Direct measurements of the interaction between layers of insulin adsorbed on hydrophobic surfaces

Abstract

The forces acting between layers of insulin adsorbed on mica surfaces rendered hydrophobic by deposition of dioctadecyldimethylammonium bromide have been investigated. The forces have been determined after different adsorption times, and after dilution at different desorption times and pH values. The adsorption of insulin has a dramatic effect on the surface forces. The long-range attraction, which is present when no insulin molecules are adsorbed, disappears. Instead a repulsive double-layer force dominates the long-range interaction. An attractive force was in some cases observed at intermediate distances (6–12 nm). The actual value of this force was dependent on how fast the measurements were carried out, indicating that molecules in the interlayer region rearranged or were pushed out from between the surfaces slowly. At small separations a repulsive force caused by steric and dehydration effects predominates. The upper limit of the range of the hydration force between insulin coated surfaces is about 1 nm.