SURFACE BEHAVIOR OF SPREAD SODIUM EICOSANYL SULFATE MONOLAYERS - 3 - EICOSANYL SULFATE MONOLAYERS SPREAD ON A NACL SUBPHASE

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.