Using the photothermal effect to improve membrane separations via localized heating

A new strategy is presented to achieve local heating of a membrane during a separation process, thereby increasing the membrane fluxes without affecting the membrane selectivity. By incorporating gold nanoparticles into a membrane structure, it is possible to heat the membrane during the separation process by light irradiation, applying the principle of photothermal heating. Gold nanoparticles are generated inside a premade cellulose acetate membrane. This composite material is characterized by electron microscopy and ultraviolet-visible spectroscopy. The effect of the photothermal heating of the gold nanoparticles on the membrane temperature, flux and selectivity is investigated by irradiation with a continuous wave argon-ion laser operating at 514 nm in a specially adapted filtration cell. It is shown for membranes containing a gold to polymer weight ratio of no more than 2% that water fluxes are increased by 15% while pure solvent fluxes can be increased up to 400% for ethanol and isopropanol. There is no significant effect of the photothermal heating on the rejection of the dye bromothymol blue in ethanol filtrations. To increase fluxes of a given membrane without lowering its selectivity is a highly desired but rarely found effect in membrane technology. The described method is most applicable for solvents with low thermal conductivity values, as energy loss by transfer to the medium is then occurring at a slow rate.