Bridging the gap between atomistic and continuum models to predict dielectric and thermodynamic properties of confined fluids

Nanoconfined fluids are ubiquitous and play a prominent role in nature and technological applications. Understanding the physics of the confined fluids and obtaining atomic-level insights into their unusual properties is essential to develop and design novel nanofluidic applications related to energy, water, and health. For systems involving multiple length and time scales, atomistic simulations become forbiddingly expensive. On the other hand, classical continuum theories fails to accurately describe the fluid properties at atomic level. Thus, there is a need for a multiscale frame work to maintain the balance between accuracy and rigor of atomistic simulations and efficiency of continuum frameworks. In this work, we present an empirical potential-based quasi-continuum theory (EQT) that provides a framework to seamlessly integrate atomistic details into a continuum-based models. The main idea in EQT is to bridge the gap between atomistic and continuum models by incorporating molecular correlations, interatomic interactions, and anisotropic effects at a continuum level. We show that EQT can be used in classical density functional theory to predict the thermodynamic properties for confined fluids. Moreover, we present a hierarchical coarse-grain (CG) approach in which we coarse grain the degrees of freedom of polar liquids from the detailed all-atom (AA) level to the cheaper particle-based CG level, and to the continuum-based level. Our goal is to devise CG interaction potentials for polar liquids that reproduces not only the structure but also accurately describe the dielectric permittivity and its anisotropic nature in the confinement. Using the CG potentials in EQT we show that neglecting the tensorial form of the dielectric permittivity in the Poisson equation leads to incorrect screening and orientational polarization profiles near interfaces. Thus, using extensive molecular dynamics simulations, statistical-mechanical theories and multiscale methods, we study the out-of-plane (z-axis) and in-plane (x-y) dielectric response of protic and aprotic fluids confined inside slit-like graphene channels. We find a universal reduction in perpendicular permittivity for all the fluids. Whereas, the parallel dielectric response of polar liquids is enhanced and is proportional to dipolar correlations and density oscillation next to the interface. The perpendicular reduction and in-plane enhancement of the dielectric permittivity is attributed to the favorable in-plane (x-y plane) dipole-dipole electrostatic interactions of the interfacial fluid layer. These findings have important consequences in, developing accurate coarse-grained force fields and improving the solvent-implicit approaches often used in biology and continuum theories such as the Poisson- Boltzmann (PB) equation for accurate prediction of capacitance in the electric double-layer capacitors.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2020

Author dc:creator

• Motevaselian, Mohammad Hossein

Contributors dc:contributor

• Aluru, Narayana R
• Tajkhorshid, Emad
• Nam, Sungwoo
• Sing, Charles

• Dielectric Permittivity
• Langevin Function
• Confined Fluids
• Parallel-plate Capacitor
• Multiscale method
• Molecular Dynamics Simulations
• Statistical Mechanics
• Thermodynamic Properties
• Coarse Graining
• Relative Entropy
• Protic and Aprotic Fluid
• Confined Lennard-Jones Fluids Mixture
• Empirical Potential-based Quasi-continuum Theory (EQT)

Statement dc:rights

• Copyright 2020 Mohammad Hossein Motevaselian

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/108507

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/108507