The aims of the present study were to elucidate the potential mechanism of propofol emulsion destabilization following the addition of lidocaine, and to evaluate the effects of various electrokinetic stabilizers on the physicochemical properties of lidocaine-propofol emulsions. The assessments included pH observations and determination of the maximum globule diameter (MGD) and zeta potential (ZP). The MGD of propofol emulsions increased up to several tens mum following the addition of 50 mg of lidocaine to 200mg of propofol, and the proposed destabilization mechanism involves localization of protonated lidocaine molecules between lecithin molecules in propofol emulsions, which consequently leads to increased ZP. The ZP of propofol emulsions containing acidic amino acid or neutral amino acid increased following the addition of lidocaine, and a charge reversal occurred. Therefore, the MGD of emulsions increased to several tens (m. However, the MGD of emulsions that contained basic amino acids or basic compounds remained below 5 (m, despite the addition of large amounts lidocaine (50 mg), and the ZP did not pass through the point of zero charge. In conclusion, our results provide not only further insight into the physical stability of propofol emulsions containing lidocaine, but also a better understanding of the administration of propofol in existing applications.
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