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.

Monolayers of mixture of alkylaminomethyl rutin and lecithin at the air/water interface

“…The value of A limt (46.04 Å 2 / molecule at 15°C) is higher than that for quercetin-3-O-palmitate (43 Å 2 /molecule at 10°C) [3] and is not in agreement with that for single fatty acid (22.0 Å 2 /molecule at 20°C) in the neutral subphase (pH 6.0) [25][26][27]. This result indicates that rutin moiety as the hydrophilic head group contributes prominently to the sur- face-occupied area per molecule.…”

Langmuir monolayers and Langmuir–Blodgett films of 1-acyl-1,2,4-triazoles

“…Besides fatty acids of 12 [1,4,5], 14 [1,[6][7][8][9][10][11][12], 16 [6][7][8][9], and 18 [1,6,9,[13][14][15][16][17] carbon atoms, which are investigated with respect to their Langmuir isotherms and to their compression on the water surface up to the film collapse of the monolayer, also fatty acids that have odd numbers of carbon atoms, e.g. from C15 to C21 [6,[18][19][20][21][22][23], or more than 18 carbon atoms, e.g. from C20 to C24 [5][6][7]10,13,[24][25][26] have been investigated.…”

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air–water interface

“…These techniques have revealed that even a small change in the isotherm corresponds to a phase transition [11][12][13][14]. A cusp point followed by a horizontal line in the π-A isotherms indicates a first-order phase transition whereas a small kink in these isotherms reveals higher-order transition between two phases.…”