Dirhamnolipid ester - formation of reverse wormlike micelles in a binary (primerless) system.

David Liese,Gebhard Haberhauer,Jochen Kleinen,Hans Henning Wenk,Xin Lu

doi:10.3762/bjoc.16.232

Abstract

We report new dirhamnolipid ester forming reverse wormlike micelles in nonpolar solvents without the addition of any primer. Therefore, these compounds represent a rare case of a binary system showing this gel-like behavior. In this study, the influence of the concentration of the rhamnolipid ester and the ester alkyl chain length on the rheological properties of the reverse wormlike micelles in toluene was investigated in detail. Highly viscoelastic solutions were obtained even at a relatively low concentration of less than 1 wt %. The phase transition temperatures indicate that the formation of reverse wormlike micelles is favored for dirhamnolipid esters with shorter alkyl chain lengths. Oscillatory shear measurements for the viscoelastic samples reveal that the storage modulus (G') and the loss modulus (G'') cross each other and fit the Maxwell model very well in the low-ω region. As is typical for wormlike micelle systems, the normalized Cole–Cole plot of G''/G''max against G'/G''max was obtained as a semicircle centered at G'/G''max = 1. The formation of network structures was also verified by polarized light microscopy. The sample was birefringent at ambient temperature and anisotropic at an elevated temperature. Differential scanning calorimetry analysis yielded a transition enthalpy of about ΔHSG/GS = ±7.2 kJ/mol. This value corresponds to a strong dispersion energy and explains the formation of the highly viscous gels by the entanglement of wormlike micelles through the interaction of the alkyl chains.

Highlights

Surfactants have both hydrophilic and hydrophobic groups and are amphiphilic molecules
In the present contribution we report the formation of viscoelastic reverse wormlike micellar solutions of RL ester with nonpolar solvents without the addition of any primer
Due to the biotechnological production, the alkyl chain length in the lipid part varies from C8 to C12, with a little portion of double bonds

Summary

Introduction

Surfactants have both hydrophilic and hydrophobic groups and are amphiphilic molecules. The phase behavior and rheological properties were studied depending on the concentration and alkyl chain length of the rhamnolipid ester and on the temperature. The solubility of the dirhamnolipid esters was investigated in various solvents (75 mg/mL) with respect to the alkyl chain length. The more polar short chain length dirhamnolipid esters and 7 form a gel-like texture in toluene, indicating the formation of a network structure of the surfactant molecules.

Results

Conclusion