Effect of chirality on monoacylglycerol ester monolayer characteristics: 3-monostearoyl-sn-glycerol

Abstract

The effect of chirality on the thermodynamic behavior, the morphological features, and the 2D lattice structures of 3-monostearoyl-sn-glycerol monolayers is studied. The present study focusses on the influence of the alkyl chain length on the chiral discrimination. Surface pressure-area (π-A) isotherms, Brewster angle microscopy (BAM), and particularly, grazing incidence X-ray diffraction (GIXD) are the experimental basis of the presented results. The π-A isotherms of the enantiomeric 3-monostearoyl-sn-glycerol monolayers measured between 25 and 38 °C resemble those of the racemic 1-monostearoyl-rac-glycerol monolayers, thus indicating small energetic differences between the enantiomeric and the racemic forms. The absolute ΔS values increase as the temperature decreases and thus, the ordering of the condensed phase increases at lower temperatures. The extrapolation to zero ΔS provides a critical temperature T of 42.1 °C (315.3 K), above which the monolayer cannot be compressed into the condensed state. Despite the great tendency of the 3-monostearoyl-sn-glycerol domains to develop irregular deviations in shape and inner texture, regular domains similar to those of the racemic monoacylglycerol esters are also formed. GIXD measurements performed over a large range of lateral pressures at four different temperatures (5, 10, 15 and 20 °C) indicate the dominance of the chiral nature. Contour plots with three clearly separated diffraction signals are observable in a large pressure range which is shifted to higher lateral pressures with increasing temperature. The comparison with the contour plots of the homologous 3-monopalmitoyl-sn-glycerol monolayers reveals the stronger dominance of the chiral nature with increasing alkyl chain length and thus, demonstrates the stronger influence of the lattice symmetry. The lattice data obtained by fitting the contour plots with 3 or 2 peaks demonstrate the resemblance to orthorhombic structures with NN tilted molecules at low pressures and NNN tilted molecules at high pressures. The temperature and alkyl chain length dependence of the distortion at zero tilt angle, d, of the enantiomeric and racemic monoacylglycerols shows an increased influence of chirality on the lattice distortion for the shorter-chain compound (equivalent to increased temperature). This demonstrates that lattice distortion and lattice symmetry are differently influenced by chirality.

Strong Headgroup Interactions Drive Highly Directional Growth and Unusual Phase Co-Existence in Self-Assembled Phenolic Films

“…Analysis of the distortion as a function of sin 2 t (tilt angle) enables extrapolation of d 0 , the distortion at the zero tilt angle. ,− This analysis for ODG shows a nonzero value of d 0 at 0, indicative of the herringbone arrangement , of the alkyl chains (Figure S3) which also correlates well with the high Q xy position for the peak at 1.64 Å –1 . The peaks observed at Q xy 0.96 and 1.86 Å –1 correspond to the [1,0] r and [1,2] r peaks, respectively, with the peaks at Q xy 1.28 Å –1 and Q xy 1.63 Å –1 being indexed as [1,1] r and [0,2] r , where the “r” subscript denotes the rectangular cell indices (Figure S4 for indexed peak).…”

Effect of the Alkanoyl Group Position at the Glycerol Backbone on the Monolayer Characteristics Demonstrated by 2-Monopalmitoyl-rac-glycerol

“…As shown in previous papers, there is considerable diversity in the mesoscopic domain topography between the monoalkanoylglycerol esters with the aliphatic chain positioned at one end of the glycerol backbone and those with the aliphatic chain in the middle of the glycerol backbone (2-position). Previous BAM studies of 2-monopalmitoyl- rac -glycerol monolayers have shown that at temperatures ≥13 °C the contrast between the fluid and the condensed phase is very low and, only at the best, the condensed phase appears as diffuse patterns . A correlation was suggested between the disordered packing of alkyl chains observed in the diffraction patterns and the absence of long-range orientational order.…”

“…Previous BAM studies of 2-monopalmitoyl-rac-glycerol mono-layers have shown that at temperatures ≥13 °C the contrast between the fluid and the condensed phase is very low and, only at the best, the condensed phase appears as diffuse patterns. 13 A correlation was suggested between the disordered packing of alkyl chains observed in the diffraction patterns and the absence of long-range orientational order. In the present paper, attention was extended particularly to the monolayer characteristics of 2-monopalmitoyl-rac-glycerol in the temperature range below 13 °C.…”

“…Besides fatty acids, the monoalkanoylglycerolesters are the most studied amphiphiles in the bulk and at the air–water interface. Palmitoyl- and stearoyl-glycerol, as the most used homologues of saturated monoalkanoylglycerols for practical applications, have been intensively studied. − Systematic studies of the influence of the chemical structure as well as chirality effects on structural properties provide general information on the two-dimensional characteristics. − In previous papers, racemic 1-monopalmitoyl- rac -glycerol and 1-monostearoyl- rac -glycerol monolayers have been used allowing conclusions on other homologues 1-monoalkanoyl- rac -glycerol monolayers. The studies of the two-dimensional lattice structures in the accessible temperature and pressure regions resulted in the construction of the generic π– T phase diagram of monoalkanoylglycerolester monolayers.…”