Effect of xanthan gum on the interfacial and foam properties of an acidic sophorolipid solution: experimental and theoretical studies

Abstract

The interaction mechanism between surfactants and polymers has important guiding significance in the field of high-performance fluid design. However, research on the interactions between biological surfactants and microbial polysaccharides remains limited. In this paper, the effect of xanthan gum on the interfacial properties of an acidic sophorolipid solution, such as surface tension, viscosity, rheology and foam properties, was systematically studied by means of experimental and theoretical methods. The results showed that the addition of 0.1 wt% xanthan gum had little effect on the critical micelle concentration of acidic sophorolipid, and increased the contact angles of the aqueous solution with PTFE and PP. At the same time, the interaction between acidic sophorolipid and xanthan gum reduced the bulk viscosity of the xanthan gum solution. Furthermore, the shear-thinning characteristics of the mixed system did not change, but the yield value gradually decreased with increasing acidic sophorolipid concentration. Significantly, the addition of xanthan gum slightly reduced the foaming property of acidic sophorolipid but significantly improved the stability of the foam. Foam counting and image analysis showed that xanthan gum not only made the foam small and evenly distributed, but also inhibited the aggregation of foam effectively as acidic sophorolipid concentration increased. Finally, in order to reveal the molecular mechanism underlying the interaction between xanthan gum and acidic sophorolipid, DFT calculations were used to simulate their interaction. The results indicated that robust hydrogen bonding interactions involving carboxyl and hydroxyl groups exist between acidic sophorolipids and xanthan gum, thereby contributing to the enhancement of foam stability. These findings would provide useful references for biosurfactant applications in complicated formulations across diverse fields.