CAREER: Understanding the Interactions Between Surfactants and Metallic Nanoparticles Using Molecular Simulation

Nanoparticles are approximately 1000 times smaller than the width of a human hair. Metallic nanoparticles have been found to be useful for many applications, such as for destroying cancer cells in our bodies, enhancing the imaging of human organs, catalyzing chemical reactions to reduce unwanted byproducts, improving the performance of paints and coatings, and fabricating nanoscale electronic devices and photovoltaic cell components.

In each of these cases, nanoparticles of application-specific shape are required. Surfactants (soap-like substances) tend to attach strongly and pack densely on to metal surfaces. This enables the synthesis of metallic nanoparticles of a desired shape when surfactants are attached to specified surface regions of the growing nanocrystal to control the relative growth rates of its facets.

Attached surfactants also influence how these nanoparticles cluster together to form ordered arrangements, a key enabler of nanomanufacturing processes. Currently, there are significant gaps in our understanding of surfactant/metallic nanoparticle interactions that have stymied progress in the envisioned applications. Part of the difficulty lies in the extremely small size of nanoparticles, putting the observation of many of the processes at work out of reach of experimental studies.

Another challenge lies in the sheer number of factors that affect the attachment and packing of surfactants on metallic nanoparticles. In this work, we will use computer models to perform a systematic study of how surfactants of different chemical makeup attach to different faces of metallic nanoparticles. In this project, computer models will probe the behavior of the systems at the length-scale of a molecule, thus providing a detailed understanding of these dynamic processes, making it possible to optimize nanoparticle growth conditions for the desired applications.

This research will be accompanied by a significant educational component. An educational strategy-based computer game will be created for middle and high school students to guide them towards STEM-based fields. Elements of the research will be incorporated in the teaching curriculum, and educational outreach activities will be performed to engage members of the Appalachian region of Ohio.

The overall research objective of this CAREER project is to develop a fundamental understanding of the interactions between metallic nanoparticles (MNPs) and surfactant molecules in aqueous media using molecular simulation. MNPs have increasingly attracted interest owing to their unique electrical, thermal, and optical properties, and have found applications in bioimaging, drug delivery, molecular sensors, nanofabrication, photothermal therapy, and heterogeneous catalysis.

These diverse technological applications often involve the interactions of MNPs with surfactants. Due to their strong affinity towards metal-water interfaces, surfactants adsorb at these interfaces in high-density ordered morphologies. These surface structures are stabilized by hydrophobic interactions between the alkyl tails of surfactants as well as the strong polar head-metal interactions.

Straightforward molecular dynamics simulations have been unsuccessful in studying these morphologies because of the kinetic barriers associated with their formation. In this research, we will employ novel free energy estimation strategies in molecular simulation to study the thermodynamic stability of the adsorption morphologies of surfactants on the facets of MNPs in a range of shapes and sizes.

We will develop rare-event sampling techniques to identify the molecular processes involved in the formation of these morphologies over timescales relevant to nucleation processes. Finally, to understand how the adsorption morphologies affect the interfacial properties of MNPs, we will study surfactant-mediated aggregation of MNPs via molecular simulations.

The PI is invested in several educational objectives that are closely tied to the research. To increase motivation and curiosity among middle and high school students for the STEM fields, the PI plans to develop an educational strategy-based computer game which will require a player to make engineering and economic decisions in simulated scenarios, including the use of surfactants to reduce petroleum pipeline corrosion.

The PI already has developed an initial version of the software and tested it on an initial cohort of students. The PI will work closely with a local high school to enlighten students about the STEM fields through various activities, including engaging the students in STEM-related discussions and inviting them to visit the experimental and computational research groups at Ohio University.

Educational outreach activities will be performed to engage members of the Appalachian region of Ohio. The PI will incorporate aspects of the research stemming from this CAREER program into curriculum development at graduate and undergraduate levels, and will recruit undergraduate students, especially those from underrepresented sections, for research.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.