Interfacial tension of a decomposed biopolymer mixture

Many biological systems as well as food products contain both proteins and polysaccharides, which are often thermodynamically incompatible. In this communication, measurements are presented of the interfacial tension in a segregated polysaccharide/protein mixture. The interfacial tension is an important quantity in determining the phase separation kinetics and the response to shear. As the protein/polysaccharide system, an aqueous gelatin/dextran mixture in 1.0 NI sodium iodide was chosen to suppress gelation and achieve a low density difference between the coexisting phases. First, the phase threshold was determined by making dilution lines. From the relative volumes of the coexisting phases, the location of the critical point was estimated. Interfacial tensions of phase-separated mixtures, varying in distance from the critical point, were measured using the spinning drop method. The interfacial tension close to the critical point was less than 1 muN/m, and it increased, in a nonlinear fashion, to a value of up to 20 muN/m farther from the critical point. For the scaling relation of the interfacial tension with the density difference and the polymer concentration difference, we found scaling exponents of 2.7 +/- 0.3 and 1.5 +/- 0.1, respectively, which are in agreement with the critical mean-field scaling exponents of 3 and 3/2, respectively.