Browsing by Author "Ballal, Deepti"

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    Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface
    (AIP Publishing LLC, 2013) Ballal, Deepti; Chapman, Walter G.
    Aqueous solutions of alcohols are interesting because of their anomalous behavior that is believed to be due to the molecular structuring of water and alcohol around each other in solution. The interfacial structuring and properties are significant for application in alcohol purification processes and biomolecular structure. Here we study aqueous mixtures of short alcohols (methanol, ethanol, 1-propanol, and 2-propanol) at a hydrophobic surface using interfacial statistical associating fluid theory which is a perturbation density functional theory. The addition of a small amount of alcohol decreases the interfacial tension of water drastically. This trend in interfacial tension can be explained by the structure of water and alcohol next to the surface. The hydrophobic group of an added alcohol preferentially goes to the surface preserving the structure of water in the bulk. For a given bulk alcohol concentration, water mixed with the different alcohols has different interfacial tensions with propanol having a lower interfacial tension than methanol and ethanol. 2-propanol is not as effective in decreasing the interfacial tension as 1-propanol because it partitions poorly to the surface due to its larger excluded volume. But for a given surface alcohol mole fraction, all the alcohol mixtures give similar values for interfacial tension. For separation of alcohol from water, methods that take advantage of the high surface mole fraction of alcohol have advantages compared to separation using the vapor in equilibrium with a water-alcohol liquid.
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    Isolating the non-polar contributions to the intermolecular potential for water-alkane interactions
    (AIP Publishing LLC., 2014) Ballal, Deepti; Venkataraman, Pradeep; Fouad, Wael A.; Cox, Kenneth R.; Chapman, Walter G.
    Intermolecular potential models for water and alkanes describe pure component properties fairly well, but fail to reproduce properties of water-alkane mixtures. Understanding interactions between water and non-polar molecules like alkanes is important not only for the hydrocarbon industry but has implications to biological processes as well. Although non-polar solutes in water have been widely studied, much less work has focused on water in non-polar solvents. In this study we calculate the solubility of water in different alkanes (methane to dodecane) at ambient conditions where the water content in alkanes is very low so that the non-polar water-alkane interactions determine solubility. Only the alkane-rich phase is simulated since the fugacity of water in the water rich phase is calculated from an accurate equation of state. Using the SPC/E model for water and TraPPE model for alkanes along with Lorentz-Berthelot mixing rules for the cross parameters produces a water solubility that is an order of magnitude lower than the experimental value. It is found that an effective water Lennard-Jones energy εW/k = 220 K is required to match the experimental water solubility in TraPPE alkanes. This number is much higher than used in most simulation water models (SPC/E—εW/k = 78.2 K). It is surprising that the interaction energy obtained here is also higher than the water-alkane interaction energy predicted by studies on solubility of alkanes in water. The reason for this high water-alkane interaction energy is not completely understood. Some factors that might contribute to the large interaction energy, such as polarizability of alkanes, octupole moment of methane, and clustering of water at low concentrations in alkanes, are examined. It is found that, though important, these factors do not completely explain the anomalously strong attraction between alkanes and water observed experimentally.
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    Microstructure and Interfacial Properties of Aqueous Mixtures
    (2014-11-07) Ballal, Deepti; Chapman, Walter G; Biswal, Sibani L; Kolomeisky, Anatoly B
    Understanding the properties of aqueous mixtures has important implications in applications ranging from enhanced oil recovery to biochemical processes. While there has been considerable effort invested in understanding the bulk properties of aqueous mixtures, very few studies have concentrated on their behavior in interfacial systems. Interfacial properties, which are important for applications like coatings and chemical separations, are defined by the molecular structuring of the fluid at the interface. The goal of this thesis is to understand and alter the wetting of solid surfaces by aqueous mixtures. In particular, we study the partitioning of aqueous mixtures of polar and non-polar molecules to different surfaces. What makes aqueous mixtures interesting is the hydrogen bonding nature of water that plays very different roles in the partitioning of polar and non-polar components of the aqueous mixtures. In this thesis, hydrogen bonding is modeled using a thermodynamic perturbation theory due to Wertheim. The theory, included in a classical Density Functional Theory framework, is used to study the molecular structure and interfacial properties of the system. We extend and apply the theory to study a number of aqueous mixtures. Key contributions of this thesis include 1. Predicting the interfacial properties of aqueous mixtures of short alcohols close to a hydrophobic surface 2. Extension of the first order perturbation theory to study the competition between intra and intermolecular hydrogen bonding of molecules in the presence of an explicit water-like solvent 3. Studying the effect of physical conditions and surface chemistry on the wetting of different surfaces by water-oil mixtures 4. Analyzing molecular simulation models for water-alkane interactions through a solubility study
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    Wertheim's association theory applied to one site patchy colloids: Beyond the single bonding condition
    (American Institute of Physics, 2012) Marshall, Bennett D.; Ballal, Deepti; Chapman, Walter G.
    We apply Wertheim's theory to develop an equation of state for one site patchy colloids where the patch can bond multiple times. We allow for the possibility of ring formation without the introduction of empirical parameters and show that for moderate patch coverage the infinite series of chain graphs is well represented by the first two terms. The theory is found to be in excellent agreement with new NVT and NPT Monte Carlo simulations. The approach described here can easily be converted to the form of a density functional theory to describe inhomogeneous patchy colloid systems.