X-ray photoelectron spectroscopy analysis of di-(2-ethylhexyl) phosphoric acid activated membranes

Surface chemical characterization of activated composite membranes, which consist of a polyamide/polysulfone support containing different amounts of di-(2-ethylhexyl) phosphoric acid as carrier, was performed by X-ray photoelectron spectroscopy (XPS) in order to obtain information about the nature of the chemical bonding between the carrier and the membrane top layer. XPS spectra of the top layer of the polymeric support (polyamide) show bands in the C 1s, N 1s, O 1s, and P 2p regions. The N 1s and O 1s signals of the polyamide layer were asymmetric and could be deconvoluted in two peaks that correspond to the coexistence of free and hydrogen bonded polyamide. To support this assignment, primary amides such as benzamide and n-butyramide, which can associate themselves forming hydrogen bonding, and a tertiary amide, N-benzoyl morpholine, unable to form hydrogen bonding, were also studied by XPS. The N Is asymmetric signals of benzamide and n-butyramide were deconvoluted in two peaks due to the coexistence of free and hydrogen bonded species, while the N Is signal of N-benzoyl morpholine is symmetric and corresponds to the existence of free amide alone. As a result of the addition of di-(2-ethylhexyl) phosphoric acid to the polymeric matrix, the N Is signal intensities decrease, while the P 2p signal intensities increase with carrier concentration to a maximum corresponding to surface site saturation. Upon acid addition, the polyamide was protonated and an expected chemical shift of the N Is signal to higher binding energies was observed due to the increase of the positive charge of the nitrogen atom. This type of chemical interaction allows to fix the carrier in the membrane without its complete immobilization, On the other hand, the surface concentration of N and P, determined by XPS, indicates that a concentration of 400 mM of the carrier in the casting solution is sufficient to saturate the surface of the membrane. (C) 2000 Academic Press.