Formulation practical - Emulsions.

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14th November 2003

Formulation practical – Emulsions

Introduction

An emulsion consists of two immiscible liquids, one liquid dispersed in another, the dispersed droplets being termed the disperse phase, which is a finely subdivided liquid that is uniformly distributed throughout the surrounding liquid, the continuous phase. Emulsions can be water-in-oil (w/o) or oil-in-water (o/w) systems. Multiple emulsions can also be formed, for example, an initial water-in-oil emulsion can be prepared, and then emulsified with an external water phase. This would produce a water-in-oil-in-water (w/o/w) emulsion. An oil-in-water-in-oil (o/w/o) emulsion would be produced in a similar way. An emulsion is stabilised by an emulsifier, which produces an interfacial film between the oil/water interphase, which is mechanically strong enough to prevent the emulsion from breaking. Emulsifiers have both hydrophilic and hydrophobic properties, but neither is completely dominant, however it should be more soluble in the continuous phase, so that it can adsorb quickly around the dispersed drop as a film, which should not thin out when two droplets collide, and thus will not permit coalescence (the fusion of droplets to separate the two liquid phases).

Surfactants are the most commonly used emulsifying agents. They adsorb at the water-oil interface to form dense monomolecular film and decrease the interfacial tension. Combination of surfactants forms a more rigid, stabilizing film at the interface. For example, a mixture of two non-ionic surfactants, Span 80 (lipophilic) and Tween80 (hydrophilic), Tween 80 penetrates between the Span 80 molecules, so that the interfacial film is more close-packed and is strengthened. This increases the stability of the o/w emulsion and decreases particle coalescence. The Hydrophile - Lipophile Balance (HLB) has been devised to measure the relative contributions of the hydrophilic and lipophilic regions of the molecule.

HLB = Σ (hydrophilic group numbers) – Σ (lipophilic group numbers) + 7

The values for HLB range from 0 to 20, where the low numbers, <10, represent the lipophilic (poorly water-soluble) surfactants and the high numbers, >10, represent the hydrophilic (water-soluble) surfactants. The HLB of a mix of two surfactants can be calculated as follows:

HLBmixture = f . HLBA + (1-f) . HLBB                 Equation (1)

Where        HLBmixture = HLB of a mix of two surfactants

HLBA and HLBB = HBL of surfactant A (Span 80; HLB 4.3) and surfactant B (Tween 80; HLB 15.0) respectively

        f  = fraction of Span 80

        1-f = fraction of Tween 80

Emulsions are thermodynamically unstable. The stability of the final product must be considered. Three major phenomena associated with physical stability are creaming/caking (particles float/sink in the emulsion depending on their density) flocculation (coagulation; coacervation; particles cluster together and float/sink) and coalescence (cracking; fusion of particles).

As the disperse phase droplets are brought closer together, creaming/caking can occur, which may facilitate the more serious problem of coalescence. Creaming is governed by Stoke’s Law:

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v = 2gr2 (ρd - ρc)        where         v = velocity of creaming                Equation (2)

        9η                        g = gravity

                                r = radius of particle

                                η = viscosity of continuous phase

                                ρd = density of disperse phase

                                ρc = density of continuous phase

Aim of experiment

  • Determine whether a given emulsion is an oil-in-water (o/w) or water-in-oil (w/o) system
  • Determine the optimum HLB value for an emulsion
  • Measure the viscosity of the continuous medium and investigate the effect of increasing the viscosity of the continuous phase on emulsion stability
  • Investigate the effect of reducing the size of the disperse phase droplets on emulsion stability
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