Emulsions — Recent Advances in Understanding

BERNARD P. BINKS

Surfactant Science Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK

1.1 Introduction

This chapter reviews the progress in the understanding of emulsions over the last ten years or so. The emphasis is on the factors affecting the type and subsequent stability of emulsions, and on the associated properties of surface-active molecules adsorbed at the oil-water interface. The field of emulsions is a vast area and so the literature covered is selective rather than comprehensive.

An emulsion may be defined as an opaque, heterogeneous system of two immiscible liquid phases ('oil' and 'water') where one of the phases is dispersed in the other as drops of microscopic or colloidal size (typically around 1 µm). There are two kinds of simple emulsions, oil-in-water (O/W) and water-in-oil (W/O), depending on which phase comprises the drops. Emulsions made by agitation of the pure immiscible liquids are very unstable and break rapidly to the bulk phases. Such emulsions may be stabilised by the addition of surface-active material which protects the newly formed drops from re-coalescence. An emulsifier is a surfactant which facilitates emulsion formation and aids in stabilisation through a combination of surface activity and possible structure formation at the interface.

This book is concerned with macromulsions and not ITLµITLemulsions. The latter are thermodynamically stable dispersions of oil and water, which means that they form spontaneously and are stable indefinitely. Being optically clear, their characteristic size lies in the range 5–50 nm. Most (macro)emulsions require the input of considerable amounts of energy for their formation and can only be stable in a kinetic sense. However, many systems of oil + water + surfactant which form microemulsions may be emulsified to emulsions and there is a growing interest in relating the properties of these emulsions to the known equilibrium phase behaviour of the corresponding microemulsions. Several recent reviews on microemulsions exist and an aim of this chapter is to discuss, as far as is possible, the behaviour of emulsions, stabilised by low molecular weight surfactants, in terms of the aggregation and adsorption in the micellar/microemulsion systems.

Several books on emulsions have appeared in the last decade. The ones devoted solely to emulsions include chapters on emulsion stability, food emulsions, crude oil emulsions, rheology, pharmaceutical emulsions and perfluorochemical emulsions as blood substitutes. Emulsions are so widely encountered in a huge variety of industries, e.g. agrochemical, food, pharmaceutical, paint, printing, petroleum, etc., that two World Congresses on Emulsions have been held in 1993 and 1997. The proceedings have been published and papers covered areas from manufacturing and stability to wetting and adhesion to applications. It would be impossible to review all aspects of emulsion science and technology in one article, and so the author refers to the many review articles which exist on the topics not to be discussed here, although some are the basis of subsequent chapters. These include emulsion formation, rheology, multiple emulsions, solid-stabilised emulsions, techniques for measurement, parenteral (fluorochemical or phospholipid-stabilised) emulsions, food emulsions, crude oil emulsions, and applications. This list is not exhaustive but it serves to illustrate the scope of the subject.

The chapter is organised into the following sections: Emulsion Type and the System Hydrophile–Lipophile Balance (HLB), Phase Inversion, Emulsion Stability, Gel Emulsions, and Forces between Oil–Water Interfaces.

1.2 Emulsion Type and the System Hydrophile–Lipophile Balance

1.2.1 Emulsions of Two Liquid Phases

Whether an emulsion is O/W or W/O depends on a number of variables like oil:water ratio, electrolyte concentration, temperature, etc. For most of this century, emulsion chemists have known that surfactants more 'soluble' in water tend to make O/W emulsions and surfactants more 'soluble' in oil tend to make W/O emulsions. This is the essence of Bancroft's rule, which states that the continuous phase of an emulsion tends to be the phase in which the emulsifier is preferentially soluble. The word 'soluble' is misleading, however, for two reasons. Firstly, a surfactant may be more soluble in, say, oil than in water in a binary system, but in the ternary system of oil + water -F surfactant it may partition more into water. A good example of this is with the anionic surfactant Aerosol OT (sodium bis-2-ethylhexylsulfosuccinate) which dissolves in heptane at 25 °C up to at least 0.5 m but has a solubility limit in water of only ~0.03 m]. An emulsion made from equal volumes of water and heptane at 25 °C is O/W, however. Secondly, no distinction is made between the solubility of monomeric or aggregated surfactant in oil or water. We will see that this is an important omission.

The first quantitative measure of the balance between the hydrophilic and hydrophobic moieties within a particular surfactant came in 1949 when Griffin introduced the concept of the HLB, or hydrophile–lipophile balance, as a way of predicting emulsion type from surfactant molecular composition. A major problem of the HLB concept is that the HLB numbers assigned to the neat surfactant take no account of the effective HLB of a surfactant in situ adsorbed at an oil-water interface. Thus, for example, a nonionic surfactant of low HLB number (and hence predicted to stabilise W/O emulsions) may form O/W emulsions at low enough temperatures. It therefore became clear that the prevailing conditions of temperature, electrolyte concentration, oil type and chain length and cosurfactant concentration can all modify the geometry of the surfactant at an interface and thus change the...