Emulsion Formation and Stability

Tharwat F. Tadros

• Publisher: Wiley-VCH
• Year: 2013
• ISBN:
  9783527647965
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

The importance of emulsification techniques, their use in the production of nanoparticles for biomedical applications as well as application of rheological techniques for studying the interaction between the emulsion droplets is gathered in this reference work. Written by some of the top scientists within their respective fields, this book covers such topics as emulsions, nano-emulsions, nano-dispersions and novel techniques for their investigation. It also considers the fundamental approach in areas such as controlled release, drug delivery and various applications of nanotechnology.

Chapter 1

Emulsion Formation, Stability, and Rheology

Tharwat F. Tadros

1.1 Introduction

Emulsions are a class of disperse systems consisting of two immiscible liquids [1–3]. The liquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous phase). Several classes may be distinguished: oil-in-water (O/W), water-in-oil (W/O), and oil-in-oil (O/O). The latter class may be exemplified by an emulsion consisting of a polar oil (e.g., propylene glycol) dispersed in a nonpolar oil (paraffinic oil) and vice versa. To disperse two immiscible liquids, one needs a third component, namely, the emulsifier. The choice of the emulsifier is crucial in the formation of the emulsion and its long-term stability [1–3].

Emulsions may be classified according to the nature of the emulsifier or the structure of the system. This is illustrated in Table 1.1.

Table 1.1 Classification of Emulsion Types

Nature of emulsifier	Structure of the system
Simple molecules and ions	Nature of internal and external phase: O/W, W/O
Nonionic surfactants	—
Surfactant mixtures	Micellar emulsions (microemulsions)
Ionic surfactants	Macroemulsions
Nonionic polymers	Bilayer droplets
Polyelectrolytes	Double and multiple emulsions
Mixed polymers and surfactants	Mixed emulsions
Liquid crystalline phases	—
Solid particles	—

1.1.1 Nature of the Emulsifier

The simplest type is ions such as OH⁻ that can be specifically adsorbed on the emulsion droplet thus producing a charge. An electrical double layer can be produced, which provides electrostatic repulsion. This has been demonstrated with very dilute O/W emulsions by removing any acidity. Clearly that process is not practical. The most effective emulsifiers are nonionic surfactants that can be used to emulsify O/W or W/O. In addition, they can stabilize the emulsion against flocculation and coalescence. Ionic surfactants such as sodium dodecyl sulfate (SDS) can also be used as emulsifiers (for O/W), but the system is sensitive to the presence of electrolytes. Surfactant mixtures, for example, ionic and nonionic, or mixtures of nonionic surfactants can be more effective in emulsification and stabilization of the emulsion. Nonionic polymers, sometimes referred to as polymeric surfactants, for example, Pluronics, are more effective in stabilization of the emulsion, but they may suffer from the difficulty of emulsification (to produce small droplets) unless high energy is applied for the process. Polyelectrolytes such as poly(methacrylic acid) can also be applied as emulsifiers. Mixtures of polymers and surfactants are ideal in achieving ease of emulsification and stabilization of the emulsion. Lamellar liquid crystalline phases that can be produced using surfactant mixtures are very effective in emulsion stabilization. Solid particles that can accumulate at the O/W interface can also be used for emulsion stabilization. These are referred to as Pickering emulsions, whereby particles are made partially wetted by the oil phase and by the aqueous phase.

1.1.2 Structure of the System

1. O/W and W/O macroemulsions: These usually have a size range of 0.1–5 µm with an average of 1–2 µm.

2. Nanoemulsions: these usually have a size range of 20–100 nm. Similar to macroemulsions, they are only kinetically stable.

3. Micellar emulsions or microemulsions: these usually have the size range of 5–50 nm. They are thermodynamically stable.

4. Double and multiple emulsions: these are emulsions-of-emulsions, W/O/W, and O/W/O systems.

5. Mixed emulsions: these are systems consisting of two different disperse droplets that do not mix in a continuous medium. This chapter only deals with macroemulsions.

Several breakdown processes may occur on storage depending on particle size distribution and density difference between the droplets and the medium. Magnitude of the attractive versus repulsive forces determines flocculation. Solubility of the disperse droplets and the particle size distribution determine Ostwald ripening. Stability of the liquid film between the droplets determines coalescence. The other process is phase inversion.

1.1.3 Breakdown Processes in Emulsions

The various breakdown processes are illustrated in Figure 1.1. The physical phenomena involved in each breakdown process are not simple, and it requires analysis of the various surface forces involved. In addition, the above-mentioned processes may take place simultaneously rather than consecutively and this complicates the analysis. Model emulsions, with monodisperse droplets, cannot be easily produced, and hence, any theoretical treatment must take into account the effect of droplet size distribution. Theories that take into account the polydispersity of the system are complex, and in many cases, only numerical solutions are possible. In addition, measurement...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface
• List of Contributors
• Chapter 1: Emulsion Formation, Stability, and Rheology
• Chapter 2: Emulsion Formation in Membrane and Microfluidic Devices
• Chapter 3: Adsorption Characteristics of Ionic Surfactants at Water/Hexane Interface Obtained by PAT and ODBA
• Chapter 4: Measurement Techniques Applicable to the Investigation of Emulsion Formation During Processing
• Chapter 5: Emulsification in Rotor–Stator Mixers
• Chapter 6: Formulation, Characterization, and Property Control of Paraffin Emulsions
• Chapter 7: Polymeric O/W Nano-emulsions Obtained by the Phase Inversion Composition (PIC) Method for Biomedical Nanoparticle Preparation
• Chapter 8: Rheology and Stability of Sterically Stabilized Emulsions
• Index