Browsing by Author "Xu, Zhenghe"

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    Application of Extended DLVO Theory: Modeling of Flotation and Hydrophobicity of Dodecane
    Mao, Laiqun (Virginia Tech, 1998-05-23)
    The extended DLVO theory was used to develop a flotation model by considering both hydrodynamic and surface forces involved in the process. A stream function was used to estimate the kinetic energies for thinning the water films between bubbles and particles, which were compared with the energy barriers, created by surface forces, to determine the probability of adhesion. A general expression for the probability of detachment was derived from similar mechanism for chemical reaction, and the kinetic energy for detachment was estimated with French and Wilson's model. The hydrophobic force parameter (K132) calculated from the rate constants of single bubble flotation tests showed that, K132 for bubble-particle interaction were close to the geometric means of K131 for particle-particle interactions and K232 for bubble-bubble interaction, indicating that the combining rules developed for dispersion forces may be useful for hydrophobic forces. The model was used to predict flotation results as functions of several important parameters such as contact angle, double-layer potentials, particle size, bubble size, etc. The predictions were consistent with experience, and could be explained in view of the various subprocesses considered in the model development. Furthermore, the model suggested optimum conditions for achieving the maximum separation efficiency. The extended DLVO theory was also used to determine the hydrophobic force between two oil/solution interfaces from the equilibrium film thicknesses of dodecylammonium chloride (RNH3Cl) solutions obtained using Thin Film Balance (TFB) technique. The results showed that, the oil droplets were inherently hydrophobic, and the hydrophobic force played an important role in the stability of emulsions. This force decreased with increasing surfactant concentration, and also changed with pH and the addition of electrolyte. The interfacial area occupied by molecules indicated that, the dodecane molecules might present between two surfactant ions at interface, thus the hydrophobicity of oil/solution interface was less sensitive to the addition of the surfactant than that of air/solution interface. Thermodynamic analysis suggested that, there might exist a relationship between the interfacial hydrophobicity and the interfacial tension.
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    A study of hydrophobic interaction in fine particle coagulation
    Xu, Zhenghe (Virginia Tech, 1990)
    A stability theory for lyophobic colloids was put forth in the 1940’s by Derjaguin, Landau, Verwey, and Overbeek. This theory, known as DLVO theory, has gone through the test of time and survived as a pillar of colloid science. In the present work, this theory has been used for describing the behavior of fine coal and silica particles in aqueous media. It has been found, however, that the classical DLVO theory is applicable only to weakly hydrophobic solids but not to very hydrophobic ones. The coagulation experiments conducted with very hydrophobic particles suggest that there exists a strong attractive force that has not been considered in the theory. This non-DLVO force has been estimated in the present work based on the data obtained from coagulation experiments. Contributions from the non-DLVO force, which is referred to as hydrophobic interaction energy (VH), have been related to the nondispersion component of work of adhesion of water on solids (Wnda). An expression for VH which is now a function of Wnda, has been added as a third term in the DLVO equation in order to better describe the stability of colloidal suspensions regardless of the hydrophobicity of the particles involved. A population balance model for a system of isotropic turbulent flow has been developed. Both aggregate growth and breakage have been considered in the model and their rate constants have been derived from a phenomenological approach. Numerical procedures have been proposed for solving the coagulation kinetic equations. Computer simulations show that the model is fairly flexible and the results are in reasonable agreement with experiment.