LANGMUIR TROUGH STUDY OF SURFACTANT MIXTURES USED IN THE PRODUCTION OF A NEW ULTRASOUND CONTRAST AGENT CONSISTING OF STABILIZED MICROBUBBLES

Span-type surfactants (sorbitan fatty acid esters) and Tween-type surf actants (sorbitan polyoxyethylene fatty acid esters) are employed by u s to generate stabilized microbubbles for use in diagnostic ultrasound . After sonication of an aqueous surfactant solution, only some mixtur es of Span-type and Tween-type surfactants at certain conditions can f orm stable microbubbles. This work investigated the stability of the s urfactant-stabilized microbubbles by using a Langmuir trough to measur e the pi-A isotherms of the surfactant monolayer. The experimental res ults, which agreed with a theoretical analysis of the microbubble stab ility, indicate that the surfactant-stabilized microbubbles have a sol id-condensed monolayer ''skin'' which functions to reduce the surface tension, prevent coalescence between microbubbles, and increase their aqueous compatibility. The higher surface pressure obtained for the ca se of a microbubble preparation, compared with that of unsonicated mix tures, indicates that sonication enhances the structure of surfactant monolayer and makes the microbubbles extremely stable.