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

Abstract

Grazing incidence x-ray scattering ͑GIXS͒ and Brewster angle microscopy ͑BAM͒ are used to determine the -T phase diagram of Behenic acid monolayers supported on the surface of water ͑pHϭ2.0͒ over the temperature range of 3°C to 20.6°C. The phase diagram is constructed from measurements taken during isothermal compressions in which the surface pressure relaxed to a stable value at each surface density, and during temperature scans at fixed average surface density. The phase diagram is different than those previously reported for Behenic acid primarily because of the surface pressure relaxation. For temperatures less than 12°C the phase diagram exhibits similar phases and topology as the published diagrams, although the location of the phases in the -T plane is different. Temperature scans combined with the isotherms, and the Clausius-Clapeyron relation are used to determine three coexistence lines that meet in a triple point. Changes in entropy across the phase boundaries are determined. Near room temperature ͑20.6°C͒ only one phase is measured over the range of surface pressure from 0 dynes/cm to the collapse pressure in contrast to reported measurements on monolayers out of equilibrium ͑i.e., when the surface pressure is not allowed to relax͒ that exhibit several phases near room temperature at pressures higher than the collapse pressure. Discrepancies are observed between the average area per molecule (A T ) and the area per molecule determined from measurements of the unit cell (A X ) in the close packed regions of the phase diagram. It is conjectured that the -A X plane is a better representation of the ordered equilibrium monolayer phases than the -A T plane. Isotherms plotted in the -A X plane are used to determine the compressibility of the ordered phases and the nature of the phase transitions.

Langmuir monolayers as disordered solids: Strain-state calculations applied to stearic acid

“…There is ongoing discussion as to the nature of various phases reported in the literature, but certain general features are consistently found. 1,2,[4][5][6][7][8] At low temperature and high surface pressure there exists a crystalline phase with two molecules per unit cell aligned along the film normal, as evidenced by Davydov splitting. 8 At low surface pressure the amphiphiles exhibit nearestneighbor tilt ͑NN͒, while at intermediate pressure next nearest-neighbor tilt ͑NNN͒ is observed.…”

“…The details of the surface pressure area (A) phase diagram are influenced by the subphase composition ͑pH and ionic composition͒, 9 the molecular structure ͑length and head group͒, 1,2,8 and the method used to apply pressure. 4 This paper presents a calculational procedure to determine the equilibrium phase for a given surface pressure . As a model system we have chosen stearic acid monolayers, due to their prominence in these systems.…”

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

“…[67][68][69][70] In any event, they would certainly not be observable for liquid LM phases of behenic acid, or similar systems. 11,18,19,28,71,72 …”

“…[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.…”