Multiple emulsions rheology

R Michaut, P. Perrin, and P. Hebraud Interface Composition of Multiple Emulsions Rheology as a Probe. Langmuir 20, 8576 (2004). 

Vasiljevic D, Vuleta G, Dakovic LJ, Primorac M. Influence of emulsifier concentration on the rheological behavior of w/o/w multiple emulsions. Pharmazie 1994 49 933—934. 

Most emulsions, unless very dilute, display hoth plastic and pseudoplastic flow behaviour rather than simple Newtonian flow. The flow properties of fluid emulsions should have little influence on their biological behaviour, although the rheological characteristics of semisolid emulsions may affect their performance. The pourability, spreadability and syringeability of an emulsion will, however, be directly determined by its rheological properties. The high viscosity of w/o emulsions leads to problems with intramuscular administration of injectable formulations. Conversion to a multiple emulsion (w/o/w), in which the external oil phase is replaced by an aqueous phase, leads to a dramatic decrease in viscosity and consequent improved ease of injection. 

It should be emphasised that polymeric surfactants prevent the coalescence of water droplets in the multiple emulsion drops, as well as coalescence of the latter drops themselves. This is due to the interfacial rheology of the polymeric surfactant films. As a result of the strong lateral repulsion between the stabilising chains at the interface (PHS chains at the W/O interface and PEO chains at the O/W interface), these films resist deformation under shear and hence produce a viscoelastic film. On approach of the two droplets, this film prevents deformation of the interface so as to prevent coalescence. 

A more sensitive rheological techniques for following the stability of multiple emulsions is to use oscillatory techniques. In this case, a sinusoidal strain or stress is applied to the sample, which is placed in the gap of the concentric cylinder or cone-and-plate geometry the resulting stress or strain sine wave is followed at the same time. For a viscoelastic system, as is the case with multiple emulsions, the stress and strain sine waves oscillate with the same frequency, but out of phase. 

Invivition adsorption is encountered in porous solid panicles that are wet by the continuous phase. The latter diffuses into the pores, thereby increasing 0,. Osmotic diffusion occurs in multiple emulsions, in which there is an electrolytic unbalance between the innermost internal phase and the continuous phase. Thi.s condition induces migration of liquid from the region of high osmotic pressure to low osmotic pressure. Transference of continuous phase into the droplets pro-duce.s an increase of 0. whereas transference from the droplets to the continuous phase produces a decrease of ( ),. Both effects account for an increase or decrease of viscosity, respectively. Invivition and osmotic diffusion are of great importance in concentrated systems in which a relatively small increase of leads to large increases of viscosity and on the complexity of the rheological behavior. 

The consistency of the multiple emulsion, which is very important for cosmetic applications, can be evaluated using rheological methods, in the same manner as used for emulsions (see above). These methods can be applied for the primary as well as the final multiple emulsion, which may also contain a gel phase. 

In this chapter the effects of pressure balance and interfadal rheological properties on the stability of multiple emulsions are discussed. 

Numerous applications of multiple emulsions in various fields have been reported. More applications need to be realized if multiple emulsions stability is to be fully understood and approaches to stabilize multiple emulsions fully rationalized. The stability of multiple emulsions is influenced by numerous formulation and process variables. As demonstrated in this chapter, long-term multiple emulsion stability is dependent on the osmotic and Laplace pressures of the inner droplets as well as on the pressure balance between them described by the Walstra equation. Stability also equally, in some cases even more, depends on the strength of the interfacial film formed on the interface of droplets of multiple emulsions. This property can be characterized by interfacial rheology. 

Jiao J, Burgess DJ. 2003. Rheology and stability of W/O/W multiple emulsions containing Span 83 and Tween 80. AAPS Pharm Sci 5(1) 62-73. 

In a first step toward a more general understanding of multiple emulsions stability and dynamics, in this chapter we study extremely stable multiple emulsions. We show how to conveniently and precisely probe the droplets properties. Additionally we describe the structural and rheological properties of such stable multiple emulsions. 

We will consider first the dynamical rheological properties of concentrated inverse emulsions (stabilized by Span 80 in dodecane) for various volume fractions ( ). The experimental results are given in the form of the plot G Rd/( in relation to ( ) (Figure 2.8, hollow symbols). From the value of the interfacial tension, y = 3.5mN-m measured by tensiometry, and using 3, we calculate values for a and ( )c equal to 1 and 0.66, respectively. We will therefore use this measured value of a in the subsequent analysis of concentrated multiple emulsions. 

---