SURFACE BEHAVIOR OF SPREAD SODIUM EICOSANYL SULFATE MONOLAYERS - 2 - SURFACE STRESS-RELAXATION BEHAVIOR

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.