Probing interfacial properties of polymeric and soluble surfactants -- New tools, new insights.

摘要

Surface-active materials such as amphiphilic molecules, copolymers, proteins, and particles are used throughout engineering and science to modify both equilibrium and dynamic properties of fluid-fluid interfaces in applications as varied as wettability modification, emulsification/foaming, and detergency. A significant challenge facing processing engineers in various industries is the ability to create, control or prevent the formation of foam. The rational incorporation of foaming/de-foaming surfactants into industrial formulations thus requires fundamental knowledge of the physico-chemical mechanisms responsible for properties and performance of the processes/products.;Over the past 60+ years, a general belief has developed that the surface shear viscosity of a given surfactant solution is correlated to the stability of the foam it produces. Published surface shear viscosity measurements for the heavily-studied foaming surfactant sodium dodecyl sulfate (SDS), however, show almost no agreement. Motivated by a critical re-evaluation, in this dissertation, we use a sensitive technique designed specifically to excite surface shear deformations alone, making the most sensitive and precise measurements to date of the surface shear viscosity of SDS solutions. Our measurements reveal the surface shear viscosity of SDS to be immeasurably low, and below the sensitivity limit of our technique, giving an upper bound on the order of ~10-8 N·s/m. Multiple control and complementary measurements support our conclusion, for SDS and a wide variety of high- and low-foaming soluble surfactants of polymeric, ionic, and non-ionic character. These results cast serious doubt on previous measurements of surface shear viscosity for soluble, small-molecule surfactants in general, and particularly on any foam stability/surface shear viscosity correlations based upon on them.;In the second part of the dissertation, we focus on the self-assembly of polymer-based surfactants and nanoparticles on a liquid substrate. These materials are central to many applications, including dispersion stabilization, creation of novel 2D materials and surface patterning. Very often these processes involve compressing interfacial monolayers of particles or polymers to obtain the desired material microstructure. At high surface pressures, however, even highly interfacially-active objects can desorb from the interface. Methods of measuring directly the energy which keeps the polymer or particles bound to the interface (adsorption/desorption energies) are therefore of high interest for these processes. We demonstrate a technique to quantify desorption energies directly, by comparing surface pressure-density compression measurements using a Wilhelmy plate and a custom-microfabricated deflection tensiometer. Focusing on poly(ethylene oxide)-based polymers and nanoparticles, we find that the adsorption energy of PEO homopolymer chains scales linearly with molecular weight, and can be tuned by changing the sub-phase composition. Moreover, the desorption surface pressure of PEO-stabilized nanoparticles corresponds to the saturation surface pressure for spontaneously-adsorbed monolayers, yielding trapping energies of ~ 103 kBT.