Soluble complexes in aqueous mixtures of low charge density comb polyelectrolyte and oppositely charged surfactant probed by scattering and NMR

Journal of Colloid and Interface Science 312 (2007) 21–33 www.elsevier.com/locate/jcis

Soluble complexes in aqueous mixtures of low charge density comb polyelectrolyte and oppositely charged surfactant probed by scattering and NMR Luis A. Bastardo a,∗ , Joseph Iruthayaraj a , Maria Lundin a , Andra Dedinaite a,f , Aušvydas Vareikis b , Riˇcardas Makuška b , Albert van der Wal c , István Furó d , Vasil M. Garamus e , Per M. Claesson a,f a Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b Department of Polymer Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania c Lever Faberage Europe Global Technology Centre, Unilever R&D, P.O. Box 114, 3130 AC Vlaardingen, The Netherlands d Department of Chemistry, Physical Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden e GKSS Research Centre, Max-Plank Strasse, 21502 Geesthacht, Germany f YKI, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden

Received 21 May 2006; accepted 1 September 2006 Available online 12 September 2006

Abstract A low charge density polyelectrolyte with a high graft density of 45 units long poly(ethylene oxide) side-chains has been synthesized. In this comb polymer, denoted PEO45 MEMA:METAC-2, 2 mol% of the repeating methacrylate units in the polymer backbone carry a permanent positive charge and the remaining 98 mol% a 45 unit long PEO side-chain. Here we describe the solution conformation of this polymer and its association with an anionic surfactant, sodium dodecylsulfate, SDS. It will be shown that the polymer can be viewed as a stiff rod with a cross-section radius of gyration of 29 Å. The cross section of the rod contracts with increasing temperature due to decreased solvency of the PEO side-chains. The anionic surfactant associates to a significant degree with PEO45 MEMA:METAC-2 to form soluble complexes at all stoichiometries. A cooperative association is observed as the free SDS concentration approaches 7 mM. At saturation the number of SDS molecules associated with the polymer amounts to 10 for each PEO side-chain. Two distinct populations of associated surfactants are observed, one is suggested to be molecularly distributed over the comb polymer and the other constitutes small micellar-like structures at the periphery of the aggregate. These conclusions are reached based on results from small-angle neutron scattering, static light scattering, NMR, and surface tension measurements. © 2006 Elsevier Inc. All rights reserved. Keywords: Comb polymer; Comb polyelectrolyte; Polymer–surfactant association; Polyelectrolyte–surfactant aggregate; Small-angle neutron scattering; Light scattering; NMR; Surface tension; Polyelectrolyte; Surfactant; SDS

1. Introduction Polymer–surfactant interactions have attracted a great deal of attention for the benefit that the combination of these molecules can provide in several industrial applications [1–4]. Among the family of polymers, poly(ethylene oxide), PEO, has received considerable interest due to its useful properties, including the decreased water solubility at elevated temperatures * Corresponding author.

E-mail address: [email protected] (L.A. Bastardo). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.09.004

and the low protein and soil adsorption to surfaces coated with such polymers [5–10]. Hence, different research groups have recurred to the use of PEO, also known as poly(ethylene glycol) (PEG), in order to incorporate its useful features in other polymers and polyelectrolytes [11–14]. For instance, polyelectrolytes mixed with oppositely charged surfactants often phase separate in a certain mixing range [15–17], and the incorporation of PEO into the polyelectrolyte structure may prevent precipitation. This is the case for ethoxylated poly(ethylene imine) [18] and diblock copolymers of PEO and poly(sodium methacrylate) [13]. Methacrylate-PEO copolymers have received considerable attention lately due to good colloidal sta-

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bility at high ionic strength, prolonged blood circulation times, and low toxicity, which makes them attractive in drug and gene delivery systems [11,13,14,19]. Complexes formed by anionic surfactants and cationic random copolymers of methoxy poly(ethylene oxide) monomethacrylate (MePEOMA), and MAPTAC were investigated by Nisha et al. [14], who used polymers with 68, 89, and 94 mol% PEO content where 8 ethylene oxide units were incorporated in each PEO chain. The authors concluded that the polyelectrolyte–surfactant complex consists of a core of cationic polymer neutralized by surfactants, surrounded by a shell of poly(ethylene oxide) units. These complexes precipitate in water when the content of PEO is below 68%, while no precipitate is formed at any surfactant concentration when the content of PEO is above this value. On the other hand, no evidence of interaction was found between negatively charged comb polymers of sodium methacrylate (MA) and PEO in mixtures with anionic SDS [20]. Here, the comb polymer was characterized by a ratio of MA:PEO of 63:37 by weight. The PEO chain length was varied to be 7, 11, or 22 EO units. A systematic study of the effects of block length and surfactant structure on the interactions between poly(ethylene oxide)b-poly(sodium methacrylate) (PEO-b-PMA) and single-, double-, and triple-tail surfactants has been reported [21]. It was found that in contrast to the nonethoxylated PMA samples, which precipitated at certain surfactant concentrations, PEO-b-PMA/surfactant complexes formed stable dispersions with a size range 100–200 nm. The lengths of the polyion and the nonionic blocks of the polymer, as well as the structure of the surfactants, were found to strongly affect the properties of these systems. In another work on PEO-b-PMA and hexadecyltrimethylammonium bromide (HTAB); the effects of solution pH and ionic strength were considered [22]. A “critical salt concentration” csc was found where the PEO-b-PMA/HTAB aggregates disintegrated. This concentration was found to depend on the nature of the salt. For cations the csc varies in the order K+ ≈ Li+ ≈ Na+ > N(CH3 )+ 4 ; while for anions the order is I− > Br− > F− . Further, a decrease in pH was reported to result in an increase in the aggregate size together with changes in their morphology. More recently, the adsorption of a low charge density polyelectrolyte containing poly(ethylene oxide) side chains on silica was investigated [23]. The polyelectrolyte used in that study was a random copolymer of poly(ethylene oxide) monomethylether methacrylate and methacryloxyethyl trimethylammonium chloride (PEO45 MEMA:METAC-2), where the backbone has 2 mol% of charged methacrylate units in the form of quaternary ammonium groups and 98 mol% of methacrylate units containing PEO side-chains that are approximately 45 units long. It was found that even small changes in the salt content and the pH strongly influenced the amount of polyelectrolyte adsorbed on silica surfaces. It was concluded that the interactions between the PEO45 MEMA:METAC-2 and silica are influenced by both electrostatic forces and by hydrogen bonding between the EO units of the side-chains and the uncharged silanol groups. Interfacial properties of comb polyelectrolytes

with a polylysine backbone and PEO chains have also been investigated recently [24,25]. In the present study we focus on the characterization of the properties of PEO45 MEMA:METAC-2 in solution. The effects of ionic strength, temperature and addition of an oppositely charged surfactant are investigated using surface tension, nuclear magnetic resonance (NMR), static light scattering, and small-angle neutron scattering (SANS). Our system is different compared to those used in previous studies in that the PEO45 MEMA:METAC-2 comb polymer has both a high graft density and long side-chains. 2. Materials and methods 2.1. Materials The comb polyelectrolyte used in this study, PEO45 MEMA: METAC-2, was synthesized and characterized as reported in [23]. The subscript 45 refers to the number of EO units in the side-chains and the digit 2 refers to the molar percentage of charged units in the backbone. MEMA stands for methacrylate, and this building block carries the 45 unit long PEO side-chain (polydispersity 1.1, as reported by the manufacturer) that is terminated with a methylether group. METAC denotes methacryloxyethyl trimethylammonium chloride, which carries the permanently charged quaternary ammonium group. The comb polymer thus carries a large number of relatively long hydrophilic side-chains and a small number of charged units. The structure of PEO45 MEMA:METAC-2 is shown in Fig. 1. Sodium dodecylsulfate, SDS, was obtained from Sigma, and deuterated sodium dodecylsulfate, d-SDS, was obtained from Cambridge Isotope Laboratories. Deuterium oxide (D2 O) was chosen as solvent in most SANS experiments to minimize the incoherent background from hydrogen and to obtain a high scattering contrast. Some SANS experiments were also carried out in a solvent mixture of 82% H2 O and 18% D2 O, which contrast matches the polymer. NMR measurements were performed in D2 O, while H2 O was the solvent used for surface tension and static light scattering measurements.

Fig. 1. Structure of PEO45 MEMA:METAC-2 monomer units. The polymer is a random comb polymer with 98% of the segments carrying a PEO side-chain and the remaining 2% a permanent cationic charge.

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2.2. Small-angle neutron scattering Small-angle neutron scattering experiments were performed at the SANS1 instrument at the FRG1 research reactor at GKSS Research Centre, Geesthacht, Germany [26]. The neutron wavelength was 8.1 Å and the wavelength resolution was 10% (full-width-at-half-maximum value). The range of scattering vectors (0.005 < q < 0.25 Å−1 ) was obtained using four sample-to-detector distances (0.7–9.7 m). The samples were kept at 25.0 ± 0.5 ◦ C in quartz cuvettes with a path length of 1 or 2 mm. The raw SANS spectra were corrected for backgrounds from the solvent, sample cell, and other sources by conventional procedures [27]. The two-dimensional isotropic scattering spectra were azimuthally averaged, converted to absolute scale, and corrected for detector efficiency by dividing by the incoherent scattering spectra of pure water, which was measured with a 1 mm path length quartz cell. The smearing induced by the different instrumental settings is included in the data analysis. For each instrumental setting the ideal model cross section was smeared by the appropriate resolution function when the model scattering intensity was compared to the measured one by means of least-square methods [28]. The parameters in the models were optimized by conventional least-square analysis and the errors of the parameters were calculated by conventional methods [29]. 2.3. Static light scattering Static light scattering experiments were performed with a light scattering equipment from Brookhaven consisting of a BI200SM goniometer and a BI-9000AT digital autocorrelator. An argon ion laser (Lexel laser, Model 95-2) operating at a wavelength of 514 nm with vertically polarized light was used as light source. The measurements were done at 24 different angles in the range 30◦  θ  150◦ , corresponding to q-values in the range 8.40 × 10−4  q  31.5 × 10−4 Å−1 . For each angle five different measurements were performed and subsequently averaged. The data were normalized to absolute scale using toluene as in Refs. [30,31]. As a final step the static light scattering data were “converted to neutron units” by multiplying by (ρm )2 /KSLS , where KSLS is the light scattering contrast and (ρm )2 is the neutron contrast. The average molecular weight and the second virial coefficient were calculated via a conventional Zimm plot.   2 2  q Rg 1 KC = + 2A2 C, (1a) 1+ Rθ Mw 3 where KC/Rθ is plotted as a function of q 2 for different concentrations. The data were extrapolated to zero concentration, C, and zero scattering vector, q. Here Rθ is the excess Rayleigh ratio, Mw the weight average molecular weight, Rg the radius of gyration, and A2 the second virial coefficient. For vertically polarized incident light the optical constant is given by 4π 2 n2 (dn/dc)2 , K= λ40 NA

(1b)

23

where n is the refractive index of the solvent (1.335); dn/dc is the refractive index increment of the solution (0.158 ml/g), λ0 the wavelength of the laser light in vacuum, and NA is the Avogadro number. The comb polymer concentration was varied from 0.98 to 3.9 g/dm3 . 2.4. NMR All diffusion NMR experiments were performed at 20 ◦ C in a Bruker DMX200 spectrometer equipped with a wide-bore magnet and a commercial (Bruker) gradient probe with 9.6 T/m maximum gradient. The diffusion data were collected by conventional stimulated echo experiments [32], with the gradient strength linearly ramped up to 4 T/m, using 16 equidistant steps. Fitting the conventional Stejskal–Tanner expression [33] to the diffusional decay of the 1 H CH2 peak of SDS, or to the EO peak of the polymer, provides the nominal diffusion coefficients that were calibrated to their correct values by diffusion data obtained in water samples under identical conditions. The 1 H spectra for chemical shift and solvent effect measurements were recorded in a Bruker DMX500 spectrometer. It should be noted that the 1 H NMR spectrum (not shown) consists of narrow lines for the EO peak (caused by the known high internal mobility of PEO chains) and broad lines for the backbone protons. In relation to the large polydispersity of the polymer, this feature excludes any significant population of the methacrylate monomer with or without attached PEO side-chain. 2.5. Analysis of scattering data The experimental data were analysed using two different approaches, the indirect Fourier transformation method, and by fitting calculated scattering curves for specific structures to the measured curves. 2.6. The indirect Fourier transformation method Information on the distribution of scattering units can be obtained by applying the indirect Fourier transformation (IFT) method [34]. We used this method to analyse the data over the whole q-range in order to get information on the entire structure of the aggregates, and at high q-values to obtain information on the local structure within the aggregates (i.e., the aggregate cross section). This method has the advantage that it does not invoke a detailed structural model of the system and, once the dimensionality of the object has been chosen, provides model independent information [35]. On the length scales where the scattered intensity is controlled by the local stiffness of the aggregates, it is possible to decouple the scattering into two contributions; one that originates from the overall chain structure and another that reflects the local cross-section structure [36]. The asymptotic behaviour of the scattering function, dΣ(q)/dΩ, for q  1/Rg , where Rg is the radius of gyration of the cylindrical cross section of the aggregates, can be expressed as   ∞   π π I (q) = 2π pCS (r)J0 (qr)r dr = ICS (q), q q 0

(2)

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where J0 is the zero-order Bessel function and ICS (q) is the cross-section scattering intensity. The normalized cross-section distance distribution function pCS (r) is given by [37]  C ρ(r )ρ(r + r ) dr , pCS (r) = (3) 2πML where C is the concentration of scattering units (surfactants and polymer segments) within the aggregate, ρ is the average difference in scattering length density between the scattering units and the solvent and the position vectors r and r + r are lying in the cross-section plane. We obtained an estimate of the distance distribution function pCS (r) by applying the IFT method. From pCS (r) it is possible to calculate integral parameters of the aggregate cross section [38], such as the mass per unit length, ML , and the cross-section radius of gyration RCS,g , which is given by  ∞ 2 r pCS (r) dr 1/2 RCS,g = 0 ∞ (4) . 2 0 pCS (r) dr The cross section forward scattered intensity ICS (0) is given by ∞ ICS (0) = 2π

pCS (r) dr

(5)

0

and ML is then calculated via ML =

ICS (0) . 2 ρm

(6)

If one adopts a model for the comb polymer as a rod with a homogeneous cross section, the volume fraction of polymer segments, φ, within the volume of the rod is given by φ=

ML , 2 ρ 2πRCS

(7)

where ρ is the density of the polymer (ρ ∼ = 1 g/ml), and RCS is the radius of the cylindrical cross section which, in the homogeneous approximation, is related to the radius of gyration of the cross section, RCS,g , by RCS = 21/2 RCS,g . The analysis within this model framework was carried out by the software developed by Pedersen, for details see [39]. 2.7. Modeling of scattering data An alternative way of analysing the scattering data is to invoke a detailed structural model, calculate the scattering curve for this model and compare the calculated curve to the experimental data. This approach can provide a detailed structural picture, but one has to be aware that there is not one unique structure that corresponds to a given scattering curve. Thus, a large data set is of importance for determining the significance of the proposed structure. In this case this is provided by measurements at different contrasts (different solvents, and replacing SDS with d-SDS). Additional information is also relevant, in the present case obtained by NMR and surface tension measurements. In the case of short cylinders L/ R < 10 with elliptical cross section with semi axes a and b scattering intensities are written as [39]

π/2 π/2 I (q) ∼ 0

2J1 (qr(a, b, φ, α)) qr(a, b, φ, α)

0

sin(qL cos α)/2 × (qL cos α)/2

2 dφ sin α dα,

(8a)

where J1 is the first-order Bessel function and qr(a, b, φ, α) = [a 2 sin2 φ + b2 cos2 φ]1/2 sin α. For an isotropic solution of rigid and long cylinders (L/R > 10), the scattering cross section is written via decoupling approximation as I (q) ∼ Prod (q, L)SCS (q),

(8b)

where Prod (q, L) is the form factor of an infinitely thin rod of length L given as [39] Prod (q) = 2Si(qL)/(qL) − 4 sin2 (qL/2)/(q 2 L2 ) and x Si(x) = t −1 sin t dt

(9)

0

and SCS (q) is the scattering function for the cross section of the cylinder. For an elliptical cross section with semi axes a and b it is given by [39] 2 SCS (q) = π

π/2

2J1 (qr(a, b, θ )) qr(a, b, θ)

2 dθ

(10)

0

with r(a, b, θ) = [a 2 sin2 θ + b2 cos2 θ ]1/2 . We note that expression (8a) is exact, whereas the approximate Eq. (8b) requires L/ R > 10, as discussed in Ref. [40]. For a core–shell cross section with outer radius Re + a and shell thickness Re it is given by [39] SCS (q) =

[(Re + a)2 2J1 (q(Re + a))/q(Re + a) [(Re + a)2 + (ρe − 1)a 2 ]2 (ρe − 1)a 2 2J1 (qa)/qa]2 + , [(Re + a)2 + (ρe − 1)a 2 ]2

(11)

where J1 is the first-order Bessel function and ρe is the ratio between the scattering contrasts of the core and the shell compared to the solvent used. The data analysis within this framework was carried out by the software developed by Pedersen [39]. 2.8. Analysis of NMR data Intermolecular interactions influence the NMR chemical shift; conventionally, this is called the solvent effect. Although difficult to quantify, the solvent effect typically increases with the number of electrons within the atoms involved. Hence, the effect is small for 1 H nuclei. For this reason heavier nuclei such as 13 C, 19 F, or 31 P were traditionally exploited for providing information about the intermolecular environment in self-aggregating or associating systems. Hence, 13 C and 19 F chemical shifts were used for investigating micellization and

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the cmc [41]: the molecular environment is aqueous for the hydrophobic surfactant tails in the monomer state while it is aliphatic in the micellar state. Although small, 1 H solvent effects can nowadays be accurately quantified with high-field NMR spectrometers. The diffusion of elongated objects is often described in Kirkwood’s model where the translational diffusion coefficient of a rod-like object becomes [42,43] D=

kB T ln(L/b) , 3πηL

(12)

where kB is the Boltzmann constant, T is the absolute temperature, η is the viscosity of the solvent, while L and b are the rod length and diameter, respectively. Association phenomena that change the apparent object size can be conveniently monitored via the diffusion coefficients of the molecular components. If, as normally is the case, the molecules are in fast (on the diffusion NMR time scale ≈ 100 ms) exchange between associated and nonassociated states, then the measurement returns the average diffusion coefficient. In the simplest model this quantity becomes the population average given by D = Passoc Dassoc + (1 − Passoc )Dfree ,

(13)

where Dfree and Dassoc are the two diffusion coefficients characterizing the nonassociated and associated states, respectively, and Passoc is the fraction of molecules in the associated state. In the present system, the SDS molecules are in fast exchange between two such pools, while the polymer molecules always participate in a complex. Hence, by assigning Dfree to the diffusion coefficient of SDS monomers (4.75 × 10−10 m2 /s, measured independently in a 0.3 cmc SDS solution), and by setting Dassoc to the experimental polymer diffusion coefficients, the variation of Passoc upon increasing the SDS concentration can be obtained. 3. Results and discussion We will first discuss the data obtained for the PEO45 MEMA :METAC-2 comb polyelectrolyte in absence of SDS, focusing on the effect of salt and temperature on the conformation of the single chain. Next, we will consider the association between this comb polyelectrolyte and an oppositely charged surfactant. 3.1. PEO45 MEMA:METAC-2 samples 3.1.1. Light scattering The weight average molecular weight of the PEO45 MEMA: METAC-2 comb polyelectrolyte was evaluated by a conventional Zimm plot in 10 mM NaCl to be 350 kDa. This analysis also provided the second virial coefficient (1.3 × 10−4 mol cm3 /g2 ), showing that the pair interaction, as expected, is repulsive [44], and the radius of gyration, 230 Å. 3.1.2. Small-angle neutron scattering (SANS) The structure of the PEO45 MEMA:METAC-2 comb polyelectrolyte was investigated by SANS at different salt concen-

Fig. 2. Scattering data for PEO45 MEMA:METAC-2 samples (0.2%) in D2 O at different NaCl concentrations. The legend on the left shows the different NaCl concentrations in the samples.

trations using D2 O as solvent. The resulting scattering curves are shown in Fig. 2. Several features can be observed in the curves, the first of which is that the curves at different salt concentrations overlap. This demonstrates that the PEO45 MEMA: METAC-2 conformation in solution is not affected by the ionic strength even at the highest electrolyte concentration studied (100 mM NaCl). Thus, the conformation of the polymer is not dictated by electrostatic repulsion, but rather due to repulsion between the PEO side-chains. The scattering curves for PEO45 MEMA:METAC-2 present a slope of close to q −1 at q-values below 2.5 × 10−2 Å−1 , which is characteristic for rod-like structures [37]. We can thus already now conclude that the PEO45 MEMA:METAC-2 copolymer resembles a stiff rod, and that the stiff nature of the polymer arises from the repulsion between the PEO side-chains. The effect of temperature on the scattering curve for a 0.2% PEO45 MEMA:METAC-2 solution in D2 O solution is illustrated in Fig. 3a. The shape and intensity of the scattering curve are influenced by temperature at high q-values, but the slope of the curve at low q-values is not affected. Hence, PEO45 MEMA:METAC-2 adopts a rod-like structure also at 70 ◦ C. 3.1.3. Indirect Fourier transformation, IFT, analysis The IFT analysis in 2 dimensions returns the pair distance distribution function of scattering units in the cross-section of the comb polymer. The results obtained for the same samples at 25 and 70 ◦ C are shown in Fig. 3b. It is evident that as the temperature is increased, the peak of the curve is shifted to lower values of the distance between scattering units (from 35 to around 25 Å) and the maximum size of the comb polymer cross section decreases from 95 to 90 Å. The calculated radius of gyration for the cross section is 29 ± 1 Å at 25 ◦ C, decreasing to 27 ± 1 Å at 70 ◦ C, which, for a rod with a homogeneous cross section, corresponds to a radius of 41 and 38 Å, respectively. The mass/unit length was found to be 7.8 ± 0.6 × 10−14 g/cm, which, in the homogeneous cross-section model, corresponds to a volume fraction of polymer of 0.15 at 25 ◦ C and 0.17

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(a)

(b) Fig. 3. (a) Small angle neutron scattering data for a PEO45 MEMA:METAC-2 sample (0.2%) in D2 O at different temperatures 25 ◦ C (empty squares), 70 ◦ C (filled squares). No salt was added. (b) Pair distance distribution function (p(r)) of scatters in the cross section of the PEO45 MEMA:METAC-2 rod, 25 ◦ C (empty squares), 70 ◦ C (filled squares).

at 70 ◦ C. Of course, this model is crude, but it demonstrates the large solvent content within the volume occupied by the comb polymer. Similar results were obtained by performing a Guinier analysis of the data [45], which resulted in a radius of gyration of the cross section of 29 Å for the sample at 25 ◦ C and 25 Å for the sample at 70 ◦ C. The decrease in the cross section of the comb polymer rod with increasing temperature is due to the less favorable interaction between PEO and water at elevated temperatures [10]. It is the same molecular mechanism that causes phase separation of aqueous solutions of nonionic PEO based surfactants [4,46] and polymers [4,5] on heating. We note that the solution conformation of PEO45 MEMA:METAC-2 is very different from the random coil-like structure of a homopolymer of PEO in water, but both PEO and the PEO45 MEMA:METAC-2 comb polymer respond to an increase in temperature by contracting its dimensions.

Fig. 4. Small angle neutron scattering data for PEO45 MEMA:METAC-2 sample (0.2%) in D2 O. Individual symbols represent the data obtained by SANS. The lines represent fits obtained using the following models from bottom to top: stiff rods with elliptical cross section, with circular cross section and graded cylinders. The curves were plotted with some off set for the sake of clarity.

3.1.4. Modeling of scattering curves The slopes of the scattering curves suggest that models for stiff rods should be used for fitting the data. To this end we tried stiff rod models with (a) monodisperse circular cross section, (b) “graded cylinders,” with polydisperse circular cross section [47], and (c) elliptical cross section. Support for the use of the latter two models comes from the shape of the Pcs(r) function for the pure PEO45 MEMA:METAC-2 (Fig. 3b), where it is important to point out that in general it is very hard to distinguish between an elliptical cross section and a polydisperse circular cross section. We note that an elliptical cross section model has previously been applied when analysing scattering from solutions of stiff rod-like micelles [40]. As seen from Fig. 4, the scattering curves for the pure polyelectrolyte are best described by the elliptical cross-section model, whereas the monodisperse circular cross-section model fits the data in the least accurate way. The same is true for the polyelectrolyte in the presence of low surfactant concentrations. At higher surfactant concentrations (at and above 4 mM SDS), the use of a persistence length parameter in the model allows a better fitting of the data. However, the persistence length of the polymer chain is almost of the same order of magnitude as its length, so it could not be appropriately determined. We note that the PEO side-chains attached to the combpolymer, as indicated in the materials section, are close to monodisperse, i.e., all of them have the same length. Hence, it is no reason to believe that we have a significant polydispersity in cross section. However, one may envisage that the size of the cross-section fluctuates due to the thermal motion of the side-chains, providing an effective polydispersity. To conclude, the model applying an elliptical cross section provides the best fit to the data, but this fact does not prove that the cross section is elliptical. Keeping this in mind we decided to use the elliptical cross-section model when fitting the data. Already now we note that the comb polymer can be modeled as a stiff rod with an elliptical cross-section with a short axis of 41 Å and a long

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axis of 79 Å. The corresponding data returned by the graded cylinder model was a radius of 39 Å. 3.1.5. Polymer length For the comb polymer in absence of SDS we find by NMR that the self-diffusion coefficient is 2.1 × 10−11 m2 /s (which compares well with the average mutual diffusion coefficient 1.5 × 10−11 m2 /s obtained by dynamic light scattering without extrapolation to zero concentration). By setting the diameter to 80 Å as provided by SANS, Eq. (12) estimates the rod length from the self-diffusion value to 250 Å (note that various minor modifications to the Kirkwood Model [48] all provide a length estimate <300 Å). Both the large solvent content within the volume of the rod and the polydispersity of the sample may shift down the size estimate; hence, the actual length of the polymer rod might exceed somewhat the 300 Å upper limit. 3.2. PEO45 MEMA:METAC-2 mixed with SDS It is well known that homopolymers of PEO associate with SDS to form a structure where SDS micelles are bound along the polymer chain [8]. This is normally referred to as a “beadand-necklace” structure. In these PEO-SDS complexes the PEO chain is highly curled to facilitate the interaction with the SDS micellar surface. The density of the PEO side-chains on the PEO45 MEMA:METAC-2 polymer is very high, causing the polyelectrolyte to adopt a stiff rod-like conformation that opposes the bending of the backbone around SDS micelles. The PEO side chains, on the other hand, are reasonably flexible as indicated by the narrow EO lines (in contrast to the broad backbone lines) in the 1 H NMR spectra (data not shown). Further evidence of the side-chain flexibility comes from the scattering data, where a cylinder radius considerably smaller than the extended length of the PEO side-chain, around 200 Å, is determined. Given all these facts, it is not clear a priori how SDS will interact with this type of polymer structure. Some light on this issue is shed by surface tension, NMR and SANS measurements using aqueous mixtures of PEO45 MEMA:METAC-2 and SDS. 3.2.1. Surface tension measurements In order to obtain some information on the association between PEO45 MEMA:METAC-2 and SDS, the surface tension isotherms in absence and presence of 0.2 wt% of the comb polymer were determined. The data, presented in Fig. 5, show a surface tension isotherm in absence of the polymer that agrees with literature values; in particular no dip in the isotherm is observed demonstrating a high purity of the SDS sample. Further, we note that PEO45 MEMA:METAC-2 in itself is slightly surface active, lowering the air-water interfacial tension to 60 mN/m. This value is slightly lower than the surface tension of PEO– water solutions, which is 65 mN/m [49]. Addition of surfactants results in a further decrease in surface tension, but not such a dramatic decrease as observed for strongly associating polyelectrolyte–surfactant systems [50]. By plotting surface tension versus log concentration it is possible to obtain the cmc value in presence of polymer without much ambiguity (Fig. 5).

Fig. 5. Surface tension (γ ) isotherms of SDS in absence (2) and in presence (1) of 0.2% PEO45 MEMA:METAC-2 in H2 O. No salt was added. The bottom figure shows the region around the cmc in more detail.

In the concentration range 8–19 mM of SDS a linear fit with r 2 of 0.977 is obtained. Above 19 mM SDS the slope of the surface tension isotherm is similar to that obtained for pure SDS. The point at which these slopes intersect is taken as the cmc value in presence of polyelectrolyte (18 mM). The shift of the cmc of SDS from 8.3 to about 18 mM in the presence of PEO45 MEMA:METAC-2 demonstrates that association between the polyelectrolyte and surfactant does occur. At saturation about 10 mM of SDS is bound to the polymer, which corresponds to about 10 SDS molecules for each PEO side-chain. Samples containing mixtures of PEO45 MEMA:METAC-2 and SDS were also investigated by NMR to gain further insight into the association behavior. 3.2.2. NMR-investigations As shown in Tables 1 and 2, the SDS lines exhibit significant solvent effects in the various investigated solutions, but this effect is very different in pure SDS micelles and in SDS associated with the polymer. Moreover, the solvent effect in the polymer complex is independent of the SDS concentration.

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Table 1 1 H chemical shifts of different SDS and PEO MEMA:METAC-2 lines, all relative to the chemical shift of water, in ppm units. The experimental error of shift data 45 is approximately ±0.001 ppm (for the ω-CH3 and EO lines) and ±0.002 ppm (for the ω-CH2 and N-CH3 lines) Polymer

SDS 0.3 cmc

SDS 25 mM

Polymer + 2 mM SDS

Polymer + 20 mM SDS

SDS α-CH2 SDS ω-CH3 Polymer EO 1.078 Polymer N-CH3 1.399

0.724 3.925

0.747 3.895

0.728 3.914 1.077 1.398

0.729 3.913 1.076 1.397

Table 2 The 1 H solvent effect provided by the SDS chemical shifts relative to those obtained in monomeric solution of SDS (see Table 1), in 0.001 ppm units SDS 25 mM Polymer + 2 mM SDS Polymer + 20 mM SDS SDS α-CH2 SDS ω-CH3

23 ± 4 −30 ± 2

4±4 −11 ± 2

5±4 −12 ± 2

Hence, the average intermolecular environment of SDS molecules in the complex is significantly more polar than in micelles and not dominated by other SDS molecules. This finding excludes SDS aggregates attaching to the polymer as the major structural surfactant entity. The self-diffusion data reported in Fig. 6a include the diffusion coefficient for both the polymer and the surfactant as a function of surfactant concentration. Addition of surfactant results in a decrease in the self-diffusion coefficient of the aggregate, indicating a moderate increase in dimension and mass. The decrease in surfactant self-diffusion coefficient is caused by the presence of two distinct states, free surfactant monomers and surfactants associated with the comb polyelectrolyte. Note that the decrease in surfactant self-diffusion below the cmc of SDS is another direct proof of SDS binding to the polymer. From the SDS data it is straightforward to calculate the bound fraction of surfactant, and the results are provided in Fig. 6b. Note that the diffusion coefficient of an SDS micelle is approximately 1 × 10−10 m2 /s [51]. Hence, on the basis of the measured diffusion coefficients we cannot exclude the existence of free SDS micelles. However, from the surface tension data we conclude that no free micelles are present until the total surfactant concentration has reached 18 mM. Thus, up to this point the two-state model is appropriate for determining the amount of surfactant bound to the polyelectrolyte. The binding isotherm of SDS on PEO45 MEMA:METAC-2 can then be obtained from the bound fraction and total surfactant concentration. The results are illustrated in Fig. 7, where the bound amount of SDS is plotted as a function of the free SDS concentration in solution. The samples were chosen to have the same total concentration of SDS as the samples studied with SANS, and the amount of bound SDS determined by NMR was used when calculating the scattering contrast between the polymer-surfactant complex and the solvent. At low free SDS concentrations the bound amount of SDS is very small, for instance at a free SDS concentration of 3.8 mM the bound concentration is only 0.2 mM. However, as the free SDS concentration approaches 7 mM, a weakly cooperative binding of SDS is observed. We note that surface tension measurements show that no free micelles are present up to a total SDS concentration of 18 mM, thus free micelles are expected only at the highest

(a)

(b) Fig. 6. (a) The diffusion coefficients of SDS (2) and of PEO45 MEMA: METAC-2 (0.2%) (!) recorded upon increasing SDS concentration. (b) The fraction of SDS bound to the comb polymer as derived from the diffusion data via Eq. (13).

concentration studied with NMR. The fact that the cooperative binding sets in close to the cmc demonstrates that the free energy of association to the polymer is only marginally favored over the formation of free micelles. The free energy lowering due to the SDS association with the PEO45 MEMA:METAC-2 polymer, compared to the one due to formation of free SDS micelles can be estimated to be [4]   cac G = RT ln (14) . cmc A cac-value in the range 6–7 mM, as determined by NMR (Fig. 7), results in a G-value in the range from −0.3 to

L.A. Bastardo et al. / Journal of Colloid and Interface Science 312 (2007) 21–33

Fig. 7. Binding isotherm of SDS on the PEO45 MEMA:METAC-2 comb polymer. The concentration of the comb polymer was 0.2%. NMR was used for determining the bound amount, except at saturation where the surface tension data was used.

−0.8 kJ/mol. For comparison, the free energy gain of forming SDS micelles along a homopolymer chain of PEO has similarly been estimated to be −1.45 kJ/mol [52].

29

Fig. 8. Normalized scattering intensity as a function of scattering vector for samples with different concentrations of added surfactant, from bottom to top no SDS, [SDS] = 1 mM, PEO45 MEMA:METAC-2 sample (0.2%) in D2 O [SDS] = 2 mM, [SDS] = 4 mM, [SDS] = 8 mM. The concentration of PEO45 MEMA:METAC-2 was 0.2 wt%, and no salt was added. Individual symbols represent data obtained with SANS (q > 0.005) and SLS (q < 0.004 Å−1 ). The curves were offset for the sake of clarity. The lines represent the results from fits using models for stiff rods with elliptical cross section (0–2 mM), and stiff rods with elliptical cross section and two shells (4 and 8 mM). The parameters of the model fit are provided in Table 3.

3.2.3. Small-angle neutron and static light scattering In this section we first describe the results obtained for mixtures of PEO45 MEMA:METAC-2 and SDS in D2 O, where both components contribute to the scattering. Next, we discuss the results obtained for PEO45 MEMA:METAC-2 mixed with dSDS in D2 O, where the surfactant is close to contrast matched and the data provide information on the structure of the polymer. Similarly, measurements using PEO45 MEMA:METAC-2 mixed with d-SDS in a solvent mixture of 82% H2 O and 18% D2 O, where the comb polymer is contrast matched, provide information on the structures formed by the SDS molecules. The data obtained are finally analyzed by the indirect Fourier transformation method and by comparing with the scattering curves from specific model structures. 3.2.4. Samples containing SDS in D2 O The normalized scattering data for PEO45 MEMA:METAC2 with different concentrations of SDS (0–8 mM) in D2 O are illustrated in Fig. 8. Clearly, the scattering of PEO45 MEMA: METAC-2 in the presence of no and low concentrations of SDS (up to 2 mM) are very similar. In the q-range 0.005 < q < 0.025 Å−1 , the slopes of the SANS curves (in the SDS concentration range 0–4 mM) display a close to q −1 dependence, which is typical of rod-like aggregates. For the sample at 8 mM, in the same q-range, the slope is q −0.9 . This reduction of the slope is an indication of repulsive interactions between the aggregates [53]. We note that even incorporation of a small amount of surfactant, determined to be 0.2 mM at a total surfactant concentration of 4 mM, which corresponds to one SDS molecule for five EO chains, or, equivalently, to 10 surfactants for each polymer charge, has a significant impact on the structure of the complex.

Fig. 9. Scattering data for PEO45 MEMA:METAC-2 samples (0.2%) in D2 O at different SDS concentrations, shown in the inset of the figure.

At higher surfactant concentrations (15 mM and above) the shape of the scattering curves becomes more complex (see Fig. 9), presenting a shoulder at high q-values that is due to the presence of surfactant associated with the polyelectrolyte, as evidenced by the data presented below. We note that according to surface tension data no free micelles are present in the sample containing 15 mM SDS, but some free micelles are present at 23 mM SDS. The similarity of the scattering curves obtained at 15 and 23 mM SDS demonstrates that the scattering contribution from the free micelles is negligible also at 23 mM SDS. Repulsive interactions between the aggregates are also evident in these samples, as indicated by the low slope (q −0.7 ) at q values between 0.005 < q < 0.019 Å−1 . The peak observed at the highest surfactant concentration, 92 mM, is due to the presence of free SDS micelles.

30

L.A. Bastardo et al. / Journal of Colloid and Interface Science 312 (2007) 21–33

Fig. 10. Scattering data for PEO45 MEMA:METAC-2 samples (0.2%) in D2 O at different d-SDS concentrations. From bottom to top: [d-SDS] = 4, 23, 46, and 92 mM. The curves were offset for the sake of clarity. Individual symbols represent data obtained with SANS. The lines represent the results from fits using models for stiff rods with elliptical cross section. The parameters of the model fit are provided in Table 3.

3.2.5. SDS match point: Samples containing d-SDS in D2 O The SANS data of PEO45 MEMA:METAC-2 mixed with d-SDS in D2 O are shown in Fig. 10. In this solvent the dSDS is contrast matched and the polyelectrolyte is responsible for the scattering. Only small changes in the scattering curves are observed upon addition of surfactant. The PEO45 MEMA:METAC-2/d-SDS scattering curves have the same slope at low q-values (0.005 < q < 0.030 Å−1 ) as observed for the comb polymer sample without SDS (i.e., q −1 ), confirming the rod-like structure of the comb polymer for all surfactant concentrations. At intermediate q-values (0.040 < q < 0.085 Å−1 ) the slope is proportional to q −4 for samples containing up to 4 mM of d-SDS. However, at higher surfactant concentrations (23, 46, and 92 mM) the value of the slope in this q-range changes to q −3 , which is an indication of a less well-defined interface. This observation shows that incorporation of SDS in the aggregate causes a structural change in the polymer, and we suggest that some PEO chains stretch further away from the backbone. We note that the shoulder in the scattering curve observed with 15 and 23 mM SDS is absent in presence of 23 mM d-SDS. This leads to the firm conclusion that this feature is due to scattering from SDS. 3.2.6. Polymer match point: Samples containing d-SDS in 82% H2 O and 18% D2 O The SANS scattering curves for PEO45 MEMA:METAC-2 mixed with d-SDS in a mixture of 82% H2 O and 18% D2 O, which is the solvent composition that contrast matches the polymer, are shown in Fig. 11. Hardly any scattering is observed for the samples in the d-SDS concentration range 0–4 mM. The absence of scattering from these samples does not mean that there is no surfactant molecules associated with the polyelectrolyte, but that the number of molecules associated with the comb polymer is small, which is consistent with NMR data (Fig. 6b). At a d-SDS concentration of 15 mM there is a visible scattering from the surfactant molecules. The scattering increases with

Fig. 11. Scattering curves of PEO45 MEMA:METAC-2 (0.2%) samples in a mixture of 82% H2 O and 18% D2 O at different d-SDS concentrations. The legend on the left shows the d-SDS concentration in the sample.

increasing surfactant concentration and a shoulder at intermediate q-values is observed already at 15 mM. The position of the shoulder is around q = 0.1 Å−1 at 15 and 23 mM SDS, which corresponds to the position of the shoulder observed at the same concentration when both the polymer and the surfactant are visible, Fig. 11. At higher surfactant concentrations, scattering from free micelles dominates, and a peak at smaller q-values appears. The peak position corresponds to the average distance between free SDS micelles [8,54]. As expected, this peak shifts to larger q-values, i.e., smaller distances, as the concentration of free micelles increases with the total surfactant concentration. It is located at 0.054 Å−1 (corresponding to a distance of 116 Å) for the sample at 46 mM, and at 0.066 Å−1 (98 Å) at 92 mM. We note that according to surface tension data no free micelles are present at a total surfactant concentration of up to 18 mM. The IFT analysis in 2 dimensions returns the pair distance distribution function of scattering units in the cross-section of the comb polymer/surfactant aggregates. The results obtained at some selected surfactant concentrations are displayed in Fig. 12. These data are somewhat affected by interactions, which preclude us from drawing quantitative conclusions and the following discussion is thus qualitative only. With increasing surfactant concentration a progressive increase in the pair correlation function at small distances, while a minimum develops at intermediate distances, is observed in the data for the samples where the surfactant is visible (Figs. 12a and 12c). These features are not present when the surfactant is contrast matched (i.e., only the polymer is visible, Fig. 12b), confirming that the features are due to the surfactant only. The data thus suggest that the surfactants are present in two distinctly different environments, corresponding to the different peaks observed in the figure. The most plausible suggestion is that surfactants initially are adsorbed at the comb polymer backbone, and that the cooperative association step is due to formation of micellar-like spherical aggregates at the periphery of the comb polymer/SDS complex (peak at small dis-

L.A. Bastardo et al. / Journal of Colloid and Interface Science 312 (2007) 21–33

(a)

31

(b)

(c) Fig. 12. Pair distance distribution function, PCS (r) of scatters in the cross section of PEO45 MEMA:METAC-2/SDS complexes at different SDS concentrations. (a) Samples with SDS in D2 O where both components are visible. (b) Samples with d-SDS in D2 O where only the polymer is visible. (c) Samples with d-SDS in 82% H2 O and 18% D2 O where only the surfactant is visible. The surfactant concentrations used are indicated on the legends to the right of the figures.

tances in Figs. 12a and 12c). The polymer cross section contracts marginally upon addition of a small amount of SDS whereas it expands in presence of higher concentrations of SDS, Fig. 12b. Negative values at the intermediate “r” interval (Fig. 12c) indicate interactions between SDS subunits within the cross section of the polyelectrolyte/SDS aggregates. 3.2.7. Modeling of scattering curves: Samples with SDS in D2 O Just as for the situation without added surfactant, the model based on “stiff rods with elliptical cross section” was chosen in order to analyse the scattering data. The results of the analyses of the PEO45 MEMA:METAC-2/SDS system are reported in Fig. 8 and Table 3, and the agreement between measured and calculated scattering curves is quite convincing over the whole q-range, except at very small values where the presence of a minute amount of larger aggregates affects the measurements.

In fact, the disappearance of the up-turn at low q-values at SDS concentrations at and above 8 mM (see Fig. 8) may indicate disintegration of aggregates due to SDS incorporation. A rod-like complex structure with an elliptical cross-section is found to fit the data for PEO45 MEMA:METAC-2/SDS complexes at surfactant concentrations up to 2 mM. At 4 mM and higher SDS concentrations the stiff rod elliptical cross-section model could not be used to explain the data. Instead, a core–shell rod-like model had to be employed. The data is well described with an outer shell with a radius (Re + a) equal to 56–59 Å, and an inner shell radius of 16 Å. The outer shell has also an elliptical cross section. The difference between the inner and the outer shell is that the outer shell has a significantly lower scattering length density as quantified in Table 3. Here, the scattering length density of the inner shell corresponds to the one calculated using the SDS-bound data obtained by NMR, while ρe was calculated by the model fitting program.

32

L.A. Bastardo et al. / Journal of Colloid and Interface Science 312 (2007) 21–33

Table 3 Summary of fitting parameters for SLS/SANS data for PEO45 MEMA: METAC-2 (0.2%) and SDS/d-SDS at different concentrations in D2 O with no salt present (25 ◦ C). Here “a” and “b” are the radius of the cylinder crosssection short and long axis, respectively; sigma is the relative standard deviation with respect to the length; ρe is the ratio between the scattering contrast of the core and the shell in D2 O; Re is the thickness of the outer shell. The cylinder length was fixed to 300 Å (as determined by NMR) for all samples. The value chosen for the cylinder length only affects the scattering at low q-values SDS (mM) a (Å) b (Å) Sigma ρe Re (Å)

(a)

d-SDS (mM)

0

1.0

2.0

4.0

8.0

4.0

23.0

46.0

92.0

41 79 1.52 – –

41 79 1.60 – –

41 79 1.62 – –

16 32 1.92 3.8 40

16 30 1.33 3.9 43

41 75 1.52 – –

43 83 1.51 – –

44 93 1.51 – –

44 107 1.51 – – (b)

We propose that the inner shell consists of a mixture of SDS associated with the polymer backbone and parts of the PEO side-chains, whereas the outer shell consists of PEO chains that are forced to extend further away from the backbone due to the presence of SDS within the aggregate. Similar structures have been proposed for comb like copolymer-surfactant complexes with shorter PEO chains, where the surfactant have been suggested to be adsorbed to the core of the polyelectrolyte, creating a hydrophobic inner domain [14,55]. The data obtained at surfactant concentrations above 8 mM cannot be fitted by any simple model. 3.2.8. Modeling of scattering curves: Samples with d-SDS in D2 O A model fit for stiff-rods was used for analysing the scattering curves for the samples where the surfactant was contrast matched. The parameters used for fitting the scattering curves are shown in Table 3. The small axes of the cylinders elliptical cross section, returned by the analysis were 41, 43, 44, and 44 Å for the samples at 4, 23, 46, and 92 mM, respectively. While the long axis size increased from 75 to around 107 Å for the highest surfactant concentration, confirming the stretching of the PEO side chains in the presence of surfactant. 3.2.9. The structure of PEO45 MEMA:METAC-2/SDS aggregates All the data obtained is consistent with a comb polymer surfactant aggregate structure evolution with surfactant concentration as illustrated in Fig. 13. The surfactant initially binds to the comb polymer backbone. At higher surfactant concentrations also some micellar-like aggregates are formed at the periphery of the aggregate. The structural changes in the polymer due to surfactant association are relatively small, but some stretching of the grafted PEO chains is evident. 4. Conclusions The comb polyelectrolyte PEO45 MEMA:METAC-2 adopts a stiff rod conformation in aqueous solutions. The polymer conformation is insensitive to the electrolyte concentration, demonstrating that the repulsion between the PEO side-chains, rather

(c) Fig. 13. The structure of the comb polymer surfactant aggregate at different surfactant concentrations. (a) No added surfactant, (b) below the cooperative association step, (c) at saturation. The stretching of the PEO side chains is, for clarity, exaggerated in the figure.

than electrostatic repulsion, dictates the conformation. An increase in temperature results in a contraction of the crosssection of the polymer cylinder as the solvency for PEO is decreased. PEO45 MEMA:METAC-2 forms soluble complexes with the anionic surfactant SDS at all mixing ratios. The association is characterised by a low binding of surfactant until a cooperative association step is observed when the free SDS concentration approaches 7 mM. The initial association of the surfactant and the polymer results in some stretching of the PEO side-chains in the complex. Further incorporation of surfactant results in the development of two distinct surfactant populations. An inner shell of surfactant bound to the comb polymer backbone, and an outer layer of small spherical micellar-like surfactant aggregates. Acknowledgments This work was supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract No. RII3-CT-2003-506839. P.C., I.F., and A.D. acknowledges financial support from the Swedish Research Council, VR. The Swedish–Lithuanian collaboration was initiated by support from the Swedish Royal Academy of Sciences and later supported by the European Commission through the project SOCON, contract No. MRTN-CT-2004512331. L.B. and I.J. acknowledges financial support from the

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VINNOVA competence centre “Surfactants based on natural products, SNAP.” Karin Schillén and Ali Naderi are thanked for valuable discussions and for some static light scattering measurements. References [1] E.D. Goddard, Polymer–surfactant interaction. Part II—Polymer and surfactant of opposite charge, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interactions of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, FL, 1993, p. 172. [2] E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interactions of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, FL, 1993. [3] S. Guillot, D. McLoughlin, N. Jain, M. Delsanti, D. Langevin, J. Phys. Condens. Matter 15 (2003) 230. [4] B. Jönsson, B. Lindman, K. Holmberg, B. Kronberg, Surfactants and Polymers in Aqueous Solution, Wiley, West Sussex, 1998. [5] S. Bekiranov, R. Bruinsma, P. Pincus, Phys. Rev. E 55 (1997) 577. [6] W. Brown, K. Schillén, M. Almgren, J. Phys. Chem. 95 (1991) 1850. [7] B. Cabane, R. Duplessix, Colloids Surf. 13 (1985) 19. [8] B. Cabane, R. Duplessix, J. Phys. (Paris) 43 (1982) 1529. [9] B. Hammouda, D. Ho, S. Kline, Macromolecules 35 (2002) 8578. [10] R. Kjellander, E. Florin, J. Chem. Soc. Faraday Trans. I 77 (1981) 2053. [11] T.K. Bronich, A.V. Kabanov, V.A. Kabanov, K. Yu, A. Eisenberg, Macromolecules 30 (1997) 3519. [12] A. Hashidzume, Y. Morishima, K. Szczubialka, Amphiphilic polyelectrolytes, in: S.K. Tripathy, J. Kumar, H.S. Nalwa (Eds.), Handbook of Polyelectrolytes and Their Applications, vol. 2, American Scientific Publishers, Stevenson Ranch, 2002, p. 1. [13] A.V. Kabanov, T.K. Bronich, V.A. Kabanov, K. Yu, A. Eisenberg, J. Am. Chem. Soc. 120 (1998) 9941. [14] C.K. Nisha, P. Basak, S.V. Manorama, S. Maiti, K.N. Jayachandran, Langmuir 19 (2003) 2947. [15] L.A. Bastardo, V.M. Garamus, M. Bergström, P.M. Claesson, J. Phys. Chem. B 109 (2005) 167. [16] L.A. Bastardo, R. Mészáros, I. Varga, T. Gilányi, P. Claesson, J. Phys. Chem. B 109 (2005) 16196. [17] A. Naderi, P.M. Claesson, M. Bergström, A. Dedinaite, Colloids Surf. A Physicochem. Eng. Aspects 253 (2005) 83. [18] Y. Li, R. Xu, S. Couderc, D.M. Bloor, J. Warr, J. Penfold, J.F. Holzwarth, E. Wyn-Jones, Langmuir 17 (2001) 5657. [19] M.C. Deshpande, M.C. Garnett, M. Vamvakaki, L. Bailey, S.P. Armes, S. Stolnik, J. Controll. Rel. 81 (2002) 185. [20] H. Middleton, R.J. English, P.A. Williams, G. Broze, Langmuir 21 (2005) 5174. [21] T.K. Bronich, A.M. Popov, A. Eisenberg, V.A. Kabanov, A.V. Kabanov, Langmuir 16 (2000) 481. [22] S.V. Solomatin, T.K. Bronich, T.W. Bargar, A. Eisenberg, V.A. Kabanov, A.V. Kabanov, Langmuir 19 (2003) 8069. [23] J. Iruthayaraj, E. Poptoshev, A. Vareikis, R. Makuska, A. van der Wal, P.M. Claesson, Macromolecules 28 (2005) 6152.

33

[24] M. Heuberger, T. Drobek, N.D. Spencer, Biophys. J. 99 (2005) 495. [25] M. Heuberger, T. Drobek, J. Vörös, Langmuir 20 (2004) 9445. [26] H.B. Stuhrmann, N. Burkhard, G. Dietrich, R. Jünemann, W. Meerwinck, M. Schmitt, J. Wadzack, R. Willumeit, J. Zhao, K.H. Nierhaus, Nuclear Instrum. Methods Phys. Res. A 356 (1995) 124. [27] J.P. Cotton, Introduction to an investigative tool for colloidal and polymeric systems, in: P. Lindner, T. Zemb (Eds.), Neutron, X-Ray and Light Scattering, North-Holland, Amsterdam, 1991. [28] J.S. Pedersen, D. Posselt, K. Mortensen, J. Appl. Crystallogr. 23 (1990) 321. [29] J.S. Pedersen, P. Vyskocil, B. Schönfeld, G. Kostorz, J. Appl. Crystallogr. 30 (1997) 975. [30] L. Bastardo, P. Claesson, W. Brown, Langmuir 18 (2002) 3848. [31] C.A. Ericsson, O. Söderman, V.M. Garamus, M. Bergström, S. Ulvenlund, Langmuir 20 (2004) 1401. [32] J.E. Tanner, J. Chem. Phys. 52 (1970) 2523. [33] E.O. Stejskal, J.E. Tanner, J. Chem. Phys. 42 (1965) 288. [34] O. Glatter, J. Appl. Crystallogr. 10 (1977) 415. [35] O. Glatter, J. Appl. Crystallogr. 13 (1980) 577. [36] J.S. Pedersen, P. Schurtenberger, J. Appl. Crystallogr. 29 (1996) 646. [37] L.A. Feigin, D.I. Svergun, Structure and Analysis by Small-Angle X-Ray and Neutron Scattering, Plenum, New York, 1987. [38] O. Glatter, in: A.J.C. Wilson (Ed.), International Tables for Crystallography, vol. C, Academic Publishers, Dordrecht, 1992, p. 89. [39] J.S. Pedersen, Adv. Colloid Interface Sci. 70 (1997) 171. [40] V.M. Garamus, J.S. Pedersen, H. Maeda, P. Schurtenberger, Langmuir 19 (2003) 3656. [41] I. Furó, J. Mol. Liq. 117 (2005) 117. [42] M. Doi, S.F. Edwards, The Theory of Polymer Dynamics, Clarendon Press, Oxford, 1986. [43] J.D. Kirkwood, J. Polym. Sci. 12 (1954) 1. [44] D.F. Evans, H. Wennerstrom, The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet, second ed., Wiley–VCH, Weinheim, 1999. [45] I. Wataoka, H. Urakawa, K. Kajiwara, M. Schmidt, M. Wintermantel, Polym. Int. 44 (1997) 365. [46] K. Shinoda, A. Carlsson, B. Lindman, Adv. Colloid Interface Sci. 64 (1996) 253. [47] S. Rathgeber, T. Pakula, A. Wilk, K. Matyjaszewski, K. Beers, J. Chem. Phys. 122 (2005), 123904. [48] J.D. Kirkwood, P.L. Auer, J. Chem. Phys. 19 (1951) 281. [49] M.J. Schwuger, J. Colloid Interface Sci. 43 (1973) 491. [50] C. Monteux, M.F. Llauro, D. Baigl, C.E. Williams, O. Anthony, V. Bergeron, Langmuir 20 (2004) 5358. [51] B. Lindman, M.C. Puyal, N. Kamenka, R. Rymdén, P. Stilbs, J. Phys. Chem. 88 (1984) 5048. [52] E. Pettersson, D. Topgaard, P. Stilbs, O. Söderman, Langmuir 20 (2004) 1138. [53] V.M. Garamus, J.S. Pedersen, H. Kawasaki, H. Maeda, Langmuir 16 (2000) 6431. [54] M. Bergström, J.S. Pedersen, Phys. Chem. Chem. Phys. 1 (1999) 4437. [55] K.N. Jayachandran, S. Maiti, Polymer 43 (2002) 2349.