Phase behavior of DSPC/PEG 40 St mixtures at higher emulsifier contents.

Phase behaviors of 1,2-Distearoyl-sn-glycero-3-phosphocholine (DSPC) and polyoxyethylene(40)stearate (PEG40 St) were investigated with Langmuir monolayer isotherms and Brewster angle microscopy (BAM) imaging at DSPC/PEG40 St molar ratios ranging from 9:1 to 5:5. Two plateaus were found in the Langmuir isotherms which were relatively shorter for the 9:1 mixture and extended significantly by increasing the PEG40 St content, indicating that the PEG40 St squeezed out whereas more emulsifier retained in the monolayer at higher PEG40 St contents. A strong hysteresis was observed when the mixed monolayers were subjected to compression-expansion cycles. The degree of hysteresis for the first cycles also increased with increasing PEG40 St content in the monolayer. Gray scale intensities in the Brewster angle microscopy images were determined for pure DSPC and pure PEG40 St and a scale was established to better interpret the morphologies for the mixtures. Bud and vessels formed during the PEG40 St squeezed out upon compression. Upon expansion, PEG40 St and DSPC is reappeared on the monolayer. When considered BAM images together with the Langmuir isotherm, PEG40 St molecules were found to be well distributed within the DSPC molecules at lower DSPC/PEG40 St mole ratios and mostly phase separated at higher mole ratios. It was concluded that higher PEG40 St content would be advantageous for the design of an efficient and cheaper ultrasound contrast agents.