Theory and experiment on the transport of surfactant from micellar solutions to a clean air/water interface: Experimental evidence of direct micelle adsorption route
Item
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Title
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Theory and experiment on the transport of surfactant from micellar solutions to a clean air/water interface: Experimental evidence of direct micelle adsorption route
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Identifier
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d_2009_2013:23b75fb183be:10050
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identifier
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10085
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Creator
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Huang, Fenfen,
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Contributor
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Charles Maldarelli
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Date
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2009
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemical engineering | Adsorption | Diffusion | Kinetic | Micelle | Monomer | Surfactant
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Abstract
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This dissertation studies the transport of surface active (surfactant) molecules, dissolved in an aqueous phase, to an initially clean air/water interface, and the reduction in surface tension which accompanies this adsorption. The study focuses on the transport in the case in which the bulk concentrations of surfactant are above a critical value for which aggregates of surfactant molecules (micelles) form in the bulk phase (the critical micelle concentration or CMC). The study will examine micellar solutions which form approximately spherical aggregates, using a polyethoxylated surfactant as a model.;This thesis proposes a second route for transport of surfactant to a clean air/water interface from a micellar solution: The direct adsorption of the micelle onto the surface. A model for the kinetics of micelle attachment to the surface is developed, and the equilibrium distribution of monomer and micelle on the surface as a function of the bulk concentration is computed from this kinetic expression. A theory is developed to describe the dynamic adsorption when a clean interface is created in a micellar solution which includes the direct adsorption route. The simulations demonstrate that the direct route circumvents the limiting monomer kinetic barrier and allows ultrafast tension reductions. Dynamic tension experiments for micellar solutions are undertaken and compared to the theory to determine the kinetic constants for direct micelle adsorption. The simulations show excellent agreement with the measured tension reductions over a range of micellar concentrations.;The thesis also reports experiments that provide verification of the direct adsorption of micelles onto a clean air/water interface. These experiments tag the micelles in the bulk solution with a hydrophobic dye, Nile Red, which partitions into the micelles. By comparing the rate at which the dye accumulates on the surface - first for micellar solutions in which the dye is in the micelle, and secondly, for sub-micellar solutions, in which the dye is present as a free molecule in solution - the direct micelle adsorption route is validated. The rate at which the dye accumulates on the surface is measured either from the dynamic tensions themselves or by fluorescence measurements in which donor fluorophores of a phospholipids dye on the surface, through fluorescence resonance energy transfer (FRET), non-radiatively transmit energy to Nile Red adsorbing on the surface. Both techniques demonstrate that in the case of sub-micellar solutions the Nile Red adsorbs slowly to the surface with a delayed induction period, while in the case of micellar solutions the Nile Red incorporated in the micelles is quickly brought to the surface in overwhelming amounts that then desorb back into the bulk at equilibrium. This difference is shown to validate the hypothesis of direct micelle adsorption.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Engineering
