Surface charge of acidic sophorolipid micelles: effect of base and time

Abstract

Acidic sophorolipids, SL–COOH, bio-derived glycolipids, are known to form micelles whose interactions vary as a function of pH. Upon partial ionization of the COOH group, intermicellar interactions take place. Here, we explore the nature of these interactions by using small angle neutron scattering (SANS) on SL–COOH solutions to which increasing amounts of NaOH are added. The effect of the nature of the base is also explored by replacing NaOH with aqueous NH3, KOH and Ca(OH)2. Time effects up to 36 days are also discussed. All SANS data have been successfully fitted using an appropriate model of core–shell prolate ellipsoids of revolution with an interaction potential, U(r), which combines hard-sphere and screened Coulomb (described by a repulsive Yukawa potential) potentials. Modelling quantifies the effect of the base in terms of micellar size, effective surface charge and interfacial hydration, thus showing the possibility of tuning them at will.