Recently, microcapsules have been extensively investigated and used in many applications, such as in the energy storage, pharmaceutical, military camouflage and defence and agricultural industries (Zhao & Zhang, Reference Zhao and Zhang2011; Yi et al., Reference Yi, Yang, Gu, Huang and Wang2015; Jiang et al., Reference Jiang, Yang, He, Xie, Fan, Wu and Zhang2018; Zhao et al., Reference Zhao, Long, Du, Zhou, Wang and Miao2020). Microencapsulation contributes to environmental protection by isolating volatile, deliquescent, oxidizable and photodegradable guest compounds from the external environment (Ravanfar et al., Reference Ravanfar, Comunian and Abbaspourrad2018; Xiao et al., Reference Xiao, Wu, Fu, Wang and Lei2019; Bah et al., Reference Bah, Bilal and Wang2020).
Microcapsules can be fabricated using a variety of methods, including solvent evaporation (Asghari-Varzaneh et al., Reference Asghari-Varzaneh, Shahedi and Shekarchizadeh2017), spray drying (Lavanya et al., Reference Lavanya, Kathiravan, Moses and Anandharamakrishnan2020), sol–gel (Yin et al., Reference Yin, Fu, Zhou and Fu2020), in situ polymerization (Nguon et al., Reference Nguon, Lagugné-Labarthet, Brandys, Li and Gillies2018), interfacial polymerization (Jerri et al., Reference Jerri, Jacquemond, Hansen, Ouali and Erni2016), complex coacervation (Demirbağ & Aksoy, Reference Demirbağ and Aksoy2016) and surfactant exchange (Chatterjee et al., Reference Chatterjee, Tran, Godfred and Woo2018). Among these methods, interfacial polymerization has attracted much attention because of its many advantages. This method can be easily carried out at the oil–water interface of an emulsion, and it is easy to control the extent of polymerization to produce microcapsules with various sizes and shell thicknesses. However, as conventional emulsions are usually stabilized by surfactants, the microcapsule shells formed on surfactant-stabilized emulsions are composed of organic polymer matrices with impurities of surfactant molecules. Such a composition of microcapsules has many drawbacks, such as flammability, poor thermal and chemical stability and low mechanical strength (Cai et al., Reference Cai, Wei, Huang, Lin, Chen and Gao2009; Li et al., Reference Li, Chen, Li and Sanjayan2014; Kumar et al., Reference Kumar, Al-Aifan, Parameshwaran and Ram2021). Surfactants can also be harmful to humans and the environment. Hence, using solid particles instead of surfactants to stabilize emulsions and then fabricating microcapsules on this basis is a candidate approach to overcoming the above drawbacks of microcapsules.
The use of solid particles as emulsifiers to stabilize emulsions can be traced back to the beginning of the twentieth century, which were called later Pickering emulsions (Pickering, Reference Pickering1907). In Pickering emulsion systems, the emulsifiers are investigated using various natural and artificial materials, such as cellulose (Dai et al., Reference Dai, Wu, Zhang, Chen, Ma and Huang2020), carbon black (Katepalli et al., Reference Katepalli, John and Bose2013), silica (Hassander et al., Reference Hassander, Johansson and Törnell1989) and clay minerals (Ganley et al., Reference Ganley, Ryan and van Duijneveldt2017; Machado et al., Reference Machado, de Freitas and Wypych2019; Gonzalez Ortiz et al., Reference Gonzalez Ortiz, Pochat-Bohatier, Cambedouzou, Bechelany and Miele2020) and their derivatives. Solid particles used for stabilizing emulsions are low cost and environmental friendly. They improve emulsification stability and can be recovered from the emulsification system by centrifugation (Fan & Striolo, Reference Fan and Striolo2012; Ganley et al., Reference Ganley, Ryan and van Duijneveldt2017; Yang et al., Reference Yang, Fang, Chen, Zhang, Xie and Chen2017). This may contribute to fabricating microcapsules with fewer defects by preventing the oil–water interface destruction caused by demulsification, and it may also reduce the cost of fabricating microcapsules. Moreover, the most vital aspects in terms of determining the performance of a microcapsule are the composition and structure of its shell (Li et al., Reference Li, Zhang, Wang, Tang and Shi2012; Yin et al., Reference Yin, Ma, Liu and Zhang2014). In this context, microcapsules with shells composed of polymers and solid particles can be easily fabricated based on Pickering emulsions, where the solid particles may serve as an enhancer of the performance of microcapsules.
The fabrication of microcapsules based on Pickering emulsions and the derived shells of microcapsules composed of polymers and solid particles have been reported in many studies. The solid particles used in those systems contain various organic and inorganic compounds, such as cellulose, chitosan, CaCO3 and silica (Wang et al., Reference Wang, Zhou, Cao, Liu and Zhu2012; Yin et al., Reference Yin, Ma, Liu and Zhang2014; Zhang et al., Reference Zhang, Wang, Meng, Wang, Wu and Chen2014; Zhang et al., Reference Zhang, Tam, Wang and Sèbe2018; Bago Rodriguez & Binks, Reference Bago Rodriguez and Binks2019). Among them, silica and its modified products are the most intensively studied solid particles in the preparation of Pickering emulsions and for subsequent use in the composition of the microcapsule shell. However, most of the original particles, including silica, might not be well hybridized in the polymer matrix of the microcapsule shell, so these particles usually require additional steps for modification and improvement of their compatibility. In addition, most of these particles are spherical in shape, resulting in nondirective combination in the polymer matrix of the microcapsule shell. Therefore, in the manufacture of microcapsules, a solid particle in its original form (without modification) with affinity to the polymer matrix is desirable.
Kaolinite (Kln), as a layered clay mineral, can be used as an emulsifier in Pickering emulsion systems. Previous works of our group (Liang et al., Reference Liang, Li, Dai, Tang, Cai and Zhen2018; Cai et al., Reference Cai, Li, Tang, Zhen, Xie and Zhu2019; Tang et al., Reference Tang, Xie, Li, Zhen, Cai and Zhang2019) and other groups (Li et al., Reference Li, Fu, Ouyang, Tang and Yang2015) have revealed that both original and modified Kln can stabilize emulsions. Because of the asymmetric structure of Kln (i.e. the octahedral surface of the platelets contains μ-hydroxy groups and the tetrahedral surface contains siloxane groups), the hybrid structure of Kln and the polymer matrix may be distinctive. To account for this, the aim of this work was to develop a novel microcapsule with a shell composition of Kln and polyurea to investigate the profile of the Kln–polyurea shell and to explore the possibility of Kln enhancing the controlled-release performance of microcapsules.
Herein, a strategy to prepare Kln–polyurea microcapsules was proposed. A schematic illustration of the preparation of such microcapsules is shown in Fig. 1. Firstly, an oil phase containing isophorone diisocyanate (IPDI) and a water phase containing Kln were prepared separately, then they were mixed in a tube, followed by emulsification with a high-shear homogenizer. After emulsification, a Kln-stabilized Pickering emulsion was obtained. Eventually, the Kln-stabilized Pickering emulsion (EKln) was used as a template for fabrication of Kln-armoured polyurea microcapsules (CKP) through interfacial polymerization of IPDI at the oil–water interfaces. The Kln is hybridized with polyurea forming a Kln-embedded and armoured polyurea structure, which is beneficial for prolonging the slow release of encapsulated Sudan Red (III) (SR) from CKP. This method takes full advantage of Kln as an emulsifier for Pickering emulsions and subsequently as an enhancer for microcapsule formation, and it is expected to shed light on advanced applications of clay minerals in composites.
Fig. 1. Schematic of the formation of Kln-armoured polyurea microcapsules.
Experimental
Materials
Kln was collected from Maoming area, China, and was purified by sedimentation to collect the <2 μm fraction (Bergaya & Lagaly, Reference Bergaya and Lagaly2013). It was then used as an emulsifier to stabilize Pickering emulsions and also in the composition of the final microcapsule shell. The chemical composition (wt.%) of Kln, determined with a RIGAKU ZSX100e X-ray fluorescence (XRF) spectrometer, was as follows: SiO2 46.71%, Al2O3 37.11%, Fe2O3 0.51%, TiO2 0.16%, MgO 0.21%, K2O 0.58% and Na2O 0.11%.
IPDI (99%) and dibutyltin dilaurate (DBTDL; 95%) were purchased from Shanghai Macklin Biochemical Co., Ltd, China. Liquid paraffin (chemical grade) and anhydrous ethanol (analytical grade) were provided by Xilong Chemical Co., Ltd, China. SR was obtained from Shanghai Yuanye Biological Technology Co., Ltd, China. The water used in this work was deionized using water purification equipment (AXLB1015, Chongqing Asura Technology Development Co., Ltd, China).
Preparation of the Kln-stabilized Pickering emulsion
A total of 4 mL of the liquid paraffin was mixed with 0.4 g IPDI and 0.07 g DBTDL in a beaker with continuous stirring. The obtained mixture was then stored and used as the oil phase for the Pickering emulsion. An aqueous phase containing Kln emulsifier was produced by adding 0.4 g Kln to 16 mL deionized water and stirring for 5 min. Finally, the Kln-stabilized Pickering emulsion was prepared by blending the above oil phase and aqueous phase in a glass tube, followed by emulsification using a high-shear homogenizer (IKA-T18, IKA (Guangzhou) Instrument Equipment Co., Ltd, China) at 12,000 rpm for 3 min. The prepared sample was labelled as EKln.
Preparation of the Kln–polyurea microcapsule
To accelerate the interfacial polymerization of IPDI with water on EKln droplets, the prepared EKln was transferred to a three-necked flask and placed in a thermostat (DF-101S) at 70°C for 5 h. Subsequently, the system was adjusted to 50°C and stored for another 12 h. After the reaction was complete, the mixture was centrifuged (TDZ5-WS, Xiangyi Centrifuge Instrument Co., Ltd, China) at 4000 rpm for 5 min. The upper fraction was then separated with a separation funnel. The CKP was finally obtained after washing it with deionized water to eliminate the adsorbate and drying it at 70°C for 24 h. The formed CKP itself was not a pesticide, but it may be used as carrier to encapsulate lipophilic compounds including pesticides.
To tailor the shell thickness of CKP, IPDI dosages of 0.3, 0.4, 0.5 and 0.6 g loaded in the oil phase were applied following the procedure used to prepare the EKln. The EKln containing various dosages of IPDI was denoted as EKln-xIPDI, where the subscript x refers to the dosage of IPDI. Similarly, the resulting corresponding CKP with various shell thicknesses were denoted as CKP-xIPDI. For example, CKP-0.6IPDI represents a Kln–polyurea microcapsule fabricated at an IPDI dosage of 0.6 g.
To tune the diameter of CKP, the dosages of Kln and the emulsifying rates of the high-shear homogenizer for EKln were adjusted. The EKln prepared at a Kln dosage of 0.4 g and at an emulsifying speed of 12,000 rpm was denoted as E0.4Kln-12r. Correspondingly, the CKP fabricated base on E0.4Kln-12r was denoted as CKP-0.4Kln-12r. Similarly, CKP-0.2Kln-10r and CKP-0.1Kln-8r were obtained.
SR was selected as the lipophilic compound encapsulated into CKP to study its encapsulation and release performance. To fabricate SR-encapsulated CKP, the SR was dispersed into a liquid paraffin matrix at a concentration of 1 mg mL–1 during the preparation of the oil phase for EKln. Then, the oil phase containing SR was used following the preparation procedure of EKln and subsequently for CKP.
Characterization of the Kln-stabilized Pickering emulsion and the Kln–polyurea microcapsules
Fourier-transform infrared (FTIR) spectroscopy in the frequency range 400–4000 cm–1 was carried out using a Thermo Nexus 470 spectrometer and following the KBr disc method. A total of 32 scans were obtained for each sample at a resolution of 4 cm–1. The morphologies and the shell thicknesses of CKP were investigated using a HITACHI S-4800 scanning electron microscope (SEM). To test the shell thicknesses of CKP, CKP was first ground in a mortar, then washed with petroleum ether to remove encapsulated paraffin, centrifuged at 4000 rpm to obtain the solid component and finally dried and sprayed with gold. Optical micrographs of the EKln droplets and CKP were obtained on a Leica DM RX-type microscope. Before observation of EKln droplets under the microscope, EKln was dropped onto a glass slide and then deionized water was added to separate the overlapped droplets. For characterization of CKP, the wet CKP obtained from the separating funnel after washing and before drying was placed on a glass slide and then was separated using anhydrous ethanol.
Characterization of the encapsulation and release performance of Kln–polyurea microcapsules
The encapsulation performance of SR in CKP was evaluated by calculating the concentration of SR encapsulated in CKP as a percentage of the initial concentration of SR in the oil phase. To determine the concentration of SR encapsulated in CKP, CKP was cut open to allow for oil-phase release and then was centrifuged at 12,000 rpm to remove the shell of CKP. The concentration of SR encapsulated in CKP was finally obtained by testing the concentration of SR in the supernatant liquid.
The release performance of SR from CKP was characterized by determining the concentration of SR in anhydrous ethanol. Specifically, 5.0 g of CKP were first added into a dialysis bag (3500 Da) and then was transferred to 150 mL of anhydrous ethanol. After a certain time, 300 μL of the release medium were removed to determine the concentration of SR in anhydrous ethanol. Determination of SR was performed using a Perkins Elmer Lambda 750S UV/VIS spectrophotometer (190–2500 nm) at a wavelength of 507 nm (wavelength accuracy of ±0.15 nm).
Results and discussion
Characterization of the Kln-armoured polyurea microcapsules
CKP were fabricated based on the Kln-stabilized Pickering emulsion templates. Previous work has shown that Kln was distributed in the continuous phase (water in this work) of the Pickering emulsion, forming a three-dimensional network (Liang et al., Reference Liang, Li, Dai, Tang, Cai and Zhen2018; Cai et al., Reference Cai, Li, Tang, Zhen, Xie and Zhu2019; Tang et al., Reference Tang, Xie, Li, Zhen, Cai and Zhang2019). Due to the distribution of IPDI in liquid paraffin in this work, the IPDI interaction with water was at the oil–water interface. As the reaction proceeded, formation of the polyurea bulk took place via polymerization of diisocyanate (–NCO) with melamine (–NH2) in the emulsion system (Yi et al., Reference Yi, Yang, Gu, Huang and Wang2015). After the interfacial polymerization, a CKP containing Kln and polyurea might have been formed.
The composition of the CKP shell was investigated by characterization with FTIR. Kln and IPDI were also tested as control samples to analyse the chemical bonds in CKP (Fig. 2). The characteristic bonds of polyurea in CKP (Fig. 2c), which involved two bands at 3376 and 1560 cm–1, assigned to the N–H stretching and deformation vibrations, respectively, were detected. Some splitting bands at ~2951 cm–1, assigned to –C–H on the hexatomic ring in polyurea and the methylene groups in paraffin (Zhan et al., Reference Zhan, Chen, Chen and Hou2016), were also identified. Furthermore, a stretching vibration typical of the –C=O– bond in polyurea was also detected at 1638 cm–1. The results obtained from FTIR characterization suggested the formation of polyurea in CKP.
Fig. 2. FTIR spectra of (a) Kln, (b) IPDI and (c) CKP.
Moreover, the chemical groups observed in Kln were also detected in CKP without shift, including the Si–O stretching vibration bands at 1031 cm–1, the inner-surface and internal O–H stretching vibration bands at 3695 and 3620 cm–1, respectively, and bands arising from metal–oxygen (metal–hydroxyl) vibrations in the lattice of Kln at <800 cm–1 (Fig. 2a) (Li et al., Reference Li, Fu, Ouyang, Tang and Yang2015). In addition, a weak band at 2265 cm–1 associated with the –N=C=O group was identified in CKP; the same band with higher intensity was also detected in IPDI (Fig. 2b). These results suggest that the CKP was composed of Kln and polyurea with no chemical bonds between Kln and polyurea, and that some unreacted IPDI remained in CKP.
To further ascertain the role of the Pickering emulsion for CKP formation and to investigate the mechanism of Kln hybridization with polyurea, a group of CKP were fabricated at a fixed Kln amount of 0.4 g and an emulsifying speed of 1200 rpm and with IPDI dosages at 0.3, 0.4 and 0.5 g. The obtained CKP were characterized using optical microscopy and SEM. The CKP-0.3IPDI was well dispersed in anhydrous ethanol and was spherical in shape (Fig. 3a). A total of 90% of the diameter of CKP-0.3IPDI was distributed within the range of 46.5–113.2 μm (counts in the viewing area), with an average diameter being ~86.4 μm. The shell thickness of CKP-0.3IPDI was 0.53 μm (Fig. 3b). Compared with CKP-0.4IPDI and CKP-0.5IPDI, the shell thicknesses increased to ~0.67 and 0.92 μm, respectively (marked in Fig. 3c,d), suggesting that the shell thickness of CKP might be tailored by varying the dosage of IPDI. Based on this approach, the encapsulation and release performance of CKP might be controlled by tailoring the shell thickness.
Fig. 3. Morphologies of CKP fabricated at various dosages of IPDI: (a) optical micrograph of CKP fabricated at an IPDI dosage of 0.3 g; SEM images of the shell profile of CKP fabricated at IPDI dosages of (b) 0.3 g, (c) 0.4 g and (d) 0.5 g; the insets correspond to magnifications of the selected areas in (b), (c) and (d).
Furthermore, the SEM images of CKP-0.3IPDI, CKP-0.4IPDI and CKP-0.5IPDI show a smooth internal surface and a rough external surface (Fig. 3b–d). Such a phenomenon is ascribed to the habitation where Kln particles distributed in the previous EKln. That is, most of the Kln particles are distributed in the aqueous phase of EKln, while fewer Kln particles are dispersed in the oil phase. As a result, more Kln particles hybridized with the polyurea shell towards the side of the water phase. The insets in Fig. 3b–d are the corresponding magnifications of the selected areas. As indicated by the arrows in these insets, some Kln platelets are embedded and assembled within the polyurea matrix, forming a Kln-armoured polyurea shell. This combination of Kln with polyurea suggests that the Pickering emulsion acted as a template during the formation of the Kln-armoured polyurea shell. In addition, such a Kln-armoured polyurea shell may be beneficial for tuning its encapsulation and release performance.
The microcapsule size is another major factor that affects its encapsulation and release performance (Rule et al., Reference Rule, Sottos and White2007; Zhao et al., Reference Zhao, Fei, Cao, Zhang and Liu2019). Previous studies have shown that the amount of emulsifier and shear force are the main parameters affecting the size of the emulsion droplets, thereby determining the size of the microcapsules (Yow & Routh, Reference Yow and Routh2009; Ozturk et al., Reference Ozturk, Argin, Ozilgen and McClements2015). Herein, both the dosage of Kln and the emulsifying speed of the high-shear homogenizer were varied to tailor the size of the EKln during emulsion preparation. On this basis, the sizes and morphologies of the EKln and those derived from CKP were further investigated.
The optical micrographs of selected EKln in various sizes prepared under certain dosages of Kln and emulsifying speeds are shown in Fig. 4a–c. Most of the Kln particles were distributed around the oil droplets to reduce the tension at the oil–water interface and to protect the emulsion against coagulation. Meanwhile, a small number of Kln particles were dispersed in the aqueous phase because of the hydrophilicity of Kln particles (Dickinson, Reference Dickinson and Wedlock1994; Liang et al., Reference Liang, Li, Dai, Tang, Cai and Zhen2018). Compared to the shapes of oil droplets in these emulsions, some of the oil droplets in E0.4Kln-12r at a high dosage of Kln (0.4 g of Kln in 16 mL water and 4 mL paraffin) and a high emulsifying speed (12,000 rpm) are non-spherical, as indicated by the arrows in Fig. 4a. In contrast, the oil droplets in E0.2Kln-10r and E0.1Kln-8r were almost entirely spherical in shape. This suggests that a high dosage of Kln and a high emulsifying speed produced non-spherical droplets in the Kln-stabilized emulsion system. As a result, some non-spherical CKP particles were formed (e.g. CKP-0.4Kln-12r in Fig. 4d). Due to aggregation of Kln particles, the individual microcapsules were more easily aggregated for CKP-0.4Kln-12r when compared with CKP-0.2Kln-10r and CKP-0.1Kln-8r (Fig. 4d–f).
Fig. 4. Optical micrographs of (a–c) EKln and (d–f) CKP fabricated at various dosages of Kln and emulsifying speeds: (a, d) 0.4 g and 12,000 rpm; (b, e) 0.2 g and 10,000 rpm; (c, f) 0.1 g and 8000 rpm; the insets correspond to the diameter distributions of CKP in (d), (e) and (f).
As CKP was derived from EKln, the size of CKP was determined by the size of the oil droplets in EKln. The diameters of the CKP and the oil droplets in EKln were statistically analysed. Both the CKP and the oil droplets in EKln are polydisperse (Fig. 4). The diameters of E0.4Kln-12r and CKP-0.4Kln-12r were in the range of 20–60 μm, those of E0.2Kln-10r and CKP-0.2Kln-10r were in the range of 50–90 μm, while those of E0.1Kln-8r and CKP-0.1Kln-8r were in the range 80–140 μm (insets of Fig. 4a–f). Interestingly, the average diameters of the oil droplets in E0.4Kln-12r, E0.2Kln-10r and E0.1Kln-8r were ~41.3, 68.9 and 105.1 μm, respectively, while the average diameters of the correspondingly derived CKP were ~42.0, 69.8 and 106.1 μm, respectively (i.e. they were larger by ~0.7, 0.9 and 1.0 μm than those of EKln). Such an increase in diameter of CKP is in agreement with the shell thickness measured from SEM images (Fig. 3b–d), further confirming the formation of Kln-armoured polyurea shells upon Pickering emulsion templates via interfacial polymerization.
Based on the above discussion, control of the dosages of IPDI and Kln and the emulsifying speed of the high-shear homogenizer are effective approaches to tailoring the shell thickness and the size of CKP. This method may be promising for the tuning of encapsulation and release performance for the fabricated CKP.
Tuning of encapsulation and release performance through tailoring of the diameter and shell thickness of Kln-armoured polyurea microcapsules
With Kln serving as an emulsifier, oil-in-water Pickering emulsions were obtained at the oil volume fraction (V oil / (V oil + V water)) of 0.2 in present study. Pickering emulsions with the dispersed oil phases were the precursors for microcapsule preparation; the encapsulated substances in those derived microcapsules were oil phases. Thus, the CKP might be used to carry lipophilic guests. In view of the abovementioned strategies for tailoring the diameter and shell thickness of CKP, the encapsulation and release performance might be effectively tuned.
SR, a chemical dye, was selected as the lipophilic guest and was eventually encapsulated in CKP. In the system in which CKP is used to encapsulate SR, the encapsulation performance of SR in CKP was evaluated by calculating the concentration of SR encapsulated in CKP as a percentage of the initial concentration of SR in the oil phase (Eq. 1).
$$EE = \displaystyle{{c_1} \over {c_0}}$$where c 1 is the concentration of SR encapsulated in CKP and c 0 is the initial concentration of SR in the oil phase.
The encapsulation efficiencies (EE) of SR in CKP with various shell thicknesses and diameters were calculated using Eq. 1 (Table 1). The EE of SR in CKP-xIPDI decreased from 85.4% to 76.2% with increasing shell thickness from ~0.5 to ~1.0 μm. This might be ascribed to the fact that a high concentration of IPDI reacted more vigorously and rapidly with water than low concentrations of IPDI, so that the spreading rate of SR in the oil phase with a high concentration of IPDI was faster than that with low concentration of IPDI. In addition, as the shell thickness of CKP-xIPDI increased with increasing concentration of IPDI, the thicker shell of CKP-0.6IPDI could hold more SR. Consequently, the concentration of SR encapsulated in the oil phase of CKP decreased with the increase in the shell thickness of CKP.
Table 1. Encapsulation efficiencies (EE) of SR in various CKP formulations, and the diameters and shell thicknesses of SR-encapsulated CKP formulations.
a The samples are SR-encapsulated CKP formulations.
b CKP-0.6IPDI and CKP-0.4Kln-12r are the two marks of the same sample.
c As the diameter of SR-encapsulated CKP formulations is polydisperse, the given diameter is the average statistic of the area observed under the optical microscope. The diameter of CKP encapsulated with SR is smaller than that of CKP without SR (unit: μm).
d ST = shell thickness. As SR-encapsulated CKP has a wide ST distribution, the given values are an approximations (unit: μm).
Tailoring the diameter of CKP is another approach for tuning the EE of SR in CKP. As the diameter of CKP increased from ~42.0 μm (CKP-0.4Kln-12r) to 106.1 μm (CKP-0.1Kln-8r), the EE of SR in CKP increased from 76.2% to 88.4%. This increase may be due to the fact that the larger the diameter of CKP, the greater the volume of the oil phase inside the microcapsule, and thus the higher the amount of SR. Alternatively, more SR was adsorbed by high dosages of Kln (0.4 g Kln for CKP-0.4Kln-12r) during the emulsifying process, so that the residual amount of SR in CKP-0.4Kln-12r was less than that encapsulated in CKP-0.2Kln-10r and CKP-0.1Kln-8r.
To study the release kinetics of SR from CKP, the Korsmeyer–Peppas model (Korsmeyer et al., Reference Korsmeyer, Gurny, Doelker, Buri and Peppas1983; Ritger & Peppas, Reference Ritger and Peppas1987) was applied to analyse the experimental data (Eq. 2):
$${\rm k}t^n = \displaystyle{{M_t} \over {M_\infty }}$$where Mt/M∞ is the released SR fraction at time t, k is the diffusion kinetic constant and n is the diffusion exponent of the SR transport mechanism. Diffusion exponent n values equal to 0.5 indicate Fickian diffusion as the SR release mechanism; n values between 0.5 and <1.0 are indicative of an anomalous SR release mechanism; for n values equal to 1.0, the SR release mechanism belongs to case II transport with zero-order release; for n values greater than 1.0, the SR release mechanism is related to super case II transport; and finally, n values <0.45 are most commonly observed in cylindrical matrices (Tsoi, Reference Tsoi2013).
Dialysis is a commonly used method to evaluate the release performance of sustained-release materials (Amatya et al., Reference Amatya, Park, Park, Kim, Seol and Lee2013; Weng et al., Reference Weng, Tong and Chow2020). In this work, anhydrous ethanol was used as a medium for SR release from CKP. The SR-encapsulated CKP with various shell thicknesses and diameters were set as the drug carriers to allow the release of SR in anhydrous ethanol. The SR release data from different CKP were fitted using Eq. 2, and the release kinetic parameters are given in Fig. 5a,b. The kinetic data for CKP with various shell thicknesses can be nicely fitted by Eq. 2, as evidenced by the high values of R 2 (>0.96). In contrast, the fitting of the kinetic data for CKP of various diameters is less satisfactory than that for CKP of various shell thicknesses (R 2 >0.92).
Fig. 5. Release kinetics of SR from CKP with various (a) shell thicknesses and (b) diameters.
CKP-0.6IPDI shows a lower k value than those CKP with thinner shell thicknesses. In particular, k was reduced drastically from CKP-0.5IPDI (24.84) to CKP-0.6IPDI (17.46). Except for the increase in the shell thickness itself, the shell structure of Kln-embedded and -armoured polyurea might be the other critical reason for the observed decrease. As the diameter of the Kln ranged from 0.2 to 2.0 μm, when the shell thickness of CKP increased from a few tenths of a micron to >1 μm, Kln platelets would embed in or armour the polyurea matrix more efficiently. The Kln-armoured polyurea structure caused diffusion of SR from the internal surface to the external surface, following a zigzag path rather than a linear path to bypass the Kln platelets in the polyurea matrix.
In the release system of CKP, the n values are >0.5, indicating that the SR release mechanism does not conform to anomalous and super case II transport. As the values of n are <0.45 for all CKP with various shell thicknesses, the release models in those CKP are similar to those in cylindrical matrices. Unexpectedly, the n vale in the CKP-0.1Kln-8r release system is equal to 0.5, conforming to Fickian diffusion. Even so, the release model is quite complicated and needs to be explored further. This is because the CKP was not a pure polymer–drug system but rather a core–shell system, and its shells are organic–inorganic hybrid compounds (Kln hybridized with polyurea). Hence, several parameters, including the electrostatic interactions, ionic strength and concentration (Prasannan et al., Reference Prasannan, Tsai, Chen and Hsiue2014), should be considered.
In practice, to reduce the dose frequency, prolonged release of a drug is usually desirable. Herein, the released rate of SR from all CKP is rapid, occurring within 4 h, and the complete release of SR from CKP can be controlled at from 20 to 45 h (22% SR remained not given for CKP-0.6IPDI). The sudden release of SR in the first 4 h might be ascribed to the following two reasons: first is the large concentration difference between SR inside and SR outside of the CKP in the period of its initial release; and second is the SR adsorbed on the shell of the CKP released in the first period of time when the CKP was placed in anhydrous ethanol. The prolonged release of SR from CKP might be attributed to the Kln-armoured polyurea shell structure of CKP, as evidenced by the shell profiles of CKP in SEM images (Fig. 3b–d).
The aforementioned discussion suggests that the encapsulation and release performance of CKP might be effectively tuned through tailoring the diameters and shell thicknesses of CKP, achieved by varying the dosages of IPDI and Kln and the emulsifying speed during the preparation of Kln-stabilized Pickering emulsions.
Conclusions
Core–shell CKP were fabricated by interfacial polymerization in a Kln-stabilized Pickering emulsion. Kln was hybridized with polyurea, forming a Kln-embedded- and -armoured polyurea structure. The fabricated CKP has good dispersibility in anhydrous ethanol and is spherical. Control of the dosages of IPDI and Kln and the emulsifying speed of the high-shear homogenizer in the emulsifying system are effective for tailoring the shell thickness and diameter of CKP. The shell thickness of CKP can be tailored from ~0.5 to 1.0 μm and the diameter of the microcapsules can be tuned from ~20.0 to 160.0 μm.
By tailoring the shell thickness and the diameter of CKP, the encapsulation and release performance of liposoluble compounds in CKP may also be tuned. When lipophilic SR was encapsulated in CKP, the CKP exhibited excellent sustained-release properties. The complete release of SR from CKP can be controlled at from 20 to 45 h in anhydrous ethanol. As for the CKP prepared at IPDI dosage of 0.6 g, 22% of SR still remained after 45 h in anhydrous ethanol. Owing to the dual role of Kln as an emulsifier and enhancer and the controllable encapsulation and release performance of microcapsules, this work may have implications for the development of organic–inorganic composite vehicles for drugs and pesticides.
Financial support
The authors wish to acknowledge financial support from the National Natural Science Foundation of China (No. 42062003; No. 41572034), the Guangxi Natural Science Foundation (No. 2019GXNSFBA245052; No. 2018GXNSFAA294012), the Engineering Research Center of Non-metallic Minerals of Zhejiang Province (No. ZD2020K05) and the Zhejiang Biochar Engineering Technology Research Center (No. 2019ZJB08).
