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Influence of linkage type (ether or ester) on the monolayer characteristics of single-chain glycerols at the air–water interface?

Physical Chemistry Chemical Physics Pub Date: 2020-09-25 DOI: 10.1039/D0CP04153J

Abstract

O-1-Alkylglycerols are ubiquitous constituents in various biological materials but their biological significance is still largely unknown. So far, reports about the striking role of structural features on the interfacial properties of 1-O-alkylglycerol monolayers are quite rare. Therefore, in the present paper 1-O-alkylglycerol monolayers are comprehensively characterized on mesoscopic and molecular scales in the accessible ranges of temperature and surface pressure. Two Bragg peaks found for the condensed monolayer phase of the racemates at all pressures investigated indicate an orthorhombic structure with NN-tilted alkyl chains at lower pressures and NNN-tilted chains at higher pressures. In contrast to the continuous change of the tilt angle, as observed for many amphiphile monolayers, the tilt angle in 1-O-alkyl-rac-glycerol monolayers shows a jump-like transition from the L₂ (NN tilt direction) to the Ov phase (NNN tilt direction) with the consequence of different slopes of 1/cos(t) vs. π in the two phases. This is the most striking difference to the behavior of the corresponding ester compound 1-stearoyl-rac-glycerol, having an oblique phase between the two orthorhombic phases L₂ and Ov at low temperatures. The generic phase diagrams of the 1-O-alkyl-rac-glycerol and 1-acyl-rac-glycerol monolayers are essentially different. The influence of chirality on the monolayer structure is weak and becomes even weaker at high temperatures (rotator phases) and high lateral compression. The GIXD results of the enatiomeric pure compounds show the expected oblique lattice structure characterized by three Bragg peaks at almost all lateral pressures measured. The results of the GIXD studies are complemented by other monolayer characteristics such as π–A isotherms and mesoscopic domain topographies. The π–A isotherms of 1-O-alkyl-rac-glycerols are similar to those of the corresponding 1-acyl-rac-glycerols indicating that the change from the ester linkage to the ether linkage does not affect significantly the thermodynamic features. However, pronounced differences in the topological structure are observed. 1-O-hexadecyl-rac-glycerol monolayers form three-armed domains whereby each arm is subdivided into two segments with different molecular orientation. Also fascinating chiral discrimination effects are observable, demonstrated in the case of S-enantiomers by always clockwise curved spirals at the domain periphery. The 1?:?1 racemic mixtures exhibit both clockwise and counterclockwise curved spirals.