Date of Award

2018

Document Type

Open Access Dissertation

Department

Physics and Astronomy

Sub-Department

College of Arts and Sciences

First Advisor

Scott Crittenden

Abstract

In this dissertation, I provide a thorough description of my development of theoretical, analytical, and experimental techniques pertaining to novel measurements of the electrostatic double layer (EDL) using atomic force microscopy (AFM), among other techniques. The EDL refers to the structure formed by ions that exists at the interface between a solid and a liquid. The EDL is an important physical element of many systems and its behavior has been of interest to scientists for many decades. Because many areas of science and engineering are moving to test, build, and understand systems at smaller and smaller scales, this work focuses on nanoscopic experimental investigations of the EDL. In that vein, AFM will be introduced and discussed as a tool for making high spatial resolution measurements of the solid-liquid interface, culminating in a description of the development of a method for completely characterizing the EDL using direct force measurements with AFM.

I first explore, in a historical fashion, the development of the various models and theories that are used to describe the electrostatic double layer. Later, various experimental techniques and ideas are addressed as ways to make measurements of interesting characteristics of the EDL. Finally, I will introduce a newly developed approach to measuring the EDL system with AFM. Using this approach, AFM measurements were made of the diffuse double layer potentials using a two-electrode system consisting of a metal coated atomic force microscopy cantilever and a metal substrate.

The variation in the diffuse layer potential was measured as a function of probesample potential difference, ▲V. From these measurements, values of the bulk solution potential were inferred using a model of two capacitive elements in series. The expected variation of diffuse layer potential was obtained for a model of the bulk solution potential varying linearly with ▲V in 0.1 mM, 0.5 mM, and 1 mM solutions of NaCl. It was found that the diffuse layer potentials varied roughly linearly with applied sample bias up to 100 mV. The values of the diffuse layer potentials varied from −46 mV to −11 mV, −89 mV to −8 mV, and from −71 mV to −13 mV for the 0.1 mM, 0.5 mM, and 1 mM solutions, respectively, for applied biases ranging from 0 to +400 mV. The results suggest that measurements of electrostatic double layer potentials can be made with atomic force microscopy without the need for a reference electrode.

Specifically, I show that AFM can be used to measure the two most commonly referenced potential drops in a solid-liquid interface system, referred to as the diffuse layer potential and the Stern potential. Also, in concert with the aforementioned model, the potential of the solution itself with respect to an external measurement point can be calculated. Experimental data validates the implementation of such a model and, together, the results indicate that AFM can be used to fully characterize the electrostatic environment of certain solid-liquid interface systems.

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Physics Commons

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