We study experimentally the energy of adsorption, DeltaE, of nanoparticles and microparticles at the oil-water interface by monitoring the decrease of interfacial tension as the particles bind. For citrate-stabilized gold nanoparticles assembling on a droplet of octafluoropentyl acrylate, we find DeltaE = -5.1 k(B)T for particle radius R = 2.5 nm and DeltaE proportional, variant R(2) for larger sizes. Gold nanoparticles with (1-mercaptoundec-11-yl)tetra(ethylene glycol) ligand have a much larger binding energy (DeltaE = -60.4 k(B)T) and an energy barrier against adsorption. For polystyrene spheres with R = 1.05 microm, we find DeltaE = -0.9 x 10(6) k(B)T. We also find that the binding energy depends on the composition of the oil phase and can be tuned by the salt concentration of the nanoparticle suspension. These results will be useful for controlling the assembly of nanoparticles at liquid interfaces, and the method reported here should be broadly useful for quantitative measurements of binding energy.

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Composite materials consisting of a polymer matrix with embedded functional modifiers (like filaments, whiskers, or nanoparticles), have potential use in chemical, biological, optical and microelectronic technology. [1][2][3][4] The properties of such hybrid materials depend on the physical properties and chemical nature of the dispersed and continuous phases, as well as morphology, which can dictate the spatial distribution of the dispersed modifiers. Control over the spatial distribution of nano-objects can be achieved using structure-guiding matrix scaffolds, such as block copolymers. [4][5][6][7][8][9][10][11][12] As an alternate method, besides the in-situ reduction technique, [13][14][15][16] the self-assembly of cadmium selenide, silica, gold and other nanoparticles into two-and three-dimensional ordered structures has been achieved recently, [5][6][7][8][9]11,12,17] led by the theoretical studies of Balazs and coworkers. [18][19][20] Kramer and coworkers [5][6][7][8] used polystyrene ligands to localize gold nanoparticles inside the polystyrene (PS) microdomains of a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer, while mixtures of PS and P2VP ligands directed the nanoparticles to the interface between the PS and P2VP microdomains. Bockstaller, et al. have shown that the location of particles within the same domain is affected by the size of the nanoparticle.[9] Large silica nanoparticles were found to segregate to the center of one domain, whereas smaller gold nanoparticles segregated to the interface between microdomains. Here, we demonstrate a simple approach to control the spatial distributions of gold nanoparticles in a lamellar forming PS-b-P2VP diblock copolymer. Solvent and thermal annealing treatments were then applied to control the specific location of the functionalized gold nanoparticles in the diblock copolymer.Gold nanoparticles have been investigated extensively over the past decade due to their synthetic availability and advanced functionalization chemistry. [21][22][23][24][25][26] For our studies, gold nanoparticles of ca. 2-3 nm diameter were synthesized according to the Brust-Schiffrin method, [27] and functionalized with a mixture of hydrophilic and hydrophobic ligands by ligand exchange. Dodecanethiol-covered gold nanoparticles were agitated in dry dichloromethane, with 11-mercapto-1-undecanol, at room temperature for 48 h. The composition of the ligands on the nanoparticles was controlled by the concentration of 11-mercapto-1-undecanol used in the ligand exchange. The nanoparticles were then concentrated, washed repeatedly with dichloromethane and methanol to remove the unbound ligands, then dispersed in toluene. The final ratio of dodecanethiol to 11-mercapto-1-undecanol was determined by 1 H-NMR spectroscopy integrations. The solution of gold nanoparticles was aged at room temperature and centrifuged before use. The average core diameter of the gold nanoparticles was 2.4 nm, as determined by small-angle X-ray scattering, shown in Figure S1 in the Supporting...