Surfactant-Free Microemulsions in Fragrance Tinctures PERICA BOŠKOVIĆ, VESNA SOKOL, MATEA DUJMOVIĆ, MARTINA GUDELJ, and ANTE PRKIĆ, Faculty of Science, Ruđera Boškovića 33, Split 21000, Croatia (P.B., M.G.), Facult y of Chemistry and Technology, Ruđera Boškovića 33, Split 21000, Croatia (V.S., M.D., A., P.) Accepted for publication February 11, 2021. Synopsis Tinctures are alcoholi c or hydroalcoholic solutions prepared from vegetable or chemical substances. The concentration of the solute varies up to a maximum of 50%, e.g., vanilla tincture. Tinctures are very useful in the perfume industry because they contain ethanol, which can dissolve molecules such as fragrant molecules, and consequently form hydroalcoholic formulations together. Recently, we have shown that nanostructures exist in the monophasic water–ethanol–citronellol system, and it is therefore a question whether such nanostructures exist in a system where a perfume molecule of citral is present as the oil component instead of citronellol. In this study, the single-phase region was mapped conductometrically and then measured by Dynamic Light Scattering (DLS) and UV/Vis spectroscopy to determine the presence of nanostructures similar to classical microemulsions. In early 1970s, Barden ’ s group began working on microemulsions without the presence of a surfactant. They were the fi rst to characterize such solutions with many methods and experiments (1). Later, microemulsions without the presence of a surfactant become a signifi cant medium for various enzyme-catalyzed reactions as well as other chemical reac- tions. For example, Zoumpanioti et al. (2) studied the catalytic activity of lipases trapped in ternary systems without surfactants, consisting of water, short-chain alcohol, and hex- ane. Both enzymes effectively catalyze the esterifi cation of acids. The authors have shown that the stability of encapsulated enzymes at low water contents in the system is higher than that found in microemulsions with classical surfactants. We have recently shown that nanostructures exist in microemulsions without surfactants based on fragrance tinctures, composed of monophasic water–ethanol–citronellol mixtures (3). Consequently, the question of the existence of such nanostructures in other perfume tinctures arises. To answer that question, we have formulated tinctures with citral mole- cules that are structurally very similar to citronellol molecules. In recent years, stati c and dynamic percolation models for describing the microstructure of microemulsions have become very common. In the static percolation theory, the result Address all correspondence to Perica Bošković at pboskovic@pmfst.hr. J. Cosmet. Sci., 72, (May/June 2021) 292–297 292

of percolation is considered to be the formation of a bicontinuous phase, whereas in the dynamic percolation model, micelles are formed and ions are exchanged as a result of a “fusion” reaction (4–6). Both models consider t hat percolation occurs when the critical amount of the aqueous phase in the single-phase system ij p w is reached, the so-called percolation threshold. The connecting law between the electrical conductivity and the concentration of the aqueous phase in a single-phase system before and after the percolation threshold ij p w with its critical coeffi cients is shown in the literature (7). In this article, a nanos tructural transition based on the percolation theory of a surfactant- free microemulsion composed of citral, ethanol, and water has been studied by measuring electrical conductivity, DLS, and UV/Vis spectroscopy. EXPERIMENTAL Ethanol (Al drich, p.a.) and citral (Aldrich, p.a.) without prior purifi cation and redistilled water were used to prepare the mixed solvent. Mixed solvents of different mass composi- tions were prepared in glass vials by adding the individual components by weighing. The monophasic region in the ternary phase diagram was determined by adding a specifi c mass of water with a syringe to the initial mass of the mixed solvent of ethanol and citral until the phase separation occurred, thus constructing a binodal curve. Measurements were performed in a thermostatic bath at 298.15 K. A Wayne Kerr device, typ e 6430 A (Wayne Kerr, London, United Kingdom), was used to measure the resistance of working solutions. Its measurement accuracy is 0.02%, the voltage can vary in the range from 0 to 2 V, and the frequency can vary from 20 Hz to 500 kHz. The Anton Paar Litesizer 500 (Anton Paar, Graz, Austria) was used to mea- sure the hydrodynamic radii. Before each measurement, the microemulsion solutions were fi ltered through a 0.2-μm fi lter. Agilent Cary 60 two-beam spectrophotometer (Agilent Cary, Santa Clara, CA) and a 1.0-cm-wide cuvette were used for measurements. RESULTS AND DISCUSSION Fi gure 1A shows the r esults of determining the separation region of the two-phase and single-phase regions, and the binodal curve of the ternary citral–ethanol–water system at 298.15 K. The experimental data are in agreement with those in the literature (8). The plot of κ versus φw at RE/C = 2.3 shown in Figure 1C represents a typical example of percolation phenomena. The conductivity is initially low, indicating the existence of dis- crete inverse micelles in a single-phase system. As the proportion of water in the inverse micelle system increases and when the percolation threshold is reached, there is a sharp increase in electrical conductivity. A sharp increase in electrical conductivity above the percolation threshold and linear growth with a further increase in the water content in the system result in two possible effects: either the formation of microemulsion aggre- gates and charge changes by the fusion reaction between them or the existence of the bi- continuous phase which forms free channels for charge exchange. From the calculation of the experimental data, the obtained result for the critical percolation coeffi cient s is 0.80 for the experimental path RE/C = 2.3 (Figure 1D), indicating the presence of a SURFACTANT-FREE MICROEMULSIONS 293