Mixed protein–polysaccharide interfacial layers: effect of polysaccharide charge distribution

The influence of the polysaccharide charge distribution on the structure, thickness, and charge reversal of the interfacial layers, formed by adsorbed positively charged protein and oppositely charged polysaccharide, has been investigated using a lattice-based self-consistent field (SCF) approach. We compare the adsorption behaviour of a uniformly charged polysaccharide model with that consisting of a short and a long block carrying different charge densities. For homogeneously charged polysaccharide we observe a resulting interfacial layer that is closer to a mixed protein + polysaccharide film, rather than a multi-layer. We also find that the maximum adsorption of polysaccharide occurs at an optimal value of its charge, above and below which the adsorbed amount decreases. In contrast, for heterogeneously charged chains, as their charge is increasingly located on the shorter block, a much thicker interfacial layer results. In such cases the weakly charged longer blocks extend well away from the surface into the solution. The interfacial film begins to resemble a multi-layer with a primary protein and a distinct secondary polysaccharide layer. When the weakly charged long blocks still have a sufficient amount of negative charge, we also observe a reversal of the sign of surface potential from a positive to a negative value. Our SCF calculated values for the reversed surface potential are of the order of −25 mV, in good agreement with several experimental results involving ζ-potential measurements on particles covered with such protein + polysaccharide films.