**EE** (%) = **Weight of essential oil in microcapsules Weight of essential oil added** <sup>×</sup>**<sup>100</sup>** (2)

#### *2.5. Determination of Antioxidant Activity*

The antioxidant activity of rosemary essential oil loaded and unloaded microcapsules was assessed using 1.1-diphenyl-2-picrylhydrazyl (DPPH), a free radical, following the procedure outlined by Ling et al. [26]. Briefly, a 0.1 mM solution of 1.1-diphenyl-2 picrylhydrazyl (DPPH) radical solution in 90% ethanol was prepared and 2.5 mL of this solution was mixed vigorously with different REO concentrations (25–7500 µg/mL in ethanol) and a mass of 100 mg of the loaded and unloaded microcapsules. After 30 min of incubation in the dark at room temperature, absorbance (A) was measured at 517 nm using a Rayleigh UV–VISIBLE 1800 spectrophotometer (Model Jasco 560, Pekin, China). The percentage of the radical scavenging capture (RSC) was calculated based on the following Equation (3), where Acont and Asample are the absorbance values of the control and the sample, respectively.

$$\mathbf{RSC}(\%) = \frac{\mathbf{Acont} - \mathbf{Asample}}{\mathbf{Acont}} \times 100 \tag{3}$$

#### *2.6. REO Release Kinetics*

The REO release study was carried out using a method described by El Hosseini et al. [23]. The REO-loaded microcapsules (250 mg) were placed in a flask containing 30 mL of the release medium (1% Tween 80 in distilled water). The suspensions were slowly stirred at ambient temperature (20 ◦C). At predetermined time intervals, samples from the release medium were taken. The REO concentration was determined by spectrophotometry at 257 nm using a Rayleigh UV–VISIBLE 1800 spectrophotometer (Model Jasco 560, Pekin, China). The concentration of rosemary essential oil in the release medium at different sampling times was determined using a calibration curve of free REO in 1% Tween 80. The cumulative percentage of the REO release (%CR) was calculated using Equation (4) [27]. To forecast and correlate the REO release behaviour from both microparticles, it is imperative to include the proper model. Therefore, the data from the release study were adjusted to an established empirical model by Korsmeyer and Peppas using Equation (5) [23].

$$\% \mathbf{CR} = \sum\_{t=0}^{t} \frac{\mathbf{M}\_t}{\mathbf{M}\_0} \times \mathbf{100} \tag{4}$$

$$\mathbf{^m}l\_{\mathbf{m}\_{\otimes}} = \mathbf{k} \times \mathbf{t}^{\mathfrak{n}} \tag{5}$$

where **<sup>m</sup><sup>t</sup> /m<sup>∞</sup>** is the percentage of the drug released at time t and for all time; k is the description of the macromolecular network system and n is the release exponent indicating the release mechanism. A linear form of the equation can be acquired through plotting ln ( **m<sup>t</sup> /m<sup>∞</sup>** ) against ln(t), whose linear coefficient is k and whose angular coefficient is n. The determined n value is used to discover which of the three mechanisms can describe the release behaviour: the n value is less than 0.43; the release follows the Fickian law and the n value is between 0.43 and 0.85; the non-Fickian release mechanism is established, and when the n value is greater than 0.85 it indicates the case II of transport release with a polymer chain relaxation [28]. Using OriginPro 2018 software, we calculated the squared correlation coefficient (R<sup>2</sup> was used to confirm the accuracy of the model).

#### **3. Results**

#### *3.1. Identification of Rosemary Essential Oil Chemotype*

Essential oil of *Rosmarinus officinalis* L. from the eastern region of Morocco was characterized by gas chromatography-mass spectroscopy (GC-MS) [22]. As can be seen in Table 2, 32 compounds were totally identified, representing 99.9% of the total oil content. The most abundant compounds of essential oil are mainly concentrated in both groups, monoterpenes hydrocarbons, and oxygenated monoterpenes. The main compounds are 1.8-cineole

(29.71%), α-thujene (14.17%), camphor (13.09%), β-pinene (9.94%), and camphene (6.46%). The high ratio of these compounds was previously revealed by some researchers [29,30] who worked on the same oil and according to the literature, the essential oil of *Rosmarinus officinalis* L. used in this work can be classified as a 1.8-Cineol chemotype. The volatility of these substances restricts their use even though they can slow down microbial growth or free radicals. Thus, encapsulating them in microcapsules is undoubtedly a better way to delay their volatilization, which can increase the range of their application.


**Table 2.** Chemical profile of *Rosmarinus officinalis* L. essential oil (R<sup>T</sup> is the retention time).

#### *3.2. Characterization of Microcapsules*

3.2.1. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR)

The attenuated total reflection Fourier transform infrared was performed on REO, REO-loaded microcapsules and non-loaded microcapsules to identify the characteristic bands of the chemical structures. The obtained spectra are shown in Figure 2. The REO spectrum presents characteristic bands from 1030 to 1135 cm−<sup>1</sup> assigned to out-of-plane C-H wagging vibrations from terpenoids. The stretching vibrations of C–O present in the carbohydrates correspond to peaks from 1190 to 1035 cm−<sup>1</sup> . The spectral region from 1700 to 1500 cm−<sup>1</sup> is associated with C=C bending. The peaks attributed to the C–H bending (aliphatic domain) appear in the 2900 cm−<sup>1</sup> frequency. Carboxylic acid (O–H) stretching has characteristic absorption between 3000–2500 cm−<sup>1</sup> . The FTIR spectrum of the pure rosemary essential oil shows the expected characteristic C–H stretch (~2900 cm−<sup>1</sup> ), C=O stretch (~1700 cm−<sup>1</sup> ), broad O–H stretch (~3400 cm−<sup>1</sup> ), and C–O stretch (~1100 cm−<sup>1</sup> ) of terpenoid components.

The "fingerprint" characteristic bands of sodium alginate include the spectral region from 800 to 1000 cm−<sup>1</sup> and corresponds to the C–C stretching of the alginate skeleton and deformation mode [31]. The bands that occur at 1417 and 1617 cm−<sup>1</sup> correspond to its carboxylic groups, symmetric and asymmetric stretching, respectively [32], and have been observed in all the capsule spectra. The broad band around 3600 cm−<sup>1</sup> in the ATR-FTIR spectrum of hybrid CA-MTN microcapsules is associated with the Al–OH and Mg–OH vibrations. The intensive peak at a wave number of 1036 cm−<sup>1</sup> represents the Si–O stretching vibrations, which are mainly associated with montmorillonite clay. The peak at 1640 cm−<sup>1</sup> is attributed to the bending vibration of water that is physically adsorbed [33]. The broad band at 3400 cm−<sup>1</sup> corresponds to the O–H stretching of interlayer adsorbed water. Regarding the spectra of the REO-loaded microcapsules, some peaks characteristic of rosemary essential oil aromatic domain (2600–3200 cm−<sup>1</sup> ) appeared on the spectra of loaded microcapsules and do not appear on the non-loaded microcapsules. Some peaks

typical of calcium alginate (1417–1617 cm−<sup>1</sup> ) were intensified compared to the non-loaded microcapsules suggesting interactions between the polymer, clay and rosemary essential oil. In the ATR-FTIR spectra, the peak of sodium alginate carboxyl anions was shifted from 1627 cm−<sup>1</sup> for CA-MTN microcapsules to 1636 cm−<sup>1</sup> for CA-MTN-REO and from 1630 cm−<sup>1</sup> to 1651 cm−<sup>1</sup> for CA and CA-REO, respectively. This can be explained by the REO incorporation. Our presumption is that REO molecules were adsorbed on the clay surface and its interlayer space which induced interaction and the creation of new bonding. In the work of Volic et al. [31], the combination of soy protein, sodium alginate and thyme oil created an aggregation of the soy protein involving a hydrogen bonds disruption. *Polymers* **2023**, *15*, x FOR PEER REVIEW 8 of 16

**Figure 2:** ATR-FTIR spectral of rosemary essential oil (REO), calcium alginate (CA), calcium alginate-montmorillonite (CA-MTN), calcium alginate-rosemary essential oil (CA-REO), and calcium alginate-montmorillonite-rosemary essential oil (CA-MTN-REO). **Figure 2.** ATR-FTIR spectral of rosemary essential oil (REO), calcium alginate (CA), calcium alginatemontmorillonite (CA-MTN), calcium alginate-rosemary essential oil (CA-REO), and calcium alginatemontmorillonite-rosemary essential oil (CA-MTN-REO).

#### The "fingerprint" characteristic bands of sodium alginate include the spectral region 3.2.2. Thermogravimetric Analysis (TGA)

from 800 to 1000 cm−1 and corresponds to the C–C stretching of the alginate skeleton and deformation mode [31]. The bands that occur at 1417 and 1617 cm−1 correspond to its carboxylic groups, symmetric and asymmetric stretching, respectively [32], and have been observed in all the capsule spectra. The broad band around 3600 cm−1 in the ATR-FTIR spectrum of hybrid CA-MTN microcapsules is associated with the Al–OH and Mg–OH vibrations. The intensive peak at a wave number of 1036 cm−1 represents the Si–O stretching vibrations, which are mainly associated with montmorillonite clay. The peak at 1640 cm−1 is attributed to the bending vibration of water that is physically adsorbed [33]. The Thermal analysis of the unloaded and loaded microcapsules showed the weight loss of microcapsules related to moisture. According to Wang et al. [34], alginate polymer has two sorts of water; unbounded water whose thermal event occurs below 100 ◦C and bounded water whose thermal event occurs above 100 ◦C. Thermogravimetric studies have shown that the stability of several antioxidants such as essential oils decreases with the increase in temperature [35]. The thermograms (Figure 3) present the weight loss pattern of CA, CA-MTN microcapsules, CA-REO and CA-MTN-REO microcapsules. REO is sensitive to heat and therefore the thermal property was carried out up to 150 ◦C.

broad band at 3400 cm−1 corresponds to the O–H stretching of interlayer adsorbed water. Regarding the spectra of the REO-loaded microcapsules, some peaks characteristic of rosemary essential oil aromatic domain (2600–3200 cm−1) appeared on the spectra of loaded microcapsules and do not appear on the non-loaded microcapsules. Some peaks typical of calcium alginate (1417–1617 cm−1) were intensified compared to the non-loaded microcapsules suggesting interactions between the polymer, clay and rosemary essential oil. In the ATR-FTIR spectra, the peak of sodium alginate carboxyl anions was shifted from 1627 cm−1 for CA-MTN microcapsules to 1636 cm−1 for CA-MTN-REO and from 1630 cm−1 to1651 cm−1 for CA and CA-REO, respectively. This can be explained by the REO incorporation. Our presumption is that REO molecules were adsorbed on the clay surface and its interlayer space which induced interaction and the creation of new bonding. In the All of the microcapsules show thermal stability up to 42 ◦C. A weight loss then takes place and slows at a constant rate to 83 ◦C. This weight decrease of 30% is merely due to the REO present on the capsule's surface and a part of the external moisture losses [36]. Between 83 and 117 ◦C, a fast weight loss appears in both loaded microcapsules compared to the non-loaded microcapsules. Until 117 ◦C, the evaporation of the outer mist and the diffusion of the REO within the core capsule occur faster in CA microcapsules than in CA-MTN hybrid microcapsules. At that stage, the loss is important and exceeds 48%. The study continues until total decomposition. The results have shown that the presence of montmorillonite clay improved the thermal stability of the hybrid microcapsules compared with CA microcapsules. This improvement in the thermal stability is related to the reduced motions of alginate networks following the clay filling which slows the diffusion of the inner

work of Volic et al. [31]*,* the combination of soy protein, sodium alginate and thyme oil

Thermal analysis of the unloaded and loaded microcapsules showed the weight loss of microcapsules related to moisture. According to Wang et al. [34]*,* alginate polymer has two sorts of water; unbounded water whose thermal event occurs below 100 °C and bounded water whose thermal event occurs above 100 °C. Thermogravimetric studies have shown that the stability of several antioxidants such as essential oils decreases with

created an aggregation of the soy protein involving a hydrogen bonds disruption.

3.2.2. Thermogravimetric Analysis (TGA)

particles through the capsule membrane [37]. Therefore, REO could be better preserved in the CA-MTN hybrid microcapsules for encapsulation purposes. tern of CA, CA-MTN microcapsules, CA-REO and CA-MTN-REO microcapsules. REO is sensitive to heat and therefore the thermal property was carried out up to 150 °C.

the increase in temperature [35]. The thermograms (Figure 3) present the weight loss pat-

*Polymers* **2023**, *15*, x FOR PEER REVIEW 9 of 16

**Figure 3.** TGA curves of calcium alginate (CA), calcium alginate-montmorillonite (CA-MTN), calcium alginate-rosemary essential oil (CA-REO), and calcium-alginate-montmorillonite-rosemary essential oil (CA-MTN-REO) microcapsules. **Figure 3.** TGA curves of calcium alginate (CA), calcium alginate-montmorillonite (CA-MTN), calcium alginate-rosemary essential oil (CA-REO), and calcium-alginate-montmorillonite-rosemary essential oil (CA-MTN-REO) microcapsules.

#### All of the microcapsules show thermal stability up to 42 °C. A weight loss then takes 3.2.3. Particle Size and Morphology

place and slows at a constant rate to 83 °C. This weight decrease of 30% is merely due to the REO present on the capsule's surface and a part of the external moisture losses [36]. Between 83 and 117 °C, a fast weight loss appears in both loaded microcapsules compared to the non-loaded microcapsules. Until 117 °C, the evaporation of the outer mist and the diffusion of the REO within the core capsule occur faster in CA microcapsules than in CA-MTN hybrid microcapsules. At that stage, the loss is important and exceeds 48%. The study continues until total decomposition. The results have shown that the presence of montmorillonite clay improved the thermal stability of the hybrid microcapsules compared with CA microcapsules. This improvement in the thermal stability is related to the reduced motions of alginate networks following the clay filling which slows the diffusion of the inner particles through the capsule membrane [37]. Therefore, REO could be better preserved in the CA-MTN hybrid microcapsules for encapsulation purposes. The size and morphological characterization of the microcapsules are important because they can have an impact on the physicochemical properties such as water sorption and release of the encapsulated agent, as well as the aesthetic quality which could be a desirable feature for pharmaceutical and food products [38]. Because the microcapsules were generated by ionotropic gelation of the emulsion through a needle of 1 mm diameter into a calcium gelling bath under magnetic stirring they differ in size and shape. The average size of hydrated microcapsules is shown in Figure 4 and Table 3 and were of 1.71 ± 0.1 mm for CA and 1.64 ± 0.5 mm for CA-MTN. The size of the microcapsules indicates that they are micrometric. The shape and morphology of the microcapsules were determined from the aspect ratio and circularity factor. The circularity values equal or closely equal to zero indicate a perfect sphere. The values are of 0.92 and 0.94 for CA and CA-MTN microcapsules, respectively, and show the morphological characteristics of a sphere.

3.2.3. Particle Size and Morphology **Table 3.** Size and morphology parameters of CA and hybrid CA-MTN microcapsules.


1.71 ± 0.1 mm for CA and 1.64 ± 0.5 mm for CA-MTN. The size of the microcapsules indicates that they are micrometric. The shape and morphology of the microcapsules were determined from the aspect ratio and circularity factor. The circularity values equal or sphere.

**Figure 4.** Photograph of CA (**a**) and hybrid CA-MTN (**b**) microcapsules. Magnification ×10*.*  **Figure 4.** Photograph of CA (**a**) and hybrid CA-MTN (**b**) microcapsules. Magnification ×10.

closely equal to zero indicate a perfect sphere. The values are of 0.92 and 0.94 for CA and CA-MTN microcapsules, respectively, and show the morphological characteristics of a

**Table 3.** Size and morphology parameters of CA and hybrid CA-MTN microcapsules. **Microcapsule CA Microcapsules CA-MTN Hybrid Microcapsules**  Diameter (mean±SD) (mm) 1.73 ± 0.01 1.64 ± 0.05 Circularity (mean±SD) 0.92 ± 0.01 0.94 ± 0.01 Aspect Ratio (mean±SD) 1.05 ± 0.01 1.17 ± 0.06 The SEM micrographs (Figure 5) show that the CA microcapsule (Figure 5a) has an irregular form and a wrinkled surface. High magnificence SEM micrographs; 1220 X and The SEM micrographs (Figure 5) show that the CA microcapsule (Figure 5a) has an irregular form and a wrinkled surface. High magnificence SEM micrographs; 1220× and 1370× (Figure 5c,e), we can observe that an alignment of lines attributed to the polymer chains and its external surface is covered with a network of fissures and cracks. However, the hybrid CA-MTN (Figure 5b) microcapsules show an improved spherical form, a more compact surface and less irregularity. On the micrographs Figure 5d,f, the polymeric chains in the hybrid CA-MTN microcapsules are associated with the montmorillonite aggregates to form a crumpled network. The hybridation improved the aesthetic aspect of the microcapsules and the polymer matrix strength by filling its interstitial voids and therefore increasing the "time hosting" of the encapsulated agent in the hybrid CA-MTN microcapsules. *Polymers* **2023**, *15*, x FOR PEER REVIEW 11 of 16

**Figure 5.** SEM micrographs of CA ((**a**): 229 X, (**c**): 1.22 K X, (**e**): 1.37 K X) and CA-MTN ((**b**): 150 X, (**d**): 1.59 K X, (**f**): 2.24 K X) microcapsules. **Figure 5.** SEM micrographs of CA ((**a**): 229 X, (**c**): 1.22 K X, (**e**): 1.37 K X) and CA-MTN ((**b**): 150 X, (**d**): 1.59 K X, (**f**): 2.24 K X) microcapsules.

The results in Table 4 showed that the loading capacity and encapsulation efficiency are influenced by the essential oil content. The loading capacity increased when we in-

with an increase in the essential oil amount. The loading capacity depends on the weight of the REO in the microcapsules and the weight of the microcapsules. Our assumption is that the clay filling densifies the microcapsule's matrix network, creating more space for the oil to fit and consequently improving the loading capacity by 2% for the 3% REO concentration. However, the encapsulation efficiency decreases with the increase in essential oil concentration for CA microcapsules. This can be explained by the fact that the polymer network is insufficient to host a higher amount of REO, suggesting a saturation of the material capacity. While for the hybrid CA-MTN microcapsules, thanks to the combination alginate-montmorillonite, they showed a better entrapment of 2% REO concentration equal to 94% compared to CA microcapsules, which is equal to 81%. These findings are supported by El Hosseini et al. [23] who worked on the encapsulation of sunflower in sodium alginate microparticles. The results indicated the highest encapsulation efficiency for the lowest sunflower concentration and the highest loading capacity for the highest sunflower concentration. Piornos et al. [39] reported that the encapsulation efficiency is related to the system of encapsulation strength, which can explain the improvement from

3.2.4. Loading Capacity and Encapsulation Efficiency

adding the montmorillonite clay in this work.

## 3.2.4. Loading Capacity and Encapsulation Efficiency

The results in Table 4 showed that the loading capacity and encapsulation efficiency are influenced by the essential oil content. The loading capacity increased when we increased the rosemary essential oil concentration (3%), 71% and 73% for CA and hybrid CA-MTN microcapsules, respectively, whereas the encapsulation efficiency decreases with an increase in the essential oil amount. The loading capacity depends on the weight of the REO in the microcapsules and the weight of the microcapsules. Our assumption is that the clay filling densifies the microcapsule's matrix network, creating more space for the oil to fit and consequently improving the loading capacity by 2% for the 3% REO concentration. However, the encapsulation efficiency decreases with the increase in essential oil concentration for CA microcapsules. This can be explained by the fact that the polymer network is insufficient to host a higher amount of REO, suggesting a saturation of the material capacity. While for the hybrid CA-MTN microcapsules, thanks to the combination alginate-montmorillonite, they showed a better entrapment of 2% REO concentration equal to 94% compared to CA microcapsules, which is equal to 81%. These findings are supported by El Hosseini et al. [23] who worked on the encapsulation of sunflower in sodium alginate microparticles. The results indicated the highest encapsulation efficiency for the lowest sunflower concentration and the highest loading capacity for the highest sunflower concentration. Piornos et al. [39] reported that the encapsulation efficiency is related to the system of encapsulation strength, which can explain the improvement from adding the montmorillonite clay in this work.

**Table 4.** Loading capacity (LC) and encapsulation efficiency (EE) for CA and hybrid CA-MTN microcapsules.


#### *3.3. Determination of the Antioxydant Activity*

The microcapsules' ability as antioxidants was tested. In the present study, the elaborated unloaded microcapsules, CA and CA-MNT, as well as the loaded microcapsule CA-MNT–3% REO were tested for their capability to reduce the free radical 1.1-diphenyl-2-picrylhydrazyl (DPPH); the REO was also evaluated for its antioxidant activity. The results of the scavenging activity performed are highlighted in Figure 6. As can be seen from these results, the unloaded microcapsules showed the lowest scavenging activity, presenting a value of 8.6% and 4.8% for CA and hybrid CA-MNT, respectively. These results can be explained by the fact that calcium alginate has the ability to react with the free radical compared to bentonite clay, which increases the free radical inhibition. Furthermore, there was a slight increase in the free radical scavenging activity of CA-REO compared to the control CA (from 8.6 to 10%); on the other hand, the CA-MNT-REO exhibited the highest scavenging activity, reaching 12.8%, which was ten times higher than that of CA-MNT (4.8%).

Based on the literature data, the same results were found by Singh et al. [40] reporting that the chitosan-gelatin-REO microcapsules exhibited a higher antioxidant property than the unloaded microcapsules. Similarly, Teixeira-Costa et al. [41] revealed that the essential oil encapsulated in the system chitosan/alginate polyelectrolyte complexes inhibits greatly compared to the control polyelectrolyte complexes. These results are mainly due to the presence of oxygenate monoterpenes and hydrocarbon monoterpenes which have significant redox properties and play crucial roles in scavenging free radicals and in the breakdown of peroxide. Our results demonstrated that the combination alginate/montmorillonite microcapsules can contribute to the protection of the essential oil against oxidation, leading to the formation of an oxygen barrier and increasing its stability.

*Polymers* **2023**, *15*, x FOR PEER REVIEW 13 of 16

**Figure 6.** DPPH-scavenging activities of rosemary essential oil (**A**) and of unloaded and REO loaded CA and hybrid CA-MTN microcapsules (**B**). **Figure 6.** DPPH-scavenging activities of rosemary essential oil (**A**) and of unloaded and REO loaded CA and hybrid CA-MTN microcapsules (**B**).

**Table 4.** Loading capacity (LC) and encapsulation efficiency (EE) for CA and hybrid CA-MTN mi-

**Microcapsule CA Microcapsules CA-MTN hybrid Microcapsules** 

LC (%) 26.62 ± 0.19 49.93 ± 0.03 71.36 ± 0.1 28.58 ± 0.21 54.83 ± 0.12 73.6 ± 0.14 EE (%) 92.73 ± 0.14 81.946 ± 0.04 81.9 ± 0.04 83.58 ± 0.23 94.59 ± 0.12 83.59 ± 0.13

The microcapsules' ability as antioxidants was tested. In the present study, the elaborated unloaded microcapsules, CA and CA-MNT, as well as the loaded microcapsule CA-MNT‒3% REO were tested for their capability to reduce the free radical 1.1-diphenyl-2-picrylhydrazyl (DPPH); the REO was also evaluated for its antioxidant activity. The results of the scavenging activity performed are highlighted in Figure 6. As can be seen from these results, the unloaded microcapsules showed the lowest scavenging activity, presenting a value of 8.6% and 4.8% for CA and hybrid CA-MNT, respectively. These results can be explained by the fact that calcium alginate has the ability to react with the free radical compared to bentonite clay, which increases the free radical inhibition. Furthermore, there was a slight increase in the free radical scavenging activity of CA-REO compared to the control CA (from 8.6 to 10%); on the other hand, the CA-MNT-REO exhibited the highest scavenging activity, reaching 12.8%, which was ten times higher than that of CA-MNT

**1% 2% 3% 1% 2% 3%** 

#### Based on the literature data, the same results were found by Singh et al. [40] reporting *3.4. REO Release Kinetics*

crocapsules*.* 

(4.8%).

**REO Concentratio n** 

*3.3. Determination of the Antioxydant Activity* 

that the chitosan-gelatin-REO microcapsules exhibited a higher antioxidant property than the unloaded microcapsules. Similarly, Teixeira-Costa et al. [41] revealed that the essential oil encapsulated in the system chitosan/alginate polyelectrolyte complexes inhibits greatly compared to the control polyelectrolyte complexes. These results are mainly due to the presence of oxygenate monoterpenes and hydrocarbon monoterpenes which have significant redox properties and play crucial roles in scavenging free radicals and in the breakdown of peroxide. Our results demonstrated that the combination alginate/montmorillonite microcapsules can contribute to the protection of the essential oil against oxidation, leading to the formation of an oxygen barrier and increasing its stability. *3.4. REO Release Kinetics*  The rosemary essential oil release from the microcapsules depends on the medium of release; the latest penetrates into the microcapsules to dissolve the entrapped essential The rosemary essential oil release from the microcapsules depends on the medium of release; the latest penetrates into the microcapsules to dissolve the entrapped essential oil which diffuses through the microcapsules' membrane. The release studies were performedto show the difference in the release pathway as a function of the microparticle formulationand the REO concentration. The experiment was carried out in 1% Tween 80-water to avoid water saturation due to the low solubility of rosemary essential oil in water. As shown in Figure 7*,* two phases were discerned: an initial phase of fast release correlated to the REO adsorbed on or near the surface of the microcapsules. Contrariwise, the second phase corresponds to the slow release of the REO and stays nearly constant over time. At that stage, the release can be due to the diffusion of the REO dispersed into the microcapsules [42]. The profiles suggest that the release rate of microcapsules with higher REO concentrations is slower than that of microcapsules with lower REO concentrations. The amount of REO released from CA microcapsules (Figure 7A) was approximately equal to the amount released from hybrid CA-MTN microcapsules (Figure 7B), but the time of release differed from 28 to 85 h for CA and hybrid CA-MTN, respectively. oil which diffuses through the microcapsules' membrane**.** The release studies were performed to show the difference in the release pathway as a function of the microparticle formulation and the REO concentration. The experiment was carried out in 1% Tween 80 water to avoid water saturation due to the low solubility of rosemary essential oil in water. As shown in Figure 7*,* two phases were discerned: an initial phase of fast release correlated to the REO adsorbed on or near the surface of the microcapsules. Contrariwise, the second phase corresponds to the slow release of the REO and stays nearly constant over time. At that stage, the release can be due to the diffusion of the REO dispersed into the microcapsules [42]. The profiles suggest that the release rate of microcapsules with higher REO concentrations is slower than that of microcapsules with lower REO concentrations. The amount of REO released from CA microcapsules (Figure 7A) was approximately equal to the amount released from hybrid CA-MTN microcapsules (Figure 7B), but the time of release differed from 28 to 85 h for CA and hybrid CA-MTN, respectively.

**Figure 7.** Kinetical release profiles of rosemary essential oil from CA-REO (**A**) and hybrid CA-MTN-REO (**B**) microcapsules in w/o medium*.* **Figure 7.** Kinetical release profiles of rosemary essential oil from CA-REO (**A**) and hybrid CA-MTN-REO (**B**) microcapsules in w/o medium.

To explain the mechanism of the REO release from the microcapsules, the release data CR (%) were fitted to the Korsmeyer and Peppas semi-empirical model. The obtained kinetic parameters are listed in Table 5. Release-kinetics correlation coefficients (R2) of both microparticles were close to 0.9. The diffusional exponent (n) ranged between 0.28 and 0.29 for hybrid CA-MTN and CA microcapsules, respectively. These data indicate that the rosemary essential oil release pathway follows a Fickian diffusion, in which the release is governed by a diffusional behaviour. Therefore, the diffusional process of essential oil in microcapsules can be influenced by the presence of clay particles which pre-To explain the mechanism of the REO release from the microcapsules, the release data CR (%) were fitted to the Korsmeyer and Peppas semi-empirical model. The obtained kinetic parameters are listed in Table 5. Release-kinetics correlation coefficients (R<sup>2</sup> ) of both microparticles were close to 0.9. The diffusional exponent (n) ranged between 0.28 and 0.29 for hybrid CA-MTN and CA microcapsules, respectively. These data indicate that the rosemary essential oil release pathway follows a Fickian diffusion, in which the release is governed by a diffusional behaviour. Therefore, the diffusional process of essential oil in microcapsules can be influenced by the presence of clay particles which prevents the

vents the movement of the polymer chains and limits the rate of the internal diffusion of

the release exponent related to the drug release mechanisms, k is the rate constant and R2 is the

**Kinetic Parameters CA-REO Microcapsules CA-MTN-REO Hybrid** 

n 0.29 0.28 k 0.18 0.12 R2 0.93 0.90

Based on the previous results, it can be said that the process of encapsulation is a well-established and efficient method to entrap and maintain molecules. It proposes a plethora of benefits and can be achieved by a myriad of techniques whose main objective is to sustain the release of encapsulated components such as essential oils. REO-loaded

**Microcapsules** 

the essential oil in the hybrid microcapsules compared to CA microcapsules.

coefficient of correlation.

**4. Conclusions** 

movement of the polymer chains and limits the rate of the internal diffusion of the essential oil in the hybrid microcapsules compared to CA microcapsules.

**Table 5.** Release kinetic parameters (n and k) for the CA and hybrid CA-MTN microcapsules; n is the release exponent related to the drug release mechanisms, k is the rate constant and R<sup>2</sup> is the coefficient of correlation.


#### **4. Conclusions**

Based on the previous results, it can be said that the process of encapsulation is a wellestablished and efficient method to entrap and maintain molecules. It proposes a plethora of benefits and can be achieved by a myriad of techniques whose main objective is to sustain the release of encapsulated components such as essential oils. REO-loaded microcapsules were prepared by o/w emulsions drop wised into a calcium gelling bath. The TGA thermograms show that hybrid microcapsules CA-MTN have higher thermal stability than CA microcapsules due to the presence of the clay. The rate loss of rosemary essential oil is greater in CA than in hybrid CA-MTN microcapsules which proves the high entrapment capacity of the hybrid system. The microcapsule size and morphology analysis have shown that the obtained particles are micrometric and spherical. Concerning the loading capacity and the encapsulation efficiency, the studies indicate that with a higher value of REO concentration, the loading capacity increased while the encapsulation efficiency decreased for both systems. The REO release is three times slower in hybrid CA-MTN microcapsules than in CA microcapsules which suggests that the hybrid CA-MTN have a higher retention capacity due to the wide specific area of the clay. The kinetic release is of a diffusional type and is correlated with the Korsmeyer–Peppas kinetic model. This work's main result is to establish a clear influence of the clay filling with sodium montmorillonite from eastern Morocco to obtain the hybrid formulation CA-MTN which shows better thermal stability, higher loading capacity/encapsulation efficiency and slower release of rosemary essential oil using the simple and inexpensive method of ionotropic gelation without including any harmful substance in the whole process. These developed microcapsule systems loaded with rosemary essential oil can be used in active food packaging to protect the food from external factors, and in cosmetics and beauty care products as well as in non-eco-toxic pesticides.

**Author Contributions:** Conceptualization, D.B. and A.T.; methodology, A.T.; software, C.H.; validation, A.T. and M.-L.F.; formal analysis, K.E.; investigation, D.B.; resources, A.T.; data curation, D.B.; writing—original draft preparation, D.B.; writing—review and editing, A.T., C.H. and M.A.; visualization, M.A.; supervision, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors sincerely thank CNRST, MESRSFC-Morocco and Mohamed Premier University for their financial support of this research through the PPR15-17 and the ANPMA/CNRST/UMP-247/20 projects. Part of this study was funded by Conseil Départemental d'Eure-et-Loir et Région Centre Val de Loire.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We are thankful to the analytical platform in the chemistry department in the Faculty of Sciences of Oujda for the analysis services. The authors are also grateful to Eswaramoorthy Muthusamy, head of the Nanocomposites-Catalysis laboratory in JNCASR in India, for the opportunity to work with his team.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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