| The control over surface tension at fluid interfaces is an issue in systems where the surface to volume ratio is high, such as in the creation of micro-drop arrays and other emerging microfluidics technologies, as well as in more traditional disciplines including the petroleum, agricultural chemicals, food, cosmetics, paper, pharmaceutical and textiles industries. Surfactants are often added to solutions to reduce the surface tension or to manipulate the stress conditions as interface is formed. Since the formation of a fluid interface is a dynamic process, the effectiveness of a surfactant is determined by the rate at which it is delivered to the interface by mass transfer, as well as the thermodynamics which determine how well a given surfactant reduces the surface tension at equilibrium. Most surfactants are ionic. As a charged surfactant adsorbs, it establishes a surface charge density. Therefore, there is an electrostatic element to the dynamic surface tension. In this thesis, the role of electrostatics in determining the dynamic surface tension for ionic surfactants is studied in four parts.; In part I, the evolution of the surface concentration, surface potential and surface tension for adsorption of a charged amphiphile at an interface is studied numerically for typical surfactant material parameters. The surface potential is related to the instantaneous surface charge density determined by the surfactant surface concentration using the Gouy-Chapman model. The sublayer concentration obeys a Boltzmann distribution in instantaneous equilibrium with the surface potential. At equilibrium, this model reduces to the Davies adsorption isotherm. The surface tension evolution is studied for both diffusion-controlled adsorption and mixed kinetic-diffusion controlled adsorption. The occurrence of the shift of controlling mechanism from pure diffusion control at dilute concentrations to mixed kinetic-diffusion control at elevated concentrations is strongly influenced by ionic strength and surfactant valence. As repulsion increases, kinetic barriers are predicted to be controlling at lower concentrations.; In part II, this model is compared to experiments on the dynamic surface tension of aqueous solutions of sodium bis (2-ethylhexyl) sulfosuccinate (Aerosol-OT) obtained by the pendant bubble technique. The equilibrium and dynamic surface tension are studied as a function of concentration and ionic strength controlled by the addition of either the monovalent salt sodium chloride or the divalent salt calcium chloride.; In part III, the system of a soluble, ionic surfactant penetrating an insoluble monolayer is studied numerically. This system provides a controlled means of understanding how intermolecular interactions change equilibration time scales. Signature trends in the time scales for diffusion controlled mass transfer of an ionic surfactant as a function of initial surface coverage of the insoluble monolayer are derived. Mixed kinetic-diffusion controlled mass transfer is also explored.; Finally, in part IV, surfactants whose surface activity can be manipulated in situ are studied using the pendant bubble technique for application in the active control over surface tension. Specifically, the dynamic surface tension of the redox-active ferrocenyl surfactant Fc-CH2-N +-(CH3)2(CH2)14CH 3Br− is studied. The surface activity of this molecule changes as a function of the redox state. At dilute concentrations, the oxidized form of this molecule reduces surface tension strongly in comparison to the reduced state. At higher concentrations, this trend is reversed. This behavior is shown to result from a complex interplay of electrostatics and surface phase behavior as a function of redox state. |
