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Why are certain emulsions and foams more stable than others ?

Foams are dispersions of air bubbles in water and emulsions are dispersions of oil drops in water (or water drops in oil). These dispersions are stabilised by surfactant monomolecular layers adsorbed at the surface of bubbles/drops. Depending on the surfactant used, their lifetime can vary from minutes to days for foams and to years for emulsions. Despite their wide range of applications (food, pharmacy, cosmetics, paints, oil recovery, elaboration of new composite materials, etc..), the control of their stability remains essentially empirical. The problem is complex, because several mechanisms act in parallel :
- gravity, producing foam drainage or emulsion creaming, followed by coalescence (fusion of bubbles/drops) when the water film between bubbles/drops becomes very thin.
- Ostwald ripening (under the effect of Laplace pressure, the small bubbles/drops disappear and the large ones grow)
Until now, it was admitted that the stability was controlled by the coalescence step, because it did not seem that ripening could be affected by the type of surfactant used. The existing ripening studies all showed large differences with respect to theoretical predictions, but these discrepancies were attributed to the fact that ripening largely depends on the water volume fraction, the existing theories being only valid in the limit of dilute bubbles/drops. On the other hand, the surfactant layer properties, in particular their elasticity, were suspected to play an important role in coalescence, during which the surfaces are strongly distorted. The surface elasticity in question is associated to gradients of surface tension, therefore of surface pressure (Marangoni effect). This elasticity depends of the frequency at which it is measured, the surfactant layers being usually strongly viscoelastic.

In our experiments, we have evidenced that the ripening step was very long, and thus controls the lifetime of the systems studied. We have shown that the dispersion lifetime is well correlated with the surface elasticity measured at low frequencies (ripening times are long, >min) and not correlated with the elasticity measured at high frequencies (coalescence times are short, <s). The surface elasticity therefore appears to control the stability of foam and emulsions, but not as expected : the elasticity that matters is not the high frequency one, but the low frequency one. For example, a foam made with the surfactant C12E6 (alkyl poly-oxyethylene) has a lifetime of 30’, whereas a foam made in the same conditions with the surfactant C12G2 (alkyl glucoside) has a lifetime of 2 days : the low frequency elastic modulus for C12E6 is much smaller than for C12G2 (figure 1) in line with the difference in lifetimes. The high frequency modulus is larger (note the inversion around 1Hz in figure 1), confirming the small influence of coalescence on the lifetimes. Quite similar results were found with emulsions.

Figure 1 illustrates the possible role of elasticity in the ripening process : the bubbles that grow stretch the surface layer, whereas those that shrink compress the layer ; it is therefore natural to find that surface elasticity plays a role in the process.

Figure 1. Possible role of elasticity during ripening

It remains to explain how does the ripening depend on elasticity, a very difficult task, as the bubbles in foams (and the drops in concentrated emulsions) are far from being spherical as in the figure, and from the geometrical point of view, the problem is already very complex.

1. D. Georgieva, V. Schmitt, F. Leal-Calderon, D. Langevin “On the possible role of the surface elasticity in emulsion stability” Langmuir, 2009, DOI : 10.1021/la804240e
2. D. Georgieva, A. Cagna, D. Langevin “Link between surface elasticity and foam stability”, Soft Matter, 2009, DOI : 10.1039/B822568K