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Growing a particle-stabilized aqueous foam
Journal Pre-proofs Growing a particle-stabilized aqueous foam Andrew T. Tyowua, Bernard P. Binks PII: DOI: Reference:
S0021-9797(19)31431-6 https://doi.org/10.1016/j.jcis.2019.11.103 YJCIS 25720
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
2 October 2019 25 November 2019 25 November 2019
Please cite this article as: A.T. Tyowua, B.P. Binks, Growing a particle-stabilized aqueous foam, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.11.103
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© 2019 Published by Elsevier Inc.
GROWING A PARTICLE-STABILIZED AQUEOUS FOAM
Andrew T. Tyowua1,2* and Bernard P. Binks1*
1Department
of Chemistry and Biochemistry, University of Hull, Hull. HU6 7RX. UK 2Applied
Colloid Science and Cosmeceutical Group,
Department of Chemistry, Benue State University, PMB 102119, Makurdi, Nigeria
Submitted to: J. Colloid Interface Sci. on 15.10.19; revised on 25.11.19
Contains ESI
*Corresponding authors: [email protected] (Tel. +2349091818649) [email protected] (Tel. +44-1482-465450)
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ABSTRACT Hypothesis Certain gas-filled colloidal particles expand upon heating. If such particles are surface-active and stabilize aqueous foams, do the foams grow with temperature as particles expand? Experiments Aqueous foams were stabilized with hollow micro-spherical particles that are partially wetted by water and grow upon heating. Foams were prepared using two different approaches, both of which led to their growth. In the first, water was heated to various temperatures (40–80 °C) and aerated in the presence of the particles. In the second, water at room temperature was aerated in the presence of the particles and then heated to various temperatures (40–85 °C). Findings Regardless of the method, foam volume began to increase on raising the temperature to the onset of particle growth (60 °C) as expected and increased with increasing temperature. However, placing the particles on hot water (80 °C) and waiting for several minutes (≤ 2.5) before aeration resulted in more growth. The volume of foam after growth remained unchanged after cooling for over six months, giving rise to ultra-stable foams.
Keywords: foam, drainage, creaming, bubble coarsening, bubble coalescence, foam growth, surface tension, adsorption, wetting, contact angle
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INTRODUCTION Inspired by the work of Ramsden [1] and Pickering [2], many particle-stabilized materials like emulsions and foams have been prepared. These materials are more stable than their surfactantstabilized counterparts. This is because particles can be irreversibly adsorbed compared with surfactant molecules which adsorb and desorb from liquid drops and air bubble surfaces. This property is directly related to the magnitude of the free energy of desorption which can be very high for particles (> 103 kBT) and relatively low for surfactants (around several kBT), where kB is Boltzmann’s constant and T is the absolute temperature. Various particles have been used in the preparation of emulsions and foams. For emulsions, some of these include fumed silica [3, 4], calcium carbonate [5], laponite clay [6], bentonite clay, polystyrene [7], whey protein microgel [8], titania [9], starch granules [10] and polytetrafluoroethylene [7]. Similarly, silica [11], calcium carbonate [12], sericite clay [13] and chitin nanocrystal [14] particles have been used in the preparation of foams. In these materials, the particles coat the liquid drops (emulsions) and air bubbles (foams) and prevent them from coalescence [15]. Whether or not this will happen depends on the wettability of the particles, quantified in terms of the threephase contact angle θ [16]. In line with the findings of Finkle and co-workers [17], the poorly wetted liquid is the dispersed phase in emulsions containing equal volumes of the component liquids. On this basis, oil-in-water (o/w) emulsions form for θ < 90° while water-in-oil (w/o) emulsions form for θ > 90° in water-oil systems. Similarly, in liquid-air systems, foams are formed for θ close to 90° while liquid marbles [18, 19] are formed for θ >> 90°. When the particles are completely wetted by either of the component phases (θ ≈ 0°), no stable materials are formed [20]. Particle-stabilized materials are ultra-stable to complete phase separation and are very important in many formulations [16]. For example, particle-stabilized emulsions are important in the cosmetic industry where they act as the basis for body creams. In the pharmaceutical industry, they are the basis for the formulation of syrups. In medicine, they are used as carriers of bioactive components. They are also the basis for many food products. Foam applications range from food (e.g. whipped cream), cosmetics, oil recovery to fire extinguishing. Foams are also precursors of many macro-porous materials widely used as thermal insulators, artificial implants and shock absorbers [21, 22]. They also act as vehicles for drug delivery and tissue engineering.
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Although bubble coarsening is absent in particle-stabilized foams, especially at high particle concentration [23], many particle-stabilized foams undergo an initial drainage and creaming, leaving a large fraction of the liquid phase beneath the foam. The stabilization of air bubbles in a system devoid of drainage and creaming will be very advantageous in foam applications especially for the creation of macro-porous materials as the foam structure will not collapse. Gonzenbach and co-workers [21] achieved this in aqueous systems using a mixture of surfactant and particles. We aim to show here that this can also be achieved through foam growth when hollow micro-spherical particles that expand with heat are used.
EXPERIMENTAL Materials Water Water was passed through an Elga Prima reverse osmosis unit and then a Milli-Q reagent water system to remove polar impurities. The treated water has pH ≈ 6 and a resistivity of 18 MΩ cm at 20 °C. Particles Expancel® micro-spherical plastic particles known as 031 WUFX 40 from Expancel, Akzo Nobel (Sweden) were used. According to the manufacturer, the particles consist of an acrylic copolymer shell (thickness ~2 μm) encapsulating a gas (isobutane) and have a true density of 1000 kg m‒3 (unexpanded) and 30 kg m‒3 (expanded). Based on the manufacturer information, the internal pressure of the core increases and the thermoplastic shell softens, leading to a dramatic increase in the volume (e.g. initial diameter 12 μm and 40 μm when expanded) when heated, but the gas remains intact. Also, particle expansion begins between 80 and 95 °C and reaches a maximum between 115 and 135 °C. Once expanded, the particles do not relax to their original volume when cooled to room temperature (23 °C). Methods Surface tension measurement A Krüss K12 digital tensiometer was used to measure the surface tension of water. Measurements were done at 20 °C using the du Noüy ring method. Three separate measurements were averaged and corrected according to Zuidema and Waters [24]. Prior to 4
each measurement, the glass vessel containing the water sample was washed with alcoholic KOH solution, rinsed with Milli-Q water and dried in an oven while the du Noüy ring was heated to glowing in a Bunsen flame. Scanning electron microscopy of particles The particles were imaged using a Ziess EVO 60 scanning electron microscope (SEM) in three situations: dry (unexpanded), dry (heated at 80 °C, expanded) and heated at 80 °C with water present (expanded). The particles were attached to the SEM stub using a carbon-impregnated self-adhesive disk. Excess particles were then removed with a jet of low-pressure compressed air. The prepared stub was then coated with ~2 nm of gold using a Polaron 7640 Sputter coater and examined at 20 kV and 100 pA under high vacuum mode. Some of the particles resemble a deflated football (Figure 1). The reason for this is not obvious, but could be due to the vacuum in the SEM chamber. This indicates that their shell is elastic otherwise they would not deform. The SEM image of the particles also shows that they are polydisperse. Measurement of particle diameter, with ImageJ software, from 51 particles on the SEM image of the dry unexpanded particles gave an average diameter of 6 µm with a standard deviation of 3 µm. The distribution of particle diameter is shown in Figure 2. Based on the bar chart, the diameter lies between 1 and 14 µm with the mode (majority) at 3 µm. However, the normal distribution curve shows that the mode is 6 µm, consistent with the average particle diameter. This can be compared with the size (D, 0.5) 10 −16 μm given by the manufacturer. Heating the particles alone to 80 °C or with water present increased their average diameter from 6 μm to 10 μm or 14 μm, respectively with a standard deviation of 3 μm. Effect of heat and the duration of heating on the particles The effect of heat on the particles was investigated in the presence and absence of water. The required mass (150 mg) of particles was closed in a screw cap glass vial (inner diameter 18 mm, height 72 mm) and heated on a water bath maintained at different temperatures (40–85 °C) for a period of 2.5 min. Similarly, the required mass (150 mg) of the particles was placed on water (3 cm3) in the glass vial and heated in a water bath maintained at the said temperatures for the same period. The height of the particle layer was measured in each case. Three separate experiments were done and an average height of the particle layer is reported along with the standard deviations. To investigate the effect of the duration of heating, the particles (150 mg) were heated in screw 5
cap glass vials in a water bath maintained at 80 °C for 30 min and the height of the particle layer was measured first at 2.5 min and then after every 5 min. During the experiment, water (as confirmed by anhydrous CuSO4) was seen to condense on the lid of the glass vials. Similarly, the particles (150 mg) were placed on water (3 cm3) at room temperature contained in the screw cap glass vials. The glass vials were tightly closed and heated in the water bath maintained at the same temperature for the same time period. The height of the particle layer was measured. Three separate samples were used and an average height of the particle layer is reported with the standard deviations. Particle immersion test with agitation The required volume of water (3 cm3) was measured into a screw cap glass vial (inner diameter 18 mm, height 72 mm) followed by placing the required mass (50 mg) of particles on its surface. In the closed glass vial, it was noted whether water wets the particles or not. Next, the water-particle mixture was hand shaken vigorously (30 s) and observed for foam formation. The experiment was further performed with particles heated alone (80 °C for 30 min) or with water so as to know whether or not foaming will occur after heating the particles. Contact angle of particles with water Firstly, the particles (110 mg) were compressed in a 13 mm diameter steel die with a hydraulic press (Research and Industrial Instrument Co., UK) under a pressure of ~1.5 × 108 N m−2 into relatively smooth disks (diameter 13 mm, thickness 0.7 mm). Secondly, a water drop (20 μL) was formed in air on the particle disk using an Eppendorf micropipette. The advancing contact angle was then measured by analyzing the profile of the water drop using a Krüss DSA Mk 10 apparatus. Liquid (10 µL) was withdrawn from the drop using the Eppendorf micropipette and the profile of the drop was analyzed similarly for the receding contact angle. The experiment was repeated with three separate disks and liquid drops and an average of the measurements is reported along with the standard deviations. Effect of particle concentration on foam stability The particle immersion test with agitation experiment revealed that the particles are able to stabilize an aqueous foam. As a result, the stability of the foam to drainage was investigated in batch foams prepared at different particle concentrations (0.5 ‒10 wt.%). The required volume (3 cm3) of water was measured into a screw cap glass vial (inner diameter 18 mm, height 72 mm) followed by addition of the required mass of the particles. Foaming was done in the closed 6
vial by hand shaking the water-particle mixture vigorously (30 s) in a vertical motion. The volume of foam obtained and the residual volume of water were estimated at each particle concentration immediately after foaming. The stability to drainage was assessed by noting the volume of water released with time. Growing foam: heating water before aeration The required volume (3 cm3) of water was heated from room temperature (23 °C) to the required temperature (40‒80 °C) in screw cap glass vials (inner diameter 18 mm and height 72 mm) using a water bath (Grant UK). This was followed by addition of the required mass of the particles (equivalent to 5 wt.%). After about 30 s, the mixture was removed briefly from the water bath and hand shaken vigorously (30 s). For water at 80 °C, the water-particle mixture was left in the water bath for 0.5‒5 min before aeration. This was to investigate the influence of particle residence time on foam formation at this temperature. After 2 h, an optical microscope (Olympus BX-51) was used to ascertain whether foams were formed at these temperatures or not. Growing foam: heating pre-prepared foam Foams stabilized by 5 wt.% particles and prepared at room temperature (23 °C) were heated in closed glass vials in a water bath (maintained at 40−85 °C) for 2.5 min and the height of the foam layer in the glass vials was noted. The foams began to “grow” at temperatures above 55 °C, leading to an increase in height of the foam layer. Three foam samples were used for the experiment and an average height of foam layer is reported along with the standard deviation. To investigate the effect of duration of heating on foam, a foam sample was heated in a water bath (maintained at 85 °C) for 2.5−40 min. During the experiment, the height of the foam layer was measured after 2.5 min and eventually after every 5 min. The experiment was done in triplicate and an average height of foam layer is reported with the corresponding standard deviations. Thereafter, a foam sample was heated in a water bath at 85 °C for 5 min, cooled and left to dry under ambient conditions and examined using SEM. Fragments of the dried foam were examined using a binocular microscope and a suitable size was selected for SEM study. The fragment was gently attached to an SEM stub using rapid-curing expoxy resin. The sample was then coated with ~5 nm of gold using a Polaron 7640 Sputter coater and examined at 15 kV and 100 pA in high vacuum mode.
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Characterization of foam Foams were characterized in terms of their microstructure using an optical microscope. A small foam sample was placed on a dimpled glass slide (Fisher Scientific) and viewed using an Olympus BX-51 optical microscope fitted with a DP50 digital camera using Image-Pro Plus 6.0 software (Media Cybernetics). Optical micrographs of the foams were used to measure the size of air bubbles by averaging the diameter of 100 bubbles using ImageJ software. Photographs of glass vials containing the foam and other photos were taken with a Canon Power Shot SX230 HS camera. RESULTS AND DISCUSSION Effect of heat on particles in the absence and presence of water According to the manufacturer, the micro-spherical particles expand when heated and so this was investigated. A photograph of particle samples heated alone and in the presence of water at various temperatures is shown in Figure S1. The height of the particle layer is plotted against temperature for the different samples in Figure S2. In the absence of water, the particles were sensitive to temperatures > 60 °C, leading to an increase in the height of the particle layer which increases with increasing temperature. With water, particle sensitivity occurred at a slightly lower temperature (> 55 °C), leading to water absorption and a five-fold increase in the height of the particle layer. The shift in temperature might be due to a specific interaction between the particles and water. By 85 °C, a 12-fold increment was obtained. Clearly, a higher particle layer is obtained when the particles are heated in the presence of water compared with when they are heated alone. These temperatures are lower than that reported by the manufacturer (80–95 °C) at which particle expansion begins. Results of a similar experiment where the particles were heated alone (80 °C) and in the presence of water for different times are shown in Figures S3 and S4, respectively. There is an initial 9 to 19-fold increment in the height of the particle layer (Figure S3) when heated alone or in the presence of water, respectively. Thereafter, the particle layer doubles in height for every 5 min and its height plateaus eventually when heated alone (Figure S4). In the presence of water, the particle layer triples for every 5 min, reaching a maximum at 20 min and decreases slightly thereafter. Heating also causes the particles to form aggregates similar to heating βlactoglobin solution at 85 °C [25]. The height of the particle layer remained unchanged even after cooling to room temperature (23 °C), indicating that particles do not relax to their original size once they grow and expand. 8
Particle immersion test with agitation The particle immersion test has been used previously [13, 26] to determine whether a liquid wets a given powdered particle sample or not. Particles that are wetted completely by a liquid do not form any foam or emulsion as reported by Binks and co-workers [13, 26, 27]. Here, water wetted the particles partially at room temperature. Some of the particles (probably the larger ones) sank to the bottom of the vial, but the majority of them remained on its surface. Foam was obtained upon agitation (Figure S5). These results are in line with those reported previously on fumed silica [26] and sericite [13] particles. By contrast, foaming does not occur when the particles are heated in the presence of water (Figure S6). This can be compared with when the particles are heated alone, where foaming was still possible (Figure S6) with visible adsorbed particles around air bubbles. This indicates that the particles lose their surface-activity at the water-air interface following heating in the presence of water. This might be due to the fact that the particles are too big or heavy to adsorb on bubble surfaces following expansion and possible water absorption [28]. Contact angle and foam formation It is difficult to measure the contact angle small solid particles make with a liquid-air interface especially when irregular in shape. As a result, the apparent contact angle of liquid drops on a disk of the particles [29, 30] or a substrate coated with the particles [26, 31] is measured. Although the Expancel® particles are not irregular in shape, we have measured the apparent contact angle of a water drop in air on their compressed disks. An advancing and a receding angle of 64.8 ± 0.3° and 30.7 ± 0.2°, respectively, were measured in line with these particles stabilizing aqueous foams [16]. We acknowledge that particles may deform during compression, but suggest that their surface chemistry remains unchanged as they are not coated. Additionally, the results obtained are in good agreement with literature values on particlestabilized foams [26, 30, 32]. Foam stability at different particle concentrations A photograph of aqueous foams prepared at different particle concentrations at room temperature (23 °C) is shown in Figure 3 along with their optical microscope images. The initial foam volume and the residual volume of water, measured immediately after foaming, are plotted against particle concentration in Figure 4. More gas bubbles are incorporated as the concentration of particles increases, like in many [13, 26, 27, 33] particle-stabilized foams, 9
leading to an increase in foam volume and a decrease in the residual volume of water. The volume of foam and the residual volume of water remained unchanged for over six months. The plot of bubble size against particle concentration (Figure 5) shows an initial increase with a maximum at 7 wt.% and a decrease thereafter. At relatively low particle concentration, the bubbles coalesce as they are partially coated leading to size variation. Coalescence is suppressed as the concentration increases and the bubble size remains unchanged once formed. This is akin to what happens in emulsion systems in the so called “emulsifier-poor” regime where drop size increases with emulsifier concentration and in the “emulsifier-rich” regime where the size is independent of emulsifier concentration [15]. The large error bars indicate that the bubbles are polydisperse with a wide range of bubble size. Growing foam: heating water before aeration Particles (5 wt.%) were placed on the surface of water samples at different temperatures (23‒80 °C) and aerated to obtain foam. Photographs and optical microscope images of the foams are shown in Figure 6. The volume of foam and residual volume of water soon after foaming are plotted against water temperature in Figure 7. The foam volume increased by 9% with a corresponding decrease in the residual volume of water at 65 °C, reaching 65% at the highest temperature (80 °C). This is thought to be due to particle growth as this corresponds to the temperature at which the particles are sensitive to heat when in water (Figure S2). The foams were allowed to cool to room temperature (23 °C), but their volume remained unchanged for over six months, indicative that they are stable to coalescence and disproportionation within this time period. The average bubble size is plotted against temperature in Figure S7. The bubbles are relatively large (diameter > 100 µm) and there appears to be no relationship between their size and the aeration temperature. For water at 80 °C, the particle contact time before aeration was varied from 0.5‒5 min so as to investigate the foamability as they grow. Photographs of the foams and their corresponding microscope images are shown in Figure S8. The foam volume and the residual volume of water are plotted against particle contact time in Figure S9. The foam volume increased by 47% at 5 min taking up 90% of water, but the microscope image contains few bubbles with plenty of unadsorbed particles. These particles are fifteen times larger than their original size, meaning that they lost their ability to foam as they grow. The bubble size increases with particle contact time, reaching a plateau value at 3.5 min (Figure 8a). The increment can be associated with the growth of adsorbed particles (Figure 8b). The foams were left to cool to room temperature and 10
their volume remained unchanged for over six months, meaning that they are stable to coalescence and disproportionation within the time period. This is similar to heating βlactoglobulin at 85 °C where it denatures and forms aggregates that formed aqueous foams stable to drainage and coalescence [25]. Contrarily, this behavior is uncommon with many foams stabilized by particles or surfactant. For instance, aqueous foams stabilized by silica particles or sodium tetradecyl sulfate [34], which undergo severe creaming, do not grow with temperature. In fact aqueous foams stabilized by sodium tetradecyl sulphate are more stable at lower temperatures [34]. Similarly, aqueous foams stabilized by sodium dodecyl sulphate do not grow with increasing temperature and are more stable at room temperature [35]. The drainage stability of aqueous foams stabilized by polymer molecules also decreases with increasing temperature [36]. Growing foam: heating pre-prepared foam Foam samples prepared at room temperature (23 °C) were heated to various temperatures (Figure S10) and the height of the foam layer is plotted as a function of temperature in Figure 9. The foam samples are sensitive to temperatures above 55 °C, leading to an increase in the height of the foam layer as temperature increases. The foam volume doubled at 60 °C and tripled at 85 °C taking up all the water. After cooling, the foam volume did not change for over six months indicating that they do not coalesce and disproportionate. The bubble size in the foams increases with increasing temperature (Figure 10), reaching a maximum at 60 °C with a slight decrease at 70 °C. However, there were no obvious air bubbles beyond this temperature. Another foam sample was heated to 85 °C for different times (Figure S11) to investigate the effect of the duration of heating on foam growth. The foam volume grew by five-fold at 2.5 min, but decreased by 20% for every 5 min of heating thereafter (Figure S12). Initially, the residual water phase was absorbed completely, but was released gradually as heating continued. An SEM image of a foam sample after heating for 5 min is shown in Figure 11, where cavities left by air bubbles can be clearly seen. This means the foams can serve as precursors for macroporous materials. Although foam growth is uncommon with other particlestabilized foams like those from silica particles, the growth of foam observed here is entirely due to the expansion of the micro-spherical plastic particles upon heating.
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CONCLUSIONS Unlike previous experiments [34-36], this work shows that stable aqueous foams can be obtained at elevated temperatures. This was achieved by stabilizing the foams with Expancel® micro-spherical plastic particles that expand upon heating. The foam grew because the particles stabilizing the air bubbles expand with heat. The particles expand more when heated in the presence of water than when heated alone. On this basis, foams were prepared in two ways. First, foams were prepared with water at various temperatures (23–80 °C) at a fixed particle concentration (5 wt.%). The foam volume grew with increasing temperature especially at ≥ 65 °C, corresponding to the onset of particle growth. The growth was higher when the particles were left on the water surface for a longer time interval (1–5 min) before aeration. Secondly, foams were prepared with water at room temperature and heated to various temperatures (40– 85 °C). Foam growth began at 60 °C and increased with increasing temperature. When heated at the highest temperature (85 °C) for different times (2.5–40 min), a five-fold growth occurred after 2.5 min. This was followed by a 20% decrease for every 5 min of heating. For both methods, the foam volume remained unchanged for over six months, meaning that they are stable to coalescence and disproportionation. Unlike the ultra-stable aqueous foams reported by Gonzenbach et al. [21] using a mixture of oxide particles and short chain surfactant, these foams are stabilized by particles alone.
ACKNOWLEDGEMENTS We thank the Tertiary Education Trust Fund (TetFund) of Nigeria for PhD sponsorship of ATT and the University of Hull, UK for his postdoctoral sponsorship.
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A.T. Tyowua, Liquid marbles: Formation, Characterization and Applications, CRC Press, Boca Raton, 2019. B.P. Binks, Particles as surfactants‒similarities and differences, Curr. Opin. Colloid Interface Sci. 7 (2002) 21-41. https://doi.org/10.1016/s1359-0294(02)00008-0. U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Ultrastable particlestabilized foams, Angew. Chemie Int. Ed. 45 (2006) 3526-3530. http://dx.doi.org/10.1002/anie.200503676. J.C.H. Wong, E. Tervoort, S. Busato, U.T. Gonzenbach, A.R. Studart, P. Ermanni, L.J. Gauckler, Macroporous polymers from particle-stabilized foams, J. Mater. Chem. 19 (2009) 5129-5133. https://doi.org/10.1039/b908926h. A. Cervantes Martinez, E. Rio, G. Delon, A. Saint-Jalmes, D. Langevin, B.P. Binks, On the origin of the remarkable stability of aqueous foams stabilized by nanoparticles: link with microscopic surface properties, Soft Matter 4 (2008) 1531-1535. https://doi.org/10.1039/b804177f. H. Zuidema, G. Waters, Ring method for determination of interfacial tension, Ind. Eng. Chem. Anal. Ed. 13 (1941) 312-313. http://dx.doi.org/10.1021/i560093a009. A. Moro, G.D. Báez, P.A. Busti, G.A. Ballerini, N.J. Delorenzi, Effects of heat-treated β-lactoglobulin and its aggregates on foaming properties, Food Hydrocoll. 25 (2011) 1009-1015. https://doi.org/10.1016/j.foodhyd.2010.09.021. B.P. Binks, A.T. Tyowua, Influence of the degree of fluorination on the behavior of silica particles at air-oil surfaces, Soft Matter 9 (2013) 834-845. https://doi.org/10.1039/c2sm27395k. B.P. Binks, A. Rocher, M. Kirkland, Oil foams stabilized solely by particles, Soft Matter 7 (2011) 1800-1808. https://doi.org/10.1039/c0sm01129k. G.R. Wiese, T.W. Healy, Effect of particle size on colloid stability, Trans. Faraday Soc. 66 (1970) 490-499. http://dx.doi.org/10.1039/tf9706600490. B.P. Binks, S.O. Olusanya, Pickering emulsions stabilized by colored organic pigment particles, Chem. Sci. 8 (2017) 708-723. https://doi.org/10.1039/c6sc03085h. B.P. Binks, S.K. Johnston, T. Sekine, A.T. Tyowua, Particles at oil-air surfaces: powdered oil, liquid oil marbles, and oil foam, ACS Appl. Mater. Interfaces 7 (2015) 14328-14337. http://dx.doi.org/10.1021/acsami.5b02890. D.J. French, P. Taylor, J. Fowler, P.S. Clegg, Making and breaking bridges in a Pickering emulsion, J. Colloid Interface Sci. 441 (2015) 30-38. https://doi.org/10.1016/j.jcis.2014.11.032. H.A. Wege, S. Kim, V.N. Paunov, Q. Zhong, O.D. Velev, Long-term stabilization of foams and emulsions with in-situ formed microparticles from hydrophobic cellulose, Langmuir 24 (2008) 9245-9253. 10.1021/la801634j. J.S. Guevara, A.F. Mejia, M. Shuai, Y.-W. Chang, M.S. Mannan, Z. Cheng, Stabilization of Pickering foams by high-aspect-ratio nano-sheets, Soft Matter 9 (2013) 1327-1336. https://doi.org/10.1039/c2sm27061g. G.C. Valenzuela, K. Wong, D.E. Connor, M. Behnia, K. Parsi, Foam sclerosants are more stable at lower temperatures, Eur. J. Vasc. Endovascular Surg. 46 (2013) 593599. https://doi.org/10.1016/j.ejvs.2013.08.012. Y.-X. Zhang, Z.-L. Wu, Z.-J. Wu, D.-F. Yang, Effects of temperature on foam stability and separation efficiencies of foam formed by high concentration SDS aqueous solution during foam separation, Chem. J. Chinese U. 26 (2012) 536-540. M.S. Pradhan, D.S.H.S.R. Sarma, K.C. Khilar, Stability of aqueous foams with polymer additives: II. Effects of temperature, J. Colloid Interface Sci. 139 (1990) 519-526. https://doi.org/10.1016/0021-9797(90)90124-7. 14
Figure 1. SEM micrograph of Expancel 031 WUFX 40 particles. The particles were: (a) at room temperature (23 °C), (b) heated alone at 80 °C for 5 min and (c) heated at 80 °C in the presence of water for 5 min prior to imaging.
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Figure 2. Percentage of particles versus particle diameter (bar chart, left axis) or normal distribution of particle diameter (broken curve, right axis) of 51 Expancel 031 WUFX 40
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Figure 4. Initial volume of foam () and residual volume of water immediately after foaming () at room temperature (23 °C) versus concentration of particles for foams shown in Figure 3. The error bars are standard deviations of three separate measurements. 4
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Figure 7. Initial volume of foam () and residual volume of water () soon after aeration versus temperature for foams obtained by hand shaking 5 wt.% of particles with water (3 cm3) at different temperatures shown in Figure 6. The error bars are standard deviations of three separate measurements. 4
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Figure 8. (a) Bubble diameter versus contact time of particles with water before aeration for foams (Figure S8) obtained by hand shaking 5 wt. % of particles and 3 cm3 of water (at 80 °C) where the particles were allowed to remain on the water surface for different times before aeration. The error bars are standard deviations of 100 separate measurements. (b) Schematic of foams at ambient and elevated temperatures showing particle growth upon heating. 300 (a)
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Figure 11. (a) Photograph of an aqueous foam stabilized by 5 wt. % particles after heating in a water bath maintained at 85 °C for 5 min. The foam was removed from the glass vial and dried (3 days) at ambient conditions. (b) Corresponding SEM micrograph of the dried foam.
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GRAPHICAL ABSTRACT
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Andrew Terhemen Tyowua: Methodology, Investigation, Visualization, and WritingOriginal Draft. Bernard Paul Binks: Funding Acquisition, Conceptualization, Supervision, Resources, Validation, and Writing-Review and Editing.
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S0021-9797(19)31431-6 https://doi.org/10.1016/j.jcis.2019.11.103 YJCIS 25720
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
2 October 2019 25 November 2019 25 November 2019
Please cite this article as: A.T. Tyowua, B.P. Binks, Growing a particle-stabilized aqueous foam, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.11.103
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© 2019 Published by Elsevier Inc.
GROWING A PARTICLE-STABILIZED AQUEOUS FOAM
Andrew T. Tyowua1,2* and Bernard P. Binks1*
1Department
of Chemistry and Biochemistry, University of Hull, Hull. HU6 7RX. UK 2Applied
Colloid Science and Cosmeceutical Group,
Department of Chemistry, Benue State University, PMB 102119, Makurdi, Nigeria
Submitted to: J. Colloid Interface Sci. on 15.10.19; revised on 25.11.19
Contains ESI
*Corresponding authors: [email protected] (Tel. +2349091818649) [email protected] (Tel. +44-1482-465450)
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ABSTRACT Hypothesis Certain gas-filled colloidal particles expand upon heating. If such particles are surface-active and stabilize aqueous foams, do the foams grow with temperature as particles expand? Experiments Aqueous foams were stabilized with hollow micro-spherical particles that are partially wetted by water and grow upon heating. Foams were prepared using two different approaches, both of which led to their growth. In the first, water was heated to various temperatures (40–80 °C) and aerated in the presence of the particles. In the second, water at room temperature was aerated in the presence of the particles and then heated to various temperatures (40–85 °C). Findings Regardless of the method, foam volume began to increase on raising the temperature to the onset of particle growth (60 °C) as expected and increased with increasing temperature. However, placing the particles on hot water (80 °C) and waiting for several minutes (≤ 2.5) before aeration resulted in more growth. The volume of foam after growth remained unchanged after cooling for over six months, giving rise to ultra-stable foams.
Keywords: foam, drainage, creaming, bubble coarsening, bubble coalescence, foam growth, surface tension, adsorption, wetting, contact angle
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INTRODUCTION Inspired by the work of Ramsden [1] and Pickering [2], many particle-stabilized materials like emulsions and foams have been prepared. These materials are more stable than their surfactantstabilized counterparts. This is because particles can be irreversibly adsorbed compared with surfactant molecules which adsorb and desorb from liquid drops and air bubble surfaces. This property is directly related to the magnitude of the free energy of desorption which can be very high for particles (> 103 kBT) and relatively low for surfactants (around several kBT), where kB is Boltzmann’s constant and T is the absolute temperature. Various particles have been used in the preparation of emulsions and foams. For emulsions, some of these include fumed silica [3, 4], calcium carbonate [5], laponite clay [6], bentonite clay, polystyrene [7], whey protein microgel [8], titania [9], starch granules [10] and polytetrafluoroethylene [7]. Similarly, silica [11], calcium carbonate [12], sericite clay [13] and chitin nanocrystal [14] particles have been used in the preparation of foams. In these materials, the particles coat the liquid drops (emulsions) and air bubbles (foams) and prevent them from coalescence [15]. Whether or not this will happen depends on the wettability of the particles, quantified in terms of the threephase contact angle θ [16]. In line with the findings of Finkle and co-workers [17], the poorly wetted liquid is the dispersed phase in emulsions containing equal volumes of the component liquids. On this basis, oil-in-water (o/w) emulsions form for θ < 90° while water-in-oil (w/o) emulsions form for θ > 90° in water-oil systems. Similarly, in liquid-air systems, foams are formed for θ close to 90° while liquid marbles [18, 19] are formed for θ >> 90°. When the particles are completely wetted by either of the component phases (θ ≈ 0°), no stable materials are formed [20]. Particle-stabilized materials are ultra-stable to complete phase separation and are very important in many formulations [16]. For example, particle-stabilized emulsions are important in the cosmetic industry where they act as the basis for body creams. In the pharmaceutical industry, they are the basis for the formulation of syrups. In medicine, they are used as carriers of bioactive components. They are also the basis for many food products. Foam applications range from food (e.g. whipped cream), cosmetics, oil recovery to fire extinguishing. Foams are also precursors of many macro-porous materials widely used as thermal insulators, artificial implants and shock absorbers [21, 22]. They also act as vehicles for drug delivery and tissue engineering.
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Although bubble coarsening is absent in particle-stabilized foams, especially at high particle concentration [23], many particle-stabilized foams undergo an initial drainage and creaming, leaving a large fraction of the liquid phase beneath the foam. The stabilization of air bubbles in a system devoid of drainage and creaming will be very advantageous in foam applications especially for the creation of macro-porous materials as the foam structure will not collapse. Gonzenbach and co-workers [21] achieved this in aqueous systems using a mixture of surfactant and particles. We aim to show here that this can also be achieved through foam growth when hollow micro-spherical particles that expand with heat are used.
EXPERIMENTAL Materials Water Water was passed through an Elga Prima reverse osmosis unit and then a Milli-Q reagent water system to remove polar impurities. The treated water has pH ≈ 6 and a resistivity of 18 MΩ cm at 20 °C. Particles Expancel® micro-spherical plastic particles known as 031 WUFX 40 from Expancel, Akzo Nobel (Sweden) were used. According to the manufacturer, the particles consist of an acrylic copolymer shell (thickness ~2 μm) encapsulating a gas (isobutane) and have a true density of 1000 kg m‒3 (unexpanded) and 30 kg m‒3 (expanded). Based on the manufacturer information, the internal pressure of the core increases and the thermoplastic shell softens, leading to a dramatic increase in the volume (e.g. initial diameter 12 μm and 40 μm when expanded) when heated, but the gas remains intact. Also, particle expansion begins between 80 and 95 °C and reaches a maximum between 115 and 135 °C. Once expanded, the particles do not relax to their original volume when cooled to room temperature (23 °C). Methods Surface tension measurement A Krüss K12 digital tensiometer was used to measure the surface tension of water. Measurements were done at 20 °C using the du Noüy ring method. Three separate measurements were averaged and corrected according to Zuidema and Waters [24]. Prior to 4
each measurement, the glass vessel containing the water sample was washed with alcoholic KOH solution, rinsed with Milli-Q water and dried in an oven while the du Noüy ring was heated to glowing in a Bunsen flame. Scanning electron microscopy of particles The particles were imaged using a Ziess EVO 60 scanning electron microscope (SEM) in three situations: dry (unexpanded), dry (heated at 80 °C, expanded) and heated at 80 °C with water present (expanded). The particles were attached to the SEM stub using a carbon-impregnated self-adhesive disk. Excess particles were then removed with a jet of low-pressure compressed air. The prepared stub was then coated with ~2 nm of gold using a Polaron 7640 Sputter coater and examined at 20 kV and 100 pA under high vacuum mode. Some of the particles resemble a deflated football (Figure 1). The reason for this is not obvious, but could be due to the vacuum in the SEM chamber. This indicates that their shell is elastic otherwise they would not deform. The SEM image of the particles also shows that they are polydisperse. Measurement of particle diameter, with ImageJ software, from 51 particles on the SEM image of the dry unexpanded particles gave an average diameter of 6 µm with a standard deviation of 3 µm. The distribution of particle diameter is shown in Figure 2. Based on the bar chart, the diameter lies between 1 and 14 µm with the mode (majority) at 3 µm. However, the normal distribution curve shows that the mode is 6 µm, consistent with the average particle diameter. This can be compared with the size (D, 0.5) 10 −16 μm given by the manufacturer. Heating the particles alone to 80 °C or with water present increased their average diameter from 6 μm to 10 μm or 14 μm, respectively with a standard deviation of 3 μm. Effect of heat and the duration of heating on the particles The effect of heat on the particles was investigated in the presence and absence of water. The required mass (150 mg) of particles was closed in a screw cap glass vial (inner diameter 18 mm, height 72 mm) and heated on a water bath maintained at different temperatures (40–85 °C) for a period of 2.5 min. Similarly, the required mass (150 mg) of the particles was placed on water (3 cm3) in the glass vial and heated in a water bath maintained at the said temperatures for the same period. The height of the particle layer was measured in each case. Three separate experiments were done and an average height of the particle layer is reported along with the standard deviations. To investigate the effect of the duration of heating, the particles (150 mg) were heated in screw 5
cap glass vials in a water bath maintained at 80 °C for 30 min and the height of the particle layer was measured first at 2.5 min and then after every 5 min. During the experiment, water (as confirmed by anhydrous CuSO4) was seen to condense on the lid of the glass vials. Similarly, the particles (150 mg) were placed on water (3 cm3) at room temperature contained in the screw cap glass vials. The glass vials were tightly closed and heated in the water bath maintained at the same temperature for the same time period. The height of the particle layer was measured. Three separate samples were used and an average height of the particle layer is reported with the standard deviations. Particle immersion test with agitation The required volume of water (3 cm3) was measured into a screw cap glass vial (inner diameter 18 mm, height 72 mm) followed by placing the required mass (50 mg) of particles on its surface. In the closed glass vial, it was noted whether water wets the particles or not. Next, the water-particle mixture was hand shaken vigorously (30 s) and observed for foam formation. The experiment was further performed with particles heated alone (80 °C for 30 min) or with water so as to know whether or not foaming will occur after heating the particles. Contact angle of particles with water Firstly, the particles (110 mg) were compressed in a 13 mm diameter steel die with a hydraulic press (Research and Industrial Instrument Co., UK) under a pressure of ~1.5 × 108 N m−2 into relatively smooth disks (diameter 13 mm, thickness 0.7 mm). Secondly, a water drop (20 μL) was formed in air on the particle disk using an Eppendorf micropipette. The advancing contact angle was then measured by analyzing the profile of the water drop using a Krüss DSA Mk 10 apparatus. Liquid (10 µL) was withdrawn from the drop using the Eppendorf micropipette and the profile of the drop was analyzed similarly for the receding contact angle. The experiment was repeated with three separate disks and liquid drops and an average of the measurements is reported along with the standard deviations. Effect of particle concentration on foam stability The particle immersion test with agitation experiment revealed that the particles are able to stabilize an aqueous foam. As a result, the stability of the foam to drainage was investigated in batch foams prepared at different particle concentrations (0.5 ‒10 wt.%). The required volume (3 cm3) of water was measured into a screw cap glass vial (inner diameter 18 mm, height 72 mm) followed by addition of the required mass of the particles. Foaming was done in the closed 6
vial by hand shaking the water-particle mixture vigorously (30 s) in a vertical motion. The volume of foam obtained and the residual volume of water were estimated at each particle concentration immediately after foaming. The stability to drainage was assessed by noting the volume of water released with time. Growing foam: heating water before aeration The required volume (3 cm3) of water was heated from room temperature (23 °C) to the required temperature (40‒80 °C) in screw cap glass vials (inner diameter 18 mm and height 72 mm) using a water bath (Grant UK). This was followed by addition of the required mass of the particles (equivalent to 5 wt.%). After about 30 s, the mixture was removed briefly from the water bath and hand shaken vigorously (30 s). For water at 80 °C, the water-particle mixture was left in the water bath for 0.5‒5 min before aeration. This was to investigate the influence of particle residence time on foam formation at this temperature. After 2 h, an optical microscope (Olympus BX-51) was used to ascertain whether foams were formed at these temperatures or not. Growing foam: heating pre-prepared foam Foams stabilized by 5 wt.% particles and prepared at room temperature (23 °C) were heated in closed glass vials in a water bath (maintained at 40−85 °C) for 2.5 min and the height of the foam layer in the glass vials was noted. The foams began to “grow” at temperatures above 55 °C, leading to an increase in height of the foam layer. Three foam samples were used for the experiment and an average height of foam layer is reported along with the standard deviation. To investigate the effect of duration of heating on foam, a foam sample was heated in a water bath (maintained at 85 °C) for 2.5−40 min. During the experiment, the height of the foam layer was measured after 2.5 min and eventually after every 5 min. The experiment was done in triplicate and an average height of foam layer is reported with the corresponding standard deviations. Thereafter, a foam sample was heated in a water bath at 85 °C for 5 min, cooled and left to dry under ambient conditions and examined using SEM. Fragments of the dried foam were examined using a binocular microscope and a suitable size was selected for SEM study. The fragment was gently attached to an SEM stub using rapid-curing expoxy resin. The sample was then coated with ~5 nm of gold using a Polaron 7640 Sputter coater and examined at 15 kV and 100 pA in high vacuum mode.
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Characterization of foam Foams were characterized in terms of their microstructure using an optical microscope. A small foam sample was placed on a dimpled glass slide (Fisher Scientific) and viewed using an Olympus BX-51 optical microscope fitted with a DP50 digital camera using Image-Pro Plus 6.0 software (Media Cybernetics). Optical micrographs of the foams were used to measure the size of air bubbles by averaging the diameter of 100 bubbles using ImageJ software. Photographs of glass vials containing the foam and other photos were taken with a Canon Power Shot SX230 HS camera. RESULTS AND DISCUSSION Effect of heat on particles in the absence and presence of water According to the manufacturer, the micro-spherical particles expand when heated and so this was investigated. A photograph of particle samples heated alone and in the presence of water at various temperatures is shown in Figure S1. The height of the particle layer is plotted against temperature for the different samples in Figure S2. In the absence of water, the particles were sensitive to temperatures > 60 °C, leading to an increase in the height of the particle layer which increases with increasing temperature. With water, particle sensitivity occurred at a slightly lower temperature (> 55 °C), leading to water absorption and a five-fold increase in the height of the particle layer. The shift in temperature might be due to a specific interaction between the particles and water. By 85 °C, a 12-fold increment was obtained. Clearly, a higher particle layer is obtained when the particles are heated in the presence of water compared with when they are heated alone. These temperatures are lower than that reported by the manufacturer (80–95 °C) at which particle expansion begins. Results of a similar experiment where the particles were heated alone (80 °C) and in the presence of water for different times are shown in Figures S3 and S4, respectively. There is an initial 9 to 19-fold increment in the height of the particle layer (Figure S3) when heated alone or in the presence of water, respectively. Thereafter, the particle layer doubles in height for every 5 min and its height plateaus eventually when heated alone (Figure S4). In the presence of water, the particle layer triples for every 5 min, reaching a maximum at 20 min and decreases slightly thereafter. Heating also causes the particles to form aggregates similar to heating βlactoglobin solution at 85 °C [25]. The height of the particle layer remained unchanged even after cooling to room temperature (23 °C), indicating that particles do not relax to their original size once they grow and expand. 8
Particle immersion test with agitation The particle immersion test has been used previously [13, 26] to determine whether a liquid wets a given powdered particle sample or not. Particles that are wetted completely by a liquid do not form any foam or emulsion as reported by Binks and co-workers [13, 26, 27]. Here, water wetted the particles partially at room temperature. Some of the particles (probably the larger ones) sank to the bottom of the vial, but the majority of them remained on its surface. Foam was obtained upon agitation (Figure S5). These results are in line with those reported previously on fumed silica [26] and sericite [13] particles. By contrast, foaming does not occur when the particles are heated in the presence of water (Figure S6). This can be compared with when the particles are heated alone, where foaming was still possible (Figure S6) with visible adsorbed particles around air bubbles. This indicates that the particles lose their surface-activity at the water-air interface following heating in the presence of water. This might be due to the fact that the particles are too big or heavy to adsorb on bubble surfaces following expansion and possible water absorption [28]. Contact angle and foam formation It is difficult to measure the contact angle small solid particles make with a liquid-air interface especially when irregular in shape. As a result, the apparent contact angle of liquid drops on a disk of the particles [29, 30] or a substrate coated with the particles [26, 31] is measured. Although the Expancel® particles are not irregular in shape, we have measured the apparent contact angle of a water drop in air on their compressed disks. An advancing and a receding angle of 64.8 ± 0.3° and 30.7 ± 0.2°, respectively, were measured in line with these particles stabilizing aqueous foams [16]. We acknowledge that particles may deform during compression, but suggest that their surface chemistry remains unchanged as they are not coated. Additionally, the results obtained are in good agreement with literature values on particlestabilized foams [26, 30, 32]. Foam stability at different particle concentrations A photograph of aqueous foams prepared at different particle concentrations at room temperature (23 °C) is shown in Figure 3 along with their optical microscope images. The initial foam volume and the residual volume of water, measured immediately after foaming, are plotted against particle concentration in Figure 4. More gas bubbles are incorporated as the concentration of particles increases, like in many [13, 26, 27, 33] particle-stabilized foams, 9
leading to an increase in foam volume and a decrease in the residual volume of water. The volume of foam and the residual volume of water remained unchanged for over six months. The plot of bubble size against particle concentration (Figure 5) shows an initial increase with a maximum at 7 wt.% and a decrease thereafter. At relatively low particle concentration, the bubbles coalesce as they are partially coated leading to size variation. Coalescence is suppressed as the concentration increases and the bubble size remains unchanged once formed. This is akin to what happens in emulsion systems in the so called “emulsifier-poor” regime where drop size increases with emulsifier concentration and in the “emulsifier-rich” regime where the size is independent of emulsifier concentration [15]. The large error bars indicate that the bubbles are polydisperse with a wide range of bubble size. Growing foam: heating water before aeration Particles (5 wt.%) were placed on the surface of water samples at different temperatures (23‒80 °C) and aerated to obtain foam. Photographs and optical microscope images of the foams are shown in Figure 6. The volume of foam and residual volume of water soon after foaming are plotted against water temperature in Figure 7. The foam volume increased by 9% with a corresponding decrease in the residual volume of water at 65 °C, reaching 65% at the highest temperature (80 °C). This is thought to be due to particle growth as this corresponds to the temperature at which the particles are sensitive to heat when in water (Figure S2). The foams were allowed to cool to room temperature (23 °C), but their volume remained unchanged for over six months, indicative that they are stable to coalescence and disproportionation within this time period. The average bubble size is plotted against temperature in Figure S7. The bubbles are relatively large (diameter > 100 µm) and there appears to be no relationship between their size and the aeration temperature. For water at 80 °C, the particle contact time before aeration was varied from 0.5‒5 min so as to investigate the foamability as they grow. Photographs of the foams and their corresponding microscope images are shown in Figure S8. The foam volume and the residual volume of water are plotted against particle contact time in Figure S9. The foam volume increased by 47% at 5 min taking up 90% of water, but the microscope image contains few bubbles with plenty of unadsorbed particles. These particles are fifteen times larger than their original size, meaning that they lost their ability to foam as they grow. The bubble size increases with particle contact time, reaching a plateau value at 3.5 min (Figure 8a). The increment can be associated with the growth of adsorbed particles (Figure 8b). The foams were left to cool to room temperature and 10
their volume remained unchanged for over six months, meaning that they are stable to coalescence and disproportionation within the time period. This is similar to heating βlactoglobulin at 85 °C where it denatures and forms aggregates that formed aqueous foams stable to drainage and coalescence [25]. Contrarily, this behavior is uncommon with many foams stabilized by particles or surfactant. For instance, aqueous foams stabilized by silica particles or sodium tetradecyl sulfate [34], which undergo severe creaming, do not grow with temperature. In fact aqueous foams stabilized by sodium tetradecyl sulphate are more stable at lower temperatures [34]. Similarly, aqueous foams stabilized by sodium dodecyl sulphate do not grow with increasing temperature and are more stable at room temperature [35]. The drainage stability of aqueous foams stabilized by polymer molecules also decreases with increasing temperature [36]. Growing foam: heating pre-prepared foam Foam samples prepared at room temperature (23 °C) were heated to various temperatures (Figure S10) and the height of the foam layer is plotted as a function of temperature in Figure 9. The foam samples are sensitive to temperatures above 55 °C, leading to an increase in the height of the foam layer as temperature increases. The foam volume doubled at 60 °C and tripled at 85 °C taking up all the water. After cooling, the foam volume did not change for over six months indicating that they do not coalesce and disproportionate. The bubble size in the foams increases with increasing temperature (Figure 10), reaching a maximum at 60 °C with a slight decrease at 70 °C. However, there were no obvious air bubbles beyond this temperature. Another foam sample was heated to 85 °C for different times (Figure S11) to investigate the effect of the duration of heating on foam growth. The foam volume grew by five-fold at 2.5 min, but decreased by 20% for every 5 min of heating thereafter (Figure S12). Initially, the residual water phase was absorbed completely, but was released gradually as heating continued. An SEM image of a foam sample after heating for 5 min is shown in Figure 11, where cavities left by air bubbles can be clearly seen. This means the foams can serve as precursors for macroporous materials. Although foam growth is uncommon with other particlestabilized foams like those from silica particles, the growth of foam observed here is entirely due to the expansion of the micro-spherical plastic particles upon heating.
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CONCLUSIONS Unlike previous experiments [34-36], this work shows that stable aqueous foams can be obtained at elevated temperatures. This was achieved by stabilizing the foams with Expancel® micro-spherical plastic particles that expand upon heating. The foam grew because the particles stabilizing the air bubbles expand with heat. The particles expand more when heated in the presence of water than when heated alone. On this basis, foams were prepared in two ways. First, foams were prepared with water at various temperatures (23–80 °C) at a fixed particle concentration (5 wt.%). The foam volume grew with increasing temperature especially at ≥ 65 °C, corresponding to the onset of particle growth. The growth was higher when the particles were left on the water surface for a longer time interval (1–5 min) before aeration. Secondly, foams were prepared with water at room temperature and heated to various temperatures (40– 85 °C). Foam growth began at 60 °C and increased with increasing temperature. When heated at the highest temperature (85 °C) for different times (2.5–40 min), a five-fold growth occurred after 2.5 min. This was followed by a 20% decrease for every 5 min of heating. For both methods, the foam volume remained unchanged for over six months, meaning that they are stable to coalescence and disproportionation. Unlike the ultra-stable aqueous foams reported by Gonzenbach et al. [21] using a mixture of oxide particles and short chain surfactant, these foams are stabilized by particles alone.
ACKNOWLEDGEMENTS We thank the Tertiary Education Trust Fund (TetFund) of Nigeria for PhD sponsorship of ATT and the University of Hull, UK for his postdoctoral sponsorship.
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Figure 1. SEM micrograph of Expancel 031 WUFX 40 particles. The particles were: (a) at room temperature (23 °C), (b) heated alone at 80 °C for 5 min and (c) heated at 80 °C in the presence of water for 5 min prior to imaging.
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Figure 2. Percentage of particles versus particle diameter (bar chart, left axis) or normal distribution of particle diameter (broken curve, right axis) of 51 Expancel 031 WUFX 40
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Figure 3. Photograph (after 3 weeks) of glass vials containing aqueous foams prepared at room temperature (23 °C) stabilized by varying concentrations (given, wt.%) of particles and corresponding optical micrographs.
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Figure 4. Initial volume of foam () and residual volume of water immediately after foaming () at room temperature (23 °C) versus concentration of particles for foams shown in Figure 3. The error bars are standard deviations of three separate measurements. 4
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Figure 6. (upper) Photograph (2 h after preparation) of aqueous foams obtained by hand shaking 5 wt.% of particles and 3 cm3 of water at the indicated temperature (°C). (lower) Corresponding optical micrographs of the above foams at the indicated temperature (°C).
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Figure 7. Initial volume of foam () and residual volume of water () soon after aeration versus temperature for foams obtained by hand shaking 5 wt.% of particles with water (3 cm3) at different temperatures shown in Figure 6. The error bars are standard deviations of three separate measurements. 4
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Figure 8. (a) Bubble diameter versus contact time of particles with water before aeration for foams (Figure S8) obtained by hand shaking 5 wt. % of particles and 3 cm3 of water (at 80 °C) where the particles were allowed to remain on the water surface for different times before aeration. The error bars are standard deviations of 100 separate measurements. (b) Schematic of foams at ambient and elevated temperatures showing particle growth upon heating. 300 (a)
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Figure 11. (a) Photograph of an aqueous foam stabilized by 5 wt. % particles after heating in a water bath maintained at 85 °C for 5 min. The foam was removed from the glass vial and dried (3 days) at ambient conditions. (b) Corresponding SEM micrograph of the dried foam.
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GRAPHICAL ABSTRACT
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Andrew Terhemen Tyowua: Methodology, Investigation, Visualization, and WritingOriginal Draft. Bernard Paul Binks: Funding Acquisition, Conceptualization, Supervision, Resources, Validation, and Writing-Review and Editing.
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