Virus passage through track-etch membranes modified by salinity and a nonionic surfactant

Why do viruses sometimes not pass through larger pores in track-etch filter s? Increasing the salinity (0.8 to 160 mM Na+) decreased phi X174 and PRD1 passage through track-etch polycarbonate membranes (sodium dodecyl sulfate coated but not polyvinylpyrrolidone coated) and PRD1 passage through polyes ter membranes. Undiminished passage when 0.1% Tween 80 was added implied th at nonionic virus adsorption occurred and indicated that high levels of sal inity decreased virus passage by decreasing electrostatic repulsion that pr evented adsorption.