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

In this work, succinic gemini surfactants, dl- and meso-2,3-bis(alkyl)succinic acids (alkyl: C6H13-C13H27), were successfully synthesized by oxidative coupling of enolates of fatty acid tert-butyl esters with copper(II) bromide followed by treatment with CF3COOH. Focusing on the influence of stereochemistry (dl- and meso-) of succinic geminis, their monolayer behaviors at the air-water interface were explored using surface pressure-area (I -A) isotherms, the compression modulus of monolayers (epsilon (s)), and Brewster angle microscope (BAM) analysis. meso-2,3-Bis(undecyl)succinic acid showed a unique isotherm where the surface pressure drastically decreased at A = similar to 0.56 nm(2) (I = 21.9 mN m(-1)) regardless of compression rates and subphase temperatures, while dl-isomer showed the common isotherm of gas -> liquid-expanded -> liquid-condensed phase transitions. BAM analysis on meso-2,3-bis(undecyl)succinic acid films at the air-water interface showed that small islands of aggregates appear just after the maximum pressure (A = similar to 0.56 nm(2)), and on further compression needle-shaped assemblies appear that can grow in size. It was reasonably concluded that hydrophobic interactions can operate more effectively in meso-isomers than in dl-isomers, and that meso-molecules can 'jump up' to cause a transition from monolayer to bilayer. This is the first finding of the 'jumping-up' phenomenon of gemini surfactants having meso-stereochemistry.