R. Wustneck et al., SURFACE BEHAVIOR OF SPREAD SODIUM EICOSANYL SULFATE MONOLAYERS - 3 - EICOSANYL SULFATE MONOLAYERS SPREAD ON A NACL SUBPHASE, Journal of colloid and interface science (Print), 203(1), 1998, pp. 90-96
Sodium eicosanyl sulfate monolayers spread on a 10(-2) M/dm(3) NaCl su
bstrate were characterized by pi/A isotherms, Brewster angle microscop
y, and surface stress relaxation experiments. The measurements were ca
rried on a conventional Langmuir trough and with a pendant drop techni
que where the monolayers are spread on the drop surface. Axisymmetric
drop shape analysis (ADSA) was used to calculate the surface tension f
rom the shape of the drop. Results of monolayer stability show that th
e alkyl sulfate monolayers are slightly soluble in the adjacent subpha
se. The effects of monolayer desorption influences the surface coverag
e calculated from Langmuir through experiments more than those from th
e drop measurements. In contrast to the behavior observed for the same
monolayers spread on water at pH 3 the monolayer shows a well-defined
crystalline-like phase. This phase can be already observed at a very
low surface coverage. The maximum surface pressure before the monolaye
r collapse is much higher than on water. The surface dilational rheolo
gical behavior confirms the formation of more viscoelastic surface str
uctures at higher surface coverage and a less viscoelastic behavior at
lower surface coverage. The results confirm the assumption of a more
condensed monolayer formed on a NaCl solution in comparison to an expe
nded monolayer formed on water, which is possibly due to H+ and Na+ io
n competition. (C) 1998 Academic Press.