Anomalous colloidal stability of protein coated polystyrene latex beads studied by small angle light scattering

Results of small angle laser light scattering experiments carried out on polystyrene latex beads coated with lysate of Plasmodium falciparum (Pf) antigen (Ag) and human seropositive sera (10:1 ratio) are reported for various NaCl concentrations (0-300 mM). The protein coated beads showed time-dependent coagulation. The normalised intensity of scattered light I(s)(t)/I(0) showed I(s)(t)/I(0)=1+(Gammat)(delta) behaviour with the coagulation rate, Gamma and exponent, delta showing anomalous dependence on NaCl concentration. The coagulation rate exhibited strong increase up to NaCl concentration of 50 mM, above this and up to 300 mM the coagulation rate was found to remain independent of NaCl concentration yielding non-DLVO behaviour. The same was true for delta which increased from 1.04+/-0.06 to 6.94+/-0.07 as NaCl concentration was raised from 0 to 50 mM. Above 50 mM it remained constant with delta=6.94+/-0.07. Results are discussed through Smoluchowski aggregation kinetics and theoretical construction of interparticle interaction potentials relevant to our problem.