Article pubs.acs.org/Langmuir
Hydrophobic Modification of Chitin Whisker and Its Potential Application in Structuring Oil Yao Huang,† Meng He,† Ang Lu,† Weizheng Zhou,‡ Simeon D. Stoyanov,§,∥,⊥ Eddie G. Pelan,§ and Lina Zhang*,† †
Department of Chemistry, Wuhan University, Wuhan 430072, China Unilever Research and Development Shanghai, 66 Lin Xin Road, Shanghai 200335, People’s Republic of China § Unilever Research and Development, 3133 AT Vlaardingen, Netherlands ∥ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, 6703 HB Wageningen, Netherlands ⊥ Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom ‡
S Supporting Information *
ABSTRACT: A facile approach was developed to modify chitin whiskers by reacting them with bromohexadecane, and the potential application of modified whiskers in structuring oil was evaluated. The results of Fourier transform infrared spectra (FT-IR), wide-angle X-ray diffraction (XRD), elemental analysis, solid 13C NMR, and differential scanning calorimeter (DSC) confirmed that the long alkyl chains were successfully introduced to the chitin whiskers and endowed them with improved hydrophobicity and thermal transition. By hot pressing the modified whiskers, the highly hydrophobic whisker sheets were constructed, showing high contact angles close to 150°. The hydrophobic interaction between the long alkyl chains and chitin backbone induced the crystal alignment with micro− nano structure, leading to the surface roughness and high hydrophobicity of the sheets. Furthermore, the modified whiskers could form a stable dispersion in sunflower oil, displaying a remarkable thickening effect. The viscosity of the oily suspension exhibited temperature dependence and shear-thinning behavior, suggesting great potentials to fabricate oleogel without adding any saturated fat. Furthermore, the intrinsic biocompatibility of α-chitin structure benefits its application in foodstuff, cosmetics, and medical fields.
1. INTRODUCTION Recently, the demand for reducing the saturated or trans fatty acids in foodstuff has grown extensively, for health and nutritional considerations.1 When the main structurant (saturated triacylglycerol hardstocks) in edible oil is reduced, however, the sensory (mouth feel) and flavor of food is inevitably impaired. Therefore, structuring alternatives are desirable and necessary.2 Generally, the so-called organogels are used as the structuring alternatives, which are small organic molecules or mixtures, such as saturated alcohols/acids,3 waxes/wax esters,4 lecithin/sorbitan tristearate,5 γ-oryzanol/βsitosterol,6 etc. Natural polymers in the nanofibril or nanocrystal form have attracted great interest due to their excellent biocompatibility, biodegradability, and mechanical reinforcement.7 However, only a few reports have been published regarding natural polymer oleogels, owing to their hydrophilic nature and insolubility. To date, the only polymer that has been reported to be suitable for the gelation of vegetable oil is ethyl cellulose.8−10 Chitin is a natural polymer consisting of N-acetyl-Dglucosamine, which widely exists in the skeletons of crustaceans and cell walls of algae and fungi as a structural backbone.11 As an important polysaccharide with fascinating biofunction12 and hierarchical structure,13 chitin has attracted intensive interest. © 2015 American Chemical Society
Compared to cellulose and other natural polymers, chitin has more advantageous properties, such as intrinsic biocompatibility, antimicrobial activity, low immunogenicity, activating plant immune receptors,14 promoting the fibroblastic synthesis of collagen,15 and accelerating wound healing.16 Therefore, chitin has practical applications in food, cosmetics, agriculture, pharmacy, textile, and paper industries as a degradable biomaterial.17 Presently, cellulose nanocrystals have been creatively applied in monodispersed emulsion by stabilizing the oil−water interface due to their large aspect ratio and excellent adsorption capacity.18−21 However, no direct dispersion of chitin whiskers in the oil phase has been reported to date. As mentioned above, the intrinsic hydrophilicity hinders the further application of chitin whiskers as oleogel candidates. Therefore, surface modification by various approaches is essential for the successful application of chitin whiskers in a nonaqueous system. It has been reported that cellulose nanocrystals can form stable suspensions in organic solvents via partial silylation22 or surface esterification,23 and the hydrophobicity of chitin can be improved via alkylation,24 Received: November 24, 2014 Revised: January 7, 2015 Published: January 10, 2015 1641
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Langmuir acetylation,25 butylation,26 fluorination,27 O-sulfation,28 etc. Nevertheless, only a few types of reaction have been reported for chitin whiskers, e.g., reaction with phenyl isocyanate (PI), alkenyl succinic anhydride (ASA), and 3-isopropenyl-α,α′dimethylbenzyl isocyanate (TMI).29 In this work, we attempted to study the physical properties of hydrophobically modified chitin whiskers and evaluate their performance in oil thickening. The chitin whiskers were modified with long chain alkyl group via a relatively safe and facile method by reacting with bromohexadecane. And the intrinsic structure and bioactivity of α-chitin remained intact during the modification. Furthermore, we applied the modified chitin whiskers in structuring sunflower oil and evaluated their hydrophobicity and rheological behavior. The sunflower oil could also be replaced with the more universal silicone oil. Our findings would be important to extend the application scope of chitin whiskers in a nonaqueous system and the relevant chemical industries, such as medicine, foodstuff, and cosmetics.
days to remove extra bromohexadecane, DMSO, and byproducts like hexadecanol. Afterward, the product was washed with water several times to remove sodium salt and finally washed with acetone to get air dried at ambient temperature. Finally, fluffy powder samples were obtained. By changing the molar ratio of bromohexadecane to anhydroglucose unit (AGU) of chitin from 3:1, 6:1, and 12:1, corresponding samples were obtained, denoted as D3, D6, and D12, respectively. A calculated amount of D6 was added into the commercial sunflower oil and stirred at 60 °C overnight to obtain homogeneous dispersions with different concentrations of 0.03g/mL, 0.05 g/mL, 0.10 g/mL, and 0.15 g/mL (the content refer to the modified whisker, including the grafted moieties). To evaluate the effect of degree of substitution (DS) on the structuring effect, 0.15g/mL dispersions of D3 and D12 were prepared under the same conditions. The oily suspensions were stored in refrigerator at 5 °C as oleogels before use. 2.3. Characterization. Infrared spectroscopy of chitin whiskers and the modified samples was recorded on a Fourier transform infrared (FT-IR) spectrometer (model 1600, PerkinElmer Co., U.S.A.) at room temperature. Wide angle X-ray diffraction (XRD) measurement was carried out on a WXRD diffractometer (D8-Advance, Bruker, U.S.A.) with Cu Kα radiation (k = 0.15406 nm) at 40 kV and 30 mA. The XRD patterns was recorded in the range 2θ = 5−30° at a scanning speed of 2° /min. The samples were deposited on a controllable heater accessory at certain temperatures for 10 min before test. Solid-state 13C cross-polarization, magic-angle spinning, and dipolar decoupling (CP/MAS) NMR measurements were carried out at room temperature on a Bruker Avance-III spectrometer operating at 75.47 MHz. The contact time for cross-polarization (CP) was 3 ms, and the spin rate of the 4 mm rotor is 5 kHz. Transmission electron microscopy (TEM) images of chitin whiskers and modified whiskers were observed on a JEOL JEM-2010 (HT) electron microscope, with an accelerating voltage of 200 kV. The lyophilized samples were redispersed in deionized water with ultrasonication. Subsequently, a drop of the dilute suspension (0.3 mg/mL) was deposited on carboncoated copper grid. Thermogravimetric analysis (TGA) and differential thermogravimetric (DTG) for the modified whisker was performed on a TG instrument (Perkin−Elmer Co., U.S.A.) at a heating rate of 10 °C/min from 30 to 600 °C under a nitrogen atmosphere. The samples were cut into small particles and dried in vacuum oven at 40 °C for 48 h before test. Differential scanning calorimeter (DSC) experiments were performed on a NETZSCH DSC 200PC (NETZSCH, Germany) with a heating rate of 5 °C/min under a nitrogen atmosphere. The temperature was controlled with liquid nitrogen, and the dried samples were put in a tightly sealed aluminum cell. The samples were first heated to 150 °C to eliminate thermal history and then quickly cooled to −20 °C. The DSC curves were recorded when the samples were reheated from −20 °C to 150 °C. The rheology behaviors of the D6 oily dispersion were carried out on ARES-RFS III rheometer (TA Instruments, U.S.A.). The rheometer was equipped with two force transducers allowing the torque in the range from 0.004 to 1000 g·cm. Dynamic temperature ramp tests were carried out at a frequency of 1.0 rad/s and a strain of 10% to determine the complex viscosity at different temperatures. The dispersion was first heated to 80 °C in order to destroy all possible crystal memory, and then cooled down to 5 °C at a rate of 2 °C/min during when the data were collected. Steady rate sweep tests were conducted at 25 °C, with a shear rate range between 10−3 and 103 s−1. A parallel-plate and a double-concentric cylinder geometry (R1/R2 = 32 mm/34 mm) were used to measure dynamic and steady parameters, respectively. Water contact angle (CA) was measured and calculated on a Data Physics Instrument (OCA20) in dynamic mode. One drop of water (2 μL or 5μL) was put on the surface of the films with an automatic piston syringe and photographed. The powder sample of CW, D3, D6, and D12 was hot-pressed at 60 °C to form a sheet with a flat surface before testing.
2. EXPERIMENTAL SECTION 2.1. Materials. The raw chitin (RC) powder was purchased from Zhejiang Golden-Shell Biochemical Co. Ltd. (China). The weightaverage molecular weight (Mw) measured by dynamic light scattering 5% LiCl/DMAc (w/w) was 5.0 × 105,30 with a degree of acetylation (DA) = 94%, calculated from IR spectra31 as follows: A1560/A2875 = 0.0125DA + 0.2
(1)
All other chemical reagents (analytical grade) and sunflower oil were purchased from commercial sources in China and used as received. 2.2. Preparation, Modification and Application of Chitin Whisker. Suspension of chitin whiskers was prepared according to a reported method with minor modification.29 The raw chitin powder were purified and subjected to hydrolysis by 3 M H2SO4 (30 mL per gram of chitin) at 90 °C for 6 h with vigorous stirring. The resultant suspension was diluted with distilled water and centrifuged to remove excessive acid in the supernatant. Subsequently, the suspension was dialyzed until the pH reached 7. A further ultrasonic treatment (800 W) was performed on an ultrasonic cell disruptor (JY92-IID, Ningbo Scientz Biotechnology Co., Ltd., China) for better dispersion of the chitin whiskers in water. Centrifugation of the chitin whisker suspension at 7200 rpm was performed to remove the precipitate, since most of the whiskers were converted to nanoscale. Finally, the chitin whisker suspension was lyophilized for use, denoted as CW. Reaction between chitin whisker and bromohexadecane was conducted according to Song et al.32 A representive scheme of the reaction is shown in Figure 1. A 2-g portion of chitin whiskers was dispersed in 80 mL dimethyl sulfoxide (DMSO) with the assistance of ultrasonication to obtain homogeneous suspension. An 8-g portion of NaOH was added to the above dispersion as catalyst and stirred at below 5 oC for 1 h to avoid deacetylation of the chitin molecule,33 and then a calculated amount of bromohexadecane was added dropwise into the solutions. The mixture was stirred at 60 oC for 4 h, then neutralized by HCl and washed with ethanol in Soxhlet extractor for 3
Figure 1. Schemetic illustration of reaction between chitin whisker and cetyl bromide. 1642
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Figure 2. 13C NMR CP/MAS solid-state NMR spectra of chitin whisker (CW) and modified chitin whisker (D3, D6, and D12). Inserted: An enlarged view of 50-110 ppm (left) and chemical structure of modified chitin (top).
Figure 3. TEM images of chitin whisker (CW) and modified chitin whisker (D6 and D12) in water. Scale bars are 200 nm.
3. RESULTS AND DISCUSSION
0.0073. Since the reaction between bromohexadecane and chitin is a SN2 bimolecular nucleophilic substitution, dipolar aprotic solvents (DMSO) are much more efficient than protic solvents (H2O) to avoid side reactions and to facilitate the main reaction,34 thus DMSO was chosen as the optimum solvent. In the FT-IR spectra of CW, D3, D6, and D12 (SI Figure S1), the characteristic absorption peaks of α-chitin and aliphatic chains appeared in the modified whiskers. While the relative intensity of OH stretching at 3400 cm−1 decreased, indicating that the cetyl group was successfully introduced into the chitin whisker by reacting with the OH group. The intensity ratio of amide I vs amide II increased when the ratio of bromohexadecane
3.1. Structure and Surface Morphology of Modified Chitin Whisker. To confirm the occurrence of the reaction shown in Figure 1 and the chemical structure of modified whiskers, some basic data are listed as follows. Elemental analysis and corresponding DS data (Supporting Information, SI, Table S1) indicated that as the ratio of bromohexadecane increased, the N content decreased and the C and H contents increased. From D3 to D12, the DS values increased from 0.58 to 1.41. Water was used to substitute the solvent DMSO, but the resultant sample of H6 had a much lower DS values as 1643
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Figure 4. (a) DSC curves of chitin whisker (CW) and modified chitin whisker (D3, D6, and D12); (b) XRD patterns of D6 under different temperatures (25, 45, and 60 °C).
Figure 5. Microphotographs of solid modified chitin whisker (D6) observed under crossed polarizer’s light at different temperature.
of chitin whisker are around 300 and 20 nm, respectively. After modification, the needle like morphology of the chitin whisker was almost retained even for D12 with high DS, suggesting that the reaction occurred preferentially on the surface of chitin whiskers due to its large surface area. Although the length and width of the chitin whiskers were almost maintained, the outer shell of D6 and D12 showed a coating of long alkyl chains on the backbones of the chitin whiskers, supported by the results from structural analysis. Moreover, the aggregation was aggravated after modification, as a result of the surface hydrophobicity and weak dispersibility of modified whiskers in water. Therefore, the modified whiskers had relatively hydrophobic nanostructures. 3.2. Physical Properties of the Modified Whiskers. The results from TG and DTG profiles of chitin whiskers before and after modification (SI Figure S3) showed that their decomposition temperature appeared in the range from 350 to 400 °C. After modification, the thermal stability decreased slightly, but the decomposition temperature was still over 350 °C. To further illustrate the thermal behavior, a DSC curve is given in Figure 4a. For the modified whiskers, side chain melting endotherms were observed at around 50 °C. The intensity and temperature of the endothermic peak increased with an increase of DS, owing to the contribution of the long
increased, suggesting possibly the changes in crystal form during the reaction. According to the XRD results (SI Figure S2), a strong peak at 22° appeared in the modified whiskers and its intensity increased as the ratio of bromohexadecane increased, confirming a transition of crystal form with the introduction of cetyl group. Due to the poor solubility of modified whisker in common solvents, we used solid state NMR to characterize the structures. Figure 2 shows the 13C NMR spectra of CW, D3, D6, and D12. Obviously, the cetyl group was successfully introduced to the chitin chain. With an increase of the DS values, the intensity of C1−C8 decreased obviously, as a result of the decrease of chitin content and higher across-polarization efficiency of alkyl carbon. The high DS values (1.41) indicated that alkylation occurred on both C3 and C6, thus the corresponding peaks shifted to a lower magnetic field by about 10 ppm after esterification,35 resulting in a broadened peak between 70−90 ppm for the modified whisker. In addition, a side peak of C1 appeared in a modified sample, and its intensity increased as the DS values increased, similar to reported cellulose esters with long alkyl chains,36 possibly ascribed to the change of hydrogen bonding type. TEM images of chitin whisker before and after modification are shown in Figure 3. The mean length (L) and diameter (D) 1644
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Figure 6. Contact angles of the hot pressed sheets prepared from chitin whiskers (CW in a,e) and modified whiskers (D3 in b,f; D6 in c,g; and D12 in d,h) with a water drop of 5μL (a−d) and 2μL (e−h).
Figure 7. SEM images of hot pressed chitin whisker and modified whisker. CW (a, e), D3 (b, d), D6 (c, g), and D12 (d, h). Surface (a−d) and the fracture cross section (e−h) are present in (a−h). Scale bars are 5 μm.
highly hydrophobic nature of the modified whiskers. To illustrate the morphology of the sheets, SEM images of both the surface and cross-section are present in Figure 7. Compared to the chitin whiskers, tremendous flaky crystals appeared on both the surfaces and the fracture cross-sections of the modified whisker sheet. According to the results of Figures 4 and 5, such flaky crystals were induced by the orderly stacking of long alkyl chain. The side chain melting occurred during the hot pressing, leading to the generation of the flaky crystals during cooling. Similar morphology has been reported in our previous work,40,41 in which rough surfaces with micronano binary structure were fabricated to trap abundant air, resulting in high hydrophobicity since the water contact angle of air is considered as 180°.42 In our findings, the hydrophobic interaction between the long alkyl chains and the hydrophobic pyranose rings of chitin as well as the covalent bonding between them facilitated the crystal alignment wrapping around the whiskers to form micronano structure, contributing to the high hydrophobicity, as well as the low surface energy of cetyl chain.43 Overall, introduction of cetyl group successfully endowed chitin whisker with high hydrophobicity. 3.3. Oil-Structuring Performances. Figure 8 shows the photographs of D6 dispersion in sunflower oil. The modified chitin whiskers were well dispersed in sunflower oil by stirring. And after being deposited in refrigerator at 5 °C for 30 min, the dispersions turned out to be oleogels. The oily dispersion of D6 with a concentration of 0.08 g/mL formed a milk-like fluid at room temperature, whereas after being deposited in a refrigerator at 5 °C, a reversible oleogel formed. When the concentration is 0.15 g/mL, however, after being taken out of the refrigerator, it remained as a gel at room temperature (25 °C). On the contrary, sunflower oil maintained its fluid state until the temperature went down below −10 °C. Moreover, the
alkyl chain. However, the powder of D3−D12 just became sticky instead of melting into liquid after being deposited at 60 °C for 6 h, possibly ascribed to the existence of chitin as a supporting backbone. XRD of D6 at different temperatures (25, 45, and 60 °C) is shown in Figure 4b. When D6 was heated, the peaks corresponding to cetyl chain gradually weakened and disappeared when the temperature reached 60 °C, while the chitin peaks remained, confirming the melting of grafted moieties. To further prove this thermal transition in a more specific temperature range, microphotographs of solid D6 observed under crossed polar light are shown in Figure 5. During heating from 48.4 to 57.3 °C, the anisotropic crystal pattern disappeared gradually, and during cooling from 55.1 to 50.2 °C, it emerged again, indicating the reversibility of this transtion. Similar thermal transition induced by side chain melting has also been reported on galactopyranoside derivatives with long chain alkyl groups37 and hydroxypropyl cellulose derivatives with alkyl side chains (18 carbons).38 This crystal structure was attributed to the orderly alignment of cetyl group on the surface of chitin whiskers, since long alkyl chain tends to be orderly stacked below melting temperature and thus forms a crystalline phase composed of paraffin-like crystallites.39 3.2. Hydrophobicity of the Modified Whiskers. On the basis of the thermal transition around 50 °C, the powder of modified whiskers were hot pressed at 60 °C with a pressure of 0.1 MPa to obtain sheets with flat surfaces. The contact angles of water on the resultant sheets are shown in Figure 6. Compared to chitin whisker, the water contact angles of the modified whiskers were greatly increased, and the contact angle increased with an increase of DS. Influenced by gravity, a 5μL water drop showed slightly lower contact angles on the sample than that of a 2μL water drop. However, in both situation, the contact angles of D6 and D12 were over 140°, indicating the 1645
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crystallization behavior at lower temperature has been proven to play an important role in the formation of oleogel.3 To further investigate the thickening effect of modified whiskers and the rheological behavior of the oily dispersion, rheological measurements were applied. Figure 9a shows the shear viscosity (Eta) dependence of shear rate for D6 dispersion with different concentrations. Compared to sunflower oil, the viscosity of D6 dispersions was remarkably enhanced, indicating an excellent thickening effect of the modified whiskers, due to the high hydrophobicity and crystal structure. The shear viscosity at 10−1s−1 varied from 1.124 Pa·s (D6 content of 0.03 g/mL) to 146.86 Pa.s (D6 content of 0.15 g/mL), much higher than that of sunflower oil (0.035 Pa.s). In addition, all the oily dispersions exhibited typical shear-thinning behavior, meeting the standard as an oil structurant. To expand the application of the modified whisker, sunflower oil was replaced with the more universal silicone oil. The shear rheology of D6 dispersion in silicone oil (SI Figure S3) revealed that the modified whisker was also effective in thickening the silicone oil. The dependence of complex viscosity (Eta*) of the oily dispersion on temperature is shown in Figure 9b. The modified whiskers exhibited a remarkable thickening effect on sunflower oil. All oily dispersions (including sunflower oil) lost their viscosity when heated and gained it during cooling, showing typical thermal reversibility, which is often seen for organogels.47 For the oily dispersion of D6, when the temperature went down from 60 °C to about 10 °C, a platform appeared, indicating the occurrence of gelation. As the concentration of D6 increased from 0.03 to 0.15 g/mL, the Eta* at the platform increased notably from 31.39 Pa·s to 5201.43 Pa·s, and the gelation temperature increased from 8 °C to 12 °C. The gelation temperature with a variation of 4 °C means that the product appearance and stability could be controlled to meet different requirements. To investigate the influence of DS values, D3, D6, and D12 were all dispersed in sunflower oil with a fixed concentration of 0.15 g/mL. The corresponding photographs and Eta* values of the dispersion at different temperatures are present in Figure 10. At lower temperature (below 10 °C), the Eta* increased with the increase of DS value. However, at room temperature, D12 with the highest DS value did not possess the best thickening effect, neither did D3 (as shown in the right picture of Figure 10). It was noted that the D12 oily dispersion displayed translucence while the other two were definitely opaque. Usually, the oil structurant must be relatively insoluble so that it can crystallize or self-assemble to
Figure 8. 0.08g/mL D6 suspension and sunflower oil taken out from refrigerator at 5 °C (top) and deposited at room temperature (RT) for 30 min (bottom).
oily dispersion of the modified whiskers remained stable without visible precipitates for at least 6 months, as a result of the nano effect and high hydrophobicity. In aqueous solution, nano whiskers with strong hydrogen bonding are easily flocculated,44 resulting in poor dispersion, while in oil, the modified chitin whiskers with impaired hydrogen bond could be well dispersed and form a network structure, following the generally accepted mechanical reinforcing principle.45 On the basis of this theory, the critical volume fraction for a threedimensional network (VRc) depends on the aspect ratio (L/D) of the chitin whiskers as VRc = 0.7/(L/D).46 Seeing that the L/ D value was 15 (calculated from the TEM results), and the density of densely packed D6 powder was measured to be 0.618g/mL, the corresponding VRc was 4.67%, equal to 2.89 g/ mL D6 in oil. In this work, the D6 concentrations in oily dispersion were all above that percolation threshold, ensuring the formation of a network structure. Meanwhile, the concentration dependent gelation behavior of the oily dispersion further confirmed the existence of the network resulting from the hydrophobic interaction between the oil medium and the modified whiskers. However, the side chain, long saturated alkyl groups are intrinsically oleophilic, and their
Figure 9. Dependence of shear viscosity (Eta) on shear rate (a) and dependence of complex viscosity (Eta*) on temperature (b) for modified whisker (D6) oily dispersion with different concentrations. 1646
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ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-27-87219274. Fax: +86-27-68762005. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 10. (Left) Dependence of complex viscosity (Eta*) on temperature for sunflower and modified whisker with different DS values (D3, D6, and D12) with a concentration of 0.15 g/mL. (Right) The oily dispersion of 0.15 g/mL D3, D6, and D12 taken out from refrigerator at 5 °C (top) and after being heated at 30 °C for 30 min (bottom).
ACKNOWLEDGMENTS This work was supported by the Major Program of National Natural Science Foundation of China (21334005), the National Natural Science Foundation of China (20874079 and 51203122), Fundamental Research Funds for the Central Universities (2014203020202), and the Unilever Fund Project (URC-UCIC-2011-024).
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form mesoscale structures. Meanwhile, it must be relatively soluble to interact with solvent molecules.2 Probably, when DS reached a certain point (as for D12), the modified whiskers were too soluble in the sunflower oil, thus the mesoscale structure was not easily formed. Nevertheless, D6 with a moderate DS value met the equilibrium between solubility and insolubility in sunflower oil, therefore, it exhibited the best thickening effects at room temperature. As for lower temperature below the gelation point, the Eta* was related to the concentration of solutes in the oleogel. Therefore, D6 with a moderate DS value exhibited the best thickening performance in oil and the viscosity of the resultant dispersion showed temperature dependence and shear thinning. Furthermore, the intrinsic biocompatibility and bioactivity of α-chitin endowed them with great potential as an oil structurant for food, cosmetic, and pharmaceutical applications.
REFERENCES
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4. CONCLUSIONS Long alkyl chains were successfully introduced to modify chitin whiskers by reacting them with bromohexadecane in DMSO. The modified chitin whiskers exhibited a thermal transition at around 50 °C and formed orderly aligned crystal structure at lower temperatures. Thus, by hot pressing the modified whiskers, a rough surface consisting of micro−nano flaky crystals appeared on both the surface and the cross-section of the sheet. Thus, the introduction of the long alkyl chains not only decreased the surface energy, but also induced the surface roughness, leading to the high hydrophobicity. The modified chitin whiskers could be well dispersed in sunflower oil, and exhibited high stability and a remarkable thickening effect. Viscosity of the oily suspension showed temperature dependence and shear-thinning behavior, and a solidified oleogel formed at below 10 °C. By adjusting the DS values and concentration of the modified whisker, the viscosity of the oily dispersion could be tuned to meet different requirements. The oily dispersion of modified whiskers exhibited high stability for at least 6 months, due to the nanosize, large aspect ratio, and oleophilic side chain. This is the first time that chitin whiskers have been applied as an oil thicker, which demonstrated its great potential in medicine, cosmetics, foodstuff, and other related chemical industries. 1647
DOI: 10.1021/la504576p Langmuir 2015, 31, 1641−1648
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DOI: 10.1021/la504576p Langmuir 2015, 31, 1641−1648
