Emulsions

Tharwat F. Tadros

• Editorial: De Gruyter
• Año: 2016
• ISBN:
  9783110452266
• Edición: 1
• Categoría: Tecnologia e ingegneria
• Categoría: Scienza dei materiali

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

1Emulsions: Formation, stability, industrial applications

1.1General introduction

Emulsions are a class of disperse systems consisting of two immiscible liquids [1–4]. 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 non-polar 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 formation of the emulsion and its long term stability [1–4].

There are many examples one could quote of naturally occurring emulsions: milk and the O/W and W/O emulsions associated with oil bearing rocks are just two examples. Emulsion types can be classified on the basis of the nature of the emulsifier or the structure of the system as shown in Tab. 1.1.

Tab. 1.1: Classification of emulsions.

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

1.2Nature of the Emulsifier

The simplest type is ions such as OH–, which 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 non-ionic surfactants, such as alcohol ethoxylates with the general formula CxH2x+1−O−(CH2−CH2−O)nH, which can be used to emulsify oil in water or water in oil. In addition they can stabilise the emulsion against flocculation and coalescence. Ionic surfactants such as sodium dodecyl sulphate can also be used as emulsifiers (for O/W), but the system is sensitive to the presence of electrolytes. Surfactant mixtures, e.g. ionic and non-ionic, or mixtures of non-ionic surfactants, can be more effective in emulsification and stabilization of the emulsion. Nonionic polymers, sometimes referred to as polymeric surfactants, e.g. pluronics, with the general formula HO−(CH2−CH2−O)n−(CH2−CH(CH3)−O)m−(CH2−CH2−O)n−OH or PEO−PPO−PEO, are more effective in stabilisation 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 stabilisation of the emulsion. Lamellar liquid crystalline phases that can be produced using surfactant mixtures are very effective in emulsion stabilisation. Solid particles that can accumulate at theO/Winterface can also be used for emulsion stabilisation. These are referred to as Pickering emulsions, whereby particles are partially wetted by the oil phase and partially by the aqueous phase.

1.3Structure of the system

(i)Macroemulsions O/W and W/O: These usually have a size range of 0.1–5 μm with an average of 1–2 μm. These systems are usually opaque or milky due to the large size of the droplets and the significant difference in refractive index between the oil and water phases.

(ii)Nano-emulsions: These usually have a size range 20–100 nm. Like macroemulsions they are only kinetically stable. They can be transparent, translucent or opaque, depending on the droplet size, the refractive index difference between the two phases and the volume fraction of the disperse phase.

(iii)Double and multiple emulsions: these are emulsions-of-emulsions, W/O/W and O/W/Osystems. They are usually prepared using a two-stage process. For example aW/O/W multiple emulsion is prepared by forming aW/Oemulsion,which is then emulsified in water to form the final multiple emulsion.

(iv)Mixed emulsions: these are systems consisting of two different disperse droplet that do not mix in a continuous medium.

(v)Micellar emulsions or microemulsions: these usually have the size ranging from 5 to 50 nm. They are thermodynamically stable and strictly speaking they should not be described as emulsions. A better description is “swollen micelles” or “micellar systems”.

The present book will only deal with macroemulsions, their formation, stability and industrial applications.

Several breakdown processes may occur on storage depending on: particle size distribution and density difference between the droplets and the medium; the magnitude of the attractive vs repulsive forces which determines flocculation; solubility of the disperse droplets and the particle size distribution which determines Ostwald ripening; stability of the liquid film between the droplets that determines coalescence; phase inversion, where the two phases exchange, e.g. an O/W emulsion inverting to W/O and vice versa. Phase inversion can be catastrophic, as in the case when the oil phase in an O/W emulsion exceeds a critical value. The inversion can be transient when for example the emulsion is subjected to temperature increase.

1.4Breakdown processes in emulsions

The various breakdown processes are illustrated in the Fig. 1.1. The physical phenomena involved in each breakdown process is not simple, and it requires analysis of the various surface forces involved. In addition, the above processes may take place simultaneously rather then 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 of surfactant and polymer adsorption in an emulsion is not easy, and one has to extract such information from measurement at a planer interface.

Below a summary of each of the above breakdown processes is given, and details of each process and methods of its prevention is given in separate sections.

Fig. 1.1: Schematic representation of the various breakdown processes in emulsions.

1.5Creaming and sedimentation

This process, with no change in droplet size, 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, with the larger droplets moving faster to the top (if their density is lower than that of the medium) or to the bottom (if their density is larger 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.

1.6Flocculation

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 attraction, which is universal to all disperse systems. The main force of attraction arises from the London dispersion force that results from charge fluctuations of the atoms or molecules in the disperse droplets. The van der Waals attraction increases with a decrease in the distance separating the droplets, and at small separation distances the attraction becomes very strong, resulting in droplet aggregation or flocculation. The latter occurs when there is not enough repulsion to keep the droplets apart to distances where the van der Waals attraction is weak. Flocculation may be “strong” or “weak”, depending on the magnitude of the attractive energy involved. In cases where the net attractive forces are relatively weak, an equilibrium degree of flocculation may be achieved (so-called weak flocculation), associated with the reversible nature of the aggregation process. The exact nature of the equilibrium state depends on the characteristics of the system. One can envision the build-up of aggregate-size distribution, and an equilibrium may be established between single droplets and large aggregates. With a strongly flocculated system, one refers to a system in which all the droplets are present in aggregates due to the strong van der Waals attraction between the droplets.

1.7Ostwald ripening (disproportionation)

This results from the finite solubility 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 larger solubility compared to the larger ones (due to the effects of curvature). 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.

1.8Coalescence

This refers to the process of thinning and disruption of the liquid film between the droplets which may be present in a creamed or sedimented layer, in a floc or simply during droplet collision, with the result of fusion of two or more droplets into larger ones. This process of coalescence results in a considerable change of the droplet size distribution, which shifts to larger sizes. The limiting case for coalescence is the complete separation of the emulsion into two distinct liquid phases. The thinning and disruption of the liquid film between the droplets is determined by the relative magnitudes of the attractive versus repulsive forces. To prevent coalescence, the repulsive forces must exceed the van der Waals attraction, thus preventing film rupture.

1.9Phase 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 during which multiple emulsions are produced. For example, with an O/W emulsion, the aqueous continuous phase may become emulsified in the oil droplets, forming a W/O/W multiple emulsion. This process may continue until the entire continuous phase is emulsified into the oil phase, thus producing a W/O emulsion.

1.10Industrial applications of emulsions

There are a number of industrial applications of emulsions worth noting: food emulsion, e.g. mayonnaise, salad creams, deserts, beverages etc.; personal care and cosmetics, e.g. hand creams, lotions, hair sprays, sunscreens, etc.; agrochemicals, e.g. self emulsifiable oils which produce emulsions on dilution...