Synergistic action of TiO₂ particles and surfactants on the foamability and stabilization of aqueous foams

Abstract

Small particles can be activated via a synergistic effect with surfactants and adsorbed to the air–water interface to generate and stabilize foams, which has been applied extensively to develop new materials and techniques. Here, we studied the synergistic effects of TiO₂ particles with ionic surfactant SDS (sodium dodecyl sulfate), ionic AOT (sodium di-2-ethylhexylsulfosuccinate), and cationic CTAB (cetyltrimethylammonium bromide) in aqueous solution, and their impacts on the foamability and foam stability were assessed by measuring the foam volume and observing the number of particles adsorbed onto the foam surface. The results showed the interactions of TiO₂ particles with SDS and CTAB surfactants were synergistic in both foamability and foam stability. The degree of synergy of the CTAB–TiO₂ mixed system was stronger than that of the SDS–TiO₂ mixed system as a whole. However, the interaction of TiO₂ with AOT (a double carbochain anionic surfactant) in the TiO₂–AOT mixed system was generally not synergistic. Unexpectedly, a maximum synergistic effect among these mixed systems occurred in the TiO₂–AOT system at an AOT concentration of approximately 5 mM. This study provides further understanding for the mechanism of foaming and stability of foam modulated by surfactant and colloidal particles and provides a useful reference for future applications.

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Introduction

Foams stabilized with particles in aqueous surfactant solutions have brought significant technological impacts, including particles assembling at the air–liquid interface to form dry water (water in air)^1–3 and liquid marbles (large liquid droplets encapsulated in air),^4–6 as well as colloidosomes⁷ and anisotropic particles.⁸ Furthermore, this technology has been applied in industrial processes extensively. Oil recovery is enhanced using foam behavior mediated by the interaction between nanoparticles and mixed surfactant in brine solution.⁹ Wastewater is cleared by removing hydrophobic contaminations that attach to air bubbles.¹⁰ Porous ceramics and porous metals are fabricated using particle-rich foam precursors.¹¹ Therefore, a comprehensive understanding to the mechanism of foamability and foam stability mediated by surfactant and particles in aqueous solution would be beneficial to further practical applications.

Small particles (nanometres to several micrometres), with a suitable wettability at their surfaces, can be strongly attached to liquid–vapor or liquid–liquid interfaces, and thus behave as the foam or the macroemulsion stabilizers, respectively.^12–14 This process results in irreversible adsorption,^1,15 unlike that of surfactant molecules with adsorption and desorption on a rapid timescale. And this adsorption, namely, small particles accumulated at the two-phase interface need much higher adsorption free energies than the thermal energy kT.¹⁵ Usually, particles promote their adsorption free energy via the activation of surface or the modification of surface wettability, which can be characterized by the contact angle of the particle at the aqueous side of the interface. There are three approaches to obtain higher surface activation for small particles or change their surface wettabillity in general. The first is to prepare “Janus” particles with asymmetric surface chemistry, i.e., regionally hydrophilic on one side and hydrophobic on the other side.^16,17 The “Janus” particles, however, involve complicated preparation processes and are difficult to produce in large quantities. Another solution is to modify the wettability of the non-surface-active or low-surface-active small particles using a homogeneous surface coating, such as, modifying SiO₂ from extremely hydrophilic to very hydrophobic,¹⁸ but the high cost of activating particles makes this method less commercially viable. The third is to use the interaction of amphiphilic compounds with particle to make surface active.^19,20 Surfactant, as typical amphiphilic compound, is usually added to solutions to change the activation of particles by the adsorption of surfactant molecules onto particle surface. This method is comparatively less complicated and less expensive, and is therefore practically significant.

Recently, Binks et al.²¹ used surfactant SDS (sodium dodecyl sulfate) to successfully achieve the inversion of dry water to aqueous foam by changing the activation of silica surface particles in aqueous system, whereby the hydrophobic surface of the particle was converted into hydrophilic surface by adsorbing SDS molecules. AOT, an ionic surfactant with double carbon chain, was used as a usual charge control agent for particles charging in apolar dispersions^22,23 is considered for the activation of CaCO₃ nanoparticles and investigated their ability to stabilize aqueous foam.²⁴ Mineral oxide nanoparticles and cationic CTAB were investigated for their synergistic effect on stabilization of CO₂ foam for the application in enhanced oil recovery.²⁵ In addition, the mechanism of particle stabilizing aqueous foams was revealed by the synergy between SiO₂ particles and surfactants at different length scale.²⁶ Langevin and co-workers also studied the mechanism of foam stabilization based on the interaction of SiO₂ particles and surfactant in the mixed aqueous solution.²⁷ Nevertheless, although these works have described the surface of particle activated by the adsorption of surfactant to stabilize aqueous foams at the interface, the investigated objects mainly focused on anionic surfactant (SDS in particular), and SiO₂ particles but less involved for other surfactants and particles. As a common material, TiO₂ particles have very important applications in food, cosmetics, medicine and functional materials,^28–30 while the studies of TiO₂ particles in stabilizing aqueous foam are relatively inadequate. Therefore, the systematic investigation of the surface activation of TiO₂ particles by interaction with different surfactants in aqueous solution is advantageous to further understand the mechanism of particle stabilizing aqueous foams.

In this paper, we investigated the surface activation of TiO₂ by the interaction with anionic surfactants SDS, AOT (sodium di-2-ethylhexylsulfosuccinate), and cationic surfactant CTAB (cetyltrimethylammonium bromide) in aqueous solution. The synergistic interactions between TiO₂ particles and SDS, AOT, and CTAB in both foamability and foam stabilization were analyzed by measuring volume of foam and observing the adsorption of particle on the surface of foam. Possible reasons for the discrepancy of these synergistic effects of particles with these surfactants were suggested. The aim of this paper was to obtain a further insight into the activation of the particle via surface active agent to stabilize aqueous foam at water–air interface. Meantime, the study would also provide the references for potential applications of TiO₂-surfactant aqueous systems in relevant industries.

Methods

Materials

TiO₂ particles with a primary particle diameter of 110 nm were supplied by Sigma-Aldrich. The particles were originally delivered as a dispersion in deionized water. Obtaining dry particles needed a dehydration process through a multistage solvent swap using ethanol as an intermediate solvent, followed by vacuum drying at 30 °C. Fig. 1 shows a scanning electron microscopy (SEM, Quanta 200, FEI) image of a dried aqueous dispersion of the particles, where the particles fused with each other and formed micron-sized agglomerates. SDS and CTAB (each 99% of purity) were from Shanghai Geneland Biotech Co., Ltd., China, and AOT of 99% purity was the commercial production of Sigma-Aldrich. All surfactants were used as received. Deionized water with a resistance of 18.2 MΩ cm at 25 °C was obtained using a deionized water system (LDD-01, Ludao, Shanghai, China).

Fig. 1 SEM image of TiO₂ particles dispersed in pure water after drying.

Preparation of aqueous surfactant dispersions without and with TiO₂ particles

A series of different concentrations of surfactants (0.01–50 mM) were dispersed in deionized water. The dried TiO₂ particles were weighed and added into these aqueous surfactant solutions. The mixed solutions without and with particles were respectively sealed into 50 ml beaker and then dispersed using constant temperature magnetic stirrer (85-2, Changzhou, China) at 2400 rpm for 2–5 min.

Preparation and measurement of aqueous foams

The aqueous surfactant dispersion without TiO₂ particles (10 ml) was transferred to a 100 ml cylindrical graduated flask. The flask was stoppered and then shaken up and down vigorously about 30 times. The foam volume immediately after shaking and 30 min after shaking were recorded as V₀ and V₃₀, respectively. Similarly, a 10 ml dispersion of aqueous surfactant solution with TiO₂ particles was placed into a 100 ml cylindrical graduated flask with a stopper, and the same procedure was repeated, except shaking was performed 50 times to facilitate full interaction between particles and surfactants. And foam volume immediately after shaking was recorded as V_0P, and that 30 min after shaking was as V_30P. V₀ and V_0P were taken as the measure of foamability of aqueous surfactant solution without and with particles, respectively. V₃₀ and V_30P were the measure of foam stability of aqueous surfactant solution without and with particles, respectively. These experiments were repeated three times.

Characterization of aqueous foams with optical microscopy

To observe foams stabilized by a surfactant alone and by a combination of TiO₂ particles and surfactants, each 5 ml dispersion was transferred to a 10 ml bottle and aerated at 5000 rpm for 5 min using a YSDF-400 centrifuge (Yuesheng, Shanghai). The foam was removed and placed on a microscope slide (without coverslip), and micrographs were taken (before moisture evaporated) using a Motic BA400 microscope system (Motic, Xiamen).

Results

Fig. 2 showed foam volumes, as the function of surfactant concentration, in aqueous solution with SDS alone immediately after shaking (V₀), SDS alone 30 minutes after shaking (V₃₀), SDS plus TiO₂ immediately after shaking (V_0P), and SDS plus TiO₂ 30 minutes after shaking (V_30P), the particle concentrations of which were 0.01% (Fig. 2a) and 0.5% (Fig. 2b). Foam volumes were all small at SDS concentrations below 0.5 mM with and without TiO₂ particles in Fig. 2. Beyond this concentration, the foam volumes with SDS alone (V₀ and V₃₀) increased quickly at first, and reached a maximum peak value near the concentration of 8 mM, which is the critical micelle concentration (CMC) of SDS in pure water,³¹ then decreased with further increase of SDS concentration. For the TiO₂ particle concentration of 0.01%, foam volumes of V_0P and V₀, as well as V_30P and V₃₀, showed similar trends. Beyond 8 mM, the foam volumes of V_30P were less than those of V₃₀. However, for higher particle concentration of 0.5% (see Fig. 2b), the curves of V_0P and V_30P monotonously increased with the increase of SDS concentration, which was obviously different from the corresponding curves of 0.01% particle concentration. The V_0P and V_30P values were larger than V₀ and V₃₀ values in the entire range of surfactant concentrations, except near the CMC of SDS, where the values of V₀ and V₃₀ were the highest. Thus, we initially inferred that TiO₂ particles can affect foamability and foam stability in SDS aqueous solutions and this influence was strengthened as particle concentration increased. The curves of V₀ and V₃₀, as well as those of V_0P and V_30P, almost overlapped in Fig. 2b, which denoted the systems of SDS-aqueous solution without and with TiO₂ particles both had strong foam stabilities in water.

Fig. 2 Foam volumes without and with TiO₂ particles were shown at different SDS concentration. (a) Particle concentration of 0.01%, where the standard deviations for V₀, V₃₀, V_0P and V_30P were ±2.8 cm³, ±2.8 cm³, 2.5 cm³, and ±2.0 cm³, respectively. (b) Particle concentration of 0.5%, where the standard deviations for V_0P and V_30P were each ±3.0 cm³.

The four foam volumes were also measured with 0.01% and 0.5% of TiO₂ particles dispersed in AOT-aqueous solution at different AOT concentrations compared with AOT alone, as shown in Fig. 3. The value of AOT alone (V₀) increased with the increase of AOT concentration, while the value of AOT alone (V₃₀) showed three phases, i.e., going up with the increase of AOT concentration from 1 mM to 2 mM, rapidly falling from 2 mM to 5 mM, then quickly rising again above 5 mM. And the CMC of AOT in pure water is just near 2 mM.³² With 0.01% of TiO₂ particles dispersed AOT-aqueous solutions, the values of V_0P and V_30P showed similar changes as those observed for the corresponding V₀ and V₃₀ values, and the values of V₃₀ and V_30P were generally smaller than those of V₀ and V₃₀, which was similar to that observed for the TiO₂–SDS mixed aqueous solutions. When particle concentration was increased to 0.5%, the values of V_0P and V_30P were still no larger than those of V₀ and V₃₀, except from 5 mM to 10 mM where V_30P was beyond V₃₀, which were obviously different from those of TiO₂–SDS solution at same particle concentration of 0.5%. Therefore, it was indicated that TiO₂ particles did not provide a strong active effect on foamablity and foam stability as a whole in AOT aqueous solutions.

Fig. 3 Foam volumes without and with TiO₂ particles were shown at different AOT concentration. (a) Particle concentration of 0.01%, where the standard deviations for V₀, V₃₀, V_0P and V_30P were ±3.2 cm³, ±2.3 cm³, ±3.1 cm³, and ±2.2 cm³, respectively. (b) Particle concentration of 0.5%, where the standard deviations V_0P was ±3.0 cm³ and that of V_30P was ±2.6 cm³.

Considering the influence of the low particle concentration of 0.01% on the foam volumes above, V_0P and V_30P were measured only with 0.5% of TiO₂ particles dispersed in aqueous solutions of CTAB, and the results were given in Fig. 4. The values of V₀ steadily increased with increasing CTAB concentrations, where the minimum concentration of CTAB is required to stabilize foams approached 0.9 mM, which is its CMC in water.³³ When TiO₂ particles were added into CTAB aqueous solutions, the values of V_0P and V_30P presented remarkable increases beyond the CMC, which were much higher than the values of V₀ and V₃₀ in the whole range of CTAB concentration, although the values slightly decreased above 10 mM. Similar to the foam volumes of the mixed system of SDS plus TiO₂ in Fig. 2b (0.5% of TiO₂), the curves of V_0P and V_30P almost overlapped, implying that TiO₂ particles, with the participation of CTAB, provided an excellent stabilizing effect on the foam at the air–water interface.

Fig. 4 Foam volumes without and with TiO₂ particles were shown at different CTAB concentration. Particle concentration was 0.5%, where the standard deviations for V₀, V₃₀, V_0P, and V_30P were ±2.5 cm³, ±2.4 cm³, ±2.8 cm³, and ±2.8 cm³, respectively.

Using S_f% = (V_0P − V₀) × 100%/V₀ to denote the degrees of foamability (shaken immediately with surfactant alone (V₀) and surfactant plus TiO₂ particles (V_0P)) and S_sf% = (V_30P − V₃₀) × 100% to denote foam stability (shaken after 30 min with surfactant alone (V₃₀) and surfactant plus TiO₂ particles (V_30P)), we further studied the effect of interaction of TiO₂ particles with surfactants on the foamability and foam stability in aqueous solutions. The values of S_f and S_sf above zero indicated the interaction of particles with surfactants was synergistic, otherwise the interaction was antagonistic. The relations of S_f and S_sf with surfactant concentration were shown in Fig. 5, where the data used to calculate S_f and S_sf were from Fig. 2b, 3b and 4. The S_f and S_sf values for TiO₂–CTAB were higher than those for TiO₂–SDS from 0.5 mM to 10 mM; however, this was reversed when surfactant concentration was beyond 10 mM. Therefore, the synergistic effects of TiO₂ and CTAB were stronger than those of TiO₂ and SDS at moderately higher surfactant concentrations.

Fig. 5 Effect of surfactant concentration on (a) S_f (foamability) and (b) S_sf (foam stability), calculated using the data in Fig. 2b, 3b and 4, for TiO₂ particle plus SDS, AOT and CTAB systems, respectively.

Discussion

For the SDS–TiO₂ mixed aqueous solution (Fig. 2b), foam volumes for SDS alone were small at surfactant concentrations below 0.2 mM, the cause of which may be fewer free surfactant molecules that would be unable to form a sufficiently dense network at the air–water interface surrounding the bubbles to prevent coalescence. With the increase of surfactant molecules, the curves of V₀ and V₃₀ presented para-curve with the maximum values near the CMC of 8 mM. The ascending part of the curves was due to the presence of more surfactant molecules that spread a dense layer at air–water interface to prevent the drainage between bubbles, and the descending of the curves was linked to the increase in the lifetime of the micelles with concentration, leading to the reduction of the supply of monomeric SDS molecules required for the stabilization of the fresh bubble at the interfaces.^34,35 As was observed, at low concentrations, i.e., from 0.2 mM to 0.5 mM, the foam stabilization mainly depended on SDS molecules and TiO₂ particles only had a slight effect on foams. Beyond 0.5 mM, the values of V_0P and V_30P were obviously larger than those of V₀ and V₃₀ (excluding those values near the CMC of SDS) at the same concentration, the improvement of which just came from the synergistic effect between SDS molecules and TiO₂ particles. The hydrophilic headgroups of SDS molecules were adsorbed onto the surfaces of TiO₂ particles and hydrophobic carbochains were directed toward air during the interaction, rendering the surfaces more hydrophobic, and thus they were adsorbed easily onto the surfaces of foams as shown in Fig. 6b. Therefore, the V_0P and V_30P curves nearly monotonously ascended with the increase of SDS concentration in Fig. 2b.

Fig. 6 Optical micrographs of foams were stabilized by surfactant alone and by a mixture of TiO₂ particles and surfactant, taken 30 min after aeration. The first set: (a) 1 mM SDS and (b) 0.5 wt% TiO₂ + 1 mM SDS. The second set: (c) 5 mM AOT and (d) 0.5 wt% TiO₂ + 5 mM AOT. The third set: (e) 10 mM CTAB and (f) 0.5 wt% TiO₂ + 10 mM CTAB.

As for the AOT–TiO₂ mixed system (Fig. 3b), the values of V₀ monotonously increased from 1 mM and were higher than the corresponding values for SDS in aqueous solution, which accounted for a double chain anion surfactant being more easier to aggregate at interface than its single chain analogues. Unlike the curves of SDS alone and CTAB alone (see Fig. 2b and 4), the foam curves of V₀ and V₃₀ of AOT showed an abnormal fall of V₃₀ in the range of 2 mM to 5 mM where was just the initial formation of AOT micelles,³² which might be the cause for the decrease of V₃₀. However, at this concentration region, adding TiO₂ particles into AOT aqueous solution, it caused the value of V_30P to quickly increase, even exceeding that of V₃₀. Additionally, the value of S_sf near 5 mM reached a maximum value among the three mixed systems, which was certified in Fig. 6d where the number of particles adsorbed onto the surface of foam was significantly higher than that in Fig. 6b and f. Why did such a strong synergistic effect between TiO₂ and AOT in foam stability occur in the range of 2 mM to 5 mM? In relation to the abnormal fall in foam stability of AOT alone, we guessed that TiO₂ particles became so hydrophobic in this phase because of the electrostatic interaction with AOT hemimicelles or quasimicelles. Because these hemimicelles or quasimicelles were not closed, their charged hydrophilic head groups would adsorb onto TiO₂ particles by electrostatic interaction with counter charges of the surface of TiO₂ particle, and bulky hydrophobic tails would extend to the interface, causing TiO₂ particles easily to adsorb onto the surface of the foam to prevent coalescence, which was described in Fig. 7. After AOT formed stable-closed micelles at AOT concentration above 5 mM, the hydrophilic head groups of the outer micelles lead to particles desorbing from the air–water interface.

Fig. 7 Schematic representation of the likely cause was responsible for the strengthening synergistic effect between TiO₂ and AOT in foam stability in the region of 2 mM to 5 mM of AOT concentration, where TiO₂ particles were easily adsorbed onto the surface of foam.

Different from the interaction of CaCO₃ and CTAB, which was no synergistic effects in aqueous solution,²⁴ a strongly synergistic action occurred between TiO₂ and CTAB in both foamability and foam stability, as showed in Fig. 4 and 6f. The foam volume plus TiO₂ particles was much larger than that of CTAB alone above CMC (Fig. 4). And, the values of S_f and S_sf were all larger than zero in Fig. 5. Moreover, the foamability and foam stability of the TiO₂–CTAB mixed system were superior to those of the TiO₂–SDS mixed system in a wide range of surfactant concentration, i.e., the surfaces of TiO₂ particles, due to the interaction with cationic CTAB surfactant, were more hydrophobic than those with anionic SDS surfactant. Besides, the surface of SiO₂ was also activated by CTAB in aqueous system, where the hydrophobicity of particles were enhanced by the adsorption of the hydrophilic headgroup of the molecule onto the particle surface while hydrophobic chain is directed towards interface.²¹ When the concentration was above 10 mM, a second layer of surfactant gradually formed on the surface of TiO₂ particles by chain–chain interactions with the first layer of molecules, which reduced the hydrophobicity of TiO₂. Therefore, the values of V_0P and V_30P for the TiO₂–CTAB mixed system began to decline with the increase of surfactant concentration from 10 mM. Contrastively, the foam values of V_0P and V_30P of the TiO₂–SDS mixed system monotonously increased with the increase of SDS concentration, implying that only a single layer of surfactant molecules was adsorbed onto the surface of TiO₂ particles, even at higher SDS surfactant concentrations.

Conclusion

The synergistic effects of TiO₂ particles with surfactants SDS, AOT, and CTAB were studied by measuring foam volumes in aqueous solutions with surfactant alone and surfactant-TiO₂ mixed systems, calculating the degrees of the foamability S_f and foam stability S_sf, and observing the adsorption of activated TiO₂ particles on the surface of foams using optical micrographs. The results indicated the interaction between TiO₂ and SDS, as well as that between TiO₂ and CTAB, was significantly synergistic above the surfactant concentration of 0.5 mM; the effect of TiO₂ and AOT was only synergistic in foam stability near the CMC of AOT, and asynergistic in both foamability and foam stability at other concentrations. The system of TiO₂ and CTAB generally provided more synergistic effects than that of TiO₂ and SDS. However, the strongest synergistic effect occurred between TiO₂ and AOT near the concentration of 5 mM. AOT hemimicelles or quasimicelles at the CMC likely accounted for this case. Though SDS and CTAB surfactants were all advantageous to enhance the hydrophobicity of the surfaces of TiO₂ particles during the interaction, SDS molecules may be adsorbed as a single layer onto the particle surfaces, and CTAB molecules maybe formed a bilayer to reduce the hydrophobicity of particle at higher concentrations.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge the financial support of the Natural Science Foundation of Guangdong Province (2015A030310178), the National Natural Science Foundation of China (31401589), Zhanjiang Science and Technology Plan (2014B01068) and Guangdong Medical College Research Foundation (XB1355).

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