© Copyright 1997 American Chemical Society
FEBRUARY 19, 1997 VOLUME 13, NUMBER 4
Letters Why Do Ethoxylated Nonionic Surfactants Not Foam at High Temperature? A. Bonfillon-Colin* and D. Langevin Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600 Pessac, France Received June 5, 1996. In Final Form: December 19, 1996X It is well-known that the foamability of nonionic surfactants is reduced above the cloud point temperature. Above this temperature, the surfactant aqueous solution separates into two phases, a surfactant-rich phase and a surfactant-poor phase. The surfactant-rich phase plays the role of an antifoam. We show that the antifoam mechanism of action is the bridging of the foam films made with the dilute phase by tiny drops of the surfactant-rich phase which merge into the air-water surfaces of these foam films.
Ethoxylated nonionic surfactant aqueous solutions present a rich phase diagram when temperature is varied: micellar and many different liquid crystalline phases are observed.1 We will be concerned here by the water-rich part of the phase diagrams where the micellar phases are present. When the temperature rises, the interactions between the micelles become attractive, because of the loss of the hydration water of the polar heads responsible for steric repulsion between these micelles. A critical point is reached at a well-defined critical temperature and critical concentration.2 The solution separates into a micellar-rich phase and a micellar-poor phase. Because the phase separation is slow, the solution first becomes cloudy. The coexistence curve between the two phases is very flat. This means that the temperature of the phase separation does not depend appreciably on the surfactant concentration. The critical temperature is therefore a cloud point for the solutions in this concentration range. It is well-known that the foamability of these solutions X Abstract published in Advance ACS Abstracts, January 15, 1997.
(1) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; Mc Donald, M. P. J . Chem. Soc., Faraday Trans. 1 1983, 79, 975. (2) Claesson, P. M; Kjellander, R.; Stenius, P.; Christenson, H. J. Chem. Soc, Faraday Trans. 1 1986, 82, 2735.
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is reduced above the cloud point.3 This phenomenon is very general and is also encountered in polymer systems with cloud points.4 This property is used in the design of the so-called “cloud point antifoams”, which now have a wide commercial availability. In the case of surfactants, it has been shown by Koretskaya that the surfactant-rich phase indeed plays the role of an antifoam.3 Ross and Nishioka assume that the antifoam action is due to the spreading action of the surfactant-rich phase at the surface of the foam films.4 We will show that the surfactant-rich phase does not spread on the surfactant-poor phase and that the antifoam mechanism is rather due to bridging of the foam films by the drops of the surfactant-rich phase. Let us recall briefly the mechanisms of action of an antifoam. A first mechanism was proposed by Ewars and Sutherland5 for liquid antifoams. In this mechanism, the antifoam droplets enter the air-water surface of a soap film and spread (Figure 1). The spreading effect is due to the surface tension difference between the water surface and the antifoam liquid, which produces a shear force. It drags the underlying liquid away from the soap films and this causes a faster thinning and leads to film rupture. A second mechanism was proposed later by Berg and co(3) Koretskaya Kolloid Zh. 1977, 39, 571. (4) Ross, S.; Nishioka, G. Colloid Polym. Sci. 1977, 255, 560. (5) Ewars, W. E.; Sutherland, K. L Aust. J. Sci. Res. 1952, 5, 697.
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Figure 2. Evolution of the foam height as a function of time above cloud point: b, surfactant-poor phase; 9, water added; 4, surfactant-rich phase.
Figure 1. Schematic drawing of the two antifoam mechanisms: (a) spreading mechanism: The drop first enters one of the film surfaces and spreads. This spreading produces a shear force and drags the underlying liquid away from the film. (b) Bridging mechanism: The drop first enters one of the film surfaces and forms a lens. Upon further thinning of the film, the lens enters the opposite film surface and an oil bridge is formed. The bridge is unstable because the capillary forces lead to a dewetting of the bridge and the film ruptures. The arrows in the film indicate the direction of the capillary forces.
workers and is also applicable to solid antifoams.6 In this mechanism, the antifoam particles still enter the airwater surface of foam films but do not spread. When the film thickness becomes comparable to the particle size, if the contact angle between the liquid and the particle is larger than 90°, the soap film is unstable and breaks. This contact angle condition is equivalent to a positive bridging coefficient.7
B ) γaw2 + γow2 - γoa2 where γaw is the air-water surface tension,γoa the airantifoam surface tension, and γow the antifoam-water surface tension. In a recent study on silicone-based antifoam, where solid hydrophobic silica particles are dispersed in silicone oil, it has been shown that the two mechanisms are active: spreading of oil and bridging by the silica particles. In addition, it has been shown that the particles are more effective if their size is not smaller than a few micrometers; indeed, below this value, the buoyancy force leading the particles to enter the air-water interface is dominated by Brownian motion, and the chance for the particles to approach the surface is reduced.8 The surfactant used in the present study is tetraethylene glycol mono-n-decyl ether (C10E4). It was purchased from Nikko Chemicals and used as received. The cloud point is 20 °C. The foaming and the foam stability of the aqueous solutions were studied using the Ross-Miles test. (6) Frye, G. C.; Berg, J. C. J. Colloid Interface Sci. 1989, 127, 222. (7) Garett, P. R. J. Colloid Interface Sci. 1989, 76, 587. (8) Bergeron, V.; Cooper, P.; Fischer, C.; Kahn, J.; Langevin, D.; Pouchelon, A. Submitted for publication.
Figure 3. Picture of a C10E4 monolayer from Brewster angle microscopy: (a, top) T ) 15 °C; (b, bottom) T ) 25 °C. (The droplets have a typical size of around 5 µm. They have a circular section, and the shapes seen on the picture are distorted by the optics of the instrument).
We first checked if, as in the Koretskya experiments, the surfactant-rich phase played the role of an antifoam above the cloud point. A solution with a concentration of 8 wt % was prepared and studied at 24 °C. After complete phase separation, the dilute phase was extracted and studied. As shown in Figure 2, the removal of the surfactant-rich phase restores the foamability, despite the reduction in surfactant concentration. The lifetime of the foam is comparable to the lifetime of the foam made from the initial solution below the cloud point. If one adds to the top of the foam a quantity as small as 1 mL of surfactant-rich phase, the foam breaks rapidly, in less than 30 s (note that the addition of 1 mL of water to the top of the foam does not change the lifetime of the foam, so that the antifoam action in the first case cannot be attribued to the mechanical action of the drops).
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We have measured the surface tensions of both surfactant phases at 24 °C with a Wilhelmy plate. The tension of the dilute phase is 31.64 ( 0.1 mN/m, and the tension of the concentrated phase is 31.1 ( 0.1 mN/m. The interfacial tension between these phases is very low at this temperature, too small to be measured: γ < 0.2 mN/ m. This leads to a bridging coefficient which is largely positive: B ∼ 33 mN/m2. However , the spreading coefficient of the concentrated phase on the dilute one is small, but possibly positive: S ) 0.4 ( 0.3 mN/m (S ) γaw - γow - γoa). In order to clarify the mechanism responsible for the antifoam action, we have studied the surface solution with a Brewster angle microcoscope. Details about the setup can be found elsewhere.9 In these experiments, a surfactant solution of concentration 4 wt % was used. Five degrees below the cloud point (T ) 15 °C)the surface is homogeneous, as seen in Figure 3a. A few degrees above the cloud point (T ) 25 °C), the surface becomes heterogeneous, and droplets of the surfactant-rich phase merge into the surface (Figure 3b). These droplets have a typical size of about 5 µm. If one keeps heating, more droplets merge at the surface without changing their size. The pictures clearly show that the droplets enter the surface. Since the spreading coefficient is small, one cannot exclude a “pseudo partial” spreading, that is to say, that the spreading of a thin layer of surfactant-rich phase stabilized by the short range forces (disjoining pressure).10 In this case, one would have a thin layer of micellar phase floating on a surfactant monolayer, below which other micelles (9) Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991, 7, 335. (10) Brochard, F.; Di Meglio, J. M.; Que´re´, D.; de Gennes, P. G Langmuir 1991, 335, 7.
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are present with a lower concentration. This seems unlikely. We rather think that S is negative, a situation which is not excluded due to the error bars in the surface tension experiments. In any case, the liquid droplets are present and effective for the bridging mechanism. If a thin surfactant-rich layer spreads, it will enhance the antifoam action, as in the case of the silica-silicone oil antifoam systems. It has been proposed by Ross that there is a sharp dramatic reduction in Gibbs elasticity that occurs above the cloud point and could account for the loss in foam stability. we have determined the Gibbs elasticity of our surfactant monolayer by measuring the surface tension of the solution below the critical micellar concentration (cmc) and using the Gibbs equation. Recent neutron reflectivity experiments on similar surfactant solutions show that the monolayer does not change after cmc.11 We have no evidence of any signifiant changes in the Gibbs elasticity below and above the cloud point. In conclusion, the loss in foam stability of our nonionic surfactant solution is due to the antifoam action of the droplets of the surfactant-rich phase which bridge the foam films made from the surfactant-poor phase and produces the rupture of these films. Acknowledgment. We gratefully thank Sylvie Henon for the use of the Brewster angle microscope. We also thank the reviewers for pointing out the work of refs 3 and 4. LA950439I (11) Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352.
