Langmuir 2004, 20, 5673-5678
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Articles Surfactant-Free “Emulsions” Generated by Freeze-Thaw Gary R. Burnett,† Rob Atkin, Stuart Hicks, and Julian Eastoe* School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K. Received January 9, 2004. In Final Form: March 30, 2004 The stability of surfactantless dispersions of surface chemically pure alkanes was studied in the presence and absence of dissolved gas. It was found that simply freezing and thawing a sample of oil and water results in a dispersion. A mechanism based on fingering of the insoluble oil into the aqueous phase, due to local surface tension gradients, followed by separation and nucleation into droplets, is proposed to account for this observation.
Introduction Recent articles have reported that removal of dissolved gas gives rise to enhanced stability of emulsions in certain mixtures consisting of only dodecane and water.1 Such a finding could profoundly impact the future direction of colloid and interface science. Intriguingly, it was found that reintroduction of gas to an already formed gas-free dispersion did not greatly affect the system stability.1 This observation warrants further investigation as it may prove to be key for understanding the phenomenon. Therefore, it is appropriate to examine these results in more detail and to explore the generality of “surfactantless emulsification”. It is common knowledge that oil and water are immiscible. For bulk phases, this incompatibility is associated with a surface (interfacial) free energy
∆G ) γ∆σ
(1)
where G is the Gibbs free energy, γ is the interfacial tension, and σ is the surface area of oil-water contact. Hence, mixing oil into water is always accompanied by a rise in surface energy, and for pure components alone, phase separation ensues. Added surface-active molecules, which lower the interfacial tension and may also present a barrier to coalescence, can increase the emulsion lifetime.2 The contribution of dissolved gases in the operation of the hydrophobic effect3-10 has led to suggestions that * Corresponding author. Telephone: +44 (0)117 928 9180. Fax: +44 (0)117 925 1295. E-mail:
[email protected]. † Current address: GlaxoSmithKline, Consumer Healthcare R&D, St George’s Avenue, Weybridge, KT13 0DE, U.K. (1) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714. Maeda, N.; Rosenberg, K. J.; Israelachvili, J. N.; Pashley, R. M. Langmuir 2004, 20, 3129-3137. (2) Hunter, R. J. Foundations of Colloid Science, Oxford University Press: Oxford, U.K., 1985. (3) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1999, 15, 1562. (4) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192. (5) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736. (6) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657. (7) Alargova, R. G.; Kochijashky, I. I.; Zana, R. Langmuir 1998, 14, 1575. (8) Sierra, M. L.; Zana, R. J. Colloid Interface Sci. 1999, 212, 162.
degassing may actually stabilize an emulsion.1 For a density matched oil-in-water emulsion, the dispersion lifetime is a function of the rate of diffusion of droplets toward one another, and the rate of thinning of the film that forms between the droplets.2 If oil droplets behave in a manner consistent with other lyophobic colloids, then the hydrophobic effect leads to attraction between droplets and emulsion instability. By this argument, removal of dissolved gas reduces, or eliminates, hydrophobic attraction, and the surface charge acquired by the oil droplet11 is sufficient to prevent coalescence. Degassing can be accomplished by freezing the oil-water mixture, applying a vacuum, and then thawing while still under vacuum, referred to here as freeze-pump-thaw (F-P-T). Repetition of this method results in removal of almost all dissolved gas and can lead to droplet formation without external agitation.1 This paper presents experiments performed to investigate the phenomenon of surfactant-less emulsification. First, the reproducibility and repeatability of published work1 is checked. Next, variables such as the identity of the oil and simple modifications to the F-P-T technique are analyzed. The presence of oil droplets is established by microscopy and turbidity. The primary conclusions from this work are that degassing does not increase the stability of surfactantless emulsions, nor does it enhance dispersion. This is consistent with the effect of reintroduction of dissolved gas described by Pashley.1 Finally, a tentative mechanism is proposed to account for these observations. Experimental Section n-Dodecane was used as received to repeat previous work,1 and also in a purified form. Untreated n-dodecane was of the same purity (Puriss (>99%)) and from the same source (Fluka) as used by Pashley.1 Purified n-dodecane was prepared by repeated passing of n-dodecane (Aldrich 99+%) through a column containing activated, basic aluminum oxide (Aldrich). This method has been shown to be highly effective for removing surface-active material from oils.12 Other alkanes used (nheptane, n-octane, n-nonane, n-decane, and n-undecane) were (9) Alfridsson, M.; Ninham, B.; Wall, S. Langmuir 2000, 16, 10087. (10) Ishida, N.; Sakamoto, M.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 5681. (11) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045. (12) Goebel, A.; Lunkenheimer, K. Langmuir 1997, 13, 369.
10.1021/la049923o CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004
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purified using the same procedure. All water used was purified with a Purite RO100 deionizer and had a constant surface tension of 72.7 mN m-1 and conductivity
