A VARIETY OF LIPOPHILIC AMINES INCORPORATED IN LIQUID MEMBRANES EXHIBIT POTENTIOMETRIC RESPONSES TO NEUTRAL PHENOLS

A variety of lipophilic amines incorporated in PVC matrix liquid membr anes exhibited anionic potentiometric responses to phenolic compounds at the pH conditions under which the phenols exist mainly or exclusive ly in their undissociated, neutral forms. The examined lipophilic amin es include a macrocyclic pentaamine, tri(decyl)amine, 4,7-diphenyl-1,1 0-phenanthroline (bathophenanthroline), 4-octadecylpyridine, and sapph yrin. The potentiometric selectivities of the membranes based on lipop hilic aliphatic amines (B) reflected the acidity (hydrogen bond donor activity) and lipophilicity (extractability) of the phenols (ArOH). si milarly as membranes based on lipophilic quaternary ammonium salts (Q( +)X(-)). The anionic responses were explained on the basis of a decrea se in the charge separation of protonated amines (BH+) and their count eranions (X-) across the membrane interface. Possible processes leadin g to a decrease in the charge separation between BH+ and X- are (i) co mplexation between ArOH and BH+X-, followed by proton dissociation and ejection of HX into the aqueous phase, as well as (ii) complexation b etween ArOH and B. The membrane based on sapphyrin showed a high poten tiometric selectivity to catechol, possibly due to geometrical discrim ination of the oi tho dihydroxy structure of catechol by the nitrogen( s) on the rigid macrocyclic structure of sapphyrin.