Colloidal particles act in many ways like surfactant molecules, particularly if adsorbed to a fluid–fluid interface. Just as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile–lipophile balance (HLB) number, so can that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however, between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. This review attempts to correlate the behaviour observed in systems containing either particles or surfactant molecules in the areas of adsorption to interfaces, partitioning between phases and solid-stabilised emulsions and foams.

Particles as surfactants—similarities and differences

Emulsions stabilised solely by colloidal particles

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

Demulsification techniques of water-in-oil and oil-in-water emulsions in petroleum industry

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.

Nianxi Yan

Papers

On water-in-oil emulsions stabilized by fine solids

Characterization and demulsification of solids-stabilized oil-in-water emulsions Part 1. Partitioning of clay particles and preparation of emulsions

Adsorption and desorption of clay particles at the oil-water interface

Effect of pH on Adsorption and Desorption of Clay Particles at Oil–Water Interface

Measurement of Contact Angles for Fumed Silica Nanospheres Using Enthalpy of Immersion Data.