Self-Organization Properties and Microstructures of Sodium N-(11-Acrylamidoundecanoyl)-L-valinate and -L-threoninate in Water Sumita Roy and Joykrishna Dey  Department of Chemistry, Indian Institute of Technology, Kharagpur –721 302, India Received April 25, 2005; E-mail: joydey@chem.iitkgp.ernet.in A new class of N-acylamino acid surfactants, sodium N-(11-acrylamidoundecanoyl)-L-valinate (SAUV) and -L- threoninate (SAUT), were synthesized and characterized in aqueous solutions. The self-assembly properties were studied by surface tension, fluorescence probe, light scattering, and microscopic techniques. The interfacial properties of the amphiphiles have been investigated at the air/water interface. The critical aggregation concentration (cac) and free energy change of aggregation for both amphiphiles were determined. The results of fluorescence probe and surface ten- sion studies have indicated that initially flat bilayer structures are formed, which upon a further increase of surfactant concentration transform into closed vesicles. The pH and temperature dependence of the bilayer aggregates have been studied. The role of intermolecular hydrogen bonding between amide groups upon aggregation towards microstructure formation in solution has been elucidated. The micropolarity of the hydrophobic domains of the bilayer self-assemblies was estimated by fluorescence probe methods. Dynamic light scattering measurements were performed to determine the mean size of the aggregates. The circular dichroism spectra of SAUV and SAUT suggested the formation of chiral aggregates in dilute solution. The transmission electron micrographs revealed the presence of closed vesicles and twisted ribbons in the aqueous solutions of the amphiphiles. Optical microscopic images also showed the existence of twisted ribbons and rope-like helical structures in the case of SAUV. The spontaneous self-assembly of molecules into supramo- lecular architectures is a result of various noncovalent interac- tions such as hydrophobic, hydrogen-bonding, electrostatic, dipole–dipole, and van der Waals interactions. The hydropho- bic interaction between long hydrocarbon chains of amphiphil- ic molecules is essential in the formation of supramolecular aggregates. The geometrical characteristics of surfactant mo- lecules determine the type of self-assembly structure taken in solution. Depending upon the packing parameter, the surfac- tant can self-assemble to form spherical, rod-like micelles, bi- layers, vesicles/liposomes, lamellar structures, and so forth in solution. 1 It has been found that a change in the headgroup and the hydrocarbon tail affects the morphology of the self-assem- blies. Vesicles are usually formed by double-chain surfactants. However, recent reports have shown that vesicles are also formed from single-tailed amphiphiles. 2,3 In fact, some fatty acid soaps form vesicles upon partial protonation. 4–6 On the other hand, introduction of electric charges can increase the average curvature, facilitating formation of vesicles from the lamellar phase. 7,8 In addition, short-range attractive interac- tions such as hydrogen bonding can also be a driving force in the formation of bilayer self-assemblies. 9–13 For example, hydrogen-bonding interaction between the protonated and nonprotonated headgroups of amine oxides 9 and fatty acid soaps, 4–6 and amide hydrogen bonding in N-acylamino acid surfactants (NAAS) 14–19 have been proposed to explain the mechanism of vesicle formation. Recently, optically active NAAS have attracted considerable attention because of their chirality, which is an important phe- nomenon in nature. 17a,20 Further, NAAS are currently used as detergents, foaming agents, and shampoos as they are mild, nonirritating to human skin, and easily biodegradable. 21 They have also been shown to be useful in stereoselective synthe- sis. 22 One of the most important properties of these supramolec- ular assemblies is chiral recognition. The chiral recognition properties of the monolayers of NAAS have recently been re- ported. 20 On the other hand, the chiral discrimination by these surfactants in solution has been exploited by separation scien- tists. 23,24 NAAS are a class of surfactants that show interesting aggregation properties in solution. NAAS have been reported to self-organize in water 15–18,20a and also in organic solvents 13,14,25 to form various types of supramolecular structures. These supramolecular assemblies generate bilayers in water with shapes like planar membranes, tubules, helices, ribbons, and rods. It has been demonstrated that the chirality of amphiphilic molecules imparts stability to the hydrophobic aggregates. In this work, we investigate the aggregation behavior of sodium N-(11-acrylamidoundecanoyl)-L-valinate (SAUV) and -L-threoninate (SAUT) (see Chart 1 for structures). The sur- face and interfacial properties of the surfactants have also been compared. Fluorescence probe methods have been used to characterize the self-assemblies. The mean size of the self- assemblies has been measured by the dynamic light scattering H N O O - O O H N R Na + R = CH(CH 3 ) 2 , SAUV R = CH(CH 3 )OH, SAUT Chart 1. General chemical structure of SAUV and SAUT. Ó 2006 The Chemical Society of Japan Bull. Chem. Soc. Jpn. Vol. 79, No. 1, 59–66 (2006) 59 Published on the web January 13, 2006; DOI 10.1246/bcsj.79.59