Surface characterization of di-(2-ethylhexyl)dithiophosphoric acid-activated composite membranes by x-ray photoelectron spectroscopy

Surface chemical characterization of a set of activated composite membranes containing different concentrations of di-(2-ethylhexyl)dithiophosphoric acid (DTPA) as carrier was performed by x-ray photoelectron spectroscopy (XPS). As a result of the addition of DTPA to the membrane samples, the N 1s signal intensities decrease, whereas the P 2p signal intensities increase with increasing carrier concentration. Upon acid addition, the polyamide component of the membrane was protonated and a new peak at higher binding energies was observed in the N 1s XPS signal, due to the increase of the positive charge of these protonated nitrogen atoms. The S 2p signals indicate the coexistence of two contributions assigned to the presence of the acid carrier ionically bonded to the protonated amide and to free acid molecules.