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Master Theses

Experiments and Molecular Simulations of Oiling-out phenomenon. Danyang Mei (2009)

Oiling out in mixture of phenol-n-hexane was studied by experiments and molecular simulations. Oiling out encompasses the crystallization of phenol from solution of n-hexane via the formation of liquid droplets of phenol dispersed inside n-hexane that at a later stage crystallize. Oiling out was found to take place at intermediate mole fractions from $X_{phenol}:X_{n-hexane} \approx 10 : 90$ to $X_{phenol}:X_{n-hexane} \approx 30 : 70$, while direct crystallization took place at other concentrations. It was also found that application of mixing diminished the size of liquid-liquid separation zone. Direct microscopic snapshots captured the formation and coalescence of liquid phenol droplets in n-hexane as well as the formation of crystals by decreasing the temperature. The liquid liquid phase separation and solid liquid separation regions for phenol-n-hexane mixture were identified using the Focused Beam Reflectance Method (FBRM) that provides valuable information about the conditions of temperature and concentration for liquid-liquid and solid-liquid phase separation. Detailed study was performed for the selection of the chord lengths of particles, which is a critical quantity in the operation of FBRM. Gas Chromatography experiments revealed the phenol distribution in mixtures of n-hexane-phenol.

Direct molecular dynamics was performed in pure n-hexane, phenol and several of their mixtures at various temperatures. The simulations revealed clustering in pure phenol systems that is enhanced by lowering the temperature. Their structure and degree of solidification was characterized by radial distribution functions and clustering profiles. Diffusion coefficients and the Arrhenius activation energy were computed, that show that pure phenol and n-hexane diffuse with different rate. The simulations test the quality of force field employed and validate its use for large scale simulations even in the micrometer range.

Fragmentation mechanisms of liquid droplets charged with ions. Kirkland Mainer (2006)

Fragmentation of binary mesoscopic clusters composed of H2O/CH3OH and charged with ions (Na+, K+, Cs+, F, Cl, I) of similar sign are studied by molecular simulation. The fragmentation involves the departure of a cluster containing one or more ions from the parent droplet by a break-down of the non-covalent interactions of the solvent. The fragmentation occurs because of the high charge of the droplet. The molecular mechanism of fragmentation is examined by the theories relating to activated processes. Free energy profiles are built along the course of the fragmentation event by following the process with a novel reaction coordinate. This reaction coordinate is named transfer reaction coordinate (TRC) and includes the positional degrees of freedom of all the solvent molecules and ions in its construction. It is very sensitive to the location of the ions, and many tests show that it can successfully distinguish configurations that correspond to the barrier top of the free energy profile. These configurations finally determine the dynamics of the fragmentation process and, therefore, the rate of the process. In this research, the behaviour of the ions in the clusters is characterized by a variety of static quantities such as radial distribution functions, radial density functions, and solvation number, as well dynamic quantities such as solvent residence correlation functions and mean square displacement. Only uneven fragmentation is observed at 200K. However, both even and uneven fragmentation are observed at 300K. In clusters composed of 100% H2O, uneven fragmentation results in distinct product distributions characteristic of the nature of the solvation of each ion. This is not the case for methanol. The transition state in methanol does not retain any solvation characteristics of the fragmenting ion and occurs much closer to the main body of the droplet. The free energy barrier decreases drastically with methanol content at 200K. In binary systems, the positive ions fragment both evenly and unevenly with slightly more methanol than the negative ions. At 300K, uneven fragmentation is found to be the predominant reaction channel. However, even fragmentation is more prominent in binary systems.

Nucleation mechanisms and cluster size distributions in vapour of mixture of methanol-ethanol. Faikah Ogger (2005)

Clusters of methanol and ethanol formed above neat liquid samples were entrained in a supersonic jet of helium and probed in the expansion using 118 nm vacuum ultraviolet laser single-photon ionization/time-of-flight (TOF) mass spectrometry. Almost every cluster ion observed in the TOF mass spectra could be represented by the formula H(ROH)n+, where R = CH3 or C2H5, and n = 1–5. Formation of these species is attributed to a well-established ionization pathway where each protonated (n–1)-mer originates from its n-mer neutral parent. Signals in the TOF mass spectra due to the protonated trimers H(CH3OH)3+ and H(CH3CH2OH)3+ were found to be the most intense and provides direct evidence that these particular cluster ions are "magic-number" structures. The possible relationships between the observed ion data and the neutral cluster vapor phase distributions are discussed. In this context, methanol and ethanol vapor cluster distributions at 298.15 K and at several pressures>=the equilibrium vapor pressure were computed using the grand canonical Monte Carlo and molecular dynamics techniques. Lastly, differences between these experiments and the results of bimolecular reaction studies are discussed.

Crystallization of NaCl in aqueous environment. William Nowak (2003)