Effect of charge and hydrophobicity on molecular adsorption at oil-in-water nanoemulsion interfaces

Nanoemulsions (NEs) are mixture of two immiscible liquids, commonly oil droplets in water (O/W) or water droplets in oil (W/O) which are stabilized by relatively low concentration of surfactants. Different types of NEs are widely used in industrial and environmental remediation applications, including oil recovery, drug delivery, food processing, water purification and cleaning up oil spills. However, there are only limited studies pertaining to adsorption of organic molecules at the NE interfaces. For example, the effect of surfactant charge on the adsorption of organic compounds is not well-established. Second harmonic generation (SHG) has been found to be an effective method to study micro- and nano- particle/liquid interfaces. It is an interface-specific technique that can probe equilibrium and dynamic processes occurring at a variety of interfaces. In-situ measurements of kinetics and isotherms for the adsorption of organic molecules at different types of NE interfaces can be studied using SHG spectroscopy. The overall goal of this project has been to deduce the role of coulombic and hydrophobic interactions on the adsorption of organic molecules at the interface of NEs. To achieve this, the objectives of this thesis project are (1) preparing oil-in-water (O/W) NEs using charged, neutral, and aromatic surfactants and characterizing the NEs before and after molecular adsorption using dynamic light scattering, (2) characterizing the dye and surfactant molecules by measuring their surface tension to assess their hydrophobicity, (3) carrying out in-situ measurements of adsorption kinetics and isotherms of charged and neutral dyes for the prepared NEs using SHG to determine Gibbs free energy of adsorptions. Our findings suggest that coulombic interaction is pivotal for molecular adsorption, and other type of interactions can also minimally impact SHG dye adsorption. Different adsorption characteristics in the context of NE interfacial charge properties and their stability following molecular adsorption has been explored in this study.