Physicochemical screening of antimicrobial agents as potential preservatives for submicron emulsions.

Antimicrobial agents should be added to lecithin-stabilized submicron emulsions when these preparations are non-sterile products or when packed in multidose containers. Eleven antimicrobials were introduced to a standard submicron emulsion. The emulsions were adjusted to pH 5.0 or 8.2 prior aseptic filtration or thermal sterilization, respectively. The physicochemical stability of the preparations was observed during storage for 2 years at room temperature. Parabens showed the best compatibility but satisfying stability was also observed in emulsions containing phenylethanol, m-cresol and benzalkonium chloride. Partitioning studies revealed poor correlation between aqueous solubility and content of the preservatives in the aqueous phase of the emulsion. Only 1.2% of the total content of benzalkonium chloride was found in this phase and incorporation of this compound into different microscopic structures of the emulsion is proposed as a reason for such effect. Preliminary studies on the efficacy of antimicrobial preservation was performed for emulsions containing parabens, benzalkonium chloride or chlorocresol and the negative results bring conclusion that higher concentration of antimicrobials or their combination may be required for efficient preservation of submicron emulsions.