Alkyl alkenyl succinic anhydride modified chitosan nanoparticles as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of functional biopolymer materials, and relates to alkyl alkenyl succinic anhydride modified chitosan nanoparticles as well as a preparation method and application thereof. The nano-particles are formed by self-assembling after chitosan is subjected to hydrophobic modification through alkyl alkenyl succinic anhydride and the pH of a system is adjusted under the condition from acidity to neutrality or alkalescence. The degree of substitution of the nanoparticles is 1-40%. The invention also provides a preparation method and application of the nanoparticles. The nanoparticles are suitable for various alkyl alkenyl succinic anhydrides with the carbon chain length of C6-C16, and a new technical scheme is provided for constructing an oil-water dispersion system stabilized by bio-based particles.

Description

Alkyl-olefinic succinic anhydride modified chitosan nanoparticles, their preparation methods and applications Technical Field

[0001] This invention belongs to the field of functional biopolymer materials technology, and relates to alkyl-olefin succinic anhydride modified chitosan nanoparticles, their preparation methods, and applications. Background Technology

[0002] Emulsions, as an important dispersion system, are widely used in food, pharmaceuticals, cosmetics, and chemicals. Traditional emulsions typically rely on small-molecule surfactants to reduce the interfacial tension between oil and water, but their interfacial film strength is limited, and they suffer from insufficient safety and environmental adaptability in some applications. Pickering emulsions achieve stability through the adsorption of solid particles at the oil-water interface, offering advantages such as high interfacial stability and strong anti-agglomeration ability compared to traditional emulsions. However, existing Pickering emulsion stabilizers are mostly inorganic or synthetic particles, with limited biocompatibility, making it difficult to meet the application requirements in the food and pharmaceutical fields.

[0003] Bio-based polysaccharide materials, due to their natural origin, good biocompatibility, biodegradability, and low immunogenicity, have become an ideal choice for developing green Pickering emulsion stabilizers. Chitosan, a natural polysaccharide and the only cationic polysaccharide found in nature, possesses excellent biocompatibility and biodegradability. Obtained from the deacetylation of chitin, it exhibits unique properties such as antibacterial activity, bioadhesion, and pH responsiveness. However, the natural chitosan molecular chain contains a large number of hydrophilic hydroxyl and amino groups, resulting in insufficient interfacial wettability and difficulty in forming a stable adsorption layer at the oil-water interface. When used directly as a Pickering emulsion stabilizer, the emulsion is prone to droplet coalescence, flocculation, and phase separation, exhibiting extremely poor stability.

[0004] To improve the interfacial wetting properties of chitosan, existing technologies have attempted to modify it by introducing hydrophobic groups, such as using hydrophobic agents like alkyl alkenyl succinic anhydride to chemically modify the polysaccharide. For example, OSA-modified high amylose corn starch can effectively enhance its gel-forming ability. Although such composite systems combine the hydrophobicity of starch and the cationic properties of chitosan, practical application obstacles remain, including complex preparation processes, the need for precise control of the ratio of the two components, poor batch-to-batch consistency, and high cost. There is a lack of systematic regulation of the transformation process of modified chitosan from a dissolved state to a particulate state under different pH conditions, especially a lack of technical solutions for constructing nanoparticles with controllable wetting states through pH-induced self-assembly to stabilize oil-water interfaces. Therefore, there is an urgent need to provide a new technical approach that modifies chitosan by hydrophobic modification and combines it with pH-induced self-assembly to achieve a controllable transformation of modified chitosan from a dissolved state to a nanoparticle state. This would allow for the formation of a stable particle interface layer at the oil-water interface, thus solving the problems of insufficient wettability and difficulty in controlling stability of bio-based particle interfaces in existing technologies. Summary of the Invention

[0005] The purpose of this invention is to provide chitosan nanoparticles prepared by hydrophobic modification of alkyl alkenyl succinic anhydride combined with pH-induced self-assembly. By adjusting the degree of hydrophobic modification and the pH conditions of the system, the modified chitosan is transformed from a dissolved state to a nanoparticle state, thereby obtaining a material system that can form a stable particle interface layer at the oil-water interface.

[0006] To achieve the above objectives, the present invention provides the following technical solution: alkyl-alkenyl succinic anhydride-modified chitosan nanoparticles, wherein the nanoparticles are formed by self-assembly of chitosan after hydrophobic modification with alkyl-alkenyl succinic anhydride and by adjusting the pH of the system from acidic to neutral or weakly alkaline conditions; the degree of substitution of the nanoparticles is 1–40%.

[0007] Further, the alkyl alkenyl succinic anhydride is selected from one or more of allenyl succinic anhydride, octenyl succinic anhydride, nonenyl succinic anhydride, decenyl succinic anhydride, decaenyl succinic anhydride, dodecenyl succinic anhydride, tetradecenyl succinic anhydride, and hexadecenyl succinic anhydride; the alkyl chain has a carbon number of C6 to C16.

[0008] Furthermore, the present invention also provides a method for preparing the alkylalkenyl succinic anhydride modified chitosan nanoparticles, comprising the following steps: S1. Dissolving chitosan in an aqueous organic acid solution to obtain a chitosan solution; adding an alcohol solvent to the chitosan solution to form a homogeneous reaction system; S2. Under certain temperature conditions, adding alkylalkenyl succinic anhydride to the reaction system in a mass ratio of 0.2:1 to 1.2:1 to carry out an amidation reaction; S3. After the reaction is completed, rapidly adjusting the pH of the system to 7.0–8.0, and allowing it to stand for 0.5–1 h to precipitate solids, which are then washed and dried to obtain modified chitosan powder; S4. Redissolving the modified chitosan powder in an alkaline solution or an organic acid solution, then slowly adjusting the pH to 5.0–8.5, and subsequently ultrasonically treating it at a power of 200–400 W for 5–15 min to finally form a stable nanoparticle suspension.

[0009] Further, in step S2, the mass ratio is selected according to the alkyl chain length: for C6 to C8 anhydrides, the mass ratio is 0.4:1 to 1.2:1; for C8 to C10 anhydrides, the mass ratio is 0.3:1 to 1.0:1; for C12 to C16 anhydrides, the mass ratio is 0.2:1 to 0.8:1.

[0010] Furthermore, in step S4, the ultrasonic power of the ultrasonic treatment is 200–400 W, a pulse mode is adopted, the pulse ratio is 2 seconds on and 2 seconds off, and the ultrasonic time is 5–15 minutes.

[0011] Furthermore, the present invention also provides an oil-in-water Pickering emulsion comprising the alkylene succinic anhydride modified chitosan nanoparticles as a stabilizer, an oil phase as a dispersed phase, and an aqueous phase as a continuous phase; wherein the mass concentration of the modified chitosan nanoparticles in the aqueous phase of the emulsion is 0.1–1.0%.

[0012] Furthermore, the volume ratio of the oil phase to the water phase is 1:2 to 2:1.

[0013] Furthermore, the oil phase is selected from one or more of vegetable oils, animal oils, mineral oils, and synthetic oils.

[0014] Furthermore, the present invention also provides a method for preparing the Pickering emulsion, comprising: using the modified chitosan nanoparticle suspension as the aqueous phase, adding the oil phase at a predetermined oil-water volume ratio, and employing a two-stage high-speed shear homogenization: the first stage is shearing at 8000–15000 rpm for 1–3 minutes, and the second stage is shearing at 15000–30000 rpm for 1–5 minutes.

[0015] Furthermore, the present invention also provides the application of the alkyl-olefinic succinic anhydride modified chitosan nanoparticles or the Pickering emulsion in the fields of food, drug delivery and cosmetics.

[0016] Compared with existing technologies, the present invention has the following advantages: 1. Advantages over OSA-starch systems: The modified chitosan nanoparticles of the present invention have significant advantages over octenyl succinic anhydride modified starch. The particle size is smaller, with an average particle size of 281 nm under optimal conditions, while starch-based particles are typically in the micrometer range, resulting in a 3–5 times improvement in interface coverage efficiency. The present invention retains the inherent pH-responsive characteristics of chitosan, enabling intelligent release, a function not possessed by starch-based systems.

[0017] 2. Advantages compared to the OSA-starch / chitosan composite system: This invention employs a single-component system of directly modified chitosan, simplifying the preparation steps compared to the OSA-starch / chitosan composite system, reducing the process from a two-step composite process to a single direct modification step. Performance is more stable; the single-component approach avoids batch-to-batch performance differences caused by fluctuations in the ratio of the two components in the composite system. Cost is lower; reducing one raw material and one process step significantly lowers raw material costs and energy consumption.

[0018] 3. Improvement compared to unmodified chitosan: The modified chitosan nanoparticles of this invention exhibit a qualitative leap in interfacial properties and emulsion stability compared to unmodified chitosan. The interfacial adsorption energy is increased from 1.2 × 10⁻⁶. -16 J increased to 46×10 -16 J is increased by approximately 38 times. The contact angle is increased from 51° to 108°, achieving a transformation from hydrophilic to amphiphilic. Emulsion stability is significantly improved; the unmodified chitosan-stabilized emulsion completely separates within one day, while the modified chitosan-stabilized emulsion of this invention maintains an emulsification index above 0.82 even after 7 days.

[0019] 4. Excellent Rheological Properties and pH Response Characteristics: The Pickering emulsion of this invention exhibits typical weak gel characteristics at pH 7.5. The storage modulus G' is consistently greater than the loss modulus G'' across the entire testing frequency range, and the loss tangent tan δ can be as low as 0.08, indicating predominantly elastic behavior. It exhibits significant shear-thinning behavior, which is beneficial for processing and application. It possesses significant thixotropy, with a hysteresis loop area ratio of 30%–70%. It retains the pH-responsive characteristics of chitosan; the particles are positively charged under acidic conditions and negatively charged under alkaline conditions, enabling pH-controlled intelligent release.

[0020] 5. Structure-Activity Relationship and Industrialization Prospects This invention establishes for the first time the quantitative relationship between chain length, degree of substitution, contact angle, interfacial adsorption energy, and emulsion rheological properties in alkyl-olefinic succinic anhydride-modified chitosan systems. The relationship between the degree of substitution and the contact angle was clarified, revealing that the contact angle exhibits a non-monotonic but regular change with the degree of substitution. Based on the measured contact angle data, the adsorption energy (ΔE) of different modified chitosan particles at the oil-water interface was quantified for the first time. Particles with a moderate contact angle (80°–100°) can form the most stable interfacial film, providing key theoretical parameters for predicting emulsion stability. The study established a chain effect relationship of "high adsorption energy (ΔE) → dense interfacial particle layer → interfacial modulus → emulsion storage modulus (G') and significant shear thinning behavior."

[0021] The modified chitosan nanoparticles and Pickering emulsion of this invention utilize natural bio-based raw materials, conforming to the principles of green chemistry and sustainable development, and meeting the safety requirements for food-grade and pharmaceutical-grade applications. The preparation process is simple, easily scalable for industrial production, and the raw material cost is relatively low. In the food industry, they can be used as nutrient delivery carriers, fat substitutes, and intelligent release systems; in the pharmaceutical delivery field, they can be used for solubilizing and oral delivery of poorly soluble drugs; and in the cosmetics field, they can be used as emulsion bases for skincare and sunscreen products, possessing broad market application prospects and significant economic and social value. Attached Figure Description

[0022] Figure 1 shows the variation curves of the degree of substitution of modified chitosan nanoparticles under different mass ratios; Figure 2 shows the static contact angle measurement results of modified chitosan nanoparticles at the air-water interface under different degrees of substitution: (a) 0%, 51.9 ± 0.30°; (b) 7%, 65.87 ± 0.50°; (c) 9%, 68.23 ± 1.33°; (d) 12%, 107.87 ± 1.53°; (e) 16%, 74.97 ± 2.38°; (f) 22%, 91.17 ± 1.68°; Figure 3 shows the results of different pH values. Figure 4 shows the particle size variation of modified chitosan nanoparticles under different conditions; Figure 5 shows the change in adsorption energy of modified chitosan nanoparticles as the degree of substitution increases; where 0 represents unsubstituted chitosan, and 1-5 represent substitution degrees of 7%, 9%, 12%, 16%, and 22%, respectively; Figure 6 shows a comparison of macroscopic appearance photographs of Pickering emulsions prepared under different storage times; where D1, D2, D3, and D5 represent storage days; Figure 7 shows steady-state flow test diagrams under different pH conditions; where (a) represents the increase in shear rate, and (b) represents the decrease in shear rate; Figure 8 shows the frequency scan side view of OSA-substituted chitosan under different substitution degrees; (a) storage modulus G'; (b) loss modulus G''; OSA-CSN 1-5 represent substitution degrees of 7%, 9%, 12%, 16%, and 22%, respectively. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in more detail below with reference to embodiments and accompanying drawings. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Specifically, Examples 1-4 are based on octenyl succinic anhydride (OSA) for detailed explanation, while Examples 5-6 demonstrate the application of nonenyl succinic anhydride (NSA) and dodecenyl succinic anhydride (DDSA), respectively, to support the universality of the scope of protection of this invention.

[0024] Example 1. An octenyl succinic anhydride modified chitosan nanoparticle, the preparation method includes: 1. Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan is dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent is added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride is slowly added to the reaction system in a mass ratio of 0.2:1 of octenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction is completed, the pH of the system is adjusted to 7.5 by adding an alkaline solution to allow the modified chitosan to precipitate; the obtained product is washed and dried to obtain octenyl succinic anhydride modified chitosan with a degree of substitution of 7%, and the data points under the feed ratio are shown in Figure 1; (4) The modified chitosan particles are dispersed in 0.1 M NaOH; the pH of the system is adjusted to 7.5 by slowly adding 0.1 M HCl solution to allow self-assembly; then, the particles are ultrasonically treated for 6 min to obtain a particle size of 767. The modified chitosan nanoparticles with a diameter of nm have a charge of 6.95 mV, and the relevant trend is shown in Figure 3.

[0025] The static contact angle of the nanoparticle powder at the air-water interface after compression was measured using a contact angle meter, and the result was 66° (see Figure 2). The adsorption energy (ΔE) reached 38 × 10⁻⁶. -16 J.

[0026] 2. Preparation and characterization of Pickering emulsion: (1) The above nanoparticle suspension was used as the aqueous phase (particle concentration of about 0.5% w / v) and mixed with the oil phase at an oil-water volume ratio of 1:1. The water-in-oil Pickering emulsion was obtained by shearing at 8000 rpm for 2 minutes using a high-speed dispersion homogenizer.

[0027] (2) The resulting emulsion is an oil-in-water Pickering emulsion, in which the modified chitosan nanoparticles are stably adsorbed at the oil-water interface and remain stable for 1 day (see Figure 4). The emulsification index EI ≈ 0.92, and G' is greater than G'', exhibiting weak gel properties.

[0028] (3) The emulsion possesses basic emulsifying ability, confirming that low-substitution (7%) OSA-CS nanoparticles can serve as a Pickering stabilizer. However, the droplet size is large and unevenly distributed, resulting in limited long-term storage stability. Observable stratification occurs after standing for 1 day, indicating that the interfacial film strength and steric barrier effect need to be improved. This corresponds to the interfacial adsorption capacity reflected by the low hydrophobicity (contact angle 66°) and large particle size (767 nm) of the particles.

[0029] Example 2: An octenyl succinic anhydride modified chitosan nanoparticle was prepared as follows: 1. Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan was dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent was added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride was slowly added to the reaction system in a mass ratio of 0.4:1 to octenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction was completed, the pH of the system was adjusted to 7.0 by adding an alkaline solution to allow the modified chitosan to precipitate; the obtained product was washed and dried to obtain octenyl succinic anhydride modified chitosan with a degree of substitution of 9% (see Figure 1); (4) Same as Example 1: the modified chitosan particles were dispersed in 0.1 M NaOH; the pH of the system was adjusted to 7.5 to allow self-assembly; then, the modified chitosan nanoparticles with a particle size of 790 nm and a charge of – were obtained by ultrasonic treatment for 6 min. 6.90 mV, see Figure 3. Compared with Example 1, it can be seen that as DS increases from 7% to 9%, the surface charge of the particles changes from positive to negative at the same pH, clearly demonstrating the regulatory effect of the degree of substitution on the ionization behavior of the particles.

[0030] The modified chitosan particles exhibit a contact angle of 68° at the air-water interface (see Figure 2). The adsorption energy (ΔE) reaches 46 × 10⁻⁶. -16 J. Slightly increased compared to Example 1, consistent with the trend of enhanced hydrophobicity.

[0031] 2. Preparation and characterization of Pickering emulsion: (1) The particle concentration in the aqueous phase was approximately 0.5% w / v, the oil-to-water volume ratio was 1:1, the homogenization speed was increased to 13000 rpm, and shearing was performed for 2 minutes; (2) The resulting emulsion was an oil-in-water Pickering emulsion, wherein the modified chitosan nanoparticles were stably adsorbed at the oil-water interface and remained stable for 1 day. The emulsification index EI ≈ 0.90, and G' was greater than G'', exhibiting weak gel properties.

[0032] (3) The emulsion prepared in this embodiment exhibits superior macroscopic stability (EI), droplet uniformity, and gel strength compared to Example 1. This is attributed to the increased degree of particle substitution (DS) (from 7% to 9%), resulting in enhanced hydrophobicity (contact angle increased from 66° to 68°) and a significant increase in interfacial adsorption energy (ΔE reaches 46 × 10⁻⁶). -16 J). The negative charge (-6.90 mV) carried by the particles at pH 7.5 further enhances the spatial stability between droplets through electrostatic repulsion. However, since the contact angle is still in the hydrophilic range, there is still room for improvement in the mechanical strength and compactness of the interfacial film, which is consistent with the phenomenon that slight stratification can still be observed in the emulsion after 1 day.

[0033] Example 3. An octenyl succinic anhydride modified chitosan nanoparticle, the preparation method is as follows: 1. Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan is dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent is added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride is slowly added to the reaction system in a mass ratio of 1:1 of octenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction is completed, the pH of the system is adjusted to 7.5 by adding an alkaline solution to precipitate the modified chitosan; the obtained product is washed and dried to obtain octenyl succinic anhydride modified chitosan with a degree of substitution of 22%, as shown in Figure 1, which is within the preferred range (15-25%); (4) As in Example 1, the modified chitosan particles are dispersed in 0.1 M NaOH; the pH of the system is adjusted to 7.5 to allow self-assembly; then, the particles are ultrasonically treated for 6 min to obtain a particle size of 281 μm. The modified chitosan nanoparticles with a mass of nm have a charge of –12.77 mV (see Figure 3), indicating that the particle surface carries a strong negative charge under alkaline conditions, which is beneficial for maintaining colloidal stability through electrostatic repulsion.

[0034] The measured contact angle was 91° (see Figure 2), which falls within the preferred range of 80-100°, indicating that the particles exhibit ideal amphiphilic equilibrium. At this angle, the adsorption energy (ΔE) reaches 14 × 10⁻⁶. -16 J.

[0035] 2. Preparation and characterization of Pickering emulsion: (1) Particle concentration 0.5% w / v; oil-water volume ratio 1:1, homogenization speed 13000 rpm, shearing for 2 minutes; (2) The resulting emulsion is an oil-in-water type Pickering emulsion, wherein the modified chitosan nanoparticles are stably adsorbed at the oil-water interface. Optical microscopy observation shows that the emulsion droplets are small and uniformly distributed, and remain stable for 5 days. The emulsification index EI ≈0.87, and G' is also greater than G''. The storage modulus (G') can reach about 350 Pa at 1 Hz, showing significant gel-like solid behavior. It exhibits weak gel characteristics.

[0036] (3) The emulsion prepared in this embodiment achieved optimal performance in terms of macroscopic stability, microstructure uniformity, and rheological mechanical strength. This is attributed to the fact that the key parameters of the particles were all within the optimal range: the appropriate degree of substitution (DS=22%) achieved an ideal amphiphilic balance; the contact angle (91°) was close to 90°, enabling the particles to achieve optimal wetting at the oil-water interface, thereby obtaining high interfacial adsorption energy (ΔE=14×10). -16 The smaller particle size (281 nm) and uniform distribution facilitate the formation of a dense interfacial layer; the stronger surface negative charge (-12.77 mV) effectively prevents droplet aggregation through electrostatic repulsion. The synergistic effect of these factors enables the particles to form an interfacial film with both high mechanical strength and good steric stability, thereby endowing the emulsion with excellent long-term stability (5 days) and significant gel-like rheological properties, fully meeting the performance requirements of high-end applications for emulsion systems.

[0037] Example 4. An octenyl succinic anhydride modified chitosan nanoparticle, prepared by the following method: Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan was dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent was added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride was slowly added to the reaction system in a mass ratio of 1:1 of octenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction was completed, the pH of the system was adjusted to 7.5 by adding an alkaline solution to precipitate the modified chitosan; the obtained product was washed and dried to obtain octenyl succinic anhydride modified chitosan with a degree of substitution of 22% (see Figure 1).

[0038] Modified chitosan particles were dispersed in 0.1 M NaOH; the pH of the system was adjusted to 7.0 (different from the previous example) to induce self-assembly. Subsequently, ultrasonic treatment for 6 min yielded modified chitosan nanoparticles with a diameter of 215 nm and a charge of 16.53 mV (see Figure 3). This indicates that even highly substituted products can still carry a positive charge at near-neutral pH due to the partial protonation of the amino groups, fully demonstrating the complex pH-responsive behavior of the system.

[0039] The modified chitosan particles exhibit a contact angle of 91° at the air-water interface. At this point, the adsorption energy (ΔE) reaches 8 × 10⁻⁶. -16 J.

[0040] 2. Preparation and characterization of Pickering emulsion: (1) The modified chitosan nanoparticles were dispersed in an aqueous phase (0.5 w / v%) as the continuous phase; an oil phase was added to the continuous phase, and the oil phase ratio was controlled at 100 v%. The emulsion was subjected to high-speed shearing at 20,000 rpm for 2 min; (2) The resulting emulsion was an oil-in-water Pickering emulsion, wherein the modified chitosan nanoparticles were stably adsorbed at the oil-water interface and remained stable for 3 days. The emulsification index EI ≈ 0.83, and G' was also greater than G''.

[0041] The emulsion prepared in this example exhibits stability and rheological properties between those of Examples 2 and 3. This result reveals the crucial regulatory role of ambient pH in emulsion performance. Although the particles have a similar degree of substitution (DS=22%) and contact angle (91°) as those in Example 3, they exhibit a higher degree of stability due to their positive surface charge (+16.53 mV) at pH 7.0 and higher interfacial adsorption energy (ΔE=8×10⁻⁶). -16 The relatively low J indicates a weakened interfacial anchoring strength of the particles under these pH conditions. The near-neutral charge state also reduces electrostatic repulsion between droplets, potentially inducing slight flocculation, which explains the observed droplet clusters and slightly larger average particle size in the microstructure. Nevertheless, the good amphiphilicity imparted by the near-90° contact angle still supports the emulsion's stability (3 days) compared to samples with lower DS. This example fully demonstrates that the interfacial behavior and emulsion properties of particles with the same chemical composition can be controlled over a wide range by adjusting the pH, showcasing the flexibility and responsiveness of the present invention.

[0042] Example 5: This example aims to illustrate that nonenyl succinic anhydride (NSA, C9), with a longer carbon chain, can also be used to prepare high-performance modified chitosan nanoparticles according to the method of the present invention. The preparation method is as follows: Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan is dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent is added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride is slowly added to the reaction system in a 1:1 mass ratio of nonenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction is completed, the pH of the system is adjusted to 7.5 by adding an alkaline solution to allow the modified chitosan to precipitate; the obtained product is washed and dried to obtain modified chitosan, and its degree of substitution is determined to be 16% by potentiometric titration; (4) The modified chitosan particles are dispersed in 0.1 M NaOH; the pH of the system is adjusted to 7.5 to allow self-assembly; then, the particles are ultrasonically treated for 6 min, and the particle size is determined to be 355 nm and the charge is -10.35 mV by dynamic light scattering.

[0043] The static contact angle of the nanoparticle powder at the air-water interface after compression was measured using a contact angle meter, and the result was 88°. At this point, the adsorption energy (ΔE) reaches 22 × 10⁻⁶. -16 J.

[0044] 2. Preparation and characterization of Pickering emulsion (1) The above nanoparticle suspension was used as the aqueous phase (particle concentration approximately 0.5% w / v) and mixed with the oil phase at an oil-to-water volume ratio of 1:1. The mixture was sheared at 8000 rpm for 2 minutes using a high-speed dispersion homogenizer to obtain an oil-in-water Pickering emulsion; (2) The resulting emulsion was an oil-in-water Pickering emulsion, wherein the modified chitosan nanoparticles were stably adsorbed at the oil-water interface and remained stable for 5 days. The emulsification index EI ≈ 0.85, and G' was also greater than G''.

[0045] This embodiment demonstrates that modifying chitosan with nonenyl succinic anhydride (NSA, C9) can also yield a high-performance Pickering emulsion stabilizer. The resulting particles exhibit a suitable degree of substitution (16%), an ideal contact angle close to 90° (88°), and high interfacial adsorption energy (ΔE = 22 × 10⁻⁶). -16 These characteristics collectively ensure good interfacial stability and a 5-day shelf life for the emulsion. Compared to OSA modification with a slightly shorter carbon chain (see Examples 2 and 3), NSA-modified particles exhibit comparable emulsifying properties and stability under similar conditions, demonstrating the universality of the method described in this invention for alkyl alkenyl succinic anhydrides (C8-C12) of different chain lengths, and the feasibility of finely controlling the hydrophobicity of particles and the final emulsion properties by selecting anhydrides of specific chain lengths.

[0046] Example 6. This example aims to illustrate that modified chitosan nanoparticles with excellent performance can also be prepared using dodecenyl succinic anhydride (DDSA, C12) with a longer carbon chain according to the method of the present invention. The preparation method is as follows: 1. Preparation and characterization of modified chitosan nanoparticles: (1) Chitosan is dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent is added to the chitosan solution to form a homogeneous reaction system; (2) Under certain temperature conditions, the anhydride is slowly added to the reaction system in a mass ratio of 1:1 to dodecenyl succinic anhydride to chitosan to carry out a grafting reaction; (3) After the reaction is completed, the pH of the system is adjusted to 7.5 by adding an alkaline solution to precipitate the modified chitosan; the obtained product is washed and dried to obtain modified chitosan, and its degree of substitution is determined to be 18% by potentiometric titration; (4) The modified chitosan particles are dispersed in 0.1 M NaOH; the pH of the system is adjusted to 7.5 to allow self-assembly; then the particles are ultrasonically treated for 6 minutes. The particle size was measured to be 499 nm and the charge was -8.92 mV by dynamic light scattering.

[0047] The static contact angle of the nanoparticle powder at the air-water interface after compression was measured using a contact angle meter, and the result was 103°. At this point, the adsorption energy (ΔE) reaches 28 × 10⁻⁶. -16 J.

[0048] 2. Preparation and characterization of Pickering emulsion: (1) The above nanoparticle suspension was used as the aqueous phase (particle concentration approximately 0.5% w / v) and mixed with the oil phase at an oil-to-water volume ratio of 1:1. The mixture was sheared at 20,000 rpm for 2 minutes using a high-speed dispersion homogenizer to obtain an oil-in-water Pickering emulsion; (2) The resulting emulsion was an oil-in-water Pickering emulsion in which the modified chitosan nanoparticles were stably adsorbed at the oil-water interface and remained stable for 5 days. The emulsification index EI ≈ 0.90, and G' was also greater than G''.

[0049] This embodiment demonstrates that modification with dodecenyl succinic anhydride (DDSA, C12), which has a longer carbon chain, can yield particles with stronger hydrophobicity (contact angle 103°) and higher interfacial adsorption energy (ΔE = 28 × 10⁻⁶). -16 J). Although its contact angle slightly exceeds the preferred range of 80-100°, it still possesses sufficient amphiphilicity, and the negative surface charge (-8.92 mV) provides electrostatic stabilization, thus forming an extremely stable emulsion. This emulsion not only has the highest emulsification index (0.90) among all embodiments, but also exhibits the finest and most uniform droplets, demonstrating excellent interfacial stability and mechanical strength. This further confirms the broad applicability of the present invention to alkylalkenyl succinic anhydrides of different chain lengths (C8-C12), and shows that by selecting longer-chain anhydrides, particles can be endowed with stronger hydrophobic driving force and interfacial anchoring ability, thereby customizing emulsion performance for applications requiring high stability.

[0050] The EI value of the emulsions prepared in Examples 1-6 of this invention changes over time. Note: Superscripts a and b indicate that different letters in the same column have significant differences at the p < 0.05 level; only a and only b indicate significant differences; a and a, b and b, a and ab, b and ab indicate no significant differences.

[0051] Comparative Example: An attempt at emulsifying unmodified chitosan. The preparation method differs from that in Example 1 in the following ways: (1) Following the particle preparation steps in Example 1, no alkyl alkenyl succinic anhydride was added, and the other conditions remained unchanged. The degree of substitution (DS) of the product was measured to be 0%; (2) Following step (3) of Example 1, the pH of the system was adjusted to 7.5 using NaOH solution. During this process, it was observed that the unmodified chitosan, lacking hydrophobic segments, could not form stable nanoparticles through self-assembly under alkaline conditions. Instead, it rapidly aggregated and formed visible flocculent precipitates. After centrifugation, washing, and drying, unmodified chitosan powder was obtained; (3) The above unmodified chitosan powder was taken and dispersed in 0.1 M NaOH solution and the pH was adjusted to 7.5, following step (4) of Example 1. The resulting system was extremely unstable and macroscopic phase separation or precipitation occurred within a short time. Effective particle size and Zeta potential data could not be obtained through dynamic light scattering.

[0052] Contact angle measurements show that the value is less than 60°, indicating strong hydrophilicity.

[0053] When attempting emulsification, a stable emulsion could not be formed, and the oil-water phases separated rapidly after shearing ceased. This indicates that unsubstituted chitosan does not possess sufficiently strong amphiphilicity to form any macroscopically stable emulsion.

[0054] This comparative example illustrates that introducing specific hydrophobic segments through alkyl alkenyl succinic anhydride to regulate the degree of substitution, thereby endowing chitosan with suitable amphiphilicity and interfacial wettability, is an indispensable key step in its successful conversion into a highly efficient Pickering emulsion stabilizer, highlighting the non-obviousness and significant progress of the technical solution of this invention.

Claims

1. Alkyl-olefinic succinic anhydride-modified chitosan nanoparticles, characterized in that, The nanoparticles are formed by self-assembly of chitosan after alkylalkenyl succinic anhydride hydrophobic modification, by adjusting the pH of the system from acidic to neutral or weakly alkaline conditions; the degree of substitution of the nanoparticles is 1–40%.

2. The alkyl-olefinic succinic anhydride-modified chitosan nanoparticles according to claim 1, characterized in that, The alkyl alkenyl succinic anhydride is selected from one or more of allenyl succinic anhydride, octenyl succinic anhydride, nonenyl succinic anhydride, decenyl succinic anhydride, decaenyl succinic anhydride, dodecenyl succinic anhydride, tetradecenyl succinic anhydride, and hexadecenyl succinic anhydride; the alkyl chain has a carbon number of C6 to C16.

3. The method for preparing alkylalkenyl succinic anhydride modified chitosan nanoparticles according to claim 1 or 2, characterized in that, Includes the following steps: S1. Chitosan is dissolved in an aqueous solution of an organic acid to obtain a chitosan solution; an alcohol solvent is added to the chitosan solution to form a homogeneous reaction system; S2. Under certain temperature conditions, alkylalkenyl succinic anhydride is added to the reaction system at a mass ratio of 0.2:1 to 1.2:1 to chitosan to carry out an amidation reaction; S3. After the reaction is completed, the pH of the system is rapidly adjusted to 7.0–8.0, and the mixture is allowed to stand for 0.5–1 h to precipitate a solid. After washing and drying, modified chitosan powder is obtained. S4. The modified chitosan powder is redissolved in an alkaline solution or an organic acid solution, and the pH is slowly adjusted to 5.0–8.

5. Then, it is ultrasonically treated at 200–400 W for 5–15 min to finally form a stable nanoparticle suspension.

4. The preparation method according to claim 3, characterized in that, In step S2, the mass ratio is selected according to the alkyl chain length: for C6 to C8 anhydrides, the mass ratio is 0.4:1 to 1.2:1; for C8 to C10 anhydrides, the mass ratio is 0.3:1 to 1.0:1; for C12 to C16 anhydrides, the mass ratio is 0.2:1 to 0.8:

1.

5. The preparation method according to claim 3 or 4, characterized in that, In step S4, the ultrasonic power of the ultrasonic treatment is 200–400 W, the pulse mode is adopted, the pulse ratio is 2 seconds on and 2 seconds off, and the ultrasonic time is 5–15 minutes.

6. A water-in-oil type Pickering emulsion, characterized in that, The mixture comprises alkyl-olefinic succinic anhydride modified chitosan nanoparticles as a stabilizer, an oil phase as a dispersed phase, and an aqueous phase as a continuous phase; wherein the mass concentration of the modified chitosan nanoparticles in the aqueous emulsion phase is 0.1–1.0%.

7. The Pickering emulsion according to claim 6, characterized in that, The volume ratio of the oil phase to the water phase is 1:2 to 2:

1.

8. The Pickering emulsion according to claim 6, characterized in that, The oil phase is selected from one or more of vegetable oils, animal oils, mineral oils, and synthetic oils.

9. A method for preparing the Pickering emulsion according to any one of claims 6 to 8, characterized in that, include: The modified chitosan nanoparticle suspension according to claim 1 or 2 is used as the aqueous phase. An oil phase is added at a predetermined oil-water volume ratio, and two-stage high-speed shear homogenization is performed: the first stage is shearing at 8000–15000 rpm for 1–3 minutes, and the second stage is shearing at 15000–30000 rpm for 1–5 minutes.

10. The use of the alkylene succinic anhydride modified chitosan nanoparticles according to claim 1 or 2, or the Pickering emulsion according to any one of claims 6 to 8, in the fields of food, drug delivery, and cosmetics.

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