Materials used to prepare the microcapsules were as follows. Camellia oil (CO: Oshima Tsubaki Co. Ltd., Japan) was microencapsulated as a core material. Oleic acid (OA: Wako Pure Chemicals Co., Ltd., Tokyo, Japan) was dissolved into camellia oil in order to stably form the microcapsule shell regardless of fluctuation of components in natural camellia oil.

Chitosan (Wako Pure Chemicals Co., Ltd., Tokyo, Japan) was used to form the microcapsule shell using heterocoagulation with oleic acid. Citric acid (Wako Pure Chemicals Co., Ltd., Tokyo, Japan) was used to dissolve chitosan in the water phase. Soybean lecithin (SBL: Wako Pure Chemicals Co., Ltd., Tokyo, Japan) was used as an oil soluble surfactant.

Figure 1 shows the flow sheet for preparing the microcapsules.

Camellia oil (CO) and Oleic acid (OA) of given volume were mixed together to form the oil dispersed phase (O), in which soybean lecithin (SBL) was dissolved beforehand.

The oil dispersed phase was added into the continuous water phase (W) and stirred with the rotor stator homogenizer to form the (O/W) emulsion.

Then, the chitosan aqueous solution of a given concentration was added to the (O/W) emulsion. The (O/W) emulsion was stirred with the impeller for a given time (ca. 4 h) to form the microcapsules. The fundamental operation described above was performed changing mainly the revolution velocity to form the (O/W) emulsion. The microcapsules prepared thus were collected with filtration paper and characterized. The experimental conditions are shown in Table 1 .

In order to investigate whether the oil droplets were microencapsulated with chitosan due to heterocoagulation or not, first the (O/W) emulsion before and after microencapsulation was observed.

Figure 2 shows the diameter distributions of oil droplets in the (O/W) emulsion which was formed by stirring with the rotor stator homogenizer under the experimental conditions such as Nr = 5000 rpm, t = 5 min and T = 40˚C.

The oil droplets with the mean diameter of 3.9 µm and the value of CV = 0.11 were formed, where CV is the coefficient of variation and defined as the ratio (σ/dp) of the standard deviation (σ) of diameter distribution to the mean diameter (dp).

The smaller the CV values are, the more uniform the diameters become.

Also, Figure 3 shows the optical microscopic photograph of the oil droplets in the (O/W) emulsion before microencapsulation.

The mean diameter and the CV value were 3.9 µm and 0.11, respectively. Furthermore, the (O/W) emulsion formed was found to be extremely stable, because the diameters of oil droplets were kept almost constant for 24h. This stability of the (O/W) emulsion may be due to the repulsive force based on carboxyl group.

Figure 4 shows the optical microscopic photographs of the oil droplets after microencapsulation. The mean diameter and the CV value of the oil droplets after a few days were dp = 3.9 µm and CV = 0.11, respectively and those of the oil droplets after a week were dp = 4.0 µm and CV = 0.12, respectively. From this result, it was found that the (O/W) emulsion was kept almost unchanged. Accordingly, it may be imagined that microencapsulation of oil droplets was satisfactorily performed.

In this study, as it was tried to microencapsulate camellia oil due to heterocoagulation between OA with carboxyl group and chitosan of polycation, the ζ-potentials of oil droplets before and after microencapsulation were measured. Figure 5 shows the ζ-potentials of oil droplets measured before and after microencapsulation. The ζ-potentials of oil droplets before and after microencapsulation are −54.6 mV and 59.6 mV, respectively. From these results, it may be capable of being thought that the oil droplets were miroencapsulated with chitosan due to heterocoagulation between OA and chitosan. Furthermore, as the oil droplets are microencapsulated with chitosan positively charged, adhesion of these oil droplets onto the negatively charged human hair will be promoted.

In order to analyze the chemical composition on the surface of oil droplets, the results of FTIR analysis for chitosan and the oil droplets after microencapsulation are shown in Figure 6. As the adsorption peaks at = 1500 cm⁻¹ and 3400 cm⁻¹ are due to NH₂ of chitosan, the oil droplets are found to be microencapsulated with chitosan. From the results obtained above, it is confirmed that the microcapsules containing camellia oil could be prepared according to the formation mechanism presented in this study, which was shown in Figure 7. Namely, as the oil droplets made from CO and OA were charged negatively, the microcapsule shell was formed with heterocoagulation between OA and chitosan of polycation.

Here, when the microcapsules are going to be applied as a spray type hair dressing material, the diameters of microcapsules are very important to use them efficiently.

So, in order to investigate the controllable region of the diameters of the microcapsules, the mean diameters of the microcapsules were measured by changing the revolution velocity to form the (O/W) emulsion.

On preparing the microcapsules with the heterocoagulation method under the experimental conditions adopted in this study, the diameters of the microcapsules approximately equal to those of the oil droplets in the (O/W) emulsion.

Figure 8 shows the dependence of the mean diameters of the microcapsules on the revolution velocity (Nr) to form the (O/W) emulsion.

From Figure 8, the following experimental equation could be obtained.

dp ~ Nr − 1 . 17 (1)

This dependency agrees well with that (~Nr^−1.20) for the (O/W) emulsion formed in the disruptive dominant field [25] [26] . Accordingly, the diameters of the microcapsules are able to be controlled using Weber Number (ρNr²di³/ɤ) as a criteria, where ρ, di, ɤ are the density of the continuous water, the impeller diameter, the interfacial tension, respectively. In this study, the mean diameters of the microcapsules could be changed in the region from 1.5 µm to 4.5 µm with the revolution velocity.

Also, the CV values for all the microcapsules prepared in this study were in the range from 0.09 to 0.16. As a result, the diameters of the microcapsules were found to be more uniform.

It is well known that, as the human hair is negatively charged, the positively charged particles are easily adhered on the surface of human hair. So, it was investigated whether the microcapsules adhered well on the surface of human hair or not.

Figure 9 shows the optical microscopic photographs of human hair on which the microcapsules adhered after drying at room temperature. From these photographs, it was found that the microcapsules adhered well on the surface of human hair.

Also, Figure 10 shows the optical microscopic photographs of microcapsules on the glass plate after drying due to blowing. The microcapsules appeared due to flattening larger than those before blowing as shown in Figure 4, but were found to be stable even after drying.

From these results, it is expected that the microencapsulated camellia oil can be applied as a hair dressing material.