Protein-polysaccharide complexes as surfactants

A study has been made of surface pressure (pi) and maximum shear stress (Ps) (at a constant shear rate) of decane-water interface on which bovine serum albumin (BSA) and its complexes with dextran sulfate (DS) were adsorbed. The pi values have been monitored by the Wilhelmy plate technique. The Ps values have been measured with a surface viscoelastometer. The changes of pi with time for BSA and BSA-DS complexes in general takes place in two stages which can be attributed to two corresponding limiting factors: diffusion of molecules (or particles) to the interface, and rearrangements of adsorbed molecules. The diffusion-controlled stage for the BSA-DS complexes was found to be several times as long as for BSA. The relaxation time of the second stage is a function of protein concentration Cp, for the complexes having maximum value at Cp = 10(-3)-10(-2) g/l. These maxima are coincident with midpoints of the surface pressure isotherms. The latter one of the BSA-DS complexes is shifted towards greater protein concentrations in comparison with that of BSA. The Ps assumes maximum values at Cp corresponding to the midpoints of surface pressure isotherms. These values for the complexes are 15-20 times greater the that for free BSA.