Pickering emulsions stabilised by Laponite clay particles

TLDR

Ripening is suggested to be arrested when Laplace pressures equalize across drops. The study describes the type and stability of Pickering emulsions stabilized by disc‑like Laponite RD clay particles. Phase‑diagram analysis of aqueous clay–salt dispersions shows that stable toluene‑in‑water emulsions form only when the particles flocculate. The emulsions exhibit drop diameters of 10–28 µm that are independent of clay concentration but vary with oil volume fraction, undergo rapid Ostwald ripening that ceases when Laplace pressures equalize, and remain oil‑in‑water across a range of oils even at high oil fractions due to the clay’s hydrophilicity.

Abstract

The type and stability of Pickering emulsions stabilised by disc-like Laponite RD clay particles are described. By establishing the phase diagram of aqueous dispersions as a function of clay and salt (NaCl) concentration, we deduce that toluene-in-water (o/w) emulsions, stable to creaming and coalescence for at least 6 months, are only formed under conditions where the colloidal particles are flocculated. The initial average drop diameter is independent of clay concentration but depends markedly on oil volume fraction, ranging from 10 to 28 μm. Changes in the drop size distributions with time are shown to be due to Ostwald ripening, which, due to the irreversible nature of particle adsorption at oil/water interfaces, is rapid at first and ceases completely at long times. It is suggested that ripening is arrested when the Laplace pressure inside drops of different sizes becomes equal. For optimum conditions, emulsions prepared using a variety of oils including non-polar alkanes and polar alcohols are always o/w even at high oil phase volume fraction, reflecting the hydrophilic nature of this synthetic clay in oil–water systems.