Simultaneous adsorption of poly(N-vinyl-2-pyrrolidone) and hydrocarbon/fluorocarbon surfactant from their binary mixtures on hydrophilic/hydrophobic silica

The simultaneous adsorption of lithium dodecyl sulfate (LiDS)/lithium perfluorooctanesulfonate (LiFOS) and poly(N-vinyl-2-pyrrolidone) (PVP) from PVP-LiDS and PVP-LiFOS binary mixed aqueous solutions on hydrophilic/hydrophobic silica was investigated. The conformation and layer thickness of PVP adsorbed on silica were also estimated by using ESR and photon correlation spectroscopy (PCS), respectively. By introducing a sedimentation field-flow fractionation (sedFFF) separation step prior to the PCS sizing to fractionate the adsorbate into cuts of uniform particle size, it is possible to improve the accuracy in these size measurements. The adsorption of PVP is enhanced in the presence of LiDS at low LiDS concentration owing to the formation of a surface complex of PVP and LiDS, followed by a decrease at high LiDS concentration. However, this increment on hydrophobic silica is significantly greater than that on hydrophilic silica. A similar trend is also observed in the PVP-LiFOS system. In both the PVP-LiDS and PVP-LiFOS systems, the conformation of PVP adsorbed, in particular on hydrophilic silica, is changed remarkably from loops and tails to trains with the surfactant concentration. The results obtained from ESR show a good relationship with the thickness of PVP adsorbed obtained from PCS.