Complex coacervation of food grade antimicrobial lauric arginate with lambda carrageenan.

In this study, the complex coacervation mechanism of Lauric arginate ester (LAE) with λ-carrageenan was studied using turbidimetry, light scattering and electrophoresis. The complexes formed were found to have a bilayer-like structure using small angle X-ray scattering (SAXS) and cryo-TEM (transmission electron microscopy). It was observed that mixing LAE with Sodium dodecyl sulfate (SDS) could significantly reduce the interactions between mixed micelles and λ-carrageenan. The interactions between LAE/SDS and λ-carrageenan were found to be predominantly entropy driven. Mixed micelles of LAE/Tween 20 and LAE/SDS showed significantly less interactions with carrageenan compared to pure LAE micelles. Interfacial properties of complexes were measured using surface tension measurements. It was observed that pure LAE showed good foaming behavior and when mixed with increasing amounts of carrageenan the foaming capacity decreased. Reduction in foam volume was due to reduced availability of free LAE molecules for foam stabilization and due to hydrophilic nature of complexes.

Self assembly of oppositely charged latex particles at oil-water interface.

In this study we explore the self assembly of oppositely charged latex particles at decane water interfaces. Two spreading protocols have been proposed in this context. In the first method oppositely charged particles are mixed prior to spreading at the interface, this is called "premixed-mixtures". In the second protocol negatively charged particles are first spread at the interface at known coverage followed by spreading positively charged particles at known coverage and this is called "sequential-mixtures". In premixed mixtures depending on particle mixing ratio (composition) and total surface coverage a number of 2d structures ranging from 2d crystals, aggregate-crystal coexistence and 2d-gels are observed. A detailed phase diagram of this system has been explored. In sequential-mixtures for the first time we observed a new phase in colloidal monolayers called 2d-bi crystalline domains. These structures consisted regions of two crystal phases of oppositely charged particles separated by a one dimensional chain of alternating positive and negative particles. Phase diagram of this system has also been explored at various combinations of first spread and second spread particles. A possible mechanism leading to formation of these 2d bi crystalline structures has been discussed. A direct visualization of breakage and reformation of particle barriers separating the crystal phases has been demonstrated through videos. Effect of salt in the water sub phase and particle hydrophobicity on domain formation is also investigated.

Stabilization of Pickering Emulsions with Oppositely Charged Latex Particles: Influence of Various Parameters and Particle Arrangement around Droplets.

In this study we explore the fundamental aspects of Pickering emulsions stabilized by oppositely charged particles. Using oppositely charged latex particles as a model system, Pickering emulsions with good long-term stability can be obtained without the need for any electrolyte. The effects of parameters like oil to water ratio, mixed particle composition, and pH on emulsion type and stability are explored and linked to the behavior of the aqueous particle dispersion prior to emulsification. The particle composition is found to affect the formation of emulsions, viz., stable emulsions were obtained close to a particle number ratio of 1:1, and no emulsion was formed with either positively or negatively charged particles alone. The emulsions in particle mixtures exhibited phase inversion from oil-in-water to water-in-oil beyond an oil volume fraction of 0.8. Morphological features of emulsion droplets in terms of particle arrangement on the droplets are discussed.

A model for the prediction of droplet size in Pickering emulsions stabilized by oppositely charged particles.

Colloidal particles irreversibly adsorb at fluid-fluid interfaces stabilizing what are commonly called "Pickering" emulsions and foams. A simple geometrical model, the limited coalescence model, was earlier proposed to estimate droplet sizes in emulsions. This model assumes that all of the particles are effective in stabilization. The model predicts that the average emulsion drop size scales inversely with the total number of particles, confirmed qualitatively with experimental data on Pickering emulsions. In recent years, there has been an increasing interest in synthesizing emulsions with oppositely charged particles (OCPs). In our experimental study, we observed that the drop size varies nonmonotonically with the number ratio of oppositely charged colloids, even when a fixed total number concentration of colloids is used, showing a minimum. We develop a mathematical model to predict this dependence of drop size on number ratio in such a mixed particle system. The proposed model is based on the hypothesis that oppositely charged colloids form stable clusters due to the strong electrostatic attraction between them and that these clusters are the effective stabilizing agents. The proposed model is a two-parameter model, parameters being the ratio of effective charge of OCPs (denoted as k) and the size of the aggregate containing X particles formed due to aggregation of OCPs. Because the size of aggregates formed during emulsification is not directly measurable, we use suitable values of parameters k and X to best match the experimental observations. The model predictions are in qualitative agreement with experimentally observed nonmonotonic variation of droplet sizes. Using experiments and theory, we present a physical insight into the formation of OCP stabilized Pickering emulsions. Our model upgrades the existing Wiley's limited coalescence model as applied to emulsions containing a binary mixture of oppositely charged particles.