R. Wustneck et al., SURFACE BEHAVIOR OF SPREAD SODIUM EICOSANYL SULFATE MONOLAYERS - 2 - SURFACE STRESS-RELAXATION BEHAVIOR, Journal of colloid and interface science (Print), 206(1), 1998, pp. 33-43
The surface behavior of spread sodium eicosanyl sulfate monolayers is
characterized by determining the dilational moduli from different pi/A
isotherms and from surface stress relaxation experiments in the short
-time range (<10 min). The elasticities derived from the pi/A isotherm
s differ depending on the experimental conditions. The quasi-equilibri
um isotherm displays a plateau in the range of coexistence of the cond
ensed and the expanded phases and strong increases caused by the forma
tion of a solid-like phase. In contrast, nonequilibrium pi/A isotherms
yield effective elasticities showing a maximum within the phase coexi
stence range. The formation of a solid phase cannot be detected becaus
e of the onset of monolayer collapse. Different stress relaxation expe
riments were carried out for monolayer compression and dilation using
transient drop volume jumps. Depending on the experimental run, these
experiments lead to consistent and complementary results with those de
rived from pi/A isotherms under comparable conditions. The stress reco
veries yield a relaxation time, a dilation viscosity, and a parameter
characterizing the homogeneity of the relaxation process. The stress r
elaxation is interpreted accounting for both the nonequilibrium betwee
n the monolayer and the bulk phase and the nonequilibrium within the m
onolayer. The influence of alkylsulfate hydrolysis on the presented re
sults was checked, It was found that within the time scale of the expe
riments no influence of hydrolysis could be detected. (C) 1998 Academi
c Press.