Stability of O/W Emulsion with Synthetic Perfumes Oxidized by Singlet Oxygen
We prepared O/W emulsion composed of a synthetic perfume, n-dodecane, protoporphyrin IX disodium salt (PpIX-2Na), sodium dodecyl sulfate, and water and investigated oxidative decomposition of the synthetic perfume in the emulsion and change in the stability of the emulsion by singlet oxygen (1O2) generated by photosensitization of PpIX-2Na. We used eugenol, linalool, benzyl acetate, α-ionone, α-hexylcinnamaldehyde, and d-limonene as a synthetic perfume. The stability of the O/W emulation including eugenol and linalool significantly decreased with increasing light irradiation time. The decrease in the emulsion stability may be attributable to oxidative decomposition of eugenol and linalool by 1O2 and enlargement of the oil droplet size.
Photosensitizing dyes, that is, rose bengal, methylene blue, phthalocyanine, and so on, excited by light convert the energy to oxygen molecules, leading to the formation of singlet oxygen (1O2), which is one of the activated oxygen species [1, 2]. When using these dyes in industrial fields, we have been confronted with the degradation of coexisting substances by 1O2.
Synthetic perfumes have been employed in many industrial products such as cosmetics, foods, detergents, pesticides, and coating materials. Synthetic perfumes are then used in various forms with other materials, depending on properties of mediums, solubility of perfumes, stability of perfumes, and so forth [3–5]. In industrial fields of cosmetic and food sciences, moreover, oily synthetic perfumes are mixed with surfactants to produce stable and homogenous O/W emulsion. The emulsification makes the heat resistance of synthetic perfumes high and constantly releases fragrance from the product [6, 7]. So that, many studies on emulsified perfumes have been undertaken [8–12].
We previously investigated the influence of synthetic perfumes on the stability of O/W emulsion composed of sodium dodecyl sulfate, n-dodecane, and water . As a result, shrinking oil droplets in the O/W emulsion, the synthetic perfume prevented the oil droplet from coagulation and creaming, which makes the O/W emulsion still more stable.
In the present study, we prepared O/W emulsion composed of a synthetic perfume, n-dodecane, sodium dodecyl sulfate, water, and protoporphyrin IX disodium salt as a photosensitizing dye and investigated oxidative decomposition of the synthetic perfume in the O/W emulsion and change in the stability of the emulsion by 1O2 generated by the photosensitization of protoporphyrin IX disodium.
We purchased synthetic perfumes, eugenol, linalool, benzyl acetate, α-ionone, α-hexylcinnamaldehyde, and d-limonene from Wako Pure Chemical Industries, Ltd. Their chemical structure, abbreviation, molecular weight, density, inorganic/organic value (I/O), and purity are listed in Table 1. We used the perfume having diverse chemical structures: EL for phenols, LL for alcohols, BA for esters, IN for ketones, HCA for aldehydes, and LN for hydrocarbons. Their molecular weight and density are almost the same. I/O is a useful index for evaluating hydrophilicity/lipophilicity balance of substances; hydrophilicity of a substance increases with an increase in its I/O [14, 15]. Table 1 then indicates that the hydrophilicity of the perfume increases as the following order: LN < HCA < IN < BA < LL < EL.
Sodium dodecyl sulfate (SDS, >95%) and n-dodecane (>99%), which were purchased from Wako Pure Chemical Industries, Ltd., were employed as an emulsifier and an oil phase, respectively. We also got protoporphyrin IX disodium salt (PpIX-2Na, >95%) from Aldrich Co. We used distilled water of which the electric conductivity was less than 1.0 μS/m.
All the materials were used without further purification.
2.2. Preparation of O/W Emulsion including Synthetic Perfumes and PpIX-2Na
We prepared a perfume-n-dodecane mixed solution of which the perfume concentration was 1.66 mol/L and an aqueous SDS solution at a concentration of 0.27 mol/L including mol/L of PpIX-2Na. The perfume and the SDS solutions were mixed at 1 : 40 in volume ratio, and the mixture was agitated with a vortex mixer to give homogenous O/W emulsion.
2.3. Stability of O/W Emulsion
Appearance of emulsion is related to its turbidity. Change in the turbidity of emulsion with progress time is attributable to coagulation, creaming, and resultant coalescence of its dispersed phase . A time-dependent change in turbidity of emulsion is then useful to examine emulsion stability.
In this study, the turbidity of the O/W emulsion was determined by its absorbance at 700 nm. Since no material used absorbed the light at 700 nm, the absorbance came from the turbidity of the emulsion. With being mixed vigorously, the O/W emulsion was irradiated with light emitted from a metal-halide lamp produced by USHIO Co. for a given time. Figure 1 shows a spectral output of the metal-halide lamp. After the agitation of the light-irradiated O/W emulsion for 2 minutes by a vortex mixer, a time-dependent change in 700 nm absorbance of the O/W emulsion was measured with a UV-VIS spectrophotometer (V-550, JASCO Co.) . The measurement was carried out at 30°C.
2.4. Diameter of Oil Droplets in O/W Emulsion
The O/W emulsion before and after the light irradiation was observed by a light microscope (ECLIPS ETE300, Nikon Co.) at 30°C, and the diameter of oil droplets in the O/W emulsion was directly measured from the microscopy observation.
2.5. Concentration of Synthetic Perfumes before and after Light Irradiation
After the light irradiation, the O/W emulsion was dissolved into ethanol at a volume ratio of 1 : 6. In addition, we prepared a perfume solution at a concentration of 1.66 mol/L with n-dodecane and mol/L PpIX-2Na solution with water-ethanol mixed solvent. We mixed the perfume and the PpIX-2Na solutions at a volume ratio of 1 : 40, irradiating the mixture with the light from the metal-halide lamp. Thereafter, the perfume concentrations in the O/W emulsion and in the perfume and PpIX-2Na mixed solution were detected by high-performance liquid chromatography (HPLC) coupled to ultraviolet absorbance detection at 200 nm (L-7420, HITACHI Co., Ltd.). The column used was a reverse phase C-18 column ( mm2, 5 μm particle size). The eluate used was a water-ethanol mixed solution at a volume ratio of 1 : 6, and its flow rate was 1.0 mL/min. The measurement was carried out at 30°C.
3. Results and Discussion
Figure 2 shows time-dependent changes in absorbance at 700 nm of the O/W emulsion irradiated with the light for 0 (∘), 30 (●), and 60 (■) minutes. Here, the vertical axis is a ratio of the absorbance at a progress time to that at the beginning (at the progress time of 0). As the progress time increases, the absorbance reduces exponentially in any system. In the case of EL and LL, in addition, the reduction in absorbance is significantly facilitated by an increase in light irradiation time.
The reduction in absorbance is caused by a decrease in turbidity of the emulsion because of coagulation and creaming of oil droplets. The reduction rate of absorbance is then a valuable index for evaluating emulsion stability; that is, emulsion having a smaller reduction rate is more stable. Changes in reduction rate at the beginning estimated from Figure 2 are represented in Figure 3. Here, the vertical axis depicts a ratio of the reduction rates with the light irradiation to that without the light irradiation. In the case of O/W emulsion with BA (■), IN(□), HCA (◆), and LN (♢), the reduction rate is almost constant despite the light irradiation. Besides, there is a little change in the reduction rate of the O/W emulsion without perfumes (×). In the case of O/W emulsion with EL (●) and LL (∘), on the other hand, the reduction rate increases significantly with increasing light irradiation. These findings confirm that the light irradiation depresses the stability of the O/W emulsion including EL and LL.
The light irradiation excites PpIX-2Na and generates 1O2 through its photosensitization. 1O2 is one of activated oxygen species and undergoes oxidation reactions. The most well-known 1O2-mediated reactions are (1) the ene reaction [18–20], (2) the [2 + 2] cycloaddition , and (3) the [4 + 2] cycloaddition , as shown in Scheme 1. As can be seen in Table 1, moreover, the synthetic perfume has a C=C bond. We then suggest that the synthetic perfume is easily oxidized by 1O2.
Figure 4 exhibits the concentration of the synthetic perfume in the O/W emulsion after the light irradiation. The vertical axis is a relative value of synthetic perfume concentrations after and before the light irradiation; when the concentration of the synthetic perfume after the light irradiation is less than that before the light irradiation, a relative value is below unity. Figure 4 indicates that the concentration of the synthetic perfume in the O/W emulsion decreases with an increase in light irradiation time, especially EL and LL. Moreover, Figure 5 expresses HPLC chromatograms of EL and LL in the O/W emulsion before and after the 60-minute light irradiation. In both perfume systems, we can find that the original peak of the synthetic perfume decreases and that a novel peak appears at a shorter retention time after the 60-minute light irradiation. Hence, Figures 4 and 5 reveal that EL and LL are readily decomposed by 1O2 into more hydrophilic substances.
Next, we discuss why the oxidation decomposition of EL and LL causes the destabilization of the O/W emulsion with them. Since chemical structures of synthetic perfumes are complicated, various kinds of factors of the synthetic perfume affect the stability of emulsion. Zhang et al. reported that the stability of the emulsion with the perfume was dependent on the density of the oil phase and on the phase changes taking place with changing the surfactant-perfume ratio . Moreover, we previously investigated the influence of synthetic perfumes on the stability of O/W emulsion . As a result, the perfume used in this study, except for LN, shrank oil droplets in the O/W emulsion, resulting in the enhancement of the emulsion stability. Here, we focus on the change in the size of the oil droplet in the O/W emulsion with the oxidation decomposition of the perfume.
In general, coagulation and resultant coalescence of oil droplets in O/W emulsion is induced by van der Waals attraction between oil droplets. Moreover, van der Waals attraction between oil droplets, , is closely related to the oil droplet size, defined as the following equation [22, 23]: where is Hamaker constant, and and are the radius of oil droplets and the interparticle distance between oil droplets. This equation indicates that van der Waals attraction is proportional to the radius of oil droplets. Decreasing oil droplet size then enhances emulsion stability.
Figure 6 indicates the oil droplet size before and after the 60-minute light irradiation to the O/W emulsion. As can be seen from Figure 6, the size of the oil droplet with EL and LL after the light irradiation is almost twice that before the light irradiation. This means that the light irradiation strengthens van der Waals attraction between oil droplets, leading to the enhancement of the coagulation of oil droplets in the O/W emulsion with EL and LL, as shown in Figure 3. Since the enlargement in the oil droplet size for BA is smaller than that for EL and LL, moreover, there is little influence of the oil droplet size on the stability of O/W emulsion with BA.
We can, therefore, consider from these results that the enlargement in the size of the oil droplet plays a role in the lower stability of the O/W emulsion with EL and LL decomposed oxidatively by 1O2.
Figure 7 shows the concentration of the synthetic perfume dissolved with PpIX-2Na into the water-ethanol mixed solvent after the light irradiation, as well as Figure 4. The change in perfume concentration with the light irradiation time can be observed in the case of EL, LL, and LN, but not in the case of BA, IN, and HCA; EL, LL, and LN concentrations decrease drastically with increasing light irradiation time. As compared with Figure 4, which depicts the change in perfume concentration with the light irradiation time in the O/W emulsion, it can be found that EL and LL are oxidized by 1O2 in both the O/W emulsion and the solution systems, and that LN is oxidized in the solution system, but not in the O/W emulsion system.
PpIX-2Na is enough hydrophilic to be dissolved into water, distributed into the external water phase in the O/W emulsion. In fact, we observed that the water phase was colored into red-brown coming from PpIX-2Na in the O/W emulsion. Hence, when the O/W emulsion is irradiated with the light to the O/W emulsion, 1O2 is generated in the external water phase. Moreover, EL and LL are so more hydrophilic that they can be orientated with SDS on the interface between the oil droplet and the external water phase in the O/W emulsion, like alcohols [24, 25]. On the other hand, BA, IN, HCA, and LN, which are more hydrophobic, are dissolved into the oil droplet in the O/W emulsion.
Consequently, we can consider from these findings that the oxidation decomposition model of the synthetic perfume by 1O2 in the O/W emulsion is as follows (Figure 8). When irradiated with the light, PpIX-2Na dissolved in the external water phase releases 1O2 in the water phase. 1O2 readily attacks EL and LL existing on the interface between the oil droplet and the external water phase. Nevertheless, the reaction of 1O2 with BA, IN, HCA, and LN is reduced, because these synthetic perfumes are dissolved into the oil droplet in the O/W emulsion. The similar oxidation decomposition model was proposed for the oxidation decomposition of ascorbic acid into O/W microemulsion formed by water, pentanol, and SDS .
We investigate the change in the stability of O/W emulsion including synthetic perfumes and PpIX-2Na with light irradiation time. The stability of O/W emulsion with EL and LL is depressed after the light irradiation. This may be caused by the oxidation decomposition of EL and LL by 1O2 generated by the photosensitization of PpIX-2Na and the enlargement of the size of oil droplets in the O/W emulsion.
The authors thank the Faculty of Biomedical Engineering at the Toin University of Yokohama for their financial support.
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