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Quantification of PEG(40)St squeeze out from DSPC/PEG(40)St monolayers at higher molar ratios
Mixtures of 1,2 Distearoyl-sn-glycero-3-phosphocholine (DSPC) and polyoxyethylene-40-stearate (PEG(40)St) were prepared at different molar ratios and their miscibility were investigated using Langmuir isotherms. Pure DSPC monolayer exhibited a liquid-condensed (LC) phase whereas PEG(40)St monolayer exhibited liquid-expanded (LE) phase at the air-water interface at 22 +/- 2 degrees C. At the collapse pressure of 33 mN/m, the PEG(40)St mean molecular area was calculated to be 28 angstrom(2)/molecule for 9: 1 composition and 50 angstrom(2)/molecule for 5: 5 composition, showing an increasing trend with the emulsifier content. A quantification method was developed to estimate the squeeze out amount of PEG(40)St from Langmuir isotherms of the DSPC/PEG(40)St mixtures at different molar ratios. Almost 93%, 82%, and 53% of PEG(40)St displaced for the 9:1, 7:3, and 5:5 mixtures, respectively, at the end of the first collapse plateau and showed a decreasing trend with the PEG(40)St content. Remaining PEG(40)St squeezed out at the end of the second collapse plateau, where 20% of PEG(40)St still contained within the 5: 5 composition. It was concluded that increasing PEG(40)St content would be advantageous to design more stable lipid based microbubbles.