Nanoparticles with a Bicontinuous Cubic Internal Structure Formed by

Article pubs.acs.org/Langmuir

Nanoparticles with a Bicontinuous Cubic Internal Structure Formed by Cationic and Non-ionic Surfactants and an Anionic Polyelectrolyte John Janiak,† Solmaz Bayati,† Luciano Galantini,‡ Nicolae V. Pavel,‡ and Karin Schillén*,† †

Division of Physical Chemistry, Department of Chemistry, Center for Chemistry and Chemical Engineering, Lund University, Post Office Box 124, SE-221 00 Lund, Sweden ‡ Department of Chemistry, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy ABSTRACT: Nanoparticles with an internal structure have been prepared by dispersing under dilute conditions poly(acrylic acid) with a polymerization degree n = 6000 (PAA6000) together with a cationic surfactant hexadecyltrimethylammonium hydroxide (C16TAOH) and the non-ionic surfactant penta(ethylene glycol) monododecyl ether (C12E5) in water. The nanoparticles are formed at different mixing ratios in the corresponding two-phase regions (liquid crystalline phase/dilute isotropic phase) of the C16TAPA6000 complex salt/ C12E5/water ternary phase diagram. The particles consist of polyacrylate PA−6000 polyions, C16TA+ surfactant ions, and C12E5. Their internal ordering was identified by small-angle Xray scattering (SAXS) to be either bicontinuous cubic with the Ia3d crystallographic space group or normal hexagonal depending upon the amount of C12E5. The bicontinuous cubic phase, to our knowledge never observed before in polyelectrolyte−surfactant particle systems, was inferred by SAXS experiments. The data also showed that this structure is thermoresponsive in a reversible manner. The bicontinuous cubic space group transforms from Ia3d to Im3m as the temperature decreases from 25 to 15 °C. According to dynamic light scattering and electrophoretic mobility measurements, the particles have a well-defined size (apparent hydrodynamic radii RH in the range of 88−140 nm) and carry a positive net charge. The size of the nanoparticles is stable up to 1 month. The faceted nanoparticles are visualized by cryogenic transmission electron microscopy that also reveals their coexistence with thread-like C12E5 micelles. preparation methods have been proposed.4−6 They can be formed by dispersing (or fragmenting) the liquid crystalline phase in excess water often in the presence of a non-ionic amphiphilic block co-polymer as a dispersion-stabilizing agent; e.g., see refs 7−9. Non-ionic triblock co-polymers composed of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), abbreviated as PEO−PPO−PEO, are among the most commonly used steric stabilizers for cubic-phase nanoparticles in water.10 Alone in dilute aqueous solution, these copolymers self-assemble to form either micelles11 or vesicles12 depending upon their relative block lengths. However, in specific conditions, these co-polymers as well as other amphiphilic di-, tri-, or multiblock co-polymers13−15 can form structured nanoparticles,16−18 thus providing various morphologies and novel internal structures. Among them, highly asymmetric diblock co-polymers of polystyrene and poly(acrylic acid) (PAA),19,20 semi-crystalline diblock co-polymers of PEO and poly(octadecyl methacrylate),21 and fluorinecontaining diblock co-polymers22 have been reported to be suitable for the preparation of bicontinuous or hexagonally structured nanoparticles in water/organic solvent mixtures. The disadvantage with the block co-polymer nanoparticles is that

1. INTRODUCTION Aqueous dispersions of nanoparticles have attracted a large scientific interest mainly because the particles are stable with a non-lamellar liquid crystalline internal structure. The internal order provides a large surface area, which enables the particles to solubilize quite high concentrations of hydrophobic molecules, proteins, or peptides and make them useful in a wide range of practical applications that span the fields of medicine, material, and food science to consumer products. The term non-lamellar includes the long-range ordered cubic and hexagonal structures as well as the more disordered L3 sponge structure. The cubic phases (normal or reversed) are separated into micellar cubic consisting of a cubic lattice of close-packed micelles and bicontinuous cubic consisting of a three-dimensional network of either hydrophilic (water channels) or hydrophobic material that divide space into two continuous but non-intersecting regions. The latter can be described by infinite periodic minimal surfaces (IPMS) of cubic symmetry that have a zero mean curvature everywhere. The IPMS of the simplest typology are the primitive (P), diamond (D), and gyroid (G) surfaces, which are associated with the crystallographic symmetry space groups Im3m, Pn3m, and Ia3d, respectively.1−4 Great scientific interest has been directed toward colloidal liquid crystalline particles with, e.g., reversed bicontinuous cubic structure made up of lipids (often glyceridebased monoolein or phytantriol) (Cubosome nanoparticles), because of their importance in applications and several © 2012 American Chemical Society

Received: October 3, 2012 Revised: November 1, 2012 Published: November 1, 2012 16536

dx.doi.org/10.1021/la303938k | Langmuir 2012, 28, 16536−16546

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scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and electrophoretic mobility measurements by laser Doppler microelectrophoresis. The large number of reported systems and methods allows for the preparation of nanoparticles with many different internal structures (lamellar, hexagonal, etc.). However, to the best of our knowledge, this is the first study presented in the literature concerning the preparation and characterization of dispersed polyelectrolyte−surfactant nanoparticles with a bicontinuous liquid crystalline internal structure. The determination of this new structure and its thermosensitivity increases the spectrum of properties of nanostructured polyelectrolyte− surfactant particles and enhances the range of their potential applications.

these are often created through more or less complicated preparation routes involving solvent exchange to reach a final aqueous solution of particles.13 It exists a need to find a simpler way of preparing nanoparticles with an ordered interior, which, thus, is one of the objectives of this study. There are numerous studies found in the literature on the formation of core−shell nanoparticles (so-called block ionomer complexes, polyion complex micelles, or complex coacervate core micelles) by electrostatic self-assembly of water-soluble diblock co-polymers with one polyelectrolyte block and one non-ionic block with oppositely charged surfactants in water; e.g., see refs 23−25. However, preparation of particles from mixtures of polyacrylamide diblock co-polymers and the oppositely charged surfactants dodecyltrimethylammonium bromide (C12TAB) or sodium dodecyl sulfate (SDS) does not always provide particles with an internal liquid crystalline order.25 It has also been shown that electroneutral nanoparticles (however not internally ordered) can be produced in the presence of stabilizing agents in mixtures of surfactants and oppositely charged branched polyelectrolytes, such as those formed by poly(ethylene imine) (PEI) and SDS.26,27 One particularly interesting and simple method of preparation of internally structured nanoparticles is provided using oppositely charged surfactant−polyelectrolyte aqueous systems and the mechanism of overcharging, which gives a kinetic barrier against particle aggregation. The basis for this method is the general associative phase separation phenomena, where a concentrated phase containing the polyion and the charged surfactant separates out and leaves the simple counterions in a dilute aqueous phase.28 For example, cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDAC) and anionic amphiphile mixtures with an excess of either component have been used for the formation of liquid crystalline phase nanoparticles.29,30 Another example where the same methodology has been used is in the study of alkyltrimethylammonium bromide (CnTAB)/sodium polyacrylate (NaPA)/H2O systems.31 For the preparation of these types of polyelectrolyte− surfactant particles, investigations of macroscopic phase behavior of aqueous mixtures of different alkyltrimethylammonium surfactants and NaPAn and the complex salts C16TAPA25 and C16TAPA6000 are crucial because they give a fundamental understanding of the phase separation behavior and what phases separate out.32,33 The starting point of our investigation of nanoparticles with an internal structure was the ternary phase diagram of C16TAPA6000, the non-ionic surfactant penta(ethylene glycol) monododecylether (C12E5), and water. C16TAPA6000 is the complex salt consisting of aggregated hexadecyltrimethylammonium (C16TA+) surfactant ions with polyacrylate PAn− (polymerization degree n = 6000) as polymeric counterions.33 C16TA+ and PA−6000 are thus in a 1:1 charge relation, and no simple ions are present in the system. The C16TAPA6000/ C12E5/H2O system shows both a normal hexagonal (H1) phase and a bicontinuous cubic phase (V1) with space group Ia3d and a gyroid minimal surface in equilibrium with a dilute water-rich phase.34 This knowledge assisted us to prepare nanoparticles of different internal structures. When the particles were overcharged as in refs 29 and 31, stable nanoparticles composed of − PA6000 polyions, C16TA+, and C12E5 were prepared. To characterize the internal structure of the nanoparticles, their overall size, morphology, net charge, stability with time, as well as the thermosensitivity of the internal structure, different experimental techniques were employed, i.e., small-angle X-ray

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Hexadecyltrimethylammonium bromide, pro-analysis grade (C16TAB; molar mass of 364.45 g mol−1), and poly(acrylic acid) (PAA6000; molar mass of 450 000 g mol−1) were purchased from Sigma-Aldrich. PAA6000 was dialyzed for 5−7 days using dialyze membranes with a cutoff of 10 000 g mol−1 from Spectrum Laboratories. Penta(ethylene glycol) monododecyl ether (denoted C12E5) was purchased from Nikko Chemicals and used without further purification. Hydrochloric acid ampules (Fixanal) used for titration purposes were purchased from Fluka. C16TAB was ion-exchanged to its hydroxide form (C16TAOH) using Monsophere Dowex (OH) 550 from Sigma-Aldrich using a previously established method.35 The C16TAOH solution was then titrated with a freshly prepared 0.1 M HCl solution to determine the C16TAOH concentration. All solutions were prepared with water purified using a Milli-Q system (Millipore Corporation, Bedford, MA). Two solution series of different PAA6000 concentrations were prepared: one dilute series with 0.25 mg mL−1 PAA6000 (i.e., 3.3 × 10−3 mol L−1 carboxylic acid groups) and with excess C16TAOH, n(C16TAOH)/n(COOH group) molar ratio (MR) equal to 1.4 (series 1), and one more concentrated with 0.73 mg mL−1 PAA6000 (or 9.7 × 10−3 mol L−1) and with equimolar amounts of C16TAOH and PAA6000; i.e., MR = 1 (series 2). All solutions had a final volume 3 mL. To prepare series 1, 0.5 mL of 0.02 M PAA6000 stock solution was mixed with water (the amount of water was determined by the amount of 0.048 M C12E5 stock solution that was added); whereafter, 0.7 mL of 0.02 M C16TAOH stock solution was added. Eight different solutions were prepared by the addition of 0.02−0.8 mL of 0.048 M C12E5, giving C12E5 concentrations of 0.32, 1.6, 3.2, 4.8, 6.4, 8.0, 11.2, and 12.8 mM. The solutions were left to equilibrate for 2 weeks. The milky dispersions were thereafter filtered using Millipore Millex-AA filters with a pore size of 0.8 μm to remove small clunks of phaseseparated sample. In the second series (series 2), 0.5 mL of 0.058 M PAA6000 stock solution was mixed with water (again, the amount was determined by the 0.048 M C12E5 stock solution addition); whereafter, 0.5 mL of 0.058 M C16TAOH stock solution was added. Eight different solutions were prepared by the addition of 0.02−1.0 mL of 0.048 M C12E5, giving C12E5 concentrations of 0.32, 1.6, 3.2, 4.8, 6.4, 9.6, 12.8, and 16.0 mM. After the samples had been equilibrated for 2 weeks, they were ultrasonicated, centrifuged at 4000 rpm for 15 min, and finally filtered through 0.8 μm filters. It was found that solutions with a low content of non-ionic surfactant showed aggregation upon ultrasonication. Therefore, the solutions containing between 0.32, 1.6, 3.2, and 4.8 mM C12E5 were only filtered, while the remaining solutions were prepared as described above. After filtration, all solutions where left to equilibrate at 25 °C for 2 weeks before characterization by SAXS and DLS. During this period, the solutions remained turbid but no visual creaming or sedimentation was observed. New solutions were prepared with the same recipes for electrophoretic mobility and additional DLS measurements. Samples were used 1 day after preparation. 16537


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2.2. SAXS. SAXS measurements were performed at the MAX II SAXS beamline I911-4 at MAXIV Laboratory in Lund, Sweden.36 A sample−detector distance of 2293 mm covered a momentum transfer range of 0.01 < q < 3 Å−1 [q = (4π/λ) sin θ], and the wavelength was 0.91 Å. The scattered intensity was recorded on a 165 mm diameter MarCCD detector. Calibration measurements were carried out using a LaB6 sample. The solutions were injected into thermostatted quartz capillary sample holders and equilibrated for at least 15 min before each temperature change. All measurements on the nanoparticle samples were performed at 25 °C and for one sample with MR = 1.4 and 8 mM C12E5 also at 15 °C. Two macroscopically phase-separated samples with a composition of MR = 1.4 and 8 mM C12E5 were preequilibrated at 15 and 25 °C for 2 months and measured at the same temperatures using mica window holders for solid samples. The sample equilibrated at 25 °C was also measured at 35 °C. The data acquisition time was 900 s for the nanoparticles and 300 s for the macroscopically phase-separated samples. The two-dimensional (2D) SAXS patterns were processed using the Fit2D software.37 All data have been analyzed without background subtraction. 2.3. Cryo-TEM. Cryo-TEM measurements were performed on a Philips CM120 Bio TWIN electron microscope operated at 120 kV. The microscope has a Gatan MSC791 cooled charge-coupled device (CCD) camera detection system. A 5 μL droplet of the sample solution was gently placed on a lacey carbon-coated copper grid under controlled environmental conditions using a vitrification system at 20 °C.38 The grids were blotted to remove excess fluid and to obtain a homogeneous liquid film coverage of the grid before they were rapidly plunged into liquid ethane at −180 °C to ensure rapid vitrification and avoid crystallization of water. The carbon grids were then stored in liquid nitrogen before insertion into the electron microscope. 2.4. DLS. The setup used for the DLS measurements is an ALV/ DLS/SLS-5022F, CGF-8F-based compact goniometer system from ALV-GmbH, Langen, Germany. The light source is a 22 mW He−Ne laser. The laser operating at 632.8 nm intensity is varied using a software-controlled attenuator. A perfect vertical polarization is achieved using a Glan laser polarizor prism with a polarization ratio better than 105 in front of the high-temperature cell housing. The cylindrical clean scattering cells of borosilicate glass (10 mm inner diameter) are immersed in a thermostatted vat filled with a refractiveindex matched liquid (cis-decahydronaphthalene or decaline), and the temperature is controlled to ±0.01 °C by a F32 Julabo heating circulator. In our experiments, the temperature was kept at 25 °C, which was within ±0.1 °C. The unpolarized scattered light is collected using a detection unit that includes a near-monomodal optical fiber and two high-quality avalanche photodiodes placed in a pseudo-crossgeometry. The rotary table of the goniometer has a range of scattering angles (θ) between 15° and 150°. The time-correlation function of the scattered intensity G(2)(t) (auto- or pseudo-cross-correlation mode) with an initial real sampling time of 25 ns and with a variable lag time t is obtained using an ALV-7004/E multiple tau digital correlator of 4 × 312 channels covering ≈12 decades in t. The DLS experiment measures the intensity correlation function G(2)(t) that directly reflects the dynamics in a system, e.g., Brownian motion of particles.39 The normalized version of G(2)(t), g(2)(t) = G(2)(t)/⟨Is⟩2, where ⟨Is⟩ is the time-averaged intensity of the scattered light, is a simple function of the normalized electric field correlation function g(1)(t) that also takes into account the deviation from ideal correlation and experimental geometry.40 If there are several characteristic decay or relaxation times (τ) present in the system, g(1)(t) may be described as39,41,42

g(1)(t ) =

∫0

∞

nonlinear regularization algorithm, regularized positive exponential sum (REPES), which directly minimizes the sum of the squared differences between the experimental and calculated g(2)(t) − 1 correlation functions (in the present case, pseudo-cross-correlated).41,43 In all ILT analyses, the so-called probability-to-reject term was set to 0.5.44 Data analysis was also made using the method of cumulants. It was performed using the ALV software that fits to the logarithm of the intensity correlation function to obtain the first cumulant, which represents the intensity-weighted mean relaxation rate Γ̅ ; see ref 45 and the references therein. A second useful output parameter from this analysis is the polydispersity index (PDI), which is defined as the second cumulant, i.e., the width of the size distribution, divided by the first cumulant squared, μ2/Γ̅ 2. The results from both fitting methods are in good agreement, which verifies their reliability. It was first established by measurements on the solutions at different angles over the range θ = 50−130° that the relaxation rates were linearly dependent upon q2 through the origin. This means that the relaxation rate of the correlation function is associated with a translational diffusion process and the mutual diffusion coefficient may be calculated according to D = (Γ/q2)q → 0, where q = (4πn0/λ)sin(θ/ 2) is the magnitude of the scattering vector with the refractive index of the pure solvent (here, water), n0, and wavelength of the incident laser light, λ. The DLS measurements were thereafter only performed at θ = 90°. The factor (1 − ϕ)2, where ϕ is the volume fraction of particles, has been omitted in the calculations of D, because it only becomes important in concentrated solutions.42,46 In this paper, the presented values of the apparent Z-averaged hydrodynamic radius RH were calculated from D using the first cumulant of the second-order cumulant analysis. 2.5. Electrophoretic Mobility Measurements. A Zetasizer Nano ZS instrument from Malvern Instruments, Ltd., Worcestershire, U.K., was used for electrophoretic mobility measurements in addition to some DLS measurements at θ = 173° (complementary to those performed at the ALV instrument). The goniometer system is equipped with a 4 mW He−Ne laser with an automatic laser attenuator, and the detection unit comprises an avalanche photodiode. The temperature range of the instrument is 2−90 °C. However, in this study, the temperature was set to 25 °C. The solutions are filled in disposable folded capillary cells, and the measurements are performed at a fixed scattering angle of 17° using a laser interferometric technique (laser Doppler electrophoresis), which enables the determination of the electrophoretic mobility.47 In such an experiment, an electric field is applied to a dispersion of charged particles, which then move with a velocity (v = |v|), ̅ and the Doppler-shifted frequency of the incident laser beam caused by these moving particles is monitored. The velocity of a particle with radius R moving in an applied electric field, E = |E̅ |, is v = ueE. ue is the electrophoretic mobility, which can be expressed as Henry’s equation:48 ue = (2εrε0ζ/3η0)f(κR), where ζ is the zeta potential at the particle surface, εr is the dielectric constant of the medium, ε0 is the permittivity of the vacuum, and η0 denotes the solvent viscosity. f(κR)48−50 is Henry’s function that depends upon the product κR, in which κ−1 is the Debye length [κ−1 (Å) = 3.04/(I/m)1/2, where I is the ionic strength in molal51]. In this study, the measured electrophoretic mobility values are stated as the average of three consecutive measurements together with the estimated standard deviation, s. ζ values were not calculated because the ionic strengths of the solutions are unknown. It should be recalled that no simple salt has been added. Before the measurements, three consecutive DLS measurements were performed on the same solutions.

3. RESULTS AND DISCUSSION The complex salt C16TAPA6000 consists of C16TA+ surfactant ions with polyacrylate PA−6000 polyions acting as counterions. It is produced through titration of hexadecyltrimethylammonium hydroxide (C16TAOH) with PAA6000 to a 1:1 charge relation.33 Because of the strong attraction between C16TA+ aggregates and the polyions, C16TAPA6000 is not soluble in water and the binary system demonstrates a normal hexagonal (H1)/water two-phase region at high water contents.33,34 As shown in the

∞

G(Γ) exp(−Γt )dΓ =

∫−∞ τA(τ) exp(−t /τ)d ln τ (1)

with the normalization of ∫ ∞ 0 G(Γ)dΓ = 1, where Γ is the relaxation frequency (Γ ≡ 1/τ). A(τ) is the relaxation time distribution. In this work, the distributions are displayed in the equal area representation, i.e., as τA(τ) versus log (τ/μs) with arbitrary units. We note in eq 1 that A(τ) can be obtained by performing an inverse Laplace transformation (ILT). In this work, we used a constrained 16538


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Figure 1. Ternary phase diagram for the C16TAPA6000/C12E5/H2O system. Boundaries to one-phase regions are marked with bold lines; full lines show boundaries to three-phase areas; and boundaries that are not exactly determined are indicated with dashed lines. Compositions are in weight percent. H1, V1, and Lα denote normal hexagonal, bicontinuous cubic, and lamellar liquid crystalline phases. L1′ and L1 correspond to dilute and more concentrated isotropic solution, respectively, and L1″ denotes a concentrated liquid micellar phase. This figure was reproduced with permission from ref 34. Copyright 2011 of the PCCP Owner Societies.

Figure 2. SAXS curves, with intensity in arbitrary units (without background subtraction) as a function of the scattering vector q of the nanoparticles of series 1 (MR = 1.4 and 0.25 mg mL−1 PAA6000) compared to spectra obtained from the corresponding concentrated liquid crystalline phases of the C16TAPA6000/C12E5/H2O system at high water content (i.e., from two- and three-phase regions) at 25 °C. Top panels (particles): (A) hexagonal (H1) structure (3.2 mM C12E5), (B) coexisting H1 and bicontinuous cubic (V1) structures (6.0 mM C12E5), and (C) V1 structure (8.0 mM C12E5). Bottom panels (concentrated liquid crystalline phases): (D) H1 phase, (E) H1 + V1 two phase, and (F) V1 phase. The insets show the Bragg peaks at high q in an enlarged scale. The q values and relative peak positions for the data in panels D−F are found in Table 1. This figure was reproduced with permission from ref 34. Copyright 2011 of the PCCP Owner Societies.

gives rise to changes in morphology. At low C12E5 contents, the structure of the liquid crystalline phase as determined by SAXS is H1, which shows that the morphology of the complex salt/ water binary system is retained when the non-ionic surfactant is

previously determined phase diagram for the C16TAPA6000/ C12E5/H2O system (Figure 1),34 when C12E5 is mixed with C16TAPA6000, the non-ionic surfactant strongly partitions into the liquid crystalline phase formed by the complex salt and 16539


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useful to compare the SAXS results of the particles (measured at low total concentrations) to the corresponding results for the macroscopic phases. A similar strategy has been employed in earlier studies of internally structured particles.9,21,29,30,52−55 Comparisons of panels B and C of Figure 2 to panels E and F of Figure 2, respectively, show that the relative position of the Bragg peaks are the same. However, the pattern shown in Figure 2A displays one resolved Bragg peak only, which does not allow for the calculation of relative peak positions. Instead, we notice that the position of the Bragg peak in Figure 2A coincides nicely with the first Bragg peak in Figure 2D. Before filtration, the samples contained small pieces of the nondispersed liquid crystalline phase, which allowed for investigation of birefringence using cross-polarizers. The particle samples in the H1 + L1′ two-phase (Figure 2A) and H1 + V1 + L1′ three-phase (Figure 2B) regions were found to be fully and partially birefringent, respectively, while the V1 + L1′ two phase (Figure 2C) displayed no birefringence. An increase of the amount of non-ionic surfactant in the system leads to a change from a H1 to a V1 phase with an intermediate concentration regime of mixed particles. The order of the phases follows the expected change in interfacial curvature (higher to lower) when non-ionic surfactants with short PEO groups are introduced (Figure 1). Interestingly, the C12E5/H2O binary system does not contain a bicontinuous cubic phase at 25 °C. In ref 34, this is explained by an increased curvature, equivalent to the effect of a temperature decrease in the C12E5/H2O system, imposed by the C16TAPA6000 complex salt. 3.2. Particle Imaging by Cryo-TEM. The internal structure and morphology of the nanoparticles was further examined using the cryo-TEM technique, which, at the same time, can give an idea about the overall size of the particles (however, not in a statistically valid way). Figure 3 presents selected cryo-TEM images of nanoparticles formed from PAA6000/C16TAOH/C12E5 mixtures at higher C12E5 concentrations of both solution series. The images show nanoparticles of faceted morphology and with an internal structure. Their size appears to be in the similar size range as determined from DLS (see below). The particles of series 1 with a composition corresponding to a sample in the V1 + L1′ two-phase region (i.e., a bicontinuous cubic internal structure is therefore expected) can be seen in panels A and B of Figure 3. In the background, thread-like structures can be observed, which are elongated C12E5 micelles and, thus, correspond to the L1′ phase. The matching SAXS data are found in Figure 2 (V1 phase structure exists from >8 to 16 mM C12E5). Panels C and D of Figure 3 show the corresponding particles of series 2. It was not possible to perform fast Fourier transform (FFT) analysis of the images to determine the interplanar distances within the nanoparticles and, by that, further examine their internal structure because of the low resolution of the CCD camera, the small sizes of the particles, and their beam sensitivity. Because of the latter, in particular, it was not possible to tilt the specimen, as needed for FFT, and to perform image magnification without destroying the particles. However, on the basis of the SAXS data both on the particles and the macroscopic liquid crystalline V1 phase, we draw the conclusion that the structure of the nanoparticles is bicontinuous. Cryo-TEM experiments were also performed on the PAA6000/C16TAOH/C12E5 mixtures at lower C12E5 concentrations (and with MR = 1.4 and 1, respectively), where hexagonally structured particles are expected. The images showed indeed internally structured particles with a

added. At higher C12E5 concentrations, there is phase transition to a normal bicontinuous cubic phase (V1) of the Ia3d space group.34 The hexagonal phase of the C16TAPA6000/C12E5/water system is made up of cylindrical mixed micelles of C16TA+ and C12E5 with the long polyacrylate chains wrapped around them, while the cubic phase may be visualized as a gyroid minimal surface structure that is able to accommodate the polyions. Starting from the knowledge of the macroscopic phase behavior of the C16TAPA6000/C12E5/water system, nanoparticles were prepared by mixing PAA6000, C16TAOH, and C12E5 in water under dilute conditions and dispersing the aggregates formed. Two different solution series were mixed according to the procedure described in the Experimental Section. The solutions in series 1 contain excess C16TAOH [with a global n(C16TAOH)/n(COOH group) MR = 1.4] and have a constant PAA6000 concentration of 0.25 mg mL−1, while those in series 2 contain 0.73 mg mL−1 PAA6000 and have a MR = 1. For both series, different mixing ratios were chosen, so that they coincide with those of the two-phase regions, normal hexagonal phase/dilute isotropic liquid phase (H1 + L1′) and bicontinuous cubic phase/dilute isotropic liquid phase (V1 + L1′), in the water-rich corner of the C16TAPA6000/C12E5/H2O system (Figure 1). It should be noted that the samples of series 2 do not strictly belong to the phase diagram presented in Figure 1, but we can still use it as a guide. 3.1. SAXS Analysis of the Internal Structure. Synchrotron SAXS measurements were performed on the various nanoparticle solutions of series 1 and 2 with the purpose to determine their internal structure. For both series, the same sequence of internal structures of the particles was found as the concentration of non-ionic surfactant was increased (presented from low to high C12E5 concentration): hexagonal, hexagonal together with bicontinuous cubic (Ia3d), and finally, bicontinuous cubic. This sequence was also predicted from the phase diagram in Figure 1, which demonstrates the advantage of using macroscopic phase studies when preparing nanostructured particles. Figure 2 presents three SAXS spectra of series 1, depicting the three different structural regimes. Also shown in the figure are the SAXS measurements previously performed on the macroscopic liquid crystalline phases found in the C16TAPA6000/C12E5/H2O system. The Bragg peaks and relative peak positions for panels D−F of Figure 2 are given in Table 1. Because the relative distances between the Bragg peaks are unique for each of these liquid crystalline structures, it is Table 1. Values of the Magnitude of the Scattering Vector (q) and Relative Peak Positions for SAXS Spectra Corresponding to the H1, H1 + V1, and V1 Phases Shown in Panels D−F of Figure 2 q (nm−1) H1 phase

relative peak position

1.36 2.34 2.72

√1 √3 √4

a

q (nm−1) H1 + V1 phase


1.36a 1.39 1.61 2.35a 2.53 2.66 2.78 2.90

√1 √1 √(4/3) √3 √(10/3) √(11/3) √(12/3) √(13/3)

q (nm−1) V1 phase


1.37 1.58 2.23 2.50 2.61 2.73 2.85

√1 √(4/3) √(8/3) √(10/3) √(11/3) √(12/3) √(13/3)

Bragg peaks from the H1 phase. 16540


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Figure 3. Cryo-TEM micrographs of bicontinuous cubic phase nanoparticles composed of PA−6000, C16TA+, and C12E5. Left- and right-hand columns refer to micrographs from the same sample. (A and B) Series 1 (MR = 1.4 and 12.8 mM C12E5). (C and D) Series 2 (MR = 1 and 16.0 mM C12E5). The bar corresponds to 100 nm.

contribution from the particles. In fact, the high value of the typical total scattering intensity (≈500 kHz) demonstrates that it is determined to a large extent by the particles. Moreover, a simple interpolation using data presented in ref 56 reveals that, at 25 °C, the hydrodynamic radius RH of an elongated C12E5 micelle is about 30 nm, which is much smaller than the values estimated for our samples (Figures 5 and 6). The time stability of the internally structured nanoparticles was also investigated, and the experiments gave notable results. DLS measurements of the apparent hydrodynamic radius were performed regularly for 1 month. Figure 5 presents RH obtained from cumulant analysis of the nanoparticles of series 1 (MR = 1.4) as a function of time. As noticed, the radius is constant during this time period. A similar result was obtained for series 2 (MR = 1). The PDI of the particle size estimated from cumulant analysis of the DLS data of both series varied little with time. The PDI values of series 1 vary from 0.15 to 0.23. In the case of series 2, they range between 0.13 and 0.18 for the three lowest concentrations of C12E5, whereas the value is slightly larger for the solution with 16.0 mM C12E5 (0.35). The RH values averaged over almost 30 days, ⟨RH⟩, of the nanoparticles of the two solution series as functions of the

hexagonal morphology (images not included). Also in this case, we were not able to perform FFT analyses, but the SAXS data reported above conclude that the interior of the nanoparticles found in the images consists of a hexagonal phase. 3.3. Particle Size and Stability. A common application of DLS is the determination of the hydrodynamic size of particles from measurements of the translational mutual diffusion coefficient D. For this purpose and also to characterize the polydispersity of the nanoparticles formed in the two solution series, we performed DLS measurements at 25 °C. The measured intensity time correlation functions were close to single exponential, giving monomodal relaxation time distributions obtained from ILT analysis (see the Experimental Section). This demonstrates that the particles have a welldefined size. A typical correlation function of nanoparticles of cubic internal structure is presented in Figure 4A, with its corresponding relaxation time distribution displayed in Figure 4B. It may be observed that the distribution is narrow, which, in turn, indicates a low polydispersity in particle size. Pure C12E5 thread-like micelles are also present in all particle systems (see cryo-TEM images in Figure 3) at this temperature. However, the scattering contribution to the total light scattering from the micelles is overshadowed by the 16541


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Figure 6. Time-averaged hydrodynamic radius (⟨RH⟩) as a function of the C12E5 concentration in millimolar. (Blue symbols) Nanoparticles of series 1 with MR = 1.4 and (red symbols) nanoparticles of series 2 with MR = 1. The time average for series 1 and 2 is calculated over 27 and 28 days, respectively. The error bars (given in ±s) are smaller than the symbol size. The lines are guides to the eye. Measurements were performed at 25 °C and θ = 90°.

the ALV goniometer system at θ = 90°. The hydrodynamic radii of the particles from the second batch were measured using the Malvern instrument at θ = 173° in conjunction with the characterization of the electrophoretic mobility measurements (see below). When comparing the ranges in RH, we find that there is almost no difference between the two batches (although different DLS instruments were used). Data (presented above and in Figure 6) from the ALV goniometer: 122 < RH < 140 nm (series 1) and 88 < RH < 125 nm (series 2) and from the Malvern instrument (see Table 2): 107 < RH < 139 nm (series 1) and 90 < RH < 129 nm (series 2). 3.4. Particle Charge. To determine the net charge of the dispersed liquid crystalline phase nanoparticles of both solution series, electrophoretic mobility (ue) measurements based on the technique of laser Doppler micro-electrophoresis were performed at 25 °C. Table 2 lists the measured electrophoretic mobilities of the nanoparticles of the two series at selected concentrations of C12E5 together with RH measured by DLS with the same instrument 1 day after preparation. The mobility values given in Table 2 imply that the particles carry a net positive charge that does not vary significantly within and between the two sample series and which is likely to be responsible for the long-term stability. It is interesting to note that positive ue values are also found for series 2 (MR = 1). The complexity of the system does not allow for a unique explanation; however, it is possible that a partial hydrolysis of the complex salt (protonation of the polyacrylate anion) takes place. The similarity of the ue values (as well as the internal structure) suggests that this phenomenon is responsible for the particle charge in both series. The pH measurement on two of the solutions from each series shows that the solutions are basic (9.7 and 9.5 for series 1 and 2, respectively), which confirms this hypothesis. Clark et al. list published experimental data of the dissociation degree (α) of PAA at various pH values,57 from which we can make a rough estimate that α of the PAA6000 polymer in the particle systems of series 2 is >0.95 or that a few percent of carboxylic groups are protonated. However, it is impossible to draw a final conclusion from the current data, because further investigation is needed. Our results from the electrophoretic mobility and DLS measurements are in agreement with those of an earlier study on the charge and size properties of complexes formed by different preparation methods in aqueous solutions of hyper-

Figure 4. (A) Intensity correlation function at θ = 90° for an aqueous solution of bicontinuous cubic nanoparticles (series 1, MR = 1.4 and 8.0 mM C12E5). (B) Corresponding relaxation time distribution obtained from ILT of the correlation function in panel A. The single mode is attributed to the translational diffusion of the nanoparticles.

Figure 5. Apparent hydrodynamic radius (RH) of nanoparticles as a function of time. Nanoparticles of series 1 (MR = 1.4 and 0.25 mg mL−1 PAA6000) with different concentrations of C12E5: (black symbols) 1.6 mM, (red symbols) 3.2 mM, (blue symbols) 8.0 mM, and (green symbols) 11.2 mM. The lines correspond to weighted linear regression fits. The error bars [given in ±estimated standard deviation (s) of three measurements] are smaller than the symbol size. Measurements were performed at 25 °C and θ = 90°.

C12E5 amount are presented in Figure 6. We observe that the radius can be considered to be constant with respect to the amount of non-ionic surfactant in the particles. It is only ⟨RH⟩ of the sample with the highest C12E5 concentration that deviates. In the case of particles of series 1, the ⟨RH⟩ values are (with increasing C12E5 concentration from left to right) 130 ± 1, 140 ± 1, 129.1 ± 0.7, and 122.1 ± 0.6 nm, and in the case of particles of series 2, the ⟨RH⟩ values are 122 ± 1, 125 ± 1, 119.5 ± 0.8, and 88 ± 5 nm (errors given as ±s). The low standard deviations reflect that the nanoparticles are extraordinarily stable with time. Furthermore, when comparing the ⟨RH⟩ values of the two series, we find that the particle size does not depend upon the global n(C16TAOH)/n(COOH group) molar ratio (size range of 88−140 nm). Both solution series were prepared twice to test the reproducibility of our preparation method. The hydrodynamic radii of the particles from the first batch were measured using 16542


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Table 2. Measured Electrophoretic Mobilities (ue) and Apparent Hydrodynamic Radii (RH) of Internally Structured Nanoparticles of PA−6000, C16TA+, and C12E5a series 1 C12E5 (mM) 0.32 1.6d 4.8e 6.4e 8.0f 11.2f 12.8f

d

ue (μm cm V−1 s−1)b 4.19 4.9 5.39 5.63 5.73 5.2 4.76

± ± ± ± ± ± ±

0.06 0.3 0.03 0.05 0.03 0.2 0.06

series 2 C12E5 (mM)

RH (nm)c 117 139 131 122 112 108 107

± ± ± ± ± ± ±

3 3 4 8 5 3 3

0.32 1.6d 3.2d 6.4e 9.6e 12.8f 16.0f

d

ue (μm cm V−1 s−1)b 4.55 5.17 5.63 5.66 5.93 4.68 4.69

± ± ± ± ± ± ±

0.09 0.03 0.04 0.03 0.02 0.06 0.05

RH (nm)c 111 120 129 90 92 92 103

± ± ± ± ± ± ±

6 7 9 1 4 2 2

Series 1, MR = 1.4 and 0.25 mg mL−1 PAA6000; series 2, MR = 1 and 0.73 mg mL−1 PAA6000. MR = n(C16TAOH)/n(COOH group). bAverage of three consecutive measurements. cRH from DLS using Malvern instrument (θ = 173°). Errors are ±s of three consecutive measurements. dHexagonal (H1) internal structure. eH1 and cubic (V1, Ia3d) mixed structures. fV1 structure. a

Figure 7. Temperature dependence of SAXS data for samples with MR = 1.4 and 8.0 mM C12E5. SAXS spectra (intensity versus q) of nanoparticle dispersion with temperature scans from (A) 25 °C (same spectrum as in Figure 2C) to (B) 15 °C to (C) 25 °C.

branched PEI and SDS, namely, that, when using the same preparation protocol (even though it is different from ours) and a constant polyion concentration and pH, the charge and apparent hydrodynamic size are independent of the surfactant/ polyelectrolyte ratio in a limited range.58 3.5. Thermoresponsivity of the Bicontinuous Cubic Structure. One sample of 8.0 mM C12E5 and MR = 1.4 that contained nanoparticles with a bicontinuous internal structure was chosen to study the thermoresponsivity of the structure by SAXS measurements at different temperatures. The sample was equilibrated at 25 °C for 4 weeks and was then measured in the sequence: 25, 15, and then back to 25 °C again. Each temperature change was followed by at least 15 min of equilibration before the measurement. For comparison, two unfiltered samples (i.e., containing the macroscopically phaseseparated liquid crystals) of the same composition were prepared and pre-equilibrated at 15 and 25 °C for 2 months, respectively. These were thereafter measured at the equilibration temperatures. In addition, the macroscopic phaseseparated sample equilibrated at 25 °C was measured at 35 °C. The results demonstrated a change of the internal structure of the nanoparticles when the temperature was decreased to 15 °C (Figure 7). The bicontinuous cubic structure of the nanoparticles was recovered when the temperature was raised back to 25 °C. When the macroscopically phase-separated sample was investigated, where more Bragg peaks could be assigned, the structure at 25 °C (Figure 2F) and 35 °C (data not shown) was confirmed to be the V1 (Ia3d space group) phase. Examination of the phase-separated sample equilibrated at 15 °C between cross-polarizers revealed an isotropic phase, which, by means of SAXS, is interpreted as a different bicontinuous phase. It has previously been reported that the transitions between different bicontinuous structures are common and can occur upon small

perturbations, such as changes in the temperature. Figure 8 displays the SAXS spectrum and the fit of the q values of the Bragg peaks to the corresponding Miller indices (hkl) of the Im3m space group.59 More measurements are needed because it is not possible from the current data to determine which type

Figure 8. (A) SAXS spectrum obtained from the macroscopic phaseseparated sample of MR = 1.4 containing 8 mM C12E5 and equilibrated at 15 °C. The inset shows the curve in an enlarged scale. (B) (q/2π)2 calculated from the q values of the Bragg peaks versus the Miller indices (h2 + k2 + l2) for the Im3m space group. 16543


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of IPMS of Im3m symmetry describes this new bicontinuous cubic phase. We can, however, conclude that the system displays a transition from Ia3d to Im3m symmetry upon a temperature decrease from 25 to 15 °C. Nanoparticles with thermoresponsive internal structures have been reported earlier in the literature, where, e.g., the following phase transitions have been observed: reversed bicontinuous cubic (Pn3m space group) via reversed hexagonal to isotropic phase in cubosome systems60 and bicontinuous cubic to disordered structure in a block co-polymer system.21

Engineering, Lund University, Lund, Sweden, for the cryoTEM experiments and P. Štěpánek, Institute of Macromolecular Chemistry, Prague, Czech Republic, for generously proving us with the REPES program. Beamline manager T. Plivelic is acknowledged for assistance at the MAX II SAXS beamline I911-4, MAXIV Laboratory in Lund, Sweden. M. Segad is thanked for providing the program for SAXS data analysis. S. T. Hyde, G. Olofsson, D. Lundberg, and L. Piculell are thanked for fruitful discussions. We are grateful to one of the referees who found a mistake in the notation of phases in the three-phase region of the phase diagram presented in the original publication. K. Schillén acknowledges the Swedish Research Council (VR), the Linnaeus Grant Organizing Molecular Matter (239-2009-6749) through VR, the Faculty of Science, Lund University, for funding synchrotron light and neutron scattering research, The Knut and Alice Wallenberg Foundation for general financial support, and The Crafoord Foundation for funding of the MALVERN instrument. L. Galantini and N. V. Pavel thank Sapienza University of Rome (Project C26A08SZ38).

4. CONCLUSION The results presented in this study show that it is possible to prepare nanoparticles consisting of PA−6000 polyions, C16TA+ surfactant ions, and the non-ionic surfactant C12E5 with either bicontinuous cubic or hexagonal ordered interior. This was achieved by mixing PAA6000, C16TAOH, and C12E5 in water under dilute conditions and, thereafter, dispersing the aggregates through a weak input energy in the form of a sequence of ultrasonication, centrifugation, and filtration at room temperature. Using SAXS, it was found that the internal structure of the nanoparticles formed at different n(C16TAOH)/n(COOH group) molar ratios and with different amounts of C12E5 corresponds perfectly with the macroscopic liquid crystalline phases of the two-phase regions (V1 + L1′ and H1 + L1′) of the C16TAPA6000 complex salt/C12E5/water system. This finding is supported by the cryo-TEM experiments, which showed internally structured particles with faceted morphology. The apparent hydrodynamic radii of the particles do not vary significantly with the C12E5 concentration, and the radius remained constant for approximately 1 month (i.e., within the time frame of our study). They carry a net positive charge because of an excess of C16TA+ surfactant ions, which helps to stabilize the system electrostatically. These particles are also thermoresponsive in terms of their internal structure. As the temperature is reduced, the bicontinuous cubic phase with Ia3d symmetry is changed into what is believed to be another type of bicontinuous cubic structure with the symmetry Im3m, which reverts upon increasing the temperature. Future work should be directed to further explore the macroscopic phase behavior as a function of the temperature to receive a more refined picture of the structures found at low temperatures. To our knowledge, this is the first report of bicontinuous cubic phase particles formed by two surfactants and one polyelectrolyte. We have shown that ternary phase studies of polyion−surfactant ion complex salts provide a powerful tool to understand the formation of different phase structures and can, thus, serve as a guide to design nanoparticles with an internal structure. To be able to prepare internally ordered nanoparticles in a reproducible way is important for various biomedical applications where the particle stability is crucial.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to G. Karlsson, Biomicroscopy Unit, Polymer and Materials Chemistry, Center for Chemistry and Chemical 16544


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Nanoparticles with a Bicontinuous Cubic Internal Structure Formed by

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