X-ray diffraction study of a Langmuir monolayer of C21H43OH

Abstract

The structure of a Langmuir monolayer of methyl eicosanoate as determined by xray diffraction and Brewster angle microscopyWe have studied the structure ofa monolayer ofC 2J H 43 0H on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the 11"a plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for 11" = 30 dyn/cm the transition temperature is in the range 16.3 < T < 21.3 ·C. We interpret this observation, by comparison with the properties of the lamellar crystalline n-paraffins, as a hexagonal-topseudohexagonal structural transition analogous to the crystal rotator II-to-rotator I transition. Our results imply that the hydrocarbon tails of the amphiphile molecules dominate the properties of the monolayer.

Understanding and control of nucleation, growth, habit, dissolution and structure of two- and three-dimensional crystals using `tailor-made' auxiliaries

“…<80%, so that the monolayer surface pressure is very low) was shown to be a widespread phenomenon. This was demonstrated for a variety of amphiphiles, such as long fluorocarbon chain a-amino acids (Jacquemain et al, 1990) and carboxylic acids (Durra et al, 1987;Barton et al, 1988Barton et al, , 1992, hydrocarbon chain alcohols, amides, carboxylic acids and their cadmium or calcium salts (BiShm et al, 1994). Indeed the GID spectrum of the cadmium arachidate monolayer gave direct evidence of an ordered counterionic cadmium layer bound to the carboxylate head group.…”

“…The newly developed method of GID is a powerful tool for exploring this field (Als-Nielsen & Kjaer, 1989;Barton et al, 1988;Als-Nielsen & Mohwald, 1992;Jacquemain et al, 1992) since it allows for the detection of two-dimensional crystalites formed by the amphiphilic molecules and for the determination of their size and structure almost at the atomic level. Using GID, the two-dimensional crystallinity of amphiphilic monolayers in the relaxed state (i.e.…”

A determination of the phase diagram of relaxed Langmuir monolayers of Behenic acid

“…10,11 However, some researchers have also depicted this boundary with a negative slope. 13 The literature 8,10,36 also suggests that at slightly higher temperature there should be another nearly vertical phase boundary separating slightly different untilted U(CS) and UЈ(S) phases. This boundary is usually presented as extending up- ward from a presumed triple point, which is not located in Fig.…”

C 60 -propylamine adduct monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study

“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][32][33][34] Except for a few cases, which have hexatic order, most of these phases exhibit resolution-limited Bragg peaks characteristic of 2D crystalline phases. Figure 1͑b͒ contains a schematic illustration of the kinematics for the GID experiment.…”

“…1-3 A principal challenge in this general area of physics has been to identify real, well defined physical systems that are suitable for experimental studies. Examples of two-or quasi-twodimensional systems that have been investigated for this purpose in recent years include rare gases adsorbed on solid substrates, [3][4][5][6] freely suspended thin liquid crystal films, [7][8][9][10] and Langmuir monolayers 11,12 of simple long-chain molecules such as fatty acids, [13][14][15][16][17][18][19][20][21][22][23] alcohols, [24][25][26][27] esters, 28 etc. Related statistical phenomena at surfaces and interfaces that have been often studied for more practical motivations, include wetting, [29][30][31] premelting and reconstruction of crystalline surfaces, [32][33][34][35] and surface induced order at liquid surfaces.…”