Date of Award

Fall 12-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

School

Mathematics and Natural Sciences

Committee Chair

Paige Phillips

Committee Chair Department

Chemistry and Biochemistry

Committee Member 2

Steven Stevenson

Committee Member 2 Department

Chemistry and Biochemistry

Committee Member 3

John Pojman

Committee Member 3 Department

Chemistry and Biochemistry

Committee Member 4

Yong Zhang

Committee Member 4 Department

Chemistry and Biochemistry

Committee Member 5

Anthony Bell

Committee Member 5 Department

Chemistry and Biochemistry

Abstract

Molecular modeling is a powerful tool to better understand the intermolecular interactions of carbon nanostructures. It provides structures and energies not easily obtainable from experiments and predicts properties that can be tested experimentally. Intermolecular interactions play an important role in the aggregation of various carbon nanomaterials. Three molecular modeling studies of carbon nanomaterial dispersions are presented in this dissertation, with an emphasis on illustrating how effective these theoretical techniques are in providing insight on the selection of dispersion additives. To achieve our goals, we employed molecular mechanics based methods, along with semi-empirical methods, and quantum mechanical methods, such as density functional theory. Of these techniques, molecular mechanics based methods were the more frequently applied. Chapter I serves as a brief review of computational methods with an emphasis on molecular mechanics.

The first project (Chapter II) includes theoretical studies on the dispersion of single-walled carbon nanotubes (SWNTs) via non-covalent attachment of dispersing polymers. This effort involved the investigation of the binding affinities between specific polymers and SWNTs. Dispersion of SWNTs has been of great interest for many years due to numerous applications promised by their unique combination of electronic, mechanical, chemical, and thermal properties. SWNTs are incompatible with most solvents and polymers, which results in poor dispersion of these compounds in the polymer matrix. Van der Waals attraction among tubes over a large surface area leads to significant agglomeration, thus preventing efficient transfer of their superior properties to the matrix. Improving our fundamental understanding of the interactions of polymer- SWNT interactions at the molecular level is needed for the development of new materials based on SWNTs. Structures of SWNT-polymer complexes were optimized using molecular mechanics, employing COMPASS forcefield. The optimized complexes enabled a morphological analysis of the arrangement of polymer strands on the SWNT surface and calculations of the intermolecular interaction energies. Our calculations identified a strong binding affinity between SWNTs and conjugated polymers containing heteroatoms. The inclusion of solvent effects in the theoretical calculations produced results matching experimental observations from laboratory dispersion studies.

The second project (Chapter III) consists of computational studies on the potential dispersion of metallic nitride fullerenes (MNFs), e.g. Sc3N@Cgo, using a solventcompatible complexing agent. MNFs have a unique hollow-ball shape built from 12 carbon pentagons and 30 hexagons, possessing truncated icosahedra symmetry and encapsulating a trimetallic-nitride cluster at the core of the cage. This unique structure results in its distinctive physical and chemical properties. The ability of MNFs to bring a functional metal to polymeric nano-composite systems opens up the possibility for extraordinary properties, e.g. magnetic, electroactive, and radioactive properties, which hold great promise for medical, optical, and electronic applications. Incorporation of MNF materials in a polymer support material involves the uniform dispersion of MNFs in the matrix. Due to the all-carbon cage, MNFs are very hydrophobic materials and possess minimal solubility in common organic solvents (mg/mL scale), monomers, and polymers, complicating the dispersion process. The ability to disperse MNFs in polymers is paramount to realizing the potential of these materials in future commercial applications. MNFs are difficult to chemically functionalize without altering the desirable intrinsic properties; therefore, an important aspect of this work is the focus on potential non-covalent dispersion techniques using co-additives, which is a versatile, nondamaging chemistry and preserves all of the intrinsic properties of MNF.

Here we studied the interactions between dispersing additive molecules and MNFs using molecular mechanics and specifically calculating interaction energies between MNFs and a variety of additive molecules. A series of resorcinarene and calixarene compounds were surveyed, and characteristics of suitable candidates were identified. Select resorcinarene and calixarene compounds were used in experimental MNF dispersion studies, analyzing samples by particle size measurements and NMR chemical shifts. These experimental studies supported theoretical results, and the dispersion of MNFs in DMF was achieved.

In a third project (Chapter IV), interactions of naphthenic acids with crude oil asphaltenes were examined; thereby contributing significantly to the volume of knowledge available describing the affinities of these acidic and basic components of crude oil. In this project a molecular mechanical analysis with an accepted structure of asphaltene was performed, and intermolecular interactions between asphaltene and naphthenic acids dispersants were calculated. The geometries of the asphaltene - naphthenic acid complexes were optimized and five resultant regioisomers of the asphaltene-naphthenic acid complex were analyzed. The molecular mechanical calculations suggest that the intermolecular interactions between asphaltene and naphthenic acids consist of vdW and electrostatic interactions.

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