Color changing Pickering emulsions stabilized by polysiloxane microspheres bearing phenolphthalein groups

Abstract

The benzoxazine monomer bearing phenolphthalein and alkoxysilane groups was synthesized using phenolphthalein, 3-aminopropyltriethoxysilane, and paraformaldehyde as raw materials via a Mannich reaction. After hydrolysis and condensation of the alkoxysilane groups, the novel polysiloxane microsphere bearing phenolphthalein groups (PMP) was synthesized. Furthermore, a toluene-in-water Pickering emulsion using PMP as a particulate emulsifier was prepared. The existence of phenolphthalein and oxazine structures in PMP were confirmed by Fourier transform infrared (FTIR) spectroscopy. The average diameter of PMP was 208 nm as indicated by dynamic light scattering (DLS), and the PMP shape was spherical as revealed by transmission electron microscopy (TEM). Due to the introduced phenolphthalein structure, the stable toluene-in-water Pickering emulsion presented a color changing behavior within the pH range from 9 to 12. It also exhibited doubly pH-responsive properties: two emulsification/demulsification processes occurred at pH 9 and 12, respectively. Moreover, these processes were verified to be reversible: five and three demulsification/emulsification cycles were repeated with the aid of homogenization.

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Introduction

In Pickering emulsion systems, the particulate emulsifiers play a key role because they adsorb at the oil/water interface and stabilize the emulsion.^1–4 Moreover, the properties of Pickering emulsions can be tuned by adjusting the particle surface wettability.⁵ Therefore, the design and synthesis of particulate emulsifiers have long been a hot topic. A variety of particles have been extensively developed, such as temperature,^6–8 pH,^7–16 magnetic field,¹⁷ and CO₂ (ref. 16 and 18–20) responsive particles.

Among them, pH-responsive particle emulsifiers have been paid special attention due to their easy adjustment.^21–24 They are mainly categorized into two kinds. One kind is the particles containing amino groups on the surface. Amalvy et al. synthesized the polystyrene latexes using poly[2-(dimethylamino)ethyl methacrylate–methyl methacrylate] copolymer [PDMA–PMMA] as a steric stabilizer via dispersion polymerization. Hexadecane-in-water emulsion was prepared at pH > 5.6, whereas the emulsion demulsified at pH < 5.6 due to the protonation of tertiary amine groups in PDMA chains. Moreover, this “emulsification/demulsification” transition was found to be reversible with the aid of homogenization.⁹ Fujii et al. reported the poly(4-vinylpyridine)/silica (P4VP/SiO₂) nanocomposite microgels. Pickering emulsions were formed above the pK_a of P4VP chains, while these emulsions were rapidly broken when lowering solution pH below the pK_a.¹² Ma et al. prepared the polyurethane (PU)-based nanoparticles grafted with poly(2-(dimethylamino)ethyl methacrylate). Stable O/W emulsions were obtained in the pH 3–5 and pH 11–12 ranges, whereas at pH 8–9 phase inversion led to the formation of stable W/O emulsions.¹⁵ Armes et al. synthesized the poly(2-(diethylamino)ethyl methacrylate) latexes through emulsion polymerization. Due to the protonation of tertiary amine groups through CO₂ purging, the demulsification occurred. After removing CO₂ by purging N₂, Pickering emulsions were formed. Six successful emulsification/demulsification cycles were performed on this emulsion.¹⁶ More recently, Cui et al. prepared the SiO₂ particle absorbing switchable surfactant N′-dodecyl-N,N-dimethylacetamidine at its surface. By purging CO₂, octane-in-water emulsion was formed due to the protonation of amidinium surfactant, while the demulsification took place when N₂ bubbled into the system to remove CO₂.¹⁸ Ngai et al. synthesized the poly(N-isopropylacrylamide) microgels through surfactant-free emulsion polymerization. The obtained octanol-in-water Pickering emulsion was responsive to both pH and temperature changes.⁷

The other kind is the particles containing carboxylic acid groups on the surface. For example, Binks et al. reported the polystyrene particles coated with carboxylic acid groups. With ionized carboxylic acid groups, hexadecane-in-water emulsion was obtained at pH > 9.5, while phase inversion occurred at pH < 9.5.¹⁰ Stover et al. prepared the cationic alumina-coated SiO₂ nanoparticle bounded with potassium hydrogen phthalate (KHP). In the pH range of 3.5–7.5, the ionized carboxylic acid groups of KHP lowered the positive charges at the particle surfaces, and stable xylene-in-water Pickering emulsions were formed. These emulsions also had two emulsification/demulsification transitions at pH 3.5 and 5.5 respectively, exhibiting the so-called doubly pH-responsive property. Five reversible cycles were achieved at these pH values.¹³

It is well-known that the phenolphthalein is a very sensitive pH-responsive indicator.^25,26 Its unique property motivates us to incorporate phenolphthalein into particulate emulsifier. Thus, the Pickering emulsions owing color changing and sensitive pH-responsive properties are expected to be obtained.

Recently developed benzoxazine chemistry, which can combine various phenolic derivatives and amine derivatives into one compound via Mannich reaction in one pot,^27–29 offers a facile method to synthesize this new particulate emulsifier. Herein, phenolphthalein and 3-aminopropyltriethoxysilane were chosen as raw materials to perform Mannich reaction. After hydrolysis and condensation of alkoxysilane group, the novel polysiloxane microsphere bearing phenolphthalein group (PMP) was obtained. Toluene-in-water Pickering emulsion was prepared using PMP as emulsifier through homogenization. Furthermore, the color changing, pH-responsive, and reversible properties of Pickering emulsion were studied in detail.

Experimental section

Materials

Phenolphthalein, para-formaldehyde, sodium hydrate, and sodium bicarbonate were purchased from Kermel Chemical Reagent Co. Ltd (Tianjin, China). 3-Aminopropyltriethoxysilane was purchased from Yudeheng Fine Chemical Co. Ltd (Nanjing, China). Deionized water was purified using an Ultrapure water purifier. Toluene, ethanol, and hydrochloric acid were purchased from Fuyu Fine Chemical Co. Ltd (Tianjin, China) and were used as received.

Synthesis of benzoxazine monomer bearing phenolphthalein and alkoxysilane

To a 100 mL flask, 4.42 g (20 mmol) of 3-aminopropyltriethoxysilane, 1.2 g (40 mmol) of paraformaldehyde, 3.18 g (10 mmol) of phenolphthalein, 30 mL toluene and 10 mL ethanol were added. The mixture was stirred and refluxed at 80 °C for 5 h, followed by washing with 1 M NaHCO₃ for three times and deionized water for two times. Toluene and ethanol was removed through a rotary evaporator to afford the viscous yellow product 7.2 g (yield, 89%).¹H NMR (300 MHz, CDCl₃, ppm): 3.90 (Ar-CH₂–N), 4.82 (O–CH₂–N), 2.67–2.72 (N–CH₂–CH₂), 1.66–1.59 (Si–CH₂–CH₂), 0.58–0.65 (Si–CH₂–CH₂), 3.71–3.83 (Si–O–CH₂–CH₃), 1.15–1.27 (Si–O–CH₂–CH₃).

Synthesis of polysiloxane microspheres bearing phenolphthalein (PMPs)

To a 100 mL breaker, 0.5 g (0.62 mmol) benzoxazine monomer and 15 mL ethanol were added. The mixture was sonicated at the frequency of 20 kHz for 10 min. The obtained dispersion was then added dropwise to 100 mL deionized water. Finally the aqueous dispersion was condensed to 100 mL through rotary evaporation.

Preparation of toluene-in-water Pickering emulsion

To a 20 mL vial, 6 mL (0.5 wt%) PMP dispersion and 4 mL toluene was added. The pH of the dispersion was adjusted by adding HCl (0.1 M) or NaOH (0.1 M) solution to cover the range from 7.0 to 13.0. The mixture was then homogenized for 2 min using a 200 W AD200S-H homogenizer (Angni, China) with an 18 mm head and operating at 12 000 rpm.

Characterization

FTIR spectra were recorded on a Tensor 27 FTIR spectrometer (Bruker, US) in a range of 4000–500 cm⁻¹. Samples were prepared as KBr pellets with 1% weight content.

¹H-NMR spectra was recorded using a 300 MHz Avance-300 spectrometer (Bruker, US) with CDCl₃ as deuterated solvent.

DLS was conducted by a BI-200SM instrument (Brookhaven, US) equipped with a 200 mW green laser (λ = 532 nm). The suspensions were diluted about 1000 times with deionized water.

TEM images were recorded on a TEM-100VII instrument (Jeol, Japan). The sample was first diluted 10 times with deionized water, and then transferred onto a carbon-coated copper grid.

AFM images were recorded with a Dimension 3100 system (Bruker, US) using silicon cantilevers in tapping mode at nominal resonance frequency of 330 kHz. PMP dispersions were drop-cast on freshly cleaved mica surface and the water was dried before the measurement.

Zeta potentials were measured using a Zetasizer Nano ZS instrument (Malvern, UK). The solution pH was adjusted from 7 to 13 using 0.1 M HCl and 0.1 M NaOH.

Tests of the conductivity of continuous phases were measured with a DDS-307 digital conductivity meter (Leici, China). Conductivities above 10 μS cm⁻¹ indicated that the water was continuous phase (i.e., an O/W emulsion). In contrast, conductivities below 10 μS cm⁻¹ indicated that the oil was continuous phase (i.e., a W/O emulsion).

The drop tests were used to confirm the continuous phase indicated by the conductivity measurement. An emulsion droplet was placed into either toluene or deionized water. When the droplet was dropped into the same liquid as the continuous phase, it dispersed rapidly. If dropped into the liquid of the internal droplet phase, the droplet remained intact, with little or no dispersion.

The emulsion droplet diameter was measured by a Zetasizer Nano ZS instrument (Malvern, UK). The experiment was performed at 25 °C and the measurement for each emulsion was repeated at least three times.

Optical microscope images were recorded using a DP73 optical microscope (Olympus, Japan). The emulsions were diluted 5 times before the measurement.

CLSM images were recorded on a Fluoview 500 confocal microscopy (Olympus, Japan). Rhodamine B was used as a fluorescent probe for labeling the microspheres. The emulsions were placed on the cover glasses, and a series of x/y layers was scanned.

Results and discussion

Synthesis and characterization of polysiloxane microsphere bearing phenolphthalein (PMP)

Scheme 1 illustrates the synthetic route of PMPs. Phenolphthalein reacted with paraformaldehyde and 3-aminopropyltriethoxysilane at a molar ratio of 1 : 4 : 2 to form benzoxazine monomer bearing phenolphthalein and alkoxysilane groups via Mannich reaction. When the alkoxysilane group underwent hydrolysis and condensation reaction in water under ultrasonication, we observed that the constant formation of precipitations. After using ethanol as solvent, the stable PMP dispersion was obtained. Finally, the aqueous PMP dispersion was prepared by dropping PMP ethanol dispersion into water.

Scheme 1 Synthetic route of PMPs.

Fig. 1 shows the FTIR spectra of the PMPs before (a) and after (b) hydrolysis. After hydrolysis, the peak at 925 cm⁻¹ corresponding to the benzene rings attached with oxazine and 1766 cm⁻¹ corresponding to carbonyl groups in the phenolphthalein exhibited no obvious changes, indicating the retention of oxazine and phenolphthalein structures.^26,30 The peak at 1105 cm⁻¹ due to Si–O–Si bond increased and the peak at 3352 cm⁻¹ due to Si–OH group appeared, suggesting the hydrolysis and condensation reactions of alkoxysilane groups.³¹

Fig. 1 FTIR spectra of the PMPs before (a) and after (b) hydrolysis.

Fig. 2 shows the size distribution of PMPs. The average hydrodynamic diameter of PMPs determined by DLS was 208 nm. The particle dispersion index (PDI) was 0.128, indicating a relatively broad size distribution. The TEM image revealed that the synthesized particle was spherical with the average diameter of 201 nm, which was slightly smaller than that obtained from DLS. This discrepancy was reasonable since the measurements were conducted toward different states of dispersions, wet for DLS and dry for TEM.

Fig. 2 Size distribution of PMPs (inset: TEM image of PMPs).

Color changing property of PMP dispersions

To test whether the PMP dispersions had a color changing property or not, we investigated the color changing behavior of dispersions as a function of pH. Fig. 3 shows the digital photographs of phenolphthalein solutions (a) and PMP dispersions (b).

Fig. 3 Digital photographs of phenolphthalein solutions (a) and PMP dispersions (b) (concentration: (a) 0.21 wt%, (b) 0.5 wt%).

At pH 7.0 and 8.1, PMP dispersions exhibited its original white color (see Fig. 3b). When the pH increased to 9.0, the color became pink. The similar phenomenon was also observed for phenolphthalein solution (see Fig. 3a). This was explained by the structural change of phenolphthalein. It changed from the colorless non-conjugated lactone form (I) to the red conjugated quinonoid form (II).²⁶ As the pH increased from 9 to 13, the color of the dispersion turned gradually to deep red, similar to that of phenolphthalein solution. This was attributed to the increasing concentration of conjugated quinonoid form (II). When the pH reached 14, the dispersion color partially faded. This was because that the phenolphthalein structure changed from conjugated quinonoid form (III) to the non-conjugated carbinol form (IV).²⁶

It should be noted that the color window of PMP dispersions moved to slightly higher pH value than that of phenolphthalein solutions. This might be due to the relatively low phenolphthalein concentration on the surface of PMPs, despite of the fact that the concentration of phenolphthalein in PMP dispersions (0.5 wt%) was equal to that of the phenolphthalein solutions (0.21 wt%).

Doubly pH-responsive property of Pickering emulsions stabilized by PMPs

It is well known that the solely hydrophilic or hydrophobic particles are poor emulsifiers. Only the particles having appropriate surface wettability were able to stabilize Pickering emulsions.⁵ Fig. 4 shows the digital photographs and optical micrographs of toluene-in-water emulsions stabilized by PMPs after standing for 24 h. At pH 7.0 and 8.1, oil–water separations were observed.

Fig. 4 Digital photographs and optical micrographs of toluene-in-water Pickering emulsions stabilized by PMPs after standing for 24 h (toluene : water = 4 mL : 6 mL, PMP concentration: 0.5 wt%).

Considering the zeta potentials of PMP particulate emulsifiers in aqueous dispersion were 51.2 mV and 46.1 mV, respectively (see Fig. 5), PMP particles were too hydrophilic to stabilize Pickering emulsions.¹⁶ We think that those relatively high zeta potentials were caused by the protonation of tertiary amine groups in the benzoxazine structure.

Fig. 5 Variation of zeta-potential for PMPs with pH (concentration: 0.5 wt%).

When the pH increased from 8.1 to 9.0, the stable Pickering emulsion was formed. We attributed this to surface wettability change of PMP particles. With increasing pH, the tertiary amine groups in benzoxazine structure deprotonated. Meanwhile, the emulsion color changes indicated that the lactone rings of phenolphthalein were opened and anionic carboxylic acid groups (–COO⁻) were generated. Both factors lowered the positive charges at the particle surfaces, and reduced the excessively high hydrophilicity of PMPs. As a result, the PMPs' surface wettability became appropriate to stabilize Pickering emulsion.^18,32 This was evidenced by the decreasing of zeta potential for PMPs from 46.1 mV to 28.4 mV as the pH increased from 8.0 to 9.0 (see Fig. 5), indicating the increasing emulsifying capacity of PMPs.

When the pH increased from 9.0 to 12.0, the emulsion color changed from pink to deep red, exhibiting the similar color changing property to PMP dispersions. Optical micrographs of Pickering emulsions at pH 9.0–12.0 revealed the spherical dispersed droplets which existed in the typical emulsion (see Fig. 4). Both conductivity measurement and drop test confirmed that the emulsion type was O/W.

The characterization data of the emulsions stabilized by PMPs were listed in Table 1. The mean droplet diameters at pH 10.0 and 11.0 were slightly larger than those at pH 9.0 and 12.0, which was also suggested by the optical micrographs.

Table 1 The characterization data obtained for the emulsions stabilized by PMPs

pH	Conductivity of emulsions (μS cm^−1)	Mean droplet diameter^a (μm)
a Measured using a Malvern Zetasizer instrument.
9.0	43	16 ± 10
10.0	59	12 ± 8
11.0	120	10 ± 7
12.0	530	20 ± 13

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When the pH increased from 12.0 to 13.0, the demulsification phenomena occurred. This is because that the increasing amount of anionic carboxylic groups afforded PMPs more negative surface charges, and PMPs became too hydrophilic to stabilize emulsions.^13,18 Fig. 5 also illustrated that the zeta potential decreased from −32 mV to −52 mV, indicating the reappearance of excessively high hydrophilicity.

To sum up, Pickering emulsions stabilized by PMPs had two emulsification/demulsification processes at pH 9 and 12, respectively, namely the doubly pH-responsive property.^13,33

To further investigate the interfacial structure, CLSM measurements were performed. Fig. 6 shows the confocal microcopy images of Pickering emulsions at different pH values. Typical droplet images revealed that the aggregated microspheres adsorbed at the oil/water interface, rather than single microspheres. The investigation by Briggs and Lucassen showed that those weak flocculations formed by aggregated particles had better emulsifying capacity than single particles.^34,35

Fig. 6 Confocal microcopy images of Pickering emulsion droplets at different pH values (inset: AFM image of the particles at the same pH value, the z-scales of all images are 300 nm).

Interestingly, at pH 10.5 and 11.2, more aggregations were observed compared to those at pH 9.3 and 12.0. The inserted AFM images of PMP dispersions presented more clear aggregation morphologies. It seemed that PMPs in more aggregated states had better emulsifying capacity in our case, resulting in a smaller mean droplet diameter.^33,36 This was consistent with the droplet diameter data listed in Table 1.

Reversibility of Pickering emulsions stabilized by PMPs

Since the Pickering emulsions stabilized by PMPs exhibited doubly pH-responsive property at pH 9 and 12, respectively, we examined the reversibility of the emulsification/demulsification processes at those two pH values.

Upon addition of a small amount of 1 M HCl to 10 mL of an emulsion prepared at pH 9, the demulsification took place within 1 min. Upon addition of same amount of 1 M NaOH to the solution to adjust the pH to 9, stable emulsion was reconstituted after homogenization. The emulsification/demulsification process was very sensitive to pH changes. This was attributed to the pH-sensitive and reversible nature of the phenolphthalein structure. After five cycles, oil–water separation and the flocs in aqueous phase were observed. This was because that the build-up NaCl concentration led to extensive flocculation of PMPs.⁴

Similarly, when 1 M NaOH was added to a stable emulsion at pH 12, the demulsification also occurred within 1 min. Adding the same amount of 1 M HCl brought the pH back to the original value, and the emulsion was reformed again after homogenization. After three reversible processes, flocs were also found in the aqueous phase and no emulsions were formed again (Fig. 7).

Fig. 7 Variation of the emulsion droplet diameters of the reversible Pickering emulsion (pH changes: (a) between 9 and 8, (b) between 12 and 13).

No matter adding acid or base, the average droplet diameter increased with increasing number of cycles. The similar phenomena was also observed by Wang et al. and Binks et al.^14,37 It was regarded that the increasing concentration of NaCl reduced the emulsifying capacity of PMPs.

Conclusions

In summary, the novel polysiloxane microsphere bearing phenolphthalein groups (PMP) has been successfully designed and synthesized. DLS indicated the average diameter of PMPs was 208 nm and TEM revealed the particle shape was spherical. Using this new particulate emulsifier, toluene-in-water Pickering emulsion was prepared in the pH range of 9–12.

As expected, toluene-in-water Pickering emulsion presented sensitive color changing property due to the inclusive phenolphthalein structure. The color turned from pink to deep red with the increasement of pH from 9 to 12. The emulsions also exhibited doubly pH-responsive property: two emulsification/demulsification processes occurred at pH 9 and pH 12 respectively. Moreover, these processes were verified to be reversible. Five and three demulsification/emulsification cycles were repeated when the pH value was adjusted between 8 to 9 and 12 to 13, respectively. Based on these properties, the Pickering emulsions stabilized by PMPs provide potential applications in the preparation of color changing smart coating and oil recovery.³⁸

Acknowledgements

We are thankful for the financial support from the Natural Science Foundation of Shandong Province, China (ZR2014BM001) and National Natural Science Foundation of China (21074067).

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