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Bentonite and double hydroxides as emulsifying agents

¹ Unilever Research Port Sunlight, Quarry Road East Bebington, Wirral L63 3JW, UK, and ² University of Kiel, Institute of Inorganic Chemistry, D-24098 Kiel, Germany

^* E-mail: h.mittag{at}email.uni-kiel.de

(Received 5 October 2000; revised 22 February 2001)

The use of solid-laden emulsions (Pickering emulsions) provides an interesting alternative to normal emulsions because the need for organic surfactants is removed or reduced. Combination of a bentonite with a layered double hydroxide represents an effective emulsifying system and provides high stability against coalescence. Solid contents as low as 0.1% (w/w, related to the total mass of the emulsion) are sufficient to prepare very stable O/W emulsions; the mass ratio of hydroxide/bentonite is not critical but should vary between 0.25 and 4 (corresponding to mass fractions of the hydroxide between = 0.2 and 0.8). With large hydroxide contents, stabilization is mainly caused by the envelope of hydroxide particles around the oil droplets. This shell provides a mechanical barrier against coalescence, typical of Pickering emulsions described so far. Addition of increasing amounts of bentonite provides an additional stabilization by building-up a three-dimensional network consisting of both types of particles spanning the coherent phase. A high storage modulus at mass fractions of the hydroxide between 0.2 and 0.8 indicates high flexibility for the inorganic framework which is considered to be an important factor in the high stability of these emulsions.

KEYWORDS: aluminium magnesium hydroxide, antithixotropy, bentonite, emulsions, layered double hydroxide, montmorillonite, Pickering emulsions, rheology