Coassembly of Gemini Surfactants with Double Hydrophilic Block

Article Cite This: Macromolecules XXXX, XXX, XXX-XXX pubs.acs.org/Macromolecules

Coassembly of Gemini Surfactants with Double Hydrophilic Block Polyelectrolytes Leading to Complex Nanoassemblies Foteini Delisavva,† Mariusz Uchman,*,† Miroslav Štěpánek,† Sami Kereïche,†,‡ Zofia Hordyjewicz-Baran,§ Marie-Sousai Appavou,∥ and Karel Procházka*,† †

Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 2030, 128 43 Prague 2, Czech Republic ‡ Institute of Biology and Medical Genetics, First Faculty of Medicine, Charles University and General University Hospital in Prague, Purkynie Ustav, Albetov 4, 12 801 Prague, Czech Republic § Institute of Heavy Organic Synthesis “Blachownia”, Energetykow 9, 47-225 Kedzierzyn-Kozle, Poland ∥ Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany S Supporting Information *

ABSTRACT: The electrostatic coassembly of a double hydrophilic block copolymer poly(2-vinylpyridine)-block-poly(ethylene oxide) (P2VP−PEO) with the gemini surfactant 6,6′-(ethane-1,2-diylbis(oxy))bis(sodium 3-dodecyl benzenesulfonate) (G2) was studied in acidic aqueous solutions by a combination of light scattering, SANS, Cryo-TEM, and ITC. As the critical micelle concentration of gemini surfactants is ∼102 times lower than that of analogous single-tail surfactants, polyelectrolyte chains interact with G2 vesicles instead of individual G2 molecules. The study shows that hydrophobicity, bulkiness, and stiffness of G2 domains formed upon its binding to the P2VP chain affect the coassembly and formation of nanoparticles. The study allows for presentation of a comprehensive outline of the coassembling process and elucidating the structure of the formed nanoparticles.

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INTRODUCTION

Basic principles of the electrostatic coassembly are now reasonably understood, but there is still vast room for studies of a number of factors that influence the formation and properties of individual electrostatically coassembled nanoparticles. The presence of electric charges is a prerequisite for “electrostatically driven” association, but Coulombic attraction is not the main driving force for the coassembly. Even though variations in the local dielectric permittivity close to the hydrophobic chain and different solvation of charged groups affect the selfassembling process, the enthalpy contribution of the abovementioned effects to the free energy of the system is small. The main force driving coassembly is a considerable entropy increase caused by the release of small ions into the bulk

Electrostatic interactions play a crucial role in the chemistry and physics of multiparticle systems and control the physicochemical behavior of both small ions and large charged polymers, including important biopolymers. Screened electrostatic interactions stabilize the emulsions and dispersions of compact nanoparticles,1 but they can also induce the electrostatic coassembly of oppositely charged polymeric and oligomeric species, resulting in the formation of interpolyelectrolyte complexes (IPC).2 The insoluble domains can be either ordered or disordered depending on the chemical nature of the polyelectrolyte, length and flexibility of the charged PE sequences, etc.3,4 The IPC assemblies formed in stoichiometric mixtures of oppositely charged PEs with matched positive and negative charges are usually insoluble,5,6 while the nonstoichiometric assemblies form aqueous dispersions of nanoparticles stabilized by excess charge.7,8 © XXXX American Chemical Society

Received: June 22, 2017 Revised: October 2, 2017

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DOI: 10.1021/acs.macromol.7b01330 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Chemical Structures of (a) P2VP−PEO Block Copolymer and (b) G2 Surfactant

micelle concentration (CMC), their hydrophobic groups are more densely packed, and they interact more strongly with oppositely charged surfactants both in bulk solution and at interfaces between the aqueous solution and the air. It has been found that their CMC values decrease dramatically with increasing length of their hydrocarbon tails or with increasing spacer length.35 A series of cationic alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants, denoted as m-s-m, where m is the number of the carbon atoms in the aliphatic chain, have been intensely investigated by the group of Zana.36,37 The authors observed that the long spacer-containing surfactants of the 12-s-12 type formed spherical or threadlike micelles, while 16-s-16 surfactants formed vesicles, bilayer membrane fragments, or threadlike micelles, depending on the length and chemical nature of the spacer.38,39 Zana et al. concluded that the size, shape, and other properties of the micelles, such as CMC, association number, thermodynamics of micellization, behavior at the air−water interface, and other properties useful for potential application, can be controlled by tailoring the structure of the dimeric surfactants, especially by tuning the length, rigidity, and chemical nature of the spacer.40−42, Studies performed so far show that the structure of selfassembled nanoparticles formed by PEs or by doublehydrophilic polyelectrolytes with gemini surfactants is affected by steric effects more strongly than that of PE-S containing single-chain surfactants.43−47 Close proximity of bulky head groups together with stiffness of tails caused by their crowding and dense packing in self-assembled domains affects the final structure of the nanoparticles. The formation of polygon nanoparticles has been reported in systems containing gemini surfactants.48 In this work, we investigated the electrostatic coassembly of the double-hydrophilic copolymer poly(2-vinypyridine)-blockpoly(ethylene oxide) (P2VP−PEO) with the anionic gemini surfactant 6,6′-(ethane-1,2-diylbis(oxy))bis(sodium 3-dodecyl benzenesulfonate) (G2) by a combination of scattering techniques (LS, SANS), microscopy (Cryo-TEM), and calorimetry (ITC). The aim of the paper is to point out and discuss the influence of different properties of gemini surfactants in comparison with single-tail surfactants on the coassembly behavior of DHBP/ionic surfactant systems.

solvent after charge compensation on associating PE chains and surfactants. However, neither the electrostatic attraction nor the increase in entropy controls the association number, size, shape, and inner structure of the formed self- or coassemblies. The basic characteristics of nanoparticles, their behavior, and the long-time stability of their dispersions are controlled by different factors, such as the hydrophobicity of the PE blocks, the compatibility of the charged and neutral blocks, the lengths of the blocks, and the interaction of the blocks with small counterions and with co-ions.9 Recent theoretical studies indicate that a certain hydrophobicity of the PE backbone and at least small incompatibility of the PE blocks with neutral shell-forming blocks are necessary conditions for the formation of multichain associates with sufficiently segregated domains.10,11 Last but not least, various steric effects (the flexibility of the blocks and bulkiness of their building units, interaction with external constrains) also play an important role.12 Double hydrophilic block polyelectrolytes (DHBP) consisting of a polyelectrolyte block and a neutral hydrophilic block form core−shell nanoparticles with oppositely charged DHBP13 or oppositely charged surfactants.14 Even though studies of the electrostatic assembly are less frequent than those devoted to the electrostatic stabilization of emulsions and dispersions, their number is large, and it would be futile to cite all the relevant papers. Here we mention only the most important recent reviews and seminal studies.15−22 Coassemblies formed in mixtures of high-molar-mass copolymers composed of two similarly long flexible blocks (a PE block and a neutral hydrophilic block) with single-tail surfactants such as anionic sodium dodecyl sulfate (SDS)23 or cationic dodecylpyridinium chloride (DPCl)24 are usually spherical core/shell micelles containing a dense insoluble core composed of polyelectrolyte−surfactant complex (PE−S) and a protective shell formed by the neutral water-soluble copolymer.3 In the overwhelming majority of cases, PE−S cores contain randomly embedded surfactant micelles in a PE matrix, but the association number of surfactant micelles differs from that of free micelles in bulk aqueous medium.4 If PE chains are flexible and sufficiently long, they do not constrain the arrangement of surfactant micelles as much as short and stiff chains. In these systems, an energetically favorable “crystallinelike” ordering of surfactant micelles has been detected by SAXS and SANS measurements.15,25,26 The coassembly of double-tailed surfactants with PEs has been studied by several research groups,27−32 but the number of published papers on this topic is considerably smaller than those on the interaction of PEs with single-chain surfactants. Gemini surfactants consist of two hydrophilic head groups, two aliphatic chains (tails) attached to the heads, and a spacer connecting the heads.33 Amphiphilic gemini compounds have superior surface-active properties compared to their conventional analogues.34 In particular, they have a very low critical

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EXPERIMENTAL SECTION

Materials. The block copolymer poly(2-vinylpyridine)-block-poly(ethylene oxide), P2VP−PEO, was purchased from Polymer Source, Inc., and used as obtained. The molecular weights of the PVP block and PEO block and the polydispersity index, provided by the manufacturer, were 13.5 kg/mol, 21 kg/mol, and 1.1, respectively. All the polymer solutions were dissolved in 0.1 M DCl solutions. Gemini surfactant 6,6′-(ethane-1,2-diylbis(oxy))bis(sodium 3-dodecyl benzenesulfonate), G2, was synthesized by a reaction between ethylene dibromide (Br(CH2)2Br) and 4-dodecylphenol in the presence of a phase transfer catalyst. Details on the synthesis and B


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Light Scattering. Experimental Setup. 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 o C for the scattering angles, θ, ranging from 40° to 150°, with an angular step of 10°. The refractive index increments were measured with an Optilab T-rEX refractometer in 25 °C and ASTRA6 software. Data Evaluation. The refractive index increments of the P2VP− PEO/G2 complexes were calculated from the dn/dc values for P2VP− PEO and G2 in 0.1 M HCl, (dn/dc)pol = 0.186 mL/g and (dn/dc)surf = 0.138 mL/g, as

characterization of the samples were reported previously.49 The critical micelle concentration (CMC) of G2 in aqueous solution measured by surface tension is 1.7 × 10−5 mol/L.49 Preparation of Samples. Solutions of P2VP−PEO/G2 samples for SANS and light scattering experiments were prepared by adding various amounts of freshly prepared 0.025 M stock solution of G2 surfactant in 0.1 M DCl to 1 mL of 1 mg/mL stock solution of P2VP− PEO in 0.1 M DCl under stirring at different charge ratio (Z = 0.3, 0.45, 0.6, 0.75, 0.9, 1.2, 1.35, 1.5, and 2.0) and left 24 h prior to the measurements. The solutions were filtered through 0.45 μm Acrodisc filters. The solutions for ITC were prepared in 0.1 M HCl. Isothermal Titration Calorimetry (ITC). ITC measurements were performed with a Nano ITC isothermal titration calorimeter (TA Instruments−Waters LLC, New Castle, DE). The measurement was carried out at 25 °C in a 1 mL Hastelloy C sample cell connected to a 250 μL syringe whose needle was equipped with a flattened, twisted paddle at the tip, which ensured continuous mixing of the solutions in the cell rotating at 200 rpm. Titration was carried out by consecutive 5 μL injections of an aqueous surfactant solution in 0.1 M HCl from the syringe into the sample cell filled with an aqueous PEO−P2VP block copolymer solution in 0.1 M HCl. The delay between two consecutive injections was 800 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. By this method, the differential heats of mixing were determined for discrete changes in composition. The data were analyzed using the NanoAnalyze software. Titration of G2 was performed in 0.1 M HCl and in P2VP−PEO block copolymer solution in 0.1 M HCl at 25 °C. Small-Angle Neutron Scattering. Experimental Setup. SANS experiments were carried out on KWS-2 from the Jülich Center for Neutron Science (JCNS) at MLZ (FRM-II) in Garching, Germany.50 Samples were prepared in deuterated (D2O) aqueous solution (0.1 M DCl) and poured in quartz cuvettes (QX quality from Hellma) with a 2 mm neutron pathway, thermostated at 25 °C. After prior scanning of the scattering abilities of the samples, experiments were conducted at three detector distances: 20, 5, and 1.4 m. A wavelength of 0.47 nm was selected. The collimation distance was fixed at 20 m for the 20 m detector distance and at 8 m for all the other detector distances. With the described experimental setup, it was possible to measure each sample in roughly 45 min at 25 °C. The final q-range obtained is 0.025−4 nm−1, corresponding to 2−250 nm in real space (using Bragg’s law d = 2 π/q). The data were radially averaged, and scattering curves from the three configurations were merged with no need for any arbitrary coefficient. The curves were fitted using SASFit software version 0.94.2.51 Data Evaluation. Forward scattering intensities, I(0), and gyration radii, Rg, were obtained from the Guinier region using the Guinier formula

⎡ R 2q2 ⎤ g ⎥ I(q) = I(0) exp⎢− 3 ⎦⎥ ⎣⎢

(dn/dc)pol c pol + (dn/dc)surf csurf dn = dc c pol + csurf

To check the accuracy of the calculated values, dn/dc was measured experimentally for Z = 1, yielding a value differing by less than 0.1% from the calculated value. The apparent molar masses, Mw, at the given total concentration c = cpol + csurf and the gyration radii, Rg, were calculated using the Zimm equation

⎡ 4π 2n 2(dn/dc) ⎤ c 1 ⎛ 1 2 2⎞ 0 ⎜1 + ⎢ ⎥ Rg q ⎟ = ⎠ Mw ⎝ 3 λ 4NA ⎣ ⎦ ΔR θ(q)

I(0)ρ2 NA c(Δη)2

2 ⎡ ⎛ Γ (q) ⎞⎤ g(2)(t , q) = 1 + β ⎢exp⎜−Γ1(q)t + 2 t 2⎟⎥ ⎠⎦ 2 ⎣ ⎝

(1)

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RESULTS AND DISCUSSION LS and SANS. Figure 1 shows SANS curves for P2VP− PEO/G2 mixtures at Z ranging from 0.15 to 2; the P2VP−PEO concentration was 1 mg/mL. For Z < 1, the curves do not show any structure. In the region from ca. 0.05 to 0.2 nm−1, corresponding to Bragg lengths from ca. 30 to 120 nm, I(q) decays were approximately proportional to q−3.5, indicating scattering at rough interfaces of compact particles.53 After attaining charge equivalence, the coassembled particles undergo distinct structural transition. First, a new power law regime in the region from 0.06 to 0.12 nm−1 with an exponent close to −1.5 indicates the formation of elongated scatterers. At Z = 2,

(2)

∑i ciχi M w, i ∑i ciχi M w, i /di

(6)

where Γ1(q) and Γ2(q) are the first- and second-order cumulants and β is the coherence factor. The hydrodynamic radii, RH, were calculated using the Stokes−Einstein formula from the translation diffusion coefficients obtained as Γ1(q)/q2 and extrapolated to q → 0. Cryogenic Transmission Electron Microscopy. The samples for Cryo-TEM were prepared as described earlier.52 A volume of 3 μL of the sample solution was applied to an electron microscopy grid with carbon-covered polymer supporting a film (lacey-carbon grids LC200CuC, Electron Microscopy Sciences), glow discharged for 40 s with 5 mA current. Most of the sample was removed by blotting (Whatman No. 1 filter paper) for ∼1 s, and the grid was immediately plunged into liquid ethane held at −183 °C. The sample was then transferred without rewarming into a Tecnai Sphera G20 electron microscope (FEI, Hillsboro, OR) using a Gatan 626 cryo-specimen holder (Gatan Inc., Pleasanton, CA). Images were recorded at 120 kV accelerating voltage and microscope magnifications ranging from 5000× to 14500× using a Gatan UltraScan 1000 slow scan CCD camera (yielding a final pixel size from 2 to 0.7 nm) and the low-dose mode with an electron dose not exceeding 1500 electrons/nm2. The employed under focus typically ranged between 1.5 and 2.7 μm. The employed blotting conditions resulted in a specimen thickness varying between 100 and ca. 300 nm.

where c is the total concentration of all the components in the system, NA is the Avogadro constant, Δη is the excess scattering length density, and ρ is the total density calculated under the assumption that all the surfactant is coassembled with the polymer

ρ=

(5)

where ΔRθ(q) is the excess Rayleigh ratio and n0 is the refractive index of the solvent. The autocorrelation functions of the scattered light intensity, g(2)(t), were fitted using a second-order cumulant expansion

The molar masses, Mw, of the samples were calculated using the equation

Mw =

(4)

(3)

Here di, χi, Mw,i, and ci are the density, molar ratio, molar mass, and molar concentration of the ith component, respectively. The Δη value was calculated as the volume-fraction-weighted average of the excess scattering length densities of the components. C


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The association behavior of the P2VP−PEO/G2 system strongly differs from that of a DHBP complex with a single-tail surfactant. In the case of the system studied in refs 24 and 26 large loose aggregates of DHBP chains with adsorbed surfactant micelles are formed at low Z. At a certain critical Z close to 1, segregation of the PE−S complex from the neutral blocks and formation of compact core−shell particles occur, accompanied by a decrease in the size of the particles and a substantial increase in their molar mass. In the P2VP−PEO/G2 system, the PE−S complex already forms compact core/shell particles at the lowest Z examined because the system is already well above the cmc of G2, which was found to be ca. 1.7 × 10−5 mol/L in pure water by surface tension measurements,49 approximately 2 orders of magnitude lower than the CMC values for analogous single-tail surfactants.34 In 0.1 M HCl, the expected CMC is even lower due to the screening of the electrostatic repulsion between the head groups. Hence, instead of cooperative binding of individual G2 ions to P2VP chains, P2VP chains interact with G2 vesicles (which break and form bilayer sheetssee later parts). We believe that this complex dynamic process leads to the formation of compact particles with segregated P2VP and PEO blocks. Increasing the amount of surfactant in the core causes disruption of the particles at Z = ∼1. A similar behavior was reported previously for the poly(N,N-dimethylaminoethyl acrylate)-block-poly(acrylate)/hexylene-1,6-bis(dodecyldimethylammonium bromide) system.46 For P2VP− PEO/G2, the disruption is accompanied by shape transformation of the particle core from spherical to cylindrical. As indicated by the presence of the correlation peak caused by the packing of G2 in the PE−S core, the core becomes more compact. The decrease in the size of the core is compensated by stretching of the corona blocks due to formation of the compact core and limited available volume for the corona blocks in the proximity of the core/shell interface. For more discussion see the section “Proposed Outline of the Coassembling Process”, which summarizes structural changes with increasing Z and elucidates their origin. Cryogenic Transmission Electron Microscopy (CryoTEM). Cryo-microscopy techniques are a very useful tool in polymer science because they directly image the self-assembled nanoparticles. In all microscopy studies of polymer systems and particularly in studies of reversible associates, it is necessary to

Figure 1. SANS curves for P2VP−PEO/G2 complexes (cpol = 1 mg/ mL) in 0.1 M DCl. Charge ratios, Z, are indicated above the individual curves.

the curve already shows distinct oscillations due to the cylindrical shapes of the particles. Second, the curves gradually develop a correlation peak with its maximum at 2.24 nm−1 due to dense packing of G2 in the PE−S core. The molar masses and gyration radii of the coassembled nanoparticles, obtained from fitting of the Guinier regime of the SANS curves, are plotted in Figure 2a. The structural transition around Z = 1 is accompanied by a steep decrease in the molar mass from ca. 2.2 × 107 to ca. 1.0 × 107 g/mol, but the gyration radius does not exhibit an abrupt change corresponding to the formation of core/shell particles. Mw and Rg from SLS (Figure 2b) have similar dependences. DLS measurements (inset Figure 2b) show that Rg/RH < l for Z > 0.3, confirming the compactness of the formed aggregates. Rg/RH values as low as 0.6 are typical for core−shell micelles54,55 with a low-contrast corona which contribute only to the hydrodynamic size of the particle. The increase in Rg/RH at Z > 1 is caused by elongation of the scatterers.

Figure 2. Analysis of (a) SANS and (b) SLS data: molar masses (curves 1, 1′) and gyration radii (curves 2, 2′) of P2VP−PEO/G2 complexes (polymer concentration, cpol = 1 mg/mL) as functions of charge ratios Z. Inset: Rg/RH ratios plotted as functions of Z. D


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Figure 3. Cryo-TEM images of G2 in 0.1 M DCl (A) and of P2VP−PEO/G2 at charge ratio Z = 0.3 (B), 1.0 (C), and 2.0 (D).

micrograph for Z = 2 (Figure 3D and Figure S2) shows fusion of the small spheres into wormlike particles, which is consistent with the SANS data. The spherical particles are touching in many cases, but they do not seem to fully merge into compact cylindrical particles. We assume that the bulkiness and the structure of gemini surfactants promote the formation of cylinders, but the high kinetic barrier due to the compactness of the structures that were formed at Z = 1 prevents structural reorganization. Isothermal Titration Calorimetry (ITC). ITC provides information on the enthalpy changes accompanying the formation of polyelectrolyte−surfactant complexes (Figure 4) (for raw heat rate data see Figure S3). Even though ITC is an indirect method, the combination of ITC data with information provided by scattering and microscopy techniques helps to elucidate details of the association process. In most ITC studies of PE−S complexes, a concentrated solution of the surfactant (above CMC) is added dropwise first to the solvent and later to a polyelectrolyte solution in the same solvent. In the first case, the heat effects measured at the beginning of the titration process (before the CMC of the surfactant is reached) reflect the dilution of the surfactant and the consequent dissociation of surfactant micelles (demicellization). When the surfactant is added into the PE solution, the measured heat in the early stages of the titration corresponds to the dilution of the surfactant, dissociation of surfactant micelles, and formation of the polyelectrolyte−surfactant complex. It is obvious that the difference between the curves measured with and without PE corresponds to the net effect accompanying the PE−surfactant complexation, and the changes in the shape of the difference curve indicate changes in the structure of the formed PE−S complexes.

secure that no changes occur during the preparation of samples for imaging, i.e., after their deposition on the support or during the freezing of solutions. In the Cryo-TEM technique, which we employed in our research, a thin layer of the solution is plunged into liquid nitrogen, and it is believed that the extremely fast freezing with cooling rates ca. 105 K/s minimizes structural changes of nanoparticles. However, some minor changes caused by the freezing can never be ruled out completely. Another problem unreels from the fact that the studied layer has to be very thin (typically less than 100 nm), and the presence of water/air and finally amorphous ice/air interface can affect the imaged structures. In spite of the abovedescribed potential risks, a number of papers demonstrate that structural changes of imaged nanoparticles can be avoided and that Cryo-TEM provides information on intact structure of reversible associates formed in the solution. Because in our study, the Cryo-TEM technique yields results which are in agreement with conclusions drawn from scattering studies, we believe that the images presented below show typical structures formed in solution at ambient temperatures. Figure 3A shows polydisperse spherical single-wall vesicles with radii ranging from 25 to 100 nm (average radius ca. 50 nm), which we observed in most images (see Supporting Information Figure S1). The structures are very similar to those described in pure water.49 We observed multilayer vesicles in a few images. Figure 3B shows the PE−S nanoparticles formed in the mixture with Z = 0.3. The particles are polydisperse in size, which is consistent with the scattering data. Their radii range from 10 to 50 nm. It is necessary to keep in mind that the hydrated coronas of the particles are not visible because of the low contrast. For the mixture with Z = 1.0 (Figure 3C), we observed a mixture of smaller spherical particles. The E


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against interaction with the aqueous medium. This observation correlates with the SANS results, which indicate a structural transition and the formation of elongated core/shell particles in this Z range. The minimum on the titration curve is shifted to slightly higher Z than 1.0 (this shift is more obvious on the curve measured for twice higher copolymer concentrationsee the inset in Figure 4). The shift suggests that the structural transformation depends slightly on copolymer concentration. For the curve measured at a lower polymer concentration, c = 0.5 mg/mL, the exothermic heat effects reflecting the polyelectrolyte−surfactant interaction steeply diminish in the range of Z ∈ (1.1, 1.5). The curves measured for systems with and without the copolymer approach each other, but their difference for Z = 1.5 is still ca. −12 kJ/mol. At this stage in the titration process, very compact insoluble micellar cores containing the PE−S complex have already been created and do not change much. The rapid approach of the two curves stops at Z = ∼1.5, and the difference between them decreases only slowly in the Z range from 1.5 to 2.0. However, their difference remains −5 kJ/mol for Z = 2.0. Because we did not observe any G2 vesicles in Cryo-TEM images for Z > 1, we assume that the excess G2 adsorbs predominantly in the core− shell interfacial region, and the negative charge of G2 is efficiently compensated at relatively short distances by small counterions that can penetrate into the highly hydrated PEO shell. The adsorption of G2 in the core/shell region, which is driven in this case by “hydrophobic interactions”, reduces the number of undesirable contacts of C12 tails with the aqueous medium and generates some exothermic effects. The penetration of counterions into the micellar shells and structural changes in water molecules solvating PEO units caused by increasing local ionic strength56 and consequent structural changes in the formed nanoparticles, e.g., the formation of elongated particles that we observed both by SANS and by Cryo-TEM, also contribute to the nonzero heat effects measured for Z > 1. We are aware of the fact that vigorous mechanical stirring used in ITC titrations contributes to the destruction of G2 vesicles and that the G2 sheets could be smaller, and the aggregates may slightly differ from those formed upon gentle intermixing of the components. As concerns the measured heat effects, the interaction of a higher number of smaller sheets and a lower number of larger sheets with the same number of PE chains should produce the same effects. Proposed Outline of the Coassembling Process. On the basis of the obtained results, we can propose the following scenario for the studied electrostatic coassembly. As already mentioned, gemini surfactants with C12 aliphatic tails are strongly hydrophobic and start to form stable vesicles at critical micelle concentrations that are 2 orders of magnitude lower than those of the corresponding single-tail analogues. In contrast to continuous titration the PE solutions by single tail surfactants, in which the concentration of the surfactant drops not only below CMC but also below CAC and in which PE chains interact first with individual surfactant molecules, P2VP blocks in the studied system interact always with G2 vesicles, regardless of Z. Because the coassembled polymer−surfactant particles are smaller than the original surfactant vesicles, it is obvious that the interaction of PE chains induces the rupture of G2 vesicles and the formation of extended surfactant bi- or multilayers which arrange around polymer chains. The decomposition can be significantly accelerated by vigorous

Figure 4. Enthalpy curves over charge ratio Z for titration of 0.015 M G2 solution into 0.1 M HCl in the absence of polymer (open circles) and into 0.5 mg/mL P2VP−PEO in 0.1 M HCl (black circles). Inset: enthalpy curve over Z for titration of 0.015 M G2 solution into 1 mg/ mL P2VP−PEO in 0.1 M HCl.

It is worth mentioning that it is common to decompose the complex process of the titration of PE solutions into several thermodynamic steps. For example, in the region of final surfactant concentration c, which satisfies the inequalities CAC < c < CMC, the following steps are taken into account: (i) dilution of the surfactant, (ii) demicellization of pure surfactant micelles below CMC, (iii) formation of smaller micelles in the presence of PE chains, and (iv) interaction of modified micelles with PE chains. Nevertheless, it is clear that the subtraction of the signal for pure buffer from that for PE solution always provides the net heat effect corresponding to the PE−surfactant interaction. We would like to stress that our measurement and data interpretation differ from the scheme outlined above because the final surfactant concentration in the calorimetric cell is always higher than the CMC. Hence, we can conclude that the thermal effects observed after addition of G2 to aqueous HCl do not correspond to the dissociation of G2 vesicles but rather to their reorganization (rupture and partial decomposition into bilayer sheets). The continuous reorganization process also explains why the titration curve changes smoothly with the concentration of the solution in the calorimetric cell. At the early stages of PEO−P2VP titration by G2, we observed a pronounced exothermic effect (the difference between the curves with and without the copolymer is ca. −30 kJ/mol). This heat effect, which almost does not vary in a broad range of charge ratios Z from 0.2 to ca. 0.8, is a result of the minimization of unfavorable interactions of hydrophobic P2VP chains after their interaction and complexation with hydrophobic aliphatic tails of the surfactant. Since the overall electrostatic interaction energy of the coassembly does not change much, we can conclude that the strongly exothermic heat effects indicate the formation of segregated insoluble domains of the PE−S complex and the association of PE chains. In the Z range 0.8−1.0, the exothermic heat effects strongly increase by −15 kJ/mol and reach a total value of −50 kJ/mol. This indicates the formation of more stable structures in which the hydrophobic parts of the molecules are well protected F


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Figure 5. Cryo-TEM images (A, B, and C) demonstrating the decomposition of G2 vesicles (their rupture and organization of released G2 sheets along PE chains) proceeding upon their mixing with P2VP−PEO at Z = 0.3. The samples for imaging were obtained by fast freezing for 12 h after gentle intermixing of the components. The final concentration of copolymer was 1 mg/mL.

Scheme 2. Structural Transitions of P2VP−PEO/G2 Complexes

bulkiness and stiffness of randomly oriented G2 domains which interconnect the aggregate produce steric tension, but the noncomplexed flexible parts of the P2VP chains efficiently reduce the strain almost up to Z = 0.8. At Z approaching 0.9, a low fraction of noncomplexed ionized 2VPH+ units cease to contribute to the solubility of the particles. Simultaneously, the sequences of 2VPH+ units are too short and are not able to compensate for the considerable tension caused by nonoptimum random orientation of G2 sheets interconnecting the polyelectrolyte chains. All the experimental methods unambiguously show that an important structural transition occurs at Z close to 1. ITC data indicate that particles with appreciably higher stability are formed in a narrow region of Z values. The fact that stable particles form abruptly rather than gradually, in spite of the large enthalpy gain associated with the increase in the G2 concentration, suggests a high kinetic barrier for the structural transition. The transition occurs as a result of large strain in the cores. The most probable structure of the core of newly formed

stirring (as is achieved in ITC measurements), but without mechanical interference the disintegration of G2 vesicles proceeds relatively slowly. Figure 5 shows the Cryo-TEM image of a gently intermixed solution of the copolymer with G2 at Z = 0.3 which was vitrified 12 h after mixing. The micrograph clearly shows aggregates of broken vesicles. The interaction of G2 with P2VP and formation of the charge-neutralized PE−S complex are convenient from the enthalpy point of view because both components involved in the coassembly are strongly hydrophobic and form very compact domains, preventing unfavorable interactions of both 2VP and G2 units with water. At low Z < 0.3, small G2 sheets sparsely interconnect individual block copolymer chains. The ionized parts of the P2VP chains improve the overall solubility of the copolymer chains, and therefore only loose associates are formed. The fraction of ionized 2VPH+ units gradually diminishes with Z, and the solubility of the P2VP blocks decreases. We observed the formation of fairly compact nanoparticles in a broad Z range from ca. 0.3 to 0.8. The G


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Macromolecules stable core−shell associates should be a cylinder with parallel arrangement of G2 sheets firmly interconnecting the P2VP chains. Strong interaction of G2 sheets with PE chains and unfavorable interaction of already existing charge-neutralized domains with water kinetically hinder the reorganization of the PE chains and G2 sheets and prevent the formation of large rodlike associates with well-organized insoluble cores. Only associates of limited size can form sufficiently rapidly under these conditions. The results of the study indicate that the particles formed at Z > 1 tend to reorganize in elongated structures on longer time scales, but the high kinetic barrier due to the strong attractive interaction of the components forming the subunits to be reorganized substantially retards the fusion of small spherical micelles into rodlike structures. The abovedescribed structural transitions are depicted schematically in Scheme 2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.U.). *E-mail: [email protected] (K. P.)> ORCID

Mariusz Uchman: 0000-0002-2564-1985 Miroslav Štěpánek: 0000-0002-7636-7234 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the Czech Science Foundation−Czech Republic (M.U. and S.K. grant 1700289Y) and (K.P. and M.S. grant 16-12291S) and from the Ministry of Education, Youth and Sports of the Czech Republic (Operational Programme Research, Development and Education: “Excellent Research Teams”, Project No. CZ.02.1.01/0.0/ 0.0/15_003/0000417−CUCAM). Z.H.B. and M.U. acknowledge the support from the Ministry of Science and Higher Education of Poland (PL 88772/R13/R14) and Ministry of Education, Youth and Sports of the Czech Republic (grant 7AMB13PL026) on Co-operation in the Field of Science and Technology. This work is based upon experiments performed at the KWS-2 instrument and SANS high pressure cell (beam time No. 10296 awarded to M.U.) operated by JCNS at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany.

CONCLUSIONS We studied the coassembly of diblock copolymer poly(2vinypyridine)-block-poly(ethylene oxide) with the anionic gemini surfactant 6,6′-(ethane-1,2-diylbis(oxy))bis(sodium 3dodecyl benzenesulfonate), G2, by a combination of scattering techniques, Cryo-TEM, and ITC in acidic aqueous solutions at pH 1. We showed that the specific structure of the gemini surfactant with a short ethylene linker connecting two bulky head groups and two long strongly hydrophobic aliphatic chains strongly influences the coassembling behavior, which then differs in many respects from the coassembly of PEs with single-chain surfactants. Electrostatic interaction of double charged surfactants with oppositely charged PE chains promotes the binding and the formation of the PE−S complex. The considerable hydrophobicity of the involved components supports the formation of linkers connecting the PE chains. However, the bulkiness of G2 hinders the formation of compact hydrophobic domains with the optimum arrangement of P2VP chains. The mechanism of association of the studied system differs from that of mixtures of PEs with single-tail surfactants. Because the CMC of gemini surfactants is lower by ca. 2 orders of magnitude than that of single tail surfactants, PE chains interact directly with G2 vesicles even at low surfactant chargeto-PE charge ratios Z. At Z close to 1, almost all the 2VP units are engaged in the electrostatic complex and do not contribute to the release of steric tension. The particles reorganize as a result of strong steric tension and form more thermodynamically stable particles. At Z > 1, we observe the formation of elongated rodlike structures promoted by the bulkiness and stiffness of G2 sheets, but repulsion between micellar coronas prevents the fusion of the micelles into rods. In summary, the performed study indicates that the specific structure of gemini surfactants influences the electrostatic aggregation with PEs and predetermines the structure of aggregates. However, various kinetic aspects strongly modify the coassembling process and affect also the structure of nanoparticles formed at different Z.

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Cryo-TEM images of G2 in 0.1 M DCl (Figure S1) and P2VP−PEO/G2 for Z = 2 in 0.1 M DCl (Figure S2) and ITC thermograms (Figure S3) (PDF)

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01330. H


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Coassembly of Gemini Surfactants with Double Hydrophilic Block

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