Larger Molecular Area Research Articles

Relaxation effects in the surface pressure behavior of dipalmitoyl lecithin

Measurements of the room-temperature surface pressure-area ( π A) characteristics of dipalmitoyl lecithin (DPL) films reveal that the dynamic π-A compression curve differs markedly from the static compression curve. Since the dynamic compression behavior is essentially independent of cycling rate in the experimental range of 19.2–96A˚ 2/molecule/min, it appears that the DPL monolayer does not display appreciable viscous effects, although this may not be so over a wider cycling range. Nonetheless, for the experimental range studied, dynamic compression of DPL appears to impart a metastable structure to the surface which is distinct from the static molecular structure. When surface compression is halted at a given film area during a first compression, the DPL surface pressure is found to relax from its dynamic value towards the equilibrium value at the same film area. The relaxation time of this process was only 50 sec at the relatively large molecular area of 68A˚ 2/molecule, and it increased continuously to a value of 850 sec at an area of 40A˚ 2/molecule, just prior to film collapse. At all trough areas following film collapse, the surface pressure was exceedingly stable and relaxations of only a few dyn/cm were observed even after 12,000 sec. The data thus indicate that molecular crowding inhibits the relaxation process for DPL films. Moreover, the formation of multilayers during film collapse appears to impart an even greater stability to the film pressure.

Monomolecular films of mixtures: I. Stearic acid with isostearic acid and with tri- p-cresyl phosphate. Comparison of components with octadecylphosphonic acid and with tri- o-xenyl phosphate

Pressure-area isotherms have been obtained for mixed monolayers of pure compounds whose structures are related to those of lubricant additives. Individually, stearic acid and octadecylphosphonic acid with long unbranched chains form strong films. Isostearic acid has a larger molecular area and a much lower collapse pressure than stearic acid. This difference demonstrates the great sensitivity of monolayers to a single methyl side chain. Tri- p-cresyl phosphate and tri- o-xenyl phosphate have much larger areas and lower collapse pressures. An equimolar mixture of two similar compounds, stearic acid and isostearic acid, has monolayer properties close to the calculated average. An equimolar mixture of dissimilar compounds, stearic acid and tri- p-cresyl phosphate, shows film properties of the phosphate at low pressures, expulsion of the phosphate from the monolayer at intermediate pressures, and properties of stearic acid at high pressures.