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The Complete Phase Diagram of Monolayers of Enantiomeric <i>N</i>‑Stearoyl-threonine Mixtures with Preferred Heterochiral Interactions
Langmuir monolayers
of chiral amphiphiles are well-controlled model
systems for the investigation of phenomena related to stereochemistry.
Here, we have investigated mixed monolayers of one pair of enantiomers
(l and d) of the amino-acid-based amphiphile N-stearoyl-threonine. The monolayer characteristics were
studied by pressure–area isotherm measurements and grazing
incidence X-ray diffraction (GIXD) over a wide range of mixing ratios
defined by the d-enantiomer mole fraction xD. While the isotherms provide insights into thermodynamical
aspects, such as transition pressure, compression/decompression hysteresis,
and preferential homo- and heterochiral interactions, GIXD reveals
the molecular structural arrangements on the Ångström
scale. Dominant heterochiral interactions in the racemic mixture lead
to compound formation and the appearance of a nonchiral rectangular
lattice, although the pure enantiomers form a chiral oblique lattice.
Miscibility was found to be limited to mixtures with 0.27 ≲ xD ≲ 0.73, as well as to both outer edges
(xD ≲ 0.08 and xD ≳ 0.92). Beyond this range, coexistence of oblique
and rectangular lattices occurs, as is clearly seen in the GIXD patterns.
Based on the results, a complete phase diagram with two eutectic points
at xD ≈ 0.25 and xD ≈ 0.75 is proposed. Moreover, N-stearoyl-threonine was found to have a strong tendency to form a
hydrogen-bonding network between the headgroups, which promotes superlattice
formation