acase@tulane.edu

acase@tulane.edu

"Hydrophobic hydration of non polar molecules is the principal driving force that dictates several interfacial phenomena in nature such as self assembly of surfactant molecules, fate of environmental pollutants, wetting of surfaces, solution behavior of polymers and folding of biological molecules such as proteins. However, the physics associated with hydrophobic interactions on a molecular length scale, which is central to self assembly and protein folding, is different from the macroscopic phenomena of de-mixing of oil and water or wetting of surfaces. This dissertation seeks to understand the implication of hydrophobic interactions to energy and environmental applications using different approaches. The first approach is to examine the behavior of water molecules with hydrophobic moieties at a molecular level using molecular dynamics simulations and evaluate macroscopic thermodynamic properties. The first problem addressed in this dissertation is the enclathration of gas molecules by water molecules in the presence of quaternary ammonium ions. Small polar organic molecules such as quaternary ammonium salts form crystalline inclusion compounds called semi-clathrate hydrates, where these polar molecules occupy a lattice position of the hydrogen bond network of water molecules. These crystalline structures of water are formed at ambient temperature and pressure conditions and can store as much 3%(w/w) of methane, making them potential materials for gas storage. The stability and structure of semi-clathrate hydrates of tetrabutylammonium bromide (TBAB) and methane were investigated using molecular dynamics (MD) simulations. MD simulations were done at varying conditions of temperature and pressure for methane-TBAB ratios of 0, 0.5, 1, 1.5 and 2. Thermo-mechanical properties evaluated using MD simulations were in agreement with experimental data available. Our investigation of this system shows that enclathration of methane in these semi-clathrate hydrates is thermodynamically favorable even at higher temperatures and shows signatures of hydrophobic hydration. Our estimation of free energies associated with successive inclusion of methane molecules in these cavities suggests a Langmuir-type adsorption of methane in these cages. Another problem investigated in this dissertation is the effect of chemical heterogeneity of crystalline cellulose (110) and (100) surfaces on their respective wetting behavior. Understanding the interaction of water with cellulose is important in the view of its role in consumer textiles made from cotton cellulose and potential applications of cellulose as biomaterials and as an energy source. The difference in the wetting behavior of (110) and (100) crystal surfaces is due to the asymmetry in the exposure of the hydroxyl groups by these surfaces. MD simulations were used to evaluate the contact angles of hemi-cylindrical water nanodroplets on crystalline (110) and (100) surfaces of the cellulose Iβ allomorph. While the native crystalline surfaces were completely wetted by water nanodroplets, substituting the primary hydroxyl groups with methyl and methoxy groups results in dewetting. The contact angle of a hemicylidrical water nanodroplet on the hydrophobically-modified (110) surface is greater than on the (100) surface suggesting that the (110) surface has a greater exposure of the primary hydroxyl groups. The solubility of cellulose in aliphatic N-oxides has been of particular interest because of its application in industrial processes such as Lyocell process. However, the mechanisms that dictate the dissolution of cellulose in these selective solvents are not clearly understood. Attempt is made to understand the solvation of cellulose in N-Methylmorpholine oxide (NMMO) and water from a molecular perspective. MD simulations of a model cellohexaose crystallite solvated respectively in pure water, NMMO and in an equimolar mixture suggest that while NMMO molecules preferentially cluster around the primary hydroxyl groups in cellohexaose chains, the role of water is critical in its ability to access the glycosidic oxygen. The second approach is to study the implication of introducing hydrophobicity at molecular level and experimental determination of its implication to addressing interfacial aspects of environmental remediation. Sub-micron size carbon particles derived from hydrothermal decomposition of sucrose are effective in stabilizing water-in-trichloroethylene (TCE) emulsions. Irreversible adsorption of carbon particles at the TCE-water interface resulting in the formation of a monolayer around the water droplet in the emulsion phase is identified as the key reason for emulsion stability. Cryogenic Scanning Electron Microscopy was used to clearly image the assembly of carbon particles at the TCE-water interface and the formation of bilayers at regions of droplet-droplet contact. The results from this study have broad implications to the subsurface injection of carbon submicron particles containing zerovalent iron nanoparticles to treat pools of chlorinated hydrocarbons that are sequestered in fractured bedrock. Interfacial aspects of hydrophobically modified biopolymer and its ability to enhance the stability of crude-oil droplets formed were investigated. Turbidimetric analyses show that emulsions of crude oil in saline water prepared using a combination of the biopolymer and the well-studied chemical dispersant (Corexit 9500A) remain stable for extended periods in comparison to emulsions stabilized by the dispersant alone. The hydrophobic residues attached to the polymer preferentially anchor at the oil-water interface and form a protective layer of the polymer around the droplets. The enhanced stability of the droplets is due to the polymer layer providing an increase in electrostatic and steric repulsions and thereby a large barrier to droplet coalescence. The implication of this study to current remediation methods is significant since the addition of hydrophobically modified chitosan following the application of chemical dispersant to an oil spill can potentially reduce the use of chemical dispersants. Increasing the molecular weight of the biopolymer changes the rheological properties of the oil-in-water emulsion. Emulsions stabilized by using a combination of Corexit 9500A and high molecular weight hydrophobically modified chitosan show characteristics of a weak gel. The ability of the biopolymer to tether the oil droplets in a gel-like matrix has potential applications in the immobilization of surface oil spills for enhanced removal. Carbon microspheres containing magnetite nanoparticles, synthesized using inexpensive precursors such as sucrose and iron chloride, are ferromagnetic and have affinity to the oil phase. We demonstrate that a thin layer of crude oil can be corralled and thickened by the application of nonionic surfactant. Following the application of magnetite-carbon particles, hydrophobically modified chitosan was applied to form a gel-like phase. This gel-like phase of crude oil containing magnetic carbon spheres can be removed as an aggregate using a magnet resulting in enhanced recovery of crude oil. The results from the current study point to developing potential applications for confinement, magnetic tracking and removal of surface oil. " / acase@tulane.edu

This research is focused on engineering submicron spherical carbon particles as effective carriers/supports for nanoscale zerovalent iron (NZVI) particles to address the in situ remediation of soil and groundwater chlorinated contaminants. Chlorinated hydrocarbons such as trichloroethylene (TCE) and tetrachloroethylene (PCE) form a class of dense non-aqueous phase liquid (DNAPL) toxic contaminants in soil and groundwater. The in situ injection of NZVI particles to reduce DNAPLs is a potentially simple, cost-effective, and environmentally benign technology that has become a preferred method in the remediation of these compounds. However, unsupported NZVI particles exhibit ferromagnetism leading to particle aggregation and loss in mobility through the subsurface. This work demonstrates two approaches to prepare carbon supported NZVI (iron-carbon composites) particles. The objective is to establish these iron-carbon composites as extremely useful materials for the environmental remediation of chlorinated hydrocarbons and suitable materials for the in situ injection technology. This research also demonstrates that it is possible to vary the placement of iron nanoparticles either on the external surface or within the interior of carbon microspheres using a one-step aerosol-based process. The simple process of modifying iron placement has significant potential applications in heterogeneous catalysis as both the iron and carbon are widely used catalysts and catalyst supports. Furthermore, the aerosol-based process is applied to prepare new class of supported catalytic materials such as carbon-supported palladium nanoparticles for ex situ remediation of contaminated water. The iron-carbon composites developed in this research have multiple functionalities (a) they are reactive and function effectively in reductive dehalogenation (b) they are highly adsorptive thereby bringing the chlorinated compound to the proximity of the reactive sites and also serving as adsorption materials for decontamination (c) they are of the optimal size for transport through sediments (d) they have amphiphilic chemical functionalities that help stabilize them when they reach the DNAPL target zones. Finally, the iron-carbon composite microspheres prepared through aerosol-based process can used for in situ injection technology as the process is conductive to scale-up and the materials are environmentally benign. / acase@tulane.edu

Polyethylene oxide polymers are intrinsic to oil spill dispersants used in Macondo well blowout of 2010. We believe that effective thermo-physical modeling of these materials should assist the application of lab-scale results into ocean-scales. Fully defensible molecular scale theory of such materials will be challenging. This thesis is the first step towards that challenge. Molecular dynamics simulations are useful in generating structural and phase behavior data for these versatile polymers. Microstructures of PEO polymers, hydrophobic interactions, direct numerical test of controversial Pratt-Chandler theory, concentration dependence of Flory-Huggins interaction parameter and neutron scattering experiments will be discussed. / acase@tulane.edu

With recent advancements in the synthesis and characterization of polymeric materials, scientists are able to create multi-scale novel polymers with various cases of chemical functionalities, diversified topologies, as well as cross-linking networks. Due to those remarkable achievements, there are a broad range of possible applications of smart polymers in catalysis, in environmental remediation, and especially in drug-delivery. Because of rising interest in developing therapeutic drug binding to specific treating target, polymer chemists are in particular interests in design and engineering the drug delivery materials to be not only bio-compatible, but also to be capable of self-assembly at various in-vivo physiological stimulus. Both experimental and theoretical work indicate that the thermodynamic properties relating to the hydrophobic effect play an important role in determining self-assembly process. At the same time, computational simulation and modeling are powerful instruments to contribute to microscopic thermodynamics' understanding toward self-assembly phenomenon. Along with statistical approaches, constructing empirical model based on simulation results would also help predict for further development of tailored nano-structured materials. My Research mainly focused on investigating physical and chemical characteristics of polymer materials through molecular dynamics simulation and probing the fundamental thermodynamic driving force of self-assembly behavior. We tried to surmount technological obstacles in computational chemistry and build an efficient scheme to identify the physical and chemical Feature of molecules, to reproduce underlying properties, to understand the origin of thermodynamic signatures, and to speed up current trial and error process in screening new materials. / acase@tulane.edu

To clarify the role of ab initio molecular dynamics (AIMD) simulation, this study organizes the McMillan-Mayer (MM) theorem, the potential distribution theorem, and quasi-chemical approach to provide theory for the thermodynamic effects associated with long-length scales. This multi-scale statistical mechanical (MSSM) theory implements quasi-chemical theory after utilizing the MM theorem to integrate-out the solvent degrees of freedom. The MSSM theory treats composition fluctuations which would be accessed by larger-scale calculations, and also long-ranged interactions of special interest for electrolyte solutions. The theory is applied to a primitive electrolyte solution model proposed to investigate ion pairing in the context of tetraethyammonium tetrafluoroborate in propylene carbonate. A Gaussian statistical model is shown to be an effective physical approximation for outer-shell contributions, and they are conclusive for the free energies within the quasi-chemical formulation. Gaussian statistical theory can be more effective than the Bennett numerically exact method when exhaustive sampling is not available, i.e., for finite samples. These results lead to the analysis of the asymptotical behavior of a relative information entropy and thus a new formula for the ion excess free energies. This asymptotic perspective completely avoids the computationally limiting evaluation of the outer-shell contributions. In addition, we use AIMD to obtain the charges on tetramethylammonium and tetrafluoroborate ions contacting neutral and charge carbon nanotube electrodes, and also charges tetraethyammonium and tetrafluoroborate ions in propylene carbonate solution. / acase@tulane.edu

acase@tulane.edu

The goal of this research is to study the neutralization of sulfuric acid by engine oils, and more specifically study how the presence of different oil additives affects the acid-neutralizing performance of engine oils by using capillary videomicroscopy. Nowadays the formulation of engine oils has been changing due to a trend of different regulations around the world that seek to diminish the emission of atmospheric pollution from all types of vehicles driven by internal combustion engines. In the particular case of large marine ships powered by low-speed two-stroke diesel engines, pollutant emissions are high given that the marine fuel they use can contain up to 4.5 wt. % of sulfur. But this sulfur content cap in marine fuel is bound to diminish dramatically during the ongoing decades due to regulations and therefore, the industry is coming up with new engine oil formulations accordingly as to comply with these changes. Here a technique called capillary videomicroscopy was used to study new changes to engine oil formulations. The reaction and dispersion of a sulfuric acid micro-droplet into formulations of marine cylinder lubricants (MCL) was studied by microscopically observing and measuring the shrinking of a micropipette-produced droplet in real time. It was found that MCL formulations having a base number (BN) of 40 had an acid-neutralizing performance comparable to those of having BN 70. On the other hand, the addition of fatty alcohols as final additives to MCL formulations so as to boost the MCL’s acid neutralization performance was found to be slightly effective although phase separation due to alcohol insolubility in MCL at room temperatures and other resilient phases formed upon reaction can be detrimental, hence the use of fatty alcohols for boosting any MCL formulation cannot be generalized and should be studied for each formulation. In the case of passenger car motor oils (PCMO), substitution of traditional oil additives by new sulfur-free additive species is driven by the need to prevent the catalytic converter's poisoning by eliminating any sulfur present in the exhaust gas. The effect of the polymeric dispersant on the acid neutralization performance was also studied. The formation of clear, thin and resilient shells surrounding sulfuric acid droplets upon reaction with some MCLs was noticed to be a detrimental aspect towards their acid neutralization performance and more importantly, due to the formation of potential precursors for cylinder liner engine deposits. Finally it is shown a modification of the capillary videomicroscopy technique that allowed long-term monitoring of the fate of microscopic particles while reacting or dissolving under flow, by suspending them using a balance between buoyancy and drag force from creep flow. / acase@tulane.edu

Synthesis and characterization of anisotropic micro- and nanoparticles, either in suspension or localized on a surface, are current areas of intense scientific interest because of their shape-tunable material properties with potential applications in catalysis, microelectronics, data storage and pharmaceutics. Electrochemical deposition represents a facile and versatile route to fabricate anisotropic particles since it offers a high degree of freedom in monitoring and manipulating particle growth processes. The first part of my dissertation presents an additive-mediated electrochemical approach to fabricate anisotropic copper micro- and nanoparticles. This work explores the possibility of using anisotropic copper particles as novel non-noble metal alternatives to expensive anode electrocatalysts (platinum and palladium) used in direct methanol fuel cells (DMFCs). Characterization using SEM, EDS, XRD and TEM confirms the anisotropic morphology and crystal structure of synthesized copper particles. A possible mechanism for anisotropic crystal growth is proposed based on preferential adsorption of additive ions onto selective crystal faces. Methanol oxidation is chosen as model experiment to test the electrocatalytic property of copper particles towards DMFC applications. Characterization using cyclic voltammetry demonstrates shape dependent enhancement in electrocatalytic activity of anisotropic copper particles for methanol oxidation. Chronoamperometry and thermal stability measurements indicate good catalyst stability and durability under steady-state conditions. The second part of my dissertation presents a novel electrochemical fabrication route to generate randomly rough surfaces over large areas. Surface roughness directly affects a material's performance at its functional interface. This work shows that by simple tuning of electrochemical deposition potential for a metal onto an electrode, island nucleation density can be systematically varied. Changes in nucleation density results in generation of thin films with different nanoscale surface roughness. Characterization using AFM illustrates the change in surface topography with applied potential. The fabricated roughness is successfully replicated onto other moldable soft materials (polystyrene and polyurethane) through an embossing and curing step. Roughness gradients were also generated by introducing a controlled mechanical retraction step to the process. Gradient surfaces serve as an effective probing tool for investigating a range of surface parameters in quick time using single experiment, enabling a cost-effective and high-throughput screening method. / acase@tulane.edu