Emulsions: Formation, Stability, Industrial Applications

✍ Scribed by Tharwat F. Tadros

• Publisher: De Gruyter
• Year: 2016
• Tongue: English
• Leaves: 242
• Category: Library

✦ Synopsis

Chapter 1 General Introduction
Definition of emulsions and the role of the emulsifier. Classification based on the nature of the emulsifier. Classification based on the structure of the system. General instability problems with emulsions : creaming/sedimentation, flocculation, Ostwald ripening, coalescence and phase inversion. Importance of emulsions in various industrial applications.

Chapter 2 Thermodynamics of Emulsion Formation and Breakdown
Application of the second law of thermodynamics for emulsion formation : Balance of energy and entropy and non-spontaneous formation of emulsions. Breakdown of the emulsion by flocculation and coalescence in the absence of an emulsifier. Role of the emulsifier in preventing flocculation and coalescence by creating an energy barrier resulting from the repulsive energies between the droplets.

Chapter 3 Interaction Forces between Emulsion Droplets
Van der Waals attraction and its dependence on droplet size, Hamaker constant and separation distance between the droplets. Electrostatic repulsion resulting from the presence of electrical double layers and its dependence on surface (or zeta) potential and electrolyte concentration and valency. Combination of the van der Waals attraction with double layer repulsion and the theory of colloid stability. Steric repulsion resulting from the presence of adsorbed non-ionic surfactants and polymers. Combination of van der Waals attraction with steric repulsion and the theory of steric stabilisation.

Chapter 4 Adsorption of Surfactants at the Oil/Water Interface
Thermodynamic analysis of surfactant adsorption and the Gibbs adsorption isotherm. Calculation of the amount of surfactant adsorption and area per surfactant molecule at the interface. Experimental techniques for measuring the interfacial tension.

Chapter 5 Mechanism of Emulsification and the Role of the Emulsifier
Description of the factors responsible for droplet deformation and its break-up. Role of surfactant in preventing coalescence during emulsification. Definition of the Gibbs dilational elasticity and the Marangoni effect in preventing coalescence.

Chapter 6 Methods of Emulsification
Pipe flow, static mixers and high speed stirrers (rotor-stator mixer). Laminar and turbulent flow. Membrane emulsification. High pressure homogenisers and ultrasonic methods.

Chapter 7 Selection of Emulsifiers
The hydrophilic-lipophilic-balance (HLB) and its application in surfactant selection. Calculation of HLB numbers and the effect of the nature of the oil phase. The phase inversion temperature (PIT) method for emulsifier selection. The cohesive energy ratio method for emulsifier selection.

Chapter 8 Creaming/Sedimentation of Emulsions and its prevention
Driving force for creaming/sedimentation: effect of gravity, droplet size and density difference between the oil and continuous phase. Calculation of the rate of creaming/sedimentation in dilute emulsions. Influence of increase of the volume fraction of the disperse phase on the rate of creaming/sedimentation. Reduction of creaming/sedimentation: Balance of the density of the two phases, reduction of droplet size and effect of addition of ''thickeners'.

Chapter 9 Flocculation of Emulsions and its Prevention
Factors affecting flocculation. Calculation of fast and slow flocculation rate. Definition of stability ratio and its dependence on electrolyte concentration and valency. Definition of the critical coagulation concentration and its dependence on electrolyte valency. Reduction of flocculation by enhancing the repulsive forces.

Chapter 10 Ostwald Ripening and its Reduction
Factors responsible for Ostwald ripening : difference in solubility between small and large droplets and the Kelvin equation. Calculation of the rate of Ostwald ripening. Reduction of Ostwald ripening by incorporation of a small amount of highly insoluble oil. Reduction of Ostwald ripening by the use of strongly adsorbed polymeric surfactant and enhancement of the Gibbs elasticity.

Chapter 11 Emulsion Coalescence and its Prevention
Driving force for emulsion coalescence : Thinning and disruption of the liquid film between the droplets. The concept of disjoining pressure for prevention of coalescence. Methods for reduction or elimination of coalescence : Use of mixed surfactant films, use of lamellar liquid crystalline phases and use of polymeric surfactants.

Chapter 12 Phase Inversion and its Prevention
Distinction between catastrophic and transient phase inversion. Influence of the disperse volume fraction and surfactant HLB number. Explanation of the factors responsible for phase inversion.

Chapter 13 Characterisation of Emulsions
Measurement of droplet size distribution : Optical microscopy and image analysis. Phase contrast and polarising microscopyDiffraction methods. Confocal laser microscopy. Back scattering methods

Chapter 14 Industrial Application of Emulsions
14.1 Application in Pharmacy
14.2 Application in Cosmetics
14.3 Application in Agrochemicals
14.4 Application in Paints
14.5 Application in the Oil Industry

• Highly practical: Experimental techniques for measuring the interfacial tension and methods for reduction or elimination of coalescence
  
• Discusses industrial applications of emulsions in pharmacy, cosmetics, agrochemicals, paints and oil industry
  

✦ Table of Contents

Preface
Contents
1 Emulsions: Formation, stability, industrial applications
1.1 General introduction
1.2 Nature of the Emulsifier
1.3 Structure of the system
1.4 Breakdown processes in emulsions
1.5 Creaming and sedimentation
1.6 Flocculation
1.7 Ostwald ripening (disproportionation)
1.8 Coalescence
1.9 Phase inversion
1.10 Industrial applications of emulsions
1.11 Book outline
2 Thermodynamics of emulsion formation and breakdown
2.1 The interface (Gibbs dividing line)
2.2 Thermodynamics of emulsion formation and breakdown
3 Interaction forces between emulsion droplets
3.1 Van der Waals attraction
3.2 Electrostatic repulsion
3.3 Steric repulsion
3.3.1 Mixing interaction Gmix
3.3.2 Elastic interaction Gel
3.3.3 Total energy of interaction
3.3.4 Criteria for effective steric stabilization
4 Adsorption of surfactants at the oil/water interface
4.1 Introduction
4.2 The Gibbs adsorption isotherm
4.3 Equation of state ppproach
4.4 The Langmuir, Szyszkowski and Frumkin equations
4.5 Effectiveness of surfactant adsorption at the liquid/liquid interface
4.6 Efficiency of adsorption of surfactant at the liquid/liquid interface
4.7 Adsorption from mixtures of two surfactants
4.8 Adsorption of macromolecules
4.9 Interfacial tension measurements
4.9.1 The Wilhelmy plate method
4.9.2 The pendent drop method
4.9.3 Sessile drop method
4.9.4 The Du Nouy ring method
4.9.5 The drop volume (weight) method
4.9.6 The spinning drop method
5 Mechanism of emulsification and the role of the emulsifier
5.1 Introduction
5.2 Mechanism of emulsification
5.3 Role of surfactants in emulsion formation
5.3.1 Role of surfactants in reduction of droplet size
5.3.2 Role of surfactants in droplet deformation
6 Methods of emulsification
6.1 Introduction
6.2 Rotor-stator mixers
6.2.1 Toothed devices
6.2.2 Batch radial discharge mixers
6.2.3 Design and arrangement
6.3 Flow regimes
6.3.1 Laminar flow
6.3.2 Turbulent flow
6.4 Membrane emulsification
6.5 Formulation variables and comparison of various emulsification methods
7 Selection of emulsifiers
7.1 Introduction
7.2 The hydrophilic-lipophile balance (HLB) concept
7.3 The phase inversion temperature (PIT) concept
7.4 The cohesive energy ratio (CER) concept
7.5 The critical packing parameter (CPP) for emulsion selection
7.6 Stabilisation by solid particles (Pickering emulsions)
8 Creaming/sedimentation of emulsions and its prevention
8.1 Introduction
8.2 Creaming or sedimentation rates
8.2.1 Very dilute emulsions (φ < 0.01)
8.2.2 Moderately concentrated emulsions (0.2 > φ > 0.1)
8.2.3 Concentrated emulsions (φ > 0.2)
8.3 Properties of a creamed layer
8.4 Prevention of creaming or sedimentation
8.4.1 Matching density of oil and aqueous phases
8.4.2 Reduction of droplet size
8.4.3 Use of “thickeners”
8.4.4 Reduction of creaming/sedimentation of emulsions using associative thickeners
8.4.5 Controlled flocculation
8.4.6 Depletion flocculation
8.4.7 Use of “inert” fine particles
8.4.8 Use of mixtures of polymers and finely divided particulate solids
8.4.9 Use of liquid crystalline phases
9 Flocculation of emulsions
9.1 Introduction
9.2 Mechanism of emulsion flocculation
9.2.1 Flocculation of electrostatically stabilised emulsions
9.2.2 Flocculation of sterically stabilised emulsions
9.2.3 Weak flocculation of sterically stabilised emulsions
9.2.4 Depletion flocculation
9.2.5 Bridging flocculation by polymers and polyelectrolytes
9.3 General rules for reducing (eliminating) flocculation
9.3.1 Charge stabilised emulsions, e.g. using ionic surfactants
9.3.2 Sterically stabilised emulsions
10 Ostwald ripening in emulsions and its prevention
10.1 Driving force for Ostwald ripening
10.2 Kinetics of Ostwald ripening
10.3 Reduction of Ostwald ripening
10.3.1 Addition of a small proportion of highly insoluble oil
10.3.2 Modification of the interfacial layer for reduction of Ostwald ripening
10.4 Influence of initial droplet size of emulsions on the Ostwald ripening rate
11 Emulsion coalescence and its prevention
11.1 Introduction
11.2 Forces across liquid films
11.2.1 Disjoining pressure approach
11.2.2 Interfacial tension of liquid films
11.3 Film rupture
11.4 Rate of coalescence between droplets
11.5 Reduction of coalescence
11.5.1 Use of mixed surfactant films
12 Phase inversion and its prevention
12.1 Introduction
12.2 Catastrophic inversion
12.3 Transitional inversion
12.4 The phase inversion temperature (PIT)
13 Characterization of emulsions and assessment of their stability
13.1 Introduction
13.2 Assessment of the structure of the liquid/liquid interface
13.2.1 Double layer investigation
13.2.2 Measurement of surfactant and polymer adsorption
13.3 Assessment of creaming/sedimentation of emulsions
13.4 Assessment of flocculation, Ostwald ripening and coalescence
13.4.1 Optical microscopy
13.4.2 Electron microscopy
13.4.3 Confocal laser scanning microscopy (CLSM)
13.5 Scattering techniques
13.5.1 Light-scattering techniques
13.5.2 Turbidity measurements
13.5.3 Light diffraction techniques
13.5.4 Dynamic light scattering – photon correlation spectroscopy (PCS)
13.5.5 Back-scattering techniques
13.6 Measurement of the rate of creaming or sedimentation
13.7 Measurement of rate of flocculation
13.8 Measurement of incipient flocculation
13.9 Measurement of Ostwald ripening
13.10 Measurement of the rate of coalescence
13.11 Bulk properties of emulsions. Equilibrium cream or sediment volume (or height)
14 Industrial applications of emulsions
14.1 Introduction
14.2 Food emulsions
14.2.1 Food grade surfactants
14.2.2 Surfactant association structures, micro-emulsions and emulsions in food
14.3 Emulsions in cosmetics and personal care formulations
14.4 Emulsions in pharmacy
14.5 Emulsions in agrochemicals
14.6 Rolling oil and lubricant emulsions
Index