Use of caprylic/capric triglyceride in the encapsulation of dementholized peppermint fragrance leading to smaller and better distributed nanocapsules

Abstract

Dementholized peppermint (DP) fragrance nanocapsules were prepared through the interfacial polymerization of isophorone diisocyanate (IPDI) and hexamethylene diamine (HMDA) in a nanoemulsion, in which peppermint fragrance added with/without caprylic/capric triglyceride (GTCC) was used as the core material and polyurea as the shell material. The results showed that addition of GTCC can significantly reduce the sizes of nanocapsules and improve the size distribution of the nanocapsules. Under the optimized conditions, the resulting DP-GTCC nanocapsules kept good physical stability, while DP nanocapsule dispersion was physically separated in a short time. The average nanocapsule diameter was 82.8 nm with a narrow size distribution of 0.19. The nanocapsules possessed a high DP fragrance loading of 16% with encapsulation efficiency of 81%. Besides, the addition of GTCC further improved the thermal stability of the core material. DP-GTCC nanocapsule dispersion was very stable and exhibited good transparency at a nanocapsule concentration of 0.2% or less due to its very small size.

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1. Introduction

The dementholized peppermint (DP) fragrance not only has flavor and drug effects but also has the ability to remove odors and endow consumers with a pleasant scent. However, the wide application of DP fragrance is restricted by its strong volatility.^1–3 Microcapsule technology is a commonly used way to reduce the volatility of DP fragrance as it is encapsulated by shell materials. At the same time, microcapsules have high wear resistance which can also significantly improve the sustained release effect of the flavor. Besides, microcapsules can change the physical state of the fragrance from a liquid into a solid, making the fragrance more convenient to use in industrial production.^4–6 However, microcapsules with micro-size diameters tend to be unstable in the emulsion system and delamination which greatly limits their application. Adding of stabilizers to the emulsion^7,8 and improving the viscosity of the emulsion^9,10 are two commonly used methods to improve the dispersion stability of microcapsules. But the adding of stabilizers or the increase of the viscosity of the emulsion causes some restriction and trouble for the production and application of microcapsules in the industry to a certain degree.¹¹ At present, with no stabilizers and no change of viscosity, nanoemulsion polymerization has been a popular way to prepare stable particles with small sizes and narrow size distribution. When the sizes of the microcapsules reduce to nanometer range, the stability of the emulsion is greatly improved, as well as its dispersibility, compatibility and durability. As the colloidal dispersions are further reduced to less than 100 nm, transparent or translucent microemulsions will be formed.¹² Besides, the optical transparency of the dispersions due to the small particle sizes is important to be used in nanomedicines, daily chemicals, and washing products.^13–18

Despite the great importance of nanocapsules in both instructrial and fundamental research aspect, there are few reports describing the preparation of physically stable nanocapsules with a high loading capability. Our group developed interfacial radical polymerization to successfully make DP nanocapsules with a high loading. But the resulting nanocapsules had the average size of 261 nm, much more than 100 nm.¹⁹ This was due to the large size of nanoemulsion, which was the template of the polymerization. Caprylic/capric triglyceride (GTCC) possesses the ability to initially create a stable emulsion against Ostwald ripening where the droplet size is below 100 nm, then promoting the subsequent encapsulation of oil phase to form capsules in this nanoemulsion. Besides, GTCC has good function to uniform and even the cosmetic in tiny, and it's easily absorbed by the skin, making skin smooth and shiny.^20–23 In this paper, we prepared oil/water droplets as the template in nanoemulsion with/without GTCC as core material in oil phase and then the nanocapsules were prepared through the interfacial polymerization with isophorone diisocyanate (IPDI) and 1,6 hexanediamine (HMDA), in which polyurea (PU) was formed as shell.^24–26 In contrast to the DP nanocapsules without GTCC, the nanocapsules with GTCC exhibited much smaller size and lower polydispersity. We investigated the influence of adding GTCC as core modifier material on the size and size distribution, thermal stability, and other properties of the resulting nanocapsules. It was demonstrated that use of GTCC in the encapsulation of DP led to smaller and better distributed nanocapsules. This paper may offer a good reference to the research workers who expect to prepare small size nanocapsules or transparent fragrance nanocapsules, and then to apply them in detergents and personal care products. Meanwhile, because of the huge specific surface area of small size nanocapsules, the research workers, who explore phase change material (PCM) nanocapsules for the study of energy storage material and heat transfer material, may also be interested in this paper.

2. Experimental

2.1 Materials

Isophorone diisocyanate (IPDI, Aldrich, 98%), 1,6 hexanediamine (HMDA, Sigma Aldrich, 70%) and sodium dodecyl sulfate (SDS, Sigma Aldrich, 99.0%) were used as received without further purification. Caprylic/capric triglyceride (GTCC, 99%) was purchased from LinYi LvSen Chemical Co., Ltd. The dementholized peppermint (DP, 98%) fragrance was provided by Anhui Fengle Perfume Co., Ltd and used without further purification. Ultrapure water was used for all experiments.

2.2 Preparation of nanocapsules and nanocapsule powder

Nanocapsules were prepared by a “interfacial polymerization” method. First, surfactant sodium dodecyl sulfate (SDS) was dissolved in water (below its critical micelle concentration) to prepare the aqueous phase. Then, IPDI or the mixture of IPDI and GTCC was dissolved in DP at RT to prepare the oil phase. The resulting oil solution was poured into the aqueous phase. Then the mixture was emulsified by 2% SDS as the emulsifier with an ultrasound device (SCIENTZ®JY92-IIDN, 900W) in an ice water bath for 30 min with power efficiency of 50%, resulting in forming nanoemulsion. Subsequently, the nanoemulsion was heated 3 min in a 35 °C water bath. Then HMDA (40%) which was obtained by diluting from the reagent (70%) was added dropwise into the nanoemulsion to trigger the polymerization under magnetic stirring. After 30 min, the bath temperature raised to 55 °C and the reaction was continued for 2 h to further cure the shell of nanocapsules.

Nanocapsule powder was gained by freeze drying, using a chamber-type freeze-drier (LGPT-10, Shanghai Ruishi Technology Co. Ltd). Firstly the nanocapsule dispersion (10 g) was added into toluene (20 g) with magnetic stirring for 30 s. The unencapsulated core materials (free oil) were extracted from the nanocapsule dispersion into the toluene. The nanocapsule dispersion (aqueous phase) was separated from the toluene solution which was obsoleted. Then the nanocapsule dispersion, of which free oil was removed, was frozen at −60 °C overnight, and then freeze-dried for 24 h under 75 μm Hg. After that nanocapsule powder was gained.^27–29

2.3 Particle size and its distribution analysis

The size and size distribution of nanocapsules were determined by DLS with a BI-9000 AT digital time correlator (BI-200SM, Brookhaven Co. Ltd) in the condition of 90 °C for scattering angle. Light source is a He–Ne laser with 35 mW and 633 nm.

2.4 Core–shell structure and morphological observation

The core–shell structure and morphology of nanocapsules were characterized by transmission electron microscope (TEM, H-800, Hitachi Co. Limited) under dry condition. It was performed at an accelerating voltage of 200 kV and the samples were prepared by placing a dilute drop of the measured solution onto the copper grids and allowing it to dry.

2.5 Chemical structure analysis

The chemical structures of IPDI, PU, core materials (DP or the mixture of DP and GTCC), and nanocapsule powder were characterized by Fourier transform infrared (FTIR, Nicolet 6700, Thermo Fisher Scientific Inc., USA) spectroscopy, which could infer whether the reactive monomer IPDI and HMDA completely participated in the polymerization and the core materials were encapsuled.

2.6 Encapsulation efficiency analysis

Encapsulation efficiency was determined by means of UV-visible spectrophotometer (PerkinElmer, USA) at the wavelength of 283 nm and 242 nm.

2.7 Thermal stability analysis (TGA)

The thermal stabilities of the core materials, nanocapsules, and PU shell material with increasing temperature were evaluated by the thermal gravimetric analysis (TGA, TG209F1, German Resistance Instrument Co., Ltd) under an atmosphere of N₂ from 30 to 600 °C at a heating rate of 5 °C min⁻¹.

2.8 Analysis of physical (dispersion) stability

Dispersion stability of nanocapsules was performed with a ultracentrifuge (Optima XE-90, Beckman Coulter, USA) and then it was determined by observing whether the emulsion delaminate after given high-speed centrifugation.

2.9 Transparency analysis of nanocapsules

Transparency of nanocapsules dispersion dilution was determined by measuring the absorbance at 555 nm, using a UV-visible spectrophotometer (PerkinElmer, USA). The transparency was calculated by the following equation: T = 1/10^A, where T is the transparency and A is the value of absorbance at 555 nm. According to this equation, a high T value would correspond to a transparent appearance.³⁰

3. Results and discussion

3.1 Mechanism of nanocapsule formation

Interfacial polymerization mechanism was proposed to illustrate the formation of nanocapsules. The mixture of aqueous phase and oil phase was emulsified by SDS as the emulsifier after sonication to form the o/w nanoemulsion droplets. After the addition of HMDA, the reaction was triggered. The monomer IPDI was in the oil phase of emulsion droplets, while the monomer HMDA was in the aqueous phase. Then the high active groups of –N=C=O and –NH₂ encountered each other at the oil–water interface, and rapidly reacted to form the carbamido group (–NH–CO–NH–). As the decrease of the monomer at the interface, IPDI dissolved in the core materials and HMDA in aqueous phase continuously diffused to the interface to participate in the polymerization to form polyurea long chains. When the temperature raised, the activity of N–H in –NH–CO–NH– was enhanced and further cross-linked with –N=C=O, forming the network polymer as the shell material.³¹ Finally, the core materials were encapsulated by the shell (Fig. 1).

Fig. 1 The schematic process of preparing DP nanocapsules or DP-GTCC nanocapsules by nanoemulsion polymerization, : emulsifier; : monomer IPDI; : monomer HMDA; : polyurea network; : core, i.e. DP or DP-GTCC; : shell.

3.2 Control of the nanocapsules: small size and narrow size distribution

In our group's previous work, it was demonstrated that the polarity of core material became weak with adding hydrophobic GTCC into DP as the core material, leading to a good spherical shape of the microcapsule.²⁶ Given that the polarity of the oil phase has a great influence on the emulsion system, the influence of GTCC on the size and distribution of fragrance nanocapsules was tentatively investigated. Since the interfacial polymerization mechanism of polyurea is simple and the reaction is responsive. Meanwhile, polyurea is a classic kind of resin material, which possesses properties such as fast curing and forming, good wearability. Then polyurea was chosen as shell material to prepare small size nanocapsules with a preliminary exploration. On the other hand, the monomer IPDI is a kind of aliphatic isocyanate and does not produce aniline which was highly toxic substance in the decomposition process. Then, a series of nanocapsules with different proportions of hydrophobic GTCC^22,32,33 in oil phase as the core materials^34,35 were prepared (the content of core materials DP and GTCC was 20%). Fig. 2a showed the average size and the size distribution of nanocapsules prepared in different formulations. As the nanocapsules were prepared only with DP fragrance in core materials, the average size of the nanocapsules was 210.4 nm with a wide size distribution of 0.251. As GTCC was added in oil phase as the core material, the nanocapsule size decreased rapidly. As the proportion of GTCC was further increased, the average size of nanocapsules increased again. The effect of GTCC addition on the size distribution was shown in Fig. 2b. The results showed that the addition of GTCC led to the formation of the nanocapsules with low polydispersity. As the content of GTCC in oil phase was increased to 20% of core materials, the average size of the resulting nanocapsules was dramatically reduced to 82.8 nm with a narrow size distribution of 0.190. The DP-GTCC nanocapsule dispersion contained 20% oil, including 16% DP and 4% GTCC.

Fig. 2 Effect of different proportions of GTCC on (a) the average particle size; and (b) the average polydispersity index (PDI).

3.3 Particle size and size distribution of nanocapsules

After the investigation of the effect with different proportions of GTCC on the size and distribution of nanocapsules, the proportion of GTCC in core material was fixed to be 20%, and then the cases with and without GTCC were compared. The influence with different contents of core material on the particle size and distribution was also investigated. Details of the composition were shown in Table 1. From A1 to A3, the content of GTCC in core materials is 0%. With the increase of the core material (DP), the average size of the nanocapsules increased and the size distribution became slightly broad. From B1 to B3, the content of GTCC in core materials was 4% of total weight in the nanocapsule dispersion. With the increase of the core materials (DP and GTCC), the average size increased slightly, but still below 100 nm, while the size distribution was similar. Fig. 3 showed the size and distribution of the nanocapsules formulated by A2 and B2 with the same moderate content of core material. In subsequent research, other properties of the resulting nanocapsules and the differences in cases with and without GTCC were also investigated with the same moderate content of core material, i.e. 20%. The results demonstrated that the adding of 20% GTCC in core material significantly reduced the resulting particle size of the resulting nanocapsules and improved the size distribution. The formulation A1, A2 and B2 further demonstrated that the variation of size and distribution was due to the adding of GTCC rather than the content of core material.

Table 1 Different compositions of nanocapsules

Entries	A1	A2	A3	B1	B2	B3
DP (wt%)	15.0	20.0	25.0	12.0	16.0	20.0
GTCC (wt%)	0	0	0	3.0	4.0	5.0
IPDI (wt%)	3.2	3.2	3.2	3.2	3.2	3.2
Oil (wt%)	15.0	20.0	25.0	15.0	20.0	25.0
HMDA (wt%)	1.44	1.44	1.44	1.44	1.44	1.44
Shell (wt%)	4.64	4.64	4.64	4.64	4.64	4.64
SDS (wt%)	2.0	2.0	2.0	2.0	2.0	2.0
Particle size (nm)	191.7	210.4	236.5	84.9	82.8	91.8
PDI	0.260	0.251	0.336	0.221	0.190	0.202

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Fig. 3 The size and distribution of DP nanocapsules (A2) and DP-GTCC nanocapsules (B2).

It could be proposed that the addition of GTCC led to the formation of the smaller nanoemulsion with size below 100 nm as follows. The formation of smaller nanocapsules correspondently confirmed the generation of stable small size nanoemulsion against Ostwald ripening. Ostwald ripening is a process by which the larger droplets in an emulsion grow at the expense of the smaller ones because of molecular diffusion of oil between droplets through the continuous phase. This process is driven by the Kelvin effect where the small emulsion droplets have higher local oil solubility than the larger droplets because of the difference in Laplace pressures. A quantitative description of Ostwald ripening in a two phase system is given by the Lifshitz–Slesov–Wagner (LSW) theory.^36,37

The ultimate size of a homogenized emulsion is determined by the balance between two opposing processes: droplet break-up and droplet recoalescence.^38,39 The frequency of both processes is promoted by the intense shear that occurs within a high shear homogenizer, i.e. the ultrasonic transducer in this paper. Droplet break-up occurs when the applied shear is greater than the Laplace pressure of the emulsion.^39–42 In other words, the process of droplet break-up mainly depends on ultrasonic power. In our research the ultrasonic condition remained constant in the cases with GTCC and without GTCC. However, nanoemulsions are particularly prone to a growth in particle size over time by a process known as Ostwald ripening, which is critical to the process of droplet recoalescence.³⁸ It also has been corroborated with the results in our work. Wooster et al.³⁸ examined the Ostwald ripening of mixed oil systems in detail and demonstrated that the insolubility of triglyceride oils in water provided a kinetic barrier to Ostwald ripening. Henry et al.⁴³ also demonstrated that kinetically trapped food grade nanoemulsions can be created using a long chain triglyceride as the oil phase. The surfactant plays a critical role in both droplet break-up and coalescence.⁴² Invariably the requirements of both droplet break-up and coalescence dictate that small molecule surfactants are the most suited to the formation of smaller nanoemulsions (compared to macromolecular emulsifiers) because of their greater ability to rapidly adsorb to interfaces and lower dynamic interfacial tensions.³⁸ Therefore surfactant SDS can further reduce the interfacial tension of o/w emulsion droplets, afterwards reducing the size of o/w emulsion droplets.

3.4 Morphological observation

Fig. 4 showed the morphology of the nanocapsules in series of A2 and B2 by TEM images. It was concluded that nanocapsules with and without GTCC both showed smooth spheres with visible core–shell structure and the size of both nanocapsules was relatively uniform. However, a slight aggregation of DP-GTCC nanocapsules may be triggered by unencapsulated GTCC in the nanocapsule dispersion. Also the TEM result reflected the size and distribution of nanocapsules, and it was consistent with the DLS result on the whole.

Fig. 4 Transmission electron micrographs of (a) DP nanocapsules, (b) DP-GTCC.

3.5 Fourier transform infrared (FTIR) analysis

FTIR spectroscopy was used to characterize the formation of nanocapsule as shown in Fig. 5. DP fragrance is a blend of menthol and menthone. The O–H stretching vibration absorption (3436 cm⁻¹) in menthol and the C=O stretching absorption (1712 cm⁻¹) in menthone were observed in Fig. 5a. The C=O stretching vibration absorption (1744 cm⁻¹) and the C–O–C stretching absorption (1159 cm⁻¹) in GTCC were observed in Fig. 5b. Correspondingly the appearance of the characteristic peaks of the core materials and the similar intensities in Fig. 5f and g proved the encapsulation of the core materials in nanocapsules.^26,31 Fig. 5e showed the spectrum of polyurea as shell material obtained by the polymerization of IPDI and HMDA. The 3346 cm⁻¹ was the N–H stretching vibration absorption of carbamido (–NH–CO–NH–), which moved to low frequency for the hydrogen bonding. The 1634 cm⁻¹ and 1560 cm⁻¹ were corresponding to the C=O stretching absorption and the N–H deformation absorption in –NH–CO–NH–, while the C–N stretching absorption was showed at the 1247 cm⁻¹. The appearance of above characteristic peaks in Fig. 5f and g proved the formation of the polyurea shell material. Moreover, the major disappearance of the 2260 cm⁻¹, which was the characteristic absorption peaks of –N=C=O of IPDI monomer as shown in Fig. 5d, indicated that almost all the monomer had participated in the polymerization of the nanocapsule shell and encapsulated the core materials effectively.^26,44

Fig. 5 Infrared spectra of (a) DP, (b) GTCC, (c) DP-GTCC, (d) IPDI, (e) PU, (f) DP capsules, (g) DP-GTCC capsules.

3.6 Encapsulation efficiency

In this paper, two methods were performed to determine the encapsulation efficiency in turn respectively. Firstly, we indirectly determined the encapsulation efficiency through an extraction method with toluene. Since unencapsulated core material (free oil) was measured, regardless of the loss, the encapsulated oil content was inferred and the encapsulation efficiency was calculated. A certain amount of DP was dissolved into toluene, and a series of standard samples (0.3, 0.75, 1.5, 2.25, 3 and 4.5 mg g⁻¹) were prepared. One of the standard samples was scanned via UV-visible spectrophotometer (Lamda 35, PerkinElmer) over the range of 200–700 nm first and the maximum absorbance of the core material (the absorbance value was in the range of 0.1–0.8) was 283 nm, where GTCC had no absorbance. Then, the UV absorbances of the six standard samples were tested at 283 nm, respectively. The linear relationship between the concentration of DP(x) of the six standard samples and their UV absorbance (y) was created according to the calibration curve of spectrophotometry; the equation is y = 0.15751x − 0.02856 (R² = 0.9992) as shown in Fig. S1a. Finally, nanocapsule dispersion (1.5 g) of A2 and B2 was added to toluene (20 g) with magnetic stirring for 30 s, respectively. The unencapsulated core material (free oil) was extracted from the nanocapsule dispersion into the toluene. The supernatant solution (toluene solution) was separated from the nanocapsule dispersion. Then, the supernatant solution was put into cuvette for testing. The UV absorbance (A_t) (y) of the free oil solution was tested by UV-visible spectrophotometer. The concentration of free DP solution (x) was calculated through the linear relationship, further indicating the whole free oil of the sample.⁴⁴ After that, the encapsulation efficiency (EE_t) of the core materials was determined by the following (eqn (1)):W_FD is the amount of free DP in the sample suspension, and W_D is the amount of feeding DP.

Fig. 6 TGA curve of the core materials, nanocapsules, and PU shell material.

Subsequently, we directly determined the encapsulation efficiency through an induction method with ethanol. After removal of free oil, ethanol was used both as solvent and inducer to induce encapsulated oil out, with capsules broken. Then the original encapsulated oil inside the capsules was measured, and the encapsulation efficiency was calculated. In a similar way, a certain amount of DP was dissolved into ethanol, and a series of standard samples (0.06, 0.15, 0.3, 0.45, and 0.6 mg g⁻¹) were prepared. One of the standard samples was scanned via the UV-visible spectrophotometer over the range of 200–700 nm first and the maximum absorbance of the core material (the absorbance value was in the range of 0.1–0.8) was 242 nm, where GTCC had no absorbance. Then, the UV absorbances of the five standard samples were tested at 242 nm, respectively. The linear relationship between the concentration of DP(x) of the five standard samples and their UV absorbance (y) was created according to the calibration curve of spectrophotometry; the equation is y = 1.38005x − 0.00698 (R² = 0.9991) as shown in Fig. S1b. Finally, nanocapsule powder (0.5 g) of A2 and B2 was added to ethanol (10 g) with ultrasonic treatment in an ice water bath, using the SCIENTZ®JY92-IIDN ultrasound device for 30 min with power efficiency of 50%, respectively. As mentioned before, the encapsulated oil inside the capsules was induced out, with capsules broken and dissolved in ethanol. After immersion for 2 days, the microcapsules – ethanol mixture was centrifuged with a table-type low speed centrifuge TDL-80-2B for 0.5 h, making the shell material precipitate. Then, the supernatant solution (ethanol solution) was diluted 100 fold with ethanol and put into cuvette for testing. The UV absorbance (A_e) (y) of the encapsulated oil solution was tested by UV-visible spectrophotometer. The concentration of encapsulated DP solution (x) was calculated through the linear relationship, further indicating the mass of encapsulated DP in the nanocapsule powder. After that, the encapsulation efficiency (EE_e) of the core materials was inferred by the following (eqn (2)):

W_D is the amount of feeding DP, W_O is the amount of oil, W_S is the amount of shell material, m_ED is the mass of encapsulated DP in the nanocapsule powder, m_NP is the mass of nanocapsule powder.

According to the Table 2, the encapsulation efficiency of B2 was about 81%, which had a little difference with the case of A2. It showed that the core material added with GTCC was encapsulated well. In conclusion, two measurement methods were confirmed with each other, and it was inferred that the fragrance loss during the preparation of nanocapsules was little.

Table 2 Absorbance and encapsulation efficiency of DP nanocapsules and DP-GTCC nanocapsules

Entries	Core material	Absorbance (At)	EEt	Absorbance (Ae)	EEe
A2	DP	0.465	79.11%	0.525	78.09%
B2	DP-GTCC	0.329	81.08%	0.422	80.89%

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3.7 Thermal stability

The thermal stabilities of the nanocapsules were evaluated by TGA from 30 to 600 °C, as shown in Fig. 6. The TGA curve showed that DP was so volatile that the onset evaporation temperature was only about 50 °C. DP evaporated completely when the temperature rose to 150 °C. The TGA curve of DP-GTCC had two stages. The first stage was the weight loss of DP from 50 to 150 °C, while the second stage was the weight loss of GTCC from 200 to 300 °C. The loss of mass accelerated at about 245 °C. The curve of shell material indicated that the thermal decomposition temperature of PU was about 326 °C. With the temperature increasing, the shell material decomposed and the quantity lost rapidly. The TGA curve of DP capsules showed that the weight loss of DP nanocapsules had three stages. At about 110–200 °C, a small amount of core material volatilized. When the temperature increased to about 243 °C, the volatilization of core material resulted in the pressure inside the capsules increasing and the capsule shell gradually burst, making the weight loss of core material accelerating. When the temperature was raised to about 330 °C, the thermal decomposition of the shell material further exacerbated the weight losses of the core and shell material. The TGA curve of DP-GTCC capsules showed that the temperature of significant weight loss was raised to about 253 °C, which indicated that the thermal stability was further improved as compared with DP nanocapsules. In conclusion, the polyurea shell effectively protected the DP fragrance from evaporation. Moreover, the addition of GTCC can further improve the thermal stability of the core material.

3.8 Dispersion stability

The great decrease of the size convinced us that the dispersion stability of nanocapsule suspension was improved greatly, and then tests were performed to prove our inference. The dispersion stability of nanocapsule suspension was characterized through high-speed centrifugation for reflecting its ability to resist gravity deposition.^45–47 The DP nanocapsule dispersion exhibited a state of white emulsion. After 60 min centrifugation at 5000 rpm, the DP nanocapsule dispersion was stable for several days, while the dispersion appeared significant delamination after 60 min centrifugation at 10 000 rpm. Even without centrifugation, the DP nanocapsule dispersion exhibited delamination after several days. However, the DP-GTCC nanocapsule dispersion didn't have delamination until 60 min after centrifugation at 30 000 rpm. Without a high speed of centrifugation, the DP-GTCC nanocapsule dispersion kept very stable state for more than one year. Small DO-GTCC nanocapsules are resistant to creaming because their Brownian motion is enough to overcome their low gravitational separation force. They are also resistant to flocculation because of highly efficient steric stabilization.³⁸ In contrast to the DP nanocapsules, smaller DP-GTCC nanocapsules with the addition of 4 wt% GTCC into core material demonstrated the great dispersion stability.^45,48–50

3.9 Transparency of nanocapsule dispersion

Likewise, it convinced us that the transparency of nanocapsule dispersion was improved greatly, and also tests were performed to prove our inference. Fig. 7 showed the states of nanocapsule dispersion with different concentrations of the nanocapsules. The transparency of nanocapsule dispersion was estimated by the absorbance of dispersion dilution.^30,51 The 555 nm was selected as the wavelength of determination for its good sensitivity to the human eye. Table 3 showed the test results of A2 and B2 dispersion with different dilution ratios. With the increase of the dilution ratio, the transparency of the nanocapsule dispersion was increased. The transparency of DP-GTCC nanocapsule dispersion with increasing dilution ratio was improved. As the DP-GTCC nanocapsule dispersion was diluted 100 times and had 0.16% DP fragrance still within the range of added fragrance from 0.1–0.5% in commonly used cosmetic products. It is important to transparent cosmetic products necessary to be formulated with smaller size of the fragrance nanocapsules.

Fig. 7 (a) 100-fold dilution of DP nanocapsules dispersion; DP-GTCC nanocapsules dispersion with dilution ratio of (b) 10, (c) 30, (d) 50, (e) 100.

Table 3 Absorbance and transparency of different dilution ratios

Core material	Dilution ratio	Fragrance addition	Absorbance	Transparency
DP	100	0.20%	1.013	9.705%
DP-GTCC	10	1.60%	2.524	0.299%
DP-GTCC	30	0.53%	0.970	10.715%
DP-GTCC	50	0.32%	0.553	27.990%
DP-GTCC	100	0.16%	0.225	59.566%

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4. Conclusions

In this paper, the dementholized peppermint fragrance nanocapsules with/without GTCC were prepared by interfacial polymerization of forming polyurea as shell materials. Under the optimized synthesized condition, the as prepared DP-GTCC core–shell nanocapsules presented spheres structure with average size of 82.8 nm, with a low polydispersity of 0.19. Besides, the encapsulation efficiency of prepared nanocapsules reached about 81% with a high loading of 16% DP. Moreover, due to size below 100 nm, the DP-GTCC nanocapsule dispersion exhibited great dispersion stability for a long time and good transparency at a low concentration of the nanocapsules. However, at the low concentration, the fragrance loading was still within the range of added fragrance from 0.1–0.5% in commonly used cosmetic products. Hence, the fragrance nanocapsules are hopefully used in transparent cosmetic products such as shampoo and detergent, with higher utilization level of fragrance and additional value of products. The thermal stability of the encapsulated DP both in DP nanocapsules and in DP-GTCC nanocapsules was greatly improved. An addition of GTCC in core was further enhanced. On the other hand, small size nanocapsules can penetrate the interior of fabric or fiber gaps and is well attached to the textile in the absence of adhesive. So it is believed that nanocapsules will be of great value in the aspect of perfuming finishing in fabrics. Meanwhile, as sonication can be substituted by high pressure homogrnization to form small nanoemulsion, the fragrance nanocapsules will be hopefully used in actual industrial products. Moreover, our approach to the formation of the nanocapsules with very small size below 100 nm will promise to be applied to numerous nanoencapsulation of different active core substances. Also, our group will continue to explore more green materials to prepare small size nanocapsules and overcome deficiencies caused by polyurea as the shell material in future studies.

Acknowledgements

The authors thank for financial supports by National Nature Science Foundation of China (51473032, 51503034), Fundamental Research Funds for the Central Universities (CUSF-DH-D-2016024, 2232015D3-12) and Science and Technology Commission of Shanghai Municipality for Yangfan Program (15YF1400700).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19003k

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