INTERACTION OF DPPC MONOLAYER AT AIR-WATER-INTERFACE WITH HYDROPHOBICIONS

Interaction of a Langmuir monolayer of dipalmitoylphosphatidylcholine (DPPC) with different hydrophobic ions added to the aqueous subphase w as studied using surface pressure-area and surface potential-area isot herm techniques. Strongly hydrophobic tetraphenylborate (TPB) anions c ause marked expansion of the monolayer and drastic decrease of surface potential. Both effects grow gradually with the TPB concentration, th e first shows also an increase with the ionic strength. The influence of the less hydrophobic picrate anions is qualitatively similar, but e ssentially weaker than that of TPB. The results obtained can be descri bed in terms of the competition of two effects: (i) both the hydrophob ic interaction and local electrostatic attraction between anions and a mmonium moiety of DPPC promote incorporation of hydrophobic ions into the monolayer, that results in its charging; (ii) long-range electrost atic repulsion between the charged monolayer and ions prevents further incorporation of the later. In contrast to TPB anions, the strongly h ydrophobic tetraphenylphosphonium (TPP) cations have only a minor infl uence on the DPPC monolayer. This marked difference can be explained b y taking into account dissimilarities in the degree of hydration, char ge density and migration ability between the negatively-charged phosph ate and the positively-charged ammonium moieties of DPPC head group. ( C) 1998 Elsevier Science S.A. All rights reserved.