Lattice structures and phase behavior of amphiphilic monoglycerol monolayers

Abstract

Due to the Angstrom resolution, Grazing incidence X-ray diffraction (GIXD) represents the most important technique for probing the lateral ordering in condensed monolayers at the air/water interface and allows the construction of phase diagrams of amphiphilic monolayers on the basis of two-dimensional lattice structures and tilt directions of the molecules. The high potential of GIXD is demonstrated by the structural characterization of a variety of amphiphilic monoalkanoylglycerol monolayers in Å-scale. The GIXD results have impressively shown that in the racemic 1-monostearoylglycerol monolayer with the appearance of an oblique intermediate phase (Obl) between the nearest neighbor (NN)- and next-nearest neighbor (NNN)-tilted orthorhombic phases symmetry breaking occurs at low temperatures. The generic lateral pressure-temperature phase diagram of racemic monoacylglycerol monolayers constructed on the basis of reliable two-dimensional lattice structures indicates that the new and surprising presence of the oblique phase depends only on the temperature. The significant effect of the substituted polar groups, chemical structure variations at the position of the glycerol backbone and chirality on the lattice structure in Å-scale was highlighted in a systematic overview on the structure and phase behavior of amphiphilic monoglycerol monolayers. The conspicuous effect of the position of the glycerol backbone at which the polar group is substituted is demonstrated. The monolayers of 2-monopalmitoyl-rac-glycerol behave as that of 1-monomyristoyl-rac-glycerol having a two CH₂ groups shorter alkyl chain. Further main topics discussed are chiral discrimination and crossover between homo- and heterochiral discrimination supported by quantum chemical calculations.

Keywords: Amphiphilic monoglycerols; Chiral discrimination; Grazing incidence X-ray diffraction (GIXD); Langmuir monolayer; Two-dimensional phase diagrams.