Conformational Behavior of N-Acylamino Acid Oil and N-Acylamino Acid Surfactant in Aqueous Solution

Abstract

The ¹³C- and ¹H- NMR spectra of isopropyl N-dodecanoylsarcosinate (SLIP) were measured in CDCl₃, CD₃OD and in sodium dodecanoylsarcosinate (Sar) aqueous solution. The existence of both cis and trans isomeric forms are observed for SLIP just as for Sar. The trans/cis ratio of SLIP in a dilute CDCl₃ solution is over 3.2, which is indicating that SLIP-trans conformation is more stable in an organic solvent. However, the population of SLIP-trans isomer decreases in aqueous solutions. This may be explained by the interaction of the ester group and N-acyl chain. Addition of SLIP to the water/Sar solution causes the conformational change of both SLIP and Sar. With increasing SLIP concentration, the population of SLIP-trans isomer increases and Sar-cis isomer, which is known to be preferred in the monomer state, also increases. This phenomenon is discussed in the context of the phase transition behavior of SLIP/Sar/water system. At high SLIP and Sar concentration, broadening of ¹H signal relative to 2-CH₂ of Sar occurs both for trans and cis forms, however, broadening of that of Sar N-CH₂ and N-CH₃ is observed only for trans. Analysis of conformation change by NMR was proved to be useful method for phase behavior analysis of an acylamino acid surfactant.