Biomass lignin stabilized anti-UV high-internal phase emulsions

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Biomass lignin stabilized anti-UV high-internal phase emulsions: Preparation, rheology, and application as carrier materials Kai Chen, Lei Lei, Yong Qian, Ailin Xie, and Xueqing Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04422 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Sustainable Chemistry & Engineering

Biomass lignin stabilized anti-UV high-internal phase emulsions: Preparation, rheology, and application as carrier materials Kai Chen†, Lei Lei‡, Yong Qian†,*, Ailin Xie†, Xueqing Qiu†,* †

School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and Paper

Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou, China, 510640 ‡

Department of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada

E-mail: [email protected] (Y. Qian), [email protected] (X. Q. Qiu).

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Abstract: Biomass enzymatic hydrolysis lignin (EHL) was used as emulsifier to produce oil-inwater high internal phase emulsions (HIPEs) with the assistant of sodium dodecyl sulfate (SDS). The effects of EHL concentration, SDS dosage, oil/water ratio, and pH of water phase on the microstructure, stability, and rheological properties of the HIPEs were investigated via optical microscopy and rheometer. The results showed that HIPEs with internal phase of 80 vol% had smaller diameter and tighter droplets packing when emulsified by 5.0 wt% EHL and 1.0 wt% SDS, which were very stable and displayed no significant change over a period of one month. EHL/SDS co-stabilized HIPEs with smaller droplets had higher viscosity, yield stress, complex viscosity and storage moduli values (G´). The experimental G´ values of the HIPEs were compared with the values predicted from the Princen and Kiss model and from the modified Mougel model, giving insight into the critical effects of non-ideality induced by polydispersity in highly viscous emulsions. In addition, the HIPEs exhibited an outstanding protection on UV-induced degradability of curcumin. The residual level of curcumin encapsulated in the HIPEs reached 60.3% after 72 h of UV irradiation. Meanwhile, the curcumin loaded HIPEs displayed rapid drug release and thermal stability in the PBS buffer solution. Keywords: Lignin, HIPEs, Rheology, Curcumin, UV-blocking, Thermal stability.

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Introduction High internal phase emulsions (HIPEs), usually characterized by a high viscosity and internal phase volume fraction of at least 74%, are attracting more and more attention due to a wide range of applications in cosmetics,1 food industry,2,3 liquid explosives,4 petroleum,5 paints,6 pharmaceuticals,7 and leatherworking.8 Traditional HIPEs are commonly stabilized against coalescence by a large amount of low-molecular-weight surfactants (5-50% v/v) at the oil-water interface to form dense packing layers.9-11 However, the use of low-molecular-weight surfactants become more limited in food and biomedical applications due to the increased consumption and legal requirements, such as being all natural, non-toxic, biocompatible, and eco-friendly.12,13 In addition, the relatively high cost hinders the use of several surfactants, such as Span 80 and Hypermers.14 Therefore, manufacturers have to replace or reduce the usage of low-molecularweight surfactants with label-friendly natural alternatives in the formulation.15,16 Lignin is the second most abundant botanical polymer and the unique scalable renewable feedstock consisting of aromatic monomers.17-20 It presents excellent UV-absorbing and antioxidant properties due to its aromatic skeleton and phenolic hydroxyl groups.21-23 Ugartondo et al.24 and Tortora et al.25 demonstrated that industrial lignin and their derivatives are safe to human cells and exhibit good biocompatibility. Therefore, lignin-based products have been investigated as UVblocker,21 antioxidant,26 macromolecular surfactants,27,28 nutraceutical and drug delivery.29 With the development of biorefinery, more and more byproduct enzymatic hydrolysis lignin (EHL) is produced, which maintains most of the original structure of lignin. Industrial lignin, such as EHL and alkaline lignin (AL), have been used to produce HIPEs after forming particles under acidic condition. Interconnected macroporous foams were synthesized by templating lignin particle stabilized HIPEs and exhibited excellent adsorption capacity for copper (II) ions and phenolic 3



compounds.30,31 However, lignin-based HIPEs have never been investigated for use in drug delivery systems. Curcumin are natural phenolic coloring compounds found in rhizomes of Curcuma longa.32 It has found wide application in food industry, including natural food colorant and stabilizer (in jellies).33 It is also used in pharmaceutical preparation, such as anti-inflammatory, anti-angiogenic, antioxidant, wound healing and anti-cancer, due to its vast number of biological targets, low cost, and negligible side effects.34-37 However, drawbacks, such as poor water solubility, bio-accessibility, thermal- and photo- stability, hinder the expansion of its application.38 Patil et al. reported that the stable turmeric oil-dairy emulsions could be prepared in the presence of SDS by using ultrasoundassisted emulsification.39 The emulsion droplets could be a liquid vehicle for curcumin, which could improve the bioavailability of curcumin. Zeng et al. prepared curcumin-loaded HIPEs by using gliadin/chitosan hybrids as stabilizer.7 The HIPEs displayed good creaming stability. However, the residual level of curcumin in the HIPEs was only 34.5% after 66 h of UV radiation. In this work, we report a facile approach to produce O/W HIPEs with internal phase fractions up to 80 vol% by using EHL and SDS as emulsifier. Curcumin was dispersed in the internal oil phase and effectively encapsulated in the HIPEs. The ability of HIPEs for protecting bioactive curcumin against UV irradiation and thermal degradation was evaluated. The sustained-release of curcumin from these HIPEs was also investigated. This work opens a promising route to create HIPEs by using lignin, which has a good potential in drug delivery application.

Materials and methods Materials. Enzymatic hydrolysis lignin (EHL) extracted from corncobs were supplied by 4


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Longlive Biotechnology Co. Ltd. (Shandong, China). The physicochemical parameters of EHL were shown in Table S1. Soybean oil was purchased from a local supermarket (Guangzhou, China). 98% sodium dodecyl sulfate (SDS) and curcumin were purchased from Sigma–Aldrich (Shanghai, China). All other chemicals were purchased from Guanghua Sci-Tech Co. Ltd. (Guangdong, China) and used without further purification. Water used in all experiments was purified by deionization. Preparations and characterization of HIPEs. The HIPEs were prepared using a Scientz-IID ultrasonic cell disruption system (Ningbo Scientz biological Co. Ltd., China) with output pulsing of 2 s on and 3 s off and 40% amplitude for 5 min. Four parameters, namely EHL concentration, SDS dosage, O/W ratio and the pH of the aqueous phase were investigated. Experimental factors & levels for preparation of EHL-based HIPEs are summarized in Table S2. The microstructures of HIPE droplets dispersed in water were observed using an optical microscopy (XPV-800E, Shanghai Bi Mu Instrument Co., Ltd. China). The average diameter and size distribution of droplet were obtained by analyzing the optical micrograph of emulsion droplets using a Nano Measurer 1.2 software.4 The sample size is about 200. Storage stability was characterized by analyzing the changes in HIPE droplets size after being stored at room temperature for 7 and 30 days. Rheological characterization of the HIPEs. The rheological properties of the HIPEs were studied using a rheometer (RV-I, Haake, Germany) with a cone and plate geometry (60 mm diameter, 1° cone angle, and 0.05 mm gap). The steady shear tests were performed in a flow mode at 25 °C by increasing the shear rate from 0.01 to 600 s−1. The obtained flow curves were presented in terms of viscosity (η) and shear rate. The amplitude sweep measurements were conducted by varying shear stress from 0.01 to 10 Pa at a frequency of 1 Hz. This oscillatory test would give information of the 5



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storage and loss moduli (G ´ and G", respectively) in a linear viscoelastic region (LVR). The frequency sweep measurements were performed to analyze the viscoelasticity of the samples in linear viscoelastic range at an angular frequency of 600-0.1 rad/s. Complex viscosity (|η*|), incorporating the elastic and viscous responses, was calculated by the following equation.     '2   "2 

1/ 2

 G "   G '             2

2 1/ 2

  



G*

(1)



where η´ is the dynamic viscosity, η" is the elastic (imaginary) part of the complex viscosity, G´ is the elastic (storage) modulus, G" is the viscous (loss) modulus and ω is the angular frequency. Dispersing curcumin in HIPEs and the anti-ultraviolet (UV) study. To prepare curcuminencapsulated HIPEs (HIPEs-cur), the oil phase was first prepared by dispersing curcumin in soybean oil. After stirring in the dark for 8 hours, a homogeneous dispersion with a concentration of 0.37 mg/mL was produced. HIPEs-cur were then prepared by processing the oil phase (80 vol%) and water phase (1.0 wt%, 3.0 wt%, 5.0 wt% of EHL and 1.0 wt% of SDS) in the ultrasonic cell disruption system for 5 min. For the anti-UV analysis, 10 g HIPEs-cur sample was poured into a 10×60 mm (height× diameter) dish. UV radiation experiments was performed in an UV aging chamber (WT-UVA, Dongguan Wanjia Instrument Co., Ltd., China) equipped with a light with a power of 15 W and wavelength 340 nm at 33 °C. 5 mL of methanol was used to extract curcumin from the HIPEs-cur samples after 0, 3, 8, 24, 48, and 72 h. Absorbance of the extract was measured at 425 nm by a UVVis spectrophotometer (UV-2550, Shimadzu, Japan). Same amount of curcumin dispersed in soybean oil was used as a control sample. In vitro release study of the HIPEs-cur. 1 g HIPEs-cur was dispersed in 5 ml PBS buffer solution (pH=7.4) with 0.5 wt% SDS. 50 μL mixture was taken out and diluted in 950 μL PBS 6


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buffer solution in a 2 ml tube. Then, the sample was placed in a thermostat with 130 rpm/min under 35 °C. At set time intervals, HIPEs-cur dispersion was centrifuged and the clear liquid was collected. Curcumin content was determined by high performance liquid chromatography (HPLC, Shimadzu, Japan). The column was Agilent ZORBAX SB-C18 (4.6×250 mm, 5 μm). The mobile phase was 52/48 (vol/vol) 4% acetic acid solution and acetonitrile with a flow rate of 1 mL/min. The column temperature maintained at 40 °C. The detection wavelength was 430 nm.

Results and discussion Formation and appearance of the HIPEs. Normal lignin cannot form HIPEs until it is aggregated under pH < 3.30,31 Herein, stable HIPEs were prepared by using EHL as emulsifier under neutral and basic conditions, with the help of a small dosage of SDS. Up to 80 vol% of soybean oil was used as the internal phase. It has been found that neither EHL nor SDS could stabilize such high amount of oil when used as the sole emulsifier, as shown in Fig. 1a and Fig. 1c. However, the combination of EHL (5.0 wt%) and SDS (1.0 wt%) worked in stabilizing the system, resulting in a very viscous and sticky HIPEs, as shown in Fig. 1b. EHL molecules easily form intramolecular and intermolecular aggregates due to the hydrogen bonding, π-π interaction, etc.40 Dissolving EHL in a basic solution eased the aggregation to some degree, but it could not disaggregate EHL completely. SDS could effectively be inserted into the hydrophobic cores of EHL aggregates and disaggregate them by the electrostatic repulsion between sulfonic groups of SDS. The three-dimensional net aromatic structure of EHL could be fully stretched and played better role of emulsifying the oils. In addition, SDS itself is a good emulsifier. Therefore, HIPEs were prepared successfully by using combination of EHL and SDS as emulsifiers. As shown in Fig. 1d and 1e, the emulsions could not 7



be dispersed in oil, but are distributed well in water, which demonstrated that HIPEs are O/W type emulsions. Effect of EHL content on the morphology and droplet size of the HIPEs. The first set of the HIPEs were prepared by emulsifying the oil phases under different EHL contents, while the dosage of SDS was maintained at 1.0 wt%. The morphology and droplet size of the HIPEs with EHL content varying from 1.0 wt% to 10 wt% are shown in Fig. 2. As the content of EHL increases from 1.0 wt% to 5.0 wt%, the droplet size of the HIPEs decreases from 2.70 to 2.22 μm, and the size distributions are close to being log-normal. When the EHL content is beyond 5.0 wt%, the droplet size increases rapidly, and the size distribution becomes wider, which indicates that SDS is not enough to synergize the high amount of EHL to stabilize the oil phase. On the other hand, too much addition of EHL would also cause cytotoxicity.24 Therefore, the dosage of EHL was limited at 5.0 wt%. Effect of SDS dosage on the morphology and droplet size distribution of the HIPEs. The second set of HIPEs were prepared by keeping EHL content, the oil phase volume, and pH value of water phase constant, while varying SDS dosage from 0.5 to 2.0 wt%. As shown in Fig. S1, 0.5 wt% SDS cannot synergize 5.0 wt% EHL to emulsify such high amount of oil and form HIPEs. When the dosage of SDS is beyond 0.8 wt%, stable HIPEs are obtained, as shown in Fig. 2c and Fig. 3. Both the morphology and size distribution data show that the size of EHL/SDS stabilized HIPEs droplets decreases with the dosage of SDS. In addition, more SDS is conducive to synergize with EHL to form HIPEs. However, with the aim of utilization of lignin, the dosage of SDS was kept at 1 wt%. Effect of internal phase fraction on morphology and droplet size distribution of the HIPEs. The third set of HIPEs were prepared to investigate the effect of oil/water ratio by varying the oil 8


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phase fraction from 75 to 85 vol%. As shown in Fig. 2c and Fig. 4, when the oil fraction is below 80 vol%, the HIPEs have narrow size distribution with an average diameter of 2.3 μm. Normally, HIPEs with larger oil phase fractions are difficult to flow due to the smaller droplet size and the thinner intervening film of oil droplets.4 However, when the oil fraction reached 82 vol%, the average droplets size of the HIPEs increased to 4.4 μm. When the oil phase was increased further to 85 vol%, the system could not form HIPEs, which indicates that 5 wt% EHL and 1 wt% SDS are not enough to emulsify such high oil content, as shown in Fig. S2. Effect of water phase pH value on morphology and droplet size distribution of the HIPEs. Many drugs or oils are sensitive to the pH of the dispersion system. Therefore, the effect of water phase pH on the stability of the HIPEs were investigated, while EHL content and the dosage of SDS were kept at 5.0 wt% and 1.0 wt%, respectively. The appearance and optical microscopy images of the HIPEs with pH value varying from 6.5 to 11.5 are shown in Fig. 5 and Fig. S3. When the pH value of aqueous solution was decreased from 11.5 to 9.5, droplet size of HIPEs gradually increased. When the aqueous solution possessed 6.5 < pH < 8.5, the system could not form HIPEs (Fig. S3). A possible reason for this phenomenon is that the phenol hydroxyl of EHL was protonated when the solution pH decreased. The electrostatic repulsion is therefore reduced and the intramolecular hydrogen bond enhanced, thus resulting in the aggregation of lignin molecules and decrement of the emulsifying capacity of EHL. Increasing the dosage of SDS could improve the stability of HIPEs under this condition, as shown in Fig. S4. However, the role of EHL in stabilizing the HIPEs would be weakened. Storage stability of the series EHL/SDS stabilized HIPEs. The storage stability is an important parameter for evaluating the quality of HIPEs. As shown in Fig. S5-S7, the morphology and droplet 9



diameter of the HIPEs did not significantly change after being stored for 30 days, which indicates good storage stability. Rheology of the HIPEs. The rheological properties of the HIPEs have a close relationship with their stability. Therefore, the rheological behaviors of EHL/SDS stabilized HIPEs are analyzed. Steady-shear flow properties of the HIPEs. The deformation behavior of the EHL/SDS stabilized HIPEs is shown in Fig. 6. The viscosity of EHL/SDS stabilized HIPEs decreases under increasing shear rate and exhibits a typical shear-thinning behavior because of the progressive alignment of the emulsion droplets. As shown in Fig. 6a, the viscosity of the HIPEs increases when the concentration of EHL varies from 1.0 wt% to 3.0 wt%. For system with the EHL concentration of 5.0 wt%, the critical viscosity of the HIPEs reaches a critical point. When the EHL concentration is above 8.0 wt%, the viscosity of the HIPEs reduces rapidly. The influence of SDS dosage on the viscosity of the HIPEs was also investigated, while the concentration of EHL was maintained at 5.0 wt%, as shown in Fig. 6b. The viscosity of the HIPEs increases significantly when the dosage of SDS was increased from 0.8 wt% to 1.0 wt%. This finding is in agreement with the reported results that the reduction in droplet average diameter would significantly improve the properties of the emulsions, such as the viscosity, yield stress, and storage modulus, which is caused by the thinner intervening liquid layers between the droplets result in tighter packing of droplets and higher effective viscosity.41-43 When the dosage of SDS used was above 1.0 wt%, the viscosity of the HIPEs decreased, even though the droplet size decreased. The possible reason for this observation is that SDS contributed more on stabilizing the emulsions at high SDS dosage, a part of EHL was liberated and played the role of dispersant, which results in lower viscosity. 10


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For a given EHL and SDS concentration, the emulsion samples with smaller droplets show higher viscosity when the oil content is below 80 vol%, as shown in Fig. 6c. The reason is that the intervening film of oil between droplets decreased when the oil phase fractions of HIPEs increased, which resulted in an increasing viscosity of HIPEs.4 However, in this work, when the oil content was above 82 vol%, EHL and SDS could not emulsify the oil well, resulting in the one-time increase of the size of HIPE droplets, while the viscosity decreased dramatically. HIPEs oscillatory tests: amplitude sweep testing. Amplitude sweeps give information such as yield stress or critical strain during maximum deformation that a system can tolerate without permanent structural damage. Yield stress (or critical strain) is a criterion for structural strength and shape retention under mechanical deformation. The plots of the amplitude sweeps performed on the EHL/SDS stabilized HIPEs in terms of dynamic storage and loss moduli (G´and G") variation with shear stress are shown in Fig. 7. As shown in Fig. 7, all the HIPEs reveal a typical gel-like behavior, in which G´ is greater than G". The specific values of yield stress, damping factor (tanδ), and complex viscosity (|η*|) obtained at the linear viscoelastic region (LVR) are summarized in Table S3. The 5.0 wt% EHL and 1.0 wt% SDS co-stabilized HIPEs have the largest yield stress when the oil content is 80 vol%. It further demonstrates that smaller droplet size and tighter overall droplet packing resulted in a thicker emulsion, which is more resistant to the deformation and also beneficial for the stability of HIPEs.41-43 Both |η*| and tanδ are important parameters for the storage stability of dispersions. Larger |η*| values and lower tan δ values present better stability and elastic behavior for emulsions in the linear viscoelastic region.42 Taking all of these into account, HIPEs prepared with 5.0 wt% EHL, 1.0 wt% SDS, and 80 vol% oil have better stability and elastic behavior, which is in agreement with the aforementioned results. 11



Small amplitude oscillatory tests: frequency sweep testing. The frequency sweep test could give information about the material stability over the measured frequencies at a constant amplitude. The frequency sweep tests of the prepared EHL/SDS stabilized HIPEs were conducted. As shown in Fig. 8, the G´of HIPE improves when the EHL concentration increases from 1.0 wt% to 3.0 wt% at the same angular frequency. When the EHL concentration reaches 5.0 wt%, the critical G´value of samples was observed. When the EHL concentration further increases to 8.0 wt%, the G ´ of emulsion rapidly decreases. It suggests that the droplets were strongly associated with each other due to the smaller average diameter, which resulted in an improvement of the stiffness and droplet elasticity, while G´ value increased significantly. When the concentration of EHL is kept at 5.0 wt%, the G´ value of the HIPEs with 80 vol% oil content increases as SDS concentration increases from 0.8 wt% to 1.0 wt%. However, when SDS concentration increases from 1.2 wt% to 2.0 wt%, the G´ decreases even though the droplet size decreases. This further supports the hypothesis that EHL plays role not only as emulsifier but also as dispersant here. The influence of O/W ratio on the G´ value was also investigated. As shown in Fig. 8c, the HIPEs with higher oil fraction exhibits larger G´ value. The result again indicates that the thinner intervening film of oil between droplets, the larger stiffness and elasticity of droplets results in a higher G´ value and better stability of emulsions.4 When the oil content increases to 82 vol%, the G´ value and stability of emulsions decreased due to the limited emulsifier content. Complex viscosity (|η*|) arises from a combination of viscous and elastic resistances, which represents the total resistance of the HIPEs to flow. As shown in Fig. 8, the complex viscosity of EHL/SDS stabilized HIPEs decreases with the increment of the angular frequency. It is reasonable that higher shearing frequency weakened the interaction among emulsion droplets and increased the 12


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energy dissipation, thereby reducing their resistance to flow. Experimental G ´ data fitting with Princen and Kiss, and modified Mougel models. The elasticity present in the HIPEs is the result of emulsion compression, in which process the deformation of the dispersed droplets will cause the surface area and storage modulus to increase. In order to approximate real and polydisperse emulsion droplets, Princen and Kiss44 derived the following theoretical expressions relating different rheological properties: G '  1.769

0 R32

e1/3 e  0.712

(2)

where  0 is the interfacial tension, e represents the equivalent volume fraction of the dispersed phase, while R32 is the Sauter mean radius estimated using the equivalent volume to surface area ratio. Tripathi et al.4 indicated that the film thickness of droplets could not be considered, as they are too thin, so that e is approximately equal to dispersed phase volume fraction ( d ). The expression for R32 is given as: R32 

 

i

ni ri3

i

ni ri 2

(3)

where ni is the number of droplets with equivalent radius ri. To account for the van der Waals interactions between droplets, Mougel et al.45 presented a modified Mougel model as follow: G' 

2 0 H avg d

(4)

R 2 max  d 

where R is the average droplet radius, max is the maximum volume fraction of internal phase, and d is dispersed phase volume fraction. Havg represents an average distance based on the different

interactions between droplets. In the work of Mougel et al.,45 Havg was defined as 30 nm. In the work of Tripathi et al.,4 Havg was defined as 70 nm since they employed macromolecular emulsifiers 13



(poly(isobutylene) succinic anhydride (PiBSA)-based derivative). The experimental G´ values and other estimated parameters of the HIPEs prepared with different EHL concentrations and oil fractions are shown in Table S4. The comparison of experimental values with those calculated values according to the models discussed above are shown in Fig. 9. As shown in Fig. 9, the G´ values predicted by the Princen and Kiss model are smaller than the experimental values, which agrees with the previously reported results.46,47 It was assumed that e

= d here since the film thickness could not be measured when estimating

the G´ values using Princen and Kiss model. The e values that were calculated from the experimental G´ were all larger than 1, which is unreasonable. Tripathi et al. obtained similar results and explained that a universal value of film thickness could not be used for HIPEs with different droplet sizes.4 Interestingly, the G´ values predicted by the modified Mougel model agree well with the experimental values, as shown in Fig. 9a. It should be noted that the Havg values is not constant as the emulsifier concentration increased. The reason may be that modified Mougel model is based on the dominant van der Waals interactions between droplets. When the EHL concentration was varied from 1.0 wt% to 3.0 wt%, the interaction between HIPEs droplets increased as a large amount of EHL are adsorbed on the oil surface. However, the same amount of SDS could not synergize with EHL when its concentration further increased, thus both EHL and SDS were peeled off from the oil surface, which led to the decrease of interaction between emulsion droplets. Therefore, Havg values displayed an initial increase, followed by a decline. As shown in Fig. 9b, the G´ values estimated from modified Mougel model also fit well with the experimental values when the oil fraction of the HIPEs varied from 75 to 80 vol%. Tripathi et al. have reported that for higher polydispersity emulsion, the modified Mougel model is less 14


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applicable.4 However, a slight mismatch appears in the G´ values for higher values of d with oil fraction of 82 vol%. As indicated by the droplet morphology, size distribution, and rheological analysis, EHL concentration, the main factors that influence the stability of HIPEs are the EHL concentration, SDS dosage, and O/W ratios. As shown in Fig. 10, HIPEs could not be formed with 1.0 wt% SDS alone. Interestingly, amphiphilic EHL could adsorb on the surfaces of oil droplets to form a stable layer by a synergy with SDS. When the concentration of EHL was varied from 1.0 wt% to 5.0 wt%, the HIPEs with smaller size exhibited higher viscoelasticity and stability, since the oil droplets were tightly packed and the emulsion droplets had thinner intervening liquid layers. However, when the concentration of EHL was above 8.0 wt%, the same amount of SDS was not enough to disperse EHL. Therefore, EHL tended to form intra- and inter-molecular aggregates, which might desorbed from the oil surface and resulted in the instability of HIPEs. Protective effect of HIPEs on curcumin under UV radiation and the in vitro release performance. In order to study whether or not the HIPEs encapsulation resulted in protective effects on UV-induced degradability of curcumin, the residual curcumin content in HIPEs during the UV treatment was determined and compared with other previously reported research work. As shown in Fig. 11, when EHL was added, it not only synergized with SDS to emulsify high amount of oil, but also provided UV protection. Curcumin encapsulated in HIPEs that was stabilized by 5.0 wt% EHL+1.0 wt% SDS and 3.0 wt% EHL +1.0 wt% SDS could maintain 41.7% and 60.3% of its original value after 72 h of UV radiation. In contrast, the residual ratio of curcumin in the bulk oil was only 0.9%. In the work of Zeng et al.,7 the residual ratio of curcumin in HIPEs stabilized by gliadin/chitosan hybrid particles was only 37.3% after 66 h of UV radiation. The residual ratios of 15



curcumin in HIPEs were increased significantly after adding EHL. The curcumin-rich droplets were coated by three-dimensional network of lignin, which prevents direct exposure of curcumin from UV radiation. In addition, EHL itself is a natural broad-spectrum sun blocker.21 Therefore, the photo degradation rate of curcumin is much slower at a high EHL concentration (Fig. 11a). Higher EHL concentration is also beneficial for the release rate and stability of curcumin. As shown in Fig. 11c, when the EHL concentration was varied from 1.0 wt% to 5.0 wt%, the cumulative release of curcumin from the HIPEs increased from 72.0% to 98.4% in the initial 1.5 h. EHL distributed on the surface of HIPEs tended to aggregate in PBS buffer solution and its emulsifying ability decreased. Therefore, curcumin was easily released from the HIPEs. On the other hand, the cumulative release of curcumin decreased when release time was too long, since curcumin could be degraded gradually at the temperature as low as 25 °C.48,49 Similar degradation of curcumin is also observed in this experiment, as shown in Fig. S8. Curcumin presented a good thermal stability in EHL/SDS-based HIPEs, especially those with high EHL concentration.

Conclusion In this paper, stable O/W HIPEs were produced by using EHL and SDS as stabilizers. With increasing concentration of EHL and O/W ratio, the droplet size of the HIPEs initially decreased and then increased. Higher dosage of SDS and pH value of water phase were beneficial for the dispersion of the oil phase. The rheological properties of EHL/SDS stabilized HIPEs showed that viscosity, yield stress, storage moduli, and complex viscosity had a close relationship with the droplet size and oil fraction. The HIPEs with a smaller droplet size or a larger oil phase had thinner intervening liquid layers and tighter packing of droplets. This reduced the flow of emulsions and 16


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increased the interface stability. The experimental G´ values of the HIPEs prepared with different EHL concentrations and higher oil fractions exhibited good consistency with modified Mougel model, which indicated that van der Waals interaction forces played important role in increasing the viscoelasticity. As a bioactive compound carrier, the residual level of curcumin dispersed in the HIPEs prepared with 5.0 wt% EHL and 1.0 wt% SDS were 60.3% after 72 h of UV radiation. In addition, the cumulative release of curcumin reached 98.4% in 1.5 h and exhibited some protection against thermal degradation.

ASSOCIATED CONTENT *Supporting Information Physicochemical parameters of EHL; Experimental factors & levels for preparation of EHL-based HIPEs; Viscoelastic parameters for EHL/SDS stabilized HIPEs; Estimated parameters from Princen and Kiss model and modified Mougel model; Appearance and stability of the HIPEs prepared under different conditions; Absorbance of curcumin in PBS buffer solution at 35 °C under different incubation time.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. (Y. Qian) Email: [email protected]. (X.Q. Qiu) Notes The authors declare no competing financial interest.

Acknowledgements 17



This work was financially supported by the National Natural Science Foundation of China (NSFC) (21606089, 21878113), State Key Laboratory of Pulp and Paper Engineering (201701), Guangdong Province Science and Technology Research Project of China (2017B090903003), and Guangzhou Science and Technology Research Project of China (201704030126, 201806010139).

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Figure Captions Figure 1. Appearances of the HIPEs stabilized by different emulsifier: (a)1.0 wt% SDS and 80 vol% oil; (b) 1.0 wt% SDS, 5.0 wt% EHL and 80 vol% oil; (c) 5.0 wt% EHL and 80 vol% oil; (d) HIPEs dispersed in oil; (e) HIPEs dispersed in water. Figure 2. (a-e) Optical images and (f) droplet size distributions of the HIPEs stabilized by different concentrations of EHL, 1.0 wt% SDS, 80 vol% oil at pH=12.2. The microscope image was taken with diluted HIPEs. Figure 3. (a-d) Optical images and (e) droplet size distributions of the HIPEs stabilized by 5.0 wt% EHL, different dosages of SDS, 80 vol% oil at pH=12.2. The microscope image was taken with diluted HIPEs. Figure 4. (a-c) Optical images (d) droplet size distributions of the HIPEs stabilized by 5.0 wt% EHL, 1.0 wt% SDS, different oil fractions at pH=12.2. The microscope image was taken with diluted HIPEs. Figure 5. (a-c) Optical images and (d) droplet size distributions of the HIPEs stabilized by 5.0 wt% EHL, 1.0 wt% SDS and 80 vol% oil under different pH conditions of water phase. The microscope image was taken with diluted HIPEs. Figure 6. Viscosity of the HIPEs prepared under different conditions. (a) Different EHL concentrations; (b) different SDS dosages; (c) different oil volume fractions. Figure 7. Storage and loss modulus of the HIPEs prepared under different conditions. (a) Different EHL concentrations; (b) different SDS dosages; (c) different oil volume fractions. Figure 8. Complex viscosity of the HIPEs prepared under different conditions. (a) Different EHL concentrations; (b) different SDS dosages; (c) different oil volume fractions. Figure 9. Comparison of storage modulus predicted from Princen and Kiss model, modified Mougel 25



models and experimental data measured under (a) different EHL concentrations and (b) different oil fractions. Figure 10. Mechanism diagram of the HIPEs stabilized by EHL and SDS Figure 11. (a-b) Residual curcumin levels in HIPEs-cur prepared with various EHL concentrations after UV radiation. (c) Release profiles of curcumin from HIPEs stabilized with various EHL concentrations.

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Figure 11.

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Graphical abstracts Biomass lignin stabilized O/W HIPEs provided sensitive curcumin with good UVblocking and antioxidant protections.

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Biomass lignin stabilized anti-UV high-internal phase emulsions

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