Emulsion science and technology

Emulsion Science and Technology

Edited by
Tharwat F. Tadros


Related Titles
T.F. Tadros (Ed.)

K.J. Wilkinson, J.R. Lead (Eds.)

Colloids and Interface Science
Series

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Volume 1: The Role of Surface Forces —
Part 1

2007
ISBN: 978-0-470-02432-4

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Volume 3: Colloids Stabilty and
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Fundamentals and Applications

ISBN: 978-3-527-31463-8

2005
ISBN: 978-3-527-30743-2

Volume 4: Colloids in Cosmetics and

Personal Care

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2008

Applied Surfactants

ISBN: 978-3-527-31464-5

Principles and Applications

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ISBN: 978-3-527-30629-9

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Highlights in Colloid Science
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Emulsion Science and Technology

Edited by
Tharwat F. Tadros

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The Editor
Prof. Dr. Tharwat F. Tadros
89 Nash Grove Lane
Wokingham, Berkshire, RG40 4HE
United Kingdom

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ISBN: 978-3-527-32525-2

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V

Contents
Preface XIII
List of Contributors
1
1.1

1.2
1.3
1.3.1
1.4
1.5
1.5.1
1.5.2
1.5.3
1.6
1.6.1
1.6.2
1.6.3
1.6.4
1.6.5
1.7
1.7.1
1.7.2
1.7.3
1.7.4
1.8
1.8.1
1.8.2
1.9
1.9.1

XV

Emulsion Science and Technology: A General Introduction 1
Tharwat F. Tadros
Introduction 1

Industrial Applications of Emulsions 3
The Physical Chemistry of Emulsion Systems 4
The Interface (Gibbs Dividing Line) 4
The Thermodynamics of Emulsion Formation and Breakdown 5
Interaction Energies (Forces) Between Emulsion Droplets and
Their Combinations 7
Van der Waals Attraction 7
Electrostatic Repulsion 9
Steric Repulsion 11
Adsorption of Surfactants at the Liquid/Liquid Interface 12
The Gibbs Adsorption Isotherm 13
Mechanism of Emulsification 16
Methods of Emulsification 18
Role of Surfactants in Emulsion Formation 19
Role of Surfactants in Droplet Deformation 21
Selection of Emulsifiers 25
The Hydrophilic-Lipophilic Balance (HLB) Concept 25
The Phase Inversion Temperature (PIT) Concept 27
The Cohesive Energy Ratio (CER) Concept 29
The Critical Packing Parameter for Emulsion Selection 31
Creaming or Sedimentation of Emulsions 32
Creaming or Sedimentation Rates 33
Prevention of Creaming or Sedimentation 35
Flocculation of Emulsions 37
Mechanism of Emulsion Flocculation 38

Emulsion Science and Technology. Edited by Tharwat F. Tadros
Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-32525-2

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VI

Contents

1.9.1.1
1.9.1.2
1.9.2
1.10
1.11
1.11.1
1.11.2
1.12
1.12.1
1.12.2
1.12.3
1.12.4
1.12.5
1.12.6
1.12.6.1
1.12.6.2
1.12.7
1.12.8
1.12.9
1.12.10

Flocculation of Electrostatically Stabilized Emulsions 38
Flocculation of Sterically Stabilized Emulsions 40

General Rules for Reducing (Eliminating) Flocculation 41
Ostwald Ripening 41
Emulsion Coalescence 43
Rate of Coalescence 44
Phase Inversion 45
Rheology of Emulsions 46
Interfacial Rheology 46
Measurement of Interfacial Viscosity 47
Interfacial Dilational Elasticity 47
Interfacial Dilational Viscosity 48
Non-Newtonian Effects 49
Correlation of Interfacial Rheology with Emulsion Stability 49
Mixed Surfactant Films 49
Protein Films 49
Bulk Rheology of Emulsions 50
Rheology of Concentrated Emulsions 51
Influence of Droplet Deformability on Emulsion Rheology 53
Viscoelastic Properties of Concentrated Emulsions 53
References 55

2

Stabilization of Emulsions, Nanoemulsions and Multiple Emulsions
Using Hydrophobically Modified Inulin (Polyfructose) 57
Tharwat F. Tadros, Elise Vandekerckhove, Martine Lemmens,
Bart Levecke, and Karl Booten
Introduction 57
Experimental 58
Materials 58
Methods 58

Preparation of Emulsions, Nanoemulsions and Multiple Emulsions
Investigation of Emulsion Stability 59
Results and Discussion 59
Emulsion Stability Using INUTEC1SP1 59
Nanoemulsion Stability Using INUTEC1SP1 60
Multiple Emulsion Stability Using INUTEC1 SP1 64
Conclusions 65
References 65

2.1
2.2
2.2.1
2.2.2
2.2.2.1
2.2.2.2
2.3
2.3.1
2.3.2
2.3.3
2.4

3

3.1

Interaction Forces in Emulsion Films Stabilized with Hydrophobically
Modified Inulin (Polyfructose) and Correlation with Emulsion
Stability 67
Tharwat Tadros, Dotchi Exerowa, Georgi Gotchev, Todor Kolarov,
Bart Levecke, and Karl Booten

Introduction 67

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58


Contents

3.2
3.3
3.4

Materials and Methods 68
Results and Discussion 69
Conclusions 73
References 73

4

Enhancement of Stabilization and Performance of Personal Care
Formulations Using Polymeric Surfactants 75
Tharwat F. Tadros, Martine Lemmens, Bart Levecke, and Karl Booten
Introduction 75
Experimental 76
Results and Discussion 76
Massage Lotion 76
Hydrating Shower Gel 79
Soft Conditioner 80
Sun Spray SPF19 81

Conclusions 81
References 81

4.1
4.2
4.3
4.3.1
4.3.2
4.3.3
4.3.4
4.4

5

5.1
5.2
5.3
5.3.1
5.3.2
5.4

6

6.1
6.2
6.2.1
6.2.2
6.3
6.3.1
6.3.2

6.3.3
6.4

Effect of an External Force Field on Self-Ordering of Three-Phase
Cellular Fluids in Two Dimensions 83
Waldemar Nowicki and Gra_zyna Nowicka
Introduction 83
The Model 84
Results and Discussion 85
Energies of Cluster Insertion and Transformation 85
Evolution of the System in a Gravitational Field 90
Conclusions 93
References 94
The Physical Chemistry and Sensory Properties of Cosmetic Emulsions:
Application to Face Make-Up Foundations 97
Frédéric Auguste and Florence Levy
Introduction 97
Materials and Methods 98
Selection of the Foundations to be Studied 98
Characterization Methods 98
Experimental Results and Discussion 99
Drying of the Foundation Bulk and Drift in Composition
During Drying 99
Evolution of Viscosity During Drying 100
Play-Time and Disposition of Foundation on the Skin 102
Conclusions 104
References 104

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VII


VIII

Contents

7

7.1
7.2
7.2.1
7.2.2
7.2.3
7.2.4
7.2.5
7.3
7.3.1
7.3.2
7.3.2.1
7.3.2.2
7.3.3
7.3.3.1
7.3.3.2
7.3.3.3
7.3.4
7.3.4.1
7.3.4.2
7.3.5
7.4

8

8.1
8.2
8.3
8.4
8.4.1
8.4.2
8.4.3
8.5
8.5.1
8.5.2
8.5.3
8.6

Nanoparticle Preparation by Miniemulsion Polymerization 107
Man Wu, Elise Rotureau, Emmanuelle Marie, Edith Dellacherie,
and Alain Durand
Introduction 107
Experimental 108
Materials 108
Emulsion Preparation 108
Polymerization 108
Size Measurement of the Emulsion Droplets 108
Particle Characterization 109
Results and Discussion 109
Synthesis of Hydrophobically Modified Dextrans 109
Preparation of O/W Miniemulsions 111
Control of Initial Droplet Size by Process Variables 111

Influence of Polymer Structure on Initial Droplet Size 112
Stability of Miniemulsions within Polymerization Duration 114
Mechanism and Kinetics of Miniemulsion Polymerization 114
Mechanism and Rate of Emulsion Aging 116
Variation of the Rate of Emulsion Aging with Polymerization
Conditions 118
Preparation of Defined Nanoparticles with Various Monomers 123
Poly(styrene) Nanoparticles Covered by Dextran 123
Poly(butylcyanoacrylate) Nanoparticles 126
Colloidal Properties of the Obtained Suspensions 128
Conclusions 129
References 130
Recent Developments in Producing Monodisperse Emulsions Using
Straight-Through Microchannel Array Devices 133
Isao Kobayashi, Kunihiko Uemura, and Mitsutoshi Nakajima
Introduction 133
Principles of Microchannel Emulsification 135
Straight-Through MC Array Device and Emulsification Set-Up 137
Effect of Channel Shapes on Emulsification Using Symmetric
Straight-Through MC Arrays 139
Effect of Channel Cross-Sectional Shape 139
Effect of the Aspect Ratio of Oblong Channels 139
Computational Fluid Dynamics (CFD) Simulation and Analysis 141
Effect of Process Factors on Emulsification Using Symmetric
Straight-Through MC Arrays 144
Effect of Surfactants and Emulsifiers 144
Effect of To-Be-Dispersed Phase Viscosity 146
Effect of To-Be-Dispersed Phase Flux 148
Scaling-Up of Straight-Through MC Array Devices 149

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Contents

8.7
8.8

Emulsification Using an Asymmetric Straight-Through MC Array 150
Conclusions and Outlook 152
References 154

9

Isotropic and Anisotropic Metal Nanoparticles Prepared by Inverse
Microemulsion 157
Ignác Capek
Introduction 157
Properties of Nanoscale Particles 157
Production of Nanoparticles and Microemulsions 158
General Aspects of Microemulsions 159
Droplet Dimensions 160
The Use of W/O Microemulsions 161
Nanoparticle Preparation 162
Surfactant-Based Methods 164
Coprecipitation 166
Isotropic Nanoparticles 166
Anisotropic Nanoparticles 167
Conclusions and Outlook 179
References 185

9.1
9.1.1
9.1.2
9.2
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
9.3
9.4
9.5

10

10.1
10.1.1
10.1.1.1
10.1.1.2
10.1.2
10.1.3
10.2
10.2.1
10.2.2
10.2.3
10.2.4
10.3
10.3.1
10.3.1.1

10.3.1.2
10.3.1.3
10.3.2
10.4

Preparation of Nanoemulsions by Spontaneous Emulsification
and Stabilization with Poly(caprolactone)-b-poly(ethylene oxide)
Block Copolymers 191
Emmanuel Landreau, Youssef Aguni, Thierry Hamaide, and Yves Chevalier
Introduction 191
Block Copolymers 192
Spontaneous Emulsification 193
Biodegradability and Biocompatibility 194
Block Copolymer Micelles 194
Diblock Copolymers 195
Materials and Methods 195
Materials 195
Synthesis of Block Copolymers (PCL-b-PEO) 196
Methods 196
Emulsification of Oils or PCL 197
Results and Discussion 197
Emulsions of PCL by Spontaneous Emulsification 198
Fabrication of the Emulsions 198
Particle Sizes 200
Stability of the Emulsions 201
Emulsions of Various Oils by Spontaneous Emulsification 203
Conclusions 205
References 206

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IX


X

Contents

11

11.1
11.2
11.2.1
11.2.2
11.2.3
11.2.4
11.3
11.3.1
11.3.2
11.4

12

12.1
12.2
12.2.1
12.2.2
12.2.3
12.2.4
12.2.5

12.3
12.3.1
12.3.2
12.3.3
12.4

13
13.1
13.2
13.3
13.3.1
13.3.2
13.4

Routes Towards the Synthesis of Waterborne Acrylic/Clay
Nanocomposites 209
Gabriela Diaconu, Maria Paulis, and Jose R. Leiza
Introduction 209
Experimental 211
Materials 211
Synthesis of Waterborne (MMA-BA)/MMT Nanocomposites by
Emulsion Polymerization 213
Synthesis of Waterborne (MMA-BA)/MMT Nanocomposites by
Miniemulsion Polymerization 214
Characterization and Measurements 215
Results and Discussion 217
Waterborne Nanocomposites by Emulsion Polymerization 217
Waterborne Nanocomposites by Miniemulsion Polymerization 219
Conclusions 226
References 226

Preparation Characteristics of Giant Vesicles with Controlled Size
and High Entrapment Efficiency Using Monodisperse Water-in-Oil
Emulsions 229
Takashi Kuroiwa, Mitsutoshi Nakajima, Kunihiko Uemura,
Seigo Sato, Sukekuni Mukataka, and Sosaku Ichikawa
Introduction 229
Materials and Methods 230
Materials 230
Preparation of W/O Emulsions Using MC Emulsification 230
Formation of GVs 231
Measurement of Droplet and Vesicle Diameters 232
Determination of Entrapment Yield 232
Results and Discussion 233
Preparation of GVs Using Monodisperse W/O Emulsions 233
Size Control of GVs and Entrapment of a Hydrophilic Molecule
into GVs 234
Formation Characteristics of GVs 237
Conclusions 240
References 241
On the Preparation of Polymer Latexes (Co)Stabilized by Clays 243
Ignác Capek
Introduction 243
Cloisite Clays and Organoclays 247
Radical Polymerization 260
Solution/Bulk Polymerization 260
Radical Polymerization in Micellar Systems 263
Collective Properties of Polymer/MMT Nanocomposites 281

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Contents

13.4.1
13.4.2
13.4.2.1
13.4.2.2
13.4.3
13.4.3.1
13.4.3.2
13.5
13.6
13.7

Kinetic and Molecular Weight Parameters 281
X-Ray Diffraction Studies 284
Homopolymers 284
Copolymers 288
Thermal and Mechanical Properties 290
Polystyrene and Poly(methyl methacrylate) Nanocomposites 290
Poly(ethylene oxide) Nanocomposites 293
Polymer–Inorganic Nanocomposites 296
General 301
Conclusions and Outlook 302
References 310
Index

317

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XI


XIII

Preface
Today, emulsions are applied in a wide variety of industrial products, including food,
cosmetics, pharmaceuticals, agrochemicals, and paints. With this in mind, a series
of World congresses has recently been held – the first in Paris in 1993, the second in
Bordeaux in 1997, the third in Lyon in 2002, and the most recent again in Lyon, in
2006. Following each meeting, a number of topics were selected, the details of which
were subsequently published in the journals Colloids and Surfaces and Advances in
Colloid and Interface Science.
This book contains selected topics from the Fourth World Congress, the title of
which – ‘‘Emulsion Science and Technology’’ – reflects the importance of applying
scientific principles to the preparation and stabilization of emulsion systems.
As a ‘‘introduction’’ to the subject, Chapter 1 provides a general description of the
physical chemistry of emulsion systems, with particular attention being paid to the
interaction forces that occur between emulsion droplets. The adsorption of surfactants at liquid/liquid interfaces is analyzed, and the methods and mechanism of
emulsification and role of surfactants described. Those methods applicable to
emulsifier selection are also detailed, as are the various emulsion breakdown
processes such as creaming or sedimentation, flocculation, Ostwald ripening,
coalescence and phase inversion. Methods used to prevent such breakdown processes are also detailed. Chapter 2 relates to the special application of a polymeric
surfactant (a hydrophobically modified inulin) for the stabilization of emulsions,
nanoemulsions, and multiple emulsions, while Chapter 3 provides the details of a
fundamental study of the interaction forces in emulsion films stabilized with
hydrophobically modified inulin and the correlation with emulsion stability. In
Chapter 4, the application of polymeric surfactants for enhancing the stabilization
and performance of personal care formulations – such as massage lotions, hydrating

shower gel, soft conditioners, and sun sprays – is described, while Chapter 5
provides the details of a more fundamental study of the effect of external force
fields on the self-ordering of three-phase cellular fluids in two dimensions. Here,
attention is focused on the energies of cluster insertion and transformation, and the
evolution of the system in a gravitational field. Chapter 6 relates to the application of
the physical chemistry and sensory properties of cosmetic formulations, with the

Emulsion Science and Technology. Edited by Tharwat F. Tadros
Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-32525-2

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XIV

Preface

example of facial make-up being used to illustrate the principles involved in both
drying and the evolution of viscosity. In Chapter 7, a detailed account is provided of
nanoparticle preparation using miniemulsion (nanoemulsion) polymerization, and
for which a variety of monomers (e.g., styrene and butylcyanoacrylate) are used to
illustrate the principles. In Chapter 8, the details of some recent developments in the
production of monodisperse emulsions using straight-through microchannel array
devices are provided, while Chapter 9 outlines not only the preparation of isotropic
and anisotropic nanoparticles (using inverse microemulsions) but also the properties of the nanoparticulate product. The preparation of nanoemulsions by spontaneous emulsification and stabilization of the resulting nanodroplets by block
copolymers, namely poly(caprolactone-b-poly(ethylene oxide), are described in
Chapter 10, while the routes for the synthesis of waterborne acrylic/clay nanocomposites (prepared by miniemulsion polymerization) are outlined in Chapter 11. The
preparation of giant vesicles with a controlled size and a high entrapment efficiency,
by using monodisperse water-in-oil emulsions, is detailed in Chapter 12, while the

final chapter describes the preparation of polymer latexes stabilized with clay
particles, and the possible preparation of nanocomposites, using the same approach.
Based on the above descriptions and details, it is clear that this book covers a wide
range of topics, both fundamental and applied, and also highlights the importance of
emulsion science in many modern-day industrial applications. It is hoped that the
book will be of great help to emulsion research scientists in both academia and
industry.
Finally, I would like to thank the organizers of the Fourth World Congress – and in
particular Dr Alain Le Coroller and Dr Jean-Erik Poirier – for inviting me to edit this
book.
January 2009

Tharwat Tadros

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XV

List of Contributors
Youssef Aguni
UMR 5007 CNRS – Université de Lyon
Laboratoire d0 Automatique et de Génie
des Procédés – LAGEP
Bât 308, 43 Bd du 11 Novembre
69622 Villeurbanne Cedex
France

Yves Chevalier
UMR 5007 CNRS – Université de Lyon

Laboratoire d0 Automatique et de Génie
des Procédés – LAGEP
Bât 308, 43 Bd du 11 Novembre
69622 Villeurbanne Cedex
France

Frédéric Auguste
L’Oréal – Centre de Chevilly-Larue
188 rue Paul Hochart
94150 Chevilly Larue
France

Edith Dellacherie
CNRS-Nancy-University ENSIC
Laboratoire de Chimie Physique
Macromoléculaire
1 rue Grandville
54001 Nancy Cedex
France

Karl Booten
ORAFTI Bio Based Chemicals
Aandorenstraat 1
3300 Tienen
Belgium
Ignác Capek
Slovak Academy of Sciences
Polymer Institute
Institute of Measurement Science
Dúbravská cesta 9

842 36 Bratislava
Slovakia
and
Tren4cín University
Faculty of Industrial Technologies
UI. I. Krasku 30
020 01 Púchov
Slovakia

Gabriela Diaconu
University of the Basque Country
Facultad de Ciencias Químicas
POLYMAT, Joxe Mari Korta zentroa
Tolosa Etorbidea 72
20018 Donostia-San Sebastián
Spain
Alain Durand
CNRS-Nancy-University ENSIC
Laboratoire de Chimie Physique
Macromoléculaire
1 rue Grandville
54001 Nancy Cedex
France

Emulsion Science and Technology. Edited by Tharwat F. Tadros
Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-32525-2

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XVI

List of Contributors

Dotchi Exerowa
Bulgarian Academy of Sciences
Institute of Physical Chemistry
Acad. G. Bonchev Str.
Sofia 1113
Bulgaria
Georgi Gotchev
Bulgarian Academy of Sciences
Institute of Physical Chemistry
Acad. G. Bonchev Str.
Sofia 1113
Bulgaria
Thierry Hamaide
Laboratoire de Chimie et Procédés
de Polymérisation LCPP
CPE Lyon
69622 Villeurbanne Cedex
France
Present address:
Université de Lyon
Ingénierie des Matériaux Polymères
LMPB, UMR 5223
15 Bd Latarjet
69622 Villeurbanne Cedex
France

Sosaku Ichikawa
University of Tsukuba
Graduate School of Life and
Environmental Sciences
Tennodai 1-1-1
Tsukuba
Ibaraki 305-8572
Japan

Takashi Kuroiwa
University of Tsukuba
Graduate School of Life and
Environmental Sciences
Tennodai 1-1-1
Tsukuba
Ibaraki 305-8572
Japan
and
National Food Research Institute
Food Engineering Division
Kannondai 2-1-12
Tsukuba
Ibaraki 305-8642
Japan
Isao Kobayashi
National Food Research Institute
Food Engineering Division
2-1-12 Kannondai
Tsukuba
Ibaraki 305-8642

Japan
Todor Kolarov
Bulgarian Academy of Sciences
Institute of Physical Chemistry
Acad. G. Bonchev Str.
Sofia 1113
Bulgaria
Emmanuel Landreau
UMR 5007 CNRS – Université de Lyon
Laboratoire d0 Automatique et de Génie
des Procédés LAGEP
Bât 308 43 Bd du 11 Novembre
69622 Villeurbanne Cedex
France

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List of Contributors

Jose R. Leiza
University of the Basque Country
Institute for Polymer Materials
POLYMAT, Joxe Mari Korta zentroa
Tolosa Etorbidea 72
20018 Donostia-San Sebastián
Spain
Martine Lemmens
ORAFTI Bio Based Chemicals
Aandorenstraat 1

3300 Tienen
Belgium
Bart Levecke
ORAFTI Bio Based Chemicals
Aandorenstraat 1
3300 Tienen
Belgium
Florence Levy
L'Oréal – Centre de Chevilly-Larue
188 rue Paul Hochart
94150 Chevilly Larue
France
Emmanuelle Marie
CNRS-Nancy-University ENSIC
Laboratoire de Chimie Physique
Macromoléculaire
1 rue Grandville
54001 Nancy cedex
France
Sukekuni Mukataka
University of Tsukuba
Graduate School of Life and
Environmental Sciences
Tennodai 1-1-1
Tsukuba
Ibaraki 305-8572
Japan

Mitsutoshi Nakajima
University of Tsukuba

Graduate School of Life and
Environmental Sciences
Tennodai 1-1-1
Tsukuba
Ibaraki 305-8572
Japan
and
National Food Research Institute
Food Engineering Division
Kannondai 2-1-12
Tsukuba
Ibaraki 305-8642
Japan
Waldemar Nowicki
A. Mickiewicz University
Faculty of Chemistry
Grundwadzka 6
60-780 Poznan´
Poland
.
Grazyna Nowicka
A. Mickiewicz University
Faculty of Chemistry
Grundwadzka 6
60-780 Poznan´
Poland
Maria Paulis
University of the Basque Country
Institute for Polymer Materials
POLYMAT, Joxe Mari Korta zentroa

Tolosa Etorbidea 72
20018 Donostia-San Sebastián
Spain

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XVII


XVIII

List of Contributors

Elise Rotureau
CNRS-Nancy-University ENSIC
Laboratoire de Chimie Physique
Macromoléculaire
1 rue Grandville
54001 Nancy cedex
France

Kunihiko Uemura
National Food Research Institute
Food Engineering Division
2-1-12 Kannondai
Tsukuba
Ibaraki 305-8642
Japan

Seigo Sato

University of Tsukuba
Graduate School of Life and
Environmental Sciences
Tennodai 1-1-1
Tsukuba
Ibaraki 305-8572
Japan

Elise Vandekerckhove
ORAFTI Bio Based Chemicals
Aandorenstraat 1
3300 Tienen
Belgium

Tharwat F. Tadros
89 Nash Grove Lane
Wokingham, Berkshire RG40 4HE
UK

Man Wu
CNRS-Nancy-University ENSIC
Laboratoire de Chimie Physique
Macromoléculaire
1 rue Grandville
54001 Nancy cedex
France

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j1

1
Emulsion Science and Technology: A General Introduction
Tharwat F. Tadros

1.1
Introduction

Emulsionsare a class ofdispersesystems consisting of twoimmiscible liquids[1–3].The
liquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous
phase). Several classes of emulsion may be distinguished, namely 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. In order to disperse two immiscible liquids a third
component is required, namely the emulsifier; the choice of emulsifier is crucial not only
for the formation of the emulsion but also for its long-term stability [1–3].
Emulsions may be classified according to the nature of the emulsifier or the
structure of the system (see Table 1.1).
Several processes relating to the breakdown of emulsions may occur on storage,
depending on:
.
.
.
.
.

the particle size distribution and the density difference between the droplets and
the medium;
the magnitude of the attractive versus repulsive forces, which determines

flocculation;
the solubility of the disperse droplets and the particle size distribution, which in
turn determines Ostwald ripening;
thestabilityoftheliquidfilmbetweenthedroplets,whichdeterminescoalescence;and
phase inversion.

The various breakdown processes are illustrated schematically in Figure 1.1.
The physical phenomena involved in each breakdown process is not simple, and
requires an analysis to be made of the various surface forces involved. In addition, the
above processes may take place simultaneously rather then consecutively, which in turn
complicates the analysis. Model emulsions, with monodisperse droplets, cannot be
easily produced and hence any theoretical treatment must take into account the effect of

Emulsion Science and Technology. Edited by Tharwat F. Tadros
Copyright Ó 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-32525-2

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j 1 Emulsion Science and Technology: A General Introduction

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Table 1.1 Classification of emulsion types.

Nature of emulsifier

Structure of the system

Simple molecules and ions
Nonionic surfactants
Surfactant mixtures
Ionic surfactants
Nonionic polymers
Polyelectrolytes
Mixed polymers and surfactants
Liquid crystalline phases
Solid particles

Nature of internal and external phase:
O/W, W/O
Micellar emulsions (microemulsions)
Macroemulsions
Bilayer droplets
Double and multiple emulsions
Mixed emulsions

Figure 1.1 Schematic representation of the various breakdown processes in emulsions.

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, the
measurement of surfactant and polymer adsorption in an emulsion is not simple, and
such information must be extracted from measurements made at a planar interface.
A summary of each of the above breakdown processes is provided in the following
sections, together with details of each process and methods for its prevention.
Creaming and Sedimentation This process results from external forces, usually
gravitational or centrifugal. When such forces exceed the thermal motion of the
droplets (Brownian motion), a concentration gradient builds up in the system such

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1.2 Industrial Applications of Emulsions

that the larger droplets move more rapidly either to the top (if their density is less than
that of the medium) or to the bottom (if their density is greater than that of the
medium) of the container. In the limiting cases, the droplets may form a close-packed
(random or ordered) array at the top or bottom of the system, with the remainder of
the volume occupied by the continuous liquid phase.
Flocculation This process refers to aggregation of the droplets (without any change
in primary droplet size) into larger units. It is the result of the van der Waals
attractions which are universal with all disperse systems. Flocculation occurs when
there is not sufficient repulsion to keep the droplets apart at distances where the van
der Waals attraction is weak. Flocculation may be either ‘strong’ or ‘weak’, depending
on the magnitude of the attractive energy involved.
Ostwald Ripening (Disproportionation) This effect results from the finite solubility (etc.) of the liquid phases. Liquids which are referred to as being ‘immiscible’
often have mutual solubilities which are not negligible. With emulsions which are
usually polydisperse, the smaller droplets will have a greater solubility when
compared to larger droplets (due to curvature effects). With time, the smaller droplets
disappear and their molecules diffuse to the bulk and become deposited on the larger
droplets. With time, the droplet size distribution shifts to larger values.
Coalescence This refers to the process of thinning and disruption of the liquid film
between the droplets, with the result that fusion of two or more droplets occurs to
form larger droplets. The limiting case for coalescence is the complete separation of
the emulsion into two distinct liquid phases. The driving force for coalescence is the
surface or film fluctuations; this results in a close approach of the droplets whereby
the van der Waals forces are strong and prevent their separation.
Phase Inversion This refers to the process whereby there will be an exchange
between the disperse phase and the medium. For example, an O/W emulsion may

with time or change of conditions invert to a W/O emulsion. In many cases, phase
inversion passes through a transition state whereby multiple emulsions are produced.

1.2
Industrial Applications of Emulsions

Several industrial systems consist of emulsions of which the following are worthy of
mention:
.
.
.

Food emulsions, such as mayonnaise, salad creams, deserts and beverages.
Personal care and cosmetic products, such as hand-creams, lotions, hair-sprays and
sunscreens.
Agrochemicals - self-emulsifiable oils which produce emulsions on dilution with
water, emulsion concentrates (droplets dispersed in water; EWs) and crop oil sprays.

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j 1 Emulsion Science and Technology: A General Introduction

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.
.
.

.
.
.

Pharmaceuticals, such as anesthetics of O/W emulsions, lipid emulsions, double
and multiple emulsions.
Paints, such as emulsions of alkyd resins and latex emulsions.
Dry-cleaning formulations; these may contain water droplets emulsified in the drycleaning oil, which is necessary to remove soils and clays.
Bitumen emulsions are prepared stable in their containers but, when applied the
road chippings, they must coalesce to form a uniform film of bitumen.
Emulsions in the oil industry - many crude oils contain water droplets (e.g. North Sea
oil); these must be removed by coalescence followed by separation.
Oil slick dispersants - oil spilled from tankers must be emulsified and then separated.
The emulsification of unwanted oil is a very important process in pollution control.

The above-described utilization of emulsions in industrial processes justifies the
vast amount of basic research which is conducted aimed at understanding the origins
of the instability of emulsions and developing methods to prevent their break down.
Unfortunately, fundamental research into emulsions is not straightforward, as
model systems (e.g. with monodisperse droplets) are difficult to produce. In fact,
in many cases, the theoretical bases of emulsion stability are not exact and consequently semi-empirical approaches are used.

1.3
The Physical Chemistry of Emulsion Systems
1.3.1
The Interface (Gibbs Dividing Line)

An interface between two bulk phases, such as liquid and air (or liquid/vapor) or two
immiscible liquids (oil/water), may be defined provided that a dividing line is
introduced (Figure 1.2). The interfacial region is not a layer that is one molecule

thick; rather, it has a thickness d with properties that differ from those of the two bulk
phases a and b.
By using the Gibbs model, it is possible to obtain a definition of the surface or
interfacial tension g.
The surface free energy dGs is composed of three components: an entropy term
P
s
S dT; an interfacial energy term Adg; and a composition term nidmi (where ni is the

Figure 1.2 The Gibbs dividing line.

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1.4 The Thermodynamics of Emulsion Formation and Breakdown

number of moles of component i with chemical potential mi). The Gibbs–Deuhem
equation is therefore,
dGs ẳ Ss dT ỵ Adg ỵ

X

ni dmi

1:1ị

At constant temperature and composition,
dGs ẳ Adg
 s
qG

gẳ
qA T;ni

1:2ị

For a stable interface g is positive – that is, if the interfacial area increases, then Gs
increases. Note that g is energy per unit area (mJ mÀ2), which is dimensionally
equivalent to force per unit length (mN mÀ1), the unit usually used to define surface
or interfacial tension.
For a curved interface, one should consider the effect of the radius of curvature.
Fortunately, g for a curved interface is estimated to be very close to that of a planar
surface, unless the droplets are very small (<10 nm). Curved interfaces produce
some other important physical phenomena which affect emulsion properties, such
as the Laplace pressure Dp which is determined by the radii of curvature of the
droplets,

Dp ẳ g

1
1
ỵ
r1 r2


1:3ị

where r1 and r2 are the two principal radii of curvature.
For a perfectly spherical droplet r1 ẳ r2 ẳ r and
Dp ẳ

2g
r

1:4ị

For a hydrocarbon droplet with radius 100 nm, and g ¼ 50 mN mÀ1, Dp $ 106 Pa
(10 atm).

1.4
The Thermodynamics of Emulsion Formation and Breakdown

Consider a system in which an oil is represented by a large drop 2 of area A1
immersed in a liquid 2, which is now subdivided into a large number of smaller
droplets with total area A2 (such that A2 ) A1), as shown in Figure 1.3. The interfacial
tension g 12 is the same for the large and smaller droplets as the latter are generally in
the region of 0.1 mm to few microns in size.
The change in free energy in going from state I to state II is made from two
contributions: a surface energy term (that is positive) that is equal to DAg 12 (where

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Figure 1.3 Schematic representation of emulsion formation and breakdown (see text for details).

DA ¼ A2 – A1). An entropy of dispersions term which is also positive (since the
production of a large number of droplets is accompanied by an increase in
configurational entropy) which is equal to TDSconf.
From the second law of thermodynamics,
DGform ¼ DAg 12 ÀTDSconf

ð1:5Þ

In most cases, DAg 12 ) ÀTDSconf, which means that DGform is positive – that is, the
formation of emulsions is nonspontaneous and the system is thermodynamically
unstable. In the absence of any stabilization mechanism, the emulsion will break by
flocculation, coalescence, Ostwald ripening, or a combination of all these processes.
This situation is illustrated graphically in Figure 1.4, where several paths for
emulsion breakdown processes are represented.
In the presence of a stabilizer (surfactant and/or polymer), an energy barrier is
created between the droplets and therefore the reversal from state II to state I
becomes noncontinuous as a result of the presence of these energy barriers. This is
illustrated graphically in Figure 1.5 where, in the presence of the above energy
barriers, the system becomes kinetically stable.

Figure 1.4 The free energy path in emulsion breakdown.
Solid line: flocculation þ coalescence.
Broken line: flocculation þ coalescence þ sedimentation.
Dotted line: flocculation þ coalescence þ sedimentation þ Ostwald ripening.

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1.5 Interaction Energies (Forces) Between Emulsion Droplets and Their Combinations

Figure 1.5 Schematic representation of the free energy path for
the breakdown (flocculation and coalescence) of systems
containing an energy barrier.

1.5
Interaction Energies (Forces) Between Emulsion Droplets and Their Combinations

Generally speaking, there are three main interaction energies (forces) between
emulsion droplets, the details of which are discussed in the following sections.
1.5.1
Van der Waals Attraction

The van der Waals attraction between atoms or molecules are of three different types:
(i) dipole–dipole (Keesom); (ii) dipole-induced dipole ((Debye-)interactions); and
(iii) dispersion (London interactions). The Keesom and Debye attraction forces are
vectors, and although the dipole–dipole or dipole-induced dipole attraction is large
they tend to cancel due to the different orientations of the dipoles. Thus, the most
important are the London dispersion interactions, which arise from charge fluctuations. With atoms or molecules consisting of a nucleus and electrons that are
continuously rotating around the nucleus, a temporary dipole is created as a result
of charge fluctuations. This temporary dipole induces another dipole in the adjacent
atom or molecule. The interaction energy between two atoms or molecules Ga is short
range and is inversely proportional to the sixth power of the separation distance r
between the atoms or molecules,
Ga ẳ

b
r6

1:6ị

where b is the London dispersion constant that is determined by the polarizability of
the atom or molecule.

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Hamaker [4] suggested that the London dispersion interactions between atoms or
molecules in macroscopic bodies (such as emulsion droplets) could be added, and
this would result in a strong van der Waals attraction, particularly at close distances of
separation between the droplets. For two droplets with equal radii R, at a separation
distance h, the van der Waals attraction GA is given by the following equation (due to
Hamaker),
GA ẳ

AR
12h

1:7ị

where A is the effective Hamaker constant,
1=2

1=2

A ¼ ðA11 ÀA22 Þ2

ð1:8Þ

and where A11 and A22 are the Hamaker constants of droplets and dispersion
medium, respectively.
The Hamaker constant of any material depends on the number of atoms or
molecules per unit volume q and the London dispersion constant b,
1:9ị

A ẳ pq2 b

GA is seen to increase very rapidly with a decrease of h (at close approach). This is
illustrated in Figure 1.6 which shows the van der Waals energy–distance curve for two
emulsion droplets with separation distance h.

Figure 1.6 Variation of the Van der Waals attraction energy with separation distance.

In the absence of any repulsion, flocculation occurs very rapidly to produce large
clusters. In order to counteract the van der Waals attraction, it is necessary to create a
repulsive force. Two main types of repulsion can be distinguished depending on the
nature of the emulsifier used: (i) electrostatic, which occurs due to the creation of
double layers; and (ii) steric, which occurs due to the presence of adsorbed surfactant
or polymer layers.

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