SURFACE BEHAVIOR OF SPREAD SODIUM EICOSANYL SULFATE MONOLAYERS - 1 - PI A ISOTHERMS DETERMINED ON A LANGMUIR FILM BALANCE AND ON DROP SURFACES AND BREWSTER-ANGLE MEASUREMENTS/

Spread sodium eicosanyl sulfate (SES) monolayers were characterized by pi/A isotherms and Brewster angle microscopy (BAM). During compressio n three different states can be distinguished by BAM: a dark expanded phase, a condensed phase, and bright dots. The bright dots are interpr eted as traces of eicosanol. pi/A isotherms were determined with a con ventional Langmuir film balance and with the pendant drop technique. D epending on the pH SES desorbs from the monolayer. At a constant surfa ce pressure the desorption was characterized by changes in the monolay er area. At pH 3 a reduction of 20% of the monolayer area was observed on a Langmuir film balance after 45 min. In contrast the surface pres sure of monolayers spread on surfaces of pendant drops reduces by only about 2 mN/m, even at high surface pressures. This is caused by the s mall volume of bulk phase which is faster in equilibrium with the mono layer. For the determination of the isotherms on pendant drops, two di fferent procedures were used: stepwise fast compression of the drop su rface followed by 5 min of relaxation of each monolayer, and slow comp ression of the monolayer by evaporation of the drop. The pi/A isotherm s at pH 3 determined with the Langmuir film balance and stepwise compr ession of the drop surface do not show transition points. In contrast however those obtained from slow drop evaporation shows transition poi nts and allows the realization of higher surface pressures. A steep in crease in the surface pressure before the monolayer collapse suggests the formation of a solid monolayer phase. At pH 5.5 fast desorption of SES does not allow a monolayer collapse. Slow compression of the mono layer by drop evaporation seems to be an suitable method to determine equilibrium pi/A isotherms. (C) 1998 Academic Press.