Complexes between High Charge Density Cationic Polyelectrolytes

Apr 23, 2009 - Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos. Constantinou AVenue, 11635 Athens, ...
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Complexes between High Charge Density Cationic Polyelectrolytes and Anionic Single- and Double-Tail Surfactants C. Mantzaridis, G. Mountrichas, and S. Pispas* Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece ReceiVed: October 30, 2008; ReVised Manuscript ReceiVed: February 11, 2009

Polyelectrolyte/surfactant complexes formed between well-defined linear flexible polyelectrolytes, namely, quaternized poly[3,5-bis(dimethylaminomethylene)hydroxystyrene] (Q-N-PHOS), bearing two cationic sites on each repeating unit, and two different anionic surfactants, namely, sodium dodecyl sulfate (SDS) with one hydrocarbon tail and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) with two hydrocarbon chains, are studied by means of fluorescence spectroscopy, electrophoretic, dynamic and static light scattering, and atomic force microscopy. Depending on the surfactant state in initial solutions (i.e., below or above nominal critical micelle concentration, cmc) and final (-/+) charge ratio, self-assembly in nanoparticles of variable size, stability, and effective charge is possible. Spherical, rather polydispserse complexes are formed in all cases. Critical aggregation concentrations (cac) depend on the surfactant type, while hydrophobicity of the main polyelectrolyte chain plays a role in colloidal stability of the complex nanoparticles. Introduction Over the past years growing attention has been given to constructing supramolecular structures using lower molecular weight compounds as building blocks. The ultimate goal for synthetic scientists is to manage to produce preprogrammed hierarchical structures of functional materials. The hierarchy can be achieved by inserting “information” in the building blocks, which will be used for self-assembly of the compounds in order to give the superstructures. This information can be introduced in the building blocks in the form of hydrogen bonding, steric demands, electrostatic interactions, and the hydrophilic or hydrophobic character of the molecule. It is obvious that these kinds of interactions, between the building blocks, play a key role in the self-assembly of soft matter.1,2 Polymer chemistry is able to create many novel macromolecules, through diverse synthetic strategies, which fulfill the above demands and can be used as building blocks in this intriguing field of research.3 A common pair of building blocks that are investigated intensively over the past years is the one comprised of oppositely charged polyelectrolytes and surfactants in aqueous solutions.4-11 Interest in such systems stems from their wide application in industrial formulations and processes, as well as their relevance to biological systems. The main interactions that take place in such systems are electrostatic interactions between the charges of the polyelectrolyte and the oppositely charged surfactant, which results in the formation of complexes. Secondary interactions can be found in such pairs, which contribute in the final structure, such as the hydrophobic character of the surfactant (associated with the length of its hydrocarbon chain) and the additional hydrophobic forces between polyelectrolyte and surfactant, in the case where hydrophobic groups are present on the polyelectrolyte chain. The molecular characteristics of the polyelectrolyte chain, such as its length, flexibility, charge density, and inherent hydrophobicity, play a key role in the self-assembly of polyelectrolyte/ * To whom correspondence should be addressed. Tel.: +30210-7273824. Fax: +30210-7273794. E-mail: [email protected].

surfactant systems.12,13 The quest for new pairs that will enlarge our fundamental understanding of the behavior of such selforganized complex nanosystems is ongoing. Research toward this direction will also allow the design and preparation of functional polymer-based nanoparticles with tailored characteristics, i.e., size and surface charge, for various potential nanotechnological applications.3,14 In the present work we report on the solution behavior of complexes formed between well-defined linear flexible polyelectrolytes, namely, quaternized poly[3,5-bis(dimethylaminomethylene)hydroxystyrene] (Q-N-PHOS), bearing two cationic sites on each repeating unit, and two different anionic surfactants, namely, sodium dodecyl sulfate (SDS) with one hydrocarbon tail and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) with two hydrocarbon chains, i.e., having a different hydrophobic/ hydrophilic parts ratio and steric requirements. Complex formation and structural properties are investigated as a function of polyelectrolyte molecular weight and surfactant concentration with respect to the nominal critical micelle concentration (cmc) values. The self-assembly in the mixed systems is studied by means of fluorescence spectroscopy (FS), electrophoretic light scattering (ELS), dynamic and static light scattering (DLS-SLS), and atomic force microscopy (AFM) with emphasis on the structural differences of the complexes formed under different experimental conditions, as well as their stability to the addition of salt. Experimental Methods Materials. Anionic surfactants (SDS and AOT, purity 99%) were purchased from Aldrich and used without any further purification. Their molecular structures are shown in Scheme 1. The cationic polyelectrolytes, Q-N-PHOS, were synthesized by a combination of anionic polymerization15 and postpolymerization functionalization reactions. The precursor homopolymers, namely, poly(p-tert-butoxystyrene) (PBOS) with different molecular weights, were synthesized by anionic polymerization high-vacuum techniques, in order to ensure control over the

10.1021/jp8095874 CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

Polyelectrolyte/Surfactant between Q-N-PHOS

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SCHEME 1: Molecular Structures of the Anionic Surfactants, SDS and AOT, Used in This Study

SCHEME 2: Reaction Scheme from the Synthesis of the Cationic Polyelectrolytes

TABLE 1: Molecular Characteristics of the Polyelectrolytes Used in This Study polymer

Mw (×10-4)

Mw/Mn

Mw, appa (×10-5)

Rha (nm)

Rg/Rh

Q-N-PHOS-1 Q-N-PHOS-2

4.6 13.5

1.18 1.08

3.9 6.0

76 115

1.2 1.5

a

a

Nwa 8 4.5

In 0.01 M NaCl aqueous solutions.

molecular weight and narrow polydispersity. The polymers were hydrolyzed to poly(p-hydroxystyrenes) (PHOS). In a second step postpolymerization functionalization of the PHOS precursors was achieved utilizing a Mannich-type reaction for the introduction of the dimethylamino functionality, followed by quaternization with methyl iodide and resulting in the desired cationic polyelectrolytes.16 The synthetic route for the polyelectrolyte preparation is shown in Scheme 2. Molecular characteristics of the samples used are given in Table 1. Previous investigations on the solution behavior of such cationic polyelectrolytes have shown aggregation effects due to the inherent hydrophobicity of the backbone,17 as the experimental data presented in Table 1 also indicate for the samples under investigation. The aggregates have a loose structure, judging from the values of the ratio Rg/Rh, and are formed by a small number of polyelectrolyte chains (calculated aggregation numbers, Nw, from light scattering are lower than 8, Table 1). Sample Preparation. Stock solutions of the polyelectrolytes and the surfactants with the desired concentrations were prepared gravimetrically, by direct dissolution of the solid compounds in distilled water. Two different preparation protocols were carried out for obtaining mixed solutions. The polymer concentration in both types of solutions was kept constant (polymer concentrations in the range of 1 × 10-4 to 1 × 10-3 g/mL were utilized). In the first protocol different amounts of the stock surfactant solution (c < cmc of the surfactant) was added dropwise to a fixed amount of polymer solution under stirring. The mixed solutions were diluted in a final volume of 15 mL. The final surfactant concentration was always lower than the nominal cmc of the surfactant, and these series will be referred to as BCMC series. In the second protocol, series of mixed solutions were prepared by adding a fixed amount of polymer stock solution in a varying volume of stock surfactant solution with c > cmc. Therefore, in the resulting solutions the surfactant concentration was always above the nominal cmc of the surfactant. These series of solutions will be referred to from

here on as ACMC. All solutions acquired a bluish tint after mixing of the components, indicating formation of polyelectrolyte/surfactant complexes. The solution preparation protocols followed ensure that in the BCMC solutions the resulting complexes were created through interactions between polyelectrolyte chains and free surfactant molecules. On the contrary in the ACMC solutions the complexes form primarily as a result of interactions between polymer coils and surfactant micelles. These different types of interactions are expected to result in complexes having different structural characteristics in the solutions. Ionic strength of the solutions was regulated by addition of NaCl (typically 0.01 M). In the experiments regarding complex stability to salt addition, ionic strengths up to 0.4 M were reached. In this case light scattering measurements were performed after allowing the system for a half-hour to equilibrate following each addition of salt solution. Methods. Light scattering measurements were conducted on an ALV/CGS-3 compact goniometer system (ALVGmbH), equipped with a ALV-5000/EPP multi-τ digital correlator with 288 channels and an ALV/LSE-5003 light scattering electronics unit for stepper motor drive and limit switch control. A JDS Uniphase 22 mW He-Ne laser (λ0 ) 632.8 nm) was used as the light source. Measurements were performed in the angular range from 20 to 150°. Solutions were filtered through 0.45 µm hydrophilic PTFE filters (Millex-LCR from Millipore) before light scattering measurements. The details of light scattering theory can be found elsewhere.18,19 In DLS, the intensity-intensity time correlation function G(2)(t,q) in the self-beating mode was measured. G(2)(t,q) is related to the normalized first-order electric field time correlation function g(1)(t,q) as

G(2)(t, q) ) 〈I(0, q) I(t, q) 〉 ) A[1 + β|g(1)(t, q)|2]

(1)

where t is the delay time, A is the measured baseline, and β is a parameter depending on the optical coherence of the detection. In general, |g(1)(t,q)| is related to the line-width distribution G(Γ) as

|g(1)(t, q)| )

∫ G(Γ) e-Γt dΓ

(2)

For a pure diffusive relaxation, Γ is related to the transitional diffusion coefficient D by (Γ/q2)qf0,cf0 ) D, which can be further converted into the average hydrodynamic radius 〈Rh〉 (or written as Rh) by the Stokes-Einstein equation, Rh ) kBT/ 6πη0D, where kB, T, and η0 are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. The hydrodynamic radius distribution, f(Rh), can result from the Laplace inversion of a corresponding measured G(2)(t,q) using the CONTIN program on the basis of the above equations. Polydispersities (µ2/Γ2, where µ2 is the second-order cumulant and Γ is the decay rate of the correlation function) can be evaluated from cumulants analysis, through the second-order cumulant. Autocorrelation functions were measured at least five times. One population of diffusing species was resolved in all cases. The apparent diffusion coefficient for a certain concentration, Dapp, was obtained by extrapolation to zero angle, which leads to the apparent hydrodynamic radius, Rh, via the Stokes-Einstein equation. In SLS, the weight-average molar mass (Mw) and the z-average root-mean-square radius of gyration 〈Rg2〉z1/2 (or written as Rg) can be obtained from the angular dependence of the absolute excess time-average scattering intensity, known as the Rayleigh ratio Rvv(q), on the basis of

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(

Kc Rvv(q)

)

= cf0

1 1 1 + Rg2q2 Mw 3

(

)

for qRg < 1

Mantzaridis et al.

(3)

where K ) 4π2n2(dn/dc)2/(NAλ04) and q ) (4πn/λ0) sin(θ/2), with NA, dn/dc, n, and λ0 being the Avogadro number, the specific refractive index increment, the solvent refractive index, and the wavelength of the light under vacuum, respectively. Toluene was used as the standard. The main problem that one faces with the application of static light scattering in these systems is that the dn/dc of the complexes cannot be determined accurately (and as a result the apparent mass, Mw,app, of the complexes cannot be calculated) for a number of reasons: (a) mixed polyelectrolyte/surfactant solutions are turbid or not stable at the concentrations required for dn/dc measurements (typically solution concentrations for dn/dc measurements are much higher than those employed for light scattering), (b) the exact stoichiometry of the complexes is not known, neither their compositional heterogeneity, and (c) calculation of the dn/dc of the mixed system based on the dn/dc values of the individual components is not accurate either, since after complexation the chemical nature of the components changes (small counterions are released and new “salt-like compounds” with different counterions are formed). If one wants to make this approximation and use calculated dn/dc values, the difference between the true and calculated dn/dc may be system-dependent and cannot be determined or calculated a priori (the same holds for Mw,app calculated from static light scattering). We note however that the dn/dc values calculated for the mixed systems investigated (based on the values of the pure components and the stoichiometry utilized) changes by less than 10% for the range of polymer and surfactant concentrations used and this difference cannot account for the differences observed for the scattered light intensity. Rg values follow the trend of Rh, and the Rg/Rh ratios (being in the range of 0.7-0.8) indicate formation of spherical aggregates in all cases. We present Rh values since they are determined with greater accuracy and in a more straightforward manner. ζ potential measurements were performed at 25 °C on a ZetaPlus Analyzer (Brookhaven Instruments Corp.) equipped with a 35 mW solid-state laser, operating at λ ) 660 nm. ζ potential values determined, using the Smolukowski equation relating the ionic mobilities with surface charge, are the average of 10 repeated measurements (with an error better than (3 mV). Steady-state fluorescence spectra of pyrene probe in aqueous micellar solutions were recorded with a double-grating excitation and a single-grating emission spectrofluorometer (Fluorolog-3, model FL3-21, Jobin Yvon-Spex) at room temperature (ca. 25 °C) using air-equilibrated solutions. A pyrene solution in acetone was prepared first at a concentration of 1 mM. Fixed volumes of this solution were introduced with a micropipet in dust-free glass vials, the acetone was allowed to evaporate, and an appropriate volume of the aqueous solutions containing the polyelectrolyte/surfactant complexes was introduced in order to give a final pyrene concentration lower than 3 × 10-7 M. Excitation wavelength was λ ) 335 nm, and emission spectra were recorded in the region of 350-500 nm, with an increment of 1 nm, using an integration time of 0.5 s. Slit openings of 1 nm were used for both the excitation and the emitted beam. The I1/I3 ratio is a characteristic parameter indicative of the polarity of the environment around pyrene, where I1 and I3 are the intensities of the first and the third peaks of the pyrene fluorescence spectra at 372 and 383 nm, respectively. AFM measurements were performed on a Quesant Q-Scope 250 atomic force micrsoscope (Quesant Instrument Co.) in the tapping mode, under ambient conditions. The instrument was

Figure 1. I1/I3 ratio of pyrene versus (-/+) ratio in BCMC complexes of sample Q-N-PHOS-2 with SDS.

equipped with a NSC16 silicon (W2C Si3N4) cantilever, available from Quesant, having a typical force constant of 40 N/m, a cone angle of less than 20°, and radius curvature less than 10 nm. Imaging was carried out with a 40 µm dual PZT scanner on different scanning areas, at a scanning rate of 3 Hz and with image resolution of 600 × 600 pixels in intermittent contact (broad-band mode). The z-axis calibration was performed by imaging a TGZ01 silicon grating with silicon oxide steps having a height of 18.3 nm (Mikromasch Inc.). Samples for imaging were prepared by applying a drop of the solution of the complexes on fresh, dried silicon wafers, precleaned with 2-propanol, for typically 5-10 min. After that period, excess water was blotted carefully by filter paper and samples were left to dry in air. In this way supramolecular structures were absorbed on the wafer surface from the same solutions investigated by light scattering for direct comparison. Results and Discussion Complexation between the cationic Q-N-PHOS and the two anionic surfactants is expected due to the complimentarity of the charges of the species. Since during complexation hydrophobic domains are created within the complexes, fluorescence spectroscopy was employed in order to track such changes using pyrene as the hydrophobic probe. In Figure 1 the ratio of intensities for the first and the third excitonic pyrene peaks, I1/ I3, is plotted as a function of the (-/+) charge ratio for complexes made between sample Q-NPHOS-2 and SDS (BCMC series). The decrease of the I1/I3 ratio from values around 1.75 down to 1.2 indicates the formation of hydrophobic nanodomains, as more surfactant is present in the system. The I1/I3 ratio for pure polyelectrolytes was 1.7-1.8, indicating the absence of hydrophobic domains, and is consistent with the loose aggregate structures observed in Q-N-PHOS solutions (Table 1).17 The lowest I1/I3 values are indicative of pyrene residing in a highly hydrophobic environment (similar to that of hydrocarbons).20 Similar results were obtained with the other polyelectrolyte/surfactant combinations confirming the creation of polyelectrolyte/surfactant complexes in the systems under investigation. A characteristic parameter for polyelectrolyte/surfactant complexes is the critical aggregation concentration, (cac), which denotes the concentration where surfactant binding occurs on the polyelectrolyte chain, and it is usually some orders of magnitude lower that the surfctants’ cmc in the absence of polyelectrolyte.12 Typical examples of the determination of cac in the systems under investigation is presented in Figure 2. A plot of I1/I3 for pyrene, from fluorescence measurements, versus

Polyelectrolyte/Surfactant between Q-N-PHOS

Figure 2. I1/I3 versus log c of surfactant concentration for the systems containing sample Q-N-PHOS-1 and SDS (O) and AOT (9) (cpol ) 2 × 10-4 g/mL).

SCHEME 3: (A) Idealized Schematic Showing the Complexation of SDS and AOT onto Q-N-PHOS polyelectrolytesa (B) Schematic Showing a Possible Structure for the Complexes Formed by Interaction of SDS with Q-N-PHOS Polyelectrolytes, When Surfactant Concentration Is above the Surfactant cmc (ACMC Series)

a

The scheme depicts better the case where surfactant concentration is below the surfactant cmc (BCMC series). The possibility for interaction of surfactant molecules with the two available cationic groups per polymer segment is highlighted.

surfactant concentration was used in these cases also.12,21,22 The cases for sample Q-N-PHOS-1 and both SDS and AOT are presented for comparison (structural representations are given in Scheme 3). It can be seen that I1/I3 starts from values typical of polar environments (1.7-1.8). The first observable decrease of these values (i.e., the first inflection point in the curve) denotes the cac, since it is related to the formation of hydrophobic domains in the system, due to surfactant complexation on the polyelectrolyte chains. The cac for SDS is found to be around 4.2 × 10-5 g/mL, and for the AOT complexes cac ∼ 3.5 × 10-6 g/mL. These are considerably lower that the respective cmc’s of the two surfactants (cmcSDS ) 2.6 × 10-3 g/mL and cmcAOT ) 1.9 × 10-4 g/mL). The lower cac for the AOT complexes should be correlated with the higher hydrophobic content of the particular surfactant, compared to SDS and indicates a larger tendency for cooperative association on the

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Figure 3. ζ potential of the resulting BCMC complexes versus (-/+) ratio. Complexes of sample Q-N-PHOS-1 with SDS (9) and AOT (O), respectively (cpol ) 1 × 10-3 g/mL).

polyelectrolyte chains; i.e., hydrophobic interactions play also a decisive role in surfactant complexation. It can also be observed that the I1/I3 values for the AOT complexes are consistently lower that those for the SDS, at the same surfactant concentration in the concentration region investigated. This must be also associated to a more hydrophobic environment in the AOT complexes, due to the presence of two hydrocarbon tails per complexed ionic head in this case. ζ potential measurements gave information on the effective charge of the complexes as a function of the (-/+) charge ratio. Representative results are given in Figure 3 for solutions of the BCMC series of sample Q-N-PHOS-1 and the two anionic surfactants. The positive charge of the complexes decreases in both cases as more surfactant is added, something that is expected from neutralization of the polyelectrolyte chain charge due to surfactant complexation. However, precipitation of the complexes occurs at rather low (-/+) ratios, without complete neutralization of the charges.8 The point of precipitation depends on the nature of the surfactant used, and it is lower for the AOT complexes. This behavior should be attributed to an increase of the hydrophobic character of the complexes as more surfactant is bound on the polyelectrolyte chains. At some point the existing charges on the nanoparticle are not sufficient to keep the nanoassembly in solution, and the complexes precipitate.12,13 The situation is more pronounced in the case of AOT-containing complexes, since surfactant complexation in this case binds two hydrophobic tails per complexation site, whereas in the SDS case one hydrocarbon tail is bound. The hydrophobicity of the Q-N-PHOS main chain must also play a role in the observed behavior, since according to chemical intuition, it is expected to be substantial, due to the presence of the phenyl ring, and higher compared, for example, to methyl iodide quaternized or protonated poly(vinylpyridine), due to the high number of methyl and methylene groups attached to the nitrogen atoms of the side groups.6,7 In the case of ACMC solutions (Figure 4) the complexes seem to carry an effective negative charge at all ratios studied. The absolute values increase, as the surfactant concentration increases; i.e., they become more negative. Negative effective charge on the complexes should originate from uncomplexed surfactant that is bound to the complex through hydrophobic interactions (even at low stoichiometric (-/+) charge ratios). That in turn points to the conclusion that not all positive charges are available for complexation. This may be partly due to (i) the fact that positive charges are very closely placed along the polyelectrolyte chains (i.e., the topology of the cationic groups along the polymer chains exerts additional steric constraints upon

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Figure 4. ζ potential of the resulting ACMC complexes versus (-/+) ratio for sample Q-N-PHOS-1 complexed with SDS (9) and AOT (O) (cpol ) 1 × 10-3 g/mL).

Figure 5. Rh versus (-/+) ratio for BCMC complexes of sample Q-NPHOS-1 with SDS (9) and AOT (O).

surfactant complexation), (ii) the fact that not all cationic charges are available for complexation due to polyelectrolyte chain aggregation before and, more drastically, after initial surfactant complexation, and (iii) because surfactants are adsorbed on the polymer chains as intact micelles, with no or some subsequent redistribution of the surfactant molecules within the complex, due to hydrophobic interactions. Other effects such as small counterion condensation may also contribute to a negative effective charge of the complexes. However, it is difficult to distinguish the effects and rationalize on why the effect of small counterions is more important in the ACMC case. The AOT complexes display more negative ζ potential values (Figure 4), and this observation may be associated with the larger aggregation number for AOT micelles (complexation of micelles in this case would increase the effective negative charge more profoundly). DLS measurements were performed in order to determine the size (Rh) and polydispersity of the resulting complexes. In Figure 5 representative results from the BCMC series are shown. Although only one population of diffusing species is observed in all solutions studied, the complexes are rather polydisperse, having polydispersity values, µ2/Γ2, larger than 0.2 (typically between 0.2 and 0.3). In the case of SDS complexes a rather weak trend of an increase in the Rh of the aggregates can be seen, as more surfactant is present in the system. For the AOT complexes Rh is more or less constant, attaining values in the range of 54-63 nm. The observed values for SDS complexes are higher than that determined for the pure polyelectrolyte chains (except for the lower SDS concentration investigated), possibly suggesting a secondary aggregation in these systems. For AOT complexes, Rh values are smaller than the pure

Mantzaridis et al.

Figure 6. Rh versus (-/+) ratio for ACMC complexes of sample Q-NPHOS-1 with SDS (9) and Q-N-PHOS-2 with AOT (O).

polyelectrolyte and some shrinking of the initial polyelectrolyte structure may take place in this case. Aggregation seems to be more pronounced in the case of SDS complexes, since larger values of Rh are observed, but the fact that higher (-/+) ratios have been used in this pair should be also taken into account. One would expect that complexation of individual surfactant molecules would lead to a shrinking of the dimensions of polyelectrolyte chains, due to charge neutralization and an increase in the hydrophobicity of the complex. It can be concluded from the data at hand that in the SDS systems the increase in hydrophobicity of the complex induces further aggregation into structures having larger dimensions than polyelectrolyte chains in solution.23-25 In contrast to the BCMC series, a stronger dependence of Rh on the charge ratio is obvious in the case of ACMC solutions containing SDS (Figure 6). The observed trend is a decreasing Rh of the complex nanoparticles, upon increasing surfactant concentration, which is added in the form of micelles in the mixed solutions. At the highest mixing ratios Rh becomes considerably smaller than the one corresponding to polyelectrolyte chains. Presumably, decoration of the polyelectrolyte chains with several SDS micelles takes place, resulting in a decrease in the dimensions of the aggregates, due to a more compact conformation of the polymer chain, while the excess, uncomplexed negative charges, coming from the surfactant micelles, stabilize the structures in solution. A parallel decrease in the mass of the aggregates through aggregates’ dissociation due to a better stabilization of the nanoassemblies by the SDS micelles cannot be ruled out. On the other hand for the ACMC solutions of AOT a maximum size is observed at a charge ratio close to 1. Rh variation is less pronounced in this case (Rh takes values in the range of 27-34 nm), but it is again considerably smaller than the dimensions of the polyelectrolyte chains. A scenario analogous to the one presented for the SDS complexes is also plausible here. It should be noted that rather broad size distributions were observed in these series of complexes (µ2/Γ2 values were in the range of 0.2-0.4). Furthermore, from the scattered intensity profiles of the resulting complexes more information can be acquired. In the BCMC case (Figure 7A) a trend of increasing intensity is observed as surfactant is added. These results are in agreement with the observed increase of the hydrodynamic radii. As more surfactant is added in the system, the complexes are gaining mass because of the attachment of surfactant onto the polymeric chains and their hydrophobic character is enhanced, due to charge neutralization and increase in the hydrophobic components, leading to further aggregation of the formed complexes.

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Figure 9. Scattered light intensity versus concentration of NaCl for Q-N-PHOS-2/AOT complexes (9), (-/+) ratio ) 0.03, and Q-NPHOS-1 with SDS (O), (-/+) ratio ) 0.05. Figure 7. Scattered light intensity versus (-/+) ratio profiles for (A) Q-N-PHOS-1/SDS BCMC complexes and (B) Q-N-PHOS-1/AOT ACMC complexes.

Figure 8. AFM images of complexes on silicon oxide substrates: (A) Q-N-PHOS-1/SDS complexes, ACMC series, ((-/+) ratio ) 1.1); (B) Q-N-PHOS-2/AOT complexes, BCMC series ((-/+) ratio ) 0.03).

Since the surfactant is added at very low concentrations, the increase in intensity, by about four times in the range investigated, should be mainly attributed to secondary aggregation and to a lesser extent to the additional mass of the complexed surfactant. On the contrary a decreasing trend for the scattered intensity from the ACMC complexes is obvious (Figure 7B). This can be associated with a decrease in the mass of the complexes by increasing surfactant concentration (due to an enhancement of polymer/complex solubility by the surfactant micelles). These data are in accordance with hydrodynamic radii results and can be attributed to the fact that certain selfassembled entities composed of polyelectrolyte chains decorated with surfactant micelles exist in the solution.4 Unfortunately, a more quantitative assessment of the static light scattering results could not be made (both for BCMC and ACMC series), due to the inherent difficulties encountered in these systems as explained before4c,d,26,27 (i.e., inability to experimentally determine the dn/dc of the mixed solutions), due to turbidity at the concentrations required for dn/dc measurements, and due to inability to estimate the true mass ratio of bound surfactant, instability/precipitation of the systems at higher polymer concentration and questionability of structure preservation upon dilution for extrapolating measured parameters of the system at infinite dilution). Additional information on the shape and size of the resulting complexes was acquired by atomic force microscopy. Representative images of different types of complexes adsorbed on silicon oxide substrates directly from solution are shown in Figure 8. The images show a rather broad size distribution for the particles in accordance with the results from dynamic light scattering. However, the complexes seem to have a more or

less spherical shape. Average dimensions fall in the range calculated from light scattering measurements, denoting a considerably compact structure for the complex nanoparticles, with some distortion at the z-axis after adsorption on the substrate. The distortion is expected due to the visualization of the particles in the dry state and the, mainly hydrocarbon, nature of the material. Stability of the Complexes to Increasing Salt Concentration. Additionally, some experiments were performed in solutions of the complexes at various salt concentrations in order to evaluate their stability to increasing ionic strength, since this feature of the complex nanoparticles is crucial for some applications. Figure 9 presents the case of a solution of Q-NPHOS2/AOT complexes ((-/+) ratio ) 0.03, BCMC series) and of Q-N-PHOS1/SDS complexes ((-/+) ratio ) 0.05, BCMC series), in which a low molecular weight salt (NaCl) solution was incrementally added. The scattered light intensity from the Q-N-PHOS2/AOT complexes, plotted versus the NaCl concentration, decreases as the ionic strength of the solution increases.12,27 Similar behavior is observed in the Q-N-PHOS1/ SDS case, as is shown on the same figure. This observation in principle means that the mass of the complexes decreases and this is a result of the screening of electrostatic interactions between the oppositely charged components, leading to a dissociation of the complexes. Interestingly, the hydrodynamic radius remains constant in the whole range of salt concentration studied (not shown). This indicates a parallel decrease in the density of the complexes resulting from the decrease in their mass at constant size. A decrease in the amount of bound surfactant at higher ionic strength contributes to keeping a constant size of the aggregates, due to electrostatic repulsions between liberated cationic charges of the polyelectrolyte chains, which swell the nanoparticle. Since even at 0.4 M ionic strength the scattered intensity is higher, by 2 orders of magnitude, than the one measured for polyelectrolyte solutions at the same polymer concentration, there is substantial possibility that complete dissociation of the complexes may have not taken place even at the highest concentration of salt investigated. Apparently, hydrophobic interactions existing between the surfactant tails and the polyelectrolyte backbone contribute to the stability of the complexes against an increase in the ionic strength of the solution. Conclusions The formation and structure of complexes between high charge density linear cationic polyelectrolytes of the Q-N-PHOS family and two different surfactants, SDS and AOT, were

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studied by a combination of techniques. Spherical and rather polydisperse complexes were formed in the cases investigated as revealed by DLS and AFM measurements. The complexes formed by the AOT surfactant have a lower cac value compared to those of SDS. The complexes precipitate at relatively low surfactant concentrations and (-/+) charge ratios, due most probably to the hydrophobicity of the polyelectrolyte main chain. Effective charge of the complexes depends on the mode of surfactant addition to the polyelectrolyte solution. It was possible to obtain negatively charged complexes by using micellar surfactant solutions at (-/+) charge ratios below the stoichiometric value. The size of the complexes could be also controlled by the type of surfactant used and its concentration. Increased ionic strength results in at least partial dissociation and swelling of the complex nanoparticles. The results of this study provide information on the role of polyelectrolyte charge density and backbone hydrophobicity on complex formation, structure, and stability to ionic strength. Furthermore, they give some guidelines on the ways available for constructing polymer/surfactant nanoparticles, where nanoparticle size and charge can be manipulated, through variation of several system parameters. Acknowledgment. Financial support of this work through the projects “Excellence in the Research Institutes” (Phases I and II) and PENED 03E∆805, supervised by the Greek General Secretariat for Research and Technology/Ministry of Development, is gratefully acknowledged. We thank Dr. D. Tsiourvas forprovidingaccesstoelectrophoreticlightscatteringinstrumentation. Supporting Information Available: Zimm plots, hydrodynamic radii distributions and Mw, app values for the complexes. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Faul, C. F. J.; Antonietti, M. AdV. Mater. 2003, 15, 673. (b) Kotz, J.; Kosmella, S.; Beitz, T. Prog. Polym. Sci. 2001, 26, 1199. (2) (a) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407. (b) Ikkala, O.; ten Brinke, G. Chem. Commun. (Cambridge) 2004, 19, 2131. (3) Bohrisch, J.; Eisenbach, C. D.; Jaeger, W.; Mori, H.; Muller, A. H. E.; Rehahn, M.; Schaller, C.; Traser, S.; Wittmeyer, P. AdV. Polym. Sci. 2004, 165, 1. (4) (a) Dubin, P. L.; Davis, D. D. Macromolecules 1984, 17, 1294. (b) Dubin, P. L.; Rigsbee, D. R.; Gan, L. M.; Fallon, M. A. Macromolecules 1988, 21, 2555. (c) Dubin, P. L.; The, S. S.; Gan, L. M.; Chew, C. H. Macromolecules 1990, 23, 2500. (d) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759. (5) (a) Duschner, S.; Grohn, F.; Maskos, M. Polymer 2006, 47, 7391. (b) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976. (c) Kong, L.;

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