Molecular basis for the emulsifying properties of sugar beet pectin studied by atomic force microscopy and force spectroscopy

Recently AFM has been used to measure the interactions in aqueous media between a deformable oil drop attached onto an AFM cantilever and another drop on a substrate. In this study this method has been applied to oil droplets immersed in a solution of a polysaccharide extract having emulsifying properties, sugar beet pectin (SBP). In parallel with the force spectroscopy studies, images of the SBP interfacial films were obtained, in order to probe the effects of interfacial structures on the interactions between the drops. The influence of the SBP concentration in the bulk phase was studied: at a low bulk SBP concentration, where the adsorbed SBP layer is relatively flat, non-adsorbed SBP in the bulk solution gives rise to a depletion interaction between the drops. Upon increasing the SBP concentration, a hysteresis appears in the interactions between the drops. It is hypothesised that this effect is induced by the strong liquid structural correlations occurring within the liquid film separating the droplets. This conjecture is supported by a theoretical description which incorporates depletion interactions into a model for the interactions between deformable droplets. At high SBP concentration a strong repulsive force, attributed to steric repulsion between the thick SBP layers adsorbed onto the drops, is observed. Under different bulk conditions polymer bridges were observed to be formed between the SBP-coated droplets. The results demonstrate how AFM can be used to probe different types of interactions between deformable oil drops in aqueous media at the molecular level. When combined with theoretical models, this approach will advance the understanding of molecular mechanisms that govern the stability of emulsions.