Transition between two next-nearest-neighbor phases in a mixed Langmuir monolayer. A study by grazing-incidence x-ray diffraction and Brewster-angle microscopy

Abstract

Grazing incidence x-ray diffraction and Brewster-angle microscopy measurements have been undertaken for a mixed Langmuir monolayer of octadecanoic acid and methyl octadecanoate. For the composition studied (49.7 mol % ester) there are two noncrystalline tilted phases, one at low pressure in which the chains point to their nearest neighbors (L2 phase) and a higher-pressure phase in which the tilt is toward next-nearest neighbors. The higher-pressure phase arises from a merger of the L2′ and Ov phases, which are separated in the pure acid. A continuous change between the two regions is observed rather than a first-order transition, as had been suggested. The results are discussed with reference to different proposals for the origin of two distinct next-nearest neighbor tilted mesophases: the presence of one-dimensional chain backbone order [V. M. Kaganer and E. B. Loginov, Phys. Rev. E 51, 2237 (1995)] and a coupling between tilt and distortion [E. Sirota, Langmuir 13, 3849 (1997)].

Grazing Incidence Diffraction and Brewster-Angle Microscope Studies of Mixtures of Hexadecanoic Acid and Methyl Hexadecanoate: The Unexpected Appearance of a Phase with Nearest-Neighbor Tilt

“…They employed BAM to trace the motion of the phase boundaries as fatty acids were mixed with their esters, which made it possible to identify the ester phases. The phase diagrams for methyl and ethyl n -heneicosanoate and methyl, ethyl, and propyl n -octadecanoate were all found to be in accord with that for methyl n -eicosanoate determined by Foster et al Recent GIXD measurements on a mixture of octadecanoic acid and methyl n -octadecanoate are also consistent with the BAM measurements and those on the pure ester.…”

“…Foster et al 9 argued that there are two distinct NNN phases. Studies on an acid-ester mixture 11 show a narrow region of temperature in which there is a continuous transition between the two kinds of structure.…”

“…In the one that applies to acids and to acetate esters of n -alkanols, there are three distinct noncrystalline tilted mesophases: L 2 ‘ (NNN), Ov(NNN), and L 2 (NN). In the other diagram, which applies to esters of saturated fatty acids and to saturated alcohols, , there is no NN phase and the L 2 ‘ and Ov phases are merged. The controlling factor appeared to be the maximum tilt angle, which is related to the headgroup size.…”

High-resolution x-ray scattering measurements: I. Surfaces

“…The effect of fluorinated [104] or branched [93,120,121] chains has been investigated. Mixtures of acids and alcohols [122], acids and esters [123] and acids with different chain lengths [124] have also been studied. Amino acids [3,88] and phospholipids [38,88,[125][126][127] have attracted a lot of attention.…”

Kinetics of phase transitions in monolayers: collapse

“…Much has been learned about the structures of monolayers of simple amphiphiles and progress has been made in understanding the topology of the phase diagrams in terms of Landau-de Gennes theory [1], but a full understanding of how the stability of the monolayer phases is determined by the interactions between head groups and chains is yet to be achieved. The effects of chain length and the mismatch in size between head groups and tail have been addressed recently for simplified model systems [2], which can be compared with BAM and GIXD investigations of mixtures of acids and esters [3]. Critical phenomena in monolayers are also currently being examined.…”