From Soft-Matter
Revision as of 03:36, 8 December 2008 by Morrison (Talk | contribs) (Particles as emulsion stabilizers)

Jump to: navigation, search

Back to Home

Back to Topics.


Emulsions are unbiquous ... and some of the most interesting and challenging are in foods:

Dickenson, Food Science.

Microfluidics are terrific ... where can we add the "magic" of emulsion science?

Top of Page


Macroemulsions At least one immiscible liquid dispersed in another as drops whose diameters generally exceed 1000 nm. The stability by addition of surfactants and/or finely divided solids. Considered only kinetically stable.
Miniemulsions An emulsion with droplets between 100 and 1000 nm. Reportedly thermodynamically stable.
Microemulsions A thermodynamically stable, transparent solution of micelles swollen with solubilizate. Usually requires a surfactant and a cosurfactant (e.g. short chain alcohol).

Becher, P. Emulsions, theory and practice, 3rd ed.; Oxford University Press: New York; 2001.

Top of Page

Emulsion processes

A - Inversion C - Sedimentation E - Coalescence
B - Creaming D - Flocculation F - Ripening

The stability of emulsions is determined by a variety of factors:

Electrostatic stabilization at lower volume fractions
Steric stabilization at all volume fractions
Additional factors Temperature is important – solubility changes quickly.
Steric stabilization is enhanced by solubility in both phases:
Mixed emulsifiers (cosurfactants) are common. They can come from either phase.

The creaming of emulsion can be shown by the variation in volume fraction at various heights and times as determined by measuring the speed of sound:

Morrison, Fig. 22.13

Emulsion inversion - As the concentration increases (A) the droplets get closer until they pinch off into smaller, opposite type of emulsion (B).

Top of Page

Making emulsions

Method of phase inversion e.g. Use a poor O/W emulsifier, go to high volume fractions, the emulsion inverts to smaller droplets of W/O
Phase-inversion-temperature method e.g. Heat and emulsify O/W 2-4o below the PIT, creates low s and small drops, cool to room temperature.
Solubilize vapor in micelles The energies driving the condensation, drive Ostwald ripening, therefore a formulation challenge.
Electric emulsification Charging the surface produces electrohydrodynamic instabilities.
Intermittent milling Surfactant adsorption is slow – waiting helps.

Intermittent milling

Top of Page

Breaking emulsions

From: Menon, V.B.; Wasan, D.T. Demulsification, in Encyclopedia of emulsion technology; Becher, P., Ed.; Marcel Dekker: New York; 1985, Vol. 2; pp 1-75.

Creaming Especially with a centrifuge, taking advantage of temperature and salt.
Mechanical Sometime high shear; filtering through bed whose surfaces are wetted by internal phase; ultrafiltration; dialysis;
Thermal Most emulsion a less stable hot; At the PIT many are quite unstable; freeze-thaw.
Chemical Chemically change the emulsifier; mismatch of HLB, pH; replace with strong surfactant but not strong emulsifier; addition of other solvents.

Top of Page

Bancroft's rule and the HLB

Bancroft's Rule:

The emulsifier stabilizes the emulsion type where the continuous phase is the medium in which it is most soluble.
A hydrophilic solute in an O/W emulsion.
The long tail on the surfactant is to represent the longer range interaction of a hydrophilic molecule through water.
A hydrophilic solute in a W/O emulsion.

The Hydrophile-Lipophile Schema:

Variation of type and amount of residual emulsion with HLB number of emulsifier at room temperature. Morrison, Fig. 22.5

Top of Page

Phase inversion temperatures

The "type" of an emulsifier often changes with increasing temperature; typically from being water soluble, hence a high HLB number to being water insoluble, hence a low HLB number. Therefore the type (O/W or W/O) emulsion it stabilizes changes. The temperature of the transition is called the Phase Inversion Temperature or PIT.


The HLB and the PIT are often related:


Top of Page

Interfacial properties

Morrison, Fig. 22.8

The rheology of O/W interfaces can be examined by measuring the diffusion of traced particles at the interface:

Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
For viscous liquids: <math>\left\langle \Delta r^{2}\left( \tau \right) \right\rangle =4D\tau \text{ where }D=\frac{k_{B}T}{4\pi \eta a}\,\!</math>

For elastic liquids: <math>\left\langle \Delta r^{2} \right\rangle =\frac{2k_{B}T}{3\pi a{G}'}\,\!</math>

The particles have to sit properly at the O/W interface.

Top of Page

Multiple emulsions

Rosen, p. 313
(a) W/O/W double emulsion (b) O/W/O double emulsion
Consider, for either diagram: Each interface needs a different HLB value.

The curvature of each interface is different.

Top of Page

Particles as emulsion stabilizers

Almost all particles are only partially wetted by either phase.

When particles are “adsorbed” at the surface, they are hard to remove – the emulsion stability is high, sometimes thousands of kT. Crude oil is a W/O emulsion and is old!!

Morrison, Fig. 22.3

Variation of the free energies of desorption (relative to kT) of a spherical particle 10 nm radius at a planar O/W interface of interfacial tension of 36 mN/m with a contact angle which the particle makes with the interface (measured through the oil phase) at 298 K.
Morrison, Fig. 22.4

The thermodynamics is rich.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.
Figure 7. Sketch of a particle of radius a, which is bridging between the surfaces of a film from phase 2 formed between two drops of phase 1. h is the film thickness. <math>\sigma </math> is the contact angle. Figure 8. Definitions of phases, angles, and emulsions: By definition, the particles are initially dispersed in phase 2. The contact angle, <math>\sigma </math>, is always measured across phase 2. The emulsion 1-in-2 is a Bancroft-type emulsion, in which the particles are dispersed in the continuous phase. In contrast, the emulsion 2-in-1 is of anti-Bancroft type.

Top of Page

Back to Topics.

Back to Home