Effect of MacroRAFT Copolymer Adsorption on the Colloidal Stability

Publication Date (Web): November 3, 2015. Copyright © 2015 American .... Layered double hydroxide nanoparticles customization by polyelectrolyte adso...
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Effect of MacroRAFT Copolymer Adsorption on the Colloidal Stability of Layered Double Hydroxide Nanoparticles Marko Pavlovic,† Monika Adok-Sipiczki,† Corinne Nardin,†,‡ Samuel Pearson,§,⊥ Elodie Bourgeat-Lami,§ Vanessa Prevot,*,⊥,# and Istvan Szilagyi*,† †

Department of Inorganic and Analytical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1205 Geneva, Switzerland Institut des Sciences Analytiques et de Physicochimie pour l’Environnement et les Matériaux, Université de Pau et des Pays de l’Adour, 2 Avenue du Président Angot, F-64000 Pau, France § University of Lyon, Univ. Lyon 1, CPE Lyon, CNRS, UMR 5265, Laboratoire de Chimie, Catalyse, Polymères et Procédés (C2P2), LCPP group, 43, Blvd du 11 Novembre 1918, F-69616 Villeurbanne, France ⊥ Université Clermont Auvergne, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10446, F-63000 Clermont-Ferrand, France # CNRS, UMR 6296, F-63178 Aubière, France ‡

S Supporting Information *

ABSTRACT: The colloidal behavior of layered double hydroxide nanoparticles containing Mg2+ and Al3+ ions as intralayer cations and nitrates as counterions (MgAl-NO3LDH) was studied in the presence of a short statistical copolymer of acrylic acid (AA) and butyl acrylate (BA) terminated with 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA) (P(AA7.5-stat-BA7.5)-CTPPA) synthesized by reversible addition−fragmentation chain-transfer (RAFT) polymerization. Surface charge properties and aggregation of the particles were investigated by electrophoresis and dynamic light scattering (DLS), respectively. The negatively charged P(AA7.5-stat-BA7.5)-CTPPA adsorbed strongly on the oppositely charged particles, leading to charge neutralization at the isoelectric point (IEP) and charge reversal at higher copolymer concentrations. The dispersions were unstable, i.e., fast aggregation of the MgAl-NO3-LDH occurred near the IEP while high stability was achieved at higher P(AA7.5-stat-BA7.5)-CTPPA concentrations. Atomic force (AFM) and transmission electron (TEM) microscopy imaging revealed that the platelets preferentially adopted a face-to-face orientation in the aggregates. While the stability of the bare particles was very sensitive to ionic strength, the P(AA7.5-stat-BA7.5)-CTPPA copolymer-coated particles were extremely stable even at high salt levels. Accordingly, the limited colloidal stability of bare MgAl-NO3-LDH dispersions was significantly improved by adding an appropriate amount of P(AA7.5-stat-BA7.5)-CTPPA to the suspension.

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INTRODUCTION Layered double hydroxides (LDHs) are hydrotalcite-type anionic clay materials1,2 that have attracted an enormous amount of attention in recent years due to their tunable properties, ease of synthesis, and promise in extensive applications2,3 including drug delivery,4 catalysis,5,6 wastewater treatment,7 carbon dioxide capture,8 and polymer nanocomposites.9,10 Each layer in the LDH structure is composed of repeating octahedral units of a central metal ion (a mixture of M2+ and M3+) coordinated by hydroxide ions, with anions in the interlayer galleries compensating for the net positive charge arising from the M3+ species. LDH particles are typically in the nanometer size range with a high aspect ratio. Great compositional diversity is accessible in LDHs by altering the nature and the ratio of M2+ and M3+ as well as the type of the intercalating anion(s), and extensive studies have been performed to characterize the resulting structures in the solid state.6,11−13 In contrast, colloidal properties such as surface © XXXX American Chemical Society

charge, stability, and aggregation behavior in aqueous solution have received much less attention, which is surprising given the relatively large number of contributions dealing with surface modification and the colloidal stability of cationic clays,14−16 of which LDHs are considered to be the anionic counterpart. Both the lack of detailed understanding and the importance of colloidal stability for the synthesis and application of LDHbased materials motivated the conducted research. The incorporation of LDH nanoparticles into polymer− inorganic nanocomposites is a thriving area of research due to the enhanced mechanical, thermal, and flammability properties displayed by the resulting materials.9,10 While polymer− inorganic nanocomposites are accessible through a variety of strategies, the relatively new macroRAFT-assisted encapsulating Received: September 7, 2015 Revised: November 3, 2015

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emulsion polymerization (REEP) technique, first developed by the Hawkett group to encapsulate titanium dioxide pigments17 and gibbsite clay platelets18 in polymer latexes, and recently employed to encapsulate cerium dioxide nanoparticles,19 shows great promise for synthesizing polymer−inorganic hybrid latexes containing charged inorganic particles. In the REEP approach, short charged polymers synthesized by RAFT polymerization (referred to as macroRAFT agents) are first adsorbed on the oppositely charged inorganic particle surface, and then emulsion polymerization is conducted under starve− feed conditions to grow a hydrophobic polymer shell around the inorganic particle. In addition to its stabilizing role, the adsorbed macroRAFT agent also enables emulsion polymerization to commence from the particle surface courtesy of the reactivatable RAFT function, thereby generating a uniform polymer shell. Controlling the initial dispersion state of the macroRAFT-modified inorganic particles is crucial to designing composite materials with controlled morphologies, but to date, this topic has not been thoroughly investigated. Because of their relevance to the REEP process, negatively charged macroRAFT agents are prime candidates for investigating polyelectrolyte− LDH interactions and colloidal stability relative to bare LDH. Existing literature reports on the colloidal stability of bare LDHs have revealed that surface charge properties and therefore aggregation behavior are sensitive to the ionic environment. Colloidal stability can be lost through a reduction in the positive surface charge induced by high-pH conditions20,21 or the specific adsorption of anions.22−24 In the latter case, the isoelectric point (IEP) indicates the anion concentration at which the surface charge crosses from positive to negative25 and can vary considerably depending on the nature of the anion.23 To reduce this sensitivity to the ions in solution, polyelectrolytes have been widely used to stabilize charged nanoparticles through strong adsorption on the oppositely charged surfaces,26−31 but the few reports on polyelectrolyte-stabilized LDH nanocomposites do not comprehensively describe the polyelectrolyte effect on stability and aggregation behavior.32−35 Although the colloidal stability of a similar system containing gibbsite particles and macroRAFT polyelectrolytes has already been published,18 the adsorption of macroRAFT agents on LDH particles and its influence on the dispersion stability have not been reported in the literature so far. The present work aims to provide insights into the surface charge properties, colloidal stability, and aggregation propensity of macroRAFT-adsorbed LDH and bare LDH using electrophoretic mobility, light scattering, and microscopy imaging. A nitrate-intercalated Mg2+-Al3+ LDH (denoted as MgAl-NO3LDH) was chosen, since LDHs of such metal ion compositions are the most frequently studied36 and therefore the most relevant for investigating fundamental colloidal properties. The macroRAFT agent (which will also be referred to as the polyelectrolyte) is a short statistical copolymer of acrylic acid (AA) and butyl acrylate (BA) denoted as P(AA7.5-stat-BA7.5)CTPPA (where CTPPA is an abbreviation for the 4-cyano-4thiothiopropylsulfanyl pentanoic acid RAFT function) and is representative of macroRAFT agents typically employed in the REEP process. This systematic study provides relevant insights into the dispersion properties of LDHs which are of further interest in many LDH applications.

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EXPERIMENTAL SECTION

All chemicals were purchased from Sigma-Aldrich and used as received unless otherwise specified. RAFT agent CTPPA was synthesized following a protocol previously reported in the literature.37 Preparation of MgAl-NO3-LDH Nanoparticles. LDHs containing Mg2+ and Al3+ cations and nitrate interlayer anions were prepared by flash coprecipitation followed by hydrothermal treatment.38,39 Typically, a metallic nitrate solution (with a Mg2+/Al3+ molar ratio of 2 and a total metal ion concentration of 0.3 M) was rapidly added to a NaOH (0.185 M) solution at 0 °C. The pH of the resulting solution was adjusted to 9.5, and the sample was transferred to an autoclave and held at 150 °C for 4 h. The resulting particles were collected by centrifugation, and the obtained gel was washed twice with deionized water. Redispersion was achieved by ultrasonication between each washing step. After the final wash, nanoparticles were again redispersed in deionized water and stored as a colloidal suspension of about 10 wt % at room temperature. Synthesis of the P(AA7.5-stat-BA7.5)-CTPPA MacroRAFT Copolymer. In a round-bottomed flask, acrylic acid (AA) and butyl acrylate (BA) as monomers, CTPPA as the RAFT agent, 4,4′-azobis(4cyanopentanoic acid) (ACPA) as the initiator, and 1,3,5-trioxane as the internal standard for nuclear magnetic resonance spectroscopy (NMR) analysis were dissolved in dioxane in a molar ratio of [monomer]/[RAFT]/[initiator] = 20:1:0.1 and a monomer concentration of 6 M. The solution was degassed with nitrogen for 30 min and placed in an oil bath at 80 °C. Samples were taken periodically using a nitrogen-purged syringe to determine conversion by NMR spectroscopy. After 5 h, corresponding to a degree of polymerization of 7.5 for both AA and BA, the solution was quenched in an ice bath. The resulting polymer was purified by three precipitation steps in cold diethyl ether, with redissolution in acetone between each precipitation. The final product was dried overnight in a vacuum oven, resulting in a gel-like yellow material which was analyzed by NMR spectroscopy and size exclusion chromatography (SEC) (Mn,theor = 1800 g/mol, Mn,SEC = 1800, and a dispersity (Đ) of 1.1, where Mn,SEC and Đ were obtained after methylation). Powder X-ray Diffraction. Powder X-ray diffraction patterns were recorded on an X’Pert Pro Philips diffractometer with a diffracted beam graphite monochromator and a Cu Kα radiation source in the 2θ range of 5−70° with a step of 0.013° and a counting time per step of 20 s. Electrophoresis. Electrophoretic mobility measurements were performed with a ZetaNano ZS (Malvern Instruments) device under an electric field of 4 kV/m. The experiments were carried out in plastic capillary cells (Malvern Instruments) cleaned with 2 wt % Hellmanex solution (Hellma) and rinsed with Milli-Q water (Millipore) between each experiment. A stock solution of P(AA7.5-stat-BA7.5)-CTPPA was first prepared by dissolving the polymer in water using KOH to deprotonate the AA units and attain a pH value of 9. Appropriate volume aliquots of this stock solution were then taken to prepare the polymer solutions of different concentrations. The desired ionic strength was attained using KNO3 stock solutions (also adjusted to pH 9) and water to give a final volume of 4.5 mL in each case. To these, 0.5 mL of a 100 mg/L MgAl-NO3-LDH stock dispersion was added, and each solution was left overnight to equilibrate. The samples were also equilibrated at 25 °C in the instrument for 1 min prior to the measurement. The electrophoretic mobility of each sample was measured five times and averaged. Light Scattering. Dynamic light scattering (DLS) measurements were carried out using the ZetaNano ZS (Malvern Instruments) at a 173° scattering angle. This device is equipped with a He/Ne laser operating at 633 nm as a light source and an avalanche photodiode as a detector. The hydrodynamic radius was determined by accumulating the correlation function for 30 s and then applying a second-order cumulant fit.40 The measurements were performed at 25 °C in 1 cm square plastic cuvettes (Malvern Instruments) cleaned with 2 wt % Hellmanex solution. In the time-resolved DLS experiments, typically 50 runs were performed over 25 min. Samples were prepared by mixing P(AA7.5-stat-BA7.5)-CTPPA and KNO3 stock solutions with B

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Langmuir Milli-Q water to give the desired polyelectrolyte concentrations and ionic strengths and a total volume of 1.8 mL. The aggregation experiment was initiated by injecting 0.2 mL of the MgAl-NO3-LDH stock dispersion, which was previously subjected to ultrasound treatment to minimize the initial aggregation. The final particle concentration was 10 mg/L, and the pH value was kept at 9. Such a low particle concentration allowed us to avoid multiple scattering events. In addition, combined static and DLS experiments performed on a multiangle goniometer (ALV/CGS-8, equipped with eight photomultiplier detectors and a solid-state laser (Coherent) operating at a wavelength of 532 nm) revealed that the decay constants of the autocorrelation functions show the typical linear dependence of a zero intercept with the square of the scattering vector. This fact confirms the translational origin of the diffusion coefficient. More details on the light-scattering technique can be found in the Supporting Information (Figures S1 and S2). Microscopy. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 electron microscope (FEI) operating at 120 kV. Samples were prepared by placing 3 μL of the particle dispersions at 10 mg/L LDH on carbon-coated 400-mesh copper grids, leaving them for 30 s, and then draining off the excess liquid with filter paper and allowing them to dry. Atomic force microscopy (AFM) was used to image the particles in solution, in the amplitude modulation mode with a Cypher (Asylum Research) instrument. Biolever minicantilevers (Olympus) with a nominal tip radius smaller than 9 nm and a resonance frequency of 25−36 kHz in water were used. Images were acquired at a scan rate of 0.8 Hz. The positively charged MgAl-NO3-LDH particles were deposited on oppositely charged mica (Plano) which was freshly cleaved in air with scotch tape prior to use. Specifically, the mica surfaces were immersed in an MgAl-NO3-LDH suspension (10 mg/L particle and 100 mM KNO3 concentration) for 5 min. Applying this salt concentration, the particles undergo fast aggregation, and hence the structure of the aggregates can be investigated. The samples were prepared 30 min before performing the AFM analysis.

Figure 1. Powder XRD pattern of MgAl-NO3-LDH synthesized by flash coprecipitation.

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Figure 2. (a) Size distribution of MgAl-NO3-LDH particles measured by TEM in the solid state. (b) Typical TEM image of a MgAl-NO3LDH sample.

RESULTS AND DISCUSSION Detailed studies were performed on the particle behavior in two colloidal systems. First, the surface charge properties and aggregation behavior of bare LDH (i.e., LDH in the absence of copolymer) were investigated as a function of ionic strength. Second, the colloidal stability of LDH particles in the presence of oppositely charged P(AA7.5-stat-BA7.5)-CTPPA was explored under similar conditions. Structure and Colloidal Stability of Bare LDH Particles. The MgAl-NO3-LDH nanoparticles were first characterized in the solid state by X-ray diffraction (XRD) analysis to provide insight into their crystalline structure. The XRD pattern (Figure 1) displays the typical LDH structure reflections which can be indexed with a hexagonal lattice and R3̅m symmetry, confirming the formation of the layered morphology with (i) sharp and intense basal 00l reflections in the low-angle region (2θ < 25°), (ii) broad 0kl reflections in the middle-angle region (2θ = 30−50°), and (iii) hk0 and hkl reflections in the high-angle region (2θ = 55−65°). The 003 and 006 diffraction lines at 10.2 and 20.3°, respectively, correspond to an interlamellar distance of 0.87 nm and are in good agreement with literature values for the nitrateintercalated LDH phase.1 The very minor peak observed at 14.5° corresponds to the formation of a small amount of Mg(OH)2 side phase formed in the flash coprecipitation process due to minor deviation from the ideal pH value of 10 during the initial moment of mixing. The size of the MgAl-NO3-LDH nanoparticles was measured by both TEM and DLS. Applying a Gaussian fit to the size distribution (Figure 2a) obtained by direct counting of the

particles on the TEM images (Figure 2b), an average particle diameter of 112 ± 5 nm was obtained, while DLS yielded a mean hydrodynamic diameter of 180 ± 6 nm with a relatively narrow size distribution indicated by a polydispersity index of 0.15. The uncertainties reported with the size values refer to standard deviations which were obtained from the Gaussian fit to the TEM data and the averaging process from individual DLS measurements, respectively. While these particle size values are not directly comparable (TEM is measured in the dry state, whereas DLS encompasses the solvating layer around the particles and overestimates the contribution from larger particles), the analyses are complementary in assessing the presence of well-dispersed particles with a relatively narrow particle size distribution. Such deviation in particle sizes determined in the dried state and dispersion is frequent.41,42 Since the stability studies were predominantly performed in solution, we consider the hydrodynamic diameter measured by DLS to be more relevant. After confirming the nitrate-intercalated crystal structure and the formation of well-dispersed and relatively uniform particles in solution, surface charge properties of the bare particles were probed by electrophoretic measurements at different ionic strengths (Figure 3a). KNO3 was chosen to adjust the ionic strength to avoid any displacement of the intercalated nitrate counterions in the LDH. The positive electrophoretic mobility observed at low electrolyte concentration decreased with increasing salt concentration and crossed to slightly negative C

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Δfast Δ

(2)

where Δfast and Δ are the initial aggregation rates in 1 M KNO3 solution and in the aforementioned samples, respectively. The initial aggregation rates were calculated from the rate of increase of Rh as Δ= Figure 3. (a) Electrophoretic mobility and (b) stability ratio values of MgAl-NO3-LDH as a function of the ionic strength adjusted with KNO3. The solid lines are only to guide the eyes, and IEP refers to the isoelectric point while CCC is the critical coagulation concentration.

1 dR h(t ) w dt

t→0

(3)

where w is the particle mass concentration and t is the time of the experiment. Since Δfast was measured for a large excess of KNO3, where the aggregation is controlled only by the diffusion of the particles, stability ratios close to unity correspond to unstable systems while higher values indicate slower aggregation and therefore greater stability. Figure 3b shows the stability ratios of MgAl-NO3-LDHs measured at different ionic strengths. Stable dispersions (indicated by high stability ratios) were observed at low electrolyte concentrations, but stability ratios decreased steadily with increasing ionic strength. At a certain ionic strength, defined as the critical coagulation concentration (CCC), the stability ratio reached a value of unity, corresponding to a maximal aggregation rate. In this system, the CCC occurs at 62 ± 5 mM, which is comparable to values reported for other LDHs in the presence of monovalent electrolytes.23 This aggregation behavior can be explained by the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO),45 which states that the interparticle forces in aqueous dispersions containing charged colloidal particles and electrolytes are the superposition of repulsive electric double-layer forces and attractive van der Waals forces. Accordingly, the suspensions are stable at low electrolyte concentration due to the overlap of the double layers and the resulting repulsive forces, while the double layers are screened and such forces therefore vanish at sufficiently high ionic strengths. In the latter case, attractive van der Waals forces predominate in the system, leading to rapid aggregation of the particles. The aggregation of MgAl-NO3-LDH remains fast even at concentrations beyond the IEP (Figure 3a) at which restabilization of the dispersions could be envisaged, as has indeed been reported in other related systems.30 TEM images were recorded to identify the particle−particle orientation in the aggregates (Figure 4a,b). The samples were prepared at 50 mM ionic strength (close to the CCC), and the aggregated samples were imaged in order to obtain information on the structure of MgAl-NO3-LDH aggregates. In general, face-to-face orientation was observed; however, aggregation induced by the drying process during grid preparation could not be excluded. To circumvent the drying effect and gain a representative view of the aggregate structures in solution, the samples were imaged by AFM in the wet state. These results in fact confirmed the TEM observation of primarily face-to-face particle orientation as shown for a representative cluster of four aggregated particles in Figure 4c. The corresponding height profile (Figure 4d) confirms the proposed stacking orientation and shows that the height of the individual particles was about 7 nm. Although edge-to-face aggregation has been reported for negatively charged clay platelets14,15 (and is therefore conceivable in our system), we saw no evidence of such structures. Indeed, a preference for face-to-face aggregation has also been observed in LDHs of other compositions.49

values beyond a KNO3 concentration of approximately 0.7 M, indicating that the nitrate anions adsorb on the LDH surface. Such monovalent counterion-induced charge reversal has already been reported in the literature for LDH23 and other colloidal particles.43,44 Note that a slight charge reversal such as this at high electrolyte concentrations indicates that only a minor fraction of the nitrate anions was adsorbed on the surface and the majority was dissolved in the bulk solution. The screening of the positive LDH surface charge by the anions in solution was therefore the predominant factor contributing to the decreasing electrophoretic mobility with increasing ionic strength, with adsorption having a negligible effect. The screening phenomenon could therefore be further explored by converting the mobilities to potentials (ζ) followed by comparison with theoretical models to extract the surface charge density (σ). The electrokinetic potentials were obtained from the electrophoretic mobilities using the Smoluchowski equation.45 The theoretical dependence of the potentials on the ionic strength was calculated by the Debye−Hückel model as45 σ ζ= εε0κ (1) where ε0 is the permittivity of vacuum, ε is the dielectric constant of water and κ is the inverse Debye length which contains the contribution of all ionic species in the solution. By fitting the theoretical potential values to the experimental ones (Figure S3), σ was found to be +0.011 C/m2. The fitting process was also repeated on the basis of the more accurate Poisson−Boltzmann theory,45 and the same σ was obtained within the experimental error, which is similar to previously reported values (0.005−0.015 C/m2) for LDH particles measured in potentiometric titrations.20 A reliable knowledge of σ can be important for other researchers dealing with the theoretical modeling of surface charges and adsorption processes in the presence of electrolytes. The decreasing mobility with increasing ionic strength was accompanied by a decline in the colloidal stability of the particles. Light-scattering techniques have proven to be powerful tools for investigating particle aggregation in dispersions;30,41,46,47 therefore, time-resolved DLS experiments were performed to further understand the aggregation process in this system. A fixed quantity of LDH dispersion was added to different KNO3 solutions of identical volume but increasing ionic strength. DLS measurements of the particle size commenced immediately after LDH addition. The hydrodynamic radius (Rh) was measured at different time intervals, and the colloidal stability of the dispersions was expressed as a stability ratio (W) defined as48 D

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Figure 4. (a, b) TEM images of MgAl-NO3-LDH recorded with aggregated samples. (c) AFM image of an aggregate of four MgAlNO3-LDH platelets in 100 mM KNO3 electrolyte. (d) Height profile of the aggregate measured along the red line in the AFM image.

LDH−Polyelectrolyte Interactions. With this detailed understanding of bare LDH stability as a function of ionic strength, the influence of adsorbed P(AA7.5-stat-BA7.5)-CTPPA on LDH colloidal stability was then investigated. P(AA7.5-statBA7.5)-CTPPA was synthesized by RAFT (co)polymerization of acrylic acid (AA) and butyl acrylate (BA) using CTPPA as a RAFT agent to achieve a well-defined polymer of narrow molecular weight distribution (dispersity of 1.1) and an average of 7.5 units of each monomer per chain (Figure 5a). Because of the very similar reactivity ratios of the two monomers, they were incorporated statistically with negligible composition drift. After dissolution at pH 9, each copolymer chain possessed on average 8.5 negative charges due to the deprotonation of the 7.5 AA units plus the carboxylic acid end group of CTPPA. First, three series of electrophoretic mobility measurements were performed. In each series, the concentration of MgAlNO3-LDH particles (10 mg/L) and the ionic strength (either 1, 10, or 100 mM) were fixed, and the concentration of P(AA7.5stat-BA7.5)-CTPPA was varied over more than 4 orders of magnitude (Figure 5b). The scattering intensity (44.9 kcps) of the copolymer solution at 3 mg/L concentration (the highest dose used in our study) was very similar to that measured in pure water (29.4 kcps); therefore, micelle or other aggregate formation is considered to be negligible under the experimental conditions applied in the present work. At very low polyelectrolyte concentrations, the electrophoretic mobilities were positive and close to the values obtained for the bare LDH (Figure 3a) because the polyelectrolytes could only partially neutralize the opposite surface charge at these low doses. The mobility values decreased with increasing polyelectrolyte concentration as more copolymer was adsorbed on the oppositely charged platelets. Well-defined IEP values (i.e., polyelectrolyte doses at which electrophoretic mobility reached zero) were observed for all three series. Higher ionic strength reduced the IEP value since IEPs of 36, 32, and 16 mg/g were obtained for ionic strengths of 1, 10, and 100 mM, respectively. Beyond the IEP, charge reversal occurred upon further addition of P(AA7.5-stat-BA7.5)-CTPPA. Such a charge reversal phenomenon has already been reported in the literature for similar particle−polyelectrolyte systems,28−31 including RAFT polymers with platelet-type particles,18 and can be attributed to several factors.25 Although the overall net charge of the particles is zero at the IEP, the surface still contains empty places which

Figure 5. (a) Chemical structure of the P(AA7.5-stat-BA7.5)-CTPPA macroRAFT copolymer. (b) Electrophoretic mobilities and (c) stability ratios of MgAl-NO3-LDH in the presence of P(AA7.5-statBA7.5)-CTPPA polyelectrolyte at different ionic strengths adjusted by KNO3. The mg/g unit of the polyelectrolyte dose refers to milligrams of macroRAFT per gram of MgAl-NO3-LDH.

can be filled in further with polyelectrolyte molecules. Hydrophobic interactions between the chains50 and entropic effects51 such as solvent and counterion release from the charged P(AA7.5-stat-BA7.5)-CTPPA also drive adsorption beyond the IEP. Polyelectrolyte adsorption continued until the onset of the adsorption saturation plateau (ASP), which corresponds to the point where no more macroRAFT can be adsorbed on the surface under the given experimental conditions. An ASP was observed for all ionic strengths used. Beyond this point, electrophoretic mobilities were essentially constant with increasing P(AA7.5-stat-BA7.5)-CTPPA concentration, indicating that additional polyelectrolytes remained dissolved in solution. The electrophoretic mobility absolute values at the plateau did, however, show some ionic strength dependence. Although similar profiles are observed for the 1 and 10 mM systems, the plateau in the 100 mM system corresponds to a significantly lower absolute mobility value due to the more pronounced screening of the negative surface charges by the potassium counterions. E

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Langmuir To understand the dynamics of aggregation in the polyelectrolyte system, time-resolved DLS measurements were conducted to determine stability ratios in a manner similar to that used in the bare LDH experiments. The experimental conditions (pH, temperature, ionic strength, particle concentration, and macroRAFT concentration range) were identical to those used for the electrophoretic mobility study reported above in order to directly relate the aggregation to the surface charging behavior. Figure 5c shows these three series of stability ratio measurements as a function of polyelectrolyte concentration, with each series corresponding to an ionic strength of 1, 10, or 100 mM. At very low polyelectrolyte doses, where only a small fraction of the positive charges on the particles were compensated for by adsorbed macromolecules, the stability of the dispersions was very sensitive to the ionic strength. The 1 mM KNO3 series has very high stability ratios (>1000) at low polyelectrolyte concentration (i.e., below 1 mg/g). For the ionic strength of 100 mM, stability ratios close to unity were determined for polyelectrolyte doses lower than 100 mg/g, as can be expected when one considers the behavior of the bare particles. The 10 mM system shows intermediate stability ratios (around 15 at a polyelectrolyte concentration of 1 mg/g). With increasing P(AA7.5-stat-BA7.5)-CTPPA concentration, the stability ratios in the 1 and 10 mM systems then decline steadily to reach a value of unity at polyelectrolyte doses close to the previously determined IEP values. The stability ratios in all three systems sharply increased upon further increasing the polyelectrolyte concentration to give highly stable dispersions at high doses. The slopes of the stability ratio versus polyelectrolyte dose curves slightly changed with the salt level in this regime, with steeper slopes obtained at lower ionic strength. These stability results highlight important features of the LDH−macroRAFT system. The aggregation behavior at very low doses is in agreement with the stability of the bare particles for similar ionic strengths (Figure 3b). Accordingly, the dispersions are stable in the low salt range (1 mM) due to the predominance of repulsive electric double-layer forces, while at high KNO3 concentrations these forces vanish due to the screening effect of the counterions, and hence attractive van der Waals forces cause fast aggregation as predicted by the DLVO theory. The decrease in the stability ratios at higher copolymer doses can also be explained by the weakening of the double-layer forces because the positive surface charge of MgAl-NO3-LDHs decreases as more negative macroRAFT agent is adsorbed (Figure 5b). Similar to other polyelectrolyte− particle systems,18,28−31 the suspensions were unstable near the IEPs at which the overall charge of the particles was zero. The stability ratios were close to unity in this regime, indicating diffusion-controlled aggregation of the macroRAFT-functionalized platelets. Further increasing polyelectrolyte doses beyond the ASP generated highly stable particles in all three systems due to the reestablishment of the repulsive double-layer forces between the negatively charged surfaces. Qualitatively similar results were also reported with gibbsite platelets in the presence of anionic macroRAFT agents,18 but quantitative comparison to our results is difficult due to the different composition of the particles and polyelectrolytes. The sensitivity of the polyelectrolyte-coated particles to ionic strength was further explored at even higher ionic strength, and was compared to the behavior of the bare particles. Figure 6 shows time-resolved DLS measurements carried out with bare and P(AA7.5-stat-BA7.5)-CTPPA-coated particles at elevated

Figure 6. Time-resolved DLS measurements with bare (empty symbols) and P(AA7.5-stat-BA7.5)-CTPPA-coated MgAl-NO3-LDH (at a polyelectrolyte dose of 300 mg/g, filled symbols) for two ionic strengths (1.0 and 1.7 M adjusted by KNO3). Note that the different intercepts are due to the fast aggregation of bare MgAl-NO3-LDH in the short time period between when the samples were prepared and when the DLS measurements were started.

KNO3 concentrations (1.0 and 1.7 M) at which one would expect unstable dispersions. It is clearly evident that the bare particles rapidly aggregated at these ionic strengths, which were higher than their CCC value, while the hydrodynamic radius of the macroRAFT-coated platelets remained unchanged throughout the measurement period. An average hydrodynamic radius of 85 ± 3 nm was obtained from the experiment carried out at 1.7 M, which is in good agreement with the hydrodynamic radius of the bare particles (90 ± 5 nm) in stable samples. The remarkable stability of the macroRAFT-coated LDH even at extremely high ionic strengths contrasts with that of other polyelectrolyte-stabilized systems, which have been reported to undergo fast aggregation under similar conditions.28 The enhanced stability can be rationalized by considering the polyelectrolyte adsorption characteristics. A dose of 300 mg/g (i.e., milligrams of polyelectrolyte per gram of LDH particles) applied to coat the MgAl-NO3-LDHs roughly corresponds to a polyelectrolyte surface density of 1.7 mg/m2 (calculations in the Supporting Information). While lower than the grafting density of polymer brushes (10−20 mg/m2),52 this value is higher than those reported in the literature for the adsorption of oppositely charged polyelectrolytes on latex,53 magnetite,54 or silica31 particles. Such a higher adsorbed amount compared to that for simple polyelectrolytes is most likely enabled by the copolymer structure (Figure 5a), in which the randomly distributed butyl chains enhance adsorption by promoting hydrophobic affinity with previously adsorbed copolymer chains. To further probe the effect of butyl chains on the adsorption process, the onset of the ASP was compared with the value for pure PAA of the same molecular mass under identical conditions (Figure S4). It can be clearly seen that in the presence of butyl chains, the onset of the ASP is significantly shifted toward higher polyelectrolyte doses compared to those for the PAA samples. This result unambiguously confirms that the butyl groups enhance the adsorption most probably due to hydrophobic interactions between the adsorbed P(AA7.5-stat-BA7.5)-CTPPA chains. Even if a part of the macroRAFT agent may have been intercalated in the LDH structure, we suspect that most of the polyelectrolyte was adsorbed on the outer surface of the particles, leading to a F

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CONCLUSIONS The surface charge properties and aggregation behavior of MgAl-NO3-LDH nanoparticles as a function of ionic strength were studied in both the absence and the presence of an oppositely charged P(AA7.5-stat-BA7.5)-CTPPA polyelectrolyte using electrophoresis, light scattering, and microscopy analysis. The colloidal stability of the bare particles could be explained by classical DLVO theory, with the specific adsorption of nitrate counterions also observed. The macroRAFT copolymer adsorbed on the oppositely charged MgAl-NO3-LDH surface led to charge neutralization at the IEP and subsequent charge reversal at higher doses. The speed of aggregation depended strongly on the ionic strength at low doses, while unstable systems were observed near the IEPs at all salt levels investigated. Highly stable dispersions were obtained at elevated macroRAFT concentrations at which the particles were coated with the copolymers. No aggregation could be detected even at high ionic strength in such samples, indicating a high surface density of the molecules whose adsorption was enhanced by the butyl groups of the copolymers which have a high affinity for previously adsorbed chains on the LDH surface. Face-to-face orientation of the MgAl-NO3-LDH platelets was found in the aggregates in both cases, i.e., in the absence or presence of copolymer at low doses. In conclusion, the P(AA7.5-stat-BA7.5)-CTPPA macroRAFT copolymer was found to be a powerful agent for tuning the colloidal stability of the MgAl-NO3-LDH dispersions and can be applied to formulate similar LDH samples for further applications wherever highly stable dispersions are desirable.

huge charge reversal (more than 100% in magnitude) at appropriately high doses. TEM analysis was performed on two of the samples from the 1 mM KNO3 series (Figure 5c) to visualize the aggregates formed in the polyelectrolyte system. Only low-ionic-strength samples were suitable for analysis by TEM because higher levels of KNO3 led to its crystallization during the drying of the TEM grids, which prevented clear observation of the LDH aggregates (Figure S5). Polyelectrolyte loading of 50 mg/g (Figure 7a)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03372.

Figure 7. TEM images of MgAl-NO3-LDH recorded with (a) aggregated and (b) stable samples in the presence of P(AA7.5-statBA7.5)-CTPPA copolymer of 50 and 300 mg/g. The first dose (a) corresponds to the IEP, while the latter one (b) is close to the ASP where the particles are coated with the polyelectrolyte.

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was compared to 300 mg/g (Figure 7b). Two key features are evident from these TEM images. First, aggregation was clearly promoted at the lower polyelectrolyte concentration. Second, the structure of the aggregates, i.e., the orientation of the platelets in the clusters, was the same as for bare particles (Figure 4) under the experimental conditions used. On the basis of the stability studies presented in this work, the interparticle forces involved in the aggregation of LDH can be assumed to be DLVO-type both in the absence and in the presence of polyelectrolytes. The marked influence of ionic strength on the gradient of the Figure 5c curves (depicting the stability ratio versus the polyelectrolyte dose) indicates such a DLVO-type stabilization. These findings are in good agreement with those for polyelectrolyte-stabilized latex particle systems.27 No evidence was found for other non-DLVO interparticles forces such as steric repulsion 15,31,54 or patch-charge attraction,29,55 which have also been reported to occur in certain polyelectrolyte−particle systems. It can therefore be concluded that the surface charge distribution is uniform for bare and polyelectrolyte-adsorbed LDH and that the adsorption induces a higher surface charge density in magnitude and hence a higher stabilization effect of electrostatic origin.

Detailed information and results of light scattering, electrophoretic and TEM measurements (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

Financial support from the Swiss National Science Foundation (150162), the Swiss Scientific Exchange Program (SCIEX14033), the Swiss Secretariat for Education, Research and Innovation (C15.0024), and COST Actions MP1106 and CM1303 is gratefully acknowledged. M.P. and I.S. are grateful to Professor Michal Borkovec for access to the instruments in his laboratory. We also thank Mr. Olivier Vassalli for technical support during the DLS experiments. S.P., E.B.-L. and V.P. acknowledge financial support from ANR-11-JS08-0013. G

DOI: 10.1021/acs.langmuir.5b03372 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b03372 Langmuir XXXX, XXX, XXX−XXX