Using an effective surface charge to explain surface potentials of Langmuir monolayers from dialkyldimethylammonium halides with the Gouy-Chapman theory

The use of an effective surface charge density has allowed the Gouy-Chapman (CC) theory to explain surface potential isotherms of Langmuir monolayers of dioctadecyldimethylammonium bromide (DODAB). The effective surface charg e density of DODAB monolayer increases with the electronegativity of the co unterions in the subphase. The pressure-area isotherms indicate a very cond ensed monolayer for DODAB spread on an I--containing subphase, which exhibi ts the lowest surface charge density, whereas the monolayer on a F-containi ng subphase is extremely expanded owing to the high surface charge density or electrostatic repulsion between headgroups. (C) 2001 Published by Elsevi er Science B.V.