Effects of anions in subphases on a Langmuir monolayer of N,N-dimethyl-1-octadecanamine

The surface pressure-area (pi-A) and surface potential-area (Delta V-A) iso therms of N,N-dimethyl-1-octadecanamine (C18NMe(2)) were measured on water subsolutions containing various sodium halides (NaX). The amine monolayer s howed an expanded liquid (LE) to a condensed liquid (LC) phase transition o n water at pH 7 containing 0.01 mol dm(-3) NaF, while only the LE phase exi sted on a subsolution with NaI. The Langmuir monolayer (LM) regularly expan ded and the A value at pi = 30 mN m(-1) increased in the order F- < Cl- < B r- < I-, while the Delta V value decreased in the order F- > Cl- > Br- > I- . The X--dependent pi-A isotherm of the C18NMe(2) LM is interpreted in term s of deprotonation from the protonated amine molecules at the LM surface up on compression. Similarly, the C18NMe(2) LM expanded regularly in the prese nce of AMP, ADP, and ATP. Such a phenomenon is understandable by assuming t hat a high-valent anion in subphase is bound to protonated amine molecules more strongly than a low-valent anion.