Phospholipid organic monolayer at the air-water interface was studied in the presence of magnesium ions. For this reason the domain size by Brewster angle microscopy (BAM) as well as the Gibbs energy by thermodynamic analysis is evaluated. Domain size increase is discussed with respect to the shape equation on the basis of the electrostatic energy contribution. By coupling these techniques it was found that for ion adsorption at first it is preferred domain boundary and liquid expanded (LE) phase. Only after reaching of specific concentration (30 mM) ions are adsorbed also inside of the liquid condensed (LC) domain. Evaluation of BAM images and thermodynamic analysis reveals existence of pure repulsive forces: it is illustrated by gradual increase of average domain area. Additionally, in low magnesium ion concentration region it was successful to use Langmuir adsorption isotherm for fitting the observed domain area. (C) 2008 Elsevier B.V. All rights reserved.