Surface Behavior of Spread Sodium Eicosanyl Sulfate Monolayers.: 1. π/AIsotherms Determined on a Langmuir Film Balance and on Drop Surfaces and Brewster Angle Measurements

Abstract

Spread sodium eicosanyl sulfate (SES) monolayers were characterized by π/Aisotherms and Brewster angle microscopy (BAM). During compression three different states can be distinguished by BAM: a dark expanded phase, a condensed phase, and bright dots. The bright dots are interpreted as traces of eicosanol. π/Aisotherms were determined with a conventional Langmuir film balance and with the pendant drop technique. Depending on the pH SES desorbs from the monolayer. At a constant surface pressure the desorption was characterized by changes in the monolayer 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 pressure 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 small volume of bulk phase which is faster in equilibrium with the monolayer. For the determination of the isotherms on pendant drops, two different procedures were used: stepwise fast compression of the drop surface followed by 5 min of relaxation of each monolayer, and slow compression of the monolayer by evaporation of the drop. The π/Aisotherms at pH 3 determined with the Langmuir film balance and stepwise compression of the drop surface do not show transition points. In contrast however those obtained from slow drop evaporation shows transition points and allows the realization of higher surface pressures. A steep increase 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 monolayer by drop evaporation seems to be an suitable method to determine equilibrium π/Aisotherms.