Inversion of Emulsions Stabilized Solely by Ionizable Nanoparticles

✍ Scribed by Bernard P. Binks; Jhonny A. Rodrigues

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 137 KB
• Volume: 44
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.200461846

✦ Synopsis

Emulsions occur as end products in a wide range of areas including the pharmaceutical, food, and cosmetic industries. They are mixtures of immiscible liquids where one liquid is dispersed as micrometer-sized drops in the continuous phase of the other. [1] To prevent separation, surfactants are usually added whose molecules cover the liquid-liquid interfaces. Recently, [2][3][4][5] it has been shown that certain solid nanoparticles act as excellent emulsifiers alone and, since particles are more strongly attached to interfaces, coalescence is absent. However, different types of particles are needed to prepare either oil-in-water (o/w) or water-in-oil (w/o) emulsions in mixtures containing equal volumes of the two liquids. [3,[5][6][7][8] Herein we describe a new class of solid particle emulsifier capable of stabilizing both emulsion types efficiently. The spherical nanoparticles are those of polystyrene whose surfaces are coated with ionizable carboxylic acid groups. Inversion of the emulsion type is simply effected by either a change in the pH value or salt concentration. It should be possible to prepare a new range of stable simple and multiple emulsions by tuning the type and charge of such responsive emulsifiers.

Certain small solid particles act in many ways like surfactant molecules that adsorb at fluid-fluid interfaces and stabilize drops in emulsions and bubbles in foams. [3] It is only recently, however, that their precise role has been elucidated in surfactant-free systems. Particles stabilize emulsions in two ways: [2] In the first, they adsorb at the oilwater interface and remain there forming a film around dispersed drops and impede coalescence, and in the second, additional stabilization arises when the particle-particle interactions are such that a three-dimensional network of particles develops in the continuous phase. The enhanced