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

Abstract

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 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.

Figure 1

Chemical structures of N-stearoyl-l-threonine and N-stearoyl-d-threonine amphiphiles. The chiral center of interest is indicated with a red star. The other chiral C atom is indicated with a filled blue circle.

Figure 2

(A and B) Temperature-dependent pressure–area isotherms of an N-stearoyl-threonine monolayer with x_D = 0.9 at compression (A) and at expansion (B). The hysteresis between compression and decompression curves is clearly visible. (C) The difference in transition pressure between compression and expansion, ΔΠ, as a function of x_D. The values were taken at A_a = 40 Ų and averaged over four temperatures. The error bars are the standard derivation of the data points at different temperatures. The dotted straight line represents a linear fit to the data points.

Figure 3

(A) Temperature dependence of the main phase transition pressure Π_t of N-stearoyl-threonine mixed monolayers for various x_D. Dashed straight lines indicate linear fits to the experimental data points. (B) Characteristic temperature T₀ as a function of x_D. The dashed straight line indicates a linear fit to the first three data points.

Figure 4

(A) GIXD pattern of an N-stearoyl-threonine mixed monolayer with x_D = 0.7 (Π = 35 mN/m). (B) GIXD pattern of an N-stearoyl-threonine mixed monolayer with x_D = 0.8 (Π = 25 mN/m). (C and D) Pressure dependence of the chain cross-sectional area A₀ (C) and of the tilt angle t (D).

Figure 5

Q_z-integrated intensity vs Q_xy (data points) and the corresponding fits (black lines) for an N-stearoyl-threonine mixed monolayer with x_D = 0.5 at Π = 5 mN/m (A), with x_D = 0.8 at Π = 25 mN/m (B), and with x_D = 0.9 at Π = 10 mN/m (C).

Figure 6

Schematic phase diagram of mixed monolayers of the d- and l-enantiomers of N-stearoyl-threonine. The phase transition pressures (Π_t) determined by isotherm experiments are indicated with solid blue dots. The LC phase of the enantiomers has an oblique lattice structure (denoted as “obl” in the figure), whereas the LC phase of the racemate and of mixtures with 0.27 ≲ x_D ≲ 0.73 has a rectangular lattice structure. The LC phase in the miscibility gap (0.73 ≲ x_D ≲ 0.92), denoted as “2Ø” is characterized by the coexistence of rectangular and oblique lattices. The light yellow and light blue areas indicate the coexistence of a disordered LE phase with a rectangular LC phase or an oblique LC phase, respectively. Red upright triangles, green inverted triangles, and orange stars indicate the points where the GIXD data were collected and revealed a rectangular phase, an oblique phase, or phase coexistence, respectively. The diagram has been symmetrically extended to cover the entire range 0 < x_D < 1.