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Simulation Studies of Self-Assembly and Phase Diagram of Amphiphilic Molecules

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Simulation Studies of Self-Assembly and Phase Diagram of Amphiphilic Molecules University of Central Florida STARS Electronic Theses and Dissertations, 2004-2019 2005 Simulation Studies Of Self-assembly And Phase Diagram Of Amphiphilic Molecules Geuorgui Kostadinov Bourov University of Central Florida Part of the Physics Commons Find similar works at: https://stars.library.ucf.edu/etd University of Central Florida Libraries http://library.ucf.edu This Doctoral Dissertation (Open Access) is brought to you for free and open access by STARS. It has been accepted for inclusion in Electronic Theses and Dissertations, 2004-2019 by an authorized administrator of STARS. For more information, please contact [email protected]. STARS Citation Bourov, Geuorgui Kostadinov, "Simulation Studies Of Self-assembly And Phase Diagram Of Amphiphilic Molecules" (2005). Electronic Theses and Dissertations, 2004-2019. 435. https://stars.library.ucf.edu/etd/435 SIMULATION STUDIES OF SELF-ASSEMBLY AND PHASE DIAGRAM OF AMPHIPHILIC MOLECULES by GEUORGUI BOUROV M.S. Sofia University, 1993 M.S. University of Central Florida, 2003 A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the College of Art and Science at the University of Central Florida Orlando, Florida Summer Term 2005 ABSTRACT The aim of this dissertation is to investigate self-assembled structures and the phase diagram of amphiphilic molecules of diverse geometric shapes using a number of different computer simulation methods. The semi-realistic coarse-grained model, used extensively for simulation of polymers and surfactant molecules, is adopted in an off-lattice approach to study how the geometric structure of amphiphiles affects the aggregation properties. The results of simulations show that the model system behavior is consistent with theoretical predictions, experiments and lattice simulation models. We demonstrate that by modifying the geometry of the molecules, self-assembled aggregates are altered in a way close to theoretical predictions. In several two and three dimensional off-lattice Brownian Dynamics simulations, the influence of the shape of the amphiphilic molecules on the size and form of the aggregates is studied systematically. Model phospholipid molecules, with two hydrophobic chains connected to one hydrophilic head group, are simulated and the formation of stable bilayers is observed. In addition, (practically very important) mixtures of amphiphiles with diverse structures are studied under different mixing ratios and molecular structures. We find that in several systems, with Poisson distributed chain lengths, the effect on the aggregation distribution is negligible compared to that of the pure amphiphilic system with the mean length of the Poisson distribution. The phase diagrams of different amphiphilic molecular structures are investigated in separate simulations by employing the Gibbs Ensemble Monte Carlo method with an implemented configurational-bias technique. The computer simulations of the above mentioned amphiphilic systems are done in an area where physics, biology and chemistry are closely ii connected and advances in applications require the use of new theoretical, experimental and simulation methods for a better understanding of their self-assembling properties. Obtained simulation results demonstrate the connection between the structure of amphiphilic molecules and the properties of their thermodynamically stable aggregates and thus build a foundation for many applications of the remarkable phenomena of amphiphilic self-assembly in the area of nanotechnology. iii Dedicated to my parents, Vasilka and Kostadin Bourov. iv ACKNOWLEDGEMENTS I would like to thank my advisor Prof. Aniket Bhattacharya for all time and efforts he spent guiding me through my research. In addition, I would like to thank the Committee members: Prof. Kevin Belfield, Prof. Viatcheslav Kokoouline and Prof. Hari Saha. I greatly appreciate the efforts of all my teachers from the Physics Department at UCF and especially I would like to thank Prof. Subir Bose, Prof. Michael Johnson, Prof. Brian Tonner and Prof. Alfons Schulte. I have learned a lot about the nanotechnology from the members of the Nano Interdisciplinary Research Team (NIRT): Prof. Weili Luo, Prof. David Tomanek (MSU) and Prof. Rosensweig (University of New Orleans). I greatly appreciate the editing help from my colleagues Karlo Kitanovski and Robert Macey. I also would like to thank my wife, Stanimira, for her encouragement and support during the entire candidacy, and my daughter Katerina This research was supported by a NIRT grant No. 0103587 from the National Science Foundation. v PUBLICATIONS BASED ON THIS WORK 1. G. K. Bourov and A. Bhattacharya, The role of geometric constrains in amphiphilic self-assembly: A Brownian dynamics study, J. Chem. Phys. 119, 9219 (2003). 2. A. Bhattacharya and G. K. Bourov, Effect of packing parameter on amphiphilic self- assembly: A Brownian dynamics study, in Computer Simulation Studies in Condensed Matter Physics XVI, Springer Verlag (2003). 3. G. K. Bourov and A. Bhattacharya, Brownian dynamics simulation study of self- assembly of amphiphiles with large hydrophobic heads, J. Chem. Phys. 122, 044702 (2005). 4. G. K. Bourov and A. Bhattacharya, Brownian dynamics of mixed surfactant micelles (submitted to J. Chem. Phys.). Under preparation: 5. G. K. Bourov and A. Bhattacharya, Effect of head group geometry in phase diagram of amphiphilic molecules - a Gibbs ensemble Monte Carlo study. vi TABLE OF CONTENTS LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES........................................................................................................... xiv LIST OF ACRONYMS AND ABBREVIATIONS ......................................................... xv CHAPTER ONE: INTRODUCTION................................................................................. 1 1. Self-assembly phenomena .......................................................................................... 1 2. Amphiphiles................................................................................................................ 3 3. Thermodynamics of aggregation ................................................................................ 5 CHAPTER TWO: COMPUTER SIMULATIONS .......................................................... 13 1. Bead-spring model.................................................................................................... 14 2. Verlet Algorithm....................................................................................................... 16 3. Brownian Dynamics.................................................................................................. 17 4. Gibbs Ensemble Monte Carlo method..................................................................... 19 4.1. Statistical Mechanics of the Gibbs Ensemble.................................................... 22 4.2. Acceptance rules ................................................................................................ 23 5. Configurational-Bias Method ................................................................................... 26 6. Correlation Functions................................................................................................ 28 7. Verlet and cell-list methods ...................................................................................... 29 CHAPTER THREE: STUDIES OF AMPHIPHILIC SELF-ASSEMBLY...................... 32 1. The model of amphiphiles ........................................................................................ 32 1.1. Random number generator................................................................................. 36 2. Critical Micelle Concentration.................................................................................. 38 vii 3. Shapes and distribution of Aggregates ..................................................................... 43 4. Autocorrelation Function.......................................................................................... 48 CHAPTER FOUR: STUDIES ON MIXED AMPHIPHILIC SYSTEMS AND PHOSPHOLIPIDS........................................................................................................................ 52 1. Mixtures of amphiphiles ........................................................................................... 52 2. Critical Micelle Concentration.................................................................................. 56 3. Cluster distributions in binary amphiphilic systems............................................... 59 4. Mixtures with the Poisson distribution of lengths .................................................... 64 5. Self-assembly of phospholipid molecules .............................................................. 66 CHAPTER FIVE: PHASE DIAGRAMS OF MODEL AMPHIPHILES ...................... 71 1. Phase behavior of chain molecules........................................................................... 73 2. Computational methods for study of chain molecules Phase Diagram .................... 76 3. Some details of GEMC simulation ........................................................................... 78 4. Results and Discussion ............................................................................................. 80 4.1. Obtaining the phase diagram of LJ dimers ........................................................ 80 4.2. Phase
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