Article pubs.acs.org/Langmuir
Hydrophobically Modified Halloysite Nanotubes as Reverse Micelles for Water-in-Oil Emulsion Giuseppe Cavallaro, Giuseppe Lazzara,* Stefana Milioto, and Filippo Parisi Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze, pad. 17, 90128 Palermo, Italy S Supporting Information *
ABSTRACT: An easy strategy to obtain inorganic reverse micelles based on halloysite nanotubes (HNTs) and alkyltrimethylammonium bromides has been developed. The selective modification of the HNTs external surface with cationic surfactants endows to generate tubular nanostructures with a hydrophobic shell and a hydrophilic cavity. The influence of the surfactants alkyl chain on the HNTs functionalization degree has been investigated. The dynamic behavior of the surfactant/HNT hybrids in solvents with variable polarity has been correlated to their affinity toward hydrophobic media explored through partition experiments. The water-in-oil emulsion is able to solubilize copper sulfate, proving the incorporation and the loading of hydrophilic compounds into the HNTs lumen. Here we have fabricated ecocompatible reverse micelles with tunable hydrophobic/ hydrophilic interface that might be suitable for industrial and biological applications as well as for selective organic synthesis.
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ferrocene.22 The deposition of TiO2 onto HNTs platform endows to the synthesis of supports with excellent photocatalytic activity and adsorptivity.23 Coating halloysite with polydopamine generated an efficient nanocarrier for enzyme immobilization.24 Here we propose a new strategy for designing inorganic reverse micelles inspired by halloysite surface chemistry. Generally, a “reverse micelle” structure possesses a hydrophilic core and a hydrophobic shell that delineates a nanoscale droplet of aqueous phase from a nonpolar medium.25 The confinement of water is useful in numerous applications, such as nanoparticle synthesis26,27 and enhancement of chemical reaction rates.28,29 As reported for anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT),30,31 certain amphiphilic molecules in nonpolar solvents can self-assemble in reverse micelle systems above a critical concentration. Recently, inorganic reverse micelles were successfully prepared by silicabased core−shell nanostructures.32 Copper nanorods were obtained from copper sulfate reduction in alkyltrimethylammonium bromides aqueous solution.33 In our research we prepared hybrid nanomaterials based on HNTs and alkyltrimethylammonium bromides. Because of their positive headgroup, the surfactants are selectively adsorbed onto the HNTs external surface endowing to the formation of tubular nanoparticles with a hydrophobic shell and a hydrophilic cavity. We systematically changed the alkyl chain length of the surfactants in order to investigate the effect of the hydrophilic/hydrophobic balance on the properties of the obtained hybrid materials. The
INTRODUCTION Halloysite clay is a very promising nanomaterial because of its versatile properties, such as hollow tubular morphology, large specific area, and tunable surface chemistry. In the field of catalysis, HNTs could be used as supports1 or as catalysts themselves,2 so showing potential functionalities which resemble those of pristine or functionalized carbon nanotubes.3−5 In addition, HNTs are biocompatible, nontoxic, and abundantly available at low cost. Because of these characteristics, HNTs are suitable for development of hybrid sustainable materials perspective for wastewater remediation,6−8 smart coating,2,9 green packaging,10−13 and pharmaceutical applications.14−17 HNTs are quite polydisperse in size with a length of ca. 1 μm while the external diameter and the lumen range between 50−80 nm and 10−15 nm, respectively.18 Chemically, halloysite is composed of gibbsite octahedral sheet (Al−OH) groups on the inner surface and siloxane (Si−O−Si) groups on the external surface. This different chemistry determines a positively charged lumen and a negatively charged outer surface in the 2−8 pH interval.18 Such a peculiarity influences the liquid crystalline behavior of aqueous halloysite dispersions.19 Interestingly, the selective modification of HNTs surfaces can be easily achieved by using ionic surfactants.20 It was demonstrated that the adsorption of sodium alkanoates onto HNTs lumen generates tubular inorganic micelles with efficient solubilization ability toward hydrocarbon compounds as well as good aqueous colloidal stability.7 HNTs functionalized with dioctyl sulfosuccinate sodium salt are effective in stabilizing oilin-water emulsions,21 whereas covalent modification of HNTs lumen with octadecylphosphonic acid enhanced the halloysite adsorption capacity toward hydrophobic derivatives of © XXXX American Chemical Society
Received: March 31, 2015 Revised: June 26, 2015
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DOI: 10.1021/acs.langmuir.5b01181 Langmuir XXXX, XXX, XXX−XXX
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20 kV, and the working distance was 10 mm. Minimal electron dose condition was set to avoid damage of the sample. Dynamic light scattering (DLS) and ζ-potential measurements were carried out by means of a Zetasizer NANO-ZS (Malvern Instruments) at 25.0 ± 0.1 °C. As concerns DLS studies, water, octanol, chloroform, and hexane were selected as solvents. The concentration of the dispersions was 10−3 wt %. Because of the very low stability, the prepared dispersions in hexane was not studied by DLS. This result indicates that a certain solvent polarity is needed to disperse the modified HNTs that are not completely hydrophobic as the external surface is not saturated. The field-time autocorrelation functions were analyzed by cumulants analysis, which provides the decay rate (Γ) of the diffusive mode. For the translational motion, the collective diffusion coefficient is Dt = Γ/q2, where q is the scattering vector given by 4πnλ−1 sin(θ/2) being n the solvent refractive index, λ the wavelength (632.8 nm), and θ the scattering angle (173°). Contact angle measurements were performed by using an optical contact angle apparatus (OCA 20, Data Physics Instruments). To reduce the roughness effect, the powder-like material was pressed under 8 × 103 kg cm−2 for 10 min to obtain a tablet. The contact angle was measured just after water deposition unto the substrate. The water droplet volume was 6.0 ± 0.5 μL. Temperature was set at 25.0 ± 0.1 °C for the support and the injecting syringe as well. Five measurements at least were carried out on each sample. FTIR spectra were determined at room temperature using a Frontier FTIR spectrometer (PerkinElmer). The spectral resolution was 2 cm−1. TEM micrographs were acquired with a Jeol JEM 2100 microscope operating at 200 kV. A drop of each dispersion had been deposited in a 3 mm nickel grid holey carbon coated (Taab). The grid was dried overnight before observation. No coating was applied for TEM observation. Partition Experiments. Water/chloroform partition experiments were conducted through the addition of 0.1 g of the investigated materials in the biphasic system composed of 5 cm3 of both solvents. The dispersions were kept under magnetic stirring at 1250 rpm for ca. 30 min. After 1 h of equilibration, the amount of nanoparticles suspended in both solvents was estimated through the gravimetric method.
colloidal stability of the functionalized HNTs was studied in solvents with different polarity (water, 1-octanol, chloroform, and hexane). Moreover, water/chloroform partition experiments were conducted with the aim to investigate the affinity of both pristine and modified HNTs toward a nonpolar medium. Finally, we demonstrated that modified HNTs dispersed in chloroform are able to encapsulate hydrophilic compounds into their lumen. Such capacity proves that the hydrophobization of halloysite outer surface drives to the development of efficient reverse micelles that can be appropriate for several technological purposes.
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EXPERIMENTAL SECTION
Materials. Halloysite nanotubes (HNTs), decyltrimethylammonium bromide (C10Br), tetradecyltrimethylammonium bromide (C14Br), hexadecyltrimethylammonium bromide (C16Br), hexane, chloroform, 1-octanol, and copper sulfate pentahydrate (CuSO4· 5H2O) are from Sigma. All the products were used without further purification. Water from reverse osmosis was used. Modification of HNTs. Aqueous surfactant solutions were prepared by dissolving 2 g of alkyltrimethylammonium bromide (C10Br, C14Br, or C16Br) in 250 cm3 of water. Then, 4 g of HNTs was added, and the obtained dispersion was stirred for 48 h. Successively, the dispersion was centrifuged to recover the functionalized material. The latter was washed with water several times until the presence of bromide was not detected in the washing water by AgNO3 addition. This procedure ensures that eventual free surfactant is negligible. The obtained powder was dried at 80 °C for 1 week. Encapsulation of Copper Sulfate into Modified HNTs. A dispersion of the C16Br/HNTs composite in chloroform was prepared by dissolving 0.2 g of the modified nanotubes into 60 cm3 of solvent. Then, the dispersion was mixed with a saturated aqueous solution of copper sulfate at 25 °C. The volume ratio between the chloroform dispersion and the aqueous solution was 3:1. The biphasic system was stirred for ca. 10 h. Figure 1 shows the separated phases.
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RESULTS AND DISCUSSION The silica external surface of HNTs was selectively modified with alkyltrimethylammonium bromide surfactants, which possess a positive headgroup, allowing the formation of tubular nanostructures with a large specific area as well as controllable hydrophobic/hydrophilic interface (Figure 2). First, the
Figure 1. Photo of the biphasic system composed of saturated aqueous phase (top) of copper sulfate and chloroform C16Br/HNTs dispersion (bottom) after 10 h of stirring. Finally, an aliquot (ca. 20 cm3) of the chloroform dispersion was evaporated in order to recover the hybrid material loaded with copper sulfate. Methods. The thermogravimetric (TG) analyses were performed by using a Q5000 IR apparatus (TA Instruments) under nitrogen flow of 25 cm3 min−1 for the sample and 10 cm3 min−1 for the balance. The explored temperature interval ranged between 25 and 900 °C at a heating rate of 20 °C min−1. The surface morphology of the prepared materials was studied by using a microscope ESEM FEI QUANTA 200F coupled with energy dispersive X-ray spectrometry (EDX) that endows to the elemental analysis. Before each SEM experiment, the surface of the sample was coated with gold in argon by means of an Edwards sputter coater S150A to avoid charging under electron beam. The measurements were carried out in high vacuum mode (
