Materials and methods
The S-CNCs were prepared by ours (Dong et al. 2021). 2-Octen-1-ylsuccinic anhydride (OSA, 97%) and Hexadecane (≥ 95.0%) were from Sigma-Aldrich Chemical Co and Macklin (Shanghai, China) respectively. Calcofluor White Stain and Nile red were supplied by Sigma-Aldrich (USA) and Aladdin (Shanghai, China) respectively. All other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were analytical reagents unless otherwise stated.
Hydrophobic modification of S-CNCs
Hydrophobic modification of S-CNCs was carried out according to the methods of previous researchers with some modifications (Du Le et al. 2020; Han et al. 2015). All reactions were carried out in a flask, and the S-CNCs suspension (3.0 wt%) was mixed with OSA, and the amount of OSA added was 3% of S-CNCs quality. During the esterification reaction of hydroxyl groups on the CNC backbone with OSA, the most suitable reaction pH was adjusted about 8.5 ± 0.1 with a certain concentration of NaOH solution to maintain, the temperature was maintained at 40 ℃ and the total reaction time about 7 h. When the reaction was completed, the resultant product was neutralised and then continuous centrifugal washing with 70% ethanol to remove residual OSA. Finally, the ethanol was removed by centrifugal washing to obtain the MS-CNCs suspension. Which was used to stabilize O/W Pickering emulsions in the next step.
Structural analysis of MS-CNCs
Transmission electron microscope (TEM) utilizing a Jem-2100 microscope (JEOL, Japan), operating at 200 kV, was used to observe the morphology of MS-CNCs.
According to our previous test method, whether the use of OSA is successfully grafted and MS-CNCs groups, peak values and changes in crystallinity are characterized. Fourier transform infrared (FTIR) in the wavenumbers of 4000 − 500 cm− 1 was used to characterize the synthesized MS-CNCs. The X-ray diffraction (XRD) patterns were recorded on AXS (Brooke, Germany).
Wettability
MS-CNCs Self-standing films were prepared by vacuum filtrating device. The three-phase contact angles (θow) of unmodified and OSA-modified S-CNCs particles were measured using a contact angle analyzer (OCA 20 AMP, Data physics Instruments, Germany). At 25℃, 5 µl water was dripped to the surface of the films, and images of the liquid contacting the surface of the films for 3 min was taken, which was the contact angle. Test three times, and finally take the average result.
Preparation of Pickering O/W emulsions
MS-CNCs suspension with different concentrations (0.3, 0.8, 1.0, 3, 5, 8, 10 g/l) as aqueous phase and then combined with hexadecane to prepare O/W Pickering emulsions (oil/water ratio = 2: 8 v/w). The resulting mixture was subjected to ultrasonic pulverization (XINZHI, JY 98-IIIDN, China) under 10% power for 1 minute to obtain an O/W emulsion.
Evaluation of Pickering emulsion performance
The storage stability of the emulsions was evaluated by recording the changes in the volume mean diameter (D [4,3]) of the emulsion, stability index of the emulsion ratio, Zeta potential and light microscopy stored at 25℃ for 24 h and14 days.
The volume mean diameter of the emulsions was measured with a Malvern 3000 laser granulometer equipped with a He-He laser (Malvern Instruments, U.K.), and the average droplet diameter was expressed as surface area–weighted diameter (D [4,3]).
The D [4,3] according to the droplet size distributions using the following equations:
m = 4, n = 3
Vi : the volume of a emulsion droplet
di: the diameter of a emulsion droplet
Approximately 15 mL of the emulsions were taken and stored in a test tube at 25 ℃. The emulsion ratio was calculated using our previous calculation method (Li et al. 2018).
Zetasizer Nano ZS90 (Malvern Instruments, U.K.) was used to measure the electrophoretic mobilities of the MS-CNCs stabilised emulsion droplets.
Optical microscope (Olympus Corporation, Tokyo, Japan) with 50\(\times\) or 100\(\times\) magnification was used to observe the microstructures of the O/W emulsion droplets.
LUMi Sizer (LUM GmbH Justus-von-Liebig-Str.312489 Berlin, Germany) to test the MS-CNCs stabilized emulsion stability, and worked for 20 min at 2300 ×g. Analyze its stability through the recorded emulsion transmlssion in % changes.
Viscosity tests were carried out using HAAKE MARSIII rheometer (Thermo Fisher Scientific Inc., Germany) with a plate geometry. The plate diameter is 25 mm, and the measurement gap width is set to 1 mm. The frequency was set to 1 Hz, and the steady-state scanning was carried out at a shear rate of 0.1–100 s− 1.
Confocal laser scanning microscopy (CLSM)
The inner oil phase and the MS-CNCs were stained using Nile Red (0.5 mg/mL) and Calcofluor White Stain for 6 h, respectively. The Leica TCS SP8 CARS (Leica Microsystem, Vezlal, Germany) under × 64 magnification scanning microscopy was then used to excite the Nile Red and Nile Blue A fluorescent dyes at (488/539) nm and (365/435) nm, respectively.
Ionic strength and pH conditions
The influence of pH and ionic strength on emulsion stability was investigated after storage for 24 h. The pH of prepared emulsions (8 g/l) was about 6.8. For the pH study, the emulsions were adjusted to pH 8.0, 10.0, 12.0 using 1 N NaOH and then to pH 2.0, 4.0, 5.0, and 6.0 using 1 N HCl. For the ionic strength experiment, mix the emulsion (8 g/l) with different contents of NaCl to adjust the ionic strength to 0 to 1000 mM. After the samples prepared above were storage for 24 hours, the droplet size, zeta potential, and viscosity of the emulsion were evaluated.
Statistical analysis
Data reported in the manuscript were expressed as mean with standard deviation (mean ± SD), and all tests were repeated at least three times.