Influence of adsorbed proteins on the stability of polystyrene latex particles

Abstract

Flocculation experiments on polystyrene latex (PSL) with human serum albumin (HSA) and human fibrinogen (HFb) have been performed above and below the iso-electric points (i.e.p.) of the proteins. The stability of the proteins (HFb, HSA) in solution has been determined as a function of salt concentration (NaCl, BaCl₂, La(NO₃)₃) and pH. Using a stopped flow spectrophotometer the rate constant of flocculation (or coagulation)k₁₁ has been measured at different protein and salt concentration (BaCl₂, NaCl). A model is proposed and tested to explain the enhancement of k₁₁ above the value for bare PSL when bridging occurs at pH-values above the i.e.p. of the proteins. The observed enhancement of k₁₁, being 20–30% for HSA and 50–60% for HFb, is a result of two effects: reduced hydrodynamic interaction between the flocculating particles and increased effective collision radius of the latex particles when they are partially covered with the protein. Steric stabilization by proteins occurs only in “good solvent” conditions for these proteins. At pH-values below the i.e.p. of the proteins flocculation is observed in the absence of salt. Measurements of electrophoretic mobilities of the latex particles as a function of the protein concentration demonstrated that this flocculation is mainly due to charge neutralization by adsorbed protein molecules. Restabilization by charge inversion of the latex particles occurs at relatively low protein concentration.