PDMS membranes modified by polyelectrolyte multilayer deposition to improve OSN separation of diluted solutes in toluene

New functionalized poly(dimethylsiloxane) (PDMS) membranes were prepared by the surface modification of the commercially available PERVAP4060 membrane (Sulzer (TM)) through cold plasma activation followed by the layer-by-layer assembly method. Four different pairs of polyelectrolytes, i.e., poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA), PAH/poly(sodium 4-styrene sulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDDA)/PAA, and PDDA/PSS, were used to coat the PDMS layer by the controlled deposition of successive nanolayers (20-50 nm). PERVAP4060 was systematically modified by the deposition of 10 bilayers of polyelectrolyte pairs. These membranes were characterized by contact angle measurements, scanning electron microscopy (SEM), ellipsometry, and atomic force microscopy (AFM). The nanofiltration membrane performance for the rejection of four diluted solutes-two soluble catalysts 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (R-BINAP) and tetraoctylammonium bromide (ToABr) and two linear aliphatic molecules n-Tetratetracontane (C44H90 or C44) and n-Hexadecane (C16H34 or C16)-was studied in the toluene feed solution at up to 40 bar pressure. It was shown that the organic solvent nanofiltration (OSN) mass transfer properties differ clearly depended on the layer-by-layer chemical structure and the characteristics of the solutes. Thus, the intrinsic permeance of toluene in the PEL multilayers were calculated by applying resistance-in-series model. Despite the highly diluted concentrations of the solutes, i.e., < 1 wt% for R-BINAP and ToABr, and the high solvent permeate flux, the ToABr, R-BINAP, and C44 solutes were markedly rejected by all the membranes. ToABr had the highest rejection coefficient, up to 97%. As a trend, higher the polarity of the bilayer assembly, lower was the mass transfer of the solvent compared to that of the pristine membrane. The results indicated that a solution-diffusion mechanism is likely to apply. The best rejection of R-BINAP (up to 88%) was obtained when the membrane was coated with 10 bilayers of PAH/PSS. Thus, these prepared polyelectrolyte (PEL)-modified membranes have potential applications in homogeneous catalysis industry, for example in olefin metathesis for expanding the lifetime of soluble catalysts by separating them at mild OSN condition while ensuring the permeation of products.