Charging and aggregation behavior of silica particles in the presence of lysozymes

To gain insight into the colloidal stability in the presence of proteins, we measured the electrophoretic mobility and aggregation rate constant of silica particles coated with lysozymes, and the adsorbed amount of lysozymes on the silica. We also examined model analyses, which are based on the Derjaguin, Landau, Verwey, and Overbeek theory with the effect of charge heterogeneity, to discuss the aggregation of lysozyme-coated silica. Our results show that lysozymes enhance the aggregation of silica when the lysozyme-coated silica is near the isoelectric point. When the adsorbed amount of lysozyme is low, the effect of charge-patch attractive force promotes the aggregation of silica. The effect of charge heterogeneity weakens with the increase of adsorbed amount of lysozyme. Our model which takes account of the effect of charge heterogeneity can capture the trend of the aggregation of silica in the presence of lysozyme qualitatively, but there are also large quantitative discrepancies between the theoretical prediction and experimental results. Further improvement is required to describe realistic charge heterogeneity and the effect of the surface coverage of lysozyme on the silica.