Langmuir monolayers of C17, C19, and C21 fatty acids: Textures, phase transitions, and localized oscillations

Abstract

The phase diagrams of Langmuir monolayers of heptadecanoic (C17), nonadecanoic (C19), and heneicosanoic (C21) acids have been determined from pressure-area isotherms, and from direct observations of the monolayers using Brewster angle microscopy. In this paper, we describe the observed domains, textures and phase boundaries for all mesophases presented by these fatty acids between 2° and 45 °C. The phase diagrams of the three fatty acids can be superposed moving the temperature scale according to the number of carbons in the tail of the fatty acids. The L2/Ov phase transition, which is not detected through isotherms, was observed in all the fatty acids under study. At low temperatures, CS,L2′, and L2″ phases of C21 were observed, as well as, the transitions among them. Also, we observed in C21 a new phase located among the phases L2,L2′, and L2″. This phase was found recently, in the relative same place, in the fatty acid C20 with a tilting azimuth between the nearest-neighbor and the next nearest-neighbor directions. In the crystalline phases of the monolayer of C21, L2″ and CS, we observed localized oscillations. These localized oscillations can be observed by long periods of time. The number of localized oscillations in the monolayer can be modified by long periods of relaxation or by heat treatment. Our results seem to indicate that these localized oscillations are areas with high density of defects, expelling material out of the monolayer. Therefore, they could be important in the events previous to the collapse.