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Interfacial Activity and Self-Assembly Behavior of Dissolved and Granular Octenyl Succinate Anhydride Starches Wei Liu, Yue Li, Douglas Goff, John Nsor-Atindana, Jianguo Ma, and Fang Zhong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00069 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019
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ABSTRACT: The mechanisms of granular octenyl succinate anhydride (GOSA) and dissolved OSA (DOSA) starches in emulsion stabilization were investigated. In general, DOSA starch offered better emulsification activity by generating greater z-potential, lower particle size as well as long-term stability in comparison to GOSA starch of close degree of substitution (DS). A compact interface in DOSA starches was determined, resulting from an increased surface loading value of 2.37 mg/m2 in comparison to that of GOSA of 1.6 mg/m2. Additionally, the irreversibly adsorbed and predominantly elastic interface of both DOSA and GOSA starches indicated that the DOSA starch may be a Pickering emulsifier rather than a biopolymer surfactant. This assumption was confirmed by TEM. Spherical micelles with average diameters of 100 nm were observed above the CMC of 1 mg/mL. Moreover, samples G28 (representing DS of 0.028), D28, G16 and D16 could reach equilibrium interfacial tensions of 19.4, 16.5, 20.0 and 19.3 mN/m, respectively. However, due to misleading contact angle as a result of rough surfaces and non-ignorable gravity of GOSA starch, the energy escape equation failed to be employed in this study.
KEYWORDS: granular OSA starch, dissolved OSA starch, TEM, SEM, micelle, interfacial stabilization, emulsion.
INTRODUCTION Octenyl succinate anhydride (OSA) starch is a widely used food-grade Pickering surfactant by incorporation of OSA groups to induce hydrophobicity to the starch granules.1 It has been proven that Pickering emulsions usually exhibited more stable than those emulsions stabilized by surfactants or biopolymers2. The emulsifiers are usually irreversibly adsorbed to surface, thereby lowering the total free energy and endowing the emulsion with long-term stability to coalescence.3 Dissolved octenyl succinate anhydride (DOSA) starch derived from heat treatment has attracted some interests in recent times as a result of an increased solubility.4 By state conversion from granular to macromolecular, Matos5 documented that DOSA starch led to unstable systems in comparison to granular octenyl succinate anhydride (GOSA) starch, which was completely in agreement with the Pickering emulsion stabilization theory. However, Miao, et al.6 demonstrated DOSA starch emulsified system performed stable compared with the granular starch over a 30 day storage period. Moreover, it was demonstrated that the barrier properties of granules on the surface of oil droplets were enhanced by a heat treatment.7 Additionally, Bergenståhl8 reported that the adsorbed emulsifiers, which are dissolved in the aqueous phase, could boost the effect of steric hindrance. Currently, methods for characterization of the emulsification capacity of OSA starch mainly involve zeta potential, particle size, emulsion activity and monitoring of stability during shelf-life. Although some insights into the stabilization of OSA starches could be offered by these techniques, the measurements typically involve large 3
uncertainties and the associated results are not essential factors to elucidate the mechanism. Mechanistically speaking, there are two main ways whereby surfactants can stabilize emulsions,9 including a network barrier effect and a surface attachment effect against instability. In practice, the stabilization of emulsion could attribute a combination of these two effects. Liu, et al.10 have found the network of emulsions stabilized via granular OSA starches was uncompact. However, since destabilization resulting from formation of bridges between particles may appear when surface coverage is insufficient, the adsorption behavior may be more important to understand the role of DOSA and GOSA as emulsifier The surface loading (Γ) is an effective parameter to quantify the amount of macromolecule per surface area. Interfacial rheology is also a crucial means to characterize the interfacial adsorption kinetics on the interface.11 In addition, it has been reported that adsorption energy is closely related to the stability of emulsions.12, 13 Nevertheless, this has not yet been referred directly in OSA starch-based emulsion systems. The energy binding of a granular or dissolved starch to the interface, ΔE, is calculated using the following equation: ΔE = −π𝑅' 𝛾) [1 − (𝛾' − 𝛾- )/𝛾) ]'
where R indicates the radius of starch, γ0 is the interfacial tension (IFT) of interface, γ1 and γ2 are the IFTs of the starch-water and starch-oil interfaces, respectively. As the combination of (γ2 - γ1)/γ0 is the cosine of the Young-Dupre contact angle,13 Eq 1 could be expressed as follow: ΔE = π𝑅' 𝛾12 (1 ± 𝑐𝑜𝑠𝜃)'
where 𝜃 stands for the contact angle (CA).12 From Eq 2, thermally excited escape of GOSA and DOSA starch, which is related with R2, IFT and CA could be used to evaluate the interfacial ability between GOSA and DOSA starches.14, 15 Previously, the degree of substitution (DS) was considered to be the most important factor that affected capacity of OSA starch as an emulsifier. The fact that the approved maximum additive amount of OSA is set at a level not exceeding 3% in starch by the FDA as well as European Union needs to be recognized.16 Hence, it is critical to develop a new strategy for enhancing emulsification capacity of OSA starch with limited DS. In this case, characterization of the different adsorption behaviors of varying OSA starches could provide new opportunities for the formation of emulsifier simply by altering the heating temperature, and thereby opening up new applications as biological delivery surfactants.
MATERIALS AND METHODS Materials. National Starch and Chemical Company (Bridgewater, NJ, U.S.A.) provides the waxy maize starch in this study. OSA (97%) was obtained from Sigma– Aldrich (St. Louis, MO, U.S.A.). Medium-chain triglycerides (MCTs) were purchased from KLK Oleo, Ltd (Malaysia). Sample Preparation. The GOSA starches with DS of 0.016 (named G 16, additive amount of OSA is about 1.8%) and 0.028 (named G 28, additive amount of OSA is approximately 3%) were prepared as the method of Liu, et al.17 The DOSA starch (D 16 and D 28) was prepared from GOSA starch during a heat treatment (10% w/w and 5
heated at 80oC for 0.5 h) and a spray drying procedure. The morphologies of these samples were observed using a SEM (Quanta-200, FEI company, Eindhoven, Netherlands). Emulsion preparation and stability determination. 1wt% DOSA starch solutions and GOSA starch suspensions were prepared by mixing in water for 3h at 30oC. Subsequently, 10 wt% MCTs were added. The crude emulsions were prepared using high-speed shearing (IKA T-18, IKA Corporation, Staufen, Germany) at a speed of 18,000 r/min for 5 min. The nano-emulsion was obtained following homogenization treatment at 30 MPa. Zetasizer (ZetaPALS; Brookhaven, NY, U.S.A.) was used to determine the particle size and zeta potential. Calculation of the Surface Loading Value of OSA Starches. After centrifuging at 16,000 r/min for 0.5 h, the DOSA starch systems were separated into two phases: the cream layer and the supernatant. GOSA starches were divided into three layers: the cream layer, the supernatant and the precipitate. The cream layers were carefully removed using a syringe. The starch concentrations of the supernatant and precipitate were determined by dual-wavelength methods. The adsorbed amount of DOSA starch, cads (DOSA) is obtained from the following equation: 𝑐89:(;<=>) = 𝑐[email protected]@[email protected]@ − 𝑐:DEB8F8BF
where φ is 0.1 (the oil phase volume fraction).8 The surface area (SA) of a polydisperse is (6φ/d50) exp (0.5β2), in which β = ln (d85/d15).18 The Surface Morphology of Crude Emulsion. Photographs were taken of crude emulsions with OSA starch concentrations of 0.3, 1, 4 and 10 mg/mL by a fluorescence microscope (Fluorescent DIC/500 pixels, Nikon, Japan).10 The morphology of singular GOSA and DOSA starches in nanoscale were investigated in aqueous solutions by transmission electron microscope (TEM) (S1). In addition, the method to analyze the surface morphology of droplets at nanoscale was provide in S2 (environmental scanning electron microscope). The morphology analysis on the emulsion droplet interface at19 Critical micelle concentration (CMC) determination. CAC was investigated as Lei, et al.20 A series of DOSA and GOSA starch suspension (100 to 0.001 mg/mL) were prepared. The intersection of I1/I3 versus OSA starch concentration plots was defined CAC. The radius of DOSA starch was determined as Liang, et al.21 The particle size of GOSA starch was measured as Santhiya, et al.22 Interfacial Rheological Measurement. The rheological properties of OSA starches were measured in oscillation modes with a rheometer (HR-2, TA Instruments, U.S.A.). At first, GOSA and DOSA starch dispersion (0.5 wt%) was poured into a cup. Subsequently, a ring geometry was positioned accurately at the interface. A certain volume of MCTs were then spread onto the starch dispersion surface slowly. Dynamic time sweeps were performed at a strain amplitude of = 0.1% and an angular frequency of ω = 1 rad/s. Dynamic strain sweeps were conducted with an angular frequency of ω 7
= 1 rad/s. Frequency sweeps were operated with amplitude of = 0.2%. Moreover, the critical micelle concentration was determined followed the method of Lei, Liu, Ye, Chen and Zhao.20 Dynamic FIT Measurement. The dynamic FIT was conducted using a pendant drop method (Dataphysics, OCA 15EC, Germany) at temperature of 20oC. A droplet (40 μL) of aqueous dispersion of GOSA and DOSA starches (1 wt%) was formed in MCTs. An image of the droplet was recorded over the course of 3000 s. Interfacial CA Measurement. CA was determined on fresh OSA starch tablets with sessile drop method, by depositing a starch tablet at the bottom of a transparent glass dish with MCTs. Then 15 μL of DI water was injected onto the tablet surface through syringe (KDL Corp., Shanghai, China). A digitized image software OCA 15EC was used to automatically calculate CA within 100 seconds before swelling began.23 In addition, the surface distribution of substitutions on GOSA starch was determined by fluorescence microscope (S3). Statistical Analysis. All measurements were expressed as means of triplicate determinations. Treatment means were considered significantly different at p < 0.05.
RESULTS AND DISCUSSION Stability Comparison between Emulsions Prepared with Granular and Dissolved OSA-S. The DOSA and GOSA starches were selected as stabilizers of emulsions. The distribution of Sauter mean size are shown in panel A of Figure 1. The average sizes in DOSA starch stabilized emulsions system were significantly small 8
when compared with that of GOSA starches. By considering the ζ potential of individual water, MCT and dispersions of starches (S4), the ζ potential values of emulsions prepared by GOSA and DOSA starch significantly decreased, indicating an increase. It should be noted that the DOSA starches exhibited a significant influence on the descending ζ-potential of emulsions (P<0.001, analyzed by one-way ANOVA, Figure 1B). In addition, the polydispersity index (PDI) of DOSA starch emulsions decreased relative to granular emulsions (Figure 1C). All the emulsions remained stable after 30 days at 20oC. However, the particle size of GOSA starch emulsions increased markedly as a function of storage time. It indicated that the prolonged stability of the GOSA starch as an emulsifier was weaker than that of DOSA starch (Figure 1D).
Figure 1. The characteristics of emulsions prepared by GOSA and DOSA starches: (A) The size distribution of fresh emulsions, (B) The ζ –potential, (C) Polydispersity index, (D) Particle size of emulsions during storage time (T=20oC).
Loading of GOSA and DOSA Starches at the Interface. To further understand the mechanisms of surface stabilizations, a series of crude emulsions were prepared 9
using GOSA and DOSA starches as the emulsifiers. Surface arrays at concentrations of 0.3 to 10 mg/mL were determined by fluorescence microscopy (Figure 2 a-d). Isolated granular GOSA starches were observed on the surfaces of droplets, which demonstrated the Pickering mechanisms on surface stabilization of GOSA starches.24 At all concentrations of GOSA starches, there were a greater number of void spaces on the interface.25 However, no obvious particles could be observed on the surface of DOSA starch emulsions. The interface of DOSA starch was compact and homogenous (Figure 2 e-h), which was similar to soft matter or a biomacromolecule surfactant. It has been suggested that in comparison with biopolymers emulsified system, Pickering emulsifiers are generally regard as a long-term stable surfactant.26 Emulsion generated by DOSA starch gained a better long-term stability in this study (Figure 1). To explain this phenomenon, the full covering concentration of adsorbed GOSA and DOSA starch was investigated first. It reached 4 mg/mL of GOSA starch, whereas it decreased to 0.3 mg/mL in respect of DOSA starch. In view of their pliability, the formation of an integrated interface was boosted by DOSA starches. Furthermore, the thickness of interface increased further after full coverage, which indicated the formation of a multilayer adsorption (Figure 2d).
Figure 2. The interface information of GOSA starch: (a-d) at concentrations of 0.3, 1, 4 and 10 mg/mL, respectively, scale bar, 20 µm. The interface information of DOSA starch: (e-h) at concentrations of 0.3, 1, 4 and 10 mg/mL, respectively, scale bar, 20 µm. Schematic representations: (i-j) droplet coverage by GOSA or DOSA starch.
The surface loading was subsequently examined in both states of the samples, where it was revealed that the surface loading values were slightly elevated with an increase in DS. However, a drastic increase was found when the state was converted from granular to dissolved. For example, the surface loading of G16 was 1.56 mg/m2, and increased to 2.37 mg/m2 in D16. This increased trend may be attributable to the flexible structure of amorphous starch chains in DOSA starches. After heat treatments, OSA starch molecules compacted and formed denser interfaces, due to the flexible characteristics of starch chains. The stiff granular structure and large granular scale may prevent or limit their movement to the interface. Consequently, the compact and thick interface may give the potential of DOSA starch to prevent the aggregation of oil droplets.
Table 1. Results from Adsorption Experiments with GOSA and DOSA Starches. a*
initial OSA starch (mg/ml) C0
adsorbed OSA starch (mg/ml) C1
absorption yield %
344.2b 760.0c 313.8d
136.7b 510.5c 130.5d
214.5b 683.7c 201.8d
1307.9a 336.4b 915.7c 312.0d
a* C /C is the adsorption yield. Different superscript letters in the columns indicate significant differences 1 0 (P < 0.05).
The Interfacial Shear Rheology of the GOSA and DOSA Starches. Interfacial shear rheology is a crucial means to detect the structure of adsorbed layers, which monitored not only the Ostwald ripening, but also coalescence between approaching droplets.27 Figure 3A showed time-sweep result. The interfacial G′ and G′′ of all samples rapidly achieved steady values. The G′′ is significantly lower than G′ but showed no significant differences between samples. Thus, the interfacial films of both GOSA and DOSA starches were predominantly elastic. The long-time rheological response in Figure 3 implied the appearance of compact network, resulting from interface-mediated capillary forces. However, the DOSA starches exhibited higher G′ than GOSA starches. This indicates a stable interfacial film to deformation, which was 12
contributed by the densely packed interfacial layer.
Figure 3. Interfacial rheological properties: (A) Time dependence of the interfacial G′ and G′′ (B) Strain dependence of the interfacial G′ and G′′ during strain sweep after adsorption equilibrium, (C) Angular frequency sweep.
Figure 3B shows the linear domain limit of the interfaces formed by all investigated OSA-S systems. A linear viscoelastic region could be found when deformation is between 0.1% and 0.4% in all layers; above this limit, the G′ decreases significantly, whereas the G′′ decreases slightly with deformation. The above results may be caused by rearrangements of surfactant in the interfacial network, resulting in a reduced storage of energy.27 The interfacial layer structure breaks down and shear thinning of the ruptured layer appears since the interfacial G′ becomes lower than the interfacial G′′, when the strain amplitudes are higher than 2%. It should be noted that despite similar linear domain limit values, the DOSA starches possessed larger G′ than GOSA, indicating a further stability to deformation. The interfacial linear dynamic moduli were investigated as a function of angular frequency (Figure 3C). It remain independent of the frequency over the range of tested frequencies and the elastic behavior showed G′ > G′′. In addition, G′ and G′′ are essentially independent of frequency, analogous with particulate gel networks.27 It is clear that no intersection of G′ and G′′ was observed throughout the frequencies probed, suggesting that the transformation of viscoelastic 13
characters of interfaces was quite difficultly. As expected, the G′ of DOSA starch films was improved, which corresponded with an increased emulsion capacity (Figure 1). Hence, it can be assumed that DOSA starch endowed the interfacial films high elasticity and toughness, and thereby it could be a Pickering emulsifer rather than normal biopolymer surfactant.38 Micelle Behavior of GOSA and DOSA Starches in solution. To verify whether a particle covered surface was generated, the surface behavior of DOSA as well as GOSA starches at nanoscale was assessed. Based on the flexible structure and the amphiphilic character of DOSA starch, it is possible to self-assemble into micelle. The CMC was investigated to verify whether the micelle also contributed to the thermodynamic stability of emulsions. The emission spectra of pyrene in aqueous DOSA starches are shown in Figure 4B. The fluorescence intensity versus the logarithm of DOSA starch concentrations was plotted in Figure 4C. There was a significant decrease in I338/I335 with the DOSA starch concentration increase, which was ascribed to the self-assembly behavior of DOSA starch, where pyrene transferred to the interior as a result of their high sensitivity to hydrophobic microenvironment.20 However, the CMC could not be determined in GOSA starch (Figure 4E and Figure 4F) as a result of rigid granular shape as well as a large granular size (Figure 4D). The micelles of DOSA starch were also observed by TEM (Figure 4G). The diameter of these micelles was ranging from 30 to 150 nm, which was in accordance with that obtained from HPSEC−MALLS−RI System21 (65.8 nm in Figure 5a). The DOSA micelle could attach on the surface of droplets, resulting in a predominantly elastic interface, which was in 14
accordance with the previous interfacial rheological measurement. However, due to the large molecular weight of starch as well as appearance of the OSA modification throughout the whole back bone of a starch molecule chain, the micelles formed by OSA starch may be similar to casein micelles rather than the conventional micelles formed by surfactants.28 In terms of GOSA starch, although the minimum scale of was about 100 nm, the hydrophobic interaction was not strong enough to aggregate together (Figure 4H) in GOSA starch. The above results strongly verify that DOSA starch actually was a Pickering emulsifier, of which definition was unmodified organic and inorganic nanoparticles, self-assembly particles as well as micelles. DOSA starch enhances the stability of emulsions through the combination of boundary layer and micelles.19
Figure 4. Characterization of the micelle behavior of DOSA and GOSA starches. (A) Particle size of DOSA starch micelles. (B) Fluorescence spectra of pyrene at different concentrations of DOSA starch. (C) I338/I335 ratio of intensity versus logarithm of DOSA starch micelle concentrations. (D) Particle size distribution of GOSA starch. (E) Fluorescence spectra of pyrene in the presence of increasing concentrations of GOSA starch. (F) I338/I335 ratio of intensity versus logarithm of GOSA starch micelle concentrations. (G) TEM image of nano GOSA starch granules. (H) TEM 15
image of DOSA micelles. (I-J) schematic representation of singular GOSA and DOSA starch in the emulsion, respectively.
ESEM was employed to investigate the surface behavior of GOSA and DOSA starches on oil droplets at nanoscale. The original GOSA starch showed a characteristic angular shape as shown in Figure 5C. The nano-GOSA starches maintained the granular shapes and exhibited an uneven distribution on the surfaces of the silicon oil droplet in Figure 5A. In the case of DOSA starches, the original DOSA starch particles took on an elliptical and shrunken shape (Figure 5D), which was caused by spray drying. After emulsification procedures, the shrunken DOSA-S particles disappeared. The starch molecules uniformly covered the surface to follow the curvature of the droplet (Figure 5B). However, no obvious micelle particle could be observed, maybe due to a fairly thick interface formation by multilayer adsorption.
Figure 5. ESEM of cross-linked silicone oil droplets emulsified by nano-GOSA and nano-DOSA starch: (A) G28, Scale bar, 500 nm, (B) D28, Scale bar, 1 µm. Original SEM of GOSA and DOSA starch: (C) G28, Scale bar, 50 µm (D) D28, Scale bar, 200 µm.
IFT Measurements. The IFT values of all emulsion systems are listed in Figure 6. Compared to equilibrium IFT of 22.3 mN/m in pure MCT/water system, there is only a slight decrease in native starch/MCT interface. However, the equilibrium IFTs of G28, D28, G16 and D16 were 19.4, 16.5, 20.0 and 19.3 mN/m, respectively. These decreases in IFTs are most likely caused by the transport of surfactants between the oil and bulk 16
phase.29 Furthermore, when compared with the GOSA starches, the DOSA samples reached their equilibrium relative IFT more rapidly. Therefore, IFT could further confirm the long-term stability of DOSA starches, which was consistent with the previous surface loading and interfacial rheological results.
Figure 6. The time-dependent interfacial tensions at the interfaces of granular OSA-S/MCTs and dissolved OSA-S/MCTs (OSA-S concentrations c=0.5% w/w, T= 20 °C).
Inspection of equation 2 reveals that increasing the value of R should increase ΔE. The radius of granular OSA was nearly 20um, compared to 78.6 nm of DOSA starch (Figure 5C and Figure 4E). Thus, the ΔE of GOSA emulsion should be significantly larger than that of the dissolved one. 12,30 Nevertheless, the prolonged stability of dissolved OSA starch was better than granular ones in this study (Figure 1). Equation 2 was established based on the assumption it is gravity could apply significant effect if the particle size is more than a few microns (1μm).31, 32 Actually, the gravity in DOSA starch could not be ignored. Thus, Tavacoli, et al.33 demonstrated that interface would be unstable when the surfactants exceed a certain gravity. Bo = 𝑅' ∆𝜌𝑔/γ(1 − 𝑐𝑜𝑠𝜃)
where R represent the size of emulsifier, ∆𝜌 indicates the density contrast of the 17
emulsifiers and the aqueous phase, g stands for gravity, γ suggests the o/w IFT, and 𝜃 is the CA between the emulsifiers and the o/w phase. A value of Bo > 3/4 indicates that particle will be isolated off the interface due to the gravity. Under normal circumstances, gravity does not detach particles as a result of ∆ρg/γ≈106 for micron scale particles. However, it seems to neglect that the body forces of surfactant can be transfer from one to another, resulting in arising of collective instabilities. CA Measurements. In addition to the size of emulsifier and IFT, the escape energy of a surfactant to interface in equation 2 also is related to CA.9 Figure 7 exhibits a schematic of a sessile-drop CA system. The dynamic CA was evaluated in Figure 8. The CA of native starch experienced a sudden decrease, indicating a completely hydrophilic character. Similar with native starch, GOSA starch also exhibited a rapid decline of CA as a result of lower DS. Interestingly, the D16 showed a stable trend in CA, although it possessed the same DS to G16, suggesting the dissolved state could improve the amphiphilic feature as a result that flexible structure of dissolved OSA starch chains could endow the concentricity of OSA groups. Both G28 and D28 were steady over time due to high DS. Table 2 summarizes the equilibrium CA of both GOSA and DOSA starches. The results verified that the D28 showed the best amphiphilic character (CA of 89.8), whereas lowest is the case of G16 (CA of 29.9). In general, if interfacial CA θ is <90°, particles usually express hydrophilicity and tend to stabilize o/w emulsions, whereas when θ is exceeded 90°, hydrophobic emulsifiers are suitable to stabilize w/o emulsions. 5 Nevertheless, it seems to contradict the G28, which was successful to manufacture a steady O/W emulsion (Figure 1) with an equilibrium CA 18
of 117.4 (table 1). The above contradiction may be attributed to the roughness and heterogeneity of surface of surfactants.23 Since γwo, γso and γsw (Figure 7) are thermodynamic features of oil, water and emulsifier, θ indicates a unique, theoretical CA; in practice, however, CA is complicated. On very rough surfaces or smooth but chemically heterogeneous solid surface, CA are larger than on a flat surface. In this study, compared with DOSA starches (Figure 4E), the surfaces of GOSA starches were rough (Figure 5C). We also noticed that the substitution were heterogeneously distributed on the surface of granules. The heterogeneity of OSA groups increased with increasing DS (Figure 9). Additionally, the granular OSA starch could be hydrated in the emulsion rather than a completely dried solid substrate, and thereby increasing the CA to a certain extent. 34 Accordingly, the CA of G28 determined in this study may be higher.
Figure 7. Schematic of a sessile-drop contact angle system
CONCLUSIONS We have demonstrated that stable o/w emulsions can be prepared using DOSA and GOSA starch. Compared with GOSA starches, the emulsification characteristics of DOSA starches improved significantly in terms of decreased z-potential, reduced particle size as well as prolonged stability. In conclusion, the reason why DOSA starch maintains better long-term stability is an increased surface loading value, a homogenous and impact interface as well as an intensive interfacial film of DOSA starch, and therefore strong enough to resist droplet flocculation or coalescence. The irreversibly adsorbed and predominantly elastic interface of both DOSA and GOSA starches indicated that the DOSA starch may be a Pickering emulsifier, rather than a biopolymer surfactant in this study. Spherical micelles with average diameters of 100 nm were observed above the CMC of 1 mg/mL. Thus, in addition to the influence of boundary layer, alternatively, micelle also could contribute to increase the emulsion stability. Moreover, the low IFT of DOSA starch and non-ignorable gravity of GOSA starch also suggested the potential of DOSA starch to maintain a long-term stability. However, due to rough surfaces and heterogeneous substitution distribution on the surface of GOSA starch, the CA determined in this study may be misleading, thus it cannot be employed for the energy escape equation. This study also provided a thought to improve the interfacial activity of OSA starch by a facile and green route. These results also suggest that DOSA starch possesses greater potential in the development of biocompatible materials in some delivery systems as a result of its nature and selfassembly behaviors. 21
■ SUPPORTING INFORMATION More information on DOSA starch preparation, as well as SEM, TEM and ESEM measurements was supplied as Supporting Information.
■ AUTHOR INFORMATION Corresponding Author *Email: [email protected] Tel: +86-510-85197876, Author Contributions. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.
■ACKNOWLEDGMENTS This research was supported by the National Key R&D Program of China (2016YFD0400802), the National Natural Science Foundation of China (No. 31571891, No. 21676122. The research is also supported by national first-class discipline program of Food Science and Technology (JUFSTR20180204), 111 project-B07029, and program of “Collaborative Innovation Center of Food Safety and Quality Control in Jiangsu Province”, China.
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