Microemulsionsare isotropic, transparent, solution-like mixtures of water, oil and tensioactivesubstances. 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 microemulsionscontaining 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.
Figure 2. Minimal concentration of tensioactives at which microemulsion formation occurs
Victor Cojocaru1, Ioana Ailiesei1, Ana-Maria Orbeşteanu1, Otilia-Ludmila Cinteză2
1Universitatea de Medicină şi Farmacie „Carol Davila”, Facultatea de Farmacie, Bucureşti, România
2Universitatea Bucureşti, Facultatea de Chimie, Bucureşti, România
Development of a new method for screening for optimal composition of microemulsions containing vegetable extract, based on interfacial tension determination
Materials & Methods
optimal extract:Tween 80 ratio
- 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-octanolwere 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.
- 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 (Rring=9.545 mm; Rwire=0.185 mm). The reported values were automatically corrected by the apparatus software using Huh-Mason correction. Throughout the experiments, the vegetable extract:isopropylmyristate 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.
Figure 3. Interfacial tension measurements at hydro-alcoholic extract-isopropyl myristate interface throughout addition of Tween 80
optimal Tween 80:n-octanol ratio (1:1.25)
optimal Tween 80:n-butanol ratio (1:1.19)
Figure 4. Interfacial tension measurements at the Tween 80-extract solution (at optimal ratio)-isopropyl myristate interface throughout addition of n-octanol
Figure 5. Interfacial tension measurements at the Tween 80-extract solution (at optimal ratio)-isopropyl myristate interface throughout addition of n-butanol
- 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 tensioactivesof 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
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.
Figure 1. du Noüyring method
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