Article pubs.acs.org/Langmuir
Antagonistic Effects between Magnetite Nanoparticles and a Hydrophobic Surfactant in Highly Concentrated Pickering Emulsions Alejandro Vílchez,† Carlos Rodríguez-Abreu,*,‡ Angelika Menner,§ Alexander Bismarck,§ and Jordi Esquena*,† †
Institute for Advanced Chemistry of Catalonia, Spanish National Research Council (IQAC-CSIC) and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona 18-26, 08034 Barcelona, Spain ‡ International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga, 4715-330, Braga, Portugal § Department of Material Chemistry, University of Vienna, Waehringer Strasse 42, A-1090 Vienna, Austria S Supporting Information *
ABSTRACT: Herein we present a systematic study of the antagonistic interaction between magnetite nanoparticles (Fe3O4) and nonionic hydrophobic surfactant in Pickering highly concentrated emulsions. Interfacial tension measurements, phase behavior, and emulsion stability studies, combined with electron microscopy observations in polymerized systems and magnetometry, are used to support the discussion. First, stable W/O highly concentrated emulsions were obtained using partially hydrophobized magnetite nanoparticles. These emulsions experienced phase separation when surfactant is added at concentrations as low as 0.05 wt %. Such phase separation arises from the preferential affinity of the surfactant for the nanoparticle surfaces, which remarkably enhances their hydrophobicity, leading to a gradual desorption of nanoparticles from the interface. W/O emulsions were obtained at higher surfactant concentrations, but in this case, these emulsions were mainly stabilized by surfactant molecules. Therefore, stable emulsions could be prepared in two separate ranges of surfactant concentrations. After polymerization, low-density macroporous polymers were obtained, and the adsorption and aggregation of nanoparticles was analyzed by transmission electron microscopy. The progressive displacement of the nanoparticles was revealed: from the oil−water interface, in which aggregated nanoparticles were adsorbed, forming dense layers, to the continuous phase of the emulsions, where small nanoparticle aggregates were randomly dispersed. Interestingly, the results also show that the blocking temperature of the iron oxide superparamagnetic nanoparticles embedded in the macroporous polymers could be modulated by appropriate control of the concentrations of both surfactant and nanoparticles.
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ization in the continuous phase of such emulsions.9 Highly concentrated emulsions (also denoted high internal phase emulsions, abbreviated as HIPEs) have an internal phase volume fraction higher than 0.74, which is the maximum compact packing ratio for monodisperse, spherical droplets.10 The selection of appropriate particles for the stabilization of Pickering emulsions offers a versatile approach to creating new functional nanocomposite polymers since the nanoparticles remain embedded within the materials after the polymerization of the continuous phase of the emulsions.11−14 Unfortunately, the use of Pickering emulsions generally shows some drawbacks, giving rise to materials with closed-cell structure and possessing wide pore size distributions.15 An existing strategy to improve the properties of polyHIPE materials is to use both particles and surfactants simultaneously.16−20 Although studies on this topic in diluted or concentrated emulsions have increased in the last few
INTRODUCTION It is well known that solid particles (covering the nano to micro range) can adsorb at interfaces and consequently stabilize emulsions.1,2 Such emulsions are known in the literature as Pickering emulsions and play an important role in numerous industrial applications.3 Particles, unlike surfactants, do not (at least significantly) reduce the free energy of a liquid−liquid interface by reducing the interfacial tension;4−6 the surface energy can be lowered simply because the area of such an interface is reduced when particles are adsorbed.7 In early works, it was well stablished that the phase that preferentially wets the particles becomes the continuous phase of Pickering emulsions.8 Moreover, it is also well known that in order to achieve efficient emulsion stabilization, the hydrophilic and lipophilic properties of the particles surfaces must be balanced. This occurs with particles of intermediate wettability, i.e., for contact angles with respect to the oil−water interface close to 90°. A field that recently has generated considerable interest is the use of highly concentrated Pickering emulsions for the preparation of macroporous polymers, obtained by polymer© 2014 American Chemical Society
Received: February 24, 2014 Revised: April 14, 2014 Published: April 16, 2014 5064
dx.doi.org/10.1021/la4034518 | Langmuir 2014, 30, 5064−5074
Langmuir
Article
years,5,21−23 there is a lack of systematic study to investigate the possible interactions between both emulsifiers and how these interactions might influence emulsion stability and nanoparticle spatial distribution into the continuous phase of the HIPEs. In this context, interactions between particles and surfactants with respect to emulsion stability can be classified into two types, namely, synergistic or antagonistic. Often in particle-stabilized emulsions, surfactant molecules are added to enhance the particle interfacial adsorption, increasing the emulsion stability.24,25 This is typically observed when particle adsorption at the interface is induced by modifying its wettability.26 For instance, Binks and co-workers observed synergistic stabilization when low concentrations of hydrophilic surfactant SDS24 or CTAB5 (below the CMC) was mixed with positively or negatively charged SiO2 nanoparticles, respectively. The synergistic effect has also been described with mixtures of hydrophilic particles (e.g., Laponite, clay) and hydrophobic surfactants (e.g., Span 80).21,25 Besides adsorption, the addition of surfactant at small concentrations (below CMC) can also contribute to lower the interfacial tension, thereby facilitating the emulsification and allowing both surfactant and particles to adsorb simultaneously at the oil−water interface. This can impart long-term emulsion stability.22 On the contrary, antagonistic effects can take place when both particles and surfactants compete each other for adsorption at liquid−liquid interfaces. Generally, surfactants adsorb in preference to particles; consequently, surfactant concentrations larger than the CMC can produce the desorption of particles from the interface.27,28 Pichot et al.22 studied systems stabilized with mixtures of silica nanoparticles and both hydrophilic and hydrophobic surfactants. Due to stronger competition for adsorption, they observed particle release from the interface in O/W emulsion only when hydrophilic surfactants were used. In contrast, Pickering stabilization was maintained when a hydrophobic surfactant was incorporated. Therefore, a transformation from a Pickering O/W emulsion to a surfactant-stabilized O/W emulsion was observed. In this work we evaluate the interactions between oleic acid surface-modified magnetite nanoparticles and hydrophobic surfactants in W/O HIPEs and how these interactions affect emulsion stability. Two types of magnetite nanoparticles with different sizes and different oleic acid contents have been chosen. Magnetic nanoparticles show multiple potential applications in different fields such as water decontamination29 and hyperthermia processes.30 Moreover, we present a detailed characterization of the nanoparticle spatial distribution after polymerization of the emulsion continuous phase carried out by transmission electron microscopy. Since adsorption and aggregation of the small superparamagnetic nanoparticles could be tuned by varying the surfactant concentration, the magnetic response of the resultant macroporous polymers has also been investigated.
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(hydrophilic-lipophilic balance number = 4.9) was kindly supplied by Croda (USA). This surfactant is a blend of sorbitan ester and a polyisobutylene succinic anhydride (PIBSA) alkyl derivative. The polymerization initiator was α,α′-azoisobutyronitrile, denoted as AIBN (≥96%, Merck). Pure ethanol (≥99.5%) and toluene (≥99.5%) were supplied by Merck. Milli-Q water was used in all experiments. Preparation, Functionalization and Characterization of Iron Oxide Nanoparticles. Two types of magnetite (Fe3O4) nanoparticles have been employed in this work. Nanoparticles are abbreviated hereafter as NPx, where x is the average size of the nanoparticles. NP8 coated with 12.4 wt % oleic acid was prepared using a chemical ́ et al.31 More details coprecipitation method, as reported by Ramirez are given in the Supporting Information (Figure S1). Additionally, nanoparticles with a 32 nm average size (Fe3O4 nanopowder
