Surface behavior of spread sodium eicosanyl sulfate monolayers. 3. Eicosanyl sulfate monolayers spread on a NaCl subphase

摘要

Sodium eicosanyl sulfate monolayers spread on a 10(-2) M/dm(3) NaCl substrate were characterized by pi/A isotherms, Brewster angle microscopy, and surface stress relaxation experiments. The measurements were carried on a conventional Langmuir trough and with a pendant drop technique where the monolayers are spread on the drop surface. Axisymmetric drop shape analysis (ADSA) was used to calculate the surface tension from the shape of the drop. Results of monolayer stability show that the alkyl sulfate monolayers are slightly soluble in the adjacent subphase. The effects of monolayer desorption influences the surface coverage calculated from Langmuir through experiments more than those from the 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 monolayer collapse is much higher than on water. The surface dilational rheological behavior confirms the formation of more viscoelastic surface structures 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 expended monolayer formed on water, which is possibly due to H+ and Na+ ion competition. (C) 1998 Academic Press. [References: 10]