Surface activity and aggregation of pristine and hydrophobically modified inulin

Abstract

In search of more effective and lower impact surfactants, hydrophobic modification of polymers has emerged as a promising strategy for the substitution of low molecular weight surfactants. Inulin, a polysaccharide obtained from chicory roots, has links with the sustainable chemistry model. Furthermore, hydrophobically modified inulins (HMI) can be synthesized in aqueous media. This article reports on the surface activity and aggregation of several compounds with differences in hydrophobic alkyl length and backbone linkage. HMI significantly reduces (down to 30 mN m−1) water surface tension while inulin does not. Both inulins and HMI are shown to aggregate hierarchically at three levels, namely aggregates (<20 nm), flocks (<100 nm) and precipitates (>500 nm). The smaller aggregates, which in the case of inulins can be classified as monomers and in the case of HMI as micelles, have dimensions below 15 nm. The second level of aggregation corresponds to loose flocks with dimensions up to 100 nm and the third level corresponds to the precipitate constituted by aggregation of the flocks. Temperature and basic conditions affect the flock formation and kinetics without a strong influence on the smaller aggregates structure and size. Long term standing of both HMI and inulin solutions eventually precipitates. This precipitate may occupy a large volume but corresponds to a small amount of the solute. In the case of inulins, the precipitate has been found to have a degree of crystallinity that diminishes with increasing temperature.