A first order-based model for the kinetics of formation of Pickering emulsions.

MOTIVATION AND BACKGROUND: Many physical systems are composed of two immiscible fluids containing solid particles whose role is to emulsify the two fluids. Such emulsions are called Pickering emulsions (PE). The present study introduces a theoretical framework for a first order kinetics of the creation of such emulsions and continues to verify the model experimentally using water and oil where water is the majority, or continuous, phase and oil is the minority, or dispersed, phase. These are referred to as O/W emulsions. The motivation for choosing this O/W system is to study the applicability of Pickering emulsions in marine environment and the role these emulsions can play in the cleaning of oil spills. As opposed to the use of surfactants which may be toxic to wildlife, the solid particles used to stabilize PEs are generally non-toxic. Theoretical and experimental methods are employed, as outlined below: THEORETICAL MODEL: A theoretical model based on first order kinetics is constructed. Unlike classic first order kinetics, our reaction is not chemical nor is it of 1:1 stoichiometry, but its time dependence is similar to that of first order. This behavior is a function of various system-specific parameters such as the energies of the different interfaces in the system, the solid particles' size, the densities of the components of the system, and the rate at which the system is agitated. The rate of formation of PEs is found to be proportional to 1-e-kt, where t is the time from the moment the system's components were introduced and k is a constant whose proportionality we describe analytically as a function of the various parameters in the system. EXPERIMENTAL FINDINGS: Our experimental results show exceptionally good agreement with the model, and it is shown that for the specific system tested (water, sand, light fraction petroleum), we get full emulsification of the 100ml system with 5 ml petroleum and 50 g sand within about 30s. This result is encouraging for studies that consider the use of such a system for the cleaning of oil spills.

Drops retracting while forming a rim.

HYPOTHESIS: Surfactant laden droplets may spread faster on a substrate and, subsequently, retract (spontaneously reversing the drop's direction). We hypothesize that this Marmur-Lelah type retraction can be explained by a de Gennes-type triple line fluctuation expression that is modified to represent our anisotropic surfactant adsorption. This explanation requires that the retraction originates inner to the triple line (not at the triple line itself). EXPERIMENTS: Drops of oil with surfactant (mainly tetradecane with octadecylamine) were allowed to spread on freshly cleaved mica surfaces at various concentrations and recorded with a high-speed camera. FINDINGS: At low concentrations, the subsequent retraction left a rim of liquid at the triple line location of maximal spreading whose thickness was inversely related to the surfactant concentration. This indeed agrees with de Gennes' triple line fluctuation expression that is modified to fit the anisotropic adsorption of the surfactant during the spreading period, which induces a solid-liquid interfacial energy gradient. According to this explanation, the rim may exist also at high surfactant solution concentrations, however, there can be a limit at which the surfactant solution concentration is so high that the rim's thickness is practically zero.