Predicting Micellization Behavior of Carboxylate Surfactants from Molecular Dynamics Simulations Open Access

Abstract

Surfactants are amphiphilic compounds that form aggregates such as micelles in aqueous solution at concentrations above critical micellar concentration (cmc). However, the structural disorder, polydispersity, and sensitivity to conditions of surfactant micelles present challenges to the unambiguous determination of their properties through experiments alone. The first part of the thesis aims to use molecular dynamics (MD) simulations to predict and compare the self-assembly behavior of octanoates, focusing on the effect of varying counterion identities among sodium, potassium, and tetramethylammonium cations. The second part focuses on predicting the enthalpy change and heat capacity change of micellization of anionic surfactants. Statistics from MD simulations of small systems are input in a global fitting procedure to generate equilibrium constants for surfactant micellization over micelle compositions described by the number of surfactants and the number of bound counterions. The resulting free energy statistics are used to predict the cmc, mean micelle size, and the degree of counterion binding of the investigated carboxylate surfactants. Trends in the fitted values are consistent with experimental data. Simulations at different temperatures are then conducted for each kind of surfactant to analyze enthalpy changes associated with micellization and counterion binding. Enthalpograms will be built for comparison with experimental calorimetry data by explicitly considering polydispersity, so that the model can provide insights on cluster contributions to the enthalpy change of micellization at different solution concentrations.

Table of Contents

1. Overview ...............................................................................................................................1

1.1 Micelle Self-Assembly............................................................................................................1

1.2 Isothermal Titration Calorimetry ...........................................................................................2

1.3 Molecular Dynamics Simulation .............................................................................................3

2. Counterion Effect on Long-chain Carboxylate Surfactant Assembly ............................................5

2.1 Introduction..........................................................................................................................5

2.2 Methods................................................................................................................................7

2.2.1 General Simulation Setup ....................................................................................................7

2.2.2 Cluster Definition................................................................................................................9

2.2.3 Partition-Enabled Analysis of Cluster Histogram (PEACH)......................................................10

2.2.4 Predicting Micellar Properties ..............................................................................................11

2.2.5 Maibaum’s Model.................................................................................................................11

2.3 Results and Discussion............................................................................................................13

2.3.1 Cutoff-Distances to Define Clusters.......................................................................................13

2.3.2 Micellar Properties derived by PEACH....................................................................................15

2.3.3 Modelling the PEACH-Derived Free Energy Surface.................................................................18

2.4 Conclusion .............................................................................................................................21

3. Predicting the Enthalpy Change of Micellization..........................................................................22

3.1 Introduction............................................................................................................................22

3.2 Methods..................................................................................................................................25

3.2.1 General Simulation Setup ......................................................................................................25

3.2.2 Generating Enthalpograms.....................................................................................................25

3.3 Results and Discussions............................................................................................................27

3.3.1 Failure of the Maibaum’s Model .............................................................................................27

3.3.2 Exploration of Counterion Binding Thermodynamics ...............................................................29

3.3.3 Pre-cmc Cluster Enthalpy ......................................................................................................31

3.4 Conclusion ..............................................................................................................................35

4. Future Directions........................................................................................................................36

5. References..................................................................................................................................36

6. Supplementary Materials.............................................................................................................43

About this Honors Thesis

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School	Emory College
Department	Chemistry
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Chemistry, Physical
Keyword	Computational Chemistry
Committee Chair / Thesis Advisor	James Kindt, Emory University
Committee Members	Michelangelo Grigni, Emory University Francesco Evangelista, Emory University

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