Model Studies on the Mechanism of Deactivation (Exhaustion) of Mixed Oil-Silica Antifoams Krastanka G. Marinova, † Slavka Tcholakova, † Nikolai D. Denkov,* ,† Stoian Roussev, ‡ and Martial Deruelle § Laboratory of Chemical Physics and Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria, Department of Solid State Physics, Faculty of Physics, Sofia University, 5 James Bourchier Avenue, 1164 Sofia, Bulgaria, and Usine Silicones, RHODIA Chimie, CRIT C, 55 Rue des Freres Perret BP 22, 69191 Saint Fons Cedex, France Received October 27, 2002 1. Introduction Antifoams are important components of many com- mercial products, such as detergents, paints, pharma- ceuticals, and others. 1 Antifoams are also used in various technologies, such as pulp and paper production, fermen- tation, and oil processing. It has been shown 2 that mixed liquid-solid antifoams (e.g., those comprising silicone oil and hydrophobic silica) usually have much higher activity than their individual components, if taken separately. A major problem in the practical application of antifoams is the gradual loss of their activity in the course of foam destruction. This process is termed “antifoam exhaustion” or “deactivation”, and several possible explanations have been proposed in the literature. 3-14 Most often, the explanations are as follows: (i) the antifoam globules reduce their size in the course of foam destruction and eventually become too small to rupture efficiently the foam films; 6,8,10 (ii) the antifoam, initially deposited on the surface of the foaming solution, is gradually emulsified and becomes inactive. 7 Other possibilities were discussed in refs 4, 5, and 9 (see also the discussion in ref 12). It was recently shown 12,13 that the exhaustion of mixed poly(dimethylsiloxane) (PDMS)-silica antifoams in sur- factant solutions is due to two interrelated processes: (1) segregation of oil and silica into two distinct populations of antifoam globules (silica-free and silica-enriched), both of them being rather inactive; (2) disappearance of the spread oil layer from the solution surface. This mechanism is illustrated in Figure 1: The oil droplets deprived of silica, which appear in process 1, are unable to enter the surfaces of the foam films and to destroy the foam lamellae, because their entry barrier is too high. Indeed, the existence of a certain critical value of the entry barrier was demonstrated, 14-17 which separates the fast (those that break the foam film) from slow (those unable to enter the foam film surfaces) antifoams. On the other side, the antifoam globules enriched in silica, appearing in process 1, are also inactive, because they are nondeformable and * To whom correspondence should be addressed. Phone: (+359) 2-962 5310. Fax: (+359) 2-962 5643. E-mail: ND@ LCPE.UNI-SOFIA.BG. † Laboratory of Chemical Physics and Engineering, Sofia University. ‡ Department of Solid State Physics, Sofia University. § Usine Silicones, RHODIA Chimie. (1) Defoaming: Theory and Industrial Applications; Garrett, P. R., Ed.; Marcel Dekker: New York, 1993; Chapters 2-8. (2) Garrett, P. R. In Defoaming: Theory and Industrial Applications; Garrett, P. R., Ed.; Marcel Dekker: New York, 1993; Chapter 1. (3) Exerowa, D.; Kruglyakov, P. M. Foams and Foam Films; Elsevier: Amsterdam, 1998; Chapter 9. (4) Kulkarni, R. D.; Goddard, E. D.; Kanner, B. Ind. Eng. Chem. Fundam. 1977, 16, 472. (5) Pouchelon, A.; Araud, A. J. Dispersion Sci. Technol. 1993, 14, 447. (6) Koczo, K.; Koczone, J. K.; Wasan, D. T. J. Colloid Interface Sci. 1994, 166, 225. (7) Racz, G.; Koczo, K.; Wasan, D. T. J. Colloid Interface Sci. 1996, 181, 124. (8) Wasan, D. T.; Christiano, S. P. In Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: Boca Raton, FL, 1997; p 179. (9) Garrett, P. R.; Davis, J.; Rendall, H. M. Colloids Surf., A 1994, 85, 159. (10) Bergeron, V.; Cooper, P.; Fischer, C.; Giermanska-Kahn, J.; Langevin, D.; Pouchelon, A. Colloids Surf., A 1997, 122, 103. (11) Denkov, N. D.; Cooper, P.; Martin, J.-Y. Langmuir 1999, 15, 8514. (12) Denkov, N. D.; Marinova, K. G.; Christova, C.; Hadjiiski, A.; Cooper, P. Langmuir 2000, 16, 2515. (13) Marinova, K. G.; Denkov, N. D. Langmuir 2001, 17, 2426. (14) Denkov, N. D.; Marinova K. G. Proceedings of the 3rd Euro- Conference on Foams, Emulsions and Applications; MIT: Bremen, 2000. (15) Hadjiiski, A.; Denkov, N. D.; Tcholakova, S.; Ivanov, I. B. In Adsorption and Aggregation of Surfactants in Solution; Mittal, K., Shah, D., Eds.; Marcel Dekker: New York, 2002; Chapter 23, pp 465-500. (16) Basheva, E.; Ganchev, D.; Denkov, N. D.; Kasuga, K.; Satoh, N.; Tsujii, K. Langmuir 2000, 16, 1000. (17) Hadjiiski, A.; Tcholakova, S.; Ivanov, I. B.; Gurkov, T. D.; Leonard, E. Langmuir 2002, 18, 127. Figure 1. Schematic presentation (adopted from ref 12) of the processes of antifoam exhaustion: (a) An initially active antifoam contains globules of optimal silica/oil ratio; a layer of spread oil is present on the surface of the surfactant solution. (b) The foam destruction by the antifoam globules leads to a gradual segregation of oil and silica into two inactive populations of globules (silica-free and silica-enriched); the spread oil layer disappears from the solution surface, and the antifoam becomes inactive. (c) The process of silica-oil segregation continues on further foaming of the sample, leading to the appearance of larger silica-enriched aggregates. 3084 Langmuir 2003, 19, 3084-3089 10.1021/la0267589 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/19/2003