Molecular Dynamics Simulations of Surfactant Functionalized Nanoparticles in the Vicinity of an Oil/Water Interface R. J. K. Udayana Ranatunga, Robert J. B. Kalescky, Chi-cheng Chiu, and Steven O. Nielsen* Department of Chemistry, UniVersity of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080 ReceiVed: June 10, 2010 The localization of nanoparticles (NPs) at fluid/fluid interfaces has emerged as an effective self-assembly method. To understand the fundamentals of this localization mechanism, it is necessary to quantify the physical behavior of NPs in the vicinity of a fluid interface. Conventional theories treat the NP as a rigid object whose equilibrium position is dictated by the balance of its surface tensions with the two fluids. However, most NPs are functionalized with “soft” organic surface layers which play a large role in determining the shape of the NP. Through molecular dynamics simulations, we show that the functionalizing layer also greatly alters the interfacial behavior of the NP beyond the scope of common theory. Furthermore, we characterize the effect of the surface density of functionalizing molecules on the NP deformability. Our results have implications on the experimental interpretation of NP contact angles and may be useful for future theory development. Introduction Scientific advances in recent decades have uncovered the unique properties of nanoparticles (NPs) and the wide array of materials that may be used to create them. 1,2 In many cases the success of incorporating NPs into industrial applications relies on the ability to control their spatial arrangement. 2-4 In this respect the self-assembly of NPs at a fluid/fluid interface is an elegant organizational method which allows some control over the geometry of the NP superstructure. 4,5 For example, Langmuir trough techniques allow mechanical tuning of the interparticle spacing in these two-dimensional assemblies, 6 as well as their deposition onto solid supports. 7 The study of NPs at interfaces can be decomposed into the energetics of localization at the interface, and the interparticle interactions which arise among localized NPs. In this study we focus on the affinity of individual NPs to adsorb at the interface of two immiscible bulk phases, specifically at the interface between oil and water. A simple treatment of the energetics of particles at fluid/fluid interfaces was proposed by Pieranski, who reformulated Young’s 1805 force balance argument from an interfacial free energy viewpoint. 8 According to this theory, for a spherical particle (p) the free energy change as a function of the particle position (η shown in Figure 1) can be given as where ΔG is the free energy measured relative to the particle being completely immersed in oil (η )-R), R is the radius of the particle, and γ pw , γ po , and γ ow are the particle/water, particle/oil, and oil/ water surface tensions, respectively. Equation 1 suggests that the driving force for adsorption of the particle at an oil/water interface is the removal of unfavorable contact between the two liquids. For a particle of radius R, the oil to water transfer free energy is given by ΔG ofw ) 4πR 2 (γ pw - γ po ) and the equilibrium position of the particle, η 0 , can be extracted from the minimum value of eq 1, giving the well- known Young’s equation In recent years treatments of rigid nanoscale particles at fluid interfaces have incorporated concepts such as line tension, 10,11 the Tolman length, 12 and capillary wave broadening 13 to supplement Young’s theory. In essence these treatments involve lower order terms in R which are neglected by Young’s theory but grow in significance as the particle size decreases. Equation 1 assumes a rigid spherical NP geometry. However, NPs used in experiments are usually surface ligated with a variety of nonrigid functionalizing molecules. 14 Commonly, polymers 15 or short organic molecules 16 coat NP cores in the * To whom correspondence should be addressed, steven.nielsen@ utdallas.edu. ΔG(η) ) 2πR 2 (1 + η/ R)(γ pw - γ po ) - γ ow π(R 2 - η 2 ) (1) -R < η < R Figure 1. Schematic of a surfactant functionalized NP in the vicinity of a fluid-fluid interface. The core is displaced from the interface by η. h b is the NP center to ligand head vector while n ˆ denotes the unit vector normal to the interface. For our studies the surfactant ligand is di(ethylene glycol) dodecyl ether, CH 3 -(CH 2 ) 11 -(OC 2 H 4 O) 2 -H. 9 cos(θ 0 ) ) η 0 / R ) (γ po - γ pw )/ γ ow (2) J. Phys. Chem. C 2010, 114, 12151–12157 12151 10.1021/jp105355y  2010 American Chemical Society Published on Web 06/29/2010