Effects of variables on the dispersion of cationic–anionic organomontmorillonites and characteristics of Pickering emulsion

Abstract

In this study, montmorillonites modified with different amounts of cationic and anionic surfactants by variable steps were used as research objects; physicochemical properties, dispersion of cationic–anionic organomontmorillonites and characteristics of Pickering emulsion were further investigated. XRD diffraction studies of organomontmorillonites showed that basal spacings of organomontmorillonites increased first and then dropped with the increasing amount of cationic surfactants used in the process of modification. Contact angles and swell indices showed that organic matter in the interlayer of organomontmorillonites had a more profound effect than the surface polarity on the dispersion of organomontmorillonites. The micro-macro morphology, thixotropy and volume of emulsion of the Pickering emulsion at elevated temperatures were further tested. The results illustrated that organomontmorillonites modified by three steps with a certain range of surfactants, and surface coated by cationic surfactants were weakly flocculated and had uniform dispersion in a 90 : 10 oil–water ratio Pickering emulsion. The resulting emulsion had preferable stability and thixotropy. Cationic–anionic organomontmorillonites can be widely applied in the drilling industry and cosmetic industry as stable and cost-effective lipophilic colloid.

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1. Introduction

An emulsion is defined as two immiscible liquids wherein droplets of one phase (the dispersed or internal phase) are encapsulated within sheets of the other phase (continuous or external phase).¹ There are mainly two kinds of emulsion. The first is the oil-in-water (O/W) emulsion in which oil droplets are dispersed and encapsulated within the water column. The second is the water-in-oil (W/O) emulsion in which droplets of water are dispersed and encapsulated within the oil. In either type of stable emulsion, two liquids must be mutually insoluble in each other and an emulsifying agent or a combination of emulsifiers must be present.²

Many studies focus on the surfactants/nanolaminates stabilized water-in-oil emulsion.^3–5 Compared with traditional emulsion with surfactants as emulsifier, organoclay/hydrophobic silica with better thermal stability and controllable surface polarity can also be widely used as emulsifier in water-in-oil emulsion,^6,7 and the resulting emulsion is called Pickering emulsion.⁸ The degree of dispersion of organoclay in water-in-oil emulsion depends upon at least two factors: (1) the degree of the surface coating of the organoclay laminates by organic matter; (2) the amount of organic matter in the interlayer of organoclay.⁹

Organoclay are present as additives in a wide range of industrial and consumer products, e.g. toothpaste, printing inks, varnishes, rheology modifiers, film formers^7,10–14 and drilling fluids lipophilic colloid.^15,16 However, limited studies show in which way organoclay laminates behaved as stabilizers and dispersed in the emulsion.¹⁷ What is more, systematic studies of the behavior and properties of surface-active laminates/organoclay in emulsion are lacking.¹⁸ Sekine proposed organomontmorillonites modified by distearyldimethylammonium chloride that swelled in oil, resulting in the formation of an oil gel, and allowed formation of a stable W/O emulsion.¹⁰ Dong et al. (2014) proposed Mt modified by bis(2-hydroxyethyl)oleylamine for use in oil-in-seawater emulsions.¹⁹ In Pickering emulsion, cationic and anionic organomontmorillonites (CA-OMt) have advantages in controllable surface polarity and variable amount of organic matter in the interlayer,^20–22 better thermal stability²³ compared with traditional cationic organoclay. With controllable surface polarity and variable organic matter in the interlayer, CA-OMt have the potential to be applied in different oil–water ratio Pickering emulsion, the resulting emulsion can be widely applied in drilling industry. However, to our best knowledge, no research focuses on the application of CA-OMt in Pickering emulsion.

In this study, CA-OMt was prepared from Mt with cationic and anionic surfactants and we took 90 : 10 oil–water ratio Pickering emulsion as an example to study the effects of several variables on the dispersion of cationic–anionic organomontmorillonites in low polarity solvent and characteristics of Pickering emulsion. The relationship between dispersion of CA-OMt and characteristics of Pickering emulsion was revealed. The variables included amount of surfactants and methods/steps used during the organic modification process of Mt.

2. Experiment

2.1 Materials

Na-Mt was purchased from Kazuo, Liaoning province, China. The Mt was ground by mortar and passed through a 200-mesh sieve. Cation exchangeable capacity (CEC) of Mt is 80 mmol/100 g. The XRD result in Fig. 1 showed the basal spacing of the Na-Mt was 12.6 Å.

Fig. 1 XRD pattern of Na-Mt.

The liquid organic dispersion media 0# polar oil is bought from Sinopec, which is a complex mixture of components and being used widely in oil-based drilling industry. The cationic surfactant is dodecyl trimethyl ammonium bromide (DTAB), and anionic surfactant is sodium dodecyl benzene sulfonate (SDBS). All the chemicals and oil were purchased from Shantou Xilong Co., Ltd., China. Distilled water was used throughout the experiment.

2.2 Preparation of CA-OMt and Pickering emulsion

2.2.1 Mt modified with cationic and anionic surfactants in two steps. Mt modified with DTAB equivalent to 1.0 CEC and SDBS equivalent to 0.2 CEC by solution intercalation method²³ in two modification steps was named as CA-OMt-1.0 + 0.2. Mt modified with DTAB equivalent to 1.0, 1.5, 2.0, 2.5 CEC and SDBS equivalent to 0.2, 0.4, 0.6, 0.8 CEC by solution intercalation method were denoted in the same way.

2.2.2 Mt modified with cationic and anionic surfactants in three steps. The organic modification process included three steps. From the beginning, the first part of cationic surfactants equivalent to 1.0 CEC of Mt and anionic surfactants equivalent to 0.4 CEC of Mt were used by the solution intercalation method to enhance the basal spacing of Mt. Then, the second part of cationic surfactants equivalent to 0.5 CEC were used in mechanochemistry (ball milling at 500 rpm for 30 minutes) modification process to decrease the surface polarity of CA-OMt. Prepared CA-OMt were named as CA-OMt-1.0 + 0.4 + 0.5 and CA-OMt-1.0 + 0.6 + 0.5, respectively. All the OMt samples in 2.2.1 and 2.2.2 were passed through 200 mesh-sieve.

2.2.3 Preparation of Pickering emulsion. The 90 : 10 oil–water ratio Pickering emulsion was prepared by adding 3 g CA-OMt into 90 ml oil and stirring for 5 min at 8000 rpm, then 10 ml distilled water was added into the colloid. The emulsion was stirred for another 5 min at 8000 rpm to prepare Pickering emulsion.

2.3 Characterization methods

2.3.1 XRD. XRD analysis was performed on a D/max-rA 12 kW diffraction at 40 kV and 100 mA, using a Cu tube (Cu-Kα radiation, l = 0.154 nm). Scans were recorded between 1° and 25° (2θ) with a step size of 0.02°, at a scanning rate of 4° min⁻¹.

2.3.2 Thermal stability. The thermogravimetry (TG) analysis were conducted under air atmosphere on DTA-TG simultaneously measuring device DTG-60 from Shimadzu, Japan. A heating rate of 10 °C min⁻¹ was applied until the temperature reached 900 °C.

2.3.3 Contact angle. Contact angle was measured by JC2000D, Zhongchen, Shanghai Ltd. Co. The sessile drop method was used for the surface polarity measurement of the CA-OMt. A video camera equipped with a homemade image analysis device allowed the measurement of contact angle between deionized water/0# polar oil and the surface of CA-OMt.

2.3.4 Swell index. Swell index of CA-OMt in oil was measured according to ASTM D5890-11. The emulsion was transferred to 100 ml graduated cylinder to the mark 100 ml, ageing for 24 h.

2.3.5 Micro and macroscopic morphology. An optical microscope (BX-51; Olympus, Tokyo, Japan) equipped with a camera was used for micro morphology characterization, Canon EOS wlens m2 was used for macroscopic morphology. The Pickering emulsion was diluted as appropriate.

2.3.6 Volume of emulsion of Pickering emulsion. Volume of emulsion of Pickering emulsion was characterized according to GB27798-2011. The Pickering emulsion was transferred to 100 ml graduated cylinder to the mark 100 ml.

2.3.7 Thixotropy. Thixotropy of Pickering emulsion was tested by ThermoHAAKE RV1 rotary viscosimeter from Thermo Electron (Karlsruhe) GmbH, Germany. The surface area of a hysteresis loop can be used as a measurement for the degree of thixotropy.

3. Results and discussion

3.1 Physicochemical properties of CA-OMt

3.1.1 XRD patterns of CA-OMt. XRD patterns of Mt, Mt modified by DTAB equivalent to 1.0–2.5 CEC and SDBS equivalent to 0.2–0.8 CEC in the process of intercalation method are shown in Fig. 2.

Fig. 2 XRD patterns of Mt and CA-OMt modified by different amounts of surfactants.

Na-Mt presented a basal reflection at the 2θ angle around 7.07° in Fig. 1, which corresponded to d₀₀₁ value 12.6 Å. The basal spacings of CA-OMt increased and diffractions became more regular in Fig. 2(a)–(c) with the increasing amount of SDBS used in the process of intercalation method. For Na-Mt modified by DTAB equivalent to 1.0 CEC, the addition beyond 0.4 CEC of SDBS had a significant impact on the basal spacing of corresponding CA-OMt, d₀₀₁ values of CA-OMt-1.0 + 0.6/0.8 were 59.6 Å, 57.3 Å, respectively. For Na-Mt modified by DTAB equivalent to 1.5 CEC, CA-OMt-1.5 + 0.4 showed basal spacing at 53.2 Å. When it came to Na-Mt modified by DTAB equivalent to 2.0 CEC, the addition of SDBS beyond 0.6 CEC enlarged the basal spacing of corresponding CA-OMt-2.0 + 0.8 obviously. However, for Na-Mt modified by DTAB equivalent to 2.5 CEC, the addition of SDBS hardly had an impact on the basal spacing of corresponding CA-OMt, d₀₀₁ values of CA-OMt-2.5 + 0.4 were around 39 Å. Comparison between CA-OMt modified by different amount of DTAB and SDBS showed, with higher dosage of DTAB (2.0/2.5 CEC) in the aqueous phase, a large part of SDBS attached to DTAB molecules by electrostatic attraction in solution, thus less amount of free SDBS intercalated into the interlaminate of Na-Mt which leading to smaller basal spacings of CA-OMt. Results from 3.1.1 showed basal spacing of CA-OMt increased and diffractions became more regular with increasing amount of SDBS (DTAB less than 1.5 CEC) used in the process of organic modification. However, basal spacings and diffractions of CA-OMt changed a little with DTAB over 1.5 CEC. CA-OMt-1.0 + 0.6/0.8 and CA-OMt-1.5 + 0.4/0.6/0.8 had larger basal spacing and more regular diffractions than other CA-OMt.

3.1.2 Thermogravimetry of CA-OMt. To measure the amount of surfactants in the interlayer of CA-OMt, TG of four representative CA-OMt samples were characterized.

As shown in Fig. 3, different extent of mass loss occurred of four CA-OMt in the range of 200–400 °C, at which surfactants decomposed. Mass loss were more significant for CA-OMt-1.0 + 0.4 (14.3%) and CA-OMt-1.5 + 0.4 (21.0%) with larger basal spacings (shown in Fig. 2) compared to CA-OMt-2.0 + 0.4 and CA-OMt-2.5 + 0.4 with relatively small basal spacings. Results here showed that more amount of surfactants were intercalated into the interlayer of CA-OMt-1.0 + 0.4, CA-OMt-1.5 + 0.4 than CA-OMt-2.0 + 0.4 and CA-OMt-2.5 + 0.4. And with more surfactants in the interlayer, basal spacing of CA-OMt were larger, diffractions were more regular than CA-OMt with less amount of surfactants in the interlayer. More surfactants in the interlayer of CA-OMt can also enchance the dispersion of CA-OMt in 90 : 10 oil–water ratio Pickering emulsion.

Fig. 3 TG of representative CA-OMt samples.

3.1.3 Surface polarity of CA-OMt. Contact angles of water/oil to CA-OMt are shown in Fig. 4 and 5.

Fig. 4 Contact angles of water to different CA-OMt.

Fig. 5 Contact angles of oil to CA-OMt-1.0 + 0.8.

Laminates exhibit low θ to the continuous phase and high θ to the dispersed phase in emulsion.^6,24 In Fig. 4, contact angle of polar oil to CA-OMt-1.0 + 0.8 was 15.5°, while that of water to CA-OMt-1.0 + 0.8 was 40°. So we chose polar oil as continuous phase and water as the dispersed phase in the emulsion, oil to water ratio was 90 : 10.

As shown in Fig. 4, modified by 1.0/1.5 CEC of cationic surfactants, contact angles of CA-OMt decreased gradually with the increasing amount of SDBS used in the process of modification, with same amount of SDBS used, CA-OMt modified by 1.5 CEC of DTAB showed larger contact angles than that of CA-OMt modified by 1/0 CEC of DTAB. For Mt modified with DTAB equivalent to 1.0/1.5 CEC, a monolayer of DTAB formed on the surface of CA-OMt and the alkyl chain of cationic surfactants were exposed to the surface of CA-OMt. Further adsorption between the surface of OMt and anionic surfactant can occur via chain–chain interaction resulting in a bilayer, now with headgroups of anionic surfactants exposed to the emulsion rendering the laminates more hydrophilic again,²⁵ different extent of hydrophilic surface and controllable surface polarity of CA-OMt can be achieved by absorbing different amount of SDBS on its surface. However, for CA-OMt modified with DTAB equivalent to 2.0/2.5 CEC, part of bilayer of DTAB were formed on the surface of CA-OMt, the increasing SDBS below 0.4 CEC attached to the surface of CA-OMt through electrostatic attraction with alkyl chain exposed to the surface, partially trilayer of surfactants formed and hydrophobicity of CA-OMt-2.0/2.5 increased. With more amount of SDBS used, the laminates became more hydrophilic again as discussed before. Combining the discussions in Binks²⁶ and the results in 3.1.2, surface polarities of CA-OMt were controllable. For cationic surfactants used below 1.5 CEC, surface polarity of CA-OMt increased with the increasing amount of anionic surfactants. While for cationic surfactants used beyond 1.5 CEC, surface polarity of CA-OMt first decreased and then increased with the increasing amount of anionic surfactants used. The surface polarity of CA-OMt-1.0 + 0.2, CA-OMt-1.5 + 0.2/0.4/0.6, CA-OMt-2.0 + 0.2/0.4 and CA-OMt-2.5 + 0.2/0.4 were intermediate, thus had optimal dispersion in low polarity Pickering emulsion.

3.2 Dispersion of CA-OMt in oil and Pickering emulsion

3.2.1 Effect of surfactants dosage on swell index of CA-OMt in oil. As the synergistic effects of surface polarity and the interlayer environment of CA-OMt, swell index of CA-OMt in oil are shown in Fig. 6, combining with the basal spacings of CA-OMt.

Fig. 6 Swell index of CA-OMt in oil. Left ordinate: basal spacing; right ordinate: swell index.

Fig. 6 showed the relationship between swell index and basal spacings of CA-OMt. In Fig. 6(a), with more amount of SDBS intercalated into the interlayer of CA-OMt, swell index of it increased a little to 16 ml (CA-OMt-1.0 + 0.4) first, while with SDBS beyond 0.4 CEC, due to the steady basal spacing and increasing surface polarity of CA-OMt, swell index of CA-OMt in oil decreased. The swell index of CA-OMt-1.5 + 0.4 in oil was 32 ml. With an obviously larger basal spacing and more amount of surfactants in the interlayer, CA-OMt-1.5 + 0.4 had a greater expansion and swelling in oil than that of CA-OMt-1.5 + 0.2 (28 ml), although the surface of CA-OMt-1.5 + 0.4 was more hydrophilic. The CA-OMt with more amount of organic matter in the interspaces showed larger basal spacing and a high affinity to the organic media, they easily swelled and dispersed in the oils.⁹ Further absorption of SDBS on the surface and increasing surface polarity of CA-OMt led to the falling of swell index of CA-OMt-1.5 + 0.6/0.8.

Basal spacings and amount of SDBS in the interspaces of CA-OMt modified with 2.0/2.5 CEC of DTAB changed a little with increasing amount of SDBS (shown in Fig. 6(c) and (d)), falling swell index of them was in line with the results in 3.1.2, the more hydrophilic surface of CA-OMt had a negative impact on the swelling ability in oil of themselves.

Results in 3.2.1 showed organic matter in the interspaces and surface polarity of CA-OMt had a synergistic effect on the swelling ability and dispersion of CA-OMt in low polarity solvent, and the effect of organic matter in the interlayer was the dominant role. Swelling ability and dispersion of CA-OMt increased with larger basal spacing and more amount of organic matter in the interlayer of it. With steady basal spacing of CA-OMt, increasing surface polarity had a negative impact on the swell index of CA-OMt in oil. CA-OMt-1.5 + 0.4 and CA-OMt-1.5 + 0.6 had larger basal spacing (shown in Fig. 2), more amount of organic matter in the interspaces and proper surface polarity, thus can better disperse in oil and low polarity solvent.

3.2.2 Effect of surfactants dosage on the dispersion of CA-OMt in Pickering emulsion. The dispersion of CA-OMt in the Pickering emulsion and representative morphology of prepared Pickering emulsion are shown in Fig. 7.

Fig. 7 Micro and macroscopic-morphology of CA-OMt in Pickering emulsion. The bar was 100 μm.

In emulsion consisted of 90% polar oil and 10% water phase, CA-OMt with more hydrophobic surface can better disperse in oil phase and also absorb water droplets on its surface.^7,27–30 So CA-OMt with high affinity to polar oil and intermediate surface polarity was preferred to disperse well in 90 : 10 oil–water ratio Pickering emulsion. The coalescence state of water droplets and dispersion of CA-OMt can be confirmed by the optical micro-morphology of the Pickering emulsion. In Fig. 6, evident flocculation showed in all emulsions except emulsion stabilized by CA-OMt-1.5 + 0.2/0.4/0.6, while CA-OMt showed different extent of the tendency to agglomerate in Pickering emulsion. CA-OMt modified by 1.0/2.0/2.5 CEC of DTAB, due to high hydrophilicity surfaces or poor dispersion in oil, showed cluster structure in the emulsion. What was more, water droplets absorbed on the surfaces of CA-OMt modified by 1.0/2.0/2.5 CEC of DTAB also tended to agglomerate inevitably. However, for CA-OMt modified by 1.5 CEC of DTAB with the best swelling ability in oil and proper surface polarity, there was no cluster flocculation of water droplets or particles in 90 : 10 oil–water ratio Pickering emulsion.

According to the DLVO theory, the ion surfactants excess to some extent in the Pickering emulsion would further compress the electrical double layer of CA-OMt and weaken the repulsion between laminates³¹ which also had negative effects on the dispersion of CA-OMt. For CA-OMt modified by a markedly higher amount of cationic surfactants DTAB (2.0/2.5 CEC), dispersion of them dropped significantly.

Micro-morphology of CA-OMt in Pickering emulsion corresponded to the discussions before, with DTAB used over 1.5 CEC, laminates of CA-OMt started to agglomerate. Results in 3.2.2 showed CA-OMt modified by DTAB equivalent to 1.5 CEC dispersed better in the Pickering emulsion than other CA-OMt. And out of four CA-OMt-1.5 samples, CA-OMt-1.5 + 0.4/0.6/0.8 had better dispersed state in emulsion, leading to homogeneous dispersion of water droplets. Combined the results here and the discussion in 3.2.1, Pickering emulsion containing CA-OMt-1.5 + 0.4 and CA-OMt-1.5 + 0.6 had optimal stability than other emulsion, CA-OMt-1.5 + 0.4 and CA-OMt-1.5 + 0.6 dispersed in emulsion better than other CA-OMt. The representative macroscopic-morphology of Pickering emulsion also in Fig. 7 illustrated that the emulsion containing CA-OMt-1.5 + 0.4 or CA-OMt-1.5 + 0.6 had optimal stability than others.

3.2.3 Effect of modification steps on the dispersion of CA-OMt in Pickering emulsion. Due to the discussion before, CA-OMt-1.5 + 0.4/0.6 with better dispersion in Pickering emulsion were both chosen for further study. To better enhance the dispersion of CA-OMt in low polarity 90 : 10 oil–water ratio Pickering emulsion, according to the surfactant layers on the surface of CA-OMt theory,^9,21 cationic surfactants used in the process of organic modification were divided into two parts as intercalation agent and as coating agent, and Mt was modified by three steps. Basal spacings of CA-OMt-1.0 + 0.4 + 0.5 and CA-OMt-1.0 + 0.6 + 0.5 have not changed significantly compared with CA-OMt-1.5 + 0.4 and CA-OMt-1.5 + 0.6, and the surface polarities of them have dropped due to the cationic surfactants covered on the surface of CA-OMt-1.0 + 0.4 + 0.5 and CA-OMt-1.0 + 0.6 + 0.5.

To measure the structure and size of CA-OMt laminates in the Pickering emulsion after being heated and aged for a period of time, micro-morphology of Pickering emulsion containing different kinds of CA-OMt separately was characterized. Results are shown in Fig. 8.

Fig. 8 Micro-morphology of Pickering emulsion added by CA-OMt modified by different steps at different temperatures. (a) CA-OMt-1.0 + 0.4 + 0.5; (b) CA-OMt-1.0 + 0.6 + 0.5. (c) CA-OMt-1.5 + 0.4; (d) CA-OMt-1.5 + 0.6. The bar was 20 μm.

Results in Fig. 8 showed dispersion of CA-OMt in Pickering emulsion after being heated and aged for a period of time. At 25 °C, noticeable water droplets were seen in the Pickering emulsion adhered to CA-OMt-1.5 + 0.4/0.6. Water molecules were more easily adsorbed on hydrophilic surface, which corresponded to the discussion before that surface of CA-OMt modified by two steps had higher hydrophilicity than that modified by three steps. Laminates of CA-OMt became larger at 150 °C, however, there was no stacked or agglomeration structure shown at 150 °C. Gradual desorption of surfactants on the surface of CA-OMt would happen at relatively low temperature due to weak van der Waals force between surfactants and laminates of CA-OMt, and result in agglomeration structure of CA-OMt, at 180 °C, stacked structure of CA-OMt showed in all emulsions and sizes of CA-OMt-1.0 + 0.4/0.4–0.5 were smaller than that of CA-OMt-1.5 + 0.4/0.6, distributing around 20 μm. After being heated and aged for over 32 h at high temperatures, dissociation of surfactants sped up, resulting in larger size, more stack and curly structure of CA-OMt. Modified by three steps, CA-OMt-1.0 + 0.4 + 0.5 had low surface polarity and high affinity to the emulsion, which illustrated outstanding dispersion, thus had a positive effect on the stability of Pickering emulsion itself.

3.3 Characteristics of Pickering emulsion containing CA-OMt modified by different steps

3.3.1 Volume of emulsion of Pickering emulsion containing CA-OMt modified by different steps. Pickering emulsion containing CA-OMt modified by different steps was heated and aged at different temperatures (25 °C, 150 °C and 180 °C), the volume of emulsion of Pickering emulsion is characterized and the results are shown in Fig. 9.

Fig. 9 Photographs and volume of emulsion of Pickering emulsion containing CA-OMt modified by different steps at different temperatures. From left to right: CA-OMt-1.5 + 0.4; CA-OMt-1.5 + 0.6; CA-OMt-1.0 + 0.4 + 0.5; CA-OMt-1.0 + 0.6 + 0.5.

The addition of DTAB in third step modification process played two roles. First, polar headgroups of DTAB attached to the SDBS polar headgroups, the alkyl chain of DTAB exposed to the Pickering emulsion led to the reduction of surface energy according to Bancroft theory,² enhanced the dispersion of CA-OMt in Pickering emulsion and prevented water droplets from aggregating on the surface of CA-OMt, thus formed stable Pickering emulsion.^2,10,32 Second, free DTAB molecules (less than critical micelle concentration of DTAB) in the Pickering emulsion enhanced the repulsion between laminates according to the DLVO theory, and led to weakly flocculation structure of CA-OMt, thus dispersion of CA-OMt laminates in Pickering emulsion got better after which Pickering emulsions containing CA-OMt tended to be stable.³³ In Fig. 9(a) and (b), photographs of different emulsions after setwing for 6 h at different temperatures were taken. With a larger volume of emulsion of Pickering emulsion, CA-OMt dispersed better in the emulsion. Emulsions added by CA-OMt-1.0 + 0.4 + 0.5 or CA-OMt-1.0 + 0.6 + 0.5 showed larger volume than that added by CA-OMt-1.5 + 0.4 or CA-OMt-1.5 + 0.6 at any temperatures. Fig. 9(c) and (d) showed the volume of emulsion recorded by every hour. After being heated at 150 °C, part of the CA-OMt laminates started to agglomerate, resulting in the stack structure of CA-OMt laminates, thus weakened the dispersion of CA-OMt in the emulsion. Volume of emulsion of Pickering emulsion added by CA-OMt-1.0 + 0.4/0.6 + 0.5 separately at 150 °C, 6 h was 42 ml and 33 ml, respectively, volume of emulsion of Pickering emulsion added by CA-OMt-1.5 + 0.4/0.6 separately at 180 °C, 6 h was 35 ml and 27 ml, respectively. There may be two reasons for the decreasing volume of emulsion of Pickering emulsion containing CA-OMt laminates. First, the surfactants attached to the surface of CA-OMt may dissociated from the surface of CA-OMt, so surface of CA-OMt turned from hydrophobic to hydrophilic and the attraction force between surfaces of Mt was enhanced. Mt laminates with high surface polarity tended to stack together and formed agglomeration structure, thus had a negative effect on the volume of emulsion of Pickering emulsion.

Second, cationic surfactants molecules would dissolve in the Pickering emulsion, and re-precipitation on the surface of large CA-OMt laminates in the heating and aging process due to the Ostwald ripening.³² However, Ostwald ripening is controlled by molecular solubility, it is negligible for immiscible fluids and is usually only significant for droplets under 0.1 μm,^30,31 which did not suit in the condition of this study. With continuing dissociation of surfactants from the surface of CA-OMt laminates, the volume of emulsion of Pickering emulsion containing CA-OMt dropped. Results in 3.3.1 illustrated that Pickering emulsion containing CA-OMt-1.0 + 0.4/0.6 + 0.5 showed better stability at different temperatures than that containing CA-OMt-1.5 + 0.4/0.6. Modified by three steps and covered by cationic surfactants, CA-OMt had a uniform dispersion in Pickering emulsion.

3.3.2 Thixotropy of Pickering emulsion containing CA-OMt modified by different steps. Thixotropy of Pickering emulsion reflected the dispersion of CA-OMt laminates in Pickering emulsion vice versa. Pickering emulsion with better thixotropy showed better dispersion of CA-OMt in it. Fig. 10 showed thixotropy of the four Pickering emulsions added by CA-OMt modified by different steps.

Fig. 10 Thixtropy of Pickering emulsion added by CA-OMt modified by different steps at different temperatures.

Results in Fig. 10 showed that Pickering emulsion containing CA-OMt modified by three steps had better thixotropy. At 25 °C, thixotropy of Pickering emulsion containing CA-OMt-1.0 + 0.4/0.6 + 0.5 was 0.642 Pa s⁻¹ and 0.554 Pa s⁻¹, respectively, thixotropy of Pickering emulsion containing CA-OMt-1.5 + 0.4/0.6 separately was 0.544 Pa s⁻¹ and 0.499 Pa s⁻¹, respectively. CA-OMt with better dispersion, smaller laminates in Pickering emulsion were more probable to form network structure that resulting in better thixotropy of it in Pickering emulsion. All CA-OMt had different extent of tendency to agglomerate at elevated temperatures, which led to the drop of Pickering emulsion stability and thixotropy. At 180 °C, Pickering emulsion started to behave anti-thixotropy. Thixotropy of Pickering emulsion containing CA-OMt-1.0 + 0.4/0.6 + 0.5 was 0.028 Pa s⁻¹ and −0.143 Pa s⁻¹, respectively, thixotropy of Pickering emulsion containing CA-OMt-1.5 + 0.4/0.6 was −0.131 Pa s⁻¹ and −0.251 Pa s⁻¹, respectively. Due to the severe stacked structure and agglomeration (shown in Fig. 8), Pickering emulsion containing CA-OMt modified by two steps showed bad stability and thixotropy, CA-OMt illustrated poor dispersion in the Pickering emulsion.

Furthermore, rheology of Pickering emulsion containing CA-OMt-1.0 + 0.4 + 0.5 and CA-OMt-1.5 + 0.4 under 25 °C and 180 °C showed different fittings based on the Herschel–Bulkley model:³⁴

τ = τ₀ + kγⁿ (1)

where τ is the shear stress, γ is the shear rate, τ₀ is the yield stress, k is the consistency index, and n is the flow index. If τ < τ₀ the Herschel–Bulkley fluid behaves as a solid, otherwise it behaves as a fluid. For n < 1 the fluid is shear-thinning, whereas for n > 1 the fluid is shear-thickening.

Results in Fig. 11 illustrated that Pickering emulsion containing CA-OMt-1.0 + 0.4 + 0.5 at different temperatures showed shear thinning behavior, and the correlation coefficient R² were 0.995 and 0.990, respectively. The τ₀ of Pickering emulsion containing CA-OMt-1.5 + 0.4 at 180 °C was −2.32, n was 0.21 and R² was 0.975. In general, τ₀ of a fluid cannot be negative, which indicated the Pickering emulsion had bad stability and did not fit the shear thinning model properly. Based on the thixotropy of Pickering emulsion containing CA-OMt, results from Fig. 11 further proved that CA-OMt modified by three steps had better dispersion in Pickering emulsion, thus improved the thixotropy of Pickering emulsion than CA-OMt modified by two steps.

Fig. 11 Fittings of Pickering emulsion containing different OMt based on Herschel–Bulkley rheological model.

4. Conclusion

The following conclusions can be drawn concerning the dispersion of CA-OMt in Pickering emulsion as studied here:

(1) In the presence of excess dosage cationic/anionic surfactants, dispersion of cationic–anionic organomontmorillonites in Pickering emulsion dropped and stability of Pickering emulsion fell, which corresponded to the DLVO theory. Montmorillonite modified by DTAB below 1.5 CEC showed better modification effect with increasing amount of surfactants used, including larger basal spacing, better swelling ability in oil and proper surface polarity than other CA-OMt. Pickering emulsion containing CA-OMt-1.5 + 0.4/0.6 showed better stability. (2) With the same dosage of surfactants used, CA-OMt modified by three steps and coated by a small amount of cationic surfactants showed better dispersion than CA-OMt modified by two steps in Pickering emulsion, even after being heated and aged for a period of time. Pickering emulsion containing CA-OMt modified by three steps showed better stability and thixotropy than that containing CA-OMt modified by two steps.

With proper process parameters, CA-OMt can disperse well and uniformly in Pickering emulsion, the resulting Pickering emulsion is stable with preferable thixotropy, environmentally safe and has the potential to be applied in drilling and cosmetic industry.

Author contributions

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by the Fundamental Research Funds for the Central Universities (no. 53200959784).

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