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/
R. Wustneck et al., 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/, Journal of colloid and interface science (Print), 203(1), 1998, pp. 83-89
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.