23 CHARACTERIZATION AND ACTIVITY OF ESSENTIAL OILS effect the combination of tea tree/sandalwood (FICI: 1.03) and lemon/eucalyptus (FICI: 1.00) exhibited an additive effect, and lemon/lavender (FICI: 2.00) showed no interactive effect against S epidermidis. The binary combination of lemon/sandalwood showed no interactive effect with FICI values of 1.95 and 2.13 against either bacterium (48). In this study, different interactions in the dual combination of EOs were observed. ANTIOXIDANT ACTIVITY OF ESSENTIAL OILS Free radical scavenging activity. The antioxidant amount of each EO was calculated by the IC 50 (effective concentration) value and is displayed in Table VI. The C verum EO showed the greatest (IC 50 :35.08 µg/ml) antioxidant activity. It is likely due to the high antioxidant capacity of eugenol (49). Our results were similar to previous studies with IC 50 values: 62.01 mg/ml (Origanum vulgare) (50) 57 µg/ml (28) and 2.23 mg GAE/l (51) (E globulus) 48.35 µg/ml (M alternifolia) (52) 31.30 µg/ml (L angustifolia) (30) and 35.35 µg/ml (C limon) (53). On the other hand, some studies have reported different IC 50 values. For O vulgare 23.96 and 0.332 (mg/ml) (37,38) for E globulus 10 (mg/ml) (33) for M alternifolia 12.5 (µg/ml) (28) for L angustifolia 2.51 (v/v) (31) for C limon 15.056 (µg/ml) (27) for C verum 13.3, 0.03, and 111.5 (µg/ml) (54–56) and for S album 0.87 (µg/ml) and 74.85 (mg/ml) (57,58). Several biochemical mechanisms and various factors like plant maturity, plant species, and organs (leaf, stem, e.g.), geographical origin, extraction method, and harvest season may affect the antioxidant properties of EOs (59). Table IV FIC and FICI Values in Dual Combination of EOs Against S epidermidis EOs in combination (EO1/EO2) S epidermidis ATCC 12228 FIC1/FIC2 FICI Remarks Or/Ci 1.00/0.13 1.13 NI Or/La 1.00/0.02 1.02 NI Or/Te 1.00/1.00 2.00 NI Or/Sa 1.00/0.00 1.00 AD Ci/La 0.25/0.13 0.38 S Ci/Te 0.50/0.13 0.63 AD Ci/Sa 0.50/0.00 0.50 S La/Te 1.00/0.06 1.06 NI La/Sa 0.13/1.00 1.13 NI Te/Sa 0.25/1.00 1.25 NI Eu/Le 1.00/0.02 1.02 NI Or/Eu 4.00/0.031 4.03 A Eu/Ci 0.02/0.25 0.27 S Eu/La 0.25/0.25 0.50 S Eu/Te 0.13/0.25 0.38 S Sa/Eu 1.00/1.00 2.00 NI Or/Le 1.00/0.50 1.50 NI Le/Ci 0.00/0.25 0.25 S Le/La 1.00/0.13 1.13 NI Le/Te 0.50/1.00 1.50 NI Sa/Le 0.25/0.5 0.75 AD Or: oregano, La: lavender, Le: lemon, Sa: sandalwood, Ci: cinnamon, Te: tea tree, Eu: eucalyptus FICI ≤ 0.5: synergy (S) 0.5 FICI ≤ 1: additive (AD) 1 FICI ≤ 4: no interactive effect (NI) FICI 4: antagonistic (A).

24 JOURNAL OF COSMETIC SCIENCE Total phenolic content. Phenolic compounds (like tannins, phenols, phenolic acids, flavonoids, or phenyl praponoids) of EOs are responsible for their antioxidant properties (60). In Table VI, the TPC of antibacterial effective EOs is presented. The highest TPC was detected at oregano EO with a 3.087 mg GAE/l value. The other EOs had lower contents of TPC ranging from 0.113 to 1.778 mg GAE/ml (in descending order: cinnamon laurel lemon tea tree lavender sandalwood eucalyptus). In previous studies, similar and different results were reported. For oregano 4.69 mg GAE/ml (37) for tea tree 0.034 mg GAE/g (61) and 14.79 mg GAE/g (62) for sandalwood 4.97 mg GAE/g (63) for lavender 137.52 mg GAE/l (30) for cinnamon 149.15 mg GAE/g (64), 8.3 mg GAE/g (65), and 210 GAE mg/g (56) and for lemon 0.155 mg GAE/g (66) and 14.73 mg GAE/g (67). The differences may be related to the plant chemotype or organ, climatic conditions, or measurement techniques (68,69). Also, some inorganic substances like iron sulfate, hydrazine, and hydroxyl ammonium can react with the Folin–Ciocalteu reagent and increase the phenolic concentration (70,71). CYTOTOXIC EFFECTS OF 8 ESSENTIAL OILS The cytotoxicity of various concentrations (2–0.0625% [v/v]) of EOs on 3T3 cells was evaluated by MTT assay after 24, 48, 72, and 96-hour incubation periods. The cytotoxicity of EOs is shown in Figure 1. The IC 50 values are displayed in Table VII. It is difficult to Table V FIC and FICI Values in Dual Combinations of EOs Against C acnes EOs in combination (EO1/EO2) C acnes ATCC 6919 FIC1/FIC2 FICI Remarks Or/Te 16.00/1.00 17.00 A Or/Ci 32.00/0.50 32.50 A Or/La 1.00/0.063 1.063 NI Or/Lau 1.00/2.00 3.00 NI Eu/Or 0.50/32.00 32.50 A Sa/Or 1.00/1.00 2.00 NI Ci/Te 2.00/1.00 3.00 NI Ci/La 0.063/0.015 0.078 S Ci/Eu 1.00/0.25 1.25 NI Ci/Sa 0.031/0.25 0.281 S Ci/Lau 0.50/0.063 0.563 AD Sa/Te 0.0313/8.00 8.0313 A Sa/Lau 0.25/4.00 4.25 A Te/Eu 1.00/0.25 1.25 NI Eu/La 0.125/2.00 2.125 NI La/Sa 1.00/0.016 1.016 NI Lau/La 0.031/0.016 0.047 S Lau/Eu 2.00/0.125 2.125 NI Te/La 2.00/1.00 3.00 NI Te/Lau 1.00/4.00 5.00 A Eu/Lau 0.125/2.00 2.125 NI Eu/Sa 0.016/2.00 2.016 NI Or: oregano La: lavender Le: lemon Sa: sandalwood Ci: cinnamon Te: tea tree Eu: eucalyptus Lau: laurel FICI ≤ 0.5: synergy (S) 0.5 FICI ≤ 1: additive (AD) 1 FIC I ≤ 4: no interactive effect (NI) FICI 4: antagonistic (A).