Yield stress of mixed suspension of silica particles and lysozymes: The effect of zeta potential and adsorbed amount

To investigate the interactions between colloidal particles in the presence of oppositely charged proteins, the yield stress of a mixed suspension of silica particles and lysozymes was measured as a function of lysozyme dose and pH. Further, the corresponding surface properties of silica particles covered with lysozymes were determined by measuring the adsorbed lysozyme amount and zeta potential. The present results indicated that an increase in the adsorbed lysozyme amount increases the zeta potential of silica particles from negative to positive through an isoelectric point. As expected from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the maximum value of yield stress is obtained around the isoelectric point and the yield stresses decrease with an increase in the zeta potential magnitude. However, the maximum yield stress depends on the pH, and yield stresses at the same zeta potential magnitude are different depending on the zeta potential sign even if the pH values are similar. That is, the relationship between yield stress and zeta potential is asymmetric with respect to the isoelectric point. These asymmetric results of the yield stress indicate the existence of non-DLVO forces, such as patch-bridging attraction and lateral repulsion between adsorbed lysozymes, affected by the adsorbed amount of lysozyme. Our analysis suggested that the relative adsorbed amount, defined as the ratio of the adsorbed amount to the maximum adsorbed amount, can be used as a parameter to explain the asymmetric feature of the yield stress with respect to the zeta potential. In addition to the zeta potential, the surface occupation of adsorbed substances is critical in controlling particle-particle interactions.