Tharwat F. Tadros

Formulation of Disperse Systems

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Tharwat F. Tadros

Formulation of Disperse Systems

Science and Technology

The Author

Prof. Dr. Tharwat F. Tadros89 Nash Grove LaneRG40 4HE Wokingham, BerkshireUnited Kingdom

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V

Contents

Preface XVII

1 General Introduction 11.1 Suspensions 11.2 Latexes 21.3 Emulsions 21.4 Suspoemulsions 31.5 Multiple Emulsions 31.6 Nanosuspensions 41.7 Nanoemulsions 41.8 Microemulsions 51.9 Pigment and Ink Dispersions 51.10 Foams 5

References 9

2 Surfactants Used in Formulation of Dispersions 112.1 General Classification of Surface-Active Agents 122.1.1 Anionic Surfactants 132.1.1.1 Carboxylates 132.1.1.2 Sulphates 142.1.1.3 Sulphonates 152.1.1.4 Phosphate-Containing Anionic Surfactants 162.1.2 Cationic Surfactants 162.1.3 Amphoteric (Zwitterionic) Surfactants 172.1.4 Nonionic Surfactants 182.1.4.1 Alcohol Ethoxylates 192.1.4.2 Alkyl Phenol Ethoxylates 192.1.4.3 Fatty Acid Ethoxylates 202.1.4.4 Sorbitan Esters and Their Ethoxylated Derivatives (Spans and

Tweens) 202.1.4.5 Ethoxylated Fats and Oils 212.1.4.6 Amine Ethoxylates 212.1.4.7 Amine Oxides 21

VI Contents

2.1.5 Specialty Surfactants 222.1.5.1 Fluorocarbon and Silicone Surfactants 222.1.5.2 Gemini Surfactants 232.1.5.3 Surfactants Derived from Monosaccharides and Polysaccharides 23

References 24

3 Physical Chemistry of Surfactant Solutions and the Process ofMicellisation 27

3.1 Thermodynamics of Micellisation 333.1.1 Kinetic Aspects 343.1.2 Equilibrium Aspects: Thermodynamics of Micellisation 353.2 Enthalpy and Entropy of Micellisation 373.2.1 Driving Force for Micelle Formation 383.2.2 Micellisation in Surfactant Mixtures (Mixed Micelles) 40

References 43

4 Dispersants and Polymeric Surfactants 454.1 Solution Properties of Polymeric Surfactants 464.2 General Classification of Polymeric Surfactants 504.3 Polyelectrolytes 53

References 54

5 Adsorption of Surfactants at the Air/Liquid, Liquid/Liquid, andSolid/Liquid Interfaces 55

5.1 Introduction 555.2 Adsorption of Surfactants at the Air/Liquid (A/L) and Liquid/Liquid

(L/L) Interfaces 565.3 The Gibbs Adsorption Isotherm 575.4 Equation of State Approach 605.5 The Langmuir, Szyszkowski, and Frumkin Equations 625.6 Interfacial Tension Measurements 635.6.1 The Wilhelmy Plate Method 635.6.2 The Pendant Drop Method 645.6.3 The Du Nouy’s Ring Method 645.6.4 The Drop Volume (Weight) Method 655.6.5 The Spinning Drop Method 655.7 Adsorption of Surfactants at the Solid/Liquid (S/L) Interface 665.7.1 Adsorption of Ionic Surfactants on Hydrophobic Surfaces 685.7.2 Adsorption of Ionic Surfactants on Polar Surfaces 715.7.3 Adsorption of Nonionic Surfactants 72

References 74

6 Adsorption of Polymeric Surfactants at the Solid/Liquid Interface 776.1 Theories of Polymer Adsorption 80

Contents VII

6.2 Experimental Techniques for Studying Polymeric SurfactantAdsorption 88

6.2.1 Measurement of the Adsorption Isotherm 886.2.2 Measurement of the Fraction of Segments, p 896.3 Determination of Segment Density Distribution ρ(z) and Adsorbed

Layer Thickness δh 896.4 Examples of the Adsorption Isotherms of Nonionic Polymeric

Surfactants 926.4.1 Adsorbed Layer Thickness Results 966.4.2 Kinetics of Polymer Adsorption 98

References 98

7 Colloid Stability of Disperse Systems Containing Electrical DoubleLayers 101

7.1 Origin of Charge on Surfaces 1017.1.1 Surface Ions 1017.1.2 Isomorphic Substitution 1027.2 Structure of the Electrical Double Layer 1037.2.1 Diffuse Double layer (Gouy and Chapman) 1037.3 Stern–Grahame Model of the Double Layer 1047.4 Distinction between Specific and Nonspecific Adsorbed Ions 1047.5 Electrical Double Layer Repulsion 1057.6 van der Waals Attraction 1067.7 Total Energy of Interaction 1097.7.1 Deryaguin–Landau–Verwey–Overbeek (DLVO) Theory 1097.8 Flocculation of Suspensions 1117.9 Criteria for Stabilisation of Dispersions with Double Layer

Interaction 113References 114

8 Stability of Disperse Systems Containing Adsorbed NonionicSurfactants or Polymers: Steric Stabilisation 115

8.1 Introduction 1158.2 Interaction between Particles Containing Adsorbed Nonionic and

Polymeric Surfactant Layers (Steric Stabilisation) 1168.3 Mixing Interaction Gmix 1178.4 Elastic Interaction Gel 1188.5 Total Energy of Interaction 1198.6 Criteria for Effective Steric Stabilisation 1208.7 Flocculation of Sterically Stabilised Dispersions 1218.7.1 Weak Flocculation 1218.7.2 Incipient Flocculation 1218.7.3 Depletion Flocculation 122

References 123

VIII Contents

9 Formulation of Solid/Liquid Dispersions (Suspensions) 1259.1 Introduction 1259.2 Preparation of Suspensions 1269.3 Condensation Methods: Nucleation and Growth 1269.4 Dispersion Methods 1289.4.1 Wetting of Powders by Liquids 1299.4.2 Structure of the Solid/Liquid Interface and the Electrical Double

Layer 1319.4.2.1 Electrical Double Layer Repulsion 1329.4.2.2 van der Waals Attraction 1329.4.2.3 Total Energy of Interaction 1339.4.2.4 Criteria for Stabilisation of Suspensions with Double Layer

Interaction 1359.4.2.5 Electrokinetic Phenomena and the Zeta-Potential 1359.4.2.6 Calculation of the Zeta-Potential 1369.4.2.7 Measurement of the Zeta-Potential 1379.4.3 Dispersing Agents for Formulation of Suspensions 1399.4.4 Adsorption of Surfactants at the Solid/Liquid Interface 1399.4.5 Steric Stabilisation of Suspensions 1419.4.6 Flocculation of Sterically Stabilised Suspensions 1439.4.7 Properties of Concentrated Suspensions 1449.4.8 Characterisation of Suspensions and Assessment of their

Stability 1499.4.8.1 Optical Microscopy 1509.4.8.2 Electron Microscopy 1519.4.8.3 Confocal Laser Scanning Microscopy 1519.4.8.4 Scattering Techniques 1519.5 Bulk Properties of Suspensions 1529.5.1 Rheological Measurements 1529.5.2 Sedimentation of Suspensions and Prevention of Formation of

Dilatant Sediments (Clays) 1539.5.3 Prevention of Sedimentation and Formation of Dilatant

Sediments 156References 159

10 Formulation of Liquid/Liquid Dispersions (Emulsions) 16110.1 Introduction 16110.1.1 Creaming and Sedimentation 16110.1.2 Flocculation 16210.1.3 Ostwald Ripening (Disproportionation) 16210.1.4 Coalescence 16310.1.5 Phase Inversion 16310.2 Industrial Applications of Emulsions 16310.3 Physical Chemistry of Emulsion Systems 16410.3.1 The Interface (Gibbs Dividing Line) 164

Contents IX

10.3.2 Thermodynamics of Emulsion Formation and Breakdown 16510.3.3 Interaction Energies (Forces) between Emulsion Droplets and Their

Combinations 16610.3.3.1 van der Waals Attractions 16610.3.3.2 Electrostatic Repulsion 16710.3.3.3 Steric Repulsion 17010.4 Adsorption of Surfactants at the Liquid/Liquid Interface 17210.4.1 Mechanism of Emulsification 17410.4.2 Methods of Emulsification 17510.4.3 Role of Surfactants in Emulsion Formation 17710.4.4 Role of Surfactants in Droplet Deformation 17910.5 Selection of Emulsifiers 18310.5.1 The Hydrophilic–Lipophilic Balance (HLB) Concept 18310.5.2 The Phase Inversion Temperature (PIT) Concept 18610.6 Creaming or Sedimentation of Emulsions 18710.6.1 Creaming or Sedimentation Rates 18810.6.1.1 Very Dilute Emulsions (𝜙 < 0.01) 18810.6.1.2 Moderately Concentrated Emulsions (0.2 < 𝜑 < 0.1) 18910.6.1.3 Concentrated Emulsions (𝜑 > 0.2) 18910.6.2 Prevention of Creaming or Sedimentation 19010.6.2.1 Matching the Density of Oil and Aqueous Phases 19010.6.2.2 Reduction of Droplet Size 19010.6.2.3 Use of ‘Thickeners’ 19010.6.2.4 Controlled Flocculation 19110.6.2.5 Depletion Flocculation 19110.7 Flocculation of Emulsions 19210.7.1 Mechanism of Emulsion Flocculation 19310.7.1.1 Flocculation of Electrostatically Stabilised Emulsions 19310.7.1.2 Flocculation of Sterically Stabilised Emulsions 19510.8 General Rules for Reducing (Eliminating) Flocculation 19610.8.1 Charge-Stabilised Emulsions (e.g., Using Ionic Surfactants) 19610.8.2 Sterically Stabilised Emulsions 19610.9 Ostwald Ripening 19610.10 Emulsion Coalescence 19810.10.1 Rate of Coalescence 20010.11 Phase Inversion 200

References 201

11 Formulation of Suspoemulsions (Mixtures of Suspensions andEmulsions) 203

11.1 Introduction 20311.2 Suspoemulsions in Paints 20411.2.1 Suspoemulsions in Sunscreens and Colour Cosmetics 20711.3 Suspoemulsions in Agrochemicals 219

X Contents

11.3.1 Model Suspoemulsion of Polystyrene Latex and Isoparaffinic Oilstabilised with Synperonic PE (PEO–PPO–PEO A-B-A BlockCopolymer) 225

11.3.2 Model Systems of Polystyrene Latex with Grafted PEO Chains andHexadecane Emulsions 227References 230

12 Formulation of Multiple Emulsions 23112.1 Introduction 23112.2 Preparation of Multiple Emulsions 23212.3 Types of Multiple Emulsions 23312.4 Breakdown Processes of Multiple Emulsions 23312.5 Factors Affecting Stability of Multiple Emulsions, and Criteria for

Their Stabilisation 23512.6 General Description of Polymeric Surfactants 23712.7 Interaction between Oil or Water Droplets Containing an Adsorbed

Polymeric Surfactant: Steric Stabilisation 23812.8 Examples of Multiple Emulsions Using Polymeric Surfactants 24612.9 Characterisation of Multiple Emulsions 24712.9.1 Droplet Size Measurements 24712.10 Rheological Measurements 248

References 249

13 Preparation of Nanosuspensions 25113.1 Introduction 25113.2 Nucleation and Growth, and Control of Particle Size

Distribution 25213.3 Preparation of Nanosuspensions by Bottom-Up Processes 25413.3.1 Solvent–Antisolvent Method 25513.3.2 Use of a Nanoemulsion 25513.3.3 Mixing Two Microemulsions 25613.3.4 Preparation of Polymer Nanoparticles by Miniemulsion or

Minisuspension polymerisation 25613.4 Preparation of Nanosuspensions Using the Bottom-Down

Process 25713.4.1 Wetting of the Bulk Powder 25713.4.2 Breaking of Aggregates and Agglomerates into Individual Units 26013.4.3 Wet Milling or Comminution 26013.4.4 Stabilisation of the Resulting Dispersion 26113.4.5 Prevention of Ostwald Ripening (Crystal Growth) 268

References 268

14 Formulation of Nanoemulsions 27114.1 Introduction 27114.2 Mechanism of Emulsification 273

Contents XI

14.3 Methods of Emulsification and the Role of Surfactants 27514.4 Preparation of Nanoemulsions 27614.4.1 High-Pressure Homogenisation 27614.4.2 Phase Inversion Composition (PIC) Principle 27714.4.3 Phase Inversion Temperature (PIT) Principle 27714.4.4 Preparation of Nanoemulsions by Dilution of Microemulsions 27914.5 Steric Stabilisation and the Role of the Adsorbed Layer

Thickness 28114.5.1 Ostwald Ripening 28314.5.2 Practical Examples of Nanoemulsions 28414.5.3 Nanoemulsions Based on Polymeric Surfactants 293

References 299

15 Formulation of Microemulsions 30115.1 Introduction 30115.2 Thermodynamic Definition of Microemulsions 30215.3 Mixed-Film and Solubilisation Theories of Microemulsions 30315.3.1 Mixed-Film Theories 30315.3.2 Solubilisation Theories 30515.4 Thermodynamic Theory of Microemulsion Formation 30715.4.1 Reason for Combining Two Surfactants 30815.4.2 Factors Determining W/O versus O/W Microemulsions 30915.5 Characterisation of Microemulsions Using Scattering

Techniques 31115.5.1 Time-Average (Static) Light Scattering 31115.5.2 Calculation of Droplet Size from Interfacial Area 31315.5.3 Dynamic Light Scattering (Photon Correlation Spectroscopy;

PCS) 31415.6 Characterisation of Microemulsions Using Conductivity 31515.7 NMR Measurements 31615.8 Formulation of Microemulsions 31715.8.1 The HLB System 31815.8.2 Phase Inversion Temperature (PIT) Method 31915.8.3 The Cohesive Energy Ratio (CER) Concept 32015.8.4 Cosurfactant Partitioning 322

References 322Further Reading 323

16 Formulation of Foams 32516.1 Introduction 32516.2 Foam Preparation 32616.3 Foam Structure 32716.4 Classification of Foam Stability 32816.5 Drainage and Thinning of Foam Films 32916.6 Theories of Foam Stability 330

XII Contents

16.6.1 Surface Viscosity and Elasticity Theory 33016.6.2 The Gibbs–Marangoni Effect Theory 33016.6.3 Surface Forces Theory (Disjoining Pressure π) 33116.6.4 Stabilisation by Micelles (High Surfactant Concentrations >

cmc) 33416.6.5 Stabilisation by Lamellar Liquid Crystalline Phases 33416.6.6 Stabilisation of Foam Films by Mixed Surfactants 33416.7 Foam Inhibitors 33516.7.1 Chemical Inhibitors That Lower Viscosity and Increase Drainage 33516.7.2 Solubilised Chemicals Which Cause Antifoaming 33516.7.3 Droplets and Oil Lenses Which Cause Antifoaming and

Defoaming 33616.7.4 Surface Tension Gradients (Induced by Antifoamers) 33616.7.5 Hydrophobic Particles as Antifoamers 33716.7.6 Mixtures of Hydrophobic Particles and Oils as Antifoamers 33816.8 Physical Properties of Foams 33816.8.1 Mechanical Properties 33816.8.2 Rheological Properties 33916.8.3 Electrical Properties 34016.8.4 Electrokinetic Properties 34016.8.5 Optical Properties 34116.9 Experimental Techniques for Studying Foams 34116.9.1 Studies on Foam Films 34116.9.2 Structural Parameters of Foams 34216.9.3 Foam Drainage 34216.9.4 Foam Collapse 343

References 343

17 Formulation of Latexes 34517.1 Introduction 34517.2 Emulsion Polymerisation 34617.2.1 Mechanism of Emulsion Polymerisation 34817.2.2 Block Copolymers as Stabilisers in Emulsion Polymerisation 34917.2.3 Graft Copolymers as Stabilisers in Emulsion Polymerisation 35217.3 Polymeric Surfactants for Stabilisation of Preformed Latex

Dispersions 35617.4 Dispersion Polymerisation 36017.4.1 Mechanism of Dispersion Polymerisation 36217.4.2 Influence of Polymeric Surfactant Concentration and Molecular

Weight on Particle Formation 36317.4.3 Effect of Monomer Solubility and Concentration in the Continuous

Phase 36317.4.4 Stability/Instability of the Resulting Latex 36417.4.5 Particle Formation in Polar Media 364

References 365

Contents XIII

18 Formulation of Pigment and Ink Dispersions 36718.1 Introduction 36718.2 Powder Wetting 37018.2.1 Effect of Surfactant Adsorption 37418.2.2 Wetting of Powders by Liquids 37518.2.3 Measurement of Wettability of Powders 37718.2.3.1 Submersion Test: Sinking Time or Immersion Time 37718.2.4 Measurement of Contact Angles of Liquids and Surfactant Solutions

on Powders 37818.2.5 Wetting Agents for Hydrophobic Pigments 37918.2.6 Dynamics of Processing of Adsorption and Wetting 38018.2.7 Experimental Techniques for Studying Adsorption Kinetics 38418.3 Breaking of Aggregates and Agglomerates (Deagglomeration) 38718.4 Classification of Dispersants 38818.4.1 Surfactants 38818.4.2 Polymeric Surfactants 38918.4.3 Polyelectrolytes 39018.4.4 Assessment and Selection of Dispersants 39118.4.4.1 Adsorption Isotherms 39118.4.4.2 Measurement of Dispersion and Particle Size Distribution 39218.4.4.3 Wet Milling (Comminution) 39218.4.4.4 Bead Mills 394

References 395

19 Methods of Evaluating Formulations after Dilution 39719.1 Introduction 39719.2 Assessment of the Structure of the Solid/Liquid Interface 39819.2.1 Double Layer Investigation 39819.2.1.1 Analytical Determination of Surface Charge 39819.2.1.2 Electrokinetic and Zeta-Potential Measurements 39919.2.2 Measurement of Surfactant and Polymer Adsorption 40019.3 Assessment of Sedimentation of Suspensions 40319.4 Assessment of Flocculation and Ostwald Ripening (Crystal

Growth) 40519.4.1 Optical Microscopy 40619.4.1.1 Phase-Contrast Microscopy 40619.4.1.2 Differential Interference Contrast (DIC) microscopy 40719.4.1.3 Polarised Light Microscopy 40719.4.1.4 Sample Preparation for Optical Microscopy 40719.4.1.5 Particle Size Measurements Using Optical Microscopy 40719.4.2 Electron Microscopy 40819.4.2.1 Transmission Electron Microscopy 40819.4.2.2 Scanning Electron Microscopy 40919.4.3 Confocal Laser Scanning Microscopy 40919.4.4 Scanning Probe Microscopy 409

XIV Contents

19.4.5 Scanning Tunneling Microscopy 41019.4.6 Atomic Force Microscopy 41019.5 Scattering Techniques 41119.5.1 Light-Scattering 41119.5.1.1 Time-Average Light Scattering 41119.5.1.2 Rayleigh–Gans–Debye Regime (RGD) λ/20 < R < λ 41219.5.2 Turbidity Measurements 41219.5.3 Light-Diffraction Techniques 41319.5.4 Dynamic Light Scattering (DLS): Photon Correlation Spectroscopy

(PCS) 41519.5.5 Back-Scattering Techniques 41819.6 Measurement of Rate of Flocculation 41819.7 Measurement of Incipient Flocculation 41919.8 Measurement of Crystal Growth (Ostwald Ripening) 42019.9 Bulk Properties of Suspensions: Equilibrium Sediment Volume

(or Height) and Redispersion 420References 421

20 Evaluating Formulations without Dilution: RheologicalTechniques 423

20.1 Introduction 42320.2 Steady-State Measurements 42420.2.1 Rheological Models for Analysis of Flow Curves 42420.2.1.1 Newtonian Systems 42420.2.1.2 Bingham Plastic Systems 42520.2.1.3 Pseudoplastic (Shear Thinning) System 42520.2.1.4 Dilatant (Shear Thickening) System 42520.2.1.5 Herschel–Bulkley General Model 42620.2.1.6 The Casson Model 42620.2.1.7 The Cross Equation 42620.2.2 Time Effects during Flow: Thixotropy and Negative (or Anti-)

Thixotropy 42620.3 Constant Stress (Creep) Measurements 42920.3.1 Analysis of Creep Curves 43020.3.1.1 Viscous Fluid 43020.3.1.2 Elastic Solid 43020.3.2 Viscoelastic Response 43020.3.2.1 Viscoelastic Liquid 43020.3.2.2 Viscoelastic Solid 43120.3.3 Creep Procedure 43120.4 Dynamic (Oscillatory) Measurements 43220.4.1 Analysis of Oscillatory Response for a Viscoelastic System 43320.4.2 Vector Analysis of the Complex Modulus 43420.4.2.1 Dynamic viscosity η′ 43420.4.2.2 Strain Sweep 434

Contents XV

20.4.2.3 Frequency Sweep 43420.4.3 The Cohesive Energy Density Ec 43620.4.4 Application of Rheological Techniques to Assess and Predict the

Physical Stability of Suspensions 43620.4.4.1 Rheological Techniques to Assess Sedimentation and Syneresis 43620.4.4.2 Role of Thickeners 43720.4.5 Assessment of Flocculation Using Rheological Techniques 43820.4.5.1 Strain Sweep Measurements 44020.4.5.2 Oscillatory Sweep Measurements 441

References 442Further Reading 442

21 Assessment and Prediction of Creaming, Sedimentation, Flocculation,and Coalescence of Formulations 443

21.1 Assessment and Prediction of Creaming and Sedimentation 44321.1.1 Introduction 44321.1.2 Accelerated Tests and Their Limitations 44321.1.3 Application of High-Gravity (g) Forces 44421.1.4 Rheological Techniques for Prediction of Sedimentation or

Creaming 44521.1.5 Separation of Formulation (‘‘Syneresis’’) 44521.1.6 Examples of Correlation of Sedimentation or Creaming with Residual

(Zero Shear) Viscosity 44621.1.6.1 Model Suspensions of Aqueous Polystyrene Latex 44621.1.6.2 Sedimentation in Non-Newtonian Liquids 44821.1.6.3 Role of Thickeners 44821.1.6.4 Prediction of Emulsion Creaming 44921.1.6.5 Creep Measurements for Prediction of Creaming 45021.1.6.6 Oscillatory Measurements for Prediction of Creaming 45121.2 Assessment and Prediction of Flocculation Using Rheological

Techniques 45221.2.1 Introduction 45221.2.2 Wall Slip 45221.2.3 Steady-State Shear Stress-Shear Rate Measurements 45221.2.4 Influence of Ostwald Ripening and Coalescence 45321.2.5 Constant Stress (Creep) Experiments 45321.2.6 Dynamic (Oscillatory) Measurements 45421.2.6.1 Strain Sweep Measurements 45421.2.6.2 Oscillatory Sweep Measurements 45521.2.7 Examples of the Application of Rheology for Assessment and

Prediction of Flocculation 45621.2.7.1 Flocculation and Restabilisation of Clays Using Cationic

Surfactants 45621.2.7.2 Flocculation of Sterically Stabilised Dispersions 45721.2.7.3 Flocculation of Sterically Stabilised Emulsions 458

XVI Contents

21.3 Assessment and Prediction of Emulsion Coalescence UsingRheological Techniques 459

21.3.1 Introduction 45921.3.2 Rate of Coalescence 45921.3.3 Rheological Techniques 46021.3.3.1 Viscosity Measurements 46021.3.3.2 Measurement of Yield Value as a Function of Time 46121.3.3.3 Measurement of Storage Modulus G′ as a Function of Time 46121.3.4 Correlation between Elastic Modulus and Coalescence 46221.3.5 Cohesive Energy Ec 463

References 463

Index 465

XVII

Preface

Several disperse systems can be identified, including solid/liquid (suspensions),liquid/liquid (emulsions) and their mixtures (suspoemulsions), gas/liquid (foams),nanodispersions (with particle sizes in the range 20–200 nm), microemulsions,dispersions of pigments and inks, and latexes. These disperse systems exist in manyindustrial applications such as paints, paper coatings, dyestuffs, printing inks,agrochemicals and pharmaceuticals. The formulation of these complex multiphasesystems is still an art, and in most cases they are produced by industrial chemistsusing simple trial-and-error techniques. Apart from being time-consuming, thisapproach does not provide a rational understanding on how a system is arrived at.In addition, whenever a problem arises – such as instability and separation of theformulation on storage – the formulation chemist may struggle to find a solutionfor the resulting instability.

This book has been written to set the fundamental basis of the formulation ofthe various types of disperse systems. It starts (Chapter 1) with a general introduc-tion of the different types of disperse systems, while Chapter 2 provides a briefdescription of the various surfactant classes used in the formulations. Chapter3 describes the physical chemistry of surfactant solutions, with emphasis placedon the process of micellisation, while the various dispersants and polymers usedfor stabilisation of disperse systems, and the criteria required for an effectivedispersant are summarised in Chapter 4. Chapter 5 describes the adsorption ofsurfactants at the air/liquid, liquid/liquid, and solid/liquid interfaces, with detailsgiven of the adsorption process and its effect on the surface, interfacial, andsolid/liquid tensions. In Chapter 6, an account is provided of the adsorption ofpolymeric surfactants at the solid/liquid interface, with emphasis on the generalbehaviour of polymer adsorption and its irreversibility. Chapter 7 describes thecolloid stability of disperse systems containing electrical double layers, and thecombination of electrostatic repulsion with van der Waals attraction is used todescribe the theory of colloid stability. Chapter 8 describes the stability of dispersesystems containing adsorbed nonionic surfactants or polymers referred to as stericstabilisation, while Chapter 9 describes the formulation of solid/liquid disper-sions (suspensions). The preparation of suspensions by condensation (nucleationand growth) and dispersion methods are also described, with the stabilisation ofsuspensions using electrostatic and/or steric repulsion being described in terms

XVIII Preface

of the various interaction forces. Chapter 10 deals with the formulation of liq-uid/liquid dispersions (emulsions). Here, the various methods that can be appliedfor selection of emulsifiers are described, and this is followed by an analysis of thestability/instability of emulsions, namely creaming or sedimentation, flocculation,Ostwald ripening, coalescence, and phase inversion. Chapter 11 describes theformulation of suspoemulsions (mixtures of suspensions and emulsions), and theapplication of suspoemulsions in agrochemicals, cosmetics and paints is brieflydescribed. Chapter 12 deals with formulation of multiple emulsions: water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) systems. The structure of multipleemulsions and their breakdown processes are described, and this is followed bya section on the preparation of multiple emulsions using a two-stage process.Chapter 13 describes the methods of preparation of nanosuspensions, and detailsof the application of nanosuspensions in cosmetics and drug delivery are given.The preparation of nanosuspensions, using top-up (starting from molecular units)and bottom-down (by comminution of larger particles) processes is also described.Chapter 14 deals with the formulation of nanoemulsions and the factors relatingto their transparency; the advantages of nanoemulsions in personal products andhealthcare products are also summarised. Chapter 15 deals with the formulationof microemulsions and the surfactant composition, with definition being providedof microemulsions and the origin of their thermodynamic stability. Theories of thestability of microemulsions are also outlined. Chapter 16 deals with the formula-tion of foams, starting with the factors responsible for foam formation, and thestability/instability of foams and the role of surfactants are described. Chapter 17describes the formulation of latexes and methods of their preparation by emulsionand dispersion polymerisation, while Chapter 18 deals with the formulation of pig-ments and inks, and provides details of the various pigment types and their generalproperties. The colloid stability of pigment dispersions in terms of electrostatic,steric and electrosteric forces is also described. Chapter 19 describes the methods forevaluating formulations after dilution, starting with optical microscopy and particlesize distribution using image analysis, phase contrast, differential interference con-trast and polarising microscopy. This is followed by the various scattering methods,including time average light scattering, turbidity, light diffraction, dynamic lightscattering and back-scattering techniques. Chapter 20 describes the methods usedfor the evaluation of formulations without dilution, namely rheological techniques;steady-state shear stress-shear rate measurements and the flow curves are alsodescribed, as are constant stress (creep) measurements and measurement of theresidual (zero shear) viscosity. This is followed by investigations of stress relaxationafter the sudden application of strain, and the dynamic (oscillatory) methods andevaluation of the elastic and viscous components are described. Finally, Chapter21 deals with the methods that can be applied for the assessment and prediction ofcreaming or sedimentation, flocculation and coalescence. In addition, acceleratedtests for the evaluation of stability using temperature changes and centrifugation,and their limitations, are described. The rheological methods that can be appliedfor the prediction of creaming or sedimentation, flocculation and coalescence arealso described.

Preface XIX

This book will be valuable for industrial scientists engaged in the formulation ofdisperse systems, and should provide them with a more rational approach of howto formulate a product. In addition, it should enable the formulation scientist tobetter understand the fundamental basis of the factors responsible for producinga stable formulation with an acceptable shelf life. The book should also be veryuseful for teaching the subject of formulation at academic institutions.

November 2013 Tharwat Tadros