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Nonionic Block Copolymer Antifoams K. S. Joshi,† S. A. K. Jeelani,*,† C. Blickenstorfer,‡ I. Naegeli,‡ C. Oliviero,§ and E. J. Windhab† Laboratory of Food Process Engineering, Institute of Food Science and Nutrition, Swiss Federal Institute of Technology, ETH-Zentrum, Schmelzbergstrasse 7, 8092 Zu¨rich, Switzerland, Dr. W. Kolb AG, 8908 Hedingen, Switzerland, and Department of Chemistry, UniVersity of Calabria, 87036 ArcaVacata di Rende (CS), Italy ReceiVed January 9, 2006. In Final Form: April 6, 2006 Aqueous dispersions of alkoxylated alcohol block copolymer (BCP) drops are investigated as antifoams. A model aqueous nonionic surfactant solution of Polysorbate 20 and an industrial white water suspension are used as foaming systems. Visual evidence obtained using a two-bubble technique involving a CCD camera coupled with high magnification lenses clearly revealed the role of BCP droplets in the bubble coalescence process. The enhancement of bubble coalescence decreased as the temperature increased from 25 to 60 °C, which is due to the corresponding decrease in the rigidity associated with the weak interfacial structure and reduced viscosity of the BCP drops. The antifoaming efficiency measured in the macroscopic recirculation foam column increased with temperature from about 13 to 26 °C (attaining a maximum) and decreased as temperature increased further. Oscillatory thermo-rheometric measurements showed a sudden increase in the storage modulus (G′) by several orders of magnitude, indicating gel formation initiated at about 13 °C and having a maximum at around 26 °C for an aqueous solution of the BCP above a critical concentration of around 20 wt %. Results obtained using small-angle X-ray scattering, micro-differential scanning calorimetry, and proton nuclear magnetic resonance confirmed the existence of ordered gel-like structures. Furthermore, macroscopic tests using a sparged air foam column showed a significant increase in antifoaming efficiency when highly hydrophobic particles are embedded in the BCP drops dispersed in water.
Introduction Nonionic block copolymers (BCPs) are employed as antifoams in this work. Because of their amphiphilic nature, BCPs are surface active and, in few cases, have also been used to stabilize aqueous films1 and foams.2 BCPs of the type RO-(PO)x-(EO)y, where R represents an alkyl chain and EO and PO correspond to ethylene and propylene oxides, respectively, form an alkoxylated alcohol series. One such BCP is investigated for its interactions with foam and role in bubble coalescence. Diblock EO-PO copolymers have been widely studied in the literature because of their diverse applications.3 Since the PO groups have poor biodegradability, a part of the polypropylene oxide (PPO) is replaced by the alkyl groups to satisfy the minimum requirement of hydrophobicity. Hence, instead of a pure EO-PO BCP, the alkoxylated alcohols, which have better environmental compatibility, are preferred in many antifoaming applications such as paper making, detergents, or flotation processes. It is even necessary to have the PO groups in the center of the chain, as the compounds of the type RO-(EO)y-(PO)x also fail the test for biodegradability.4 The mechanisms of foam destabilization using alkoxylated alcohols have not been completely discovered, and not much literature exists in this context. Similarities in behavior exist between diblock EO-PO copolymers or some of * Corresponding author. E-mail:
[email protected]. † Swiss Federal Institute of Technology, ETH-Zentrum. ‡ Dr. W. Kolb AG. § University of Calabria. (1) Exerowa, D.; Sedev, R.; Ivanova, I. B.; Kolarov, T.; Tadros, T. F. Colloids Surf., A 1997, 123, 277. (2) Krupers, M. J.; Bartelink, C. F.; Grunhauer, H. J. M.; Moller, M. Polymer 1998, 39, 2049. (3) Lindman, B.; Alexandridis, P. Amphiphilic Block Copolymers: SelfAssembly and Applications; Elsevier: Amsterdam, 2000. (4) Blease, T. G.; Evans, J. G.; Hughes, L.; Loll, P. Defoaming: Theory and Industrial Applications, Surfactant Antifoams; Garrett, P. R., Ed.; Marcel Dekker, Inc.: New York, 1993; Vol. 45, pp 299-324.
the symmetric triblock copolymers, but the present work would be the first attempt to investigate the BCP of alkoxylated alcohol on the mechanistic level. The structure5,6 of these molecules plays an important role in their foaming behavior. Aqueous solutions of BCP tend to be turbid above the cloud point. Ross7,8 investigated the effect of the solubility of organic liquids in an aqueous phase on foaming in terms of the commonly known Ross-Nishioka effect. The spreading coefficient was used to predict capillary effects, which cause the stabilization or drainage of the matrix phase forming the thin film between coalescing droplets or bubbles. This rule is a step beyond the Ferguson effect,9 where the foam stability/surface activity passes through a maximum as the molecular weight of the lipophile in a homologous series of solutes is increased. The use of surfactants as antifoams has been discussed previously with examples of copolymers for the machine dishwashing4 and fermentation industries.10 Several authors11-14 have investigated the foaming characteristics of aqueous solutions of various BCPs. They found that the concentration of the BCP, operating temperature, cloud point of the BCP in aqueous phase, and molecular conformation at the interface are the important parameters affecting foaming. Destabilization of foams was observed13,15 when the operating temperature exceeded the cloud point of the solution. At and above the cloud point, the surfactant (5) Binks, B. P.; Fletcher, P. D. I.; Haynes, M. D. Colloids Surf., A 2003, 216, 1. (6) Sawicki, G. C. Colloids Surf., A 2005, 263, 226. (7) Ross, S. J. Phys. Chem. 1979, 83, 2226. (8) Ross, S. Colloids Surf., A 1996, 118, 187. (9) Ferguson, J. Proc. R. Soc. London, Ser. B 1939, 127, 387. (10) C¸ alik, P.; Illeri, N.; Erdinc¸ , B. I.; Ayodogan, N.; Argun, M. Langmuir 2005, 21, 8613. (11) Pugh, R. AdV. Colloid Interface Sci. 1996, 64, 67. (12) Ne´meth, Z.; Racz, G.; Koczo, K. Colloids Surf., A 1997, 127, 151. (13) Ne´meth, Z.; Racz, G.; Koczo, K. J. Colloid Interface Sci. 1998, 207, 386. (14) Tan, S. N.; Fornasiero, D.; Sedev, R.; Ralston, J. Colloids Surf., A 2004, 250, 307. (15) Colin, A.; Giermanska-Kahn, J.; Langevin, D. Langmuir 1997, 13, 2953.
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solution separated into two phases: a clear dilute solution and a concentrated insoluble surfactant phase (coacervate phase),16 in which the surfactant formed dispersed particles or drops. Denkov17 recently reviewed the mechanistic knowledge on mixed oil and hydrophobic particle antifoams. There are a few basic differences between BCP- and oil-based antifoams. BCP antifoams have partial solubility in an aqueous phase and a cloud point unlike the poly(dimethylsiloxane) (PDMS)-based antifoams. The latter utilizes hydrophobic silica particles in combination, which are undesired in applications such as paper making and food processing. Thus, water-based and BCP antifoams are important, even though they are not yet formulated to be as efficient as the PDMS antifoams. A BCP antifoam is advantageous over an aqueous fatty alcohol suspension antifoam in that the former exists as a single phase, unlike the latter, which has the problem of long-term stability.18 In addition, the transport of 100% active BCP antifoams is economical compared to suspension antifoams, which contain 60-80% water. For the first time, the influence of the addition of an antifoam dispersion of partially soluble nonionic BCP to a foaming solution containing another surfactant is investigated in detail. The term “antifoaming” in this work implies accelerated interbubble coalescence in a liquid pool, in addition to the destabilization of surface foam. Previous results19 on the antifoams of aqueous suspensions of fatty alcohol particles indicated the importance and effect of solid particles on antifoaming efficiency. Consequently, in addition to a pure BCP dispersion antifoam, an antifoam that is a mixture of BCP and hydrophobic wax particles dispersed in water introduced into a foaming solution is also investigated. Macroscopic antifoaming efficiency tests are carried out using a sparged air foam column to determine the extent of reduction in air content in the presence of the different antifoam formulations. A novel two-bubble technique involving a highspeed camera coupled with high magnification lenses is developed and used to visually investigate the interactions of antifoam drops or particles between colliding air bubbles in an aqueous phase. This enabled us to observe the bubble coalescence process due to the movement of particles or drops of the BCP as a fresh bubble approaches a second bubble, the surface of which is covered with the BCP drops/particles. Interbubble coalescence also occurs in a sparged air foam column when bubbles collide. Two-bubble experiments have also been carried out by other authors.20-22 However, none of them visually observed the role of particles or drops in the bubble coalescence process but interpreted the results on the basis of the differences in coalescence times in the absence and presence of certain solubilized chemicals. Two-bubble experiments provide visual proof of drops or particles bridging two bubbles, leading to coalescence. This has been explained schematically in the literature.23-26 There exist visual proofs of particles being trapped in thin foam films and their role in rupturing these films.27,28 This mechanistic knowledge existing (16) Chaisalee, R.; Soontravanich, S.; Yanumet, N.; Scamehorn, J. F. J. Surfactants Deterg. 2003, 6, 345. (17) Denkov, N. D. Langmuir 2004, 20, 9463. (18) Jeelani, S. A. K.; Benoist, G.; Joshi, K. S.; Gunde, R.; Kellenberger, D.; Windhab, E. J. Colloids Surf., A 2005, 263, 379. (19) Joshi, K. S.; Jeelani, S. A. K.; Blickenstorfer, C.; Naegeli, I.; Windhab, E. J. Colloids Surf., A 2005, 263, 239. (20) Sagert, N.; Quinn, M. Chem. Eng. Sci. 1978, 33, 1087. (21) Gourram-Badri, F.; Conil, P.; Morizot, G. Int. J. Miner. Process. 1997, 51, 197. (22) Spyridopoulos, M.; Simons, S. Colloids Surf., A 2004, 235, 25. (23) Garrett, P. R. Defoaming: Theory and Industrial Applications, Surfactant Antifoams; Marcel Dekker, Inc.: New York, 1993. (24) Aveyard, R.; Cooper, P.; Fletcher, P. D. I.; Rutherford, C. E. Langmuir 1993, 9, 604. (25) Bergeron, V.; Cooper, P.; Fischer, C.; Giermanska-Kahn, J.; Langevin, D.; Pouchelon, A. Colloids Surf., A 1997, 122, 103. (26) Denkov, N. D.; Cooper, P.; Martin, J.-Y. Langmuir 1999, 15, 8514.
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in the literature deals specifically with surface foams and their films. The current work investigates the action of antifoams in the froth zone, which consists of a wet foam with spherical bubbles colliding in a liquid pool. Thus, the existing mechanisms become relevant but not complete for application in this case. The rheological properties of BCPs can be used to explain their efficiency in bubble coalescence. For BCP solutions, the point of gelation and the strength of the gels as a function of temperature can be quantified by comparing the storage modulus (G′) as a function of temperature or stress.29-31 Gelation in this work is strictly defined on the basis of rheological analysis (crossover of G′ and G′′, the latter being the loss modulus), and detailed structural information of this highly networked phaseseparated BCP dispersion is not analyzed in this work. Amphiphilic BCPs appear to change the conformation when the temperature or concentration in solution is altered. Above a critical concentration, the temperature of gelation depends on the PPO content. It is shown that there exists a direct correlation in the maximum in antifoam efficiency and the gel strength for the BCP. The present work aims at finding the mode of action of nonionic BCP antifoams in froth and the parameters affecting the performance. This knowledge would help customize the physicochemical and interfacial characteristics of BCPs to enhance the interbubble coalescence for a given system. Experimental Section Materials. Antifoams. BCPs consist of ethoxylated and propoxylated alcohols or acids or esters as a single component or a mixture. The alkoxylation varies for different antifoams. Depending on the absolute number and the ratio of the EO and PO groups, the antifoam efficacy changes. The number of carbon atoms in the basic hydrocarbon chain also influences the performance of the antifoam to a large extent. In the present work, a BCP was used, which is an ethoxylated and propoxylated straight-chain fatty alcohol with an EO/PO ratio of 1:5 and a molecular weight of about 2500. A detailed temperatureconcentration phase diagram based on the rheological characteristics of the BCP-water mixture is presented in a later section. The BCP has a cloud point at around 7 °C for a 21% (by weight) solution in aqueous phase. With a 21% BCP solution, it is possible to pass through three distinct phase regions of the BCP by varying temperature alone: (i) micelles or small micellar aggregates (transparent solution), (ii) aggregated micelles (colloidal turbid solution above cloud point), and (iii) gelled phase (white gel). The solutions are thermoreversible. The behavior of this BCP is similar to that observed by Wang et al.31 for the triblock copolymers. When a drop of BCP is suspended in water above its gel point, a partial mixing of water is expected to take place at the interface. The gel formation (phase separation) needs finite time and is achieved only when water molecules interact with the BCP. It is hard to determine the thickness of this layer. Visual observations show an interfacial layer, forming a shell for the drops as shown in Figure 1. As the BCP drop is formed in water at the tip of a capillary, the shell begins to form at the apex of the drop, since the time of contact of the apex with the water is the longest compared to the rest of the drop. The interfacial gel subsequently propagates toward the neck of the drop, eventually covering the entire surface. A residence time of approximately 10 s in quiescent conditions is needed for the interfacial gel to completely cover the suspended BCP drop. It is difficult to (27) Koczo, K.; Koczone, J.; Wasan, D. J. Colloid Interface Sci. 1994, 166, 225. (28) Tamura, T.; Kageyama, M.; Kaneko, Y.; Kishino, T.; Nikaido, M. J. Colloid Interface Sci. 1999, 213, 179. (29) Soenen, H.; Berghmans, H.; Winter, H. H.; Overbergh, N. Polymer 1997, 38, 5653. (30) Bhatia, S. R.; Mourchid, A.; Joanicot, M. Curr. Opin. Colloid Interface Sci. 2001, 6, 471. (31) Wang, Q.; Li, L.; Jiang, S. Langmuir 2005, 21, 9068.
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Figure 1. Interfacial skin formation at 25 °C. determine the exact concentration of BCP/water in the interfacial layer, but is certainly above the critical gelling concentration of a BCP solution in water. The properties of the interfacial layer are related to those of a 21% aqueous solution of the BCP. The former may differ quantitatively from the latter, but qualitatively they should follow similar trends. A mixed antifoam was also developed and tested. Hydrocarbon wax particles (Sasol Spray 30F) with a Sauter mean diameter of about 5 µm were dispersed in the liquid BCP at various concentrations (1-10% of particles by weight, based on the BCP). Foaming Solutions. The nonionic surfactant Polysorbate 20 (PS20) obtained from Fluka with no further purification was used to prepare the model foaming solution for the sparged air foam column experiments. This is polyoxyethylene sorbitan monolaurate with 12 carbon atoms in the hydrophobic chain of the molecule and has a hydrophilic-lipophilic balance value of about 16.7. A 4.27 mg/L solution of PS20 in deionized water, which was about 25 times lower than the critical micelle concentration (110 mg/L)32 was used. A recycled paper system, commonly referred to as white water by the paper manufacturing companies, was used to prepare the foaming system employed in the circulation foam column experiments. Methods. Preparation of Antifoam Dispersions. An aqueous dispersion of the BCP was prepared prior to its introduction into the foaming solution. A 1 mL portion of the BCP was added to 100 mL of deionized water while mixing it with an Ultraturex high-shear mixer. The dispersion was mixed under high shear for 30 min and then inserted in an ultrasound bath to obtain a narrow size distribution of the dispersed drops with a mean diameter of 5 µm. The size distribution of the dispersion was analyzed with a Beckman Coulter laser diffraction particle size analyzer. For preparing the mixed antifoam of hydrophobic particles in combination with the BCP suspended in the aqueous phase, an additional step was carried out. An appropriate quantity (50 mg) of the hydrophobic particles with an average particle diameter of 5 µm was added to the pure BCP (500 mg) and dispersed using a mechanical stirring device. Since the particles were highly hydrophobic and the BCP was an amphiphilic molecule, the particles were readily dispersed. For preparation of the aqueous dispersion of the combined antifoam, a procedure similar to that mentioned for the pure BCP was followed. A 200 µL portion of the aqueous dispersions of the antifoams was finally added to 1 L of the foaming solutions. Since the aqueous dispersions of the BCP were not extremely stable, the dispersions were produced fresh and kept in an ultrasound bath just before their (32) Stang, M. Zerkleinern und stabilisieren von tropfen beim mechanishen emulgieren. Ph.D. Thesis, Universita¨t Karlsruhe, Karlsruhe, Germany, 1998.
use to ensure reproducibility. All the dispersions had similar size distributions (shown in a later section) to avoid any influence of size on efficiency as an antifoam. Identical size distributions are hard to obtain because of aggregation of the hydrophobic particles in a mixed antifoam. Two-Bubble Technique. The apparatus consisted of two L-shaped stainless steel tubes of 1 mm internal diameter, with their open ends facing each other concentrically and immersed in an aqueous foaming solution filled in a cuvette as shown in Figure 2. The top ends of the tubes were connected to two microsyringes, which dispensed air. The movement of each of the syringe plungers was controlled precisely with a threaded screw. Bubbles formed at the ends of the tubes immersed in water contained in a Pyrex glass cell with dimensions of 50 × 50 × 10 mm were monitored with a camera (Sony DFW V500) coupled with a telecentric and a Mitutoyo 20× Apo microscope lens. The region of interest was where the two bubbles just touch each other, as highlighted in Figure 2. The used lens combination provided an area of focus with a width of 700 µm, enabling the tracking of the movement of drops as small as 5 µm. A single bubble from one of the tubes was formed until the bubble apex reached the center (d/2) between the two open ends of the tubes, as indicated in Figure 2. The bubble shape and size was then kept constant by preventing any air to leak or escape back into the tubes, as no backward piston movement was allowed in the syringe. The antifoam dispersion was then introduced using a syringe close to the bubble surface. Since the BCP drops had an affinity to the air/water interface, it was relatively easy to obtain a high surface concentration of the drops on the bubble. The second bubble, which was then grown slowly in a quasi-static way by pushing air through the second L-shaped capillary, was made to approach the first bubble carrying a large number of drops. The motion of the drops on the surface of the first bubble was then observed as the two bubbles almost touch and form a film. As the bubbles were approached slowly in a quasi-static condition, it was safe to assume that the inertial force acting in the direction of motion of the bubbles was negligible and did not affect bubble coalescence. Macroscopic Antifoaming Efficiency Tests. Sparged Air Column. A double-walled 100 mm internal diameter sparged air glass column was used for comparing antifoam efficiencies. The total air content for antifoam suspension added to the foaming solutions as a function of time was compared with that obtained without antifoam. The foam column and method of measurement were described in detail elsewhere.19 Circulation Flow Loop. Commercial Contifoam equipment manufactured by Ing. Franz Raab Datentechnik (Rohrbach, Austria) was used to carry out foam tests optoelectronically. The measuring principle was quite simple: a fixed volume of foaming solution was
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Figure 2. Schematic representation of the two-bubble technique. circulated by a gear pump into a 2 L jacketed glass cylinder creating foam. The height of the foam was recorded by the optical sensors as a function of time at a constant desired temperature. The initial level of the foaming liquid was used as the zero-point for measuring foam heights. The apparatus was cleaned after each measurement by circulating clean water and rinsing with deionized water. The circulation method provided a different approach in measuring antifoam efficiency where the foam was dynamically produced and would confirm the efficiency of an antifoam in a highly turbulent environment, as opposed to the sparged air test where it was relatively quiescent conditions. In contrast, this method was not able to quantify the reduction in total air content but measured the destabilization of the surface foam. In addition, there was poor control of bubble sizes and the extent of air input in circulation tests. These disadvantages did not exist in the sparged air foam column method. Rheological Measurements. A rheometer (Physica MCR300) with a Couette geometry (CC27) and a gap of 1.33 mm was used. The dynamic viscoelastic functions such as the storage (G′) and loss (G′′) moduli were measured as a function of temperature at a heating rate of 1 °C/min, an angular frequency of 1 rad/s, and a strain of 1%, the latter corresponding to the linear viscoelastic regime. Dynamic strain tests were carried out at an angular frequency of 1 rad/s. These helped in determining the linear viscoelastic regime where G′ and G′′ remain constant and independent of strain. The point of break-up of the gelled network is indicated by a systematic decrease in G′ upon increasing the strain. Small-Angle X-ray Scattering (SAXS). SAXS measurements were performed using an Anton Paar SAXS equipped with a slit camera. A 1.54 Å wavelength X-ray beam was generated by a sealed copper tube. Colloidal dispersions of BCP in water were injected in a 1 mm thick capillary, which was sealed and exposed to X-rays for 20 min at three temperatures: 1,6, and 13 °C. q-Dependent intensity spectra were recorded on Kratky-configuration plates and corrected for the empty capillary background. Nuclear Magnetic Resonance (NMR) Measurements. NMR SelfDiffusion Measurements. Self-diffusion coefficients were determined using the pulsed-field gradient spin-echo (PGSE) NMR technique. Water and surfactant self-diffusion coefficients were performed on a 21 wt % BCP in water mixture using Fourier transform PGSE experiments on a Bruker NMR spectrometer (AVANCE 300 Wide
Bore) working at 300 MHz on 1H. The self-diffusion coefficients were obtained from 1H NMR spectra following the intensity of methylene peaks belonging to PO blocks and H2O. For all temperatures (except those higher than 20 °C), a single-exponential decay was observed, as deduced from intensity versus g2 plots. No dependence on the diffusion time was observed. Consequently, values of self-diffusion coefficients (D) were obtained by a nonlinear fitting of the experimental data to the equation of Stejskal-Tanner:33 I(δ) ) I0e-KD
(1)
where, I0 is the peak intensity in the absence of gradient pulses (I0 is a function of the transversal relaxation time T2). The time function, K ) (γδg)2(∆ - δ/3) depends on the experimental variables: γ is the proton gyromagnetic ratio; g and δ are the magnitude and the width of the magnetic gradients, respectively, ∆ is the delay between the first and the second gradient pulses and represents the “diffusion time”. In the case of polymer diffusion, δ was kept constant at 2 ms, ∆ was set to 30 ms, while g was varied between 0 and 180 G/cm. The accuracy of measurements was estimated to be 3%. An airflow regulator, yielding a temperature stability of 0.3 °C, controlled the temperature in the measuring chamber. NMR Relaxation Time Measurements. NMR relaxation experiments were performed using the same instrument. The experimental conditions were 9 µs for the 90° radio-frequency pulses, a 1.5 kHz spectral width, a 1.9 kHz filter bandwidth, an acquisition of 1024 complex data points per transient (in 8 s), and an accumulation of 128 free induction decays. 1H NMR relaxation times (T2) were obtained from methylene protons by means of signal decay on Hahn spin-echo sequences.34 A nonlinear least-squares fitting procedure was used to get T2 values. The temperature was controlled with an accuracy of 0.3 °C and calibrated with a copper-constant thermocouple. The sample rotation was set at 20 Hz. Differential Scanning Calorimetry (DSC) Measurements. For the DSC measurements, a micro-DSC from Setaram (France) was employed. A 400 mg portion of the sample was weighed in crucibles (33) Stejskal, E. O.; Tanner J. E. J. Chem. Phys. 1965, 42, 288. (34) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy; Oxford University Press: Oxford, 1987; pp 61-67.
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Figure 3. Temperature-concentration phase diagram of the BCPwater mixture. Points connected by solid lines are experimental measurements based on temperature sweeps carried out for each BCP-water mixture in oscillatory rheometry. Dashed lines are drawn to distinguish the various phase regions. of 1 cm3 capacity. The samples were sealed in aluminum pans. As a reference, a sealed pan with the corresponding amount of water was used. To check water evaporation, the pans were weighed before and after the DSC measurements. The DSC thermograms were recorded in the temperature range from 5 to 30 °C. The heating rate was 0.5 °C min-1.
Results and Discussion The phase behavior of the BCP in water obtained using oscillatory thermo-rheometry is first discussed in terms of the temperature-concentration phase diagram. The temperaturedependent characteristics of BCP in water above the critical concentration for gel formation obtained from the experimental results of SAXS, micro-DSC, and 1H NMR are then presented. Macroscopic antifoaming efficiency analysis conducted in the circulation flow loop are subsequently discussed. Characteristics of the BCP drops are then related to the observed behavior at various temperatures in the macroscopic tests. The visual observations made using the two-bubble technique are then presented. Finally, the results of the effect of the presence of hydrophobic particles in the BCP on the antifoaming efficiency in the sparged air foam column are explained. Phase Behavior of the BCP in Water. A temperatureconcentration phase diagram based on extensive oscillatory thermo-rheometric and visual experiments involving a wide range of temperatures and concentrations of BCP in water is shown in Figure 3. Points along the lines indicate an onset of the respective phase transitions. Typical results for two different concentrations of BCP shown in Figure 4 indicate the various phase transitions analyzed using temperature sweeps in oscillatory rheometry used to build the phase diagram. The dotted lines in Figure 3 are used as borders to improve visual clarity of the various zones investigated experimentally. A phase diagram with details on the structure of the micellar aggregates forming the gel as normally designed with light or neutron scattering for every concentration in addition to rheological characterization30,35-39 has not been investigated here because there is no direct relevance of each of the phases to the antifoaming (35) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (36) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (37) Prud’homme, R. K.; Wu, G.; Schneider, G. K. Langmuir 1996, 12, 4651. (38) Li, H.; Yu, G.-E.; Price, C.; Booth, C.; Hecht, E.; Hoffmann, H. Macromolecules 1997, 30, 1347. (39) Kelarakis, A.; Mingvanish, W.; Daniel, C.; Li, H.; Havredaki, V.; Booth, C.; Hamley, I. W.; Ryan, R. F. Phys. Chem. Chem. Phys. 2000, 2, 2755.
Figure 4. Storage (G′) and loss (G′′) moduli as a function of temperature for (a) 20% BCP in water and (b) 30% BCP in water.
mechanisms. SAXS and 1H NMR measurements for a 21 wt % BCP in water representing the interfacial gel are elaborated on in a later section. The important aspects of the phase diagram are the lines along which the BCP-water mixture forms the gel. This gelled state represents the interfacial gelled network observed in Figure 1. An aqueous solution of BCP can exist mainly in three different phases above a critical concentration (∼20 wt %), as shown in Figure 3. This phase diagram indicates that, between 20 and 30 wt % BCP in water, the solution-to-gel transition occurs only once (L1 f G2). Beyond 85% BCP in water, no phase separation can be observed. For 30-60% BCP in water, the solution (L1) was seen to have an intermediate gel phase (G1). Upon increasing the temperature, dissolution of the gel was observed to result in a liquid-like phase or sol (L2). Upon traversing along the constant concentration line, at higher temperatures, a second transition to the gelled phase (G2) was observed. Beyond a critical temperature, the gel transformed to a sol with a liquid-like behavior (L3). L3 can be interpreted as the onset of the region where the storage (G′) and loss (G′′) moduli curves start approaching each other to become almost equal in magnitude. It was observed that the BCP solution exists in a gelled phase in subzero temperatures for a concentration range of 60-70 wt %. For the purpose of further discussions, the L1/L2 f G2 f L3 transitions are of greatest relevance since the temperatures above 25 °C are of interest in this work. When the dispersion of BCP in water is prepared at 25 °C as explained in the previous section, each individual drop develops
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Figure 5. SAXS results for 21% BCP in water at different temperatures.
the interfacial gel as indicated in Figure 1. Once the interfacial gel is formed, the drops remain stable, even if the dispersion is further diluted at 25 °C. This indicates that the composition of the BCP-water mixture in the interfacial gel remains unchanged at that temperature. On the other hand, if the temperature of the dispersion is lowered to 8 °C, the drops dissolve (a G1/G2 f L1 transition occurs), and the BCP exists in an unimer or micellar form in the solution. From the phase diagram, it can be now inferred that the interfacial gel would have a concentration between 20 and 30 wt % BCP in water. This is because, if the concentration was higher (between 30 and 60 wt % BCP), the G2 f L2 transition would have taken place at a much higher temperature, and the drops would cease to exist already at a higher temperature. For concentrations between 60 and 80% BCP in water, the drops would never exist at 25 °C because the mixture does not gel and exist in a liquid-like phase (L2). Characterization of the BCP-Water Mixture. A 21 wt % BCP in water mixture, which is just above the critical gelling concentration, is used to characterize the interfacial gel. The rheological curves are not shown again because they appear to be similar to those shown in Figure 4a, with a shift in transitions indicated by the lines in the phase diagram and having lower magnitudes of storage (G′) and loss (G′′) moduli at a given temperature, indicating a less elastic (weaker) gel. SAXS data for the 21% BCP solution shown in Figure 5 indicate the primary steps in gel build up while the BCP exists in the micellar state. At 1 °C, the 21% BCP in water exists as a transparent solution. It is observed in the SAXS measurements that, between 1 and 6 °C, the intensity peak corresponds to a size of about 25 nm. It is evident that this size is too big for it to be unimers because the BCP molecules are reasonably short chained, and thus micelles should exist at this temperature. As the temperature is further increased, an increase in the size of the micellar structures can be observed. At 13 °C, where the gel formation is initiated (L1 f G2), the size of the micelles is measured to be 36 nm. The size increase can be associated with an increase in the aggregation number for the micelles or an increase in the intermicellar aggregate distance. On the basis of the knowledge existing in the literature3,36 for such amphiphilic BCP molecules, the poly(oxyethylene) chains would associate with water molecules at lower temperatures and the poly(oxypropylene) and alkyl chain would form the core of the micelles. As the temperature is increased, the poly(oxyethylene) groups break the associative bonds with the water molecules, resulting in a phase-separated gelled state, which is a very probable situation in this case as well.
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Figure 6. Micro-DSC scans for 21% BCP in water at different temperatures.
The micro-DSC analysis presented in Figure 6 for a 21 wt % BCP solution identifies the first peak at around 7.5 °C, where micellar aggregation takes place for this system. The experiment is carried out at a constant rate of 0.5 °C/min. As is generally true, the micellization or micellar aggregation process indicated here is also an endothermic process.40 The last point in this zone is higher up, as seen in the graph, indicating the completion of micelle formation. The third zone corresponds to a local phase separation41 where the heat flow is monotonically increasing. The two phases can be interpreted as the hydrated BCP molecules in equilibrium with the excess free water that would form the continuous phase. Visual observations agree quite well with this finding, as the solution starts getting turbid at about 7.5 °C (cloud point), and, at about 13 °C, the gel formation is initiated. 1H NMR spectroscopy is then used for confirming the systematic structure or gel formation of the BCP solution. The results shown in Figure 7 provide a more detailed explanation on the phase behavior and the properties of the aqueous BCP solution. The structural analysis is studied using the water selfdiffusivity (Figure 7a) as a function of temperature for the 21% aqueous BCP solution. The signal for the water self-diffusion is highly pronounced as these measurements are carried out with high-resolution NMR. Three zones are observed to exist here. In the simple micellar state, the diffusivity of the water molecules is high. Upon the formation of structured micelles in the solution, large aggregates start obstructing the path of water molecules, and the water near a micelle has to diffuse a longer path to reach the other side of the micelle structure. This phenomenon, called the obstruction effect, was introduced by Jo¨nsson et al.42 Hydration of the micelles as well as increased size are concomitant to a decrease in the water self-diffusion coefficient. As the complete phase change happens, the diffusivity of the water molecules drops sharply (onset of the third region), indicating the presence of ordered structures, which are solid-like. In the micro-phaseseparated zone (third zone), the diffusion trend is not monotonic, and hence two different values are obtained for the fitting of the biexponential function to the data of the echo attenuation. This corresponds to two phases in equilibrium at slow exchange. Only one set of equilibrium data is plotted to show this phenomenon, except for the last set of points labeled free water and hydrated (40) Wenk, M. R.; Seelig, J. J. Phys. Chem. B 1997, 101, 5224. (41) Kositza, M. J.; Bohne, C.; Alexandridis, P.; Hatton, T. A.; Holzwarth, J. F. Macromolecules 1999, 32, 5539. (42) Jo¨nsson, B.; Winnerstro¨m, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77.
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Figure 7. 1H NMR results for 21% BCP in water at different temperatures: (a) diffusion coefficient of water; (b) diffusion coefficient of BCP and; (c) relaxation times of molecules or molecular structures.
water in Figure 7a, which show the two signals separately. The increase in the self-diffusion coefficient observed at higher temperatures corresponds well with the exothermic process occurring at about 20 °C in the micro-DSC curve. This could possibly correspond to the breaking of the hydration bonds formed with the ethylene oxide groups of the BCP, resulting in a release of energy. The self-diffusion coefficient for the BCP shown in Figure 7b corroborates with the diffusion results for water molecules. In this case, the resolution is low because of a weaker proton signal from the BCP, and hence all three zones are not clearly visible. The sharp drop in the diffusion coefficient appears at slightly higher than 13 °C. The diffusion coefficients for the BCP decrease with temperature, once again providing the evidence for micelle growth and the formation of ordered gel-like or solid-like structure. This phenomenon can be explained in a manner similar to that done for the diffusion of water molecules. The spin-spin relaxation times (T2) provide information on the micellar transition (e.g., growth, elongation, and entanglements). A steep linear decrease in T2 initiated at a certain temperature can be observed in Figure 7c. The T2 values are close to 130 ms at temperatures lower than 15 °C. Above this temperature, the relaxation time decreases with increase in temperature, attaining a constant value of 30 ms at higher temperatures for this 21 wt % BCP solution. This change is associated with a decrease in the correlation times for aggregate tumbling and BCP diffusion caused by the selfassembly of the molecules to form rigid well-ordered structures. Antifoaming Efficiency in a Flow Loop. The time-dependent relative height of foam generated using the surface-active cellulose (recycled paper) foaming system at four different temperatures (4, 8, 25, and 45 °C) in a Contifoam circulation flow loop is shown in Figure 8. This paper fiber suspension forms one of the
Figure 8. Variations with time in foam height in a Contifoam recirculation flow loop with an industrial foaming system in the presence of BCP at 4, 8, 25, and 45 °C.
industrial systems in which the antifoam has a potential application. A steady-state foam height is attained after circulating the foaming system for about 300 s at each temperature. An aqueous dispersion of the BCP drops (200 µL/L) is then introduced in the continuously circulated foaming solution. It can be observed from the Contifoam test results in Figure 8 that, for temperatures up to 8 °C, the addition of the BCP dispersion leads to foam stability, as indicated by the increase in foam height. This is due to the fact that BCP drops “dissolve” in the turbulent foaming solution and exist in a micellar state below a critical concentration of 20 wt % of BCP in water, as indicated in Figure 3. The size of the micellar aggregates up to 13 °C was determined to be less than 40 nm, as indicated previously in Figure 5. On the other hand, tests conducted at 25
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Figure 9. Size distributions of the drops of BCP (denoted by C1) and drops or particles of BCP-HP mixtures when dispersed in aqueous phase at 25 °C.
°C show a significant ability of the BCP to destabilize foam. The drop size distribution of the dispersed BCP at 25 °C is shown in Figure 9, with an average drop diameter of 2.5 µm. Thus, it is clearly observed that, for the use of BCPs as antifoams, the operating system temperature has to be above the point where the interfacial gel has completely been formed. As discussed in the previous section, the properties of the interfacial gel can be related to the BCP-water mixture with 20-30 wt % BCP in the solution. The value of G′ for the 20-30% BCP-water mixtures shown in Figure 4 passes through a maximum between 25 and 35 °C and drops to lower values at 45 °C. At maximum G′, the interfacial network has the highest elastic properties and, hence, can withstand higher deformations. This can be seen from the fact that, for a 21 wt % BCP solution at 25 °C, the G′ value is greater in magnitude than it is at 45 °C. The deviation from the constant value of G′ occurs at much larger strains at 25 °C than at 45 °C, marked by the arrows in Figure 10. The crossover point of the G′ and G′′ drops to half its value at 45 °C as compared to that for 25 °C, indicating a weaker network.43 This indicates that the interfacial network on the dispersed drops of BCP would be weaker at temperatures beyond the maximum in G′. This explains the observed maximum efficiency of the antifoam at around 25 °C in the Contifoam experiments, which decreases at 45 °C. A complete loss of efficiency was observed at 60 °C, which is not shown in Figure 8 because the steady-state foam height remains unaffected by the addition of the BCP dispersion. This can be related to the significant drop in the G′ values, which can even lead to a transition to a sol at this temperature. In addition to the gelling properties, the bulk viscosity of a pure BCP also decreases significantly with an increase in temperature, as indicated in Table 1. Thus, if the core of the drops is formed by a pure BCP, then the loss in rigidity can be a combined effect of a weaker interfacial gel and a less viscous core of the drop. Observations on Bubble Coalescence. The two-bubble apparatus is extensively used to visually analyze the interaction of droplets with approaching bubbles. Figure 11 is an example showing the sequence of events that takes place when a single BCP drop, D1, is trapped between two air bubbles, B1 and B2. In this experiment, bubble B2 was formed first, and then its surface was covered by the aqueous dispersion of the BCP. The resulting concentration of the BCP drops is 0.5 wt %. The width of each picture is 700 µm in Figure 11, and the diameter of entrapped droplet D1 is about 50 µm. In this case, the image of the drop appears to be distorted due to focusing difficulties associated with the rapidly moving drop. However, for a drop (43) Jørgensen, E. B.; Hvidt, S.; Brown, W.; Schille´n, K. Macromolecules 1997, 30, 2355.
Figure 10. Strain sweep experimental results for 21% BCP in water at (a) 15 °C, (b) 25 °C, and (c) 45 °C. Table 1. Properties of the BCP η σaw σow σoa E B1/2 S T F °C [kg/m3] [mPa s] [mN/m] [mN/m] [mN/m] [mN/m] [mN/m] [mN/m] 25 45 60
997.5 980.2 971.1
200 80 50
50.1 48 45.9
0.1 0.37 0.41
35 29.9 28.8
15 19 18
36 38 36
15 18 17
of another BCP with similar bulk viscosity, the image was much sharper, further confirming the sequence of events involved in interbubble coalescence. As can be seen in Figure 9, the size distribution of the drops used in the real application has a mean diameter of about 2.5 µm. Although the drop diameter is more than an order of magnitude larger than the average value, it provides an ideal case for visualizing the drop interaction. It is observed that, as the two bubbles approach, the liquid between the bubbles is pushed away from the region of contact. Pugh11 discussed the various situations of drainage as a function of surfactant concentration in liquid films leading to either rupture or stability. There are schematic representations of the liquid
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Figure 11. Bridging of two air bubbles with a BCP antifoam drop in water.
layer below the surface, dragged along with the surface motion of the surfactants. Most discussions and schematics on bubble coalescence in the literature involve thin films with submicrometer thickness. This work deals with relatively thick films on the micrometer level because this situation is more relevant to bubbles colliding in an aqueous phase. Schulmann and Teorell44 showed that a monolayer of oleic acid spreading on a liquid surface at a rate of 50 mm/s can influence the liquid velocity 30 µm from the surface. Thus, a fresh surface that appears as a result of the growing bubble would disturb the local system equilibrium close to the already existing bubble carrying the excess BCP on its surface. The pictures in Figure 11 show the sequence of events that took place in the region of contact between the two bubbles as B1 approaches B2 in the presence of a drop, D1, located at the periphery initially. It can be seen that drop D1 gradually moves from the periphery (Figure 11a) to the center (Figure 11b) of the region of contact between two bubbles where the interbubble distance is minimum. As soon as drop D1 bridges the bubbles, the two air interfaces close to the drop move rapidly toward each other resulting in coalescence (Figure 11c). The entire process shown in Figure 11 takes place in about 200 ms. This sequence is a good example of bridging leading to coalescence. It is observed in Figure 11b that liquid drop D1 is compressed and deformed by the two bubbles, the deformation being small because the liquid drop is relatively viscous (200 mPa‚s). Subsequent film drainage is too rapid to capture with a camera. It is important to note that the nonequilibrium condition existing when the fresh bubble approaches the existing bubble carrying the excess BCP is a necessary condition for coalescence to occur. If the fresh surface created is in constant equilibrium (rate of fresh surface creation slower than the diffusion rate of the dissolved BCP) with its local surroundings and the existing bubble, then no coalescence occurs. This is because both the surfaces are covered equally with the dissolved part of the BCP, increasing the surface (44) Schulmann, J. H.; Teorell, T. Trans. Faraday Soc. 1938, 34, 1337.
elasticity and introducing stearic hindrance forces in the film that would eventually be formed in the region of contact between the two bubbles, also preventing the BCP drop entry. Figure 12 shows an example of a dispersion of multiple droplets (higher concentration) of BCP introduced at the surface of the right bubble B4 before the left bubble B3 is produced. The BCP is only partially soluble in water and hydrophobic, so that it has a tendency to sit at the interface of the bubble. When bubble B3 starts approaching bubble B4 as shown in Figure 12a, the droplets are first dragged away from the region of contact as a result of drag forces created by liquid convection. When the bubbles approach each other further, the motion of the drops reverses, which is driven by the dominating interfacial forces. The phenomenon is clearly illustrated by Figure 12b-d, in which the drops with diameters of about 10 µm are highlighted in black for the sake of clarity. Since the concentration of BCP is higher in this case compared to that for Figure 11, the rate of coalescence of bubbles after the drops are pulled toward B3 is also higher. In this case, the coalescence process from the point where the drops are being pushed away from the region of contact because of the liquid drag is taking place in less than 200 ms. After the drops form a continuous swarm between B3 and B4 as in Figure 12d, the coalescence occurs almost instantaneously. Unfortunately, it is not clear from the images how the two bubbles are bridged and whether multiple drops are involved in the coalescence. This is because the process takes place at a speed higher than that which the camera can capture. But once the drops are pulled toward bubble B3, the two bubbles eventually touch each other and the film formed is bridged by single or multiple drops, as in the case shown in Figure 11 leading to coalescence. Again, in this case, if both bubbles are equally covered with an equal amount of BCP dispersion, then no coalescence occurs. The process of bridging and drainage occurs extremely fast. The classical entry (E), bridging (B), and spreading (S) coefficients
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Figure 12. BCP drops located on the right air bubble interact with the fresh air bubble interface approaching from the left side. The lines are drawn to improve visual clarity.
used by many authors7,25,26,45,46 to justify antifoaming efficiency are defined below:
E ) σaw + σow - σoa
(2)
B ) σaw2 + σow2 - σoa2
(3)
S ) σaw - σow - σoa
(4)
in which σaw and σoa are the surface tensions of water and the BCP, respectively, with respect to air, and σow is the interfacial tension of the BCP drop in water. The subscript “o”, normally used for the oil phase, is loosely used for the BCP as it forms the insoluble part in water. The interfacial tension of BCP with water is measured by fitting the drop shape coordinates to the Gauss-Laplace equation.47,48 It is difficult to fit the Laplace equation for the BCP drops that undergo the interfacial phase change, and hence the initial interfacial tension values where the gelation process has not been completed are considered for calculating the thermodynamic coefficients. Table 1 lists the experimental values of σaw, σoa, and σow at three different temperatures together with the calculated values of E, B, and S. The latter values provide an idea of the forces acting at the drop interface since they are not exactly at equilibrium. Table 1 shows that the entry coefficient (E) and bridging coefficient (B) are positive. A positive entry coefficient indicates that a drop entry into the bubble is thermodynamically favorable, and the positive bridging coefficient provides the criterion for unstable bridges, leading to coalescence of the bubbles bridged by a drop. The values of E and B will still be positive, even if the values of the interfacial tension σow are an order of magnitude larger than the initial values listed in Table 1. Positive values of E and B cannot further quantify how fast the film can drain leading to coalescence of bubbles after entry and bridging. This is due to the nonequilibrium condition existing during the approach of the bubbles, which depends on the kinetics of transfer of the dissolved and undissolved BCP to the fresh surface of the bubbles after their introduction in the aqueous phase film close to the surface of the already existing bubble. For instance, although (45) Garrett, P. R. J. Colloid Interface Sci. 1980, 76, 587. (46) Bonfillon-Colin, A.; Langevin, D. Langmuir 1997, 13, 599. (47) Stauffer, C. E. J. Phys. Chem. 1965, 69, 1933. (48) Miller, R.; Joos, P.; Fainerman, V. B. AdV. Colloid Interface Sci. 1994, 49, 249.
the values of E and B are positive at 60 °C in Table 1, the BCP dispersion completely loses its ability to enhance interbubble coalescence at this temperature. However, at lower temperatures (25 and 45 °C), the thermodynamic conditions of positive E and B values are consistent with the observed kinetic experiments in Figure 8. Although a maximum in efficiency is observed at 25 °C, it cannot be explained by the thermodynamic criteria. Although the spreading coefficient S is positive at all temperatures, it is observed that a drop of BCP placed at the air/liquid interface does not spread instantaneously as expected. This implies that interfacial tension forces are not sufficient to describe the spreading phenomenon in this case, and additional cohesive forces such as the ones arising because of interfacial gelation could be significant. Although positive entry and bridging coefficients imply thermodynamic necessity for the drop to enter and form an unstable bridge, they are not sufficient conditions. To establish a contact between the drop and air bubble, the drop has to break the thin aqueous film separating it from the air phase. In addition, rupture of the thin aqueous film can be resisted by short-range forces other than the interfacial forces used in the above equations, such as hydration forces or structural forces due to dissolved surfactants present at the air/aqueous interface. Close observation of the video pictures for Figure 11 showed that the approaching bubbles compress the entrapped BCP drop, deforming it partially. If the drop is not rigid enough, it could either break or get squeezed out of the region of contact between the bubbles. This was, in fact, observed at higher temperatures, such as 60 °C, in another case dealing with a similar type of BCP drop. It was hard to conduct the two-bubble experiment for the BCP considered in the present case at 60 °C in quiescent conditions because the BCP drops creamed out rapidly as a result of the increased density difference. The drop moving toward the region of contact between the two bubbles was observed to be deformed and accelerated out of the region of contact between the two bubbles as it passed between the bubbles. Unlike the case illustrated in Figure 11 corresponding to a lower temperature of 25 °C, the drops are unable to retain their shape and thus are squeezed out of the region of contact. This implies that when the drops lack a minimum rigidity, they cannot cause coalescence. Thus, the rigidity of the drop would be an important parameter affecting the ability of the antifoam to induce interbubble coalescence in addition to the interfacial forces that determine the entry and bridging coefficients of the antifoam drops. The
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Figure 14. Macroscopic tests using a sparged air foam column showing an increase in efficiency upon the addition of hydrophobic particles at 25 °C.
Figure 13. Schematic of an antifoam drop bridging two bubbles.
bulk viscosity of BCP decreases from 200 to 50 mPa‚s as the temperature increases from 25 to 60 °C, as listed in Table 1. This indicates that the drop becomes less rigid as the temperature increases. In addition, the interfacial gel weakens at higher temperatures, as explained in the previous section, reducing the rigidity of the drop further. The rigid drop observed in Figure 11 would resist deformation and penetrate air bubbles, as shown schematically in Figure 13. If the viscosity of the drop is considerably lowered and the interfacial gel is weakened, then the drop would deform, as shown by the dotted ellipse, and get squeezed out from the region of contact between the two bubbles. These conclusions derived from the two-bubble technique corroborate with the macroscopic experimental results, which indicated that the antifoam is incapable of enhancing interbubble coalescence in a foaming surfactant solution at 60 °C as opposed to that observed at 25 °C. Mixture of Hydrophobic Particles and BCP as an Antifoam. As discussed above, macroscopic foam column tests for analyzing the acceleration of interbubble coalescence induced by the antifoam corroborate well the facts observed in the two-bubble experiments. The antifoam dispersion is able to inhibit foam formation at 25 °C but is completely ineffective at increased temperatures (60 °C). To further establish the fact that increased rigidity and pinching efficiency would increase the ability of antifoam to induce interbubble coalescence, a mixture of hydrophobic particles and BCP is tested in the sparged air foam column with a PS20 foaming solution. The preparation of the antifoam dispersion is explained in the materials section, and the size distributions are shown in Figure 9. Three dispersions of BCP in water and BCP in combination with 1 and 10 wt % HP in water are prepared to obtain a size distribution that is as close as possible. As seen in Figure 9, the primary peaks of all distributions are around 2.5 µm, and the 10% HP has a secondary peek at 10 µm. This peak signifies the presence of more particle aggregates. The dispersion of the mixtures of BCP and HP in water would contain single HP particles covered with BCP, individual drops of BCP, and drops of BCP embedding multiple HP particles. It is evident from Figure 14 that interbubble coalescence is enhanced upon increasing the concentration of HP particles in combination with BCP. Details on the method of evaluation of the antifoaming efficiency in terms of the reduction in air content in the foam column are explained in detail elsewhere.19 Pure drops of BCP reduced the total air content by 15%. Upon
introducing 1% HP by weight in combination with BCP, the reduction in air content for experiments carried out at 25 °C increases to around 18% and further increases to around 35% upon addition of 10 wt % of particles. This clearly shows that particles in combination with BCP have a pronounced effect on increasing the rate of interbubble coalescence. Similar behavior of the antifoam performance is also observed in circulation tests with industrial foaming systems. This follows a trend similar to that published for the case of PDMS oil in combination with hydrophobic silica.24,26,27,49-51 These authors showed that the hydrophobic particles sitting at the oil drop interface help in pinching the thin aqueous film between oil and air. It is not expected that the particles would stay at the water/BCP interface, since they are extremely hydrophobic and have no tendency to approach an aqueous phase. It was even observed that, if the wax particle powder is forced to mix in water, the particles instantaneously cream out and can be recovered as dry powder at the air/water interface. Hence, the particles would be expected to form the core of the BCP drops in the current situation, although this point remains speculative at this stage.
Conclusions Dispersions of BCP droplets in an aqueous phase are investigated for their antifoaming effect. The two-bubble technique designed in this work helps to explore the role of drops or particles in the process of bubble coalescence. The spreading, entry and bridging coefficients of BCP antifoam drops determined using their surface and interfacial tensions measured by tensiometry are found to be positive, satisfying the necessary thermodynamic condition but insufficient in explaining the loss of antifoaming efficiency at higher temperatures. The interfacial gel formed on the drops imparts maximum rigidity to the drop at 25 °C, as the gel strength is maximum. The drops could not induce efficient bubble coalescence at higher temperatures (45 °C) because of their deformability facilitated by the reduction in bulk viscosity to less than half that at 25 °C and the weakening of the interfacial gel, as indicated by the strain sweeps. The existence of ordered gel-like structures in a BCP-water system indicated by the temperature-dependent values of the storage modulus measured by oscillatory thermo-rheometry is further corroborated by the results obtained using SAXS, micro-DSC, and 1H NMR techniques. Drop rigidity characterized by the bulk viscosity and the interfacial structure is an important parameter that influences the antifoaming efficiency. Experimental inves(49) Dippenaar, A. Int. J. Miner. Process. 1982, 9, 1. (50) Garrett, P. R.; Davis, J.; Rendall, H. Colloids Surf., A 1994, 85, 159. (51) Denkov, N. D. Langmuir 1999, 15, 8530.
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tigations using the two-bubble technique show that the deformability of the drop reduces its ability to bridge two bubbles and break the thin aqueous film to cause coalescence. The drops are squeezed by the bubbles and dragged out along with the aqueous phase, which moves away from the region of contact due to convection. The combination of hydrophobic particles with the BCP enhances the efficiency of the antifoam. Single particles covered with the BCP or drops embedding multiple hydrophobic particles are more efficient in pinching the thin aqueous films.
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Increased rigidity and bulk viscosity of the drops would further help bridging and entry. Acknowledgment. The authors thank KTI Switzerland for financial support; R. Gunde and A. Baumann from ETH Zu¨rich for sharing their expertise in interfacial tension and rheology measurements; R. Mezzenga from Nestle´ Research Center, Lausanne, for the SAXS measurements; and D. Kellenberger from Kolb AG for the Contifoam experiments. LA0600797
