(2ff) Chemically Informed Theoretical Models and Simulation Techniques to Characterize Interfacial Phenomenon

I am currently a postdoctoral research associate in Prof. Zhen-Gang Wang’s research group at the California Institute of Technology. My expertise is in macromolecules (specifically, polymers and surfactants) and their interfacial phenomenon. I use molecular simulation techniques like molecular dynamics (MD) and theoretical tools like self consistent field theory (SCFT), classical density functional theory(cDFT), and dynamic density functional theory (DDFT) to model macromolecules and characterize their aggregation and interfacial properties in selective solvents.

I obtained my PhD in chemistry from the University of Wisconsin-Madison, conducting research in Prof. Arun Yethiraj’s group. My doctoral thesis focussed on “Self-assembly of Gemini Surfactant Molecules and the Properties of Nano-confined Water”. Using molecular dynamics simulations, I studied structural features of the gemini surfactant molecules that lead to enhanced gyroid window in the self-assembled phase diagram of the gemini surfactant molecules in water. Following that, I characterized the structure of water molecules confined in different self-assembled morphologies of gemini surfactant molecules and explained specific counter-ion effects on the confined water dynamics. Results from my doctoral work are corroborated by the experimental observations of Prof. Mahesh Mahanthappa’s group (University of Minnesota). Following my doctoral work, I moved to the Institute of Physics at the University of Mainz, Germany to study the self-consistent field theory(SCFT) and the dynamic density functional theory (DDFT) under the guidance of Prof. Friederike Schmid. Using these techniques, I first investigated the effect of polymer chain dispersity on the size and distribution of micelles formed by them in the solution. Then I developed a new approach to systematically construct dynamic density functional theory technique to study the dynamics in inhomogeneous polymer systems. For my postdoctoral work in Prof. Zhen-Gang Wang’s research group at Caltech, I use my expertise in molecular dynamic simulations and field theory techniques to study surfactant mediated CO₂ bubble nucleation in polyurethane foams and to design high salinity and temperature tolerant surfactants.

Research Interests

Interface, i.e. the boundary between two phases, has properties very different from that of the bulk phases. Molecular features of the interface governs the interfacial transport, adsorption at the interface and surface to bulk partitioning. These properties affect liquid-liquid extraction, reaction at the interfaces, preferential accumulation of different components at the interface etc. Consequently, characterizing the interface and understanding the interfacial phenomenon has important implications for applications in diverse domains like atmospheric science, energy storage, pharmaceutics and personal care industry. Though advanced imaging techniques can in principle probe surfaces to the atomistic scale, mobility of molecules at the interface challenges experimental characterization of the interfacial structure and the associated dynamics. Chemically informed theoretically models like classical density functional theory (cDFT), and its dynamic variant called dynamic density functional theory (DDFT), and techniques like molecular dynamics (MD) and Montecarlo (MC) simulations complement experimental characterization. Judicious choice of theoretical and simulation techniques provide time resolved atomistic scale information and macroscopic thermodynamic data, namely surface tension, contact angles, adsorption isotherms, dynamic and energetics in adsorption layers and interfacial transport phenomena. Using the above stated theoretical and simulation techniques I seek to gain insight on the interfacial phenomenon operating in a) materials for capacitive energy storage and b) salting-out assisted liquid-liquid extraction of chemicals.

Materials for capacitive energy storage: A capacitive energy storage material stores energy though the formation of electric double layer at the electrode/electrolyte interface. Though electric double layer capacitors (EDLCs) are more durable than the conventional Li-ion batteries, the current porous-carbon electrode based EDLCs record an order of magnitude lower energy storage capacity than the Li-ion batteries. It is my objective to design materials and interfaces for EDLCs with higher energy density. To this extent I will explore the effect of pore size and geometry of the electrode and the effect of chemical nature of the electrolyte on the energy storage capacity of an EDLC. I will particularly focus on the quantum vs classical nature of the capacitance. This is relevant for the carbon-based porous electrodes which are characterized by low electronic degree of states. Recently proposed techniques like joint density functional theory shows promise to systematically characterize the quantum and classical contributions to the capacitance with modest computational resources.

Salting out assisted liquid-liquid extraction of chemicals: Addition of salt induces liquid-liquid phase separation in an otherwise miscible solution of organic solvent and water. This phenomenon is widely employed for extraction or separation of chemicals. A particularly popular example is to separate acetone from water by addition of salts like CaCl₂. Though it is obvious that the salt alters mixing properties of the solvents, there is no comprehensive understanding on the associated physical chemistry leading to the phase separation. Specifically, there is no molecular level understanding on a) energetics b) specific ion effects, c) ion partitioning between solvents with high and low dielectric constants, d) interfacial composition and associated adsorption isotherms. I seek to understand these using a judicious combination of cDFT and molecular dynamics simulations. Our recently proposed cDFT approach has potential to model chemically realistic systems. These models can be readily deployed to accurately identify the system composition leading to the liquid-liquid phase separation and associated inhomogeneous density profiles. The same models can also be used to identify interfacial energy and other free-energetic contributions governing the phase separation. Molecular dynamics simulations, at the system composition yielding liquid-liquid phase separation, can be used to systematically identify the intrinsic surface and the interfacial composition.

Teaching Interests

My academic training and the past teaching assignments has fully equipped me with the essential background to teach the core curriculum courses like thermodynamics to the graduate and undergraduate students in chemical engineering. In my research, I routinely use concepts from statistical mechanics, molecular simulations, polymer physics and colloids and interfaces to study different problems of interest to the scientific community. I propose to teach each of these topics as separate courses to senior undergraduate/graduate students. Additionally, I propose to teach a special topics course on “computational techniques to tackle current research problems in soft-condensed matter”. Objective of this course is to introduce students, few problems of current research interest and demonstrate how computational techniques can be used to address them. I was recently involved in a similar course building exercise at Caltech. It is my opinion that such a course would particularly benefit senior/junior undergraduate students who plan to take up computational/theoretical research. The same special topics course may also complement the skill set of any interested student in pursuit of their career development.