volume ratio realized by the drop. Therefore the saturation of Sodium eicosanyl sulfate monolayers spread on a 10 02 M/dm 3 the subphase by dissolved monolayer molecules was reached NaCl substrate were characterized by p/A isotherms, Brewster faster decelerating further desorption. angle microscopy, and surface stress relaxation experiments. The In a recent work white dots have been observed by Brewmeasurements were carried on a conventional Langmuir trough ster angle microscopy in the monolayer structure immediand with a pendant drop technique where the monolayers are ately after spreading (1) and interpreted as either eicosanol spread on the drop surface. Axisymmetric drop shape analysis impurities or alkyl sulfate aggregates. When these white (ADSA) was used to calculate the surface tension from the shape dots were eicosanol impurities an autocatalytic alkyl sulfate of the drop. Results of monolayer stability show that the alkyl hydrolysis had to be feared, especially at pH 3. Therefore a sulfate monolayers are slightly soluble in the adjacent subphase.
special experiment was carried out in the second part of the The effects of monolayer desorption influences the surface coverage calculated from Langmuir through experiments more than paper to clarify the nature of these dots (2). It was clearly those from the drop measurements. In contrast to the behavior shown that even for long time monolayer experiments an observed for the same monolayers spread on water at pH 3 the alkyl sulfate hydrolysis does not affect the results and that monolayer shows a well-defined crystalline-like phase. This phase the dots are alkyl sulfate aggregates rather than eicosanol can be already observed at a very low surface coverage. The maxiimpurities.
mum surface pressure before the monolayer collapse is much
Another effective way to reduce alkyl sulfate solubility higher than on water. The surface dilational rheological behavior without running to risk alkyl sulfate hydrolysis is to increase confirms the formation of more viscoelastic surface structures at the Na / counterion concentration in the subphase (3, 4). higher surface coverage and a less viscoelastic behavior at lower Chang and Rosano (3) pointed out another interesting feasurface coverage. The results confirm the assumption of a more ture of using a NaCl subphase. They claimed that alkyl condensed monolayer formed on a NaCl solution in comparison sulfate monolayers exhibit an expansion with increasing to an expended monolayer formed on water, which is possibly due acidity of the bulk phase. This was explained by the substituto H / and Na / ion competition. ᭧ 1998 Academic Press tion of the sodium counterions by competing hydrogen ions.