Physisorption of SDS in a hydrocarbon nanoporous polymer

Surface modification of nanoporous 1,2-polybutadiene of pore diameter approximately 15 nm was accomplished by physisorption of sodium dodecyl sulfate (SDS) in water. Loading of the aqueous solution and the accompanying physisorption of SDS into the hydrophobic nanoporous films were investigated in a wide range of concentrations. The loading showed varying dependence on the SDS concentration. No loading was observed for SDS concentrations below 4.0 mM. At concentrations above 5.0 mM, the initial part of loading showed a linear dependence on the square root of time, which can be interpreted as diffusion-controlled dynamics. Both the specific equilibrium loading and the final SDS adsorption reached plateau values at concentrations above 6.8 mM. The infiltration of SDS into the nanoporous film was mainly followed by gravimetry and for a few samples confirmed by X-ray photoelectron spectroscopy (XPS). The SDS adsorption isotherm can be well described by the Langmuir model, consistent with a monolayer adsorption onto the pore walls. The surface modified by the surfactant clearly showed water wettability, in contrast to the original hydrophobic nanoporous 1,2-PB. We also looked at the release process of SDS out of the nanopores in the presence of excess of water or methanol. A discussion of the very different time scales of release in the two media is presented at the end of the paper.