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Influence of Corona Structure on Binding of an Ionic Surfactant in Oppositely Charged Amphiphilic Polyelectrolyte Micelles Foteini Delisavva, Mariusz Marcin Uchman, Juraj Skvarla, Edyta Wo#niak, Ewa Pavlova, Miroslav Slouf, Vasil M Garamus, Karel Prochazka, and Miroslav Stepanek Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00700 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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Influence of Corona Structure on Binding of an Ionic Surfactant in Oppositely Charged Amphiphilic Polyelectrolyte Micelles Foteini Delisavva1, Mariusz Uchman1, Juraj Škvarla1, Edyta Woźniak1,2, Ewa Pavlova3, Miroslav Šlouf3, Vasil M. Garamus4, Karel Procházka1,* and Miroslav Štěpánek1,* 1

Department of Physical and Macromolecular Chemistry, Faculty of Science

Charles University in Prague, Hlavova 2030, 12840 Prague 2, Czech Republic 2

Department of Chemistry, The Faculty of Food Science, Wrocław University of Environmental and Life Sciences, 50-375 Wrocław, C. K. Norwida 25, Poland

3

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 16206 Prague 6, Czech Republic

4

Helmholtz-Zentrum Geesthacht, Centre for Materials and Coastal Research, D-21502 Geesthacht, Germany *corresponding authors. e-mail: [email protected], [email protected]

Abstract Interaction of polystyrene-block-poly(methacrylic acid) micelles (PS-PMAA) with cationic surfactant N-dodecylpyridinium chloride (DPCl) in alkaline aqueous solutions was studied by static and dynamic light scattering, SAXS, cryogenic transmission electron microscopy (Cryo-TEM), isothermal titration calorimetry (ITC) and time-resolved fluorescence spectroscopy. ITC and fluorescence measurements show that there are two distinct regimes of surfactant binding in the micellar corona (depending on the DPCl content) caused by different interaction of DPCl with PMAA in the inner and outer parts of the corona. The compensation of the negative charge of the micellar corona by DPCl leads to the aggregation of PS-PMAA micelles and the micelles form colloidal aggregates at a certain critical surfactant concentration. SAXS shows that the aggregates are formed by individual PSPMAA micelles with intact cores and collapsed coronas interconnected with surfactant micelles by electrostatic interactions. Unlike polyelectrolyte-surfactant complexes formed by free polyelectrolyte chains, the PMAA/DPCl complex with collapsed corona does not contain surfactant micelles. 1 ACS Paragon Plus Environment

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Introduction Complexes of polyelectrolytes with oppositely charged ionic surfactants1–3 (polyelectrolyte-surfactant complexes, PE-S) have been attracting the attention of many researchers over the past two decades as a result of the general interest in nanostructured selfassembled systems, as well as because of application of these systems in biomedical, environmental and various technical fields. PE-S complexes are stabilized by the entropic gain reflecting the release of PE and surfactant counterions upon the formation of a PE-S complex in the bulk solution.3 Both hydrophobic and electrostatic interactions of the surfactant with PE affect its cooperative binding to the PE chain.1,2 The surfactant condenses on the PE chain and forms micelles at the concentration called the critical aggregation concentration (cac) below the critical micelle concentration (cmc) of the surfactant.1 Stoichiometric PE-S with zero net charge are insoluble in water and form various ordered crystalline-like phases.2 The interaction of double hydrophilic block polyelectrolytes with surfactants leads to the formation of core/corona particles with the PE-S complex cores and corona formed by neutral, water-soluble blocks.4–6 The above described behavior has been commonly observed in dilute aqueous PE solutions. If the PE chain is located in a confined volume, cooperative binding becomes more difficult or is not possible at all due to the excluded volume effect. This effect has been studied in detail on planar PE brushes differing in grafting density.7 It was found that the affinity of binding the oppositely charged surfactant to densely grafted PE brushes is substantially reduced. The interaction of spherical PE brushes with oppositely charged surfactants was also studied, both experimentally8,9 and by computer simulations10 and the results of both types of studies indicate that the spherical brush collapses upon the uptake of the surfactant and formation of the PE-S complex. However, in some cases, reswelling and expansion of the brush was observed, which was attributed to the accommodation of large surfactant aggregates inside the brush.10 Another type of PE-containing system is the amphiphilic block polyelectrolyte micelle. Such a micelle with a collapsed core and highly segregated hydrophilic and hydrophobic blocks can be regarded as a spherical polyelectrolyte brush tethered to the hydrophobic sphere.11 So far, studies of the interactions of amphiphilic block copolymer micelles with surfactants focused mostly on polymeric micelles with neutral coronas.11–13 It was reported that surfactant molecules bind at the core/corona interface and their aliphatic 2 ACS Paragon Plus Environment

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chains can solubilize and soften the outer layer of the core. The surfactant can induce morphological transitions or even disruption of micelles with a soft core above the glass transition temperature.14 In this article, we focus on the interaction of polystyrene-block-poly(methacrylic acid) (PS-PMAA) micelles15,16 with the cationic surfactant N-dodecylpyridinium chloride (DPCl). PS-PMAA micelles have a glassy core (kinetically frozen in aqueous solutions16) and a weak polyanion corona. The morphological changes in the PS-PMAA micelles upon addition of DPCl are followed by static, dynamic and electrophoretic light scattering, by SAXS and by cryogenic transmission electron microscopy. Binding of DPCl to the micelles is followed by isothermal titration calorimetry and fluorescence quenching measurements using 5-N(dodecanoyl)aminofluorescein (DAF) as an amphiphilic fluorescent probe.

Experimental Materials. Polystyrene-block-poly(methacrylic acid) (PS-PMAA) was synthesized and characterized at the university of Texas at Austin according to the procedure reported in ref 15. The weight average molar mass of the copolymer and the mass fraction of polystyrene were Mw = 42.2 kg mol-1 and wPS = 0.58, respectively. N-dodecylpyridinium chloride (DPCl) was purchased from Sigma-Aldrich and 5-N-(dodecanoyl)aminofluorescein (DAF) from Molecular Probes (Eugene, OR). PS-PMAA micelles in 0.05 M aqueous sodium tetraborate were prepared by stepwise dialysis of the solution in 1,4-dioxane(80 vol.%)/water mixture into 1,4-dioxane(60 vol.%) / water, 1,4-dioxane(40 vol.%) / water, 1,4-dioxane(20 vol.%) / 0.05 M aqueous Na2B4O7 and finally into 0.05 M aqueous Na2B4O7, using Spectra/Por dialysis tubing with the molecular weight cut-off of 8 kD. Methods. Light scattering. The light scattering setup (ALV, Langen, Germany) consisted of a 22 mW He-Ne laser, operating at a wavelength of λ = 632.8 nm, an ALV CGS/8F goniometer, an ALV High QE APD detector and an ALV 5000/EPP multibit, multitau autocorrelator. The measurements were carried out at 25°C for scattering angles, θ, ranging from 30° to 150°. The copolymer mass concentration in the solution was cp = 1 mg mL-1. The samples were filtered using 0.45 mm Acrodisc PVDF membrane filters prior to measurements.

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The gyration radius, Rg, of the PS-PMAA/DPCl micelles was calculated by the Zimm equation neglecting the interparticle interactions (by assuming the 2nd osmotic virial coefficient, A2 =0)

∆Rθ (0) 1 = 1 + Rg2 q 2 , ∆Rθ (q ) 3

(1)

where ∆Rθ(q) is the excess Rayleigh ratio and q=(4πn0/λ)sin(θ/2) is the scattering vector magnitude (here n0 is the refractive index of the solvent). Assuming that the refractive index increment of PS-PMAA in water, µ = 0.209 mL g-1, calculated as the mass-weighted average of refractive index increment for PS, µPS = 0.257 mL g-1, and PMAA, µPMAA = 0.142 mL g-1,17 the weight-averaged molar mass of the micelles is Mw = Rθ(0)NAλ4/(4π2n02µ2cp) = 8.8×106 g mol-1, which is slightly higher than the previously found value.16 Dynamic light scattering (DLS) measurements were evaluated by fitting the electric field autocorrelation function, g(1)(t,q), related to the measured normalized time autocorrelation function of the scattered light intensity, g(2)(t,q), by the Siegert relation, g(2)(t,q)=1+β|g(1)(t,q)|2, where β is the coherence factor. The g(1)(t,q) functions fitted to the second order cumulant expansion

ln g (1) (t , q ) = −Γ1 ( q )t +

Γ2 ( q ) 2 t , 2

(2)

where Γ1(q) and Γ2(q), respectively, are the first and the second moment of the distribution function of the relaxation rates. The diffusion coefficient of the particles, D, can be evaluated by extrapolation of Γ1(q) /q2 using the equation, Γ1 ( q ) = D 1 + CRg2 q 2 , q2

(

)

(3)

where C is the structural parameter reflecting the shape, polydispersity and internal dynamics of the scattering particles. The hydrodynamic radius, RH, can be calculated from D using the Stokes-Einstein formula.


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Electrophoretic Light Scattering. The ζ-potential was measured with a Nano-ZS zetasizer (Malvern Instruments, U.K.). The ζ-potential values were calculated as the average of five consecutive measurements (each consisted of 15–100 runs) from the electrophoretic mobilities using the Henry equation in the Smoluchowski approximation, µ=ɛζ/η, where µ is the electrophoretic mobility and ɛ is the dielectric constant of the solvent. Isothermal Titration Calorimetry (ITC). ITC measurements were performed at 25 ºC with a Nano ITC isothermal titration calorimeter (TA Instruments – Waters LLC, New Castle, DE, USA), equipped with 24 karat gold reference and sample cells, both with volume of 193 µL. The sample cell was connected to a 50 µL syringe, whose needle was equipped with a flattened, twisted paddle at its tip, which ensured continuous mixing of the solutions in the cell rotating at 250 rpm. The titration was carried out by repeated 1.03 µL injections of an aqueous 20mM DPCl in 50 mM sodium tetraborate solution from the syringe into the sample cell filled with an aqueous 1 g L−1 PS-PMAA block copolymer in 50mM sodium tetraborate solution. A total of 30 consecutive injections were performed. The delay between two consecutive injections was 250 s. These injections replaced part of the solution in the sample volume and the changed concentration was considered in the calculation of the sample concentration. This method was used to determine the differential heat of mixing for discrete changes in the composition. The data were analyzed using the NanoAnalyze software. Time-Resolved Fluorometry. Time-resolved emission decays were measured in 1 cm quartz cuvettes by a Fluorolog FL 3–22 fluorometer (Horiba Jobin Yvon, France) equipped with a FluoroHub time-correlated single photon counting module. The PS-PMAA and DAF concentrations were 1 mg mL-1 and 1 µM, respectively. Emission decays were measured at the emission wavelength of 520 nm in a time window of 50 ns with a resolution of 56 ps per channel using a 472 nm NanoLED pulsed laser diode source operated at a repetition frequency of 1 MHz, producing pulses with FWHM of ~0.2 ns. The decays were fitted by triple-exponential decay models using iterative reconvolution of the data with the Marquardt– Levenberg least squares algorithm, 3  t  F (t ) = ∑α i exp −  + B i =1  τi 

(4)

The average lifetime 〈τ〉 was calculated from the pre-exponential factors α1, α2, α3 and lifetimes τ1, τ2, τ3 as


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τ =

α1τ 12 + α 2τ 22 + α 3τ 32 α1τ 1 + α 2τ 2 + α 3τ 3

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(5)

Cryogenic Transmission Electron Microscopy. Cryo-TEM micrographs were obtained with a Tecnai G2 Spirit Twin 12 transmission electron microscope (FEI, Hillsboro, OR) equipped with a cryo-specimen holder (Gatan, Pleasanton, CA). Sample preparation consisted in dropping 3 µL of the sample solution on a TEM grid covered with a holey carbon supporting film (C-flat 2/1-4C, Electron Microscopy Science; Hatfield, UK); just before the experiment, the film was rendered hydrophilic by glow discharge (Expanded Plasma Cleaner; Harrick Plasma, Ithaca, NY). The excess solution was removed by blotting (blotting time 1 s; Whatman no. 1 filter paper) and the grid was immediately plunged into liquid ethane at −181 °C. The frozen sample was transferred to the microscope and observed at –173 °C using bright field imaging at 120 kV. Small-angle X-ray scattering (SAXS). SAXS experiments were carried out on the P12 BioSAXS beamline at the PETRA III storage ring (EMBL/DESY, Hamburg, Germany) at 20°C. The beamline was equipped with a Pilatus 2M detector and synchrotron radiation with a wavelength of λ = 0.1 nm. The sample-detector distance was 3 m, permitting coverage of the q-range interval from 0.07 to 4.4 nm-1. The q-range was calibrated using the diffraction patterns of silver behenate. The experimental data were normalized to the transmitted beam intensity and corrected for nonhomogeneous detector response and the background scattering of the solvent was subtracted. The solvent scattering was measured before and after the sample scattering to control for possible sample holder contamination. 20 consecutive frames with 0.05 s exposures comprising measurement of the solvent, sample, and solvent were performed. The data were checked for radiation damage. The final scattering curve was obtained by automated data acquisition software.18 An automatic sample changer was used for a sample volume of 20 µL and cleaning-filling cycle of 1 min.

Results and Discussion Light Scattering Measurements. The addition of DPCl to the PS-PMAA micellar solution leads to the aggregation of PS-PMAA micelles when the DPCl concentration, cDPCl, exceeds ~3 mM. This surfactant concentration corresponds to the charge ratio (defined as the ratio between the molar concentrations of DPCl cations to PMAA monomeric units, 6 ACS Paragon Plus Environment

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cDPCl/cMAA), Z = 0.8 at a PS-PMAA concentration cP = 1 g L-1. Fig. 1 (curve 1) shows the Rayleigh ratio of the PS-PMAA/DPCl dispersion (polymer concentration of cP = 1 g L-1) at θ = 90° as a function of Z. Above Z = 0.8, the turbidity of the solution grows very steeply, indicating the formation of large aggregates. Curve 2 in Fig. 1 depicts the ζ-potential of PSPMAA micelles as a function of Z for Z < 0.8. Above Z = 0.8, the formed aggregates are very large and the high turbidity of the dispersion precludes the evaluation of the electrophoretic mobility of the micelles. 0

2

2 1

-20

-30 0.0

1

0.2

0.4

0.6

0.8

1.0

Rθ(90°) / m

-10

–1

3

ζ / mV

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0

Z Fig. 1. Right-angle excess Rayleigh ratio, ∆Rθ(90°) (curve 1), for PS-PMAA/DPCl solutions (copolymer concentration, cp = 1 mg mL-1) in 50 mM aqueous Na2B4O7, and zeta potential, ζ (curve 2), of PS-PMAA/DPCl micelles as functions of the charge ratio of DPCl to PMAA units, Z.

At Z < 0.4, the negative ζ potential increases slowly with Z suggesting that the surfactant preferentially binds in the inner part of the corona and do not strongly affect the surface charge of PS-PMAA micelles. Only when approaching saturation of the inner part of the corona (at Z ca. 0.6) does the negative ζ potential start to grow rapidly demonstrating that the aggregation of the PS-PMAA micelles is caused by loss of the electrostatic stabilization of the PE micelles due to charge neutralization of the upper parts of the micellar coronas.


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40 2 35

Radius / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 1 25

20 0.0

0.2

0.4

0.6

Z

Fig. 2. Gyration (curve 1) and hydrodynamic (curve 2) radius of PS-PMAA/DPCl micelles from LS measurements as functions of Z.

The dependences of the gyration and hydrodynamic radii of PS-PMAA micelles vs. Z obtained from SLS and DLS measurements are shown in Fig. 2. (The measurement for Z > 0.8 is influenced by intense multiple scattering, which precludes simple interpretation of the data.) While Rg (curve 1) grows slightly from 25 nm at Z = 0 to 27 nm at Z = 0.7, RH (curve 2) remains roughly constant up to Z = 0.4 and then decrease from 37 nm to ca. 33 nm at Z = 0.7. The different trends of the Rg and RH dependences on Z can be explained by the fact that Rg is mostly controlled by the size of the core and of the dense inner part of the corona, while RH is affected by the loose peripheral layer of ionized and extended corona chains. Thus the sorption of the surfactant to the corona-forming PE blocks, which causes an increase in Rg, is counterbalanced for RH by concomitant collapse of the uppermost part of the corona due to neutralization of the negative charge of the corona periphery (which is indicated by the ζpotential measurement) and leads to a decrease in RH. Isothermal Titration Calorimetry. A value of cmc = 9.8 mM of DPCl in sodium tetraborate buffer was determined earlier6 by ITC and its value suggests that free surfactant molecules rather than surfactant micelles interact with PS-PMAA micelles in the studied surfactant concentration region (0–3 mM). It was formerly reported that the corona formed by polyelectrolytes with fairly hydrophobic backbone is spatially heterogeneous and particularly that of PS-PMAA consists of a deprotonated and hydrophobic inner layer and an ionized hydrophilic outer layer.19 The assumption that the two regimes of DPCl binding are related to


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the bilayer structure of the PMAA corona is supported by ITC and fluorescence quenching measurements. The ITC measurement (Fig. 3, curve 1) reveals that, at low Z (Z < 0.3), the interaction of DPCl with the PS-PMAA micelles is exothermic and the corresponding enthalpy change is comparable to that for DPCl dilution connected with DPCl demicellization. However, at Z > 0.3, the binding becomes a strongly endothermic process (Fig. 3), similarly to what was observed earlier for DPCl binding to poly(methacrylic acid)block-poly(ethylene oxide).6 The positive enthalpy change indicates that, in this regime, the binding of the surfactant in the outer part of the corona is entropy-driven. The estimated value (4.3 kJ mol-1) can be compared with that for the DPCl micellization in 50 mM buffer (2.7 kJ mol-1),6 so that the endotherm can be attributed to DPCl micellization induced by the interaction with PMAA (cooperative binding of the surfactant in the outer part of the corona) connected with the entropic gain due to the release of counterions from the PMAA chains. 0.0 4

0.2

Z 0.4

0.6 1

–1

3

∆Hobs / kJ mol

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2 1 0 2

-1 -2 0.0

0.5

1.0

1.5

2.0

2.5

Csurf / mM Fig. 3. Enthalpy curves of titrating DPCl into PS-PMAA micelles (cp = 1 g l–1) in 50 mM aqueous Na2B4O7 (curve 1) and into 50 mM Na2B4O7 aqueous solution (curve 2). The corresponding Z values for the PSPMAA/DPCl system are shown in the top horizontal axis.

Time-Resolved measurements

using

Fluorometry.

Similarly

to

5-N-(dodecanoyl)aminofluorescein

ITC,

fluorescence

(DAF)

as

an

quenching amphiphilic

fluorescence probe and DPCl as a quencher reveal differences in the quenching constant below and above Z = 0.25. In our earlier studies19 we found that DAF (in very low concentrations, cDAF ca 1 µM, which are used in fluorescence measurements) binds very


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strongly into the inner PMAA corona due to hydrophobic interaction of its aliphatic tail with the deprotonized PMAA and with the PS core. Because the fluorescence of DAF can be efficiently quenched by DPCl, PS-PMAA micelles with bound DAF are suitable systems for studying DPCl penetration into the PMAA corona. Fig. 4 shows the mean fluorescence lifetimes for the probe embedded in PS-PMAA micelles (curve 1) and for the free probe (curve 2). Both dependences are plotted against the DPCl concentration and against the corresponding charge ratio Z for the PS-PMAA/DPCl system. For Z < 0.15, the mean lifetime decreases steeply with increasing Z for the embedded DAF, indicating that the quenching of the PS-PMAA/DAF system by DPCl is much more efficient than for free DAF, due to the sorption of DPCl in the inner part of the corona close to DAF. However, the 〈 τ〉 vs. Z dependence levels off at Z = 0.15. This means that the added surfactant cannot reach the binding sites of the probe, which further confirms the assumption that, at high loadings, it binds in the outer part of the corona.

0.0 5

0.1

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Counts

3

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2

3

10

2

10

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0 0.002 0.006 0.015 0.031 0.064 0.130 0.262 0.520

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50

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1 0 0.0

Z

0.2

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〈τ〉 / ns

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0.5

1.0 Csurf / mM

1.5

Fig. 4. Mean fluorescence lifetime, 〈τ〉, of DAF (5 µmol L-1) loaded in PS-PMAA micelles (cp = 1 g l–1) in 50 mM aqueous Na2B4O7 (curve 1) and into 50 mM Na2B4O7 aqueous solution, as functions DPCl molar concentration, Csurf. The corresponding Z values for the PS-PMAA/DPCl system are shown in the top horizontal axis. Insert: DAF fluorescence emission decays of DAF loaded in PS-PMAA micelles at various Z ratios (indicated at the individual decays).

As compared with ITC data, the dependence of the mean fluorescence lifetime on Z levels off sooner. This apparent discrepancy is understandable and is caused by the fact that the number of DAF molecules per PS-PMAA micelle is very low (ca. 10) so that just a few DPCl molecules located in the vicinity of DAF (i) efficiently quench the fluorescence, (ii) but simultaneously block the access of other DPCl molecules to DAF. Hence, many DPCl 10 ACS Paragon Plus Environment

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molecules, even though they bind into the inner layer, are located quite far from DPCl and do not affect its fluorescence quantum yield and therefore the curve of 〈 τ〉 vs. Z levels off sooner than the corresponding ITC curve. Nevertheless, the time-resolved fluorescence data unambiguously demonstrate that the binding of DPCl to the PMAA corona starts at low DPCl loadings close to the core/corona interface. Cryogenic Transmission Electron Microscopy. We performed Cryo-TEM measurements to follow changes in the morphology of the PS-PMAA micelles induced by the interaction with DPCl and to investigate the internal structure of the PS-PMAA/DPCl aggregates.

(b)

(a)

Fig. 5. Cryo-TEM micrographs of (a) PS-PMAA and (b) PS-PMAA/DPCl (Z = 1.1) solutions in 0.05 M Na2B4O7, PS-PMAA concentration, cP = 1 mg mL-1

Figs. 5a and 5b show Cryo-TEM micrographs of PS-PMAA micelles without the surfactant (Z = 0) and PS-PMAA/DPCl aggregates at Z = 1.1, respectively. In both cases, only the cores of the micelles are visible because of the low contrast of the hydrated corona. The image analysis of the TEM micrographs yields two pieces of quantitative information: the average particle size and interparticle distances. The average particle size is expressed in the form of a standard morphology descriptor, equivalent radius, Req. The value of Req determines the radius of a circle with the same area, Aeq as the micrographic image of the measured object.20 The distribution of interparticle distances is calculated as the pair distance distribution function (PDDF), obtained from the 1D radial profiles calculated from the 2D Fourier transforms of electron micrographs.21


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In the case of the PS-PMAA/DPCl system, the Req distribution is broader but the mean values are the same for both PS-PMAA and PS-PMAA/DPCl, Req = 11 nm, which is in good agreement with the earlier reported core radius obtained by small angle neutron scattering.22 The most probable distance between neighboring PS-PMAA micelles is 73 nm, which is three times less than the average distance calculated from the molar concentration of the PS-PMAA micelles, which is dcalc = (NAcp/Mw)–1/3 = ~210 nm. The observed value actually corresponds to the hydrodynamic diameter of the micelles, which suggests that micelles aggregate in a thin layer of solution in the TEM grid during the preparation of the Cryo-TEM sample, most probably due to the adsorption at the air-water interface. This assumption is supported by the fact that the Cryo-TEM image is heterogeneous on larger scales and suggests the formation of patches of concentrated polymer micelles as a result the approach and concentration of micelles in the polymer-rich domains. Fig. 6b depicts an analogous image of PS-PMAA/DPCl aggregates. It is evident that the packing is much denser (the most probable distance, Rdist = 35 nm) and the maxima of the PDDF function are more pronounced. This result indicates that the formation of aggregates at Z > 0.7 is accompanied by strong collapse of the micellar corona, which is already indicated for lower Z values (Z < 0.4) by DLS. The Cryo-TEM study indicates that the observed structures are influenced by interfacial phenomena affecting the thin layer of solvent prior to freezing. However, the striking differences between the frozen solutions of pure micelles and polymer-surfactant aggregates allow us to draw certain conclusions concerning the destabilization and secondary aggregation of PS-PMAA micelles at high Z. The large homogenous area filled by a large number of polymer micelles in Fig. 6b suggests that large aggregates of concentrated and closely packed PS-PMAA/DPCl micelles with collapsed PMAA/DPCl coronas exist in the solution at the ambient temperature. These multi-micellar aggregates conserve the structure of the original PS-PMAA micelles, i.e., the aggregates are loosely interconnected by PMAA/DPCl coronas, but all the individual frozen cores are essentially left intact by the aggregation process.


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100 (a)

PDDF(d)

10

75 50

5

dmax = 73 nm

0 0

100

200

d / nm

25 0

5

100

10

15

(b)

20

10

PDDF(d)

Frequency Count

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75 50

5

dmax = 35 nm

0 0

100

d / nm

200

25 0

5

10

15

20

Equivalent radius / nm Fig. 6. Equivalent distributions of core radii and pair-distance distribution functions (insert) for (a) PS-PMAA micelles and (b) PS-PMAA/DPCl aggregates at Z = 1.1

Small-Angle X-Ray Scattering. We performed the SAXS measurement to study the internal structure of the PS-PMAA/DPCl aggregates. The SAXS curve of the PSPMAA/DPCl solution (copolymer concentration, cp = 1.1 mg mL-1) in 50 mM aqueous Na2B4O7 at Z = 1.1 is shown in Fig. 7. In the low q regime (below 0.5 nm–1), the behavior is dominated by scattering from PS-PMAA micelles, which is slightly affected by excluded volume interactions for the lowest q. Since the interactions between the micelles with collapsed cores are dominated by the excluded volume effect, the latter contribution can be treated by the structure factor of hard spheres. Since the corona chains are fully collapsed, the micellar scattering shows no power law contribution typical for block copolymer micelle coronas.23 Instead, the form factor of PS-PMAA micelles can be sufficiently described by the model of a homogeneous spherical shell with the Gaussian distribution of the core radii. (The above model used for fitting the curve is described in detail in the Supporting Information.) For a fixed value of the core radius, R0 = 11 nm, the fit yields the ratio of the excess scattering length density (SLD) to the 13 ACS Paragon Plus Environment

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excess SLD of the core, ∆η1/∆η2 = 0.076, the width of the distribution σ = 6.3±0.1 nm, the shell thickness ∆R = 8.5±0.1 nm and the hard sphere interaction radius r = 36.9±0.3 nm. The latter value is in good agreement with the average distance between PS-PMAA micelles observed by Cryo-TEM.

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Fig. 7. SAXS curve for the PS-PMAA/DPCl solution (copolymer concentration, cp = 1 mg mL-1) in 50 mM aqueous Na2B4O7 at Z = 1.1 and the fit by the model of the spherical shell with the Ornstein-Zernike contribution from the short range fluctuations.

In the high q range (0.5–4.5 nm–1), the scattering curve contains a distinct contribution from short-range fluctuations, which can be fitted by the Ornstein-Zernike structure factor, IOZ(q) = Ifluct/(1+ξ2q2), providing the correlation length, ξ, about 0.25 nm. It is worth noting that the SAXS curve does not exhibit any correlation peak in the high q range. The correlation peaks are usually observed in SAXS or SANS of polyelectrolyte-surfactant complexes due to self-assembly of the surfactant into densely packed DPCl micelles in the complex.24 The absence of such a peak indicates that the formation of DPCl micelles in the PMAA corona is not possible because of the quite high density of the corona-forming PMAA chains and the insufficient volume available for accommodation of the surfactant micelles. Therefore, only a low number of DPCl micelles can interact with the uppermost periphery of the corona and act as “glue” in interconnection of the polymer micelles to form micellar aggregates. The latter process occurs at a slightly higher surfactant concentration than corresponding to the charge neutralization in the system, which is indicated by the fact that the onset of the aggregation detected by the increase of the solution turbidity is shifted to a higher Z value than the point where the zero surface charge of the copolymer micelles is attained (Fig. 1). 14 ACS Paragon Plus Environment

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Scheme 1. Interaction of PS-PMAA Micelles with DPCl

(A) Z < 0.4, PS-PMAA micelle with surfactant bound in the inner part of the corona. (B) 0.4 < Z < 0.8, PSPMAA/DPCl micelle with surfactant micelles in the outer part of the collapsed corona. (C) Z < 0.8, aggregate of PS-PMAA/DPCl micelles.

Conclusions This article describes study of the binding of cationic surfactant DPCl to the anionic corona of the diblock copolymer micelles of PS-PMAA and aggregation of PS-PMAA micelles by a combination of light scattering, SAXS, TEM, ITC and fluorometry. We have shown that secondary aggregation of the polymer micelles is due to the loss of electrostatic stabilization following formation of the neutral PMAA/DPCl complex in the corona. We can conclude that, depending on the amount of the added surfactant (expressed by the molar ratio between the amount of DPCl anions and the PMAA units, Z), there are three regimes of interaction of the DPCl cations with the PMAA corona blocks: (i) For Z < 0.4 (Scheme 1A), DPCl binds in the hydrophobic inner part of the corona, as indicated by the strong fluorescence quenching of an amphiphilic fluorescent probe (DAF) which binds at the core/corona interface. In this regime, the hydrophobic effect dominates the interaction between DPCl and PMAA. ITC measurements show that the binding is an exothermic process. Since the outer part of the corona remains unaffected, the surface charge of PS-PMAA micelles and consequently the hydrodynamic radius of the micelles controlled by the stretched chains in the outer corona change only slightly. (ii) In the range 0.4 < Z < 0.8 (Scheme 1B), DPCl binds in the ionized outer part of the corona, which leads to neutralization of the outer interface of the corona (observed by electrophoretic light scattering measurements) and consequently to a decrease in the hydrodynamic radius of PS-PMAA micelles. In this regime, the interaction of DPCl with 15 ACS Paragon Plus Environment

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PMAA is an endothermic process, which suggests that the decrease in the free energy of the system is due to the translational entropy gain from the release of counterions from the corona and that the surfactant binds cooperatively in the outermost part of the corona. (iii) For Z > 0.8 (Scheme 1C), PS-PMAA/DPCl micelles aggregate. Both SAXS and TEM measurements indicate that the micellar coronas collapsed. DPCl bound in the interior part of the corona is disordered and, unlike polyelectrolyte-surfactant complexes formed by free polyelectrolyte chains, does not self-assemble in packed surfactant micelles. The TEM and SAXS measurements indicate that, in contrast to the interaction of surfactants with flexible polyelectrolyte chains, only individual DPCl molecules interact with PMAA coronas, presumably due to the high density of the corona-forming PE chains and insufficient space for the accommodation of surfactant micelles. Some surfactant micelles interacting with PMAA at the corona periphery participate at interconnection of the polymeric micelles into larger aggregates and act as an “electrostatic glue”, but they do not penetrate inside the coronas. The large aggregates of polymeric micelles are interconnected only by collapsed PMAA/DPCl coronas and conserve the separated (dense and kinetically frozen) PS cores.

Acknowledgments FD, MU, MŠ and KP acknowledge the support from the Czech Science Foundation: Grant No. P106-12-0143. Electron microscopy at the Institute of Macromolecular Chemistry was supported by projects TE01020118 (Technology Agency of the CR) and POLYMAT LO1507 (Ministry of Education, Youth and Sports of the CR, program NPU I). The support of Clement Blanchet (EMBL) is kindly acknowledged.

References (1) Langevin, D. Complexation of oppositely charged polyelectrolytes and surfactants in aqueous solutions. A review. Adv. Colloid Interface Sci. 2009, 147−148, 170–177. (2) Kogej, K. Association and structure formation in oppositely charged polyelectrolytesurfactant mixtures. Adv. Colloid Interface Sci. 2010, 158, 68–83. (3) Berret, J.-F. Controlling electrostatic co-assembly using ion-containing copolymers: From surfactants to nanoparticles. Adv. Colloid Interface Sci. 2011, 167, 38–48.


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For Table of Contents Use Only:

Influence of Corona Structure on Binding of Ionic Surfactant in Oppositely Charged Amphiphilic Polyelectrolyte Micelles Foteini Delisavva, Mariusz Uchman, Juraj Škvarla, Edyta Woźniak, Ewa Pavlova, Miroslav Šlouf, Vasil M. Garamus, Karel Procházka and Miroslav Štěpánek


Influence of Corona Structure on Binding of an ... - ACS Publications

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