1. Results Introduction and Objectives Microemulsions are isotropic, transparent, solution-like mixtures of water, oil and tensioactive substances. They are characterized by ultralow interfacial tension between water and oil, usually achieved when using a surfactant in combination with a cosurfactant. The purpose of this study was to develop a new method for screening for optimal composition of biocompatible topical microemulsions containing a hydro-alcoholic extract, viewed as superior alternative preparations for cosmetic and pharmaceutical use. The method is based upon investigating the optimal surfactant:cosurfactant ratio and concentration of tensioactives for microemulsion formation, by determining the minimal interfacial tension at the liquid-liquid interface between hydrophilic and lipophilic phases. Based on this data, optimal microemulsion formulations were proposed. All formulations were evaluated regarding their transparency/translucency, optical birefringence and stability. Materials & Methods Materials Isopropyl myristate (from Titolchimica) was used as oil component. The role of the surfactant was assigned to Tween 80 (kindly gifted by Actavis). n- butanol and n-octanol were purchased from Merck and were used as cosurfactants. The chili pepper hydro-alcoholic extract (70% v/v ethanol) was kindly gifted by Hofigal and represented the hydrophilic phase of the microemulsion. Methods The interfacial tensions were determined by du Noüy ring method (as shown in Figure 1), using a KSV Sigma 703D tensiometer. All measurements were conducted at 30°C, using a 50 mm Boro 3.3 glass vessel and a platinum/iridium ring (R ring =9.545 mm; R wire =0.185 mm). The reported values were automatically corrected by the apparatus software using Huh-Mason correction. Throughout the experiments, the vegetable extract:isopropyl myristate ratio was maintained constant at 1:1 (w/w). Tween 80 and n- butanol/n-octanol respectively, were added to the appropriate phase using titration method. The interfacial tension experimental data was confronted against real data, obtained by titrating mixtures of chili pepper extract and isopropyl myristate (1:1 weight ratio) with Tween 80 or mixtures composed of Tween 80:n-butanol/n- octanol (at 3:1, 2:1, 1:1, 1:2, 1:3 weight ratios), using 100 μL incremental steps. The point of titration at which the system became a clear single phase represents the minimal concentration of tensioactives at which the microemulsion formation occurs. Conclusions  A n-butanol/n-octanol free system was obtained, at approximately 38% Tween 80, as a consequence of ethanol (contained by the hydro- alcoholic extract) acting as a cosurfactant  Adding a second alcohol (n-butanol or n-octanol) leads to a marked reduction of Tween 80 concentration necessary for the microemulsion formation, although the total concentration of tensioactives remains almost the same  Samples containing Tween 80:n-butanol/n-octanol at 1:2 and 1:3 ratios didn’t formed microemulsions even at a concentration of tensioactives of nearly 60%  Based on the interfacial tension measurements, optimal weight ratios were determined for the hydro-alcoholic extract:Tween 80 and Tween 80:cosurfactant (n-butanol or n-octanol)  Compared to n-octanol, n-butanol generated lower interfacial tension values. A possible explanation is that the n-butanol concentrates to a higher extent at the extract-oil interface, due to its more pronounced amphiphilic character and its smaller molecular volume  The optimal formulations were prepared and appeared to be transparent single phase non-birefringent systems, suggesting their microemulsion nature  All formulations considered microemulsions were stable after 60 days storage at a temperature below 30°C Acknowledgement References Adamson, A.W.; Gast, A.P. Physical Chemistry of Surfaces, 6th Ed.; John Wiley & Sons, Inc.: New York, 1997 Danielsson I, Lindman B, The definition of a microemulsion. Colloids Surf B Biointerfaces, 3, 391–392, 1981 Hiemenz, P.C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd Ed.; Marcel Dekker, Inc.: New York, 1997 Mosayeb A, Abedini R, The effect of non-ionic surfactants on the interfacial tension between crude oil and water. Petroleum & Coal 54 (2), 110-113, 2012 Paul BK, Moulik SP, Microemulsions: an overview. J Disp Sci Technol, 18, 301- 367, 1997 Salager JL, Morgan JC, Schechter RS, Wade WH, Vasquez E, Optimum formulation of surfactant/water/oil systems for minimum interfacial tension or phase behavior. SPE-AIME Fifth Symposium on Improved Methods for Oil Recovery, Tulsa, April 16-19, 1978 Trabelsi S, Hutin A, Argillier J-F, Dalmazzone C, Bazin B, Langevin D, Effect of Added Surfactants on the Dynamic Interfacial Tension Behaviour of Alkaline/Diluted Heavy Crude Oil System. Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, 2013 This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/159/1.5/S/132395. Development of a new method for screening for optimal composition of microemulsions containing vegetable extract, based on interfacial tension determination Victor Cojocaru 1, Ioana Ailiesei 1, Ana-Maria Orbeşteanu 1, Otilia-Ludmila Cintez ă 2 1 Universitatea de Medicin ă şi Farmacie „Carol Davila”, Facultatea de Farmacie, Bucureşti, România 2 Universitatea Bucureşti, Facultatea de Chimie, Bucureşti, România Figure 2. Minimal concentration of tensioactives at which microemulsion formation occurs Figure 3. Interfacial tension measurements at hydro-alcoholic extract-isopropyl myristate interface throughout addition of Tween 80 optimal extract:Tween 80 ratio (2.1:1) Figure 4. Interfacial tension measurements at the Tween 80-extract solution (at optimal ratio)-isopropyl myristate interface throughout addition of n-octanol optimal Tween 80:n-octanol ratio (1:1.25) Figure 5. Interfacial tension measurements at the Tween 80-extract solution (at optimal ratio)-isopropyl myristate interface throughout addition of n-butanol optimal Tween 80:n-butanol ratio (1:1.19) Figure 1. du Noüy ring method