Interfacial self-assemblies of surfactants at water-oil interfaces

Abstract

The generation of novel synthetic materials with functionality and hierarchical ordering is a major focus of current studies in materials science and engineering. Self-assembling amphiphilic materials (such as lipids or surfactants) are an important subgroup of colloids and soft matter and are being used as an ideal candidate for such purposes due to quickly forming rich supramolecular structures across nanometer and micrometer length scales. The immiscible liquid-liquid interfaces which are constrained environment have proved to offer ideal platforms for such endeavors where surface-active nanoparticles or molecules can accumulate and self-assemble. In this thesis, we focus on self-assembly of surfactants which induce formation of nano- or microstructures at the interface between the aqueous solution of a surfactant (ionic or non-ionic) and a polar oil (such as oleic acid). First, self-assembly of different types of surfactants (ionic vs. non-ionic) at immiscible water-oil interface is used for the newly emerged liquid-in-liquid 3D printing. In this printing approach, the aqueous solution of surfactants and the oil constitute the printing phase and support liquid, respectively. Considering the low viscosity of aqueous solutions, the printed liquid constructs using this technique are significantly well-defined and relatively complex in shape. Interfacial rheology is utilized to understand viscoelastic properties of the interfacial layer made between the surfactant solution and oil phase. This gel-like material formed at the interface is robust enough that makes the printed liquid construct perfusable, enabling an injected solution to flow within its structure without any change in the shape and integrity of print. The kinetics of aggregate formation at the water-oil interface is also studied in two cases; when aqueous solution is stagnant (static) and in contact with oil phase and when the aqueous solution is flowing (dynamic) over oil phase. Then relevant models are established for these two conditions and the key aspects of formation of such structures are discussed. According to the proposed models, estimates for solubilization rate of oleic acid into aqueous solution are measured for both dynamic and static conditions. Subsequently, a computational simulation (dissipative particle dynamics) is performed to study the self-assembly behavior of each component (i.e. a cationic surfactant and a polar oil) in water. These self-assemblies behaviors are validated successfully based on the other experimental or simulation studies. Finally, mesoscopic simulation of water-oil interface with presence of all three components (surfactants/oil/water) provides us with an insight into dynamics and the underlying morphological pathway for the structure formation. The significance of this work lies in the printing of low viscous solutions of self-assembling materials into relatively complex designs which is enabled by surfactant self-assemblies and can be of use in various applications such as fabrication of liquid electronics and novel media for encapsulation of cells. This printing approach can be easily applied for different types of surfactants (ionic and non-ionic), block copolymers, biocompatible surfactants, peptides, or proteins. With having relevant kinetic modeling coupled with validated computational simulation of the system, one will be able to tailor desired microstructured materials and properties by tuning the type and concentration of constituent components, and dynamics of the system (i.e. flow rate of aqueous solution).