Birefringent physical gels of N-(4-n-alkyloxybenzoyl)-L-alanine amphiphiles in organic solvents: the role of hydrogen-bonding.

A new class of amphiphiles, N-(4-n-alkyloxybenzoyl)-L-alanine was designed and synthesized. These amphiphiles have been shown to form thermoreversible gels in organic solvents such as aromatic hydrocarbons, cyclohexane, and chlorinated hydrocarbons at room temperature. The effects of amide functionality, chain length of the hydrocarbon tail, and the chirality of the head group of the amphiphiles on the ability to promote gelation in organic solvents have been studied. The n-tetradecyl derivative showed the best gelation ability, whereas the amphiphile with DL-alanine as the head group formed weak organogels. The 4-dodecyloxybenzoic-1-carboxyethyl ester derivative in which the amide group is replaced by an ester group also formed weak organogels at a slightly lower temperature (293 K). The gelation number and the gel melting temperature of the gelators in different solvents were determined. The rheological measurements suggested that the organogels of n-tetradecyl derivatives are stronger than those of amphiphiles containing n-dodecyl chains. Also the organogels of the amphiphiles, except the one with an ester group, were found to have gel-to-sol transition temperatures, T(gs), higher than room temperature (approximately 303 K), which increased with the increase of chain length and total concentration of the gelator. SEM pictures of the gels show fibrous structures. Small-angle XRD and optical microscopy were also employed to characterize the gels. The organogels of alanine derivatives, except that of 4-dodecyloxybenzoic-1-carboxyethyl ester, showed optical birefringence. The mechanism of gelation was studied using (1)H NMR and FTIR spectroscopy. Hydrogen-bonding between -CO(2)H groups as well as pi-pi interactions were found to be important for the gelation process.