Development of novel methodologies to analyze the adsorption kinetics of nonionic surfactants

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2007.Includes bibliographical references.When an aqueous surfactant solution is exposed to a clean water/air surface, it takes a finite time for the surfactant molecules to physically transport from the bulk aqueous solution to the surface in order to adsorb and reduce the surface tension. The time scales associated with the reduction in surface tension can vary between milliseconds to hours depending on the surfactant type and its concentration. Accordingly, development of a fundamental understanding of the underlying physical phenomena involved in the kinetics of surfactant adsorption will help to: (i) understand the observed Dynamic Surface Tension (DST) behavior of surfactants, and (ii) design optimal surfactant formulations for applications in which the surfactant adsorption kinetics plays a significant role in determining the effectiveness of the formulation. This thesis deals with modeling the adsorption kinetics of nonionic surfactants at prernicellar surfactant concentrations. Traditionally, the adsorption kinetics of nonionic surfactants at premicellar surfactant concentrations has been understood in the context of two models: (1) the diffusion-controlled model, which assumes that diffusion of surfactant molecules from the bulk solution to the surface is the rate-limiting step, and (2) the mixed diffusion-barrier controlled model, which hypothesizes the existence of an energy barrier for surfactant adsorption from the bulk solution to the surface, and assumes that both diffusion and the energy barrier determine the overall rate of surfactant adsorption. Although the existence of the energy barrier was hypothesized more than 50 years ago, the physical basis underlying the existence of the energy barrier has not yet been elucidated.(cont.) The first major contribution of this thesis was demonstrating that the energy barrier is associated with the adsorption of a single surfactant molecule onto a clean surface, contrary to the broadly-held view that the energy barrier is associated with collective interactions between the adsorbed surfactant molecules. This was demonstrated by developing a generalized mixed diffusion-barrier controlled model and deriving a short-time adsorption kinetics formalism for this generalized model. The short-time formalism revealed that, when adsorption takes place onto an initially clean surface, the adsorption kinetics is independent of the specific interactions between the adsorbed surfactant molecules, and is solely controlled by the energy barrier at asymptotic short times. This observation led to the important conclusion that the energy barrier is related to the adsorption of a single surfactant molecule onto a clean surface. One of the major drawbacks with the traditional procedure to determine the adsorption kinetics rate-limiting mechanism (diffusion-controlled vs. mixed diffusion-barrier controlled), including the values of the relevant adsorption kinetics parameters, from experimental DST data is that it requires a specific model for the equilibrium adsorption behavior of the surfactant, where the deduced results were found to be extremely sensitive to the accuracy of the specific equilibrium model used. As a result, it has not been possible to elucidate the underlying physical basis of the energy barrier by analyzing the experimental DST data of nonionic surfactants.(cont.) With this limitation in mind, the second major contribution of this thesis was the development of a new methodology to determine the adsorption kinetics rate-limiting mechanism, including the values of the relevant adsorption kinetics parameters, from the experimental DST data without using any model for the equilibrium surfactant adsorption behavior. The new methodology was implemented to analyze the experimental DST behavior of several alkyl poly(ethylene) oxide, CiEj, nonionic surfactants, and revealed that the energy barrier may be related to the hydrophobic effect. The third major contribution of this thesis was the development of a novel approach to determine the equilibrium adsorption properties of nonionic surfactants from experimental dynamic surface tension data, a novel concept which has never been explored in the surface tension literature. Motivated by the observed high sensitivity of the predicted DST profiles to the accuracy of the model used to describe the equilibrium surfactant adsorption behavior, a new methodology was developed to determine the Equilibrium Surface Tension versus surfactant bulk solution Concentration (ESTC) behavior of nonionic surfactants from experimental DST data when the adsorption kinetics rate-limiting mechanism is diffusion-controlled. The new methodology requires: (1) experimental DST data measured at a single surfactant bulk solution concentration, Cb, (2) the diffusion coefficient of the surfactant molecule, and (3) one equilibrium surface tension value measured at a single surfactant bulk solution concentration, to determine the entire ESTC curve corresponding to surfactant bulk solution concentrations which are less than, or equal to, Cb. The new methodology was implemented to analyze the experimental pendant-bubble DST data of C12E4 and C12E6.(cont.) For this purpose, the time scale associated with the validity of the assumption involving diffusive transport of surfactant molecules in the bulk solution in a pendant-bubble DST measurement was first determined, and the experimental DST data at those time scales was analyzed using the new methodology to predict the ESTC curves of C12E4 and C12E6. In both cases, the predicted ESTC behavior compared very well with the appropriate experimental DST results reported in the literature. The final major contribution of this thesis was the development of a novel theoretical framework to design optimal surfactant formulations that meet specific adsorption kinetics requirements, which circumvents the more widely used and time consuming experimental trail-and-error surfactant selection approach. Specifically, the new theoretical framework involves using predictive DST models in conjunction with optimization techniques to identify the most efficient surfactant formulation that meets a specific surfactant adsorption kinetics requirement. The technical feasibility of the new theoretical framework and its effectiveness was demonstrated in the context of the adsorption kinetics of nonionic surfactants. Overall, the results obtained in this thesis contribute to: (1) the development of a fundamental physical understanding of the energy barrier, (2) the development of efficient and reliable methodologies to more accurately analyze experimental DST data, and (3) the design of optimal surfactant formulations in industrial applications.by Srinivas Nageswaran Moorkanikkara.Ph.D

Microfluidic Dynamic Interfacial Tensiometry (mu DIT)

We designed, developed and characterized a microfluidic method for the measurement of surfactant adsorption kinetics via interfacial tensiometry on a microfluidic chip. The principle of the measurement is based on the deformability of droplets as a response to hydrodynamic forcing through a series of microfluidic expansions. We focus our analysis on one perfluoro surfactant molecule of practical interest for droplet-based microfluidic applications. We show that although the adsorption kinetics is much faster than the kinetics of the corresponding pendant drop experiment, our droplet-based microfluidicsystem has a sufficient time resolution to obtain quantitative measurement at the sub-second time-scaleon nanoliter droplet volumes, leading to both a gain by a factor of environ10 in time resolution and a downscaling of the measurement volumes by a factor of environ1000 compared to standard techniques. Our approach provides new insight into the adsorption of surfactant molecules at liquid–liquid interfaces in a confined environment, relevant to emulsification, encapsulation and foaming, and the ability to measure adsorption and desorption rate constants