Effect of Oily Additives on Foamability and Foam Stability. 2. Entry Barriers Asen Hadjiiski, † Slavka Tcholakova, † Nikolai D. Denkov,* ,† Patrick Durbut, ‡ Guy Broze, ‡ and Ammanuel Mehreteab § Laboratory of Chemical Physics Engineering (Formerly Laboratory of Thermodynamics and Physicochemical Hydrodynamics), Faculty of Chemistry, Sofia University, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria, Colgate-Palmolive Technology Center, 909 River Road, Piscataway, New Jersey 08854-5596, and Colgate-Palmolive Research & Development, Inc., Avenue Du Parc Industriel, B-4041 Milmort (Herstal), Belgium Received April 24, 2001. In Final Form: July 23, 2001 In the preceding paper of this series we studied the effect of several oils of different chemical structure on the foaming properties of sodium dodecylbenzenesulfonate solutions. A straightforward correlation was found between the foam stability and the so-called “entry barrier”, which prevents the emergence of pre- emulsified oil drops on the solution surface. In the present article we perform a systematic experimental study of the entry barriers for several oils by means of the recently developed film trapping technique. The latter consists of trapping oil drops in wetting films on a solid substrate, followed by a controlled increase of the capillary pressure of the meniscus that compresses the drops against the substrate. At a certain critical capillary pressure, PC CR , the asymmetric oil-water-air films rupture and the drops enter the water-air interface. This event is observed microscopically, and PC CR is determined as a function of various parameters (type of oil, surfactant concentration, drop size, and others). The entry barrier increases with the surfactant concentration, especially in the range where the surfactant micelles are expected to stabilize the asymmetric films. The results obtained with a series of alkanes (from octane to hexadecane) show that the entry barrier increases with the alkane chain length. Furthermore, it is shown that the presence of a spread oil (even as an ultrathin, molecular layer) on the surface of the foam film might lead to a significant change of the magnitude of the entry barrier. For decane and dodecane, the layer of spread oil reduces the entry barrier, whereas for hexadecane the effect is the opposite. As far as we know, such a role of oil spreading in the antifoaming action of oils has not been reported so far. Since the stability of thin liquid films is usually discussed in the literature in terms of the disjoining pressure, we estimate from the experimental data the critical disjoining pressure, ΠAS CR , at which the asymmetric oil-water-air film ruptures and the drop entry occurs. The estimates show that the curvature of the asymmetric film is very important in the overall consideration of the mechanical equilibrium in the system and there is a big difference between the numerical values of PC CR and ΠAS CR , unlike the case of planar films where PC CR ) ΠAS CR . Additionally, we find that PC CR is a weak function of the oil drop size and of the asymmetric film radius, while ΠAS CR scales as (film radius) -1 for all of the studied systems. These results are discussed with respect to the possible mechanisms of film rupture. Concerning the foam stability, PC CR is a more convenient quantity for description of the entry barriers, because its magnitude correlates with the foam height, whereas the magnitude of ΠAS CR does not. 1. Introduction In the first paper of this series 1 we studied how several oils of different chemical structure affected the foam stability and the foamability of sodium dodecylbenzene- sulfonate (SDDBS) solutions. The results from the foam tests demonstrated a straightforward correlation between the foam stability and the entry barrier, which prevents the emergence of pre-emulsified oil drops on the solution surface (the used definition of the entry barrier is explained below). On the other hand, no direct relation between the foam stability and the magnitudes of the so-called entry, E, spreading, S, and bridging, B, coefficients was observed (most of the studied oils had positive B coefficients, which means that oil bridges, once formed in the foam films, would be unstable 2-4 ). Similar results were obtained recently with other surfactant-oil couples, 5-7 and a quantitative relation between the final foam height and the entry barrier was established. 5 The primary reason for this correlation is that any mechanism of foam destruction by emulsified oil should include the stage of formation and rupture of asymmetric oil-water-air films. 1,3,8-18 As noticed long ago by Kruglyakov 8 and Kulkarni et al., 15 these asym- * To whom correspondence may be addressed. Phone: (+359) 2-962 5310. Fax: (+359) 2-962 5643. E-mail: ND@LTPH.BOL.BG. † Laboratory of Chemical Physics Engineering, Sofia University. ‡ Colgate-Palmolive Research & Development, Inc., Milmort (Herstal). § Colgate-Palmolive Technology Center, Piscataway. (1) Arnaudov, L.; Denkov, N. D.; Surcheva, I.; Durbut P.; Broze G.; Mehreteab, A. Langmuir 2001, 17, 6999-7010. (2) Garrett, P. R. J. Colloid Interface Sci. 1980, 76, 587. (3) Garrett, P. R. In Defoaming: Theory and Industrial Applications; Garrett, P. R., Ed.; Marcel Dekker: New York, 1993; Surfactant Science Series, Vol. 45, Chapter 1. (4) Denkov, N. D. Langmuir 1999, 15, 8530. (5) Basheva E.; Ganchev, D.; Denkov, N. D.; Kasuga, K.; Satoh, N.; Tsujii, K. Langmuir 2000, 16, 1000. Basheva E.; Stoyanov, S.; Denkov, N. D.; Kasuga, K.; Satoh, N.; Tsujii, K. Langmuir 2001, 17, 969. (6) Marinova, K.; Denkov, N. D. Langmuir 2001, 17, 2426. (7) Denkov, N. D.; Marinova K. Proceedings of the 3rd EuroConference on Foams, Emulsions and Applications; MIT: Bremen, 2000. (8) Kruglyakov, P. M.; Koretskaya, T. A. Kolloid. Zh. 1974, 36, 682. Kruglyakov, P. M. In Thin Liquid Films: Fundamentals and Applica- tions; Surfactant Science Series; Ivanov, I. B., Ed.; Marcel Dekker: New York, 1988; Vol. 29, Chapter 11. (9) Exerowa, D.; Kruglyakov, P. M. Foams and Foam Films; Elsevier: Amsterdam, 1998; Chapter 9. (10) Bergeron, V.; Fagan, M. E.; Radke, C. J. Langmuir 1993, 9, 1704. (11) Lobo, L.; Wasan, D. T. Langmuir 1993, 9, 1668. 7011 Langmuir 2001, 17, 7011-7021 10.1021/la010601j CCC: $20.00 © 2001 American Chemical Society Published on Web 09/29/2001