Adsorption of aqueous copper chlorophyllin mixtures on model surfaces

Measured were the colloidal and adsorption properties of copper chlorophyllin (CuChl), a complex mixture derived from chlorophyll that is used as a food additive and in other applications. CuChl has an “apparent solubility” (it is a mixture) of 0.1 g/L in PBS buffer. Dynamic light scattering of dilute solutions showed the presence of particles with diameters in the range 10−800 nm. Nanoparticle tracking analysis (NanoSight LM10) showed the majority of nanoparticles had hydrodynamic diameters 100−200 nm. Dialysis experiments revealed that 58 % of a 0.1 g/L CuChl was present as dispersed particles. QCM-D measurements showed that aqueous CuChl adsorbs on surfaces giving a maximum adsorbed coverage, Γmax, that is sensitive to the type of surface; cellulose > polystyrene>> pullulan > silica = 0. No adsorption was observed on silica. Centrifuged CuCe6, a pure chlorin, showed essentially the same adsorption behaviors as centrifuged CuChl mixtures on polystyrene, suggesting that chlorins are the dominant adsorbed species from CuChl mixtures on hydrophobic surfaces. Centrifuged CuChl and CuCe6 usually gave lower Γmax values compared to non-centrifuged samples. In all cases, rinsing with buffer after CuChl or CuCe6 adsorption resulted in the immediate removal of some of the adsorbed chlorins. With cellulose there was no further dissolution, whereas with polystyrene, the adsorbed layer eventually was entirely removed. AFM measurements showed the adsorbed materials included nanoparticles. The surfactants SDS or DTAB could displace CuChl adsorbed on cellulosic filter paper only if the surfactant concentrations were above the critical micelle concentration.