concentration until a second critical concentration, around 10 mM (designated x2), is detected. x2 is greater than the CMC of the pure surfactant and is interpreted as the point where free micelles predominate. (Note that the formation of free micelles and the beginning of the coexistence of free and polymer-bound micelles may occur

before x2.) Above this point, the surface tension of the mixed solution is lower than that of surfactant alone. Thus, in this interacting system, the curve for the polymer-surfactant mixture does not superpose with that of surfactant alonesit remains lower throughout the entire surfactant concentration range. In the previous study,13 we reported the effects of SDS on the adsorption behavior of PNIPAM at the air-water interface with the surfactant molecules contrast-matched to that of the solvent (protonated PNIPAM and deuterated SDS in D2O). We showed that in the absence of SDS, the polymer adsorbs strongly at the interface due to its partially hydrophobic nature. The presence of SDS at a concentration below the critical aggregation concentration (CAC) does not affect the polymer adsorption. Above the CAC, however, the PNIPAM chains are progressively displaced from the interface. In contrast to the PNIPAM/C10E8 noninteracting system, the polymer is depleted from the surface even though the polymer surface pressure exceeds that of SDS, indicating that the depletion is not driven by adsorption energy. The phenomenon has been explained by the complexation of the polymer with surfactants in volume and by the resulting equilibrium between the charged complex at the surface and that in the bulk phase. In this region, the adsorbed polymer forms a more dilute and extended layer, suggesting the presence of micellar aggregates within the adsorbed layer. Thus, in the SDS/ PNIPAM system, polymer-surfactant interaction at the surface reflects that in the bulk solution. To complement the previous results, the reflectivity of deuterated SDS from a solution containing protonated PNIPAM contrast-matched to the solvent (20% D2O and 80% H2O by volume) was measured, and the curves obtained are shown in Figure 4. Here, the reflectivity R is normalized to the Fresnel reflectivity, Rf, which is the reflectivity of the pure solvent, taking into consideration angular resolution and surface roughness. It was checked experimentally that in this solvent mixture, the adsorbed protonated PNIPAM is contrast-matched out by the solvent. Thus, deviation of R/Rf from unity is due solely to adsorbed deuterated SDS and shows directly the magnitude of the amount adsorbed. The increasing reflectivity curves are

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TABLE 1: Adsorption of Deuterated SDS Alone; Results of the Best-Fits using a One-Layer Model Cs (mol/L)

Nb layer (10-6 Å-2)

e (Å)

A (Å2)

ΓSDS (10-10 mol/cm2)

9.9 × 10-4 4 × 10-3 8 × 10-3 1.6 × 10-2 2.4 × 10-2

2.81 3.72 4.13 4.11 4.22

18.40 17.60 18.60 18.27 17.60

53.38 42.16 35.93 36.76 37.16

3.11 3.94 4.62 4.52 4.47

TABLE 2: Adsorption of Deuterated SDS in the Presence of PNIPAM (Cp ) 1 g/L, Mw ) 1.9 × 105); Results of the Best-Fits using a One-Layer Model Cs (mol/l)

Nb layer (10-6 Å-2)

e (Å)

A (Å2)

ΓSDS (10-10 mol/cm2)

1 × 10-3 2 × 10-3 3.14 × 10-3 5 × 10-3 8 × 10-3 8 × 10-3 1.02 × 10-2 1.35 × 10-2 2 × 10-2 2.96 × 10-2

1.47 1.51 1.50 1.72 2.25 2.36 3.10 4.30 4.12 4.15

18.20 20.00 19.95 18.30 19.35 19.80 19.00 17.90 19.40 19.30

103.16 91.39 92.23 87.69 63.39 59.07 46.86 35.86 34.53 34.46

1.61 1.82 1.80 1.89 2.62 2.81 3.54 4.63 4.81 4.82

obtained for increasing SDS bulk concentration at a constant polymer concentration, Cp ) 1 g/L (Mw ) 190 000). The continuous lines through the data are the best-fit curves using a one-layer model with the parameters given in Tables 1 and 2. Except for the lowest SDS concentration, the fits are reasonably good, and the use of a two-layer model does not improve their quality. This indicates that most of the surfactant molecules are localized in the surface monolayer; beyond the monolayer, the presence of excess surfactants that are adsorbed or trapped within the polymer layer in the loop and tail region is possible, but the amount is undetectable within the limit of the measurement. The adsorption density of SDS, ΓSDS, is given by

ΓSDS )

Nbe bNA

where b is the scattering length of deuterated SDS, Nb and e the fitted scattering length density and thickness of the layer, respectively, and NA is Avogadro’s number. Various authors have studied PVP/SDS and PEO/SDS weakly interacting systems at the air-water interface. Their neutron reflectivity data show a regular increase of SDS adsorption versus SDS concentration and a complete displacement of the polymer at the CAC.2,3,17 The case of PNIPAM/SDS is more complex due to the high surface activity of PNIPAM and because PNIPAM/SDS is a strongly interacting system. These features lead to an original behavior at the interface. Figure 5a shows the adsorption isotherms deduced from the reflectivity data stated previously. In the absence of polymer, ΓSDS increases with concentration and reaches a plateau around the CMC at about 8 mM, as expected. In the presence of PNIPAM, adsorption of SDS is strongly modified, and four regions can be distinguished in the adsorption isotherm; these are characterized by variations in the affinity for the surface of the two species as well as by formation of aggregates. Region 1: Cs e 1 × 10-3 M. SDS adsorption is very weak, increases slowly, and is significantly lower than that obtained in the absence of polymer. In this region, as can be seen in the surface tension results, the surface pressure of the surfactant is much lower than that of polymer (Πpolymer ) 30 mN/m, a value

Figure 5. (a) SDS adsorption isotherms: SDS alone (×), in the presence of PNIPAM (Cp ) 1 g/L, Mw ) 1.9 × 105) (O). (b) Adsorption density of PNIPAM vs SDS concentration (see ref 13 for experimental details).

significantly higher as compared to other partially hydrophobic polymers such as PEO or PVP); thus, the polymer adsorbs preferentially leaving few available surfaces for the SDS molecules to adsorb. Therefore, below the CAC, adsorption of the individual components at the air-water interface is governed by simple competition. Region 2: 1 × 10-3 M e Cs e 5 × 10-3 M. This concentration range is above the CAC (8 × 10-4 M) where the polymer-surfactant complex is formed in the solution.6,8,10 Note that despite desorption of the polymer (Figure 5b), SDS adsorption remains very weak. This can be explained by two compatible reasons: first, added surfactant molecules are taken up in the formation of polymer-surfactant complex in the bulk and therefore unavailable to adsorb at the interface. Second, although the polymer adsorption density is reduced, the remaining adsorbed layer maintains a monomer-rich proximal zone due to strong interaction of the monomer and the surface, leaving little space for the surfactant molecules.13 These results confirm our earlier hypothesis that polymer loss from the surface is due to complexation; thus, there is a change in chemical potential in the bulk rather than surface competition due to surface pressure effects. Recently, Chari et al.19 reported a similar behavior for a random copolymer Luvitec VA 64 (60 mol % vinylpyrrolidone and 40 mol % vinyl acetate). The polymer, although less surface active than SDS, when added to a solution of SDS above the CMC displaces the surfactant as a consequence of formation of polymer surfactant aggregates in the bulk. The excess free energy of polymer surfactant interaction for the system SDS/0.1% Luvitec VA 64, ∆G ) -1361

5166 J. Phys. Chem. B, Vol. 109, No. 11, 2005 cal/mol, is similar to that of SDS/PNIPAM, reflecting a strong interaction due to the presence of the VA moiety. Region 3: 5 × 10-3 M e Cs e 2 × 10-2 M. SDS adsorption increases abruptly. In this region, only a small amount of polymer remains adsorbed. This region may be interpreted as the near-saturation point of the polymer chains by adsorbed micelles: adsorption of the highly charged complex becomes unfavorable. Thus, we observe that SDS adsorption increases significantly only after the surface is almost completely liberated by the polymer. This region may also signify the crossover between saturation of polymer chains and formation of pure surfactant micelles in the bulk. Indeed, as the polymer-surfactant complex grows, it becomes less favorable energetically to add more surfactants to the chain due to increasing electrostatic repulsion between adsorbed micelles. Thus, a further increase in surfactant concentration once again increases adsorption at the interface and promotes formation of free micelles in solution. The resulting increase in surface pressure of SDS (see drop in surface tension in Figure 3) also helps to further displace the PNIPAM from the surface. In regions 2 and 3, formation of a PNIPAMSDS complex in solution controls the chemical potentials of both PNIPAM and SDS and thus their adsorption behavior. Interestingly, despite decreased adsorption of both species as compared to their respective adsorption alone, the surface tension remains almost constant, indicating a higher stability of the mixed layer. Region 4: Cs g 2 × 10-2 M. In this region, a plateau adsorption is obtained. This saturation indicates a new constant chemical potential of the surfactant molecules. With respect to the curve for SDS alone, two interesting points of comparisons can be made: (i) in the absence of polymer, the onset of this plateau region corresponds to the CMC (8 × 10-3 M). In the presence of polymer, this occurs at a higher concentration, suggesting an onset of predominance of free micelles, coexisting with bound micelles. Under this situation, the polymer chains are probably saturated. (ii) The adsorption density is higher than that obtained for SDS alone. This is the only region in the entire range of concentrations studied where PNIPAM enhances the adsorption of SDS. Point (i) is typical of interaction of polymer and surfactant that pushes the formation of free micelles to higher concentration, x2. This effect can be seen clearly in surface tension curves for PEO-SDS and PVP-SDS systems.11,12,16,17 For PNIPAMSDS, x2 can also be detected in surface tension measurements, and the adsorption plateau confirms the existence of this second critical concentration. Point (ii) is more surprising since it shows that even at very high surfactant pressure, PNIPAM continues to affect SDS adsorption. Here, SDS adsorption is enhanced instead of depressed. Strong PNIPAM-SDS interaction is conserved at the surface either through hydrophilic interactions between amide moieties and SDS polar heads or by the anchoring of isopropyl moieties between the SDS hydrophobic tails. Indeed, recent calorimetry studies have suggested such incorporation of isopropyl moieties in the SDS hydrophobic micellar cores.20 Thus, residual PNIPAM in the surface region acts like a cosurfactant that reduces the area per polar head of SDS from 36.9 to 34.6 Å2. The resulting mixed polymer-surfactant layer, in equilibrium with bulk polymer-surfactant aggregates as well as with free micelles, is more stable and gives surface tension values that are lower than those of pure SDS layer.

Jean and Lee

Figure 6. Schematic drawing of the polymer-surfactant structures at the air-water interface and in the bulk for different SDS concentration regions. Region 1: Cs e 1 × 10-3 M; region 2: 1 × 10-3 M e Cs e 5 × 10-3 M; region 3: 5 × 10-3 M e Cs e 2 × 10-2 M; and region 4: Cs g 2 × 10-2 M.

Figure 7. Surface molar ratio of SDS and NIPAM monomer. The experimental points are evaluated from the reflectivity data using various isotopic combinations: protonated NIPAM (plus deuterated SDS in D2O)/deuterated SDS (plus protonated NIPAM in 80:20 H2O/D2O mixture (O) and deuterated NIPAM (plus protonated SDS in 80:20 H2O/D2O mixture)/deuterated SDS (plus protonated NIPAM in 80:20 H2O/D2O mixture) (4). The solid line is the calculated ratio in the bulk solution.

Figure 6 shows a schematic representation of the structures at the interface and in the bulk for the different SDS concentration regions. Figure 7 shows the surface molar ratio of SDS and PNIPAM monomer. The solid line is the calculated ratio in the bulk solution, and the experimental points are evaluated from the reflectivity data. It can be seen that with increasing Cs, the experimentally measured surface molar ratio of SDS and PNIPAM increases in a similar manner as the composition in the bulk solution. This shows that interaction of the two species at the interface is closely related to the bulk property. Conclusion Neutron reflectometry coupled with surface tension measurements has allowed us to study the interface between air and solutions containing different polymer-surfactant mixtures. In particular, the use of different isotopic substitutions provided information on the composition of the mixed system at the interface and on the interactions of these species at the surface. In the case of PNIPAM-C10E8, adsorption is purely driven by surface pressure. The two species interact neither in the bulk nor at the surface; this leads to a simple competitive adsorption behavior at the air/solution interface.

Poly(N-isopropylacrylamide)-Surfactant Mixtures The high surface activity of PNIPAM combined with its strong interaction with SDS molecules leads to an original adsorption behavior of the PNIPAM-SDS system at the airwater interface. Surprisingly, PNIPAM chains remain adsorbed at the air-water interface over a very large range of SDS concentration up to well above the CAC, thus limiting the adsorption of surfactant. A pronounced increase in adsorption of SDS takes place only when most of the adsorbed PNIPAM has been depleted from the surface to form polymer-surfactant aggregates in the bulk. At high SDS concentrations, some PNIPAM chains are conserved at the interface to form a mixed polymer-surfactant layer with enhanced packing of SDS molecules. This latter effect may be a way to produce more stable surface layers to improve the stability of foams and emulsions. Acknowledgment. We gratefully acknowledge A. Menelle for his consistent support and help on experiments performed on the reflectometer EROS at LLB. References and Notes (1) Goddard, E. D.; Ananthapadmanahban, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993.

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5167 (2) Cooke, D. J.; Blondel, J. A. K.; Lu, J.; Thomas, R. K.; Wang, Y.; Han, B.; Yan, H.; Penfold, J. Langmuir 1998, 14, 1990. (3) Lu, J. R.; Blondel, J. A. K.; Cooke, D. J.; Thomas, R. K.; Penfold, J. Prog. Colloid Polym. Sci. 1996, 100, 311. (4) Creeth, A.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (5) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 3712. (6) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (7) Lee, L. T.; Cabane, B. Macromolecules 1997, 30, 6559. (8) Wu, X. Y.; Pelton, R. H.; Tam, K. C.; Woods, D. R.; Hamielec, A. E. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 957. (9) Tam, K. C.; Wu, X. Y.; Pelton, R. H. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 963. (10) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665. (11) Lange, H. Kolloid Z. Z. Polym. 1971, 243, 101. (12) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (13) Jean, B.; Lee, L. T.; Cabane, B. Langmuir 1999, 15 (22), 7585. (14) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 1637. (15) Lee, L. T.; Jean, B.; Menelle, A. Langmuir 1999, 15, 3267. (16) Chari, K.; Hossain, T. Z. J. Phys. Chem. 1991, 95, 3302. (17) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (18) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 33, 6269. (19) Chari, K.; Seo, Y.-S.; Satija, S. J. Phys. Chem. B 2004, 108, 11442. (20) Loh, W.; Teixeira, L. A. C.; Lee, L. T. J. Phys. Chem. B 2004, 108, 3196.