Interaction of DPPC monolayer at air-water interface with hydrophobic ions

Interaction of a Langmuir monolayer of dipalmitoylphosphatidylcholine (DPPC) with different hydrophobic ions added to the aqueous subphase was studied using surface pressure-area and surface potential-area isotherm techniques. Strongly hydrophobic tetraphenylborate (TPB) anions cause marked expansion of the monolayer and drastic decrease of surface potential. Both effects grow gradually with the TPB concentration, the first shows also an increase with the ionic strength. The influence of the less hydrophobic picrate anions is qualitatively similar, but essentially weaker than that of TPB. The results obtained can be described in terms of the competition of two effects: (i) both the hydrophobic interaction and local electrostatic attraction between anions and ammonium moiety of DPPC promote incorporation of hydrophobic ions into the monolayer, that results in its charging; (ii) long-range electrostatic repulsion between the charged monolayer and ions prevents further incorporation of the later. In contrast to TPB anions, the strongly hydrophobic tetraphenylphosphonium (TPP) cations have only a minor influence on the DPPC monolayer. This marked difference can be explained by taking into account dissimilarities in the degree of hydration, charge density and migration ability between the negatively-charged phosphate and the positively-charged ammonium moieties of DPPC head group. (C) 1998 Elsevier Science S.A. All rights reserved.