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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 (PEG(40)St) were investigated with Langmuir monolayer isotherms and Brewster angle microscopy (BAM) imaging at DSPC/PEG(40)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 PEG(40)St content, indicating that the PEG(40)St squeezed out whereas more emulsifier retained in the monolayer at higher PEG(40)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 PEG(40)St content in the monolayer. Gray scale intensities in the Brewster angle microscopy images were determined for pure DSPC and pure PEG(40)St and a scale was established to better interpret the morphologies for the mixtures. Bud and vessels formed during the PEG(40)St squeezed out upon compression. Upon expansion, PEG(40)St and DSPC is reappeared on the monolayer. When considered BAM images together with the Langmuir isotherm, PEG(40)St molecules were found to be well distributed within the DSPC molecules at lower DSPC/PEG(40)St mole ratios and mostly phase separated at higher mole ratios. It was concluded that higher PEG(40)St content would be advantageous for the design of an efficient and cheaper ultrasound contrast agents.