The Complete Phase Diagram of Monolayers of Enantiomeric N-Stearoyl-threonine Mixtures with Preferred Heterochiral Interactions

Autor: Tetiana Mukhina, Lars Richter, Dieter Vollhardt, Gerald Brezesinski, Emanuel Schneck
Rok vydání: 2022
Předmět:
Zdroj: Langmuir
Langmuir 38(41), 12521-12529 (2022). doi:10.1021/acs.langmuir.2c01936
ISSN: 1520-5827
0743-7463
Popis: Langmuir 38(41), 12521 - 12529 (2022). doi:10.1021/acs.langmuir.2c01936
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-acidbased 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 $x_D$. 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 ≲ $x_D$ ≲ 0.73, as well as to both outer edges ($x_D$ ≲ 0.08 and $x_D$ ≳ 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 $x_D$ ≈ 0.25 and $x_D$ ≈ 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.
Published by ACS Publ., Washington, DC
Databáze: OpenAIRE
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