Aqueous foam stabilized by plate-like particles in the presence of sodium butyrate

Journal of Colloid and Interface Science 343 (2010) 87–93

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Aqueous foam stabilized by plate-like particles in the presence of sodium butyrate Qian Liu, Lingyu Luan, Dejun Sun *, Jian Xu Key Laboratory for Colloid and Interface Chemistry of the Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 5 August 2009 Accepted 31 October 2009 Available online 5 November 2009 Keywords: Foam stability LDH Sodium butyrate

a b s t r a c t The addition of salt promotes the adsorption of layered double hydroxide (LDH) particles onto the air– water interface, but stable foams cannot be prepared from LDH dispersions at all the concentration of NaCl or sodium acetate. We generated stable foams using positively charged plate-like LDH particles in the presence of sodium butyrate. The effects of adding sodium butyrate to LDH on the particle zeta potential, adsorption behavior and the adsorption of modified particles at the air–water interface were studied. At a fixed LDH particle concentration, adding of a trace amount of sodium butyrate maximizes flocculation of the aqueous particle dispersion. Foams prepared under this condition of particle dispersion are most stable to coalescence and halt completely disproportionation. Also, the size of the bubbles is the smallest. The bubbles are stable when drying at 80 °C with little change in size. Laser-induced fluorescent confocal micrographs and scanning electron microscopy observations clearly confirm the adsorption of LDH particles on the foam surfaces, and the bubbles are armored by an interfacial particle multilayer. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Small solid particles can be irreversibly adsorbed at fluid–fluid interfaces, enabling the stabilization of emulsions and foams [1– 5]. Recent reviews have covered different aspects of foam stabilization by solid particles [1–6]. The enormous stability of particle-stabilized foams results from the interplay among the ability of the particles to form dense coherent particle shells around the bubbles [7–12], to stabilize the liquid films separating the bubbles and to form a three-dimensional network in the bulk aqueous phase [10]. In most cases, particles form a dense layer on bubble surfaces preventing coalescence and slowing down or halting disproportionation [13–16]. Evidence also exists that additional stabilization occurs via particle network formation between adsorbed and nonadsorbed particles, which reduces, in this case, drainage of water between the bubbles [9]. Non-spherical particles can act as effective foam stabilizers in the absence of any additives. Alargova et al. [7,17,18] have developed a novel method for the preparation of flexible high-aspect-ratio microrods from epoxy resin. Ultrastable foams stabilized by these rods have hair-like structures, in which rods are bent and entangled at the bubble surfaces. Zhou et al. [19] have reported the production of superstable foams with bimodal size distribution stabilized by modified CaCO3 particles. The rigidity and high ratio of CaCO3 rods result in the first examples of unique bubble surface structures that evolve as a function of the curvature from nestlike to armorlike. * Corresponding author. Fax: +86 531 88564750. E-mail address: [email protected] (D. Sun). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.10.081

In most cases, spherical silica [8–10,13,20,21] and latex particles [11,12,22,23] modified through prior chemical surface treatments are used as foam stabilizers. Aqueous suspensions of certain solid particles with inherent hydrophobicity are able to make extremely stable foams in the absence of any surfactant. The optimum particle hydrophobicity has been achieved by appropriate chemical synthesis [7,11,12] or surface modification [8– 10,13,20,21], or after dispersing the particles in the aqueous phase by adjusting the pH and electrolyte concentration [20,22,23]. Sometimes a system contains both particles and amphiphilic molecules (surfactants or short chain amphiphilic molecules). Gonzenbach et al. [24–26] achieved the required level of hydrophobicity by addition of short chain amphiphilic molecules which adsorbed on the particle surfaces. High-volume, stable foams were prepared from those particles. Binks et al. [27] reported the behaviors of foams stabilized by a mixture of Ludox HS-30 silica nanoparticles and the cationic surfactant di-decyldimethylammonium bromide (di-C10DMAB) at high pH. Foam stabilization changes from surfactant-dominated at low surfactant concentration to particle-dominated at intermediate concentrations and reverts to surfactant-dominated at higher concentrations. Notably, the particles employed in all the above works are almost sub-micron sized spherical particles of simple shape. In our laboratory, we have used plate-like Laponite particles to stabilize foams. We initially [28–30] prepared stable foams with cetyltrimethylammonium bromide (CTAB), tetraethylene glycol monododecyl ether, hexylamine and disk-like Laponite particle dispersions. Layered double hydroxide (LDH) compounds are important materials owing to their wide applications [31]. LDHs also known as anionic clays are

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a large family of lamellar hydroxides, which are composed of positively charged metal hydroxide layers and interlayer anions (Fig. 1). They may be represented by the general composition xþ n 2+ and M3+ are di- and tri-va½MII1x MIII x ðOHÞ2  Ax=n  mH2 O, where M lent metal cations, such as Mg2+, Co2+, Ni2+, Al3+, Fe3+ and Cr3+. An is 2þ 2+ 3+ mothe interlayer anion, NO 3 and CO3 , and x changes with M /M lar ratio of preparation. Much work has been done on the properties of LDHs in our lab. Our previous work [32] has also shown that the addition of salt promotes the adsorption of LDH particles onto the oil–water interface. Salt addition into dispersions promotes particle concentration at the oil/water interface due to the increase of attachment energy E and the decrease of the particle–particle (at the interface) and particle-interface electrostatic repulsions, which is a prerequisite for preparing stable emulsions. Surprisingly, a double phase inversion of the emulsion containing LDH particles is induced by the adsorption of SDS [33]. Most of the previous studies reported the foams stabilized by spherical particles. The effect of salt on the foams stabilized by hydrophobic silica have been studied, but the effects of salt on the adsorption of hydrophilic particles at air–water interface and on the stability of foams stabilized solely by hydrophilic particles have not been well investigated. In this paper, stable foams have been prepared from aqueous dispersions of Ni/Al LDH particles in the presence of sodium butyrate. Sodium butyrate exhibits high solubility and high critical micelle concentrations in the aqueous. This is a primary requisite to enable the surface modification of a high concentration of colloidal particles in the liquid phase. The presence of sodium butyrate is crucial for the formation and stability of foams. The adsorption of butyrate ions onto the LDH particle surfaces is first discussed and then we describe the properties of foams stabilized by a mixture of sodium butyrate and LDH particles. The stabilization mechanism of the foams is mutually supported by laser confocal microscopy imaging and scanning electron microscopy (SEM). The stable foams can be prepared by simply hand-shaking. The bubbles are stable when drying at 80 °C with little change in size. The foams will offer the potential for particular applications that use foams as intermediate structure materials. 2. Materials and methods

a

b

80 60

Intensity (%)

88

40 20 0 0

100

200 300 400 Particle Size (nm)

500

Fig. 1. Morphology and size distribution of LDH particles in aqueous suspension. (a) TEM image of LDH particles and (b) size distribution of LDH particles.

2.1. Synthesis of LDH To prepare stable suspensions of Ni/Al LDH, 350 mL NH3H2O solution (3.0 M) was quickly added into 600 mL mixed aqueous solution of NiCl26H2O and AlCl36H2O (total metal concentration 0.3 M, Ni/Al molar ratio 1:1) under vigorous stirring. The obtained precipitate was aged in air at room temperature with stirring for 45 min. After filtration, the filter cake was washed thoroughly with deionized water, then collected, redispersed in deionized water and peptized in an autoclave at 130 °C for 24 h. The interlayer anions of LDH particles are mainly Cl ions. As shown in Fig. 1, all the LDH particles are hexagonal platelike and polydisperse, and the average hydrodynamic diameter is about 86 nm (measured with a Zetasizer 3000HS instrument). The thickness of most LDH particles is about 5 nm. Sodium butyrate was purchased from Alfa Aesar with a purity of 98%. Other chemicals used in the experiments were deionized water purified by ion exchange, NiCl26H2O (analytical grade), AlCl36H2O (analytical grade) and NH3H2O (Sinopharm Chemical Reagent Co., China). 2.2. Preparation and characterization of dispersed phases The stock suspension of Ni/Al LDH (9.11 wt.%) was diluted with distilled water to control the particle concentration at 2.0 wt.%. To

obtain 25 mL of mixed dispersion, 12.5 mL of sodium butyrate solution (concentration in the range of 2–200 mM) was added to 12.5 mL of 2.0 wt.% Ni/Al LDH dispersions. The chemical composition of the Ni/Al LDH is Ni0.50Al0.47(OH)2Cl0.470.71H2O [34] . After addition of sodium butyrate, the aqueous dispersions of Ni/Al LDH/sodium butyrate mixture were further stirred for at least 12 h to attain adsorption equilibrium of butyrate groups on the Ni/Al LDH surfaces. All experiments were performed at room temperature (25 ± 1 °C). The dispersions were then transferred into a stoppered glass tube with internal diameter 1.6 cm and length 15.0 cm to observe the phase behavior of the dispersions 24 h after preparation. The prepared dispersion was centrifuged for 60 min at 37,117 g in order to separate LDH particles from supernatant. Sodium butyrate in the supernatant was analyzed for total organic carbon (TOC) with TOC analyzer (Toc-VCPN FA, CN200, Shimadzu Corporation). The adsorption amount was calculated from the difference between initial and equilibrium concentrations of surfactant, divided by the mass of the dried solid. The zeta potentials of the LDH particles in sodium butyrate solution were measured with a JS94H microelectrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The particles in the sediment phase were diluted with the supernatant to make the particles visible under the microscope before measurement.

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Dilatational viscoelasticities reported here have been measured by a drop shape tensiometer (a Tracker from IT Concept, France). For the measurement, a pendant drop of the aqueous dispersion is formed at the tip of a capillary tube, inside a glass cell containing air. The specific drop shape apparatus utilized implements an automatic feedback for the control of the interfacial area during the experiment. Surface dilational modulus (E) in compression and expansion is defined by

E¼

dc d ln A

ð1Þ

where c is the interface tension and A is the interfacial area. In oscillatory experiments, E is a complex quantity

E ¼ jEj cos h þ ijEj sin h where h is the phase angle. The real part refers to the elastic component and the imaginary part refers to the viscous component. The drop volume can be varied sinusoidally with low amplitude. The dilatational viscoelasticity measurements were performed with a relative volume variation of 10% and oscillation frequency of 0.2 Hz for 25 s. 2.3. Preparation, stability, and characterization of foams The foams were prepared from LDH/sodium butyrate aqueous dispersions at room temperature by hand-shaking for a period of 30 s. The morphology of the bubbles was observed with an Axioskop 40 microscope (ZEISS, Germany). A laser-induced confocal microscope (Olympus Fluoview 500, Japan) was used to investigate the adsorption of particles at the bubble surface. The fluorescent probe used here was Rhodamine B, which is negatively charged in basic solution and has a maximum excitation wavelength at 543 nm. Rhodamine B can adsorb onto LDH particles, which is positively charged, through electrostatic attractive interactions. In this experiment, the Rhodamine B’s concentration is 105 M. The sodium butyrate-modified LDH particles were labeled with Rhodamine B. No obvious change in stability and zeta potential of LDH particles indicated that the Rhodamine B added has minimal effect on LDH particles adsorption. Then the particle dispersions were treated with dialysis to remove the free Rhodamine B molecules in the bulk. The fluorescence of Rhodamine B was detected by the emission at 560 nm upon excitation at 543 nm. Then, foams stabilized by fluorescent labeled LDH particles were prepared. The fluorescent images of the foams were observed under the microscope. After drying at 80 °C, the powders and foams were coated with a thin gold film and imaged using a JEOL JSM-7600F scanning electron microscope.

obtained for LDH dispersions in present of NaCl or sodium acetate are reported in Fig. 2. In fact, the air/water interface presents very small values of E at low salt concentrations. Upon increasing the salt concentrations, the dilational viscoelasticity modulus appreciably increased. But at high concentrations, the values of E slightly decreased. At high salt concentration, the size of the flocs is so large that gravity cannot be neglected, and the adsorption of particles at the interface becomes difficult. The macroscopic mechanical properties of liquid interfaces can be strongly affected by the attachment of particles, significantly contributing to stabilization of foams and emulsions. Similar effects have been observed in studies concerning surfactant systems typically used for foam stability, where the formation of surface condensed phases, or solidlike aggregates, confers a strongly elastic character to the interfacial layer [35–37]. Low elasticity makes the interface film less stable and it easily collapses. Film stability is influenced by high surface elasticities, indicating that the gel-like layer at the interface inhibits film drainage and rupture [38]. Unlike the oil–water systems [32], we cannot get stable foams at any NaCl and sodium acetate concentrations. The formation of stable foams requires the adsorption of solid LDH particles on the freshly incorporated air bubbles. In order to enable their adsorption at the air/water interface, particles with a partially hydrophobic surface are needed. In the case of alumina [25], hydrophobization can be achieved by modifying the particle surface with short-chain carboxylic acids that adsorb with the carboxylate group onto alumina, leaving the hydrophobic tail in contact with the aqueous solutions. Although the addition of NaCl or sodium acetate promotes the adsorption of LDH particles onto the oil–water interface, we cannot get stable foams because they cannot modify the particles into optimum hydrophobicity. Therefore, we have chosen to investigate sodium butyrate as the modifier. We have prepared stable foams with sodium butyrate and LDH mixtures. The adsorption of sodium ions onto the LDH particle surfaces is first discussed, and then the properties of foams stabilized by a mixture of sodium butyrate and LDH particles are described. 3.2. Properties of LDH/sodium butyrate dispersions Adsorption isotherms can illustrate the adsorption behavior of surfactants on the solid particles. Fig. 3 shows the adsorption behavior of sodium butyrate on Ni/Al LDH surfaces. Obviously, the adsorption isotherms are characteristic of the Langmuir type, which is typically used to describe the adsorption behavior of species that adsorb strongly on the surface at small concentrations

20

3.1. The adsorption of LDH particles onto the air–water interface in the presence of salts

15

The dilational rheological properties of the LDH particles interfacial layers at the air–water interface were investigated by measuring the dilational viscoelasticity. To show the effect of salts on the adsorption of the LDH onto the air–water interface, the systems were investigated with and without salts in the LDH dispersions. Preliminary tests on 0.075 wt.% LDH dispersions, without salt, showed that no dilational viscoelastic effects were introduced by the LDH particles alone. This result definitely confirms that these LDH particles do not interact with the fluid interface. The dilational viscoelasticity modulus was measured for LDH dispersion/air systems at 0.075 wt.% particle concentration and different concentrations of salts (NaCl or sodium acetate). The measured values

Mod(E) (mN/m)

3. Results and discussion

NaCl Sodium acetate Sodium butyrate

10 5 0 1E-3

0.01

0.1

1

Concentration of Salt (M) Fig. 2. Dilational viscoelasticity modulus versus salt concentrations. The concentration of the LDH particles is 0.075 wt.%.

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Adsorbed amount (mmol/g)

10 8 6 4 2 0 0

20 40 60 80 100 Concentration of Sodium Butyrate (mM)

Fig. 3. Adsorption isotherms of sodium butyrate on LDH particles at 25 °C. All measurements were obtained from 1.0 wt.% LDH dispersions at pH 9.5–10.6.

and reach a saturated condition at higher concentrations. At low equilibrium concentrations of sodium butyrate, the molecules lie flatly on the particle surface. As the sodium butyrate concentration increased, due to a strong electrostatic attractive interaction, the molecules quickly cover the surfaces of the particles until complete monolayer coverage is reached. The zeta potential of colloidal particles in aqueous dispersion can also reflect the adsorption of sodium butyrate molecules on the surface of the solid particles. The original particles are strongly positively charged at about 48 mV. With the increase of sodium butyrate concentration, the adsorption of negatively charged carboxylate ions onto the LDH surface screens the positive surface charge and the magnitude of the zeta potentials dramatically decrease (as shown in Fig. 4). With further increase of the sodium butyrate concentration, the adsorption isotherms reach a plateau. That is to say the adsorption reaches a saturated state. A plateau value of zeta potentials is reached at about 15 mV for 50 mM of sodium butyrate. There is no evident change of potential and no charge reversal occurs. Therefore, addition of sodium butyrate beyond 50 mM led to strong coagulation of the particles as a result of van der Waals and hydrophobic attractive forces. Both the adsorption isotherms (Fig. 3) and the zeta potential measurements (Fig. 4) confirm that sodium butyrate adsorbs on the particle surface. Lyophobization occurs as a result of the relatively strong interaction between the anchoring group and the particle surface, thus leaving the amphiphiles’ hydrophobic tail in contact with the aqueous solution.

When the butyrate group is adsorbed, the interlayer spacing does not change appreciably, from the 0.77 nm spacing of Ni/AlLDH. This indicates that butyrate anions can be adsorbed only on the exterior surface of the LDH at all concentrations. The addition of high concentrations of sodium butyrate did not invert the zeta potential sign nor change the LDH particles isoelectric point, confirming that the butyrate anions adsorb as counter-ions rather than as specific adsorbing species around the particles. Therefore, the electrical potential of LDH particle surface remains constant at about 15 mV upon addition of amphiphiles or electrolyte. On the other hand, the zeta potential is strongly influenced by the concentration and valence of the counter-ions in the diffuse layer that screen the particle surface charge. Two factors can lead to the reduction of zeta potential. First, the particle surface charge is screened by molecules adsorbed on the particle surface [39]. As a result of this screening effect, the thickness of the electrical double layer around the particle is reduced. When the particle surface is completely covered with butyrate groups, the potential values will remain constant. Second, the adsorption of butyrate groups on LDH particles creates clusters of bridging flocs having less mobility compared to small particles, which can also result in a decrease of zeta potential [40]. The stability of the aqueous dispersions of positively charged LDH particles (1.0 wt.%) was monitored 24 h after mixing with negatively charged butyrate ions. The appearances of the LDH/sodium butyrate mixed dispersions are shown in Fig. 5a. In the absence of sodium butyrate, LDH particles in water are stable as an aqueous dispersion. At a low sodium butyrate concentration, the size of the LDH particles becomes larger (as it is shown in Fig. 5b), but there is no sediment. At higher sodium butyrate concentration, the particles can form sediment after 24 h. With further increase in sodium butyrate concentration, the sediment becomes more

a

b 50

3500 3000

Diameter (nm)

Zeta Potential (mV)

40 30 20

2500 2000 1500 1000

10 500

0

0 0

-10

0

20

40

60

80

100

Concentration of Sodium Butyrate (mM) Fig. 4. Zeta potential of 1.0 wt.% LDH particles in water as a function of the initial sodium butyrate concentration.

20

40

60

80

100

Concentration of Sodium Butyrate (mM) Fig. 5. (a) Appearances of the mixed dispersions of LDH (1.0 wt.%)/sodium butyrate at different initial concentrations (given in the figure, mM), 24 h after preparation and (b) effect of initial sodium butyrate concentration on median LDH particles diameters.

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and more compact. This change comes from the surface adsorption of sodium butyrate on the LDH particles, which is reflected by the zeta potential. The LDH particles can be well dispersed in water in the absence of sodium butyrate due to a relatively high positive zeta potential. Upon adding sodium butyrate, the potential gradually decreases in magnitude, butyrate anions electrostatically adsorb on the LDH surfaces mainly as a monolayer with their hydrocarbon chains exposed to solution. This configuration can screen the electrostatic repulsion between particles, and thus phase separation occurs.

3.3. Foams stabilized by a mixture of sodium butyrate and LDH particles Fig. 6 shows the appearances of foam systems stabilized by the mixed dispersions of LDH/sodium butyrate. The attachment of particles to the air–water interface enables the stabilization of air bubbles and consequently the formation of foams upon hand-shaking for 30 s. The LDH particles alone are poor foam stabilizer at any dispersion concentration. The positively charged LDH particles are very hydrophilic, and can hardly stay on the air–water interfaces due to low attachment energy. Also, for the sodium butyrate alone, there was no foam. With low concentrations of sodium butyrate added to the LDH particles, the particles are not sufficiently hydrophobic to adsorb to the air–water interface and are therefore not able to stabilize air bubbles. With increasing sodium butyrate concentration, particles become sufficiently hydrophobic to adsorb at the surface of freshly incorporated air bubbles. Foams stable to coalescence and disproportionation are formed, although they release water through drainage with time. At low sodium butyrate concentration range, particle hydrophobicity increases as the sodium butyrate concentration increases. Hence, the concentration of particles increases on the foam surface and there are more particles in the plateau border. The drained liquid became clearer and more transparent. However, when adsorption is saturated, the hydrophobicity does not change and most of the particles are in the foam phases. The decrease of air content in the foam at high amphiphile concentrations can be attributed to an increase in the suspension viscosity that ultimately hinders air incorporation by mechanical frothing. Distinct non-spherical bubbles, with diameters of 50–200 lm, are a feature of these systems. Their surfaces are rough as a result of ripples. Similar ripples have been observed in the case of a monolayer of silica nanoparticles at the planar air/water interface after compression which suggests that the bubbles are covered with dense particle layers compressed to a high surface pressure

Fig. 6. Photograph of vessels containing foams stabilized by a mixture of LDH particles and sodium butyrate at different initial concentrations (given in the figure, mM), 24 h after preparation. The particle concentration is fixed at 1.0 wt.%.

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that is close to the surface tension of water. Stable bubbles are probably formed by coalescence between smaller bubbles that are covered with dilute particle layers during foam formation. As the bubble area decreases, excess particles cannot be released as they are irreversibly adsorbed and so the surface corrugates to increase in area. As LDH particles in water and bare air–water surfaces are positively charged, it is anticipated that addition of salt to water should enhance the transfer of particles to the surface by reducing the energy barrier to adsorption. The particle contact angle may also increase as hydrophobicity increases, and both of these factors leading to improved stabilization of the foams. In addition, it should be noted that in both foam systems the change of separated aqueous phase from aqueous dispersion to clear water shows that increasing the sodium butyrate concentration promotes the adsorption of the LDH particles and eventually all the particles adsorb at the air–water interface or combine with the formed foams. The optical microscopy images in Fig. 7 are revealing in terms of changes in the structure of these foams at low (25 mM) and high (50 mM) salt concentrations. At low (25 mM) salt concentrations, the mean droplet size of the foam prepared by LDH is large. At high (50 mM) concentrations, the droplet size decreases as the foam’s stability to drainage increases. Above this concentration, the droplet size and the foam’s ability to resist drainage remain essentially unchanged. To confirm the adsorption of Ni/Al LDH particles at the bubble surface, laser-induced confocal scanning microscopy experiments images in Fig. 8 were taken of foams stabilized by sodium butyrate-modified LDH. The fluorescently labeled sodium butyratemodified LDH particles can be seen at the surfaces of the bubbles. In addition, flocculation is observed clearly between the armored bubbles (1) and the unadsorbed particles in the surrounding con-

Fig. 7. Optical microscope images of foams stabilized by: (a) LDH/sodium butyrate (25 mM) and (b) LDH/sodium butyrate (50 mM), immediately after preparation. The LDH concentration in all aqueous dispersions is 1.0 wt.%.

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by a translucence thin film. The dry bubbles changes foam structure from an individual ‘bubble’ type to a typical polyhedral type structure. At high (100 mM) salt concentrations, after drying, the foams look like the original fluffy fumed silica and the SEM image of the inner surface of the material in Fig. 9b shows air cells of size comparable to the original bubbles, separated by fused particle aggregates. The image show that the air bubbles have preserved their spherical shape and have not collapsed during the drying process, which indicates that the surfaces of these bubbles are very rigid. From the SEM results, we can see that increasing the sodium butyrate concentration there are more particles adsorbed on the bubble surface. This is consistent with the stability observations. Fig. 9c shows the interface containing close-packed particles. The bubbles are armored by an interfacial particle multilayer. 4. Summary

Fig. 8. Confocal fluorescence microscope images of foam stabilized by: (a) LDH/ sodium butyrate (25 mM) and (b) LDH/sodium butyrate (50 mM), immediately after preparation. The LDH concentration in all aqueous dispersions is 2.0 wt.%.

tinuous phase (2). The presence of particles packing at the plateau border interface will increase the rigidity, surface viscosity and the strength of the foam structure. More unadsorbed particles are seen in the surrounding continuous phase at low (25 mM) salt concentrations than at high (100 mM) salt concentrations. The microstructures of the foams at different sodium butyrate concentrations after drying are also given in Fig. 9, using scanning electron microscopy (SEM). At low (25 mM) salt concentrations (Fig. 9a), in the dried state, the surface of the foams was covered

Although the addition of salt promotes the adsorption of LDH particles onto the air–water interface, at all the concentrations of NaCl and sodium acetate, we cannot prepare stable foams. The behavior of foams stabilized by a mixture of positively charged particles and sodium butyrate was investigated. The presence of sodium butyrate is crucial for the formation and stability of foams. Based on the results from dispersion stability measurements, particle zeta potentials, the adsorption isotherm of sodium butyrate on the particles and the adsorption of the modified particles on the air–water interface, we conclude that foams are most stable when particles are strongly flocculated corresponding to possessing a low charge and being appropriately hydrophobic. The size of the bubbles decreases with the increase of the concentration of sodium butyrate. The morphology of the bubbles was monitored by optical microscopy, confocal fluorescence microscopy, and SEM. The results show the interface containing close-packed particles. The bubbles are armored by an interfacial particle multilayer. Acknowledgments The authors thank Prof. Xusheng Feng and Dr. Pamela Holt (Shandong University) for help in preparation of the manuscript. References

Fig. 9. SEM images of foams stabilized by butyrate anions-modified LDH particles at low and high sodium butyrate concentrations. The initial sodium butyrate concentrations are (a) 25 mM (b) 100 mM (c) the interface containing close-packed particles.

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