Microrheology to Understand the Viscosity Behavior of a Sophorolipid Biosurfactant

Jochen Kleinen,Hacer Yalcinkaya,Jan Langwald,Joachim Venzmer

doi:10.3390/colloids6010003

Abstract

The microstructure of the aqueous solutions of purified acidic Sophorolipid (SL) has previously been studied using highly sophisticated methods such as SANS and Cryo-TEM. We were interested in whether (a) the main findings also apply to commercially available SL (which is a mixture of acidic and lactonic SL) and (b) more readily available methods such as DLS can be used to gain insight into the molecular aggregation of SL. Our work was motivated by the increasing interest in biosurfactants for applications in personal and household care. Moreover, the origin behind the more or less lack of rheological response to changes in pH is of practical relevance, as it is somewhat unusual for a carboxylate-group containing surfactant. By using DLS microrheology, we could elucidate the aggregation structure and dynamics of the surfactant on a microscopic scale. Surprisingly, the different degrees of protonation only impacted the microscopic properties such as exchange kinetics and the plateau values of the storage moduli.

Highlights

Biosurfactants are surface-active materials produced by microorganisms
The unusual behavior of SL is attributed to the double hydrophilic structure of the molecule, carrying both a sugar group and a carboxylic acid group on each side of the hydrophobic part
We could show by microrheology measurements that the structure and the dynamics of the surfactant change with pH value and temperature on a microscopic scale

Summary

Introduction

Biosurfactants are surface-active materials produced by microorganisms. Various beneficial properties have been reported, such as remarkable surface activity, dispersing and foaming [5,6,7]. It is being increasingly used in cosmetic, food, medical and detergency applications [1,8,9]. The mixture of lactone (the fatty acid moiety forms a closed lactone ring) and acidic SL (the fatty acid has a free carboxylic acid end) (see Supplementary Information Figure S1) reveals a synergism leading to improved performance in interfacial activities [6]. SL might serve as a platform for surfactants of the generation, as several derivatives have been reported and all the different types (including bola form) behave distinctly different [12,13]

Methods

Results

Conclusion