Chiral Discrimination of a Gemini-Type Surfactant with Rigid Spacer at the Air−Water Interface

Abstract

Spontaneous separation of chiral phases was observed in the monolayers of a racemate of gemini-type twin-tailed, twin-chiral amphiphiles, (2R,3R)-(+)-bis(decyloxy)succinic acid and (2S,3S)-(-)-bis(decyloxy)succinic acid. The pressure-area isotherms of the interfacial monolayers formed at the liquid-air interface, and the 2D lattice structures studied through surface probe measurements revealed that the racemate exhibits a homochiral discrimination of the enantiomers in two dimensions. An enantiomeric excess (e,e) of 20% was sufficient to break the chiral symmetry at the air-water interface for a homochiral interaction. Langmuir monolayers on ZnCl2 and CaCl2 subphases manifested chiral discrimination with Zn2+ evidencing homochiral interaction with a chelate-type complex, whereas Ca2+ resulted in a heterochiral interaction forming an ionic-type complex. For the chiral asymmetric units, oblique and rectangular unit cells of the racemic monolayer had exclusive requirements of homo- and heterochiral recognitions for Zn2+ and Ca2+ ions, respectively. Monolayers transferred from the condensed phase at 25 mN/m onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. The emergence of homochiral discrimination was explained using the effective-pair-potential (EPP) approach.

Synthesis of Succinic Gemini Surfactants and the Effect of Stereochemistry on Their Monolayer Behaviors

“…Shankar et al [17,18] reported similar isotherms for (2R,3R)-(? )-bis(decyloxy)succinic acid, (2S,3S)-(-)bis(decyloxy)succinic acid and (±)-bis(decyloxy)succinic acid, but there were no maximum on their isotherms.…”

“…Furthermore, the stereochemistry at the point of attachment of spacer group would also be one of the important structural factors for the surface properties of gemini surfactants. In this decade, several studies on the correlation between the stereochemistry at the point of attachment of spacer group, physical properties and self-aggregation behavior of gemini surfactants have been reported [14][15][16][17][18][19][20][21], but more research is needed.…”

“…A more tightly packed monolayer was formed at the air-water interface with increase in hydrophobic alkanoyl chain length. Aside from those works, Shankar et al [16][17][18][19] found that enantiomerically pure tartaric acid-based surfactants form more stable vesicles and have a lower CMC compared to their racemic mixtures. They also reported the monolayer behavior of chiral gemini, (2R,3R)-(?…”

“…They also reported the monolayer behavior of chiral gemini, (2R,3R)-(? )-bis(decyloxy)-and (2S,3S)-(-)bis(decyloxy)succinic acids (otherwise, l-O,O 0 -bis(decyl)and d-O,O 0 -bis(decyl)tartaric acid, respectively), at the airwater interface [18] and observed the spontaneous separation of chiral phases in the monolayers of the racemate [17].…”

Surface dilatational properties of Gemini surfactants containing multiple hydroxyl groups

“…In a Gemini surfactant molecule, the spacer is the key structural element. The length [13], rigidity [10,14,15] and stereochemistry [16,17] of the spacer affect the selfassembly of Gemini surfactants. Once an adsorption film has been formed by a Gemini surfactant, the spacer is expected to affect the interfacial dilatational behaviour because the position of the spacer is close to the head group of the surfactant and is situated within the adsorption layer.…”