Pickering emulsions basically manipulate the complex properties of particles where it becomes impossible for them to detach due to the enormous amounts of energy dissipated during adsorption. This therefore directly correlates to the fact that typical colloidal particles attach to fluid-fluid interfaces, made up of relatively polar and apolar fluid phases. This, however, enables the adoption on colloidal particles as alternatives for molecular surfactants as they are cheap, environmentally friendly and can be mass-produced. Presently, much has been done with regard to interparticle interactions, configurations of individual particles, assembly behaviors and stability of Pickering emulsions. Nowadays, interests have been drawing attention to identifying electrostatic interactions and dynamics preceding charged particles adsorbing to a charged interface.
Recent studies have reported that surface charges on individual colloidal particles are usually non-uniform, which therefore leads to heterogeneity in electrostatic interactions between the particles. With regard to this, it is heavily felt that the surface heterogeneity of the particles can directly affect their interface-adsorption properties. Since it is likely that the particle adsorption is mainly determined by the electrostatic interactions between the particles and a fluid–fluid interface, in this paper, a postulate that the particle adsorption to the interface is a heterogeneous and non-deterministic process has been put forward.
Researchers Dong Woo Kang, Jin Hyun Lim and Bum Jun Park from Kyung Hee University in South Korea proposed a study to quantitatively and statistically investigate adsorption behaviors of negatively charged particles to the oil–water interface using optical laser tweezers. They envisioned that the surface heterogeneity of the particle had the ability to directly affect their interface-adsorption properties. Their work is now published in the research journal, Soft Matter.
The research team commenced their study by forming a sessile water drop containing charged polystyrene particles in a glass container filled with oil. They then laterally translated the optically trapped particles to the slope of the oil-water interface. Eventually, the probability of particle adsorption to the interface was measured for the various salt concentrations, upon multiple attempts to attach the particles to the interface.
The authors mainly observed that the ionic strength dependence on the adsorption behaviors of the particles measured on a microscale was consistent with their bulk properties with regard to the formation efficiency/stability of Pickering emulsions and the gravity-induced spontaneous adsorption of the particles to the interface depending on the salt concentration. Specifically, it was found that the probability logarithmically decreased as the ionic strength decreased.
The study by Kyung Hee University researchers presented thorough investigation on the effect of ionic strength on the adsorption behaviors of colloidal particles to the oil–water interface. It has been seen that non-unity of the adsorption probability under particular salt conditions and the holding time dependence offer important evidence that the particle adsorption to the oil–water interface is not deterministic but stochastic. This study opens gate for future works on the effects of the shape of particles, surface charges, holding time, the interface curvature, solution pH and chemical additives, on the adsorption dynamics to fluid–fluid interfaces.
Funded by the National Research Foundation of Korea (NRF-2014R1A5A1009799).
Reproduced from the reference of [Soft Matter, 13, 6234-6242 (2017)] with permission from The Royal Society of Chemistry.
Dong Woo Kang, Jin Hyun Lim and Bum Jun Park. Heterogeneous interface adsorption of colloidal particles. Soft Matter, 2017, 13, 6234
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