Article pubs.acs.org/Langmuir
Effects on the Self-Assembly of n‑Alkane/Gold Nanoparticle Mixtures Spread at the Air−Water Interface Brandon P. Gagnon and M.-Vicki Meli* Department of Chemistry and Biochemistry, Mount Allison University, 63C York Street, Sackville NB, Canada S Supporting Information *
ABSTRACT: Nanoparticle films formed at the air−water interface readily form rigid films, where the nanoparticles irreversibly associate into floating “islands”, often riddled with voids and defects, upon solvent evaporation. Improving the nanoparticle mobility in these films is key to achieving control over the nanoparticle packing parameters, which is attractive for a variety of applications. In this study, a variety of n-alkanes were mixed with tetradecanethiol-capped 2 nm gold nanoparticles and studied as Langmuir films at 18 and 32 °C. Pressure−area isotherms at 18 °C reveal a mixed liquid-expanded phase of nanoparticles and alkane at the air−water interface, but only for nalkanes that are equal to or exceed the nanoparticle capping ligand in carbon chain length. Transmission electron microscopy images of the corresponding films suggest that the nanoparticles are mixed with a continuous hydrocarbon phase at 0 mN/m and that the hydrocarbon is squeezed out of the nanoparticle film during compression.
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together to form a contiguous film. The order and morphology of these films corresponds to a number of conditions, including core size, for which larger (>4 nm) cores form closely packed films whereas smaller cores (2 to 3 nm) form a variety of morphologies.12,13 Compression of the films, in some cases, leads to the correction of small defects;14 however, interparticle spacing is unaffected.8,15 Other properties of these films are susceptible to changes in the experimental conditions, such as the spreading solvent9−11 and temperature.12,13 Previous studies of lipid films at air−water and oil−water interfaces and air−water interfaces in contact with n-alkane lenses have shown that hydrocarbons that would normally dewet the air−water interface can, in fact, mix into liquid expanded (LE) and, to a lesser degree, liquid condensed (LC) phases of lipid films to form mixed LE phases.7,16 In other words, adequate mixing leads to the incorporation of the hydrocarbon into the lipid film and results in a more liquidlike film. The compression of these films is then mediated by the presence of the alkane and the interaction of the mixture with water, instead of the lipid properties alone. In effect, these previous studies imply that the introduction of certain hydrocarbons could have a plasticizing effect on what might otherwise be considered glassy, rigid films. In this study, we therefore examine the ability of n-alkanes (denoted Cn, where n = 10−16) to mix into nanoparticle films spread at the air−water interface and affect the nanoparticle self-assembly.
INTRODUCTION Studies on interfaces containing oil, water, and nanoparticles are of interest for their potential to inform industrially relevant processes such as the stabilization of microemulsions with particles (i.e., Pickering emulsions)1−3 and the formation of novel materials such as nanoparticle 2D and 3D lattices.4,5 The addition of hydrocarbons to the air−water interface has also been studied to understand their effect on lipid monolayers at the air−water interface.6,7 Such studies have shown that film and superlattice formation relies not only on the interaction between particles but also on the interfacial forces involved. The addition of a soluble component to the system, such as excess alkanethiol,5,8 can alter both the interfacial and interparticle interactions, resulting in a change in film formation. However, a recent study by Lohman et al. shows that despite generally following the classic “like-dissolves-like” principal, the solubility of nanoparticles in n-alkanes has distinct peculiarities compared to simpler solutes, such as a strong preference to mix with n-alkanes of matching chain length to the nanoparticle n-alkanethiol ligand.4 Clearly, much remains to be understood in such systems where nanoparticle mixing with secondary components is a key process. In this study, we mix nalkanes with n-alkanethiol-capped gold nanoparticles and form Langmuir films of these mixtures at the air−water interface in an attempt to affect the mobility and morphology of the nanoparticle film. Nanoparticles, when deposited in solvent onto the air−water interface, readily form islands whose morphology is frozen during the initial spreading and evaporation of solvent.8−11 As the film is compressed, the initially formed islands are brought © 2013 American Chemical Society
Received: October 1, 2013 Revised: December 9, 2013 Published: December 20, 2013 179
dx.doi.org/10.1021/la4037937 | Langmuir 2014, 30, 179−185
Langmuir
Article
the carbon side of the grid. Imaging was performed on a JEOL 2011 scanning transmission electron microscope operating at 200 keV several days after film transfer. Images were collected on a Gatan 4K × 4k Ultrascan digital camera. The nanoparticle core size and overall film morphology were determined using the TEM images. Despite the delay between preparation and analysis and the possible influence of the substrate (grid), atomic force microscopy images, obtained soon after film preparation, revealed consistent morphologies (Supporting Information). Measurements from the images were made using Image Pro Plus software (MediaCybernetics Inc.).
Surface pressure−area isotherms are used to determine the extent that the film properties are affected, and transmission electron microscopy (TEM) is used to assess the resulting film morphology.
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MATERIALS AND METHODS
Materials. Gold(III) chloride trihydrate(99.9+%), sodium borohydride (reagent grade), tetraoctylammonium bromide (98%), chloroform (HPLC grade), tetradecanethiol (reagent grade), and n-alkanes (CnH2n+2, n = 10, 12−16; ≥99%) were purchased from Sigma-Aldrich Canada. Toluene (99.5%) was purchased from ACP Chemicals Inc., and benzene (99%) was obtained from Caledon Laboratory Chemicals. Anhydrous ethanol was obtained from Commercial Alcohols Canada. All chemicals were used as received. An Elga Purelab UHQ filtration system was used for the deionization of distilled water with 18.2 MΩ resistivity. Four hundred mesh copper TEM substrates, coated with carbon and Formvar, were purchased from Ladd Research Industries. Gold Nanoparticle Synthesis. Brust−Schiffrin two-phase synthesis was used.17 Aqueous HAuCl4·3H2O (0.03 M, 30 mL) solution was prepared, to which a toluene solution of tetraoctylammonium bromide (0.05 M, 80 mL) was added. The solution was stirred until the aqueous phase remained colorless while the toluene phase turned a burgundy color. To this solution, 0.84 mmol of tetradecanethiol was added and left to stir for 3 h. A fresh solution of NaBH4 (0.4 M, 25 mL) was added dropwise at a rate of 3 drops per second. The solution turned dark brown and was stirred for an additional 3 h. The organic phase was separated, concentrated to
