Viscoelastic properties of interfacial lignosulfonate films and the effect of added electrolytes

New evidence is presented, which confirmed interfacial gelling of lignosulfonates in presence of di- and trivalent cations. In this article, the viscoelastic properties of lignosulfonate films at the water-xylene interface were studied by dilatational interfacial rheology and interfacial shear rheology. Both techniques showed that increasing lignosulfonate concentration would first increase and then decrease the interfacial modulus. The same trend was observed for increasing salinity. The maximum interfacial modulus corresponded with lignosulfonate aggregation or precipitation and accounted for the best emulsion stability. The film strength increased progressively with the cation charge number. It was argued that multivalent cations provided intermolecular bridging between lignosulfonate molecules, which increased film strength and led to gelling. The decrease of interfacial film strength at high salinity was explained by two mechanisms: (1) For sodium cations, the polyelectrolyte contraction at high ionic strength yielded screening of the functional groups, which are deemed responsible for attractive interactions between lignosulfonate molecules or aggregates. (2) For calcium and aluminum cations, precipitation would reduce the effective bulk concentration, yielding a lower surface coverage. Modelling of the interfacial properties was conducted in addition, which showed that lignosulfonate adsorption was not diffusion-controlled and that lignosulfonate aggregation was affecting the adsorption process. In conclusion, our results revealed a more detailed picture of the mechanisms, which govern the interfacial behavior and properties of lignosulfonates.