Interfaces stabilized by nonionic surfactants: a combined computational and experimental study

Abstract

Nonionic surfactants can be used as stabilizers in emulsions and foams in e.g. food products. This is achieved due to their amphiphilic nature, which makes them strongly adsorb at fluid-fluid interfaces and thereby reduce the interfacial (surface) tension. The aim of this thesis is to characterize the microstructure, phases, and mechanical properties of these interfaces using a multiscale multidisciplinary approach, which integrates state of the art computational methods with surface rheological experiments. Surfactants may undergo different two-dimensional (2D) phase transitions upon compression of the interfacial layer, which is manifested by regions with different slopes in their surface pressure--area isotherm. Investigating interfacial phase transition via brute-force molecular dymanics (MD) simulation is tedious and extremely time consuming. We propose, validate, and successfully utilize a multiscale-coarse graining (MS-CG) scheme for the first time which maps an entire 3D reference system comprising of two fluid phases separated by an interface stabilized by nonionic surfactants, to an effective 2D system of surfactant center-of-masses (COMs) confined to the interface plane. This results in the calculation of an effective lateral pairwise interaction potential between surfactants which respects the equilibrium properties of the interface (surface structure and pressure), and allows to perform fast 2D Monte Carlo (MC) simulations, and even more efficient 2D density functional theory to capture large-scale 2D self-assembly and phase transition within the interface. We further show via coarse grained MD simulations that relatively large nonionic surfactants such as multiblock copolymers may self-assemble into (weak) complex (quasi) 2D microstructures upon adsorption at fluid-fluid interfaces. We perform surface rehology experiments on air-water interfaces stabilized by a class of commercially available nonionic surfactants, known as Pluronic triblock copolymers, in both linear and nonlinear regimes. The in-depth analysis of nonlinear dilatational viscoelastic properties by means of the Lissajous plots and strain-stiffening/strain rate- thickening factors points to the formation of weakly associated microstructures formed by short-ranged bonds that can easily break under applied deformations. While there exist well-established nonequilibrium MD (NEMD) simulation methods for bulk rheology, this part of the field for interfacial rheology is still in its infancy. We propose a novel method which tackles the long-standing problems in NEMD simulation of interfacial systems, that can impose in-plane oscillatory deformations to the interface, prevents the occurrence of unphysically strong secondary bulk flows, and measures the surface stress directly. This gives us the opportunity to evaluate the structural changes in the deformed state and work out their relationship to the surface stress tensor, thus constructing constitutive relations. We apply this scheme to a molecularely resolved liquid-vapor interface stabilized by model triblock copolymers, and compare the results qualitatively with those obtained from our experiments.