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

The influence of the position of the aliphatic chain at the glycerol backbone has been basically unknown. Solely the results of 2-monopalmitoyl-rac-glycerol obtained at ≥13 °C indicated an essential influence of the position of the palmitoyl group at the glycerol backbone, substantiated by a disordered packing of the alkyl chains. Therefore, the present study extends the comprehensive characterization of 2-monopalmitoyl-rac-glycerol monolayers to the low-temperature range for highlighting the effect of the position of the aliphatic chain at the glycerol backbone of monoalkanoylglycerolester monolayers. Systematic studies of the thermodynamic behavior, the morphological features, and the 2D lattice structures of 2-monopalmitoyl-rac-glycerol monolayers at ≤10 °C allow useful conclusions. Large differences between the π-A isotherms of 1- and 2-monopalmitoyl-rac-glycerol monolayers and their thermodynamic analysis indicate that the change of the substitution from position 1 to position 2 of glycerol backbone is consistent with a shortening of the alkyl chain by roughly two CH groups. Quantum chemical calculations of the molecular structure and packing calculations are in reasonable agreement with the thermodynamic results. Considerable diversity in the mesoscopic domain topography exists between the monoalkanoylglycerol esters with the aliphatic chain positioned at the end of the glycerol backbone (1-position) and those with the aliphatic chain in the middle of the glycerol backbone (2-position). The new faceted shape of the 2-monopalmitoyl-rac-glycerol domains, before they develop branched fractal-like structures at the edges, is the essential difference to the round or cardioid-like 1-monoalkanoylglycerol domains. In the low-temperature range, well-defined orthorhombic lattice structures exist at all surface pressures. Comparing all GIXD data from the three racemic compounds (1-monostearoyl-rac-glycerol, 1-monopalmitoyl-rac-glycerol, and 2-monopalmitoyl-rac-glycerol) shows that 2-monopalmitoyl-rac-glycerol behaves as 1-monomyristoyl-rac-glycerol, i.e., the shift from position 1 to position 2 of the glycerol backbone is equivalent to a shortening of the alkyl chain.