A new class of emulsion systems - Fast inverted o/w emulsions: Formulation approach, physical stability and colloidal structure

Abstract

The fast inverted oil-in-water (o/w) emulsions named SWOP (SWitch Oil Phase) emulsions have been investigated with particular reference to physicochemical characteristics. Emulsions (oil-in-water (o/w) and water-in-oil (w/o)) are widely used in cosmetic and pharmaceutical formulations. Fast inverted o/w emulsions have been introduced as an alternative. The combination of appropriate w/o emulsifiers, anionic surfactants and polymeric stabilizers is essential for the formation of these emulsions. Samples of investigated, fast inverted o/w emulsion were prepared by hot-hot emulsification procedure using the combination of a w/o polymeric emulsifier (polyglyceryl-2 dipolyhydroxystearate) with a mild surfactant (sodium lauryl glucoside carboxylate (and) lauryl glucoside) in ratio 4:1.5 with addition of a stabilizing polymer (sodium polyacrylate). In the same manner, reference o/w and w/o emulsions were prepared. Investigated and reference emulsions were compared according to their physicochemical characteristics, structural characteristics, and their stability under the foreseen storage conditions and under the stress conditions employing the centrifugation test, pH, conductivity, rheological and contact angle measurements, microscopic observation, differential scanning calorimetry and thermogravimetric analysis. The samples of investigated and reference o/w emulsions were stable during three months storage at room temperature, while the samples of reference w/o emulsion showed phase separation. Only the samples of investigated emulsion remained stable in centrifugation test taken after the six freeze-thaw cycles. Oscillatory rheology indicated that the elastic modulus was dominant for both the investigated and the reference o/w emulsions due to the presence of gel structures, but the values of maximal apparent viscosity of the investigated emulsion which increased due to the temperature changes and the higher yield stress values that were obtained for the reference o/w emulsion indicated that the reference o/w emulsion had a stronger gel structure. Analysis of all the emulsions using polarization microscopy showed that the reference o/w emulsion had a more regular and a more rigid structure than the investigated emulsion. Obtained DSC and TGA results indicated that the SWOP emulsion showed a much faster evaporation of the water than the reference o/w emulsion which is in fine agreement with results from the contact angle measurements, i.e., the inversion point for the SWOP emulsion was attained in less than 15 min in comparison with the reference o/w emulsion. Generally, the fast inverted, SWOP emulsion showed better characteristics in comparison with the reference o/w and w/o emulsions making it suitable for the wide range of applications.