Adsorption of alkyl triphenylphosphonium amphiphiles on Nafion membranes. X-ray photoelectron spectroscopy and static secondary ion mass spectrometry analysis

Abstract

Conductivity, UV, and attenuated total reflectance IR measurements show that n-alkyltriphenylphosphonium amphiphiles adsorb on a Ndion 117 membrane. Approximately 20% of the Ndion protons are exchanged for a cationic amphiphile (n-hexadecyltriphenylphoephonium). Diffusion of amphiphile through the membrane was not observed. Once adsorbed, the amphiphiles did not leach from the membrane. Surface-sensitive techniques (x-ray photoelectrion spectroscopy, static secondary ion mass spectrometry) were used to investigate the presence, concentration, and distribution of the amphiphile in the Ndion membrane. Our experiments point to an incomplete monolayer coverage of the membrane, the molar ratio of amphiphile to sulfonate groups being only slightly less than 1 in the uppermost 2-5 nm. The amphiphile is bonded to the membrane, most likely via an ionic bond with the sulfonate groups. X-ray fluorescence measurements show that the amphiphile is also present in the bulk of the membrane, at least in the uppermost micrometer. However, in the bulk the concentration of amphiphile is significantly lower than the sulfonate groups. These results show that thin and stable amphiphilic layers can be made on a solid support material using adsorption of an amphiphile and coupling via an ionic bond.