Electroacoustic Determination of Droplet Size and Zeta Potential in Concentrated Intravenous Fat Emulsions

The electrokinetic sonic amplitude (ESA) of the oil droplets in an undiluted intravenous emulsion (Intralipid, Kabi Pharmacia) has been measured using the AcoustoSizer (Matec Applied Sciences) to obtain estimates of both size and zeta potential. At natural pH, the initial median diameters were 0.27 ± 0.08 μm for Intralipid 10% and 0.25 ± 0.04 μm for Intralipid 20%. Zeta potentials were -30.4 ± 1.4 mV and -23.8 ± 1.0 mV, respectively. Calcium ions complexed specifically with the phospholipid emulsifiers, inducing an isoelectric point (iep) at 6-8 mM followed by charge reversal. The median size increased four- to fivefold at the iep and then decreased as the zeta potential became positive. The effect of sodium was nonspecific. There was no iep, the zeta potentials decreased to -4 to -10 mV at 80 mM sodium, and the median size increased twofold. The changes in particle size were compared with optical microscopy studies. These results agree with published data and show the utility of the ESA technique in measuring these stability indices for injectable emulsions.